BOOK1 - The High Pressure Die Casting Process

THE HIGH PRESSURE DIE CASTING PROCESS BOOK - 1

The High Pressure Die Casting Process

Menu des moulages sous pression des alliages d'aluminium

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INTRODUCTION Whoever works in the aluminium alloy foundry sector, unfortunately knows, only too well, the state of affairs from the point of view of documentation. I am referring to the enormous difficulty encountered when searching for technical literature or manuals that would help one to understand the diverse technical facets of foundry processes. A work of technical literature that is clear, organised and exhaustive without resorting to scientific details, which furnishes the indispensable basis for understanding the metallurgical phenomena of the aluminium casting foundry, seen from the viewpoint of the die caster, the plant manufacturer, the castings designer and the end user. Bearing in mind this already rather complex problem, then imagining looking deeper into specific sector of the aluminium foundry, such as die-casting, then the quest for bibliographic data becomes a veritably desperate undertaking. It is in this context, and with these objectives in mind, that we have taken the opportunity to take advantage of the valuable depth of experience in the aluminium alloy die-casting field of Luigi Andreoni, enriched with contributions from such worthy experts as Giorgio Pomesano and Mario Casè. We, in Edimet, as a publishing house that has taken on the duty of promoting the best technical literature on metals, and especially on aluminium, can consider ourselves well satisfied.

Mario Conserva

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1 – INTRODUCTION TO HIGH PRESSURE DIE CASTING

1.1 – The process

The transformation of metals in manufacturing items by means of melting and casting is an ancient art. The relative technology, known to the Ancient Egyptians, has through time seen a number of different methodologies : sand casting, gravity casting, HPDC and others were to follow. Die-casting is a relatively young technique (the first presses were built in this century), but it has had a very rapid evolution. Under certain conditions it is the most rapid and economic means of transforming metals into manufactured items having a high standard of finish. The evolution of die-casting has been determined by the model of development of our society, revolving around the production of consumable goods in long and extremely long production runs and has found applications in all fields of manufactured products and complex equipment that demand non-ferrous components. The machine necessary for the production of die-cast pieces is specific to the t he process of die-casting. The term die-casting is the abbreviation of casting under pressure and is synonymous with die-cast. It defines the process according to which the molten alloy is cast in a metallic mould (die) and is submitted to pressure. This causes the following effects : -

very rapid filling of the die cavity, feeding of solidification shrinkage, perfect filling of the die cavity, a fine crystalline structure.

The metals usually used in HPDC, in order of importance with respect to the volumes of manufactured items, are as follows : -

Aluminium and its alloys (AlSi, AlSiCu, AlMg) ; Zinc and its alloys (Zn + Al + Mg alloy) ; Copper and its alloys (brass, bronze) ; Tin and its alloys.

Because of the greater difficulties of handling aluminium, die-casting of such alloys has always presented a great number of problems but, at the same time, offers the greatest number of solutions to resolve them. For this reason, it can be justifiably considered that this technology is, overall, the prime mover in the die-casting field. We shall therefore make reference to this form now on in this publication. We may add, that recently die-casting has also been tried in manufacturing items in ferrous alloys. A system called Ferro D has been experimented with in America for the die-casting of stainless steel. However, the development of ferrous die-casting has been halted due to the very high melt temperatures required, resulting in an unsatisfactory die life.

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1.2 – Economic aspects of the process

We have said that the die-casting process is the most rapid and economic means (under certain conditions) for the manufacture of some items. These conditions are principally determined by the high investment in machines and equipment and the expensive dies, factors that determine high capital costs, which become less and less with the production of increasing numbers of pieces of the same type. The manufacture of die-cast pieces is therefore economically valid where the production runs are of the order of tens of thousands of pieces.


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1.3 - The state of the art of die-casting Satisfactory results can be obtained only by possessing well understood techniques in the various sectors, techniques that rest on rigorous scientific bases. The elements that compete and interact in the process to achieve constant final results on long and very long runs, must be strictly controlled and the variables involved rendered as constant as possible. These variables are manifold and varied in nature. We find such factors as physical-chemical, metallurgical, thermal, mechanical (kinematics and dynamics) and hydraulic – all factors that must harmonise for a successful outcome to the process. The above require the knowledge and the mastery of various disciplines, but for the practical production of new pieces, new dies, new machines and equipment : experience, sensibility, intuition, creativeness and a passion for the work. Today it is possible to produce pieces to high standards conforming to manufacturing specifications, with complex geometries, minimal wall thicknesses, extremely tight dimensional tolerances, excellent mechanical properties, pressure holding, high quality surface finish, bores and/or counterbores ready for threading and minimum machining allowance for subsequent operations, etc. The quality demands are therefore very high and are tending to rise ever higher. The die-caster is therefore, more and more frequently required to conform to preset qualitative standards of a more and more demanding nature, as a form of insurance or quality guarantee, under the relative certification. This implies the employment of production means of increasing sophistication and greater acquaintances of the technologies and operational techniques of the process from the designers, the dies makers, the production engineers and the operators. The containment of costs is tied to the rate of production, so the presses are often equiped with various devices to automate and mechanise the process, such as robots for the extraction of the cast piece and means for the automatic lubrification of the die cavity, etc. A decisive factor in the containment of the costs and the achieving of high quality standards is the role played by the die, which must be designed, manufactured and operated in such a way as to accommodate the forecast production rates.

HIGH PRESSURE DIE CASTING IN THE AUTOMOTIVE INDUSTRY For some decades now the automotive market has constituted the most significant market sector for aluminium castings. In Europe it represents 80% of the applications for them.In 2001 a European vehicle contains on average 90kgs of aluminium of which 70kgs are in the form of castings. Of these approximately 50% are pressure die castings.The majority of these castings i.e. more than 80% are in AlSi9Cu3 alloy.

The main applications are traditionally associated


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with the engine and transmission with a large number of housings (oil, pumps, clutch, gearbox, transmission) and above all cylinder blocks for which the swing towards aluminium is well advanced for petrol engines and already well under way for new direct injection diesel engines. But one more recent application is representing a strong development potential : it uses the optimised process known as vacuum high pressure die casting and involves bodywork components and suspension parts. They have appeared in production with the bodywork nodes of the Audi A8, front and


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rear cross members for Porsche, then larger dimensioned parts for the Audi A2 and the rear cross members of the AlfaRomeo 156 and 166 as well as the front engine cradles of the S and then the C class Mercedes.

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For all these applications, the Casting Alloys Business Unit works in partnership with its customers by drawing upon its own know-how and on that of Pechiney’s Voreppe Research Centre. The scope of work involves the metallurgy and the processing of alloys, the design and dimension calculations in static, dynamic and crash modes, and the simulation of the behaviour of the components and their production processes. It also participates in the technical and economic evaluation as well as in prototype manufacturing. These partnerships with manufacturers and component suppliers are widened through Pechiney Automotive which co-ordinates the development of such sub-assemblies which often also comprise rolled and/or extruded products. Here again the Voreppe Research Centre completes the skills mentioned above by its knowledge of and its equipment for assembly, surface treatment, corrosion resistance, etc.


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1.4 - More recent technological developments

The die-casting process has been the subject of conspicuous investigations and studies throughout the past decade. Studies and experiments have been concerned with the examination of the filling conditions of the die cavity and of the conditions which influence such filling during the casting. This has been done to establish a correlation between mechanical and technological characteristics of the cast piece and the casting conditions, in order to enunciate mathematical rules which would confer a scientific character to the operation of die-casting, equal to that of any other industrial procedure. The research carried out to date, has served to establish the broad outline of some of the typical values of the forces in play, but still much remains to be done and many difficulties present themselves in determining how much these variables are tied to the geometric form of the piece. These studies have however, led to notable progress in the construction of die casting machines, especially the parts concerning injection, and today there are machines available that very much facilitate the attainment of high class die-castings. With the incentive of meeting the quality needs touched on above, new methodologies and approaches to the die-casting of aluminium alloys have been developed. These are : -

Acurad (slow filling of the die cavity), Pore free (injection into the cavity pre-filled with oxygen), Vacuum (injection with the simultaneous aspiration of air from the cavity), Vacural (injection with alloy drawn from the furnace).

The acquired knowledge, the improvement to the machines and the new cavity filling methods permit the production of high quality castings at high production rates.


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2 – THE THEORY OF THE HIGH PRESSURE DIE CASTING OF ALUMINIUM ALLOYS

2.1 – Filling of the die cavity Various theories have been formulated around the way in which the jet of metal fills the die cavity (fig. 2.1.1). The nearest to reality is that put forward by L. Frommer in which the method of filling is as shown in fig. 2.1.2. This hypothesis has been recently verified with the high speed filming of the behaviour of casting, in a transparent die, using a substance with a specific weight and viscosity equal to that of aluminium. The jet of metal, beginning from the ingate, is projected to the opposite extremity of the cavity and then flows back along the ways of least resistance towards the ingate. It is understood therefore, that the form, the position, the dimensions and the geometric configuration of the piece influence the result of the casting. Not being predictable by the use of standard formulas, the geometry and the position of the ingate when making a die caster, remain factors which are decisive to the outcome of the casting, and ones which rest with the experience and the ability of the designer.

Fig. 2.1.1 – Filling of the die : a) filling the shot sleeve, b) first phase : injection of metal close to the cavity and escape of the air, c) second and third phase : filling of the die cavity and application of shrinkage compensation pressure, d) extraction and ejection of the casting.


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Fig. 2.1.2 – Filling of the die cavity according to Frommer


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2.2 - The escape of air from the cavity

The method and the duration of the die cavity filling are conditioned by the ability of the air contained in the die and in the shot sleeve to escape. Theorically this should happen so that the incoming metal pushes the air from of it, which then must escape (see fig. 2.1.1, note b) through openings created for this purpose in the die (air outlets or vents). We shall always have a die-casting which is more or less porous according to the way in which the air is vented and the influence of the factors we have mentioned ; we shall now examine these further.


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2.3 - Time and method of cavity filling The filling time cannot have a casual value nor can it be fi xed arbitrarily. To obtain a good diecasting it is necessary that the cavity is filled completely before the solidification of the metal in the die. The time in which this solidification occurs is related to t he following factors : solidification range (the temperature difference between the beginning and the end of solidification of each alloy) ; - temperature of the metal ; - temperature of the die ; - thickness of the piece. In the following table we give some examples of solidification times as functions of the wall thicknesses of given castings. As can be seen, these are extremely brief times, and they are inclusive within a range that is determined by such factors as the type of alloy and the thermal conditions of the metal on entry, among others. Thickness (en mn)

Time (seconds)

1,50

0,01 - 0,03

1,77

0,02 - 0,04

2,00

0,02 - 0,06

2,30

0,03 - 0,07

2,60

0,04 - 0,09

3,00

0,05 - 0,10

3,80

0,05 - 0,12

5,00

0,06 - 0,20

6,35

0,08 - 0,30

Solidification times of a pressure die-casting, according to the wall thickness.


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2.4 – Solidification of the metal in the die

One of the peculiar characteristics of the die-casting is the attainment of castings having very tight dimensional tolerances. This is possible due to the use of a metallic mould (die) and to t he fact that during the solidification, the metal is not able to shrink spontaneously spontaneously and is forced by the applied pressure to adhere to the walls of the die. However, there is shrinkage that cannot be eliminated (see the solidification diagram in fig. 2.4.1). The still molten metal coming from the biscuit plays an important role in this phenomenon ; passing into the casting, it feeds it during the solidification phase and eliminates or reduces the shrinkage that can form in correspondence correspondence to the hot spots and the thicker parts. Because this feeding of the piece happens during the solidification, it is essential that the pressure applied to the metal, and the cavity filling cycle, are completed within a tight time frame and are precisely executed. Therefore, the cavity filling f illing time is decisive in obtaining sound, high quality castings.


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2.5 - Extraction of the casting

When the casting has solidified and is dimensionally stable, it must be extracted from the die in the shortest possible time. The extraction can be carried out when the solidification is complete and the mean temperature has fallen to a value between 350 and 250°C. For the metal to cool to within the above mentioned range, it requires a certain amount of time, commonly called the cooling time or the solidification time, which is related to the temperature of the metal/die, the characteristics of the alloy being used and the type of die, according to its ability to shed heat. The magnitude of this cooling time is one of the factors that determines the production rate. The cooling must be as brief as possible, but it cannot be fixed arbitrarily. In fact, should the piece be extracted at a temperature higher than the optimal range mentioned above, the alloy will be found to be in a state of brittleness brittl eness and could incur distorsions or hot cracks.

Fig. 2.4.1 – Morphological diagram of the solidification structure of the metal inside the metal die : bt, thermal barycentre line ; tm, temperature of the metal = 670°C ; ts, temperature of the die = 200° C. A : cortical layer, thickness 0.1 – 0.3 mm, a function of the thermal gradient between the alloy and the die ; very fine compact metallic grain ; B : dendritic zone (crystalline arms proceeding from the outside towards the inside) ; coarser metal grain ; presence of interdendritic voids and porosity ; C : thermal barycentre zone, the last to t o solidify ; coarse metal grain ; interdendritic voids between the individual crystalline structures ; porosity and formation f ormation of shrinkage cavities.

Other drawbacks will occur if this time range is exceeded ; in this case the mean temperature values of the piece will fall too low, near to the temperature of the die, and the shrinkage of the metal will make the extraction more difficult, especially where the geometry of the casting involves undercuts and/or holes. In these latter cases, the projections of the form and the pins of the die, create zones of opposing shrinkage around themselves and therefore create a particular resistance to extraction.


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To make extraction of the piece possible, the die is constructed in such a way as to present adequate draft and the pins have a suitable taper. The magnitude of the draft i s based on the functionality of the piece and its geometric characteristics, including its thickness. These precautions however, are not on their own sufficient, and the extraction of the piece is not possible without suitable die lubrification.


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2.6 – Lubrification of the die

To extract a casting without incurring dimensional and structural alterations, it is necessary to coat the die surface with a thin lubricating film ; this favours the separation of the piece at the moment of the extraction from the die. The lubricant must preserve its essential characteristics after contact with the metal. Any type of lubricant brought up to these temperatures will burn and leave a residue on the surface of the die and/or the piece. The more widely used substances for this purpose are graphite, aluminium oxides or talc, dispersed in water, grease, oils or waxes. We have said that, for the combined actions of speed, temperature and pressure, the metal would tend to adhere to the die ; it is therefore the lubricant that, with its separation properties and the film on the surface of the die, which prevents this adherence. In addition to its property of separation, the die release agent carries out another important function too. The molten metal meets a certain resistance to flow when it enters the die cavity because of its own viscosity and could also become prematurely cold and slow down or stop the metal flowing behind it. It is the lubricant film that, being a poor conductor of heat, assists the flowing of metal. Let us summarise the characteristics required in a good lubricant : -

allow the extraction of the piece from the die, prevent, with its isolating function, the welding of the metal to the die (sticking, erosion), facilitate the filling of the die cavity, act as a true lubricant for the moving parts of the die.

The agent in which the chemical-physical characterising agents are dispersed must evaporate from the heat accumulated from the die, when it comes into contact with it or at any rate before the arrival of the metal. If it doesn’t happen or it happens in an incomplete manner, as soon as the residues of lubricant come into contact with the molten metal they form i nto gases. These gases expand to a volume several times greater than the volume of the cavity and must be vented, otherwise they will induce or accentuate porosity in the casting.


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2.7 – Characteristics of the die casting machine

The machines have been constructed and perfected (particularly the injection systems) to take account of all the factors that influence the process. The filling of the die cavity can be schematically subdivided into the following phases as illustrated in the already mentioned fig. 2.1.1*. -

pre-filling of the shot sleeve, proper filling of the cavity, pressure on the metal during the solidification phase.

The metal is therefore introduced little by little into the shot sleeve until it is full. In the initial part of its travel, the piston expels the air that is in the shot sleeve and carries the metal in proximity to the casting ingate. Because the air is not to stay trapped in the shot sleeve, it needs the speed of the piston not to be superior to 0,25 m/s. If this speed is exceeded, the metal regurgitates and the air stays imprisoned. Subsequently introduced into the die, the air will create blowhols or excessive porosity. With further travel of the piston, the filling of the die cavity is made. This phase must be completed in the shortest time possible compatible with the thickness of the walls of the casting. Given the order of magnitude of these time intervals, the speed of the piston will be high and will be determined by the quantity of metal to be introduced into the die in the given time. Generally the order of magnitude of this speed is in the 1 to 2 m/s range. When the piston stops because it has completed the filling of the cavity, it applies pressure on the metal. This could also be increased by a pressure multiplier. This pressure is transmitted through the still liquid metal in the cavity, pressurising it and feeding the piece during the solidification. The order of magnitude of this pressure ranges from a minimum of 300 to a maximum of 1.000 kg/cm² (30 – 100 Mpa in Internation System units). This die must be designed bearing this value in mind. The pressure, in the above-mentioned range of values, will be selected with reference to form, dimensions and structural characteristics of the piece. Beyond structural characteristics, a die-cast piece is also often required to have a satisfactory surface appearance, and this factor could at times be the principal one. Various experiments have been carried out to see what are the factors that characterise the surface finish of the piece. It has been ascertained that one of the determining factors is the speed of the jet of metal that enters the die combined with the filling time. The speed of the jet of molten metal must be of the order of magnitude 30 – 45 m/s. This speed is in turn determined by the section of the ingate and by the section and the speed of the injection piston. We can therefore summarise that the resulting die-cast piece, with regards to its structural integrity and its aesthetical appearance, is tied to the following factors : -

configuration of the ingate, thickness and section of the ingate, filling time of the die cavity, speed of the metal at the ingate, physical and technological characteristics of the alloy, speed and pressure of the metal, relationship between the temperature of the metal and the temperature of the die.

*) The figures in the diagrams refer to a horizontal cold chamber machine, but they also can be considered, with some variants, valid for vertical cold chamber machines where, of course, it is not necessary that the initial speed approaches 0,25 m/s (see also the section on die-casting machines).


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3 - HIGH PRESSURE DIE CASTING ALUMINIUM ALLOYS 3.1 – The aluminium alloys All alloys, depending on the type and quality of their constituents, impart to the piece particular physical, mechanical and technological characteristics. The aluminium alloys most commonly used in the manufacturing of castings are those shown in table 1. Listed at fig. 3.1.1 are various foundry properties, depending on the silicon content, on the binary alloys Al Si. This provides a useful guide to the selection criteria for the most appropriate alloy for the design and manufacture of the piece.

Fig. 3.1.1 – Casting properties of aluminium-silicon alloys in relation to the Al-Si binary equilibrium diagram. Diagram is based on tests carried out by the Foundry Institute of Aachen. a) fluidity ; b) eeding behaviour ; c) shrinkage sensitivity ; d) uniformity of shrinkage ; e) hot tearing tendency.

While at table 2 the aluminium alloys corresponding to the italian UNI specifications are listed together with the equivalents in the main foreign standards institutions.


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3.1.2

– Principal mechanical properties

A die-casting design office would normally take account of the following mechanical properties : -

contraction Rm, in kg/mm² (Mpa) ; yield strength Rp0,2, in kg/mm² (Mpa) ; percentage elongation, % ; Brinell hardness.


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3.1.3 – The more notable technological characteristics The following parameters are considered : -

castability ; machinability ; ability to be polished ; mechanical properties when hot ; shrinkage brittleness ; pressure holding capacity ; resistance to corrosion.

These characteristics are tied, not only as said above, to the type of alloy but also to the quantities of the elements present in the alloy. To understand the significance of the properties of an alloy, it is very useful to know the influence of the various elements present.

Table 1 – Die-casting aluminium alloys Designation the alloy

of Abbreviated designation

UNI standards

Chemical composition Percentage by weight Cu Zn 0,10 0,10

GD AS13

4514 4514

Fe 0,70

Si 12,00 + 13,50

GD-AlSi9MgFe

GD AS9GF

5074

1,30

9,00 + 10,00

0,50

GDAlSi,5Cu3,5Fe GDAlSi12Cu2Fe GD-AlSi5Fe GD-AlSi13Fe GD-AlMg7,5Fe GD-AlNi2Mn2

GD AS8,5C3,5F GD AS12C2F

5075

1,10

8,00 + 9,50

5076

1,10

11,00 + 12,50

GD AS5F GD AS13F GD AG7,5F GD AN2M2

5077 5079 5080 6253

1,20 1, 1,10 1,00 0,70

GD-AlSi13

Mg 0,10

Mn 0,40

Ni 0,10

0,40

0,40 + 0,60

0,35

0,50

3,00 + 4,00

1,00

0,30

0,40

0,30

1,75 + 2,50

0,90

0,30

0,50

0,30

0,50 0,50 0,20 0,20

0,20 0,30 6,50 + 8,00 0,03

4,50 + 6, 6,00 0,50 11,50 + 13 1 3,00 0,80 0,50 0,08 0,70 0,70 Si + Cu < 1,00 GD = die-casting – (*) Solution heat-treated and air quenched ; artificial aged. Impurity Pb

Sn

0,15

0,15

0,15 0,15 0,15 0,15 0,05

0,10 0,10 0,10 0,10 0,05

Ti 0,15

excluded

total

Fe+Mn+Ti

0,30

0,15

Ti

0,10 Ti 0,15 Ti 0,15 Ti 0,15 Ti 0,20 Ti+Mn+Si 0,10+0,20 Fe+Si+Cu


Treatment each

0,10

1,60 1,80 1,60 2,00 0,30 0,50

0,10 0,10 0,10 0,10 0,05 0,10

Minimum mechanical properties Rm Rp0,2 A

HB

GD

Mpa 180 200

Mpa 120 150

% 150 2,00

70

GDTcA* GD GD GD GD GD GD

240 220 270 180 230 200 200

180 150 160 100 130 120 100

1,5 1,0 1,0 2,5 1,5 1,5 4,0

80 85 85 50 75 60 50

GD

1,80

0,30 0, 0,30 0,30 0,20 0,50 0,05 1,90+2,1 1,90+2,1

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3.1.4 – The influence of the constituent elements on aluminium alloys We list here the effects exercised by various elements on aluminium alloys. Iron is always present in aluminium as a generally undesirable impurity in the foundry ; it is however very useful in die-casting because it diminishes the aggressiveness of the aluminium in regards r egards to the moulds. In fact iron reduces the risk of soldering to the mould. It improves the mechanical strength when hot, assisting the extraction of the casting from the mould. If the iron is present at no more than one percent, it refines the mechanical grain and reduces the shrinkage brittleness. Silicon, up to the eutectic value, improves castability, reduces shrinkage brittleness and assists the obtaining of compact castings, it doesn’t reduce r educe the resistance to corrosion in any appreciable way but it reduces the machinability. Copper increases tensile strength, hardeness and hot strength and improves machinability. On the contrary, it notably reduces the resistance to corrosion, even when present in small quantities. The presence of zinc in an alloy improves the mechanical strength, the plasticity and the machinability ; while it decidedly decreases the corrosion resistance properties and induces an appreciable hot brittleness. Magnesium increases the tensile strength and the hardness and maintains good machinability. It increases the corrosion resistance and the ability to t o be polished. However it reduces the castability and increases the shrinkage brittleness ; therefore castings in magnesium alloy can give rise to flaws which occur more frequently the higher the quantity of this element there is in the alloy. In foundry practice, the metal to produce a given casting is obtained initially from i ngots scrap and pieces of waste of the same alloy, melted in distinct furnaces and transported with different ladles for each type of alloy. The blend of scraps, ingots and waste of different alloys, or the use of f urnaces that have contained different alloys and have not been thoroughly cleaned afterward, impart impurities to the alloy, and in certain cases, can deeply alter the characteristics, even if the quantities involved are very small. This alteration can create major difficulties in the moulding of the casting, in the number of rejected castings produced or in the problems arising with castings produced from contaminated alloys when they are later machined or are put into use. Other alterations of the characteristics can be produced, if during the melt the metal has not been brought up to the prescribed temperature. On the contrary if the maximum temperature of the casting is exceeded there would be the formation of oxides and the loss of some components, such as magnesium and zinc. If the temperature of the moulding is not observed, and is very low, it can be seen in the holding furnace that the intermetallic compounds of Fe, Si and the heavier elements contained in the alloy are precipated out at the base of the crucible in the form f orm of crystals. In the absence of adequate stirring or sufficient convective motion, sedimentation phenomenon would occur which would create segregation in the bath.


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This, because of having a high content of elements heavier than the aluminium matrix, separates into strata ever more dense as the bottom of the crucible is approached. If work is carried out in such a way, the composition of the alloy will not be that prescribed and when metal for casting is ladled from the bottom of the crucible, where there elements are to be found in high percentages, difficulties would be encountered, such as a lack of castability and the presence of soldering to the mould, breaking of tooling and other machining difficulties. The difficulties in machining are due to the presence of hard spots revealing that the alloy has not been treated at the prescribed temperature or that the operation has not been fulfilled with the requisite attention to « metal cleanliness », in the sense that high percentages of oxides or intermetallic compounds are to bound because the purification operation has not been carried out in the correct manner. The presence of these hard spots degrades the quality of the castings, and that of the foundry that produces them, apart from creating rejects and contestations. In the following, we show a listing of the principal rules that should be observed for a correct operating cycle.

ALLIAGE DUCTILE POUR COULEE SOUS PRESSION SOUS VIDE CONTRIBUTION AU DEVELOPPEMENT ET A L’OPTIMISATION D’ALLIAGES RESISTANTS ET DUCTILES UTILISES EN COULEE SOUS PRESSION SOUS VIDE POUR DES PIECES DE STRUCTURE ET DE LIAISON AU SOL DESTINEES A L’AUTOMOBILE

JJ. Perrier – Aluminium Pechiney – Activité alliages de moulage R&D

La mise au point et le développement d’alliages coulés sous pression ductiles a été initiée dans les années 80 par l’industrie automobile pour les développements dans le domaine de la carrosserie automobile du type space-frame. Les exigences actuelles de l’industrie automobile en matière de pièces de sécurité pour la carrosserie et les pièces de liaison au sol peuvent être listées comme suit :

Allongements élevés compris entre 5 et 20% suivant les traitements thermiques -


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ciés à une capacité d’absorption d’énergie au choc importante (crash test) Aptitude au soudage (TIG-MIG-Laser) Très bonne résistance à la corrosion saline et sous contrainte Possibilité de traitements thermiques type T2, T4, T5, T6, T7 pour assurer divers compromis entre Rp 0,2 (limite d’élasticité) et A (allongement à la rupture). Résistance à la fatigue Facilité de recyclage Aptitude au rivetage et au sertissage

CHOIX DES FORMULATIONS D’ALLIAGES Les alliages aluminium-silicium avec additions de magnésium en proportions variables constituent le meilleur compromis en ce qui concerne la facilité de mise en œuvre. Ils sont adaptés aux traitements thermiques grâce au magnésium ce qui permet de déterminer un grand nombre de compromis R p0,2 /A associés à diverses gammes de traitements thermiques (T2, T4, T5, T6, T7). Pour les raisons citées auparavant, les alliages industriels actuellement utilisés contiennent entre 9 et 11% de silicium, et des teneurs en magnésium variant entre 0,10 et 0,50 %. Afin d’améliorer les propriétés mécaniques et de mise en œuvre, les recherches effectuées par les producteurs d’alliages européens ont abouti à un certain nombre de perfectionnements parmi lesquels il faut citer :

L’addition de strontium qui permet de modifier l’eutectique AlSi et contribue à améliorer les allongements à la rupture même aux vitesses de solidification rapides et surtout affine les phases contenant le fer et le manganèse. L’optimisation des rapports fer-manganèse pour éviter le collage. La morphologie de ces constituants jouant un rôle important identique sur la ductilité, ils doivent être de petite taille et de forme favorable.


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DEVELOPPEMENT DE L’ALLIAGE CALYPSO 61D Aluminium Pechiney s’est orienté vers une composition à teneurs en fer et manganèse moyennes. Ce qui suit montre que cette formulation présente un potentiel de ductilité important. L’alliage CALYPSO 61D a été formulé sur la base de résultats de recherche internes auxquels ont été associées des connaissances et expériences industrielles antérieures.

Composition chimique de l’alliage CALYPSO 61 D – (lingots de 1 ère fusion) Standard de composition (lingots) dans les lingots livrés par Aluminium Pechiney Mini Maxi

Si

Fe

Cu

Mn

10,0 11,0

0.30 0,50

0,02

0,40 0,50

Mg

Ni

Zn

Pb

Sn

Sr

Ti

0,10 0,40

0,04

0,09

0,03

0,03

300 500

0,10 0,15

Autres Chaque

total

0,05

0,15

Teneurs indiquées en %, sauf Sr en ppm * La teneur en Mg peut être ajustée à un niveau plus spécifique en fonction du traitement thermique prévu et des propriétés requises

Cet alliage permet de couler des pièces conformes à la désignation EN AC-43400 de la norme européenne EN 1706 :1998 moyennant l'ajustement de la teneur en Mg à 0.20 – 0.40%

CARACTERISTIQUES MECANIQUES STATIQUES CARACTERISTIQUES A L’ETAT BRUT DE COULEE - ETAT F Rm Rp0,2 A5 % MPa MPa 270-290 120-140 10-14 Etat F (brut de coulée) CARACTERISTIQUES APRES TRAITEMENT THERMIQUE Rm Rp0,2 A5 MPa MPa % 220-250 100-130 15-20

Etat T4 Etat T5 Etat T6

290-310 280-310

170-190 210-230

5-8 8-12

Domaines typiques des valeurs obtenues à partir d’éprouvettes plates extraites de pièces d’épaisseur 3 mm coulées en sous pression sous vide


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CARACTERISTIQUES DYNAMIQUES Des essais de fatigue ont été faits avec des éprouvettes usinées dans des plaques d’épaisseur 2,5 mm coulées sous pression avec assistance du vide. La limite de fatigue à 10 7 cycles en traction compression est égale à 120 Mpa avec R= 1.


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PROPRIÉTÉS D’USAGE : SOUDABILITÉ Exemple de pièce en Calypso 61 D coulée sous pression sous vide.

Document de l’IFS, Université de Braunschweig

Ce type d’alliage est utilisable pour des pièces moulées assemblées à des pièces corroyées en alliage de la famille 5000 ou 6000 en MIG ou LASER. La photo ci-contre montre un échangeur constitué de deux demi-pièces assemblées par soudure selon le procédé MIG. La qualité du cordon de soudures avec métal d’apport 4043 est excellente et l’étanchéité de la pièce assurée.

CONCLUSIONS L’alliage CALYPSO 61D du type AlSi10MgMn à fer et manganèse fixés à teneurs moyennes possède un bon potentiel de ductilité à l’état F et dans différents états de traitement thermique. Il est parfaitement apte à la réalisation de pièces coulées sou s pression destinées à l’automobile et à d’autres secteurs d’activités. Les performances potentielles de l’alliage ne pourront être exploitées, comme pour d’autres alliages équivalents, que si un grand soin est apporté à la fabrication des pièces, en particulier à la conception du système de coulée et à l’efficacité du système de mise sous vide de l’empreinte (valeur recommandée : pression résiduelle ≤ 50 hPa). Ce type d’alliage laisse au constructeur et au fondeur une grande latitude de choix de


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compromis Rp0,2 /A en fonction des besoins en adaptant : - la teneur en magnésium - les conditions des traitements thermiques. L’état T5 est particulièrement intéressant car du fait de la « trempe » de la pièce dans le moule en cours de solidification, le potentiel de durcissement est suffisant pour beaucoup d’applications.

L’état T2 qui consiste en un traitement de recuit stabilisation favorise les allongements élevés tout en conservant une limite d’élasticité de l’ordre de 100 MPa et a l’avantage

d’éliminer les déformations inhérentes aux contraintes résiduelles de trempe.

D’autres alliages des familles AlSiMg et AlMgSi sont en cours de mise au point pour répondre à des besoins spécifiques dont en particulier celui d'une ductilité encore supérieure.


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3.2 – Preparation of the alloys The alloys used in the production of die-casting pieces are generally obtained from the melting of standard alloy ingots and the contemporaneous melting of foundry returns (scraps castings, rejects and casting overflows, biscuits). The percentage of foundry returns present in the alloy is, on average, of the order of 30 to 40 percent, and in the melt for the production of a piece, where the furnace charge is not made up solely of ingots, which could happen at the start of an initial production run, the percentage of foundry returns must not exceed the above figures. If these limits are exceeded the uncertainty of the chemical composition of the final alloy is increased and inevitably appreciable amounts of undesirable compounds, such as oxides, complex intermetallic compounds, etc … are introduced. So, the use of a mixed charge of ingots and foundry returns is dictated on one part by economic considerations but on the other it also gives metallurgical advantages ; in fact the returns are endowed with a certain « working » that primary ingots do not at that stage possess ; a « working » that translates into a minute crystallisation that is nearer to the finished structure of the required casting. On the other hand, it is risky to run only foundry returns, which usually have a chemical structure altered from the original ; in fact they can be short of volatile elements such as magnesium, or contain excess amounts of heavier elements, such as iron, manganese, etc. For these reasons the ideal composition of the alloy used in a furnace supplying a die-casting press is a constant mixture o f standard alloy ingots and scrap in predetermined percentages. Naturally the ingots and the returns must be of the same alloy. During the melting phase, the maintaining of the molten state and the manipulation of the alloy, suitable precautions must be adopted to prevent contamination and degradation of the alloy. The phenomena connected with the melting of the alloy and the maintaining of the molten alloy, such as the interesting processes of operating the holding furnace, are dealt with in the chapter dedicated to the melting of the alloys. Some contamination factors are given in the following table 3 along with the precautions to be taken, and the remedies to be adopted in order to avoid undesirable metallurgical phenomena.

Tableau 2 – Alliages d’aluminium à mouler sous pression : équivalence entre les normes italiennes et étrangères. Table 2 – Die-casting aluminium alloys : Equivalence of Italian standards to other national standars. Designation of alloy

Abbreviated designation

UNI

USA

France

Germany

standards

A.A

N.F A 57-702

DIN 1712, blatt 1

ASTM B-79 Equiv. Similar GD-AlSi13 GD-AlSI9MgFe GD-AlSi8,5Cu3,5Fe GD-AlSi12Cu2Fe

Equiv.

Similar

AS 13

A S 12

GD AS13

4514

A 413.2

GD AS9GF

5074-74

360.2

A S9G

GD AS8,5C3,5F

5075-79

B 380.1

A S9U3

GD AS12C2F

DIN 1725, blatt 2 Equiv. Similar GD-AlSi12 G/GK-AlSi10Mg (Cu) GD-AlSi9Cu3

5076-74

GD-AlSi5Fe

GD AS5F

5077-74

C 443.1

A S5U3G

G/GK-AlSi5Mg

GD-AlSi13Fe

GD AS13F

5079-74

A 413.1

A S12U

G/GK-AlSi12(Cu)

GD-AlMg7,5Fe

GD AS7,5F

5080-74

518.1

A G6

GD-AlMg9

GD-AlNi2Mn2

GD AN2M2

6253-68

U.K

International

B.S 1490

I.S.O

B.S Aerospace Equiv. Similar LM6

(A-M2N2)

Equivalent AlSi12 (R614)

LM26 LM4 LM8 LM9


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Table 3 – Contamination factors in the bath and remedies Phenomena found in the bath Contamination with other alloys

Contaminating factors

Mixing of baths, ingots extraneous foundry returns

Precautions and remedies and Keep all materials separated

well

and identified Contamination with iron

Tooling not protected, excessive bath Dilution with low iron content temperature metal

Sedimentation of heavy elements

Prolonged waiting times of the metal Renew and agitate bath, high bath level of metal

Reaction with atmospheric humidity

Prolonged waiting, overheating of Degassing the alloy vacuum)

(salts,

nitrogen,

Contamination with oxides and with Introduction of oxidised material, Control of the furnace charges hydrogen oleate, with a high surface/volume ratio (flashes, chips)

Coarse crystallisation


Overheating, iron contamination

Speed of melting and casting, refining

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3.3 – Requirements for a die-casting alloy

The requirements that an alloy must meet to obtain a good die-casting are summarised according to the following points:


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3.3.1

– Castability

Castability, that is the natural disposition of the metal to fill and to take on the shape of the mould, faithfully reproducing even the smallest detail of the cavity, is the primary requisite to obtain a sound, compact casting with a good finish. An alloy with good castability will give a part free of cold shut, and will allow the obtaining of good mechanical properties. This piece is often better than one obtainable from an alloy with higher mechanical properties but which has a lower castability.


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3.3.2

– Temperature stability

A good temperature stability is necessary to allow the extraction of the piece from the mould at high temperatures, so permitting a high production rate without cracks occuring while under the inevitable stress that the piece undergoes during the opening of the cover die and especially, during ejection from the mould.


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3.3.3

– Melting point and solidification range

A low melting point is desirable because, apart from permitting a higher production rate, it means a longer life for the mould, as it is subjected to a lower degree of thermal stress. A wide solidification range is suitable as it allows a slower filling of the cavity, allowing the air to vent and permitting the gates to remain open for a longer period. By this, the pressure of the piston can be transmitted to the piece and can feed the casting during the solidification. This fact attenuates the characteristics of these alloys that, when solidifying, crystallise in a coarse manner, favouring misruns and cavities.


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3.4 – The most usual aluminium alloys

The types of alloys mainly used for die-casting are secondary alloys, obtained that is, by the remelting of scraps (for example crank-cases from automobile and/or aeronautical industries, domestic appliances, semifinished products such as sheets, extrusions, forgings, etc…). These are suitably mixed and alloyed with appropriate additives to achieve the required composition. The stacks of ingots should be marked to identify the type of alloy and the supplier must be provided with an analysis certificate. The compositions most commonly used in die-casting are shown in the previously mentioned tables 1 and 2 along with their expected mechanical properties. Each one of the above-mentioned types of alloys has its own particular features and determining physical, mechanical and technological characteristics. The particular characteristics of an alloy should be chosen and used to advantage in relation to the type and requirements of the piece to be produced. The type of alloy to be used for a particular piece, provided it has not been expressly prescribed by the customer, must be chosen and/or advised by the die caster in relation to the requirements of the piece and to the correspondence of the physical, mechanical and technological properties of the alloy. To summarise the fields of use of the various types of alloys, even only in outline, is not an easy matter, nevertheless the following are some generalised areas : - pieces requiring a resistance to corrosion, call for alloys of the AlSi and AlMg types ; pieces needing high mechanical properties require types AlSiCu or the self-hardening AlZnMg ; - a high surface finish requires the use of types AlMg, AlSiMg or AlNiMg ; - while for pieces to have good heat resistance and AlNiMn alloy should be chosen.

* Since the 20th of may 1998, the European standard NFEN 1706 has come into force « Aluminium and aluminium alloys – Die-castings – Chemical composition and mechanical properties » and the reader is invited to read it.


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4 – EQUIPMENT USED IN PRESSURE DIE-CASTING 4.1 – The die

4.1.1

– Introduction

The die is the means that directly gives the form to the casting. The precision and the value of a good die-cast piece depend on the accuracy with which the die is made in addition to its correct employment on the machine. The die is an assembly of steel components and assembled with precision. The unit makes possible the casting of pieces in long and very long production runs. The dimensions of the die depend substantially on the form and the dimensions of the casting to be made. It should also be borne in mind that these dimensions are also influenced by other factors depending on the degree of mechanical and thermal stress, and the service to which the die is to be subjected. These factors are of the major importance to the duration in service of the die, and to the quantity of pieces it can produce.


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4.1.2

– Generality

The die assembly, when in the closed position, creates a cavity having the form of the piece to be cast and some channels through which the injected molten metal is introduced. This assembly must meet and satisfy various demands; the most important of these are as follows : - exactly reproduce the form and dimensions of the piece ; ensure, at operating temperatures, the relative movement between the moving parts and the extraction of the casting ; resist the thermal and mechanical stresses derived from the injection and from the pressure applied to the molten metal ; - allow, after the solidification of the metal, the extraction of the piece without incurring and/or generating fractures or deformations in the piece. In its simplier form the die is made up of two principal parts : -

the fixed part : it is fixed to the front platen of the press ; the moving part : it is fixed to the moving platen of the press.

On the moving part, but sometimes also on the fixed part of the die, there can be some radial slides, whose presence is due to the particular configuration of the piece (presence of undercuts) and/or to the need to create holes. The employment of high pressures, from 400 to 1.000 kg/cm² (40 – 100 Mpa), on the molten metal, necessitates the need to securely maintain the various component parts of the die in the closed position. These parts are subjected to very high forces and stresses, derived from the clamping force of the press and from the component forces that the various parts exert between them. The production of each die presents problems that differ from piece to piece depending on the individual aspects : -

shape and dimensions, mechanical stresses, thermal stresses, cooling requirements, dimensions and configuration in relation to the press on which the die has to operate.

But common elements are to be found among the different dies, both in form and in function, that can be considered standard : ejectors, pins and guide bushes, assembly pins, etc. Given the impossibility of describing a die type because of the infinite possible variations and degrees of complexity that can be encountered, it is of use to schematically show an example of a die (see sections at fig. 4.12.2 – 4) that would be used to produce a simple piece (fig. 4.1.2.1), comprised of a fixed part and a moving part, and a die, fig. 4.1.2.6, that is for producing a more complex piece (4.1.2.5). This latter is comprised of a fixed part, a moving part, 3 lateral slides mechanically operated by inclined pins on the moving table and a moving slide operated through a system of racks by a hydraulic cylinder. A similar die is represented in the two sections in fig. 4.1.2.7 and 4.1.2.8. By reference to these schematic drawings, we are able to see the terminology with which the different components are identified.


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The glossary follows the more common terminology used in the foundry industry.

Fig. 4.1.2.1 – Sketch of a piece produced by die-casting. Legend for fig. 4.1.2.2 + 4 : 1 – Holding block, fixed die ; 2 – Holding block ; 3 – Holding block, movable die ; 4 – Holding block ; 5 – Brace, moving platen ; 6 – Brace, frame ; 7 – Ejector plate ; 8 – Ejector backplate ; 9 – Insert fixed part ; 10 – Insert movable part ; 11 – Male insert, movable part ; 12 –Bush, shot sleeve ; 13 – Anvil ; 14 – Die centring ; 15 – Die centring ; 16 – Flange ; 17 – Cavity ; 18 – Guide pin, knockout plate ; 19 – Core ; 20 – Pin ; 21 – Lock brace ; 22 – Ejector ; 23 – Ejection tie bar ; 24 – Cooling channel.


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Fig. 4.1.2.2 – Movable die for the die-casting shown in fig. 4.1.2.1 Sectioned views are shown in the following fig. 4.1.2.3 - 4

Fig. 4.1.2.3 – Die assembly for the die-casting in fig. 4.1.2.1 Sections a – b – c – d – e. Numbers refer to the legend in fig. 4.1.2.1


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Fig. 4.1.2.4 – Die assembly for the die-casting shown in fig. 4.1.2.1 Sections f – g. Numbers refer to the legend in fig. 4.1.2.1


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Fig. 4.1.2.5 – Carburator body produced by die casting. See the following figs. 4.1.2.6 – 8. Legend for figs. 4.1.2.6 – 8. 1) Holding block, fixed die – 2) Holding block – 3) Holding block, movable die – 4) Holding block – 5) Moulding box, brace, movable die holder – 6) Moulding box, movable die – 7) Ejector plate – 8) Ejector plate – 9) Insert fixed part – 10) Insert movable part – 11) Male insert movable part – 12) Bush, shot sleeve – 13) Counter-casting – 14) Screw – 15) Semi-die centring bush – 16) Flange – 17) Insert – 18) Guide pin, ejector plate – 19) Insert – 20) Pin – 21) Lock brace – 22)Ejector – 23) Pin – 24) Cooling jet – 25) Movable block, pin carrier – 26) Guide pin, movable block – 27) Movable block, pin carrier – 28) Guide pin movable block – 29) Lower sliding block – 30) Block guide studs – 31) Block lock pins – 32) Lock pins – 33) Core – 34) Core extractor – 35) Inclined core extraction pin – 36) Slide – 37) Slide – 38) Core pin – 39) Inclined slide operating pin – 40) Lug – 41) Ejector plate – 42) Ejector backplate – 43) Brace – 44) Ejector – 45) Guide brace, ejector plate – 46) Guide pin – 47) Cooling – 48) Counter-casting – 49) Die centring pin – 50) Block, inclined pin carrier – 51) Inclined pin – 52) Slide – 53) Inclined pin – 54) Cooling tube.


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Fig. 4.1.2.6 – Fixed die for die casting shown in fig. 4.1.2.5 Numbers refer to the legend in fig. 4.1.2.5


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Fig. 4.1.2.7 – Movable die for die casting shown in fig. 4.1.2.5 Numbers refer to the legend in fig. 4.1.2.5


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Fig. 4.1.2.8 – Movable die for the die casting shown in fig. 4.1.2.5 Numbers refer to the legend in fig. 4.1.2.5


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4.2 – The press 4.2.1 – Generating hydraulic energy 4.2.1.1 – Hydraulic fluids As has been said previously, die casting presses are operated on hydraulic energy. This is conferred to a fluid by pumps driven by electric motors. Before we look more deeply into the characteristics of a hydraulic circuit, it is necessary to speak about the hydraulic fluids. The choice of the fluid is made by reference to the determining properties of the fluid ; these characteristics are listed as follows : -

incompressibility, low viscosity, a clearly defined viscosity/temperature relationship, good antioxidant characteristics, good anti-foaming characteristics, good anti-corrosive characteristics, water repellency, good demulsification characteristics, high flash point, low environmental pollution potential.

4.2.1.2 – Characteristics of the hydraulic fluids Among the more widely used hydraulic fluids in die casting presses (see fig. 4.2.1.2.1) is polyglycol dispersed in approx. 40% water. Since the separation of the glycol from the water is difficult to achieve, these fluids, while not being toxic, are pollutant ; in fact water containing glycol cannot be discharged into sewers as it increases the COD (Chemical Oxygen Demand is the requirement of oxygen necessary to oxidise the substance completely, so making it harmless from an ecological point of view). For this reason leakages and drippings, apart from representing an economic loss to the foundry, are also sources of further expense for the necessary purification which the water must be subjected to before it is discharged from the factory. Timely intervention is therefore necessary to reduce this type of loss from a press to a minimum. 4.2.1.3 – Hydraulic circuits of the press We can talk about a hydraulic circuit and not an oleodynamic one, because as we have already mentioned, presses no longer operate with oil but with hydraulic fluids. A hydraulic circuit can be considered to have two distinct parts : -

a part which supplies energy to the fluid, a part which uses the energy possessed in the fluid.

The hydraulic energy is obtained by converting other forms of energy : in our case we convert mechanical energy supplied by an electric motor (that in turn has transformed electric energy into mechanical energy) into hydraulic energy through fluid delivery pumps. The fluid is characterised by its flow rate and a pressure (see fig. 4.2.1.3.1). A simple hydraulic circuit could be essentially composed of : -

tank, filter, pump, pressure regulator.

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The hydraulic circuit of a metal die casting press is very more complex and sophisticated ; it is normally composed of : - tank for the fluid, - intake filter, - low pressure pump, - delivery filter, - high pressure pump, - high pressure regulator, - non-return valve between low and high pressure, - pump discharge valve, - heat exchanger for the cooling of the hydraulic fluid. On certain machines, for certain user circuits, filters are also installed on the discharge side of the pumps (with the function of ensuring that the fluid is clean before reaching the users themselves. On certain presses for use in some cold countries, a thermoregulation device is also fitted (heating/cooling), which maintains the fluid at its optimal working temperature. 4.2.1.4 – Use of the hydraulic energy On die casting presses the parts that use the hydraulic energy are essentially composed of hydraulic cylinders and hydropneumatic accumulators. In practice, we have :

4.2.1.4.1 – Press group Comprised of : -

opening and closing cylinder ejection cylinder or cylinders

4.2.1.4.2 – Injection group Comprised of : -

injection accumulator, injection cylinder, multiplier accumulator, multiplier cylinder, injection group positioning cylinder.

On some machines this control apparatus is rather complex and sophisticated, such as where variations of speed must be effected during the cylinder travel or where variations of incremental speed are needed, as in the case of the « Parashot » and « Progression » injection systems. In the course of our discussion we shall analyse the structural and functional characteristics of the apparatus and of the above-mentioned components.


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Fig. 4.2.1.2.1 – Non-flammable hydraulic fluids


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Fig. 4.2.1.3.1 – Hydraulic energy generators


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4.2.2

– Construction of die casting press

A die casting machine can be characterised by a construction diagram showing the functions depending on the type of performance required. For whatever type of machine, we can identify schematically three blocks, each of wich, to effect movements, is equipped with hydraulic cylinders operated on the energy supplied by the pumps and by the pressure accumulators. The three blocks (see fig. 4.2.2.1) described hereunder.

4.2.2.1 – Hydraulic energy generator 4.2.2.2 – Press group In which we find : -

an opening and closing cylinder, an ejection device, hydraulic blocks for the operation of radial slides ;

4.2.2.3 – Injection group In this there is mounted : -

a centring device for the injection point, hydropneumatic accumulator, injection device.

This latter will be different depending on the operational method of the machine, which can be distinguished as : -

hot chamber presses, cold chamber presses.

The first type are suitable for the die casting of low melting point alloys (zinc and tin) and for magnesium ; the second are suitable for high melting point alloys (aluminium, copper, etc). Cold chamber machines can be further subdivided into : -

vertical cold chamber presses, horizontal cold chamber presses.

These different types of presses are illustrated in fig. 4.2.2.2. The horizontal cold chamber presses are by far the most widely used and for this reason, except for different specifications, the text will always be referring to them.


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Fig. 4.2.2.1 – The die casting press : E) hydraulic energy generating group ; P) press group ; I) injection group

Fig. 4.2.2.2 – Comparison of the injection systems.


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4.2.3

- General principles of operation

4.2.3.1 – Press group The press group is made up of three plates joined by four columns that also constitute the guides on which the moving plate (fig. 4.2.3.1.1) slides. This is the device that allows the opening and closing of the die ; it comprises : -

a fixed plate, a moving plate.

The die is mounted between them. A device that acts on the pillar nuts allows the adjustment of the clearance between the above plates at the height of the die (fig. 4.2.3.1.2). The closing of the die occurs with a certain force and it is this force that characterises the size of the press ; so when we speak about a 500 ton press, this means that the press is able to close the die with a force of 500 tons, that is 500.000 kg (5 MN in International System units). The four columns of the press are dimensioned as a function of this force ; the hydraulic cylinder that exerts the force for the closing of the die operates a system of toggle-joint levers (see fig. 4.2.3.1.3), which places the pillars under tensile stress. To make clear this function of the machine, we detail here some of the elementary definitions. The closing force is that force that would be needed to be exerted to separate the mating faces of the die valves when in the closed position. The function of the closing force is to oppose the force applied by the pressure agent, through the injection device, on the molten metal within the die cavity, which is translated into a force attempting to separate the two die halves (opening force). Therefore this force, counteracting the opening force applied by the injection, must be superior to this latter to prevent the die opening and allowing the metal, under pressure within the die cavity, from being projected to the outside. For this reason the closing force must be in the region of 15 – 20% greater than the corresponding opening force. The opening force is given by the product of the pressure exerted on the molten metal on the frontal surface of the die cavity ; for example, if the pressure on the metal is 500 kg/cm² (50 Mpa) and the frontal surface of the cavity is 1.000 cm², the opening force of the die would be : 500 x 1.000 = 500.000 kg (5 MN). Therefore the press must have a closing force in the order of 500.000 x 1,25 = 600.000 kg (6 MN). Fig. 4.2.3.1.4 shows the forces that interact in the die casting press. Fig. 4.2.3.1.5 illustrates some situations of inadequacy between the forces and the structure of the press and/or of the die.


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Fig. 4.2.3.1.1 – Movable and fixed plates


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Fig. 4.2.3.1.2 – Movable plate : sliding on columns. A) The motor simultaneously moves the four nuts. B) The crown wheel, driven by the motor, simultaneously moves the four nuts.

Fig. 4.2.3.1.3 – Toggle closing device.

Fig. 4.2.3.1.4 – Interactive forces in a die casting press. CF) closing force, this does not act directly but, after the closing of the die, it puts the columns under tension so tending to provoke elongation : thedie closing force = reactions 1-2, 3-4, 5-6, 7-8. The reactions 1-5, 2-6, 3-7 and 4-8 have equal and opposite values in the same columns ; OF) The opening forces are those tending to open the die and are those generated by the force of injection OF and are equal to : Final specific pressure, in kg/cm² (Mpa) x the frontal surface area of the stack mould in cm² (m²) = kg (N)


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Fig. 4.2.3.1.5 – Diagram of the forces acting on the closing system : A) Closing phase : plates of the press inadequate, excessive closing force, die support surfaces too small with respect to the dimensions of the die holder plate ; B) Injection phase : structure of the die inadequate with respect to the pressure exerted by the injection piston (excessive injection force) ; C) optimal working conditions.


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4.3 – Injection device

We find the injection device on the moving plate of the pres. The casting ejection system is actually a function of the die itself. The press furnishes the necessary movement to this function in the form of : -

speed and force of thrust (forwards) ;

-

speed and force of re-entry (backward).

So, on the moving plate of the press system, we find the injection device made up of a plate operated by 1 or 2 hydraulic cylinders having hydraulic power varying between 8 to 60 tons, depending on the size of the machine. The reliability, the regularity of movement and the levelness of the plate to which the injection system is attached, are as important as the die itself in as much as the expulsion must be effected with a certain delicacy to avoid the casting being deformed or damaged by the die extraction pins (extractors).

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4.2.4 – The injection device 4.2.4.1 – Description of the system The injection device is the heart of the die casting press ; by means of it, the filling of the die cavity is achieved in a time and at a speed determined by the functional parameters that regulate it. Also the feeding of the solidification shrinkage of the casting is determined by the method of filling it. The injection device essentially forms an integral unit with the fixed plate of the press on which is mounted a hydraulic cylinder (injection cylinder) fed from pumps and from pressure accumulators through a complex system of solenoid valves and pressure regulators. The rod that exits from the injection cylinder has an extension mounted on it (piston carrier rod) on which in turn the injection piston is mounted ; the function of this piston is to push the molten metal into the die cavity, as illustrated in fig. 4.2.4.1.1. Often, the hydraulic injection cylinder is coaxial to, or connected to, another hydraulic cylinder called a pressure multiplier. Once the die cavity has been filled, this multiplier has the function of multiplying the pressure, applied by the injection cylinder, that is tamping the metal feeding the shrinkage of the metal in the cavity while it solidifies. The injection device doesn’t act directly on the metal filling the die, but through the shot sleeve and the piston. The filling of the cavity generally occurs in three separate phases : - first phase : pre-filling with slow advancement of the piston, - second phase : proper filling of the cavity with the piston moving rapidly, third phase : feeding of the solidification shrinkage ; the pressure on the metal is exerted by the piston and eventually increased by a multiplier. This pressure will vary depending on the characteristics of the piece to be produced, with the more usual pressure range being from 350 to 1.000 kg/cm² (35 – 100 MPa). The method by which the injection piston fills the die cavity is of particular importance. The piston must push the metal into the die in such a way as not to trap any air, and in a time span which is compatible with the thickness of the piece. For this reason the injection device has been called the heart of the machine, in as much as it characteristises the performances of it ; by its ability to regulate it, by the time and the method of applying the final pressure onto themetal and from the constancy and repeatability of its cycle. Fig. 4.2.4.1.2 illustrates what we have discussed until now ; note the irregular course and the pressure peaks that occur in the injection device which is not fitted with a multiplier, and the smoother pressure course that is obtained with the multiplier. The repeatability of the movements, the constancy of the speeds, the constancy of the time in which it is established, determines the completeness and the quality of the piece.


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Fig. 4.2.4.1.1 – Basic injection diagram.


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Fig. 4.2.4.1.2 – Function of the multiplier in the three phases of injection. Phase 1 : Pre-filling, with the slow advancing o f the piston. Phase 2 : Filling of the die cavity by means o f the piston moving, this time at high speed. Phase 3 : Feeding of the solidification shrinkage ; the pressure on the metal is e xerted by the piston and eventually increased b y the multiplier. 4.2.4.2 – Components of the injection device 4.2.4.2.1 – The injection device, in its elementary form, is composed of : A) Injection cylinder ; comprising : -

injection distributor in the first phase, regulator valve for the first phase ;

B) Pressure accumulator, comprising : -

pressure regulator valve for the accumulator charge,


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-

injection valve for the second phase, - regulator for the second phase.

4.2.4.2 .2 – In the improved versions having multipliers, the following is added to the above items : C) Multiplier cylinder, consisting of : -

non-return valve, regulator valve for the multiplier back pressure.

4.2.4.2 .3 – In the more advanced modern versions, the injection device is composed of the following : A) Injection cylinder, consisting of : -

injection distributor in the first phase, regulator valve for the speed in the first phase, nitrogen pressure accumulator ;

B) Floating piston accumulator, comprising : -

accumulator pressure regulator valve, command distributor for the second phase valve, injection valve for the second phase, regulator valve for the speed in the second phase, multiplier cylinder, non-return valve, multiplier speed regulator valve, multiplier counter-pressure regulator valve.

In some particular versions with separate and independent multiplier circuits (Weingarten system), the following are added to the above : C) Multiplier floating piston accumulator, and then : -

multiplier valve opening pressure regulator, opening servo-valve, multiplier valve.

In another particular version (Italpresse system), a multiplier is not used and we find : D) Floating piston accumulator, with : -

accumulator pressure regulator valve, accumulator high pressure opening valve.

4.2.4.3 – Principles of operation of a conventional injection system In the conventional injection system, there are two feed sources for the injection cylinder : -

press pumps, pressue accumulator.

The press pumps feed the injection cylinder in the first phase ; characterised by a low speed, it does not require a high fluid flow rate. For the second phase, in which a higher speed is required and consequently a larger flow rate in a brief time span, the pumps would be insufficient ; here then, is where the pressure accumulator assists. The second phase is thus jointly fed from the pumps and the accumulator. Because of the withdrawing of fluid, the pressure in the accumulator decreases because of the relationship between the respective volumes of fluid and gas present. The level of the fluid lowers in proportion to the quantity distributed to the cylinder.


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Level and pressure in the accumulator are restored by the pumps in the dead time of the machine cycle. The pressure fall in the accumulator lust be contained within some limits, 10 – 15 atm (1 – 1,5 MPa), otherwise at the end of the second phase the residual pressure would be too low to operate the multiplier. It is important that the value of this pressure fall remains constant after the cycle because this residual pressure serves to regulate the force of the multiplier by establishing the back pressure. By reason of the effect of the multiplier in the injection cylinder, at the end of its travel, the value of the pressure will determine the force which is applied by the piston on the metal to feed the shrinkage during the solidification phase. It follows that varying the residual pressure in the accumulator will vary the force on the piston and therefore determine the value of the specific final pressure applied to the metal.

4.2.4.4 – Operation of the accumulator It has been seen that the fall of pressure in the accumulator is determined by the fluid/gas relationship. This, by its nature, is difficult to contain, as we could have losses which often go unnoticed ; besides this we could have further gas losses from emulsion with the fluid. If we have losses of gas, we will be warmed observing the pressure in the accumulator as the volume of the lost of gas will be occupied by fluid, but, after the injection it will be noticed that the residual pressure is lower. In this case it will be necessary to introduce more gas and to verify that there are no losses. The quantity of gas to introduce will be that which gives the optimum relationship between the volumes occupied by gas and fluid at that particular pressure. It is for this reason that the press manufacturers supply tables from which we could find the nitrogen charging pressures for specific working pressures. For instance, with a working pressure of 150 atm (15 MPa) a nitrogen charging pressure of 135 atm (13,5 MPa) is required. These values are tied to the volumes involved, and more precisely to the volumes of the injection cylinder chamber and the multiplier chamber. Naturally we do not take into consideration all the volume of the cylinder chamber, but only the volume relative to the piston travel in the second phase and the volume of the multiplier chamber. The supply from the pumps being negligible. In modern systems the accumulator (see fig. 4.2.4.4.1) is made up of two distincts units : -

floating piston accumulator cylinder, pressure cylinder.

There is exclusively gas in the pressure cylinder with fluid and gas respectively in the lower and upper part of the accumulator cylinder separated by a floating piston.

Fig. 4.2.4.4.1 – Accumulators. A) Conventional system : direct contact of the gas (Nitrogen) with the hydraulic fluid ; aa) First phase of injection, gas at 150 atm (15 MPa). ab) End of the second phase of injection, gas at 135 atm (13,5 MPa). B) Floating piston system : gas and hydraulic fluid physically separated. ba) Before injection, gas at 150 atm (15 MPa). bb) End of the second phase of injection, gas at 135 atm (13,5 MPa).

4.2.4.5 – Comparison between conventional and floating piston system


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Although the floating piston system, also called the D system by Bühler who conceived it, and the conventional system are the same as regards their hydraulic operation, their behaviour is clearly different. With reference to fig. 4.2.4.5.1, we notice the large capacity of the pressure cylinder in the conventional system. For the same injection cylinder volume, the cylinder containing fluid and gas in a single chamber must be of greater capacity because at the moment of instantaneous withdrawal of the fluid, this, in relation to its viscosity and the shape of the pressure cylinder, rapidly exits forming an inverted cone in the fluid. If the point of the cone reaches the exit aperture, gas could escape in addition to the fluid. This could cause the injection cylinder to be fed by a mixture of fluid and gas, which is notoriously compressible, and will provoke irregular piston travel and a loss of gas. The injection system using a floating piston allows the use of accumulators with a smaller capacity, whereby the danger of gas escaping is no longer present thanks to the physical separation between the fluid and the gas. However, also in this case, the optimal fluid/gas ratio must be respected in line with the manufacturer’s instructions (see fig. 4.2.4.5.2) because : -

an insufficient charge results in a higher or an inadmissible fall in pressure ; an excessive charge could result in insufficient fluid being available.

However, these advantages are obtained at a certain cost ; the cost of the plant is higher and the seals for the separation of the fluid and the gas are subject to a certain wear and need periodic replacement.


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4.2.4.6 – Speed of the injection system The injection cylinder, with which the piston forms an integral part, moves under the force of the hydraulic fluid that enters into the cylinder itself, forcing the metal from the shot sleeve into the die cavity. The speed with which the cylinder advances is determined by the flow rate of the incoming fluid and is proportional to the volume of the cylinder. The travel and the speed of the cylinder movement in the first phase of injection is characterised by low speed. These speeds are of the order of 0,15 – 0,25 m/s or higher in injection systems having progressively accelerated speed (for example Parashot or Progression). The speed is determined by the pressure/pump capacity ratio and by the aperture of the regulator valve controlling the speed of the first phase of injection. The flow rate of the system hydraulic f luid is characterised by the P/Q diagram (see fig. 4.2.4.6.1). The second phase, the rapid filling of the die cavity, is characterised by the high speed of the injection cylinder (up to 6 m/s), for which we take advantage of the accumulator, which delivers the hydraulic fluid at a high flow rate adding to that of the first phase. The fluid flow rate in the second phase is determined by the pressure/accumulator capacity relationship and by the aperture of the regulator valve controlling the speed of the second phase of injection. The flow rate of the fluid delivered by the accumulator is also shown in the P/Q diagram, which therefore indicates all the hydraulic characteristics of the machine. The speed of advancement of the injection piston is a function of the following parameters : -

pressure, flow rate of the hydraulic fluid, volume of the injection cylinder.

In the second phase, where the rapid advancement of the injection piston is determined by the flow rate of the fluid withdrawn from the accumulator, we need to bear in mind that, in withdrawing the fluid, the pressure in the accumulator drops and the flow rate of the fluid being delivered decreases proportionally and accordingly, so does the speed of movement of the injection piston. From the above it can be seen that it is necessary to keep the accumulator pressure fall contained within the previously quoted values and that this depends on the relationship og the gas/fluid volumes and therefore on the charge pressure of the gas in the accumulation system. This latter must be established by the press manufacturer, linked to the accumulator operating pressure (see the gas charging diagrams supplied by the press manufacturer). All of the following : -

pressure/flow rate of the pumps ; aperture of the first phase regulator valve ; accumulator charge pressure ; volume of the accumulation system ; aperture of the second phase regulator valve ; volume of the injection cylinder,

characterise the hydraulic profile data of a die casting press, and are expressed in P/Q diagrams, as the example in the previously mentioned fig. 4.2.4.6.1.


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Fig. 4.2.4.5.1 – Principles of operation of the injection systems.

Fig. 4.2.4.5.2 – Floating piston accumulators : examples of the influence of the nitrogen charging pressure. insufficient charge. B) excessive charge. C) regular charge.


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Example of a nitrogen pressure diagram. The nitrogen charge pressure in the accumulator is proportional to the working pressure of the hydraulic fluid, for example : working pressure of the hydraulic fluid 150 atm (15 MPa), - the charge pressure of the nitrogen is 135 atm (13,5 MPa) – the relationship of the nitrogen charge to the working pressure of the hydraulic fluid must in any case be that prescribed by the manufacturer.


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Fig. 4.2.4.6.1 – Pressure – capacity – speed diagrams.

4.2.4.7 – Dynamic aspects of the injection system The injection device, characterised by the injection cylinder and by the devices acting to confer the movement and speed required by the casting to be die cast, present the moving masses : -

mass m1 of the fluid in the cylinder mass m2 of the volume of the fluid in movement (in the tubing), mass m3 of the multiplier, mass m4 of the injection cylinder, mass m5 of the injection.

These masses, that in the second phase, are characterised by high speed, each acquires a kinetic energy that, in total, determines the peak of pressure at the moment in which they abruptly halt at the end of the filling of the cavity (see fig. 4.2.4.7.1). The kinetic energy possessed by the complex is represented by the formula : Ec = (m1 + m2 + m3 + m4 + m5) v² 2

4.2.4.8 – Shot sleeve and injection piston 4.2.4.7.1 – Description The shot sleeve and the injection piston (figs. 4.2.4.8.1 and 4.2.4.8.2) are two units of very simple structure. They constitute two key points of the injection device as the speed/force parameters set and applied by the injection group are transmitted by means of these to the metal. Integrity, efficiency and functionality of these units are therefore very important to the quality and the quantity of production. Despite the care taken with their construction, because of the difficult conditions in which the piston and the shot sleeve are to be found, their durability is often unsatisfactory. We will therefore analyse their operational conditions.


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The shot sleeve reaches temperatures of 100 – 150°C when in production. This temperature is not fairly distributed ; we have higher temperatures in the forward zone (the part nearest to the die), and lower temperatures around the metal pour hole (rear zone). Moreover, from a minimum of one half to a maximum of three quarters of the inside surface comes into contact with the molten metal in the filling phase and it reaches very high surface temperatures.

Fig. 4.2.4.7.1 – Pressure curves diagrams. These different temperatures induce tensions within the material owing to the forces of expansion, which translate into fatigue of the material. The points that are subjected to the most wear are therefore the two extreme zones (front and rear). The front zone (where the riser solidifies), beyond being subjected to thermal energy, the metal is also placed under high pressures : in order of 500 – 1.000 kg/cm² (50 – 100 MPa), transferring further stress to these units. The rear zone, that receives molten metal during the filling phase, is prone to erosions and welding between metal and the alloy. The piston, which slides inside the shot sleeve, is subjected to very severe conditions, because of the temperatures reached at the surface in contact with the molten alloy, while the internal and rear parts are at lower temperatures, as cooling water circulates inside it. The surface in contact with the metal also undergoes compression stress (500 – 1.000 kg/cm², 50 – 100 MPa) while the external surface is stressed by attrition as it slides on the surface of the shot sleeve at speeds that vary between 0,30 m/s and 1 – 1,5 m/s (seconf phase). For such severe operating conditions the materials with which the shot sleeves are made genrally steels for hot working, for example steel H11 hardened and tempered to obtain a hardness Hd = 140 – 160 kg/mm² (1,400 – 1,600 MPa). To obtain a better resistance to erosion owing to the aggression of the molten alloy and to the attrition, the shot sleeves are often surface treated by means of nitriding, which raises the surface hardness to 70 HCR. The surface treatment, recently perfected by Degussa, and called Tenifer Treatment QPQ, effected on H13 steel at 640°C, imparts a superficial hardness of 1.400 HV. The materials that give the best duration for pistons, combined with the least drawbacks, are copper alloys. One of the most widely used alloys is copper-beryllium-cobalt, tempered and recrystallized ; this has the following technological characteristics : -

Hardness HD : 220 – 240, Ultimate tensile strength : 75 kg/mm² (750 MPa), Elongation : 8%,


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-

Linear expansion : 1,76/1.000.000 °C, - Thermal conductibility : 0,50 Cal/cms °C (209 J/ms °C),

Fig. 4.2.4.8.1.1. – Shot sleeve

Fig. 4.2.4.8.1.2. Injection piston layout

The design of the piston and the play between it and the shot sleeve have a notable influence on the efficiency and the life. Examples can be seen in the following tables in figs. 4.2.4.8.1.3 and 4.2.4.8.1.4. An elevated thermal conductivity of the piston is desirable as it favours the absorption of heat so reducing the solidification time of the biscuit. A notable role in the duration of the working life of the shot sleeve and the piston is played by the type of lubricant and the way in which it is applied

4.2.4.8.2 – Service life of the shot sleeve Despite the care taken with the manufacture of this item, the service life of the shot sleeve is often unsatisfactory, especially in respect of large diameter shot sleeves (greater than 100 mm), in which a large quantity of metal is poured. Wear and erosion that is found in the shot sleeve are translated into rapid piston wear, creating difficulties (seizures and bad quality castings), which in turn result increased production costs. The problem is still unresolved and various bodies are carrying out research into possible solutions. For information, the following is the latest news on the experiments being carried out on the covering of the inside surface of the shot sleeve with a ceramic material called « Sialon », produced by Hitachi Metal Ltd. Technical data of Sialon : maximum exposure temperature : 1.000°C, -

coefficient of thermal expansion : 0,000005/°C, resistance to thermal shock, difference of temperature : 400°C,


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Fig. 4.2.4.9.1 – Filling of the shot sleeve and piston travel.

The filling capacity of the shot sleeve is generally expressed as a percentage (%). It is important to keep in mind the filling capacity of the shot sleeve in considering a major problem with die casting ; that is the evacuation of the air in a very short time span as required by the filling of the cavity. It is for this reason that it is preferable to work with high filling capacities (60 – 80%). But in practice, we often have to work with much lower capacities (20 – 30%), and this often gives rise to defects and difficulties such as castings affected by cavities and/or excessive porosity, produced because of a too long first phase travel and a too short second phase (see fig. 4.2.4.9.1).

4.2.4.9.1 – Travel of the first injection phase The length of piston travel in the first phase of injection (which theorically must carry the molten metal in proximity to the ingate) is given by : Useful length of the shot sleeve x filling capacity = L x t. At the end of the piston travel of the first phase of injection, we find what is generally called the intervention point of the second phase of injection. This point indicates the position reached by the metal at low sped with respect to the gate and/or to the die impression. This start point must be in such a position that the metal is as near as possible to the ingate ; if the metal is behind this position it could increase the porosity of the piece. On the other hand, if the point of the intervention carries the metal beyond the ingate, a part of the die will be filled at low speed ; therefore the filling time of the die will be longer and it will in this way create the conditions for a premature solidification of the first metal. This drawback is particularly prejudicial to the integrity of thin walled pieces. The position of the point of intervention in question is less critical when the injection device is equipped with a constant acceleration system for the first phase (Parashot or Progression). On the press, this point is set by means of a limit switch operated by the injection movement or by use of an electronic system, operated by a resolver, which constantly checks the position of the injection. The maintenance of the optimal position of the point of intervention of the second phase presupposes that the quantities of molten alloy introduced into the shot sleeve are substantially constant.

4.2.4.9.2 – Travel of the second injection phase The travel of the second phase of injection is determined by the volume of alloy necessary to fill the die cavity in relation to the section of the piston : Volume of alloy Section of the piston The quantity of poured alloy in the shot sleeve is always greater that the necessary quantity to fil l the cavity, for which the travel of the second phase is less than that given by the useful length of the shot sleeve. The excess alloy solidifies in the shot sleeve constituting the so-called biscuit, and it is from the volume of this that, in the phase of final pressure on the metal (third phase of injection) the quantity of metal necessary to feed the solidification shrinkage of the alloy in the cavity is obtained and forced towards the cavity.

4.2.4.9.3 – Third phase of injection : final pressure on alloy The injection device, beyond transferring the molten alloy from the shot sleeve to the die cavity at the correct times and in the most suitable manner, must, after filling the cavity, exert a pressure on the molten alloy. The final pressure serves to force a part of the alloy, which is in excess of the quantity strictly required by the volume of the cavity and which is still in the shot sleeve, toward the solidification front of the piece and so copensates for the shrinkage. The density of an aluminium alloy in the solid state is approximately 2,7 kg/dm² (2,700 kg/m²) and in the liquid state 2,5 kg/dm² (2,500 kg/m²) approximately ; the shrinkage on 1 dm³ is : 2,7 – 2,5 = 0,2 = 0,074 dm³ / dm³ 2,7 2,7 To better understand the concept, we say that, if the die cavity of a hypothetical piece has a volume of 1.000 cm³, then theoretically, after the filling of the cavity a further 74 cm³ must flow from the shot sleeve, through the channel and the


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ingate to compensate for the shrinkage. The pressure is applied by the piston, which receives and transmits the energy of the injection cylinder. The energy or force of the injection cylinder is given by the pressure of the hydraulic fluid that is acting on it, multiplied by the area of the cylinder piston ; for instance :

Fig. 4.2.4.9.3.1 – Pression sur le métal dans la troisième phase d’injection. Le rapport des diamètres du multiplicateur est de 1/3, dont la pression de 150 kg/cm² devient 450 Kg/cm². Il est toutefois possible de changer ce rapport en maintenant devant le multiplicateur une contre pression qui s’oppose à la poussée, réduisant la pression effective dans le vérin d’injection à la valeur désirée. -

pressure of the hydraulic cylinder : 150 kg/cm² (15MPa), internal diameter of the cylinder : 160 mm (0,160 m), area of the cylinder : 200 cm² (0,02 m²), force of the injection cylinder : 200 cm² x 150 kg/cm² = 30.000 kg, 30 tons (300.000 N) ;

this therefore, is the force transmitted from the cylinder to the injection piston and so is exerted on the molten alloy. The pressure on the molten alloy in the filled cavity, is given by : Force of injection divided by area of the piston.


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Taking the preceding example, given that the injection piston has a diameter of 80 mm, corresponding to an area of 50,2 cm², we have : Force of injection= 30.000 = 597,6 kg/cm² Area of the piston 50,2 = 59,76 MPa The pressure of the hydraulic fluid in the injection cylinder, 150 kg/cm² (15 MPa), becomes 597,6 kg/cm² (56,76 MPa) on the metal, and under this thrust, the residual metal in the shot sleeve is transfered to the cavity to feed the shrinkage. In the majority of the presses in service, the pressure of the hydraulic fluid in the injection cylinder during the pressure phase (third phase) is increased by a pressure multiplier system. The operating principle of the pressure multiplier is that of a hydraulic cylinder. The ratio between the major section and the minor section of the multiplier cylinder is generally 1/3, for which it is possible to triple the pressure of the hydraulic fluid. We would have for example : -

pressure of the hydraulic fluid : 150 kg/cm² (15 MPa), ratio of major/minor section of the multiplier cylinder : 3/1, pressure acting on the metal : 150 x 3 = 450 kg/cm² (45 MPa).

The above is shown in fig. 4.2.4.9.3.1. The data on the relationships of pressure multiplication and the relationship between the injection cylinders and the diameter of the piston is generally furnished by the press manufacturer in the form of diagrams or injection nomographs that allow the setting of parameters to be rapidly carried out on the press to obtain the desired final pressure. As an example, an injection nomograph is shown in fig. 4.2.4.9.3.2.

Fig. 4.2.4.9.3.2 – Determination of the final pressure on the metal. Po) Hydraulic pressure in the injection cylinder in kg/cm² ; FkN) Force on the injection piston in kN ; Ø Diameter of the piston ; Pg) Pressure on the metal.

4.2.4.10 – The modern injection devices


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The manufacturers of die casting presses, under pressure from the users who are required to produce ever higher quality castings, have progressively made changes to, and improved the injection devices. The problematic areas for injection systems can be briefly summarised as : - control of the speed of the first phase of injection, to avoid entrapment of air in the shot sleeve that would result in an increase in porosity in the cast piece ; - control and ample regulation of the speed of fi lling of the cavity (second phase) ; elimination or containment to a minimum of the pressure peak at the end of the second phase when the kinetic energy acquired from the masses in movement discharges itself instantly against the molten metal that has filled the die cavity ; - control and reduction to a minimum of the time necessary to reach the final pressure on the metal (third phase) ; - constancy and repeatability of the values of speed, pressures and set times. The solution to these problems has been facilitated by the introduction of electronic control devices on the various units involved in the injection cycle. In the following paragraphs, we outline and analyse the different solutions to the problems set forth above, as offered by different press manufacturers, in a kind of overview of t he state of the art of injection devices.

4.2.4.10.1 – Bühler 4.2.4.10.1.1 – Parashot system The Parashot system consists essentially of a control for the first phase of injection. This is realised by a special hydraulic progressively variable opening valve. The movement of the piston in the injection cylinder during the first phase occurs with varying incremental motion rather than with an accelerating motion. This type of movement is able to avoid the rebounding of the wave, provoked in the liquid metal inside the shot sleeve, which can obstruct the exhaustion of the air (present in the shot sleeve, because of the partial filling) toward the casting. The air entrapped in the shot sleeve is emulsified with the metal giving rise to pulsations during the filling phase, which will be transmitted to the casting so increasing its porosity. Therefore the Parashot system is able to reduce the porosity of the cast piece. The restricting of the first speed of injection to 0,25 – 0,30 m/s, aimed at avoiding the rebounding of the wave, is bypassed so allowing shorter times with a smaller temperature drop of the molten metal. This is particularly advantageous when the configuration of a particular piece requires the metal to go beyond the ingate during the first phase, giving a partial pre-filling of the cavity. The differences between the Parashot injection system and conventional systems, and a diagram of the respective curves, are shown in figs. 4.2.4.10.1.1.1 and 2. Presses having the Parashot system can come equipped with a mobile device called a « Flashtrol » for the elimination of the pressure peaks at the end of the filling of the die ; these pressure peaks are responsible of a shortening of the life of the die. The Flashtrol is made up of a hydraulic group (see fig. 4.2.4.10.1.1.3) and an injection group ; the first is comprised of a compact unit containing various accessories (oil tank, filters, gear pump, etc…) ; the injection group, with the injection diameter being selectable, is made up of an injection piston, a complete Flashtrol head and an injection piston rod. In the first phase the Flashtrol, charged, has the maximum available dampening travel ; in the second phase, in the advanced position, still pre-loaded, it has still not intervened. In the third phase, as soon as the pressure is applied on t he metal, the dampening begins ; in the final compression, that is in the phase of dampening, the piston rod moves, in relation to the piston, causing the dampening effect ; at the end of dampening, the Flashtrol head butts directly up against the injection piston. Characteristics and advantages of the machines are : highly sensitive proportional control of the first and the second phases of injection ; independent setting of the third phase pressure, of multiplying, with extremely brief commutation times and pressure formation times ; - incremental measurement system for the determining of the injection piston position ; - automatic control of the accumulator piston charge, and therefore overseeing of the efficiency; hydraulic components with optimised weight and special hydraulic functions that allow the reduction of the dynamic pressure peak during die filling.


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Fig. 4.2.4.10.1.1.1 – Comparison of filling in the traditional and the Parashot systems.


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Fig. 4.2.4.10.1.1.2 – Control of the kinetic of the piston in traditional and Parashot systems. 4.2.4.10.1.2 - The SC system This system (see figs. 4.2.4.10.1.2.1 and 2) is completely governed in real time by an electronic system similar to that used for the numerical control of machines. All the movements of the injection piston are continuously regulated by a control valve. The use of this extremely rapid response valve allows the setting of the space S, speed V and time T parameters, in an almost bounless manner. The valve is positioned in the discharge line of the injection cylinder and not only allows the injection cylinder to travel at constant speed or to accelerate, but also to operate at reduces speed. In this way, the piston can be braked, so it can be said that the system has an integral dampener. The system is not fitted with a multiplier and the final pressure on the metal (third phase) is also regulated by this continuous control valve. The real regulation ensures that the predetermined injection curve develops correctly and is reproducible injection after injection. Unfavourable conditions such as attrition of the injection piston, different viscosities of the hydraulic fluid at various temperatures and instability of the nitrogen pressure are neutralised and have no influence on the process. The processing of the digital signals is of particular importance. These machines are regulated by a tested integrated CNC system called « Datacass/Processtrol » (see fiG. 4.2.4.10.1.2.3), which fixes the points of an injection curve that can be entered on one page of the program. These curve settings, like the rest of the real curves, can be recalled at any moment and can be represented as a f unction of piston travel or time. Equipped with the necessary sensors, it is able to sense up to 50 technological parameters and some of the injection diagram parameters, 8 injection curves, of which 4 are for speed, travel, time and pressure, and 4 cavity curves. For each parameter selected, the caster must fix a tolerance band, in this way deciding if the casting is to be rejected or accepted. With remote control of the die casting parameters, the system is able t o ensure that the set values are being faithfully repeated ; in addition, it allows the precise inclusion of 7 auxiliary data (for example times of filling, speed of themetal to


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the ingate, opening foce, etc…).

Fig. 4.2.4.10.1.1.3 – Vertical section of a Flashtrol injection system piston. This numerical control of the signals is of major importance and removes the problem of drift and overcomes wear on various components. The replacement of electronic or hydraulic components without upsetting adjustments or needing to re-tune is an appreciable advantage. The apparatus is set with an electronic digital system so that the injection curve is predetermined by fixing the salient points at the optimal values. The injection apparatus, from a hydraulic point of view, is decidedly simplified and very compact. The injection cylinder and the accumulator that feeds it are bigger, for the same power, than with systems having multipliers, as the final pressure on the metal (third phase) is not obtained by multiplying the pressure of the accumulator, but is the same pressure as that used to feed the cylinder during th e second phase. Cast pieces having thin wall thicknesses (0,7 – 1 mm) require filling times of 10 – 12 milliseconds, and could require piston speed of the order of 6 m/s. At this speed, the final pressure peak, due to the kinetic energy, is very high. To obviate this, the system allows the braking of the piston just short of the complete cavity filling so preventing the pressure peak.


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Fig. 4.2.4.10.1.2.3 - Integrated Cnc Datacess/Processtrol control panel. Pieces having thick walls often demand high mechanical properties, and therefore must be without or with low porosity and with an absence of shrinkage cavities. The system allows a filling profile well suited to this : -

low acceleration at the beginning of the injection ; moderate speed of pre-filling to favour the escape of air ; strong acceleration of the piston just before the complete filling of the cavity ; final pressure (third phase) exerted by stages so gradually feeding the solidification shrinkage.

The system allows the piston to be halted in any phase of the injection cycle so giving the possibility of controlling the way in which the metal arrives at the die cavity, and the optimising, by appropriate corrections, of the configuration and the size of the casting unit.

4.2.4.10.2 – Colosio system 4.2.4.10.2.1 – Dual circuit system (V2 system) This system produces a dual circuit injection. Unlike the traditional simple circuit system, the control of the multiplier, that is of the speed of intervention and of the final pressure is achieved by means of an independent circuit, fed from its own accumulator, charged to the required pressure. In the traditional system the intervention of the multiplier is confined to the second phase of injection (filling of the die). In this system (fig. 4.2.4.10.2.1.1), given the r elease of slide valve C3, the fluid from accumulator A3 acts on piston P2 that, advancing, closes the non-return valve contained in the piston rod. Consequently the hydraulic pressure P1 is raised (multiplication). With the dual circuit system it is therefore possible to control, not only the final pressure and its duration, but also the instant of intervention of the multiplication, independent of the second phase. This allows the elimination of multiplier delay Tr, and the obtaining of a more rapid intervention. The control of the multiplier by means of back pressure is eliminated, and therefore an appreciable reduction in the time necessary to reach the final pressure Tm. The principal problems arising from having only one oleodynamic circuit for the control of all the phases are therefore overcome. A comparison between the injection diagrams in the case of a single circuit (on the left) and a dual circuit (on the right) is shown in fig. 4.2.4.10.2.1.2.


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Fig. 4.2.4.10.1.2.1 – Comparison of Bühler B and C systems

Fig. 4.2.4.10.1.2.2 – Trend of the pressure and travel curves of Parashot system.


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Fig. 4.2.4.10.2.1.1 – Colosio injection system, V2 system.

Fig. 4.2.4.10.2.1.2 – Colosio injection system, V2 System : Diagram P/t

4.2.4.10.2.2 – I.P.C System (Computerised Injection Profile) The injection is operated by particular sensors, incorporated in the cylinder, connected to a continuous modulation control valve on the flow. The electronic system allows an automatic closed ring control (see fig. 4.2.4.10.2.2.1). This means that the desired injection profile can be predetermined and entered in the program (see fig. 4.2.4.10.2.2.2). The sensors transmit the return signals of position and speed of the cylinder, in real time, to the control system. A special


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microprocessor processes the appropriate signal of the continuous control valve, instantly controlling the speed, independent of the variables of the process. The influence of variations of temperature of the molten metal, of attrition between piston and shot sleeve, the temperatures and viscosities of the hydraulic fluid, are no longer conclusive. Once programmed, the profile of the injection speed is faithfully repeated for every casting produced and for each use of the same die. Presses having the I.PC system, have an added cylinder and do not use any multiplier system. The complete programmability of the system allows to reduce the speed at the required points so resolving the problem of pressure peaks with a reduction in the wear on the components. The level of control of the process can be increased with the interfacing of a further complete SPC system, with XR curves, statistics and management of more than one press, etc.

Fig. 4.2.4.10.2.2.1 – Colosio injection system IPC.


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Fig. 4.2.4.10.2.2.2 – Colosio IPC injection profile.

4.2.4.10.3 – FRECH 4.2.4.10.3.1 – The « Ecopress » injection device The Ecopress device, conceived and produced in an extremely compact unit, thanks to the precise design of the internal galleries with reduced length of runs, allows the optimum employment of energy, avoiding overheating of the hydraulic fluid. The presses which come equipped with this system make use of extremely rapid response proportional valves. These allow the obtaining of speed pressure and speed gradients, as shown in fig. 4.2.4.10.3.1.1, and a very high reproductibility of the operating cycle. The hydraulic circuit, fed through waterglycol base fluid, thanks to a precise control of the main valve, returns constant response and intervention times. The force of injection can be continuously regulated without the need to vary the nitrogen charge pressure in the accumulator. The injection parameters are set on the panel using digital commands. The injection is produced in three phases, each of which is fed by an independent hydraulic circuit. The first phase is characterised by a progressive action with incremental varying speeds. The multiplier of the third phase can be regulated on the basis of travel. The electronic Data-Control/Variomat system of regulation and control, allows fast regulations display of parameters on a video and control of the process with the pre-selection and formulation of the maximum and minimum speed and pressure value variations.


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Fig. 4.2.4.10.3.1.1 – Frech Ecopress device : injection phases. The parameters of injection and production can be calculated, recorded, memorised on a disk, printed and recalled and reproduced at any moment. Production can be checked by reference to the supervision tables. Thanks to the use of a hydraulic block unit and to the absence of connecting pipes, the maintenance of the unit is extremely siplified. The advantages obtainable from the system are as follows : - very brief filling phase ; the very high speed of injection provides a very short filling phase ; injection times, from empty, have been measured at 9 milliseconds. - High mechanical properties of the casting. The faster the speed of the rising pressure gradient when compacting the metal, the better the mechanical properties of the casting ; a final compacting pressure of 350 bar (35 MPa), can be reached in 9 milliseconds. - Absence of need for trimming. Thanks to the optimisation obtained from the hydraulic circuit, the pressure peaks that are reached during the compacting phase are very much limited and therefore do not produce flashes. High productivity. The brief intervention times of the hydraulic circuit valves guarantee much reduced reaction times and so shorter production cycles. - Repeatability and constancy of the quality level. The insertion in the circuit of a pressure equaliser in front of the proportional valve in the first phase in the Datacontrol/Variomat system, prevents variations of speed that could be caused by the different attrition conditions of the injection piston ; in this way ensuring the smooth running of the process. The optimum use of hydraulic energy. The compact configuration of the hydraulic circuit with its very short galleries, avoids energy losses due to overheating.


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Fig. 4.2.4.10.3.1.2 – Frech Ecopress device : injection phases.

4.2.4.10.4 – IDRA 4.2.4.10.4.1 – The « Progression » system The « Progression system » allows the varying of the speed of the first and the second phase while these are being undertaken. This system can be inserted at will, so that the machine could have a traditional type of injection or one similar to the Bühler Parashot system if, in the first phase, the Porgression system is fitted. The progression system could also be fitted in the second phase, parallel to the Parashot system. As can be seen, from a conceptual point of view, the system offers many possibilities. However, it also must be said that, with the fitting of the Progression, the injection also occurs by means of the accumulator in the first phase, a fact that can, in some cases, reduce the residual pressure for the multiplier to a very low value. The variation in speed in the first and the second phase is obtained opening the relative valves at varying speeds, so that the flow toward the cylinder varies in relation to the degree of the opening of the valve. The opening of the valve is achieved through an electronically controlled electric motor.

4.2.4.10.4.2 – The « Clamar » System It uses a structure with a separate pressure multiplier cylinder and an independent hydraulic control circuit (see fig. 4.2.4.10.4.2.1). The flow of oil and accordingly, the speed in the various phases of the cycle are controlled by means of proportional valves, in which the opening of the flow control valve is directly correlated to the control tension. The cylinder is of sufficient diameter to guarantee an accompanying force, during the filling of the die cavity, with which to have a pressure


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on the metal, at the same time, of not less than 400 kg/cm² (40 MPa). At the same time, the pressure must not be so high as to jeopardise the maximum speed that, during the second phase, empty, without the back pressure of the metal, must reach the value of 7,5 – 8,0 m/s. The multiplier piston is constructed in one piece, without valves or internal passages which would be difficult for maintenance and, mounted in the vertical position, it is connected to a rod which indicates the position and is controlled by means of a proximity sensor. During the first phase (see fig. 4.2.4.10.4.2.2) the proportional valves P2 and P3 are closed and the hydraulic flow is furnished by the two-stage pump and controlled by the proportional flow valve fitted in the closing distributor. The hydraulic fluid (see fig. 4.2.4.10.4.2.3) flows through check valve R, designed and produced by the manufacturer to guarantee sections of adequate passage, and consequently, reducing charge losses. On reaching the limit switch corresponding to the end of the second phase (second phase portion in the case of machines with injection computers), the proportional valve P2, previously set to the speed value entered by the operator, is energised and the fluid contained in the accumulator charged at the pre-load pressure, passes through check valve R, imparting to the piston the second phase speed.

Fig. 4.2.4.10.4.2.1 – Injection scheme, Idra « Clamar »

A second limit switch, or alternatively the third phase portion, must be positioned at about 20 mm before the point of complete filling. On reaching this point, the proportional valve P3 of the third phase is commanded to immediately, or after a delay entered in milliseconds, moves multiplier piston M at a speed independent of that of the second phase. When, because of the effect of the resistance generated by the metal in the die in the injection cylinder, the pressure reaches that of the accumulator, check valve R is automatically rapidly closed, in 3 – 4 milliseconds, and the pressure, because of the intervention of the multiplier, increases to the required value. The third phase (fig. 4.2.4.10.4.2.4) is essentially a pressure phase in as musch as the filling of the die having occured, the movement of the injection piston is negligible and limited to the volume of shrinkage of the compaction of the metal. The final pressure value is determined by the back pressure on the circular crown of the multiplier piston C determined by the accumulator that is precharged to the pressure needed to obtain the required final pressure. The value of the precharging is taken from the injection diagram, or in the case of press equipped with an inject computer, it is calculated automatically. The system patented by Idra, called « Biconstant », eliminates the pressure peaks due to the kinetic energy of the system and is independent to the value of the hydraulic flow, as happens in the case of using pressure reducer valves for the same purpose. A typical example of a speed and pressure graph of the piston in function of time is shown in fig. 4.2.4.10.4.2.5. During the first phase it is important to check the pressure : an anomalous value, excessively high, signifies the beginning of a seizure of the injection piston for lack of lubrication or insufficient cooling. Varying the value of the first phase different profiles of speed curves are obtained, as can be seen in the graph. To avoid, as far as possible, turbulence in the inside of the shot sleeve and the relative inclusion of air, the first phase should proceed with a progressive speed (at constant acceleration), rather than with a constant speed. In this case an accumulator is used with proportional valve P2 that is opened with a « ramp » order definable by the


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operator. The effect of different values of intensity of the ramp and of the maximum progressive value are shown in fig. 4.2.4.10.4.2.6. As can be seen, it is possible to determine the final value and the inclination of the curve, and therefore the value of the acceleration. On reaching the position of the second phase limit switch valve P2 opens to the set value of between 0 and 100%, determining the speed of the metal in the ingate section selected by the technologist, and thereby obtaining the appropriate filling time. Applying an increasing control tension to the proportional valve a progressive valve speed can also be obtained for the second phase. It is possible to regulate the speed in the third phase, which has an influence on the rapidity with which the final value is reached, the possible delay of intervention, which determines a step of adjustable length D after the attainment of the line pressure Pm. This latter is very important because it contributes to determine the compaction of the casting and its structural fluidity. Moreover, it must be compared with the frontal area of the piston and the clamping force of the press. Final pressure values which are too high could cause the formation of flashes in the die with all the negative consequences they involve.

Fig. 4.2.4.10.4.2.2 – Idra « Clamar » system : 1° phase. The appropriate sizing of the piston accumulator and the nitrogen cylinder allow the running of long second phase injections without substantial decreases in pressure, so avoiding the need for a second series of accumulators with the attendant managerial and bureaucratic problems of safety valves and periodic tests to which pressure accumulators are subjected now in all countries. For the sake of an example, let us consider an OL 900 model press that is required to die cast a piece weighing 3 kg with a shot sleeve of 90 mm ; at the end of the second phase, in the accumulator, there is a pressure decrease of 7 bars (0,7 MPa) which is decidedly limited and one which can be compensated for, to obtain the required multiplied final pressure value, reducing the pre-charged pressure in the Biconstant by the same percentage value. In other words, if the charging value to obtain the final required pressure were calculated theoretically to be 45 bars (4,5 MPa), then to take account of the pressure drop due to the consumption of the second phase, it is sufficient to reduce it to 40 bars (4 MPa), thereby reaching the required multiplied pressure. In conclusion, the independent adjustability of the phases of injection, controlled by proportional valves, that assure the complete repeatability of the set values, permit the die caster to determine the operating values which are most suitable to the attainment of quality castings. The course of the speed and pressure curves is freely determined by the operator, who can adapt them to any typologies of castings, from those with thin walls, where a high injection speed is called for, to the thick walls, where a slow filling and control of the die cavity, with high final multiplication pressures are necessary. The system, using hydraulic controls woth proportional valves managed by microprocessor, allows the obtaining of pressure and speed profiles which are freely programmed without sacrificing sturdiness and simplicity of maintenance, which are characteristics which cannot be forsaken in the environment of the foundry.


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Fig. 4.2.4.10.4.2.3 – Idra « Clamar » system : 2° phase

Fig. 4.2.4.10.4.2.4 – Idra « Clamar » system : 3° phase


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Fig. 4.2.4.10.4.2.5 – Speed-pressure graph, Idra « Clamar Biconstant » system.

Fig. 4.2.4.10.4.2.6 – Idra « Clamar Biconstant », system 1st phase speed and travel diagram

Fig. 4.2.4.10.4.2.7 – Idra « Clamar Biconstant » system, 1st and 2nd phase speed and travel diagram.

Fig. 4.2.4.10.4.2.8 – Idra Clamar Biconstant system : multiplication delay and time regulation.


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Fig. 4.2.4.10.4.2.9 – Idra « Clamar Biconstant » system : regulation of the final pressure.

4.2.4.10.5 – ITALPRESSE SC System (Separate Circuit) The increased difficulty in the management of any technical solutions in the die casting production process sphere has made an improved versatility in the control of each single injection parameter ; experience has made clear that the intervention of the phases of injection can be controlled and varied without influencing the other adjustments. In this system, by means of separate circuits for the second and the third phase (see fig. 4.2.4.10.5.1), Italpresse has made the injection parameters totally independently adjustable and modifiable. The advantages can be briefly summarised as : -

very rapid increase of the final pressure even at low injection speed ; independent regulation between injection speed and final pressure ; control of the multiplier piston either as a function of the course of injection or pressure (as preferred) ; anticipatory operation of the multiplier piston (also during the second phase).

The SC system is usually accompanied by a computerised block regulation system called « Electronic System 2000 » (see fig. 4.2.4.10.5.2). The following can be entered in the « Injection System » page : -

speed : values from 0 to 99% ; pressures : values expressed in bars ; travel : from 0 to the maximum value, typical of the machine, expressed in mm ; times : from 0 to 99,9 s, in intervals of 0,1 s ; only the multiplication time is express in ms.

In the computerised blocks system « Theoretical Injection Calculations » page indications regarding the pre-setting of the machine in function of the data previously entered by the operator can be seen ; this will be data regarding some parameters to be used for the production of the casting, for example : -

specific pressure on the metal, diameter of the available shot sleeve, etc.


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The « injection curves » page graphically displays the following parameters : -

speed ; travel ; injection pressure ;

in the form of curves, whose data can be memorised together with the die data. With the « Values of Injection Control Parameters » page, it is possible to represent a « trend » of injection control parameters with reference to the tolerance range entered for each machine cycle. In this way, the operator can know, in real time, with what parameters and in which moment the casting has been produced. Castings are separately identified as being within or out of tolerance. Those reported in numerical form, in the « Injection Parameter Trends » page, can be displayed in graphic form. Observing, by reference to the graph, a gradual variation of the parameters and a consequent gradual drift within the tolerance range, the operator is able to take the necessary corrective steps, while the castings being produced are still in tolerance, with notable savings in time and a reduction of rejects. Other pages contained in the system allow the processing of production statistics, both with reference to the press and to the installed die, to « save » optimised parameters in memory, to prepare the using and regulating of peripheral units, to carry out auto-diagnosis of malfunctions, etc.

Fig. 4.2.4.10.5.1 – Italpresse Sc system : separation of the 2nd and 3rd phase circuits.


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Fig. 4.2.4.10.5.2 – Italpresse Sc system ; computerised blocks regulation « Electronic System 2000 ». 4.2.4.10.6 - TRIULZI RESS « GEM System » The particularity of this system (see figs. 4.2.4.10.6.1.1 – 4), which is aimed at the avoidance of creating injection pressure peaks, is the fact that the transmission of power from accumulator 10 to injection cylinder 4 does not occur directly, but through a twin intermediary cylinder 7. This latter cylinder, given its structure, receives the impulse from 10 on the suitably dimensioned circular crown of piston 8 in such a way as to reduce the impact and to allow it to advance at a reduced speed. The low speed allows a more precise modulation of the required accelerations. Piston 8, therefore multiplies the speed that it applies to 6 ; thereby 6 assumes twice the modulus of 8. At the end of its travel, briefer than that of 6, 8 absorbs the impact of stopping originating from 5. Piston 6, no longer fed from 8 and no longer influenced by the inertia of 5 or the fluid contained in the feedways. From this point the multiplication phase takes over by means of piston 9, which accompanies the injection with reduced speed and increased pressure, in predetermined measure and without creating uncontrolled pressure peaks and/or a difficulty of control. Of no small advantage is the fact that the discharging of the accumulator acts on the circular crown, for which reason the peak is significantly reduced and there is a meaningful saving in energy because of the small recharge necessary for accumulator 10. The buffer piston of accumulator 10 moving at a slower speed, reduces the risk of seizure and of wear to the accumulator seals. With the use of this system, by eliminating the pressure peak, it is possible to reduce the die clamping force necessary to produce the same piece.

4.2.4.10.6.1 – The GEM 2 System The purpose of the GEM system is not to reduce the injection overpressure (peak) but to eliminate it (fig. 4.2.4.10.6.1.1). Having determined the ineffectiveness of lightweighting and braking, and with the need to have more and more elevated injection speed, as required by present technology and metallurgy, remaining unchanged, research has turned to an alternative practical solution : transfering the inertial force of the injection piston to an external closed circuit twin hydraulic system (GEM). The solution (fig. 4.2.4.10.6.1.2) is that of using the accumulator to move a piston in a cylinder containing the same quantity of fluid necessary for the injection and transfering it in the same way (first, second and third phase, or progressive phase) from the GEM cylinder to the injection cylinder. In this way it is the GEM piston that finishes its travel at the end wall, against which it exhausts its force and does not transmit it to the injection piston as the quantity of fluid necessary for its movement has been exhausted. In this case the pressure peak is not transmitted but is exhausted inside the GEM cylinder (fig. 4.2.4.10.6.1.3). Where, because of a wrong regulation of the fluid contained in the GEM or of an imprecise metering, the exhaustion of the GEM travel is prevented, then the kinetic energy and the tr ansfered force is accordingly much reduced. The GEM piston, which has a diameter ratio of 1 : 5 as regards to the injection piston, will have a speed equal to 1/5 of that of the piston and if this has a speed of 4 m/s, the GEM moves at 4 : 5 = 0,8 m/s. Its kinetic energy, except from the


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masses, will take account of the relationship between the squares of 4 and of 0,8, that is 16 and 0,64. In fact, the kinetic energy Ec is in the first case 98,4 kgm (984 J) ; in the GEM it is ½ m V² = ½ x 14,3 x 0,64 = 4,6, that is 22 times inferior. Fig. 4.2.4.10.6.1.4 shows the modest peak that occurs inside the GEM. Therefore, both with the GEM totally discharges, and with the GEM having some residual fluid, having eliminated the pressure peak in the die cavity, the injection speed is able to be raised without the closing system being altered in an anomalous manner. Figs. 4.2.4.10.6.1.5 -–7 show the existence of peaks at low speed, at high speeds without a multiplication delay and at high speed with a multiplication delay. This system involves particular advantages with respect to the clamping force of the press. In fact, because of the GEM, at the end of the filling, the dynamic energy and the force of impact will be reduced to zero, or at least to the value of the force transmitted from the GEM and only then where the GEM and the riser are not well regulated.

Fig. 4.2.4.10.6.1.1 Triulzi Ress « Gem 2 » system : elimination of the injection peak.

1 – Shot sleeve 2 – Injection piston 3 – Rod 4 – Injection cylinder 5 – Piston of the injection cylinder 6 – Gem piston 7 – Gem cylinder 8 – Multiplier cylinder

Fig. 4.2.4.10.6.1.2 – Triulzi Ress « Gem 2 » system : components scheme.

9 – Multiplier piston 10 – Accumulator


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11 – Accumulator buffer piston

Fig. 4.2.4.10.6.1.3 – Triulzi Ress « Gem 2 » system : end of injection piston travel.

Fig. 4.2.4.10.6.1.4 – Triulzi Ress « Gem 2 » system : pressure peak inside the Gem.


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Fig. 4.2.4.10.6.1.5 – Triulzi Ress « Gem 2 » system : low injection speed.

Fig. 4.2.4.10.6.1.6 – Triulzi Ress « Gem 2 » system, high injection speed without multiplier delay.

4.2.4.10.7 – UBE « DDV » System


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This system (refer to figs. 4.2.4.10.7.1 – 5) covered by Japanese patent, works in the following way. Oil, at high pressure and at high speed (fig. 4.2.4.10.7.2) is supplied to the head of the Acc cylinder, while the cylinder rod (2) advances and the cavity is filled with molten metal. The oil exiting from the cylinder rod is supplied to the head of the cylinder until the sequential valve (7) does not close and the oil then travels through the inner sleeve (3) to the centre of the cylinder (1), forcing the opening of the control valve (4) and so forming the peripheral circuit. The area of head A1 is double that of the area A2, so that the quantity of oil required to have the same speed is exactly half that in the direct system. On the advancing of the cylinder rod (2), the opening in the inner sleeve (3) is closed near the end of the filling, so the oil that leaves the area of the cylinder rod passes through the circuit to the exterior of the cylinder, through the control valve (5) and the flow control valve (6), and so suddenly reduces the speed of the cylinder rod (2) to the « third » speed. Regulation of the third speed can be achieved by use of the flow control valve (6). The position of deceleration must be varied to suit the thickness of the fixed die. This is obtained by means of regulator (10) which advances or retracts the inner sleeve (3). Regulation of the force of injection occurs in the following way: F = A1P1 – (A1 – A2) F is variable with P2, that can be regulated by the release valve (8) under constant pressure of the accumulator. Regulation of the metal compression time is achieved gently and efficiently by the needle valve (9).

Fig. 4.2.4.10.7.1 – Ube DDV system : instant before the beginning of the injection. 1) cylinder – 2) cylinder rod – 3) inner sleeve – 4) control valve – 5) control valve – 6) flow control valve – 7) sequential valve – 8) release valve – 9) needle valve – 10) regulator.

Fig. 4.2.4.10.7.2 – Ube DDV system : filling of the cavity at high injection speed.


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Fig. 4.2.4.10.7.3 – Instant immediately prior to the completion of the filling.

Fig. 4.2.4.10.7.4 – Filling completed.

Fig. 4.2.4.10.7.5 – Curve type of filling speed : abscissas, time. 1) injection speed (m/s) soft start – 2) speed 2 – 3) speed 3 – 4) speed 4 – 5) speed curve in a DDV press – 6) compression time of the metal – 7) speed curve in a traditional press – 8) projection speed.

4.2.4.10.8 – WEINGARTEN – STP


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Independent circut multiplier The injection system designed by STP and manufactured by Weingarten is characterised by having an independent multiplier circuit. The multiplier is supplied by a separate circuit, with its own accumulator, and is completely independent of the injection cylinder circuit (see fig. 4.2.4.10.8.1). In this way, the multiplier is released from having its pressure determined by the second phase of injection and can be applied earlier or later than the point at which the piston stops at the end of the die cavity filling. The advantages of the STP Weingarten system are : - the unrestricted ability to establish the final pressure (multiplied) – the multiplied pressure most suitable for the compaction of more complex castings can be determined by varying the accumulator charge pressure (third phase) ; - the time required to build the final pressure is extremely short ; - the ability to determine the pressure curve as shown in diagram fig. 4.2.4.10.8.2. Thanks to the exactness and the simplicity of its design, this device offers a high level of operational reliability, repeatability, constancy and safety. This system gives the casterman the means to further improve and to maintain the quality of the cast product.

Fig. 4.2.4.10.8.1 – Weingarten system with an independent circuit multiplier.

Fig. 4.2.4.10.8.2 – Weingarten system : allows an extremely rapid attainment (and a ample scope of delay) of the final pressure in the 3 rd phase (pressure on the solidifying metal to feed the shrinkage). 4.2.4.11 – Application aspects of pressure in the die casting process


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In completing our overview of the characteristics of modern injection devices, we have seen what are the philosophies and the practical creations of press manufacturers. The objective is to contain, if not to eliminate the undesirable and uncontrolled pressure peak that can occur at the end of cavity filling through the effects of the kinetic energy determined by the masses in movement. Now, we note some of the elements with regard to the function and application aspects of the pressure exerted on the metal. In our presentation we have seen that the main aim of the pressure acting on the metal at the end of the die cavity filling is to feed the solidification shrinkage by forcing the still liquid metal of the riser to flow towards the solidification front of the piece, in this way avoiding the formation of micro and macrocavities due to the volumetric contraction of the metal when it passes from the liquid to the solid state. For the above reasons, the pressure exerted on the metal must occur at the times and with the intensities relative to the temperature conditions, to the characteristics of the alloy (solidification rage) and to the characteristics of the die (configuration and sizing of the ingate). The pressure exerted on the metal throughout all the phases of injection, as confirmed by lond experience, is of the utmost importance not only regarding the absolute value of the final pressure (third phase of injection) but also for the way in which it is applied. The pressure acting on the metal during the dynamic phase (while the metal is filling the die cavity) should not be less than 400 kg/cm² (40 MPa). This pressure linked to the diameter of the piston, must be applied in the dynamic phase of the injection device throughout the second phase. The value of the pressure, the method and the time in which it is applied to the metal are critical influences on the characteristics of the piece (morphological configuration of the structure). In fact, the thermal exchange between the metal in the solidification phase and the die is conclusively conditioned by the contact between the metal and the die itself. The solidifying metal contracts and tends to retract from the external walls of the die, creating an interspace of micrometric dimensions, but sufficient to conspicuously slow down the transfer of heat from the metal to the die. This phenomenon does not occur around pins and plugs, where the contraction tends to increase the tightening of the metal around the form. In these latter zones, the flow of heat does not decelerate, but never accelerates. The deceleration of the thermal flow in the mentioned zones has a tendency to lengthen the solidification time of the piece and thus to condition its structural morphology. For all the reasons mentioned, control and the method in which the pressure is applied has a significant influence on the characteristics of the piece (for example mechanical strength, etc), both for the size and for the metallurgical structure of the piece.


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4.4 – Control systems for die casting presses These systems are the nerve centre of the press in that on them depends the possibility of programming the cycle phases and the intervals of time between one phase and the next. The press control system is exclusively electric. The electric circuits (or power) are, or can be added or integrated with electronic circuits in the form of fixed or programmable softwares. These circuits determined the cycle of the press by means of solenoid valves and cylinders. The most common control system, the electric one, consists of the monitoring, through the use of limit switches, of the position reached by a component according to a specified logic (press program) and to consent to the action or to allow the following phase. In this way a cascade operational order is achieved, in which each movement or positioning of a component consents to the following movement and so on until the completion of the cycle. The electronic circuits offer many advantages over the purely electric circuits as they are able to memorise and to process data. For example, say we need to close a press mounted with a simple die (a fixed part and a moving part). Before this can happen it is necessary to verify that the die can be closed or not, and therefore to verify if the ejectors have re-entered or not. In the simpliest case of an electric circuit, a limit switch, closing a circuit in the « rear » position of the ejection group, would need to give consent in order that the press be closed. In the case of an electronic circuit this simple function could be widened in order to offer better safety to the same closing operation. In fact this circuit could give consent while taking into account the withdrawal of the ejector plate and the following situation : -

injection « rear » (piston retracted) ; lubricator retracted (die lubrification device) ; robot grippers (mechanical withdrawal of the casting) out of field (as regard to the die).

The above stated example illustrates the possibilities offered by electronic systems, possibilities offered by electronic systems, possibilities which go on to include control systems using programmable microprocessors. The enormous evolutionary development of programmable microprocessors in recent years has made a vast range of versatile, reliable and affordable apparatus available, which are more and more being used for the control and safety of die casting plants. The adoption of these apparatus has facilitated and allowed : -

the mechanisation and automation of the process ; the displaying of the operational parameters ; the process control ; with the employment of memories, the automatic reprogramming of the operational parameters.

The development of electronics has allowed the production of hydraulic devices such as proportional travel valves and mechanised valves with programmed opening, etc. These, integrated with microprocessors and sensors such as pressure and temperature sensors, allow the control of the process, comparing the delivery values with the values of the various parameters of each cycle. Any shifting of the most important process parameters (for example the injection parameters) can be signaleed and/or automatically corected in order to guarantee the quality constancy of the die casting. Microprocessors can be managed not only by the press but also by the auxiliary apparatus for the automation of the process, the devices for metering and transferring the molten metal, robots for withdrawing the castings and apparatus for the lubrification of the die cavity, etc. In this way, the operational systems of the process can be very much improved.


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4.5 – Control and safety devices Die casting presses are equiped with devices for accident prevention and safety. These devices consist essentially of fixed barriers that segregate the moving parts of the machine (such as the toggle linkage) and of movable barriers, such as gates that shield the area between the fixed and moving plates of the press. The function of these gates is also to protect against possible projections of molten metal from the die during the injection phase. The safety cycle of a machine could therefore be the following : -

closing of the gates (checked by a limit switch) ; closing of the press plates ; opening of the plates ; opening of the gates.

Other installed safety devices are : Die closing safety. This is a device that prevents the closing of the press plates, with the provided closing force, if the die halves are not mating perfectly. The non-mating of the die faces could have various causes resulting from an anomalous operation of the moving parts of the die (side slides, core, etc) or because of the presence of flashes, parts of the previous casting, risers etc. Safety is generally ensured with a position sensor (limit switch), which must always be carefully set up. Injection safety. This device prevents the operating of the injection if the press is not perfectly closed (press blocked with the required closing force), thereby avoiding the external projections of molten metal. This safety is obtained in different ways : - with position limit switch ; - with closing pressure sensors ; - with closing force sensors ; - Accumulator pressure safety. This is a maximum pressure safety valve for the gas and/or the hydraulic fluid.


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4.6 – Press operating cycles 4.6.1 – Basic cycle The die casting press has its own basic operating cycle that is integrated with external interventions. The basic cycle could be as follows : -

closing of the press ; charging of the metal ; injection ; solidification ; opening of the press ; return of the injection piston ; expulsion of the casting ; lubrification of the die.

Operations on this basic cycle could be carried out with manual command or in automatic sequence. Today the basic cycle sequences are more numerous to take account of the increased demands placed on the products being manufactured. For example, today there is a need for the production of castings that are cast with dies made up of 6, 8 or 10 pieces, as would be the case of a die with 4 lateral slides on the moving semi-die and likewise on the fixed semi-die.


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4.6.2 – Complex operating cycle As previously pointed out, a die that must generate a given piece can be very complex and demand that its composition and de-composition occur in an appropriate sequence. Modern presses are equiped to perform complex cycles and, by means of programming, the succession of the various phases can be modulated accordingly. The operating cycle of such a press has a series of consents and internal commands from and to the auxiliary apparatus. These could be, for example : - mechanical cup and oven measure ; - die lubricator ; - robot-arm for the withdrawal of the casting. These are integrated into the internal cycle of the press. Therefore the presses are designed to perform a multitude of programs, in order to satisfy the diverse demands of the die : it is then, these latter that determine the choice of the press and the compatible programs with which to equip it. The dies are in fact divided into a number of components in relation to the complexity of t he geometry of the piece to be produced. In turn, the sequence of the programs is more or less formulated as a function of the complexity of the die. They are equiped with movements : lateral slides (that produce parts of the form in the die cavity) and pins (that produce holes in the casting). In foundry slang the components whose axes do not lie on the dividing plane of the die are called « cores ». In particular the slides on the moving die are identified as « core1 » and « core 2 » and those on the fixed die as « core 3 » and « core 4 ». A press is in general able to undertake the following movement sequences : -

core 1 and core 2 : enter and exit with opened die (before the closing of the press) ; core 3 and core 4 : enter and exit with closed die (after the closing of the press).

When cores 1, 2, 3 and 4 are only two and must function simultaneously, the relative cylinders are connected in parallel between them, while the limit switches are generaly connected in series. Core 3 and 4 are able to enter with the opened die and exit with the closed die only if the press is equiped with this special cycle and then only if the die will allow it.


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4.6.3 – Press work parameters We call work parameters those values, controllable and adjustable, of the physical, thermal and mechanical attributes that contribute to the production of a die cast piece. In die casting these parameters are substantially physical and thermal. To perform all the needed operations it is necessary to take account of the parameters concerning the melting of the alloy and, after having transfered the same to the holding furnace, those of the solidification of the metal in the quantity dosed into the inside of a die, where it is to assume the form of the piece. The process is therefore conceptually simple enough, but very complex to achieve. In fact it is necessary that the metal is at an adequate temperature, superior to its melting point, because part of its thermal content will be lost in the operation of ladling and pouring into the shot sleeve. A further part of the heat will be lost to the shot sleeve in the time that the molten metal is in it ; this latter will be at a temperature inferior to the solidification point of the metal. Likewise losses will occur during the wait up to complete solidification and up to when the piece is sufficiently composed from a mechanical point of view. At this point we must open the die and remove the piece, which will be anyway at a higher temperature than the die to avoid that the shrinkage forces, particularly accentuated on pins and reverses of the figure, impede the expulsion of the casting. If excessive resistance, the casting being hot (the temperatures at this point will be in the order of 300 – 350°C), the force exerted during the extraction could give rise to deformations and flaws in the casting. On the other hand, if the extraction is carried out too late or too cold, than the expulsion would be opposed by such force from the die that it could result i n the fracture of the ejectors and/or pins. The following is a list of useful checks and rules for the casterman to obtain high quality castings and to get the maximum from the press by the meticulous setting up of the principal work parameters :

geometric adjustment of the press : is the adjustment of the machine to ensure the precise alignment of the axis of the metal infeed to the die ; - press closing adjustment : is the setting of the die clamping force to the correct value ; - program-cycle of the press : is the programming of the correct sequence of closing and opening of the die and of any lateral slides, in accordance with the complexity of the die ; - regulation of the speed of movement of the slides : provides for the harmonious movement of the slides in such a wy as to avoid hammerings on reaching their definitive positions ; die closing/opening speed : the speed of the small movements of the die on the final approach/initial opening of the die must be smooth in order to avoid chattering or tears and will be slower than the speed of opening and closing during the major movements (big shifts) ; safety of the die : also the approach force of the die must be limited and must not reach the full closing force if the die is not correctly mating. This adjustement is to avoid flashes or risers that may have remained in the die from the previous cycle, damaging the die ; ejection delay : the expulsion of the piece must occur with a certain delay as regards to the opening of the die ; the delay coincides with the beginning of the movement of the robot to withdraw the casting. ejection speed and travel : these parameters are regulated so that the ejections stop at a determined point and act with delicacy so as to avoid the deformations and damagings of the piece ; sometimes the ejectors stop at the end of the travel ; - ejection return : the return of the ejectors to the position of rest can be adjusted ; they could be subordinated to the withdrawal of the casting by the robot arm ; injection delay : is the delay of the application of the power on the injection piston with respect to the end of the pouring in of the molten metal ; - first phase travel : is the length of the piston travel during the first phase of injection ;


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-

first injection speed : is the speed with which the piston moves during the first phase travel ; - parashot or progression setting : is presses that are provided with this device, this setting determines the degree of acceleration of the piston during the first phase ; it therefore replaces the preceding point ; accumulator pressure : the value of the pressure of the fluid in the accumulator determines the speed of the second phase of injection and is, of course, adjustable ; second phase of injection : through the regulating of this, the speed of the piston during the filling of the die is determined, so ensuring that filling is completed in the time dictated by the thickness of the piece ; - multiplier accumulator pressure : on presses that are fitted with these, it is possible to set this parameter ; on the others the corresponding parameter is described in the penultimate paragraph ; - multiplier delay : on machines having a specific timer, the intervention of the multiplier can be advanced or delayed as the need dictates ; on other machines the intervention of the multiplier occurs as a reaction when the die is completely filled and the piston does not advance any further ; speed of the multiplier : this regulation determines the speed at which the multiplier moves and, accordingly, the time in which the final pressure is established to feed the casting shrinkage ; back pressure of the multiplier : this allows the adjustment of the pressure met by the multiplier on advancing and, therefore, determines the force of the same and, indirectly, the speed of advancement ; - machine opening delay : this setting, using a suitable timer, determines the time in which the machine remains closed ; it is in the interval of time that the solidification of the piece and the riser occurs. The riser is the final part of the casting to solidify ; - injection return : the interval of time after which has accompanied the riser out from the shot sleeve, returns to its position of rest (rear position) and is regulated through the use of an appropriate timer ; timing of the charging of accumulators : is the regulation of the time during which the machine pumps work at very high pressure, after the cycle has finished and the pressure has been decreased due to the injection and multiplication, to allow the charging of the accumulators.


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4.6.4 – Operating parameters of the holding furnace We analyse the operational parameters of the holding furnace management, in a similar way to that for the die casting presses. The correct control of these parameters is essential to the success of the casting : - maintenance of the level of the metal : it is good practice for the metal not to fall bellow 2/3 of the capacity of the furnace and absolutely never bellow the half way mark. The frequency of the topping up of the furnace must be in direct relationship with the hourly metal consumption ; - temperature of the metal : the thermocouple pyrometers installed on the furnaves are sensors, that connected on a regulator system, allow a preset temperature of the metal bath to be maintained ; these act on fuel valves (with the possibility of modulation between minimum and maximum levels) or on a remote control switch that switches electric power on or off or, in more sophisticated cases, directly varies the electric power ; - charging of metal : when the ladling is not manual, the quantity of metal drawn by the ladle can be regulated by varying either the inclination of the ladle and/or its immersion in the bath ; the quantity drawn varies between minimum and maximum limits ; if these limits are exceeded than the ladle needs to be renewed ; - speed of transfer of the ladle : the speed of the transfer of the ladle must be either so slow as to allow an excessive temperature drop nor so rapid as to provoke accidental spilage ; speed of pouring : the cylinder shot sleeve must be filled in a continuous motion, without delay ; injection delay : the regulation of the delay of the start of injection after the pouring of the metal must be as short as possible ;

delay after failing to pour : these need to be set return a ladle of metal to the holding furnace when it does not receive the consent signals to pour into the shot sleeve.


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4.6.5 – Lubricator regulation parameters The operating parameters of the device that provides for the automatic lubrication of the die are as follows:

speed of raising and lowering : the die must observe a thermal regime as constant as possible, therefore the action of the lubrocator must be very rapid and constant ; - spraying of refrigerant fluid : the quantity of refrigerant fluid is regulated by timing its flow rate ; this factor determines the lowering of the temperature of the die, which is particularly important where pins and/or particular geometric prominences are present in the die form ; spraying of the lubricant : the reasoning is similar to the previous parameter ; the objective of the operation is in this case the formation of a sufficient film of liquid ; - blow : the quantity of air blown onto the die to dry the cavities is regulated by a timer ; once the drying is finished any further blowing will result in excessive cooling of the die and therefore an alteration of its thermal regime ; time-cycle : the delay between the end of the drying blow and the closing of the die, determines the total length of the production cycle, therefore these parameters must be regulated equal to all the others ; - pressure of the spraying air : a variation in the pressure of the air that carries the refrigerant fluids and/or lubricants determines the degree of atomisation and therefore, the effectiveness of the operation.

Table 1 – Press setting card and die casting technical data Die casting card Press……………………… Tons …………………………… Name of the casting …………………………………………………………………. Alloy…………………………………… Customer code…………………………………………………………… Die …………………………………… Production rate (castings/hour) n° ………………………………………………………………………………….. Percentage rejects …………………. (castings/good) n° ………………………………………………………… Die weight Kg ………………… Dimensions mm ……………………X ………………… H …………………. Shot sleeve o int o Diameter mm ……………………………………………Length mm ……………………… Piston acc. o Cu. o …………………..mm ………………………………………………mm …………………… Knockout bar. N° ……………………. Length mm ……………………………………. Weight casting g ……………….. Casting frontal area cm²…………………………………………………… Weight piece g ………………. mm……………………………………………………..

Piece

Weight cast g …………………… mm……………………………………………………………………

min.

thickness

max.

Overflow g ………………………. med.mm……………………………………………………………………..


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Press cycle program Injection position high o med. o low o Press movements

Closing

Opening

Slow phase ……….....mm……………

(FC)

…………….mm………….(FC)

Fast phase ……….....mm……………

(FC)

…………….mm………….(FC)

Braking ……….....mm……………

(FC)

…………….mm………….(FC)

Die safety

(FC)……………..mm……………

Extractor travel

(FC)…………….mm……………

Timer settings Gate recycle time ………………………………….sec…………………………… Solidification closed sec……………………………

time

Tn°

Tn°…………………………………

Extraction delay time sec……………………………

Tn°…………………………………

Extraction return time sec……………………………

Tn°…………………………………

Injection delay time sec……………………………

Tn°…………………………………

Piston return time sec…………………………… Pressure regulation

Tn°…………………………………

Line pressure Kg/cm²……………………………………………………. Accumulator 2nd phase Kg/cm²……………………………………………………. Nitrogen charge Kg/cm²……………………………………………………. Booster accumulator Kg/cm²…………………………………………………….


pressure

pressure

pressure

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Nitrogen charge Kg/cm²…………………………………………………….

pressure

Final pressure 3rd /4th Kg/cm²…………………………………………………….. Injection regulation

phases

Travel

Speed

1st phase m/s…………….tours…………………

(FC)……..mm………………

2nd phase m/s…………….tours…………………

(FC)……..mm………………

3rd phase ….m/s……………..tours…………………

(FC)……..mm…………..

4th phase m/s……………..tours……………….

(FC)……..mm…………...…

Booster sec……………….advanced………sec…………………. Allay treatment Scorification

o……………….

Refining

o……………….

Modification

o……………….

Temperature of molten alloy

C°………………

delay……

Regulation of mechanical ladle mm………………………………….

Riser

thickness

Ladle size n° Kg………………………………………………..

Capacity

Times ……………………………………………………..

Speed

Start cycle …………………………………..

sec………………………………

Dosing sec……………………………… ………………………………….. Descent immersion ………………………………….


sec……………………………….

Traverse

Slow

traverse

Descent

%

%

%

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Slow raising …………………………………..

sec……………………………….

Raising

Raising sec………………………………. …………………………………..

Slow

Traverse sec……………………………….. …………………………………..

Backward

Pouring sec……………………………….. ………………………………….

Forward

Pouring delay

sec……………………………….

Emergency return

sec……………………………….

Press movements lubrification cycles………………………….

%

raising

%

movement

movement

%

%

sec……………………n°

Piston lubrication

sec…………………….

Die cavity lubrication

manual o automatic

o

Automatic spray lubrication cycles………………………….

sec…………………….n°

Drying cycles………………………….

sec…………………….n°

Total die lubrication cycle

Lub separator – Type/mark Percentage dilution

sec……………………

……………………….. %……………………...

Consumption per 100 pieces Cooling Functioning…………………….

Kg……………………. Existing ………………..

Fixed part ………………………………..

n°………………………..n°

Moving part ………………………………..

n°………………….…….n°

Die temperature

Cavity fixed part

C°…………………

Cavity moving part

C°…………………


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Exterior of die Technological checks Ingates

C°…………………

Ingate number ………………………………….. Thickness………………….mm……………………... Length

Total area of ingate Ratio of area of piston/area of ingate

………………….mm…………………….

mm²………………………..cm²……………………..

Cm²

:

cm²

…………..

=R

…………….

…………..

Speed to the ingate 2nd phase speed injection

m/s

x

……….

R …………..

= m/s ……………..

Final pressure on metal Pressure/injection force

:

Piston area

Kg/cm²

:

cm²

Tons

……….. ………….

………..

=

Kg/cm²

…………

N.B : On production start up you must reject the first n° ……………………castings

On restarting production you must reject the first n°……………………castings


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4.6.6 – Observance of the regulation In the previous aparagraphs we have highlighted the more representative parameters that need to be set in the production of die castings. As has been seen, there are many of them and to set them we turn to the previously mentioned « Casting Data » tables. It is necessary to be scrupulous in setting the parameters, bearing in mind that the listed values have been arrived at from exhaustive tests and from the optimisation of production through long time experimentation. When a satisfactory production run is completed the set parameters must not be varied. If it should be necessary to carry out variations, for instance of geometric type, it will be necessary to alter a few parameters, but this should be done with the utmost awareness, considering well the implications that these variations will involve. It is well to carefully check the quality level of the piece after original or subsequent parameter settings and, when satisfied that the results are the optimum obtainable, the parameter settings must be recorded to ensure repeatability.

Table 2 – Dependences between process parameters and the characteristics of the casting


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4.6.7 – Mutal dependence between the operating parameters The operating parameters, that is the measurements set on the machine and the thermal parameters, determine the outcome of the casting and its physical-mechanical properties. The various parameters are not independently variable, but are very much interdependent, and it is worth repeating : any variation to one of them determines variations in all the others. An example of a typical circumstance could be the following. Let us assume that we want to increase the production rate (number of castings produced per hour) ; to obtain this we need to turn to the overall cycle time. Generally it is the opening time of the press after the injection that needs to be varied (the solidification time) ; let us see how the variation of this parameter will reflect on the others and therefore what will be the consequences on the casting. Acting as we have said, the thermal equilibrium of the die is altered as, together with a greater quantity of alloy, a greater quantity of heat is added per unit of time. Therefore, after a certain number of cycles, the temperature of the die cavity is raised and it stabilises at this new value ; so we can say that the die has reached a new thermal equilibrium. The casting will have to be extracted at a higher temperature and that could result in deformations or directly to flaws ; in the best of cases there would only be a variation in the free shrinkage of the casting and consequential dimensional variations. What must we do then to increase the production rate ? We can not only act on the solidification time, but must simultaneously increase the ability to absorb the greater quantity of heat by proportionally increasing the die internal coolant flow rate and/or the quantity of lubricating/cooling fluid sprayed into the die cavity, in order to maintain the temperature equilibrium unchanged. In this way the production rate will be increased without altering the qualitative characteristics of the casting. As above-mentioned, the understanding of the relationships between the various parameters is of fundamental importance in being able to predict and to control a process ; a necessary precondition to the production of quality casyings that constantly conform to specifications. A detailed and exhaustive discussion of these mutual influences would certainly be beneficial, so, in the following tables (tables 2 to 18) we detail a series of brief summary notes that will help to give sufficient comprehension of the phenomena involved in the process.

Table 3 – Relationship between the geometry of the casting and the die casting parameters

Table 4 – Relationship between the thickness of the casting and the die casting parameters


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Table 5 – Relationship between strippings of the casting and the die parameters

Table 6 – Relationship between the characteristics of the die and the die casting parameters


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Table 7 – Relationship between the air vents and the die casting parameters

Table 8 – Relationship between the characteristics of the alloy and the die casting parameters


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Table 9 – Relationship between the temperature of the alloy on entering the die and the die casting parameters


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Table 10 – Relationship between the temperature of the die and the die casting parameters

Table 11 – Relationship between the characteristics of the press (injection) and the die casting


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parameters

Table 12 – Relationship between the speed of the metal to the ingate and the die casting

parameters

Table 13 – Relationship between the filling time of the die cavity and the die casting


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parameters

Table 14 – Relationship between the feeding of the shrinkage and the die casting parameters


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Table 15 – Relationship between the cooling time of the casting and the die casting parameters

Table 16 – Relationship between the extraction temperature and the die casting parameters


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Table 17 – Relationship between the preparation of the die and the die casting parameters

Table 18 – Relationship between the operating cycle time and the die casting parameters


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4.7 – Die casting equipment 4.7.1 – Melting and holding furnaces There are various types of furnaces in a die casting foundry, depending on their specific functions, this can be schematically described as follows. Fig. 4.7.1.1 illustrates how the furnaces are connected with the various materials applicable to the process ; fig. 4.7.1.2 describes some fundamental criteria that castermen must observe to establish optimum production conditions.

4.7.1.1 – Melting furnaces These are the furnaces used for the melting of ingots and scrap (foundry returns, castings, flashes and discards) ; see fig. 4.7.1.1.1 – 3. These are generally combustible fuelled (diesel oil, methane) induction furnaces. Their characteristics are defined as the charge (contents in kg) and melting capacity (in kg/h) ; this will be commensurate with the needs of the foundry, i.e. the number of presses that they must feed. The criteria of choice must be to satisfy the following demands : -

good thermal output ; contain melting losses to a minimum ; ease of access for cleaning (removal of dross) ; possibility of carrying out metallurgical treatments on the alloys ; reduced environmental pollution (smoke, discharges and noise) ; minimum maintenance (for example : duration of the refractory) ; flexibility, reliability and continuous operation.

4.7.1.2 – Holding furnaces Holding furnaces (see figs. 7.4.1.2.1 – 4) are used to supply metal at the most suitable temperature to feed the die casting machines. They are fed, at least initially, with molten metal derived from the melting furnace and function on combustible or electric energy. The possibility of being fed with solid metal (one ingot at a time), while not advised in the beginning, is now sometimes practised ; it must not however jeopardise an acceptable constancy of temperature of the metal being supplied to the presses ; an important factor for constant quality production. The size of a holding furnace must be commensurate with its absolute capacity (kf of metal) compared with the quantity of metal (kg/h) required by the presses, it is serving. The absolute capacity of the holding furnace must be such as not to require too frequent topping up and must be in harmony with the production rate of the melting furnace that is feeding it. Another criteria of choice is the form of energy that it uses. This must take account of the whole foundry’s requirements, the cost of energy and the local ecological considerations (smoke emissions and noise). The metallurgical aspect is however the prevailing technological factor and, therefore, the first consideration is the ability of the furnace to maintain a set temperature, within narrow limits, so assuring the user of the most constant feed temperature possible. High efficiency electric furnaces are the best suited for maintaining the temperature of aluminium alloys and are in fact the most recommended. With regards to alloys that tend to easily form scoria, above all aluminium, the ease with which periodic maintenance and cleaning can be carried out becomes an important factor.


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4.7.1.3 – Holding/melting furnaces (hybrid) This type of furnace, see fig. 4.7.1.3.1, which combines the two functions of the previous furnaces, can be found in small foundries, where they are prefered because of their flexibility. Since the demands on a melting furnace are, from a metallurgical standpoint, very different from those of holding furnaces in that they do not ensure a constant temperature, theur use does not allow an optimal metal bath and, for that reason, its use is not recommended.

Fig. 4.7.1.1 – Melting losses. Metal losses during the die casting process. The generally acknowledged level


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of transformation loss is 3 - 5%.

Fig. 4.7.1.2 – Rules for the operating and the maintenance of good metallurgical conditions in the holding furnaces. - Do not introduce solid metal into the furnace - The molten metal charge should be at approximately the same temperature as that drawn off for the press (± 20/30°C) - Carefully maintain the temperature - Remove the slags and the oxides from the heating bath at least every 24 hours. - Remove the oxides from the ladling bath as necessary. - When cleaning the ladle bath take care not to damage the thermocouple. - In combustible fuelled furnaces do not temper with the burner adjustment, which should be set to give an overpressure in the chamber and the suitable atmosphere (CO2 at the stack = 9/11%) - Keep the charging door closed. - Re-charge the furnace at intervals necessary to maintain the level between the minimum and the maximum. - Immediatly attend to any malfunction.


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Fig. 4.7.1.1.1 – Crucible melting furnace, Melting under t he bath in the crucible Characteristics : - Tilting crucible furnace - Tapping through a discharge spout hinged on the tilting axis - Energy source : diesel or methane gas - Capacity : max. 500 kg - Thermal efficiency : low - Melting loss : minimum - Metallurgical profile : optimum


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Fig. 4.7.1.1.2 – Double chamber melting furnace, melting on a dry earth Characteristics : - Fixed furnace - Tapping through taphole with mechanical plug - Energy source : diesel or methane gas - Melting chamber with burner regulated to the maximum temperature of the chamber - Containment and heating chamber with burner regulated to the tapping temperature - Thermal efficiency : high - Melting loss : med./high - Metallurgical profile : good


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Fig. 4.7.1.1.3 – Tilting tank melting furnace, Melting under bath Characteristics : - Tilting tank furnace - Tapping through a discharge spout hinged on the tilting axis - Energy source : fuel oil, diesel, or methane gas - Two long flame burners for indirect and radiation heating (radiance), thermoregulated to the tapping and/or the holding temperature - Capacity from 1.000 to 1.500 kg - Melting capacity : from 250 to 1.500 kg/h - Thermal efficiency : medium - Melting loss : medium - Metallurgical profile : good


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Fig. 4.7.1.2.1 – Crucible holding furnace Characteristics : - Crucible holding furnace - Thermo-regulation for maintaining the ladling temperature - Energy source : diesel or methane gas or electricity - Capacity : max. 200 kg - Thermal efficiency : low - Melting loss : minimum - Metallurgical profile : optimum


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Fig. 4.7.1.2.2 – Tank holding furnace , indirect heating Characteristics : - Reverberatory holding furnace - Thermo-regulation to maintain the temperature at the optimum for ladling - Energy source : electricity, heating with ceramic resistors - Capacity : max. 1.000 kg - Thermal efficiency : med./high - Melting loss : minimum - Metallurgical profile : good

Fig. 4.7.1.2.3 – Tank holding furnace, reverberatory heating Characteristics: - Tank holding furnace, low consumption - Capacity : from 500 to 1.500 kg - Thermo-regulation to maintain the temperature at the optimum for ladling - Heating electric power from10 to 25 kW - Thermal efficiency : high - Energy source : electricity, heating with metallic resistors - Melting loss : minimum


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- Metallurgical profile : good

Fig. 4.7.1.2.4 – Tank holding furnace, direct flame heating Characteristics : - Tank holding furnace, direct flame - Thermo-regulation to maintain the temperature at the optimum for ladling - Energy source : diesel or methane gas - Capacity : from 500 to 2.000 kg - Thermal efficiency : high - Melting loss : medium - Metallurgical profile : medium


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Fig. 4.7.1.3.1 – Melting and holding furnace, dire ct flame melting. Characteristics : - Hybrid melting and holding furnace, direct flame melting - Thermo-regulation to maintain the temperature at the optimum for ladling - Energy source : diesel or methane gas - Capacity : from 1.000 to 5.000 kg - Thermal efficiency : high - Melting loss : med./high - Metallurgical profile : good


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4.7.3 – The lubrication The lubrication of all the components making up the process is of extreme importance. It must be correct, precise and appropriate to the variety of application points. In fact production hold ups, malfunctions, breakages and premature wear are often caused by the lack of, or unsuitable lubrication. Fig. 4.7.2.4.2 details the points on the different components, as well as the characteristics of the products. 4.7.3.1 – Lubrication of the machines In die casting process, lubrication is of capital importance for the correct functioning of the machines and the tooling and to prevent their premature wear. All the components of a die casting plant operate in a hostile environment, not only characterised by temperature, but also humidity, vapours, dust, smokes and often severely stressed. The kinematics of the press, toggle linkages, the movements, column bushes and guide bearings are generally fed by their own automatic lubrication system. The importance of the lubrication of these parts is such that often the lubrication fittings are Provided with electric safety devices, lubricant tank level sensors and circuit pressure sensors that operate in such a way as to halt the machine where the lubrication is not absolutely correct. The press and equipment manufacturers indicate the lubrication points in the user manuals, showing also the frequency of application and the nature of the products to be employed. 4.7.3.2 – Lubrication of the die Sometimes, lubricants are very important with regard to the lubrication of the die, and we will discover this application in a deeper way, reading the book n°7 « Lubrication of the c avity of the die ». 4.7.3.3 – Lubrication of the parts exposed to contact with the metal Special lubricants, which are resistant to high temperatures, are employed for the protection of ladles, pistons and shot sleeves. Tables 19.1 to 3 show a schematic summary of the factors in play as a compendium of what has been discussed on the die casting process up to this stage.

Fig. 4.7.2.4.1 – Summary of the functions and adjustments of a die casting press. 1) Speed of 1 st phase – 2) Starting point of the 2nd phase – 3) Speed of the 2 nd phase – 4) Nitrogen charging of the 2nd phase – 5) Starting point of the 3 rd phase. Regulation of 3rd phase pressure : 6) Pressure regulator valve – 7) Unloading valve – 8) Nitrogen charging system – 9) 3 rd phase speed – 10) Height of injection cylinder adjustment – 11) Speed of closing – 12) Speed of opening – 13) Height st Core extraction : 14) Speed of the 1 phase – 15) Force of 1 st phase – 16) 1 st phase travel – 17) Speed of 2 nd


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phase – 18) Force of 2 nd phase - 19) 2 nd phase travel – 20) 3 rd phase speed – 21) 3 rd phase force – 22) 3 rd phase travel. Ejectors : 23) Extraction force – 24) Speed of extraction – 25)Ejector travel. Columns : 26) Locking of moving plate – 27) Traverse locking.


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Fig. 4.7.2.4.2 – Lubrication scheme of a die casting press and the dosing ladle


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Fig. 4.7.2.4.3 – Technical data characteristics of some die casting presses

Model Closing force Maximum injection force Central extraction force t Minimum die opening – O mm Maximum die opening – P mm Moving plate travel mm Central extraction travel mm Plate dimensions – AxB, CxD mm Distance between columns – GxH mm Maximum weight of casting (Al) kg Maximum castable area 400

t t

HP1 160 23 15,8

HP2 250 33 20,3

HP3 350 38 20,3

HP4 500 55 30

100

150

150

200

500

600

600

800

400

430

470

625

120

120

120

180

650 x 650

800 x 800

850 x 850

1090 x 1090

400 x 400

500 x 500

550 x 550

650 x 650

2,2

3,4

5,2

8,5

400

625

875

1250

20

15

13

8

15

18,5

22

30

With pressure on metal of bar/cm² Empty cycles N/1’ Pump motor power Kw Weight of press kg Dimensions overall

7500

12000

14500

22000

1350 x

1375 x

1455 x

2100 x

W.

4700 x

5850 x

6050 x

7150 x

2200

2330

2330

2700

x

L. mm

x

H.


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4.7.2 – Mechanized devices The demands of increasing productivity and of high and constant quality, accompanied by technological progress, have also brought automation applications into the die casting foundry. Automation, by reducing the variable nature of human intervention, certainly brings a more constant production and can bring improvements in quality and productivity. To function in an automatic regime, the press must be integrated with other apparatus that substitute for manual work. The operator, freed from this physical fatigue, is more able to observe, consider and to intervene, often more effectively, on the production tools that he is controlling. The mastery of the process rests on the following presuppositions : - reliability of the press and of the auxiliary apparatus ; dies which can sustain the production rate (remember that one of the factors that determines the adequacy of the die is an effective system of cooling).

4.7.2.1 – Metal feeders – chargers This device generally consists of a mechanised ladle that draws a precise quantity of molten metal from the holding furnace and transfers it to the press shot sleeve. The following figures show various types of mechanised ladles which are differentiated by their kinematics. The devices are governed by electronic systems and/or by microprocessors and operated by variable speed electric motors. As regards to the kinematic properties, these could be categorised as follows : -

rectilinear transfer ladles (fog. 4.7.2.1.1) ; rotary ladles (fig. 4.7.2.1.2) ; pendulum-like movement ladles (fig. 4.7.2.1.3) ;

The accuracy of the dosing and of the casting requires an easy control of : -

variation in the metered quantity, within its range, tied to the capacity of the ladle ; precision and regulation of the speed of movement ; programming and reprogramming of the cycle ; repeatability and maintainability of the geometric parameters and set kinematics ; possibility of « self-learning ».

The same problem can be faced in a different way, but with greater difficulty in regulation : the pouring of the metal can be achieved by use of tilting furnaces (the metal moves by gravity) or with pressurised furnaces (the metal is transfered along ducts pushed by gas under pressure).


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Fig. 4.7.2.1.1 – Rectilinear mechanical ladle Characteristics : - Horizontal carriage travel : 1,600/1,900/2,100 mm - Vertical traval of the ladle carrying arm : 900 mm - Power of motor for horizontal and vertical movements 2.7/2.2 HP - Capacity of applicable ladles : approx. Kg/Al 0.2/2-1/8 – 4/25 - Max. cycle empty : N/hr 280/190/130 - Electric power : 380V/50Hz


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Fig. 4.7.2.1.2 – Rotary movement mechanical ladle Characteristics : - Quantity of metal adjustable from 0.10 to 14 kg - Number of cycles : 300 per hour - Height of immersion : 650/850 mm - Distance from furnace to shot sleeve : 550/750 mm - Dimensions very much reduced


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Fig. 4.7.2.1.3 – Pendulum movement mechanical ladle Defining characteristics : - Quantity of metal adjustable - Number of cycles : - Height of immersion : - Distance from furnace to shot sleeve : - Dimensions very much reduced 4.7.2.2 - Lubricating/cooling device This device is required to carry out a delicate and difficult function ; the preparation of the die cavity by the application of a thin layer of fluid separator ; this has the function of preventing sticking of the metal to the matrix of the die and to facilitate the separation of the casting after solidification. The separating strata must be appropriately chosen and be able to be regulated by the device for thickness. The structure of the lubricators/coolers is fairly constant, while the techniques ans spraying methods vary from type to type. Being tied to the characteristics of the die cavity, the spraying needs to be different from die to die ; for this reason fundamental requisites of thse devices must be : -

highly flexible spraying programs ; adaptability of the spray heads in relation to the conformation of the die ; adjustability of movements and speed of movements ; possibility of regulating the flow rate of individual nozzles ; constancy and repeatability of the set functions ; high reliability and continuous operation.

4.7.2.3 – Casting withdrawal device


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This device consists of a more or less articulated mechanical arm ; beyond operating the actual transfer from the station or stations in the press line, it could also be equiped with sensors that check the completeness of the figure. Fundamentally we can distinguish two types of device :

4.7.2.3.1 – Mechanical arms These are the simpliest and most elementary and the functions which they undertake are : -

withdrawal from the die ; verification of the completeness of the casting ; consent to the resumption of the press operation ; depositing of the casting into a shot sleeve or onto a work surface.

Generally they have 3 degrees of freedom to reach fixed points determined. The possible movements are : -

entering between the die halves ; taking of the casting ; extraction and withdrawal from the die ; release of the casting.

4.7.2.3.2 – Manipulators These are more sophisticated mechanical arms in that they are able to undertake further functions. In the more complex examples, they have up to 7 degrees of freedom (see fig. 4.7.2.3.2.1) which allow them to reach predetermined fixed points utilising mechanical stops and/or electric limit switches. Beyond removal of the castings, these could be employed : to orient the casting from the vertical position of removal to the horizontal position in cases where the application of a completeness control device requires it or it is required by the following process operation ; - for cooling the casting by dipping it in water ; - to rotate the casting to assist dripping and drying ; - to place the casting down and position it on the shears for removal of excess, flashes, etc ; - to hold the casting or take it back and place it in a shot sleeve. This type of manipulator allows the total automation of the process and allows the operation of the press without the constant presence of the operator. The possible movements are set in a fixed sequence and examples of this are given in the following fig. 4.7.2.3.2.2


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4.7.2.3.3 – Robot These devices, like the manipulators are equiped with 7 degrees of freedom and can perform all the functions above-mentioned, with the variation that the movement sequences are programmable and the attainable points can be in any position in the operational field of the apparatus. The system of governing the robots is generally supplied by a programmable microprocessor on which the sequences and the positions of the desired points have been set. To facilitate the programming, many of these control systems are equiped with « self learning », whereby the program is taken into memory while piloting the required manœuvre manually. It is then able to be repeated automatically. The memorised programs can be filed on fixed or removable memories thereby making it possible to re-run production of the same piece at any later time, so gaining economies in preparation and start up times.

Fig. 4.7.2.3.1.1 – Mechanical arm remover Characteristics : - Thress fixed movement programs - Device for checking the completeness of the casting - Weight of casting removable : up to 6 kg - Number of cycles per hour empty : 400


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FIg. 4.7.2.3.2.1 – Robot casting remover and Manipulator Characteristics : - Seven degrees of freedom - Self-learning : field programming - Memorising of programs - Maximum casting weight : 15 kg


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Fig. 4.7.2.3.2.2 – Mechanical arm remover manipulator Characteristics : - Seven degrees of freedom that allow the following movements : - Enter into and exit from the die - Take hold of and release the casting - 90° rotation of the gripper - 180° rotation of the wrist - Raising and lowering movement of the arm - Left and right traversing of the arm - 360° rotation of the arm 4.7.2.3.4 – Robots in work operations subsequent to casting The use of robots, and particularly « multipurpose anthropomorphs » is attracting ever more interest in the die casting industry that, in addition to the automating of the press itself, also seeks to automate various operations subsequent to the casting. High flexibility, extreme reliability and the ability to be programmed make it possible to fully automate the die casting process, even including the application of the lubricant separator in the die cavity. This operation can be achieved in two ways : the robot can pick up and move an appliance fitted with the spray head or it can have the appliance permanently mounted on the wrist of the robot arm. After withdrawing the casting from the press, the robot automatically manipulates and moves it following sequence : -

it rejects the casting on receiving a signal from the press (abnormal injection) ;


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-

it presents the casting to the control sensors (generally checking the completeness of the piece) and gives a signal to the machine and restart the cycles ; it stops the machine and rejects the casting on a signal from the control sensors (casting incomplete or anomalous) ; - it cools the casting by immersing it in water ; - it presents the casting, suitably oriented, to the casting detaching device ; - it deposits the casting on the shears from removing the flashes ; - it palletises the casting ; - it delivers the piece to another work station for further processing.

Fig. 4.7.2.3.4.1 shows an overview of the activities of robots in the context of the process, and the statistical times required by the various operations. Work operations which are subsequent to the actual casting of the piece must be precisely timed, so it is not generally convenient, nor sometimes possible, given the optimum temperature of the die, to lengthen the operating cycle of the casting press to undertake such work during the casting itself. Figs. 4.7.2.3.4 – 8 illustrate some automation of the die casting press and subsequent operations, which have made possible by the employment of robots.

Fig. 4.7.2.3.4.1 – Overview of the operations/ancillary working in the automated die casting process using industrial robots. RC = Robot Cartesian – RD = Robot Dedicated – RA = Robot Anthropomorphic. The number of the operations which can be automated (for each of which the average time is given), except for the unloading and lubrication operations, depends on the « die closed » time, which, in turn depends on the size of the press: on average for presses from*300 tons, 700 tons and > 1.000 tons respectively. The standard duty, relative to the size of the anthropomorphic robot, is based on the


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pay loads of 10, 30, and 100 kg, respectively for the 300 tons, 700 tons and the > 1.000 tons presses. **Operations executed using lances moved by the same robot function that withdraws the piece from the die or by means of a specific supplementary robot. *** Operations using installed nozzles fitted in a robot.

Fig. 4.7.2.3.4.2 – Robot for the removal and the unloading of the piece and for die lubrication


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Fig. 4.7.2.3.4.2.3 – Robot control panel for regulation of the kinematic parameters.


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Fig. 4.7.2.3.4.2.4 – Robot for the removal of the casting with access to shearing and trimming operations


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Fig. 4.7.2.3.4.2.5 – Robot for the removal of the casting, shearing and depositing at the dimensiona l checking station


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Fig. 4.7.2.3.4.2.6 – Robot for the removal of the casting and successive mechanical working.


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Fig. 4.7.2.3.4.2.7 – Robot for the removal of the casting shearing, selection and starting of the accept/reject conveyer.


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Fig. 4.7.2.3.4.2.8 – Mechanical arm robots placed in series for the removal, shearing, trimming and finishing, selecting and starting the subsequent stations.

TABLE 19.1 – DIE CASTING PROCESS

Aluminium alloys – Horizontal cold chamber press : preparation of the bath.


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TABLE 19.2 – DIE CASTING PROCESS

Aluminium alloys – Horizontal cold chamber press : casting of the piece


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4.7.2.4 – Considerations on the automation of the process The length of the production run weighs heavily on the choice to adopt automatic support systems for the production process. The expenses that accompany such a choice and the extension of an automatic system is spread across the number of units produced and its unit cost is therefore reduced as low as possible (it must also be borne in mind that this choice to automate does not only mean the cost of equipment but often the cost of laborious setting up and start up that lengthen lead times). In specific and objective situations, automation could also be limited to individual functions, so we can have various degrees of automation, which we can list as follows : - 1st integration, partial automation of the press ; the press is only equiped with one ladle ; 2nd integration, partial automation of the press ; the press is equiped with a ladle and an automatic lubricator/cooler ; 3rd integration, total automation of the press ; this is equiped with a mechanical ladle, lubricator/cooler and a mechanical arm for the withdrawal of the casting. Sometimes automated production, integrated to the third level, is completed by a further operation, also automatic, for the separation of the casting and the removal of the overflow well and the flashes by means of shearing. In this case, the mechanical arm does not simply place the casting in a collection skip, but arranges it precisely on the shearing die ; obviously the shears are within its operating range. We can consider such a widely integrated complex to be a real autonomous production island. Figs. 4.7.2.4.1 – 3 represent a horizontal cold chamber die casting machine, the general system of


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lubrication and the principal characteristics.


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5 – THE DIVERSIFICATION AND EVOLUTION OF DIE CASTING Die casting presents a negative aspect inherent in the process : the presence of porosity in the cast pieces, to a greater or lesser extent depending on the conduct of the process. This porosity is due to the inclusion of gas in the metallic matrix ; essentially nitrogen and hydrogen. This inclusion significantly influences the quality of the casting, provoking reductions in the mechanical strength and pressure tightness and impeding heat treatments, processes that aim to improve the mechanical properties of the piece. To counter this phenomenon, various diverse methods have been employed. In the following paragraphs we deal with the principal ones while asking the reader to bear in mind that the die casting sector is a rich source of ongoing research. The different systems act on the porosity with the objective of reducing it ; while the downside is that such systems tend to introduce complications into the process and increase the unit cost of production. For this reason many applications have often remained limited and are only adopted in the face of particular service demands.


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5.1 – The Acurad Process Conceived in the United States of America, and developed by General Motors, it is characterised as follows : -

controlled cooling of the die ; large section ingate ; slow speed filling of the cavity ; double injection piston.

The controlled cooling of the die has, like all devices of this kind installed in the die, the function of effecting a directional, i. e. controlled. To rationally effect this course, the heat exchange and the local heat flows must be carefully studied. The above-mentioned thermal analysis was developed using the Teledeltos method, which allows the simulation and measurement of the thermal flows utilising electric similarities. The ingate is generously dimensioned, particularly concerning the thickness, in such a way as to allow a low speed to the ingate and a relatively slow filling of the die cavity with the minimum turbulence of the incoming metal. In these conditions, the modality of the filling is near to the hypothesis formulated by Brant and the lack of turbulence avoids gas segregation. The molten metal is forced into the cavity by a special piston : this consists of a internal cylindrical concentric piston inside an external tubular piston. In the injection phase the two pistons advance together in the shot sleeve pushing the molten metal into the cavity until the filling is complete. Solidification starts, the inside piston is made to advance with a high force into the still liquid heart of the casting, in this way feeding the shrinkage at the front up to the solidification of the ingate. The Acurad system is illustrated and compared with the conventional system in fig. 5.1.1. The Acurad process was used for the production of the 2,6 kg engine block of the Vega motorcar using a special alloy GDAlSi17Cu4Mg. The injection curves and a micrography of a zone of the casting is given in fig. 5.1.2. The Acurad process owes its scarce adoption to the need to undertake the careful thermal study for each individual casting and for the complications of the injection device (see fig. 5.1.3). However, in the following we summarise the characteristics of the Acurad : Advantages : -

more compact pieces ; improved mechanical strength ; improved pressure tightness ; a limited possibility of heat treatments ; improved weldability :

Disadvantages : - difficult study of the thermal flows ; thermoregulation of the die ; thick ingates ; complex injection system.


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Fig. 5.1.1 – At the top, conventional method. Below, Acurad method.

Fig. 5.1.2 – Readings registered on an osciloscope showing the silicon injection pressure in relation to the piston travel in the Acurad process 1) External piston ; 2) Main cylinder ; 3) Internal piston ; 4) Start of casting ; 5) Piston speed ; 6) Piston travel.


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Micrographic characteristic of the aluminium alloy in a cast piece produced with the Acurad process. The sample was taken from the surface sliding zone of the cylinder. The dark zones are particles of silicon (surface etched with 0,5% of HF, magnification x 90).

Fig. 5.1.3 – Acurad system, injection cylinder with double pistons.


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5.2 – Vaccum die casting Knowing that the inclusion of air and gas in the piece has a negative and determining influence on the obtainable characteristics of the cast piece, one proposed solution, is the filling of the die cavity after having exhausted the air from the shot sleeve (Nelmor and Vacucast systems). To obtain a vacuum within the cavity, the die is enclosed in a shot sleeve which is integral with the machine or the die. The most effective degree of depression is between 0,6 and 0,7 bar (60.000 – 70.000 Pa). With these systems the quantity of exhausted air, with respect to the effective volume of the cavity, is high and requires a large vacuum plants so as not to excessively lengthen the die casting cycle. Having seen the positive results of the process, studies of further solutions have been undertaken with the aim of exhausting the air directly from the die and to continue the aspiration also during the filling phase and, in this way, also extracting the gases that develop when the molten metal comes into contact with the lubrication separator. The advantages obtained with this latter system are as follows : -

improved compactness of the structure ; pieces with reduces porosity ; improvement of the mechanical properties of the piece ; improvement of the pressure tightness ; possibility of moderate heat treatments.

On the other hand, they do not influence susceptibility to the formation of shrinkage cavities and of micro shrinkage cavities (feeding defects). One of the systems that has proved to be most effective and that has found wide acceptance is illustrated in fig. 5.2.1 (the Holder system). This sytem, which has an aspiration system mounted on the die, makes use of special self-closing valves, which are of complex construction and which require frequent maintenance interventions and the replacement of parts subject to wear. The disadvantages of this system can be summarised as : -

higher cost of the die ; increased down time for maintenance ; cost of spare parts ; running costs of the vaccum plant.

It is a fact that, while there are undoubted advantages of the system, we have a significant increase in the cost of production, which must be reflected in the cost of the piece ; this means that the process is employed only when there is a need to produce pieces that have particular demands placed on them. The Vacural vaccum process consists of the aspiration of the molten metal directly from the furnace through the formation of a vacuum in the die cavity, as illustrated in fig. 5.2.2. This system imposes a higher degree of vacuum and produces castings of a lower residual porosity, which allows the use of heat treatments. Compared to this, the system presents the following disadvantages : -

extension of the time-cycle ; higher capacity vaccum plant ; special shot sleeve ; furnace of a particumar construction.

Also this system is adopted only to satisfy particumar demands of the piece.


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Fig. 5.2.2 – Vacural vacuum die casting process. Cold chamber press utilising the vacuum in the die to draw the molten metal into the cavity (« vacuum fill » or « vacuum feeding »). 1) Molten metal ; 2) Piston ; 3) Shot sleeve ; 4) Die ; 5) Vacuum casting ; 6) Flexible joint ; 7) Ducting to the vacuum chamber.


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5.3 – Pore Free Process Developed in America and in Japan, it appears that this process is destined to find increasing applications with some US auto manufacturers having imposed this process on their subordinate suppliers. The Pore Free is a simple variation on the traditional process. The basic concept on which the process relies was the result of observations and experiments undertaken with the objective of overcoming the problems of porosity due to the gas which is always present in pressure die casting. The accepted route to obviate this drawback has always been that of applying higher injection pressures to compress the gases present into volumes of more tolerable dimensions. In this way however, the gases, always present in the metal, make both heat treatments and welding impossible subsequent to casting. What is more, the pressure of the gas in such continuous systems would make the effects of an overheating of the piece that much more disastrous : the gas pockets present can coalesce into microexplosives, provoking distorsions in the piece and surface breakthroughs, giving the appearance of unacceptable imperfections. So, the origin of the Pore Free process was urgent experimentation to overcome a pressing problem. From an analysis of the gases trapped in a die casting, we find an absolute absence o oxygen; present instead are hydrogen, nitrogen, argon, carbonic anhydride and methane. It is clear therefore that the die casting process consumes the oxygen initially present by reacting with the aluminium alloy. The following step in the experimentation was to replace, i. e. expel, the gases intially present in the die with only oxygen : therefore, before the injection of the molten metal into the die, the whole cavity is filled with pure oxygen. The stable oxides formed are finely distributed throughout the mass ; in the metallic matrix, they will be found to be of diameters of around 1µ and with a dispersion in the order of 0.1 – 0.2% by weight. Their presence does not have negative consequences on the alloy and provokes a very small increase in hardness. The licence holder of the Pore Free process is International Lead Research Organisation Inc. Of New York ; applications are provided for the die casting of aluminium, magnesium, lead and zinc alloys. It is normally used for high quality castings, such as lead battery plates, light alloy rims for motor cars and other engine parts ; further applications are being studied however, such as electric motor frames, pipe fittings and alternatives to items which are presently forged. In Pore Free die casting, the cavity is filled with a reactive gas and the melting temperature, the speed of injection and the other press parameters must be very carefully selected. During the injection, the liquid metal mixes intimately with the oxygen and, with the speed of the formation of the oxides and of the solidification in the die, form a very fine dispersion which cannot be found using normal control techniques. The following lists the advantages obtainable with this technology : 10% increase in the tensile strength of the metallic matrix and an increase in the percentage elongation from 1.5 to 2 times, values obtained on an AlSi10Cu3,5Fe0,7 alloy, and increases of the same order of magnitude for other alloys ; less internal defects and high reliability, absence of internal porosity due to gas and therefore excellent pressure tightness; - tensile strength increasable by up to 30% in some aluminium alloys by means of an ageing treatment ; the same treatment could increase the yield strength by up to 100% ; - excellent appearance of the mechanically worked surfaces ; an increase in dimensional stability at high temperatures thanks to the lack of internal porosity, opening up the possibility of producing much reduced thicknesses and having castings that are able to be welded. With an auxiliary pressure in the die cavity, a further lowering of internal porosity, which is never totally eliminated, can be obtained. The results of some of the experiments are shown in figs. 5.3.1 and 5.3.3. Against the stated advantages, the following are the disadvantages of the system : having to replace the air present in the cavity with oxygen, this requires time ; however, the automating of the process sequences can reduce this time to a tolerable minimum ; the presence of anu lubricant that leaves gaseous residues is not tolerable for the success of the Pore Free process ; it is worth mentioning that no lubricant of an organic nature is acceptable ; given the strong


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presence of oxygen along with sufficiently high temperatures, this could, in fact, facilitate combustion ; it therefore becomes necessary to use solid lubricants ; also in this case, automation of the sequences will reduce the increase in time required for the application of the solid lubricant to a tolerable minimum ; the employment of oxygen and special lubricants constitute additional costs compared to the traditional die casting process ; casting channels and air vents must be designed using different criteria that are specific to the Pore Free process. The disadvantages and cost increases just described, combined with the previously listed advantages, will mean that any preference for this process over the traditional die casting process must be supported by an improved compromise between increased costs and the reduction in rejects and, as these conditions will not have general validity, its use will be able to be justified for only a narrow range of castings. A comparative analysis between castings produced using different procedures has established a classification, in ascending order of advantage, from the traditional die casting process, to the Acurad method and then to the Pore Free process ; this latter would however, result in being some 3 – 6% more expensive than the first in conditions of zero rejects. For the Pore Free process, in addition to the regulations necessary in traditional die casting, we need to add the following : -

regulation of the oxygen ; lubrication of the shot sleeve and the forward part of the piston ;

blowing of air and closing of the shot sleeve. It is almost certain that the advantages of the Pore Free system could only be achieved where the sequences are automated and do not involve the manual intervention of the operator.

Fig. 5.3.1 – Die casting process in an oxygen atmosphere, Pore Free die casting


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Fig. 5.3.2 – Pore Free Process


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Fig. 5.3.3 – Comparative data between the mechanical characteristics of die casting samples from a normal process (PC) and with the Pore Free process (PF), as a function of the injection pressure

Table 1 – Comparison between the mechanical properties measured on test pieces in T6 temper, taken from certain areas of the casting. Method of casting

Conventional

Zone where the sample was taken from Disk

Rm

Rp0,2

A

HB

Kg/mm²/Mpa 21/210

Kg/mm²/Mpa 14/140

% 5

25

Flange Disk

23/230 26/260

14/140 20/200

11 8

25 35

die casting Pore Free

die casting Flange 27/270 20/200 9 37 Taken from an average of wheels produced in an experimental aluminium alloy DX30, using conventional die casting and Pore Free die casting.


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5.4 – Partially solid alloy die casting The similar behaviour of some partially solidified alloys to fluids, has suggested its use, with the employment of adequate pressures, for producing castings. An important discovery in the behaviour of alloys in this state is that, if subjected to vigorous agitation during the solidification, they behave as a high viscosity mixture with solid fractions. With this in mind, various die casting processes have been modified to employ partially solid alloys, with the metal temperature lowered to the point at which it has surrendered half of its solidification heat ; the metal is extracted and injected into the die cavity in that state and then subjected to pressure, in a manner similar to the traditional die casting process, until it has undergone complete solidification. In this way the thermal stress of the die is reduced and the cycle time limited as there is a lesser quantity of heat to be extracted to reach solidification. Also the heat transfered through convection in the injection cylinder is reduced. Moreover, metallurgically speaking, alloys in this state are less susceptible to the absorption of air ; secondly, also the phenomenon of solidification shrinkage is less and more uniformly distributed. The casting of this partially solid mixture is called « Rheocasting » ; alloys that are in this state are shown to possess a particular property, called « thiwotropy », that consists of demonstrating a decrease in viscosity with agitation, a characteristic of some interest in the foundry. This property influences the behaviour of the alloy in an interesting manner ; if an alloy ingot is rheocast and the reheated to 40% of the solidification heat content, it maintains only a part of the rigidity of a solid and can then be handled like a pasta. Once the solid is reached by agitation, it rapidly regains the characteristics of a fluid mixture. A simple demonstration of this phenomenon was carried out by taking two ingots of A380 alloy and heating both to such a temperature that the solid fraction present was 40% ; one of the two was cast, the other was rheocast ; both maintained their original form and could be handled as solids. Both were allowed to fall to the floor ; the rheocastingsplashed onto the floor as if it were a liquid, while the normal casting cracked like a friable solid. The employment of rheocasts in die casting is done by heating the metal to a sufficiently low temperature so that it handles like a solid, but sufficiently high so that it flows like a fluid mixture under the force of the piston. Fig. 5.4.1 indicates, relative to that alloy, the volume of the solid fraction existing at the moment of treatment. This diagram is used to determine the best temperatures for the operation. The casting of this partially solid mixture is called « thixocasting ».


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Fig. 5.4.1 – State diagram of a Thixocast alloy


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5.4.1 – Thixocasting The semi-solid mixture for thixocasting is prepared in a thermally controlled furnace, fitted with a crucible and a mixing device, as in fig. 5.4.1.1. The alloy is heated to beyond the liquidus temperature ; the agitators are lowered into the molten mixture ; the rotation then begins ; the temperature is lowered at a controlled rate until the mixture reaches the desired temperature of the liquid-solid stage ; the temperature of the alloy, which in the meantime has continued to be agitated, now remains constant. The metal is then withdrawn and transfered by means of a preheated ladle, to the shot sleeve.

Fig. 5.4.1.1 – Rheocasting apparatus


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5.4.2 – Rheocasting Pre-cast ingots are preheated to the semi-solid state in electric furnaces and brought to the desired temperature ; the material obtained in this way then, is processed similar to traditional foundry alloys.


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5.4.3 – Comparisons Shown in fig. 5.4.3.1 are the positions, speeds and pressures of the pistons in three die castings using traditional processes (top and centre), and thixocasting. The significance of the indicated points is as follows : -

the speed grows and then decreases as soon as the metal reaches the ingate ; the pressure increases slightly in pushing the metal through the ingate ; negative speeds recorded indicate piston rebounds ; the piston stops, the die is full ; speed of injection.


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5.5 – Squeeze casting or squeeze forming The efforts expended on recent technological developments to eliminate porosity, oxides and internal defects in general, from a high production rate process like die casting, have brought about the introduction of a particular process that profoundly modifies the die casting machine while preserving the concept of intervention of pressure during the solidification of the alloy. From the metallurgical point of view the process stil exploits the principle of introducing into the form a material in which the germination of the crystals and a certain amount of degassing is already taking place ; from the plant engineering point of view it is close to die casting and in some respects to forging. The pressure intervenes metallurgically, opposing the possible formation of piping and shrinkage cavities, thanks to the presence of sufficient residual liquid metal. Control parameters are, among others : - casting temperature ; - time of delay between casting and forming ; - intervention of the pressure ; duration of the application of pressure (an important factor because it intervenes propitiously during the phase of solidification ; a phase in which, by preference, all the defects are formed). Among the advantages of the process, which has been experimented with both in the manufacture of important power transmission components and light alloy wheels for motor vehicles, we quote : - isotropy of metallic structure ; dimensions of the metallic grain and the distance between the arms of the dendrites (DAS, dendrite arm spacing) are very small ; - possibility of using traditional foundry alloy ; in the case of undergoing successive heat treatments, there is the possibility of obtaining mechanical properties near to those of a forming ; improved mechanical performances and tighter geometric tolerances in the cast piece.


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Fig. 5.4.1 – State diagram of a Thixocast alloy


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Fig. 5.4.1.1 – Rheocasting apparatus


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Fig. 5.4.3.1 – Trace of the magnitude og kinematics relative to die casting with traditional alloys (A and B) and partially solidified alloys.


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BOOK1 - The High Pressure Die Casting Process

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