IMPORTANCE OF GATING SYSTEM
• The design of gating system is as important as risering of a steel casting.
• It is well well know knownn th that at im ro er atin tin ra racctic tice can can re ressul ultt in defe defect ctss like like CER ERO OXID IDEE, INCLUS USIO ION NS, COLD SHUTS, MI MISSRUNS, HOT TEA EAR RS, LOCAL SHRINKAGES, and GAS CAVITIES in a steel casting.
• A gating system should be pre-designed and incorporated in method drawing as is the case of risering and not left to the discretion of the molder.
CONSTITUENTS OF GATING SYSTEM
• A gating system for steel castings can be broadly divided into:-
• 1) The entry section – consisting of the pouring basin, .
• 2) The distribution section – consisting of the runners and ingates.
FUNCTIONS OF A GATING SYSTEM
• The entry section of a gating has two functions:1) To supply liquid metal free of entrapped gases, slag and eroded sand. 2) To establish a hydraulic pressure head, which will force the metal through the rest of the gating system and into the casting. • The distribution section has five functions:1) To decrease the velocity of the metal stream. 2) To minimize turbulence, both in the gating system as well as in the mold cavity. 3) To avoid mold and core erosion. 4) To establish the best possible thermal gradient in the casting. 5)To regulate the rate of flow of metal into the mold cavity. • In addition to these, the gating system should be of such simple design as to facilitate molding, particularly with mechanical methods, at the same time involving minimum fettling cost and affording maximum casting yield. • Many of these requirements and functions are conflicting with each other. Effort should be to harmonize these so as to create conditions conducive to the production of a defect free casting.
GENERAL PRINCIPLES OF FLOW
• When considering the running systems, it is instructive to bear in mind a few idealized conditions of flow. It is true that the conditions prevalent in a mold are more complex. However, certain basic patterns of flow are fundamental and the gating system can be fully “engineered” from these. • The requirements of a gating system are the opposite of a hydraulic system. In the later case, every effort is made to reduce all frictional and kinetic losses to minimum . , case. The metal entering the mold should have the lowest possible velocity, and yet, should fill up the same at a rate first enough before the loss of temperature renders this impossible. • To obtain a understanding of the fundamentals of metal flow in gating systems, two basic fluid flow equations are of interest. The first of them is the “Law of continuity” and the second one is “Bernoulli’s Theorem”.
LAW OF CNTINUITY
• The law of continuity states that the flow rate must be the same at a given time in all portions of a fluid system. It may be written as:•
Q = A1V1 =A2V2 where Q = metal flow rate in cu.ft/sec A1 & A2 = cross-sectional area of flow channel at two different points 1 & 2 in sq.ft. V1 & V2 = metal velocity at points 1 & 2 in ft/sec.
• This would mean that if the flow channel narrows down to half its original cross-section, the metal velocity would be double, and vice versa. The law of continuity, therefore, can be used to predict quantitatively the effect of variation in channel size on the metal velocities and flow rates in a gating system.
BERNOULLI’S THEOREM
• Bernoulli’s theorem states that the energy of a liquid at a given point can be separated into three parts: energy of velocity, ( v2/2g) energy of pressure, (P1/ρ) and energy of position, (h). In the ideal case ( with no energy loss ), when liquid moves from point 1 to point 2, it neither gains nor looses energy. Thus, setting the energies equal for two positions, yields the equation as below:• (V12÷2g) + (P1÷ρ) + h1 = (V22÷2g) + (P2÷ρ)+ h2 where V1 & V2 ~ metal velocity at two different points 1 & 2, in ft/sec. g ~ acceleration due to gravity. P1 & P2 ~ static pressure in the liquid at points 1 & 2 in lb/sq,in. h1 & h2 ~ height of liquid at points 1 & 2 in ft. ρ ~ density of liquid in lb/ cu.ft. • As mentioned earlier, Bernoulli’s theorem can only be employed to calculate velocity in ideal fluid system i.e.. in systems in which the fluid suffers no energy losses. In real gating systems, besides losses due to friction, energy losses occur at all entrances and exits, bends, enlargements and contractions. The exit velocity and flow rates obtained by the above equation would, therefore, be somewhat higher than those
• A pouring basin can be compared to a tank full of water with a hole in its base. If the edges of the hole is sharp, then the crosssectional area of the issuing stream decreases to a minimum value a little below the orifice. The reason for this is that a fluid cannot turn at a sharp angle. Thus, fluid, with the exception of those at the centre line of the orifice, will be traveling in a direction inc ine to t e centre ine. ey ave to travel a little further before this direction becomes parallel with the general direction of the stream , resulting in a contraction in the stream as shown in Fig-1. A standard design of the pouring basin, generally used in steel foundries, is shown in
POURING BASIN
• As the liquid metal enters the sprue from the pouring basin and travels down, it accelerates under the influence of gravity. This acceleration has two effects:• 1) The metal stream acquires a high velocity, which, theoretically, is given by the simple equation, v2= 2gH. • 2) Due to the acceleration of the freely falling stream, the cross-sectional area reduces as the velocity increases; this is because, according to the law of continuity, volume flowing past one section must be the same as at any other section. As a result of the above, the metal pulls away from the walls of the sprue with consequent turbulence and aspiration as shown in Fig-3.
SPRUE
• If the walls of the sprue are tapered sufficiently so that metal lies firmly against them, aspiration is eliminated. The following equation may be used with advantage to arrive at the taper necessary to prevent aspiration. •
A1/A2=√Z2/Z1
• Where A1~ area of the sprue entrance. A2~ area of any other location in the sprue. Z1~ level of the pouring basin above the sprue entrance. 2
basin to the location of A2.
• Although the above equation indicates that the ideal sprue should have a parabolic taper, straight sided taper has been found to suffice in practice as shown in Fig-4. • In addition to its shape, the height of the sprue also effects its filling. It has been shown that short sprue tend to fill up completely, when the sprue: runner is1:1. The precise sprue height at which incomplete filling begins, is determined by the choke area.
SPRUE BASE
• As it leaves the sprue, the molten metal travels at its highest velocity and develops its maximum energy. At the sprue base, the direction of flow abruptly change, which causes severe turbulence. Therefore, by increasing the area of sprue base, both the velocity and the turbulence of metal can be effectively reduced. In addition, as the sprue base is filled, the molten metal acts as a cushion to absorb the impact of the falling stream. In order for the sprue base to function properly, its bottom surface must be flat. This because curved bottom surface of a sprue base will not absorb the kinetic energy of the falling stream and will deflect the molten metal up the sides of the bowl, thus causing severe turbulence. • The cross-sectional area of the sprue base should be approximately 5 times that of the sprue exit, its depth being 2 times that of the runner.
RUNNER
The function of the runner is to change the direction of the flow of metal from vertical to horizontal. Since liquid cannot turn through a right angle instantaneously, a contraction results as shown in Fig-6.
• Although little is known of the optimum radii required to suppress this type of contraction, an enlarged sprue base goes long way in meeting the above problem. Also to reduce appreciably the velocity of the metal leaving the sprue or spue base, the cross-sectional area of the runner must be larger than that of the sprue exit. As mentioned earlier, short sprues tend to fill completely, the reverse is, however, true for runners. As the metal stream proceeds along the runners, it expands as its velocity falls off, and eventually, completely fills the runners. Therefore it may be said that short sprues, and long runners are an ideal combination in a running system.
• To ensure that only clean metal enters the gates, and thereby, the mold cavity, the runners should be filled before the gates. It is, therefore, best to place runners in the drag and gates in the cope.
• The molten metal that first enters the running system is usually contaminated due to turbulence, aspiration and eroded sand. Runner bar extensions are, therefore, used with advantage to prevent this metal from entering the mold cavity. The runner extension must, however, be extended far enough beyond the last gate to prevent the backwash of unclean metal from entering the gate.
GATES
• Similar conditions of flow exist at the junction of each gate and runner bar as the junction of sprue and runner. The resulting contraction that takes place in the former is shown in Fig:-8.
• It can be seen that the contraction at the leading , considerably pronounced.
• Suffice it to say that unless the degree of contractions at various junctions, as enumerated above, are known, or suppressed altogether, it is not possible to talk with any precision about the cross-
• Research has shown that since, in the case of multiple gating, the tendency of the stream of molten metal is to flow the path of least resistance, a large portion of metal will flow through the last gate attached to the runner.
•
’ , , stream of metal, tends to continue moving in the same direction until some outside force is exerted to change it. The reduction of the crosssectional area of the runner just beyond the first gate, acts as that force. It restricts the flow of metal to a certain extent and builds up a slight back pressure, thereby making the stream of metal turn and flow through the first gate. The amount by which the cross-sectional area must be reduced at each gate is dictated by the gating ration being used.
1. When gating ratio is 1:1:1, decrease area of runner by the area of gate. 2. When gating ratio is 1:2:1.5, decrease area of the runner in proportion to the number of gates passed.
GATING SYSTEM
• Theoretically, the best way to fill a mould with liquid metal is to pour the metal straight through the riser. This will create the ideal conditions for directional solidification of castings. However, the method is not applicable , reasons. Hence, the need for a gating system. Some of the gates commonly used in steel foundry are described below:-
TOP GATE
• Top gates are usually limited to relatively small castings of simple design. The turbulence of metal as it enters the mould cavity causes erosion, which is a ma or roblem in the manufacture of steel castin s. As such to ates are used in steel foundries only for broad shapes of low heights.
BOTTOM GATE
• Bottom gating reduces the turbulence and erosion of the mould to a spots results at the gate entrance, cold metal appears in the riser.
Foundry men have devised various means of to find a compromise between these basic forms of gating. It my be stated that bottom gating is most desirable where risers or atmospheric risers are used to feed sections deep in the mould.
HORN GATE
• This gate, so called because of its shape, is a variety of bottom gating. The main objection to its use is that the metal enters mould in a fountain like jet, causing turbulence, aspiration of air etc. Horn gate is probably the greatest sin le cause of as cavities resultin from tra ed air and is not recommended for gating steel castings. Experiments have shown that the above fountain effect can be considerably reduced by enlarging the crosssectional area of the exit end of the horn gate into the mould to twice the area of its entrance from the runner.
PARTING LINE GATE
• This particular form of gating is a compromise between top and bottom gating. They are often chosen more as a molding expedient than for the intrinsic value. In this case, metal enters the mould cavity at the same level as the mould joint or parting line. Molten metal enters through the sprue and reaches the parting surface where the sprue is connected to the runner or gates in a direction horizontal to the casting. The arrangement of providing a gate at the parting line allows the use of devices that can effectively trap any slag, dirt, or sand, which passes with the metal down the sprue.
STEP GATE
This takes the advantage of bottom gating, at the same time allowing hot metal to enter directly into the riser. In some instances, it is possible to arrange a series of gates at several levels. Metal flows through the bottom ingates, until the mold is filled to the level of the next higher ingate. At this point, metal is expected to start flowing through this ingate and through successively higher ones, as the mould gates filled. However in ractice ste ates do not function in this ideal manner. The inertia of the metal falling through the sprue and the resulting low pressure areas created at the entrance of the top gates, as shown in Fig-10, carries the metal past the higher ingates and nearly all of it flows through the bottom ingates only. Through experimentation, it has been observed that by slanting the ingates upward at an angle to the casting, and designing the gates for relatively increasing resistance to flow at lower levels, step gates can be made to function properly.
WHIRL GATE
• A whirl gate is the most positive device for preventing dirt from entering the mould cavity. Although steel foundry men have used them sporadically for many years, great interest in their use has been taken only very recently as the demand for more cleaner steel castings increased. Cast irons can be effectively filtered by using variety of Filters, But for steel, development successful filters is still awaited. The following parameters have been recommended for whirl gates used for steel castings: 1) Ratio of ingate to outgate cross-sectional area should be 1.5:1. and the height about 1.5 the ingate height.
,
3) Whirl gate performance is improved increasing the angular displacement (recommended orientation: 1800 apart.)
DESIGN OF GATING SYSTEM
• There are two major steps in designing a gating system:
i) Calculation of the ingate area.
ii) Derivation of the size of other components, such as runner, sprue etc.
GATING RATIO
Gating ratios recommended by various theoreticians in the literature vary over a wide range. For steel castings, a mildly pressurized gating system is generally used. This has the following advantages: i) The gating system is kept full of metal. The back pressure due to the restriction of at the gates tends to minimize the danger of the metal pulling away from the mold walls, causing the consequent aspiration, turbulence and sand erosion. ii) In case of multiple gating system, the flow from the gates of equal area is uniform. Since the kinetic effect of the metal stream is dampened by the back pressure created. A non-pressurized gating system, wherein the area of runners and gates is larger than that of the sprue i.e.. 1:2:2 or 1:4:4, offers a rapid filling, the low velocity metal stream resulting in materially reduced mold erosion. Such systems, however, favor oxidation of metal and may be partially responsible for the formation of ceroxide defect. Also metal flow is nonuniform, when the gate area equals the runner area. A slight change in in the nonpressurized system of 1:2:2 to the gating ration of 1:2:1.5 will produce steel castings nearly free from sand erosion, will minimize oxidation in the gating system and will produce uniform flow. It is reported that general application of this ratio reduced the percentage of steel castings requiring welding from about 10 to 2%.
GATING CALCULATION
• A number of methods for calculating gating systems are available in technical literatures today. The method consist of calculating the optimum pour ng me o e cas ng, w c s cross c ec e w m n mum ra e o rise of metal in the mould. The next step is to determine the total ingate area, from which the size of the individual gate, runner and sprue are derived, depending upon the gating ratio being used.
• For determination of pouring time, the following empirical formula can be used:t = S 3√VG where t is pouring time in seconds, S is time-coefficient for steel castings(Table-1) V is mean section thickness of casting in millimeter. G is weight of casting and risers in Kg. TABLE-1
Pouring temperature & fluidity
Bottom gating
Side gating
Top gating
Normal
1.3
1.4
1.5 to 1.6
Increased
1.4 to 1.5
1.5 to 1.6
1.6 to 1.8
It has been reported that the following values for coefficient ‘S’ have found to be suitable in actual production condition of steel castings over a considerable long period. For castings weighing from 10 to 50 MT :- 1.8 to 2.8 For castings weighing from 1.0 to 10.0 MT :- 1.2 TO 2.0
• In addition to the determination of pouring time of the casting, due consideration must be given to the rate of rise of metal in the mould. As it is well known, besides casting miss-run, cold shut etc, too slow a rate of rise of metal in the mould tend to give rise to scabbing defects on the cope surface. Table-2 gives the minimum rate recommended for the rise in the mould for steel castings. Table-2
Below 4
1
6 to 10
2
10 to 40
1
Above Above 40 40
0.8 0.8
•
Having determined the optimum pouring time of the casting, the cross-sectional area of the ingate may be calculated according to the following formula:F = G ÷(0.31u√hst.t)
where F = Cross-sectional area of ingate, cm2 G = Weight of the casting and risers, Kg u = Flow coefficient t = Optimum pouring time hst = Mean ferro static pressure during pouring, cm The flow coefficient ‘u’ represent the inverse value of the resistance offered . given in Table-3. Table-3
Type of Mould
Resistance of mould High
Medium
Low
Green Sand
0.25
0.32
0.24
Dry Sand
0 30
0 38
0 0.50 50
• Table-3 represents castings made without any open risers or flow-offs in a moulding sand of average permeability, cast at normal pouring temperature. • The following factors, therefore, influences the value of coefficient ‘u’ :Factors 1. Increase in pouring temperature per 500C 2.
Open risers & FlowFlow-offs
Change in values of ‘u’ ‘u’ Up to +0.05 From + 0.05 to + 0.30 +
+
if Sprue area/Gate area>2 and Runner area/Gate area>1.5
4.
Complex multiple gating
5.
Low permeability of mould
From +0.05 to -0.10 Up to -0.05
• The mean ferro static pressure hst (Fig-12) during pouring is calculated from the equation: hst = H0 – (P2÷2C) where H0 is Height of sprue ( from top of metal level in . P is the height of the casting above the ingate level in cm. C is the total height of the casting in as cast condition in cm.
A Practical Example of Gating Calculation
• For the method drawing shown, the basic data available for gating calculation are as follows:Casting Weight:- 128 Kgs. Weight of casting including riser:-145 Kgs. Mean Casting thickness:- 75mm Height of metal level in pouring basin from ingate level:- 250mm
• Calculate Pouring time where time coefficient ‘S’ for side gating and normal pouring temperature is 1.4 (from Table-1), Casting weight with risers ‘G’ is 145Kgs and Mean section thickness ‘V’ is 75mm. t = S 3√VG = 1.4 3√145X75 = 31 seconds • Calculate Mean ferro static pressure ‘hst’ where ‘H0’ is 25cm, ‘P’ is 0 as total height of the casting is below the ingate level and ‘C’ is 7.5cm:hst = H0 – (P2÷2C) = 25 – (0÷2X7.5) = 25 – 0 = 25cm
Calculate ingate area ‘F’ where ‘G’ is 145Kgs, Flow coefficient ‘u’ is 0.6, Mean ferro static pressure ‘hst’ is 25cm and Pouring time ‘t’ is 31sconds :F = G ÷(0.31u√hst.t) = 145÷(0.31x0.6x√25x31) = 28cm2
There are two ingates for the casting and as such cross-sectional area of each 2 ingate will be 14cm i.e.. 54mm wide and 26mm thick.
• Calculate area of runner and sprue :A gating ratio i.e.. Sprue area: Runner area:Ingate area =1:2:1.5 to be used for steel castings. As such the runner area will be (ingate area÷1.5)x2 = (14÷1.5)x2 = 18.66 cm2. So the runner dimension is 54mmx36mm. In case of sprue area, since it is feeding both the ingates, total area of ingates i.e. 28cm2 to be taken into account, So the sprue area is 28÷1.5 = 18.66cm2
POINTS TO REMEMBER
1.
Contraction in the metal stream occur at the various junctions of a running system even after calculating a gating system accurately, defects in casting may appear unless steps are taken to suppress these contractions.
2.
Short, tapered sprues and long runners with a large well at sprue base, ensure the complete filling of the system with minimum turbulence, aspiration etc, thereby causing less mould erosion.
3.
Runner bar extensions, whirl gate and runners in drag & gates in cope, are effective dirt trap.
4.
Faster flow rates with low metal stream velocities ensure castings with least mould erosion.
5.
Horn gate cause more air entrapment in steel castings and, therefore, are not recommended.
6. Step gates do not function as expected. In practice, most of the metal tends to flow from the bottom gate unless means are employed to obviate the above condition. 7. Multiple gating produces less mould erosion than a single ingate system. 8. A mildly pressurizes system with a gating ratio of 1:2:1.5 has been found to give very satisfactory results in steel castings.
GATING & CASTING QUALITY
Before any of the studies on gating can be applied in production, the following fundamental precautions must be observed. It has been found that more sand inclusions in castings result following improper moulding practices, than from the failure to apply scientific gating system. 1. New facing sand must be used for forming the gates, since the latter has to withstand more erosive forces than any other portion of the mould. 2. The gates must be rammed at least as hard as the mould cavity, harder if possible. This is particularly applicable to sprue. 3. Rather than the gates cut by moulder, the gating system should form a part of the pattern equipment, wherever possible, as the former practice give rise to easily eroded sand surface.
4. Various portions of the gating system must be fully matched, for if they are not, the projections coming in the path of the stream are continually washed away into the mold cavity. 5. Most of all, the gating system must be free from loose sand prior to the entry of the molten metal. The practice of aspirating dirt with compressed air after mold assembly and placing of coverings over risers and sprue openings are excellent quality control operation. These precautions may seem too elementary to discuss and should be taken for granted. Perhaps they are taken too much for granted.
• Having discussed the common practical safeguards to be taken during preparation of mould, some of the defects commonly found in steel castings, which can be minimized by the application of proper gating practice, may be now discussed.
MOULD EROSION
Research work done on the flow of liquid steel, by taking actual motion picture, has shown that the steel flows discontinuously over a flat surface. The stream emerging from a gate generally moves with a sidewise, whip like motion. Consequently the sand on which the stream edges run is alternatively covered and uncovered by metal, thereby burning out the room temperature bonds. The temperature at this stage on these edges is not yet high enough to fuse the sand grains or bentonite with a high temperature bond. The next wave of metal, therefore encounters sand that is not bonded, which is then easily eroded by the stream and may be lodged in the cope or other surfaces of the mold. This means that gating vertical member, pouring the mould up-hill, to allow the metal to move as a body over flat surfaces, should be resorted to for obviating mold erosion on account of the above . A summery of what had been said earlier would show that, in order to minimize the mould erosion, the best gating system would be a double ingate with a central sprue, a rapid filling, low velocity system of properly proportioned runners and gates and short sprues with an enlarged well base. Besides, dirt trap in the form of whirl gates, runner bar extensions and provision of runners on the drag and ingate in the cope are effectively used by most steel foundries to ensure that, as far as possible, only clean metal enters into the mold cavity.
POROSITY
Normally, porosity or gas cavities in steel castings are not associated with gating practice. However, certain factors pertaining to gating system are sometimes responsible for isolated gas cavities. As mentioned earlier, horn gate is probably the greatest single cause for entrapped air in steel castings. Its use is, therefore, to be strongly discouraged. A changed to taper sprue and deeper pouring basin also goes a long way to minimize aspiration.
CEROXIDE
Ample opportunities for oxidation of molten steel exist in an improperly designed gating system, before it enters the mold cavity. The oxidation is caused by aspiration of air into the molten stream in the gating system and by an oxidizing atmosphere in the mould cavity. The resulting corrosive constituents, according to one school of thought, reacts with the moulding material, particularly eroded sand from the gate, to form a viscous material called ‘Ceroxide’ which is lodged usually at the cope surfaces of steel castings. Poor gating system apparently add to the amount of ceroxide in at least two ways:- turbulent flow produces excessive aspiration, and increased turbulence causes more mould erosion – both of which are contributive towards the production of more ceroxide of higher viscosity resulting in deeper and more pronounced defects in the castings.
COLD-SHUT & MISRUN
Too slow a rate of flow, as well as rate of rise of steel in the mould, results in misrun castings with wrinkles and cold-shut surfaces. Under the above conditions, temporary solidification takes place and further flow of metal is not sufficient to erase the cold-shuts by re-melting these surfaces.
Increasing the gate area is not a panacea of every misrun problem. A multiple gating system so designed, that each gate receives supply of metal uniformly, reduces the casting area served by each gate, thereby offering a effective solution.
SHRINKAGE CAVITIES & HOT TEARS
Both these defects can be caused by the existence of local hot spots, resulting from gating system. Gating practice may have marked effect on the temperature gradients in the casting. Ingates are potential hot spots, in the mould area adjacent to the ingate absorbs heat and become as hot as the metal itself, thus delaying solidification of the casting at this area. This may be very severe where a single ingate, and a slow pouring rate, is employed.
Therefore to avoid shrinkage cavities and hot tears, multiple gating should be used so as to provide a flatter temperature gradient in the casting. Two rules of thumb employed by steel foundries to minimize the above defects are: to keep the cross-sectional area of the ingate smaller than that of the casting and to cut ‘cracker ribs’ in the mold or core surface in front of the ingates.
•
A word about the inter-relationship of riser and ingate positioning in steel casting. Control directional solidification along the casting towards the riser should not be disturbed by improperly placed ingates, since the feeding range of suppression of end effect by the ingates.
REFERENCES 1. S. Bharadwaja, Indian Foundry Journal’1969 2. Bidulya, “Steel Foundry Practice” 3. Report of Sub-committee TS54 of the Technical council: Investigation of flow-phenomenon in various running and gating system. The British Foundrymen, May, 1965 4. Basic principles of gating, AFS.1967 5. Taylor, Fleming and Wulf, Foundry Engineering. 6. Caine: AFS Symposium on principle of gating,1951 7. Brigg: Gating steel castings, Foundry, June,1960 8. SFSA Research report No.31: The performance of whirl gate with liquid steel. December,1953
