Form Design of Castings and Weldments

Design for Manufacture and Assembly Form Design of Castings and Weldments

FORM DESIGN OF CASTINGS AND WELDMENTS Introduction Good casting design requires maximum coordination of end use with those of processing. Among significant processing costs are those associated with molding, gating, and raisering. This chapter deals with achieving economy in the design of casting, area of interest in cores .The object is to arrange the design shape of the castings as to keep the need for these sand cores to a minimum, or to eliminate the need for sand cores.

Redesign Of Castings Based On Parting Line Considerations: Design rules: 1. Reduce the number of parts where possible by designing one so that it performs several functions. 2. Spaced holes in machined, cast or stamped part so that they can be made in one operation with out weakening the tool. 3. Design for low labor cost operations when ever possible. 4. Dimensions should be made not from points in space but from specific surfaces or points on part itself. 5. Dimensions should all be from datum line rather than variety of points. 6. Once the functional requirements are fulfilled, lower the weight lowers the cost. 7. Use general purpose tooling rather than special tooling. 8. Avoid sharp corners. use generous fillets and radii. 9. Design apart so that as many operations are performed on it with out repositioning. 10.Design to avoid stepped parting lines in cast, molded or powder metal components. 11. Design work pieces to achieve uniform thickness, with high shrinkage rates.

Parting Line

Both expandable and permanent moulds must separable in two or more parts in order to permit with drawl of patterns. The largest section of the casting should be located adjacent to the parting plane of the mould and its size should be reduced in moving to the extremities. Design must avoid over hangs or under cuts.

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Design for Manufacture and Assembly Form Design of Castings and Weldments When parting line cannot be located at the largest dimension, either cores or loose pieces must be provided, permit the with drawl which increases cost.The location of parting line is dictated the shape of the casting .parting the casting as shown in fig1 (a) to (e) makes it possible to cast faces that are flat and parallel. The parting lines shown in fig 1(f) and (g) necessitate that the faces be tapered in order to provide draft.

SEVEN DIFFERENT PL LINES ARE POSSIBLE IN PRODUCING SIMPLE CASTING

FIG.1(a,b,c,d,e)

fig1(f) Different Parting Lines fig 1(g) Other special requirements are in fig1(a) to (f).for example the method of parting in fig 1(a) provides normal draft in hole and on the sides of casting; the hole sides are concentric. Any flash formed at the parting can be removed .Fig1 (b) provides parallel sides and normal draft in the hole and on the sides of casting. It is more difficult to maintain concentricity between the holes and the sides, because of

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Design for Manufacture and Assembly Form Design of Castings and Weldments the possible misalignment of the mold half after the pattern has been with drawn. Fig1(c),hole taper is reduced by 50%.Only a minimum of material need to be machined from hole to provide perfect straightness, a possible advantage when difficult to machine materials are involved. As in fig 1(b),misalignment of the mould halves after the pattern has been withdrawn is a potential disadvantage. Fig 1(d) is similar to1(c), except that taper remains same in the hole and is reducedby50%on outer walls. In fig .1(e),taper is reduced by 50% both in the hole and outer walls. Metal requirements in casting and machining are minimized to a greater degree than in any one of the other designs. Fig.1(f) require taper on two sides, and fig 1(g) provides sides that are parallel. These seven examples illustrate the adaptability of parting line location.

fig 2(b)

fig 2(a)

For a bell-shaped casting of fig 2. Locating the parting line at the base of the bell, as in fig 2(a), would eliminate the parting line reflection from the body of the casting. However because the core cannot be vented at the top, tapped gas may cause defects in casting metal. Placing the casting on its side so that parting line is at right angles to the base, as in fig.2(b) ,would permit the adequate venting of the core provide an improved means of gating, and eliminate the need for second riser a parting line seam is unavoidable, but it can easily removed.

Determination of parting line of moulds: 1>The parting line should be flat as far as possible and should be minimum in number and should facilitate, ramming, assembly and reliability of core arrangement. 2>mould should have minimum number of cores. 3>pattern should have minimum number of moving parts to avoid dimensional errors.

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Location of radii:

The casting seen in fig.3 shows how a minor design concession serves to avoid possible mismatch and simplifies the removal parting line.Fig3(a) was modified to eliminate radii and thus enabling parting line to located at top surface of the casting as in fig3(b). A similar concession applied to coring is shown in Fig.4 Here the possibility of core shift may be a problem, but it can be avoided by eliminating the radius at the end of the core. If such a radius is required, it can be provided easily by machining.

FIG(3) A RADIUS WHERE THE FLAT FACE JOINS THE EDGES OF THE CASTING WOULD REQUIRE PL AS (A). SEAMS ARE MISMATCHED MAY RESULT BY ELIMINATING THE RADIUS, THE PL AS IN (B)

FIGURE 4THE RADIUS OF THE JUNCTION OF THE CORED HOLE AND SAND CASTING FACE REQUIRES A SHAPED CORE, AS IN (A) MISMATCH COULD RESULT. ELIMINATION OF RADIUS AS IN (B) SIMPLIFIES THE CORE AND REMOVES THE POSIIBILITYOF MISMATCH

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Bosses and Undercuts:

It is necessary to locate a boss some distance from the parting line. The section shown in Fig. 4.1(a) illustrate the positioning a boss well below the flange whose upper surface serves as a parting line. In this design a core is required to permit removal of the pattern from the mould. In introducing a casting as shown, accurate positioning of the core is difficult, and any shifting of core results in surface irregularities. A less complicated design, shown in Fig4.1 (b) extends the boss to the flange, eliminating the undercut and the need for core.

fig4.1(a)

fig4.1(b)

an undercut created by an isolated boss on side of a sand casting requires a core as in (a) or continuation to flange in as in (b)

Design Considerations: Here the datum’s are being checked on the prototype casting. With the use of the CAD systems the accuracy of the final casting can be to a fine tolerance. If economy and best results are to be obtained, it is very important that the designer of castings give careful attention to several requirements of the process and, if possible, cooperate closely with foundry. Frequently, minor and readily permissible changes in design will greatly facilitate and simplify the casting of a component and will reduce the percentage of defects. One of the first features that must be considered by a designer is the location of the parting plane, an important part of all processes that use segmented or separable moulds. The location of the parting plane can affect each of the following:1. The number of cores, 2. The use of effective and economical gating, 3. The weight of the final casting, 4. The method of supporting the cores, 5. The final dimensional accuracy, and

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Design for Manufacture and Assembly Form Design of Castings and Weldments 6. The ease of molding. In general, it is desirable to minimize the use of cores. Often, a change in the location of the parting plan can assist in this objective, as illustrated in Figure 5. Note that the change also reduces the weight of the casting by eliminating the need for draft. Figure 6 shows another example of how a simple design change eliminated the need for a core. The location of the parting plane can also be dictated by certain design features. Figure 7 shows how the specification of round edges on a part can restrict the location of the parting plane. The specification of draft can also fix the parting plane, as indicated in Figure 8. This figure also shows that considerable freedom can be provided by simply noting the need to provide for a draft or simply letting it be an option of the foundry. Since mould closure may not always be consistent, consideration should also be given to the fact that dimensions across the parting plane are subject to more variation than those that lie within a given segment of the mould.

Figure 5 Elimination of a core by changing the location of the parting plane Controlling the solidification process is of prime importance in obtaining quality castings, and this control is also related to design. Those portions of a casting that have a high ratio of surface area to volume will experience more rapid cooling and will be stronger and harder than the other regions. Heavier sections will cool more slowly and, unless special precautions are observed, may contain shrinkage cavities and porosity or may have large grain-size structures.

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Design for Manufacture and Assembly Form Design of Castings and Weldments Ideally, a casting should have uniform thickness in all directions. In most cases, however, this is not possible. When the section thickness must change, it is best if these changes are gradual, as indicated in the various sections of Figure 9.

Figure 6

Elimination Of A Dry-Sand Core By A Change In Part Design

FIGURE 7 Effect Of Rounded Edges On The Location Of The Parting Plane

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Figure 8 (top left) Location of the parting plane specified by draft (top right) Part with draft unspecified (bottom) Various options in producing that part

Figure 9 Guidelines for section changes in castings When sections of castings intersect, two problems can arise. The first is the possibility of stress concentrators. This problem can be minimized by providing generous fillets (inside radii) at all interior corners. Excessive fillets, however, can cause the second problem, known as hot spots. Figure 10 shows that localized thick sections tend to exist where sections of castings intersect. These thick sections cool more slowly than the others and tend to be sites of localized, abnormal shrinkage. When the differences in section are large, as illustrated in Figure 11, the hot-spot areas are likely to result in serious defects in the form of porosity or shrinkage cavities. Defects such as voids, porosity, and cracks can be sites of subsequent failure and should be prevented if at all possible. Sometimes cored holes, as illustrated in Figure

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Design for Manufacture and Assembly Form Design of Castings and Weldments 12, can be used to prevent hot spots. Where heavy sections must exist, an adjacent riser can often be used to feed the section during shrinkage, as in Figure 13. If the riser is properly designed, the shrinkage cavity will lie totally within the riser and can be removed when the riser is cut off. Intersecting can cause shrinkage problems and should be given special consideration by the designer. Where sections intersect to form continuous ribs, contraction occurs in opposite directions as the various ribs contract. As a consequence, cracking frequently occurs during cooling. By staggering the ribs, as shown in Figure 14, there is opportunity for slight distortion to occur, thereby ensuring that high stresses are not built up. Large unsupported areas should be avoided in all types of casting, since such sections tend to warp during cooling. The warpage then disrupts the good, smooth surface appearance that is so often desired. Another appearance consideration is the location of the parting line. Some small amount of fin, or flash, is often present at this location. When the flash is removed, or if it is considered small enough to leave in place, a region of surface imperfection will be present. If this is in the middle of a flat surface, it will be clearly visible. However, if the parting line is placed to coincide with a corner, the “defect” line will go largely unnoticed.

Figure 10: "Hot spot”at section r2 caused by intersecting sections

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Figure 11 Hot spot resulting from intersecting sections of various thickness

Figure 12: Method of eliminating unsound metal at the centre of heavy sections in castings by using cored holes

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Figure 13 Use of a riser to keep the shrinkage cavity out of a casting

Figure 14 Method of using staggered ribs to prevent cracking during cooling

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Minimizing core requirements: Cores are separate shapes, of sand, metal or plaster that are placed in the mold to provide castings with contours, cavities and passages not otherwise practical or physically obtainable by the mold. Cores increase cost and tolerance requirements, they enable the foundry man to cast intricate internal shapes not producible by any other process. In some situations core cause severe problems during the pouring time. Some times due to high temperature of the pouring metal the binder in the core may breakdown, or sometimes the cantilevered cores may breakdown due heavy weight of the molten metal, a larger tolerance is needed on dimensions at the unsupported end of the core, because of the necessity for a small amount of slide clearance between the core and the mold at the opposite end. This clearance permits a displacement of the core when the molten metal enters the mould. The displacement is amplified as the core extends into the casting, and has a pronounced influence on dimensional discrepancies.

Designing to eliminate cores: These problems led the foundry man to minimize the cores or to eliminate them completely by redesigning the casting. A decision often depends on cost analysis. An example shown in fig15. in the original design of this casting , fig.15(a) the core is required to permit molding of the hook shape. The possible redesign shown in fig.15(b) would permit easy removal of the pattern from the sand, eliminate the need for a core, and effect a saving in molding cost. Figure 16 shows a sand cast malleable iron wheel hub for which redesign eliminated a ring core and at the same time provided a stronger casting.

fig 15(a)

fig 15(b)

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original design

fig 16(a)

new design

fig 16(b)

IT IS RESULTED IN THE STRONGER , MORE ECONOMICAL PART.

As originally designed, fig16(a), the eight ribs and eight small bosses prevented this casting from being molded with the parting line parallel to the axis of the hole. Furthermore adjacent to the flange, the casting had a cross section smaller than either the flange or the extreme end of the casting. The undercut section that was thus formed prevented the pattern from being withdrawn from the mold in a direction perpendicular to the mounting flange. A ring core , as shown was necessary to produce the shape. By revising the casting as shown in fig.16(b), the need for the ring core was eliminated and the shape could be withdrawn easily from the mould. By broadening the base of the tubular section the eight ribs were also eliminated. In the original design , the small diameter of the tubular section at the junction with the flange section was unable to withstand the forces of service. Eight strengthening ribs were required , to assure satisfactory performance of the casting in application. As redesigned, the broader base of the tubular section provided sufficient strength to permit elimination of the ribs.

Coring versus drilling: It is advisable to omit cores and to remove excess metal by other means. The choice may be based on considerations of soundness, dimensional accuracy, economy, or reducibility. For example , if a casting is to have one or more round holes, these may be produced with greater accuracy or economy by subsequent boring or drilling, rather than by core. See the next pages for, some examples on design rules and minimization of core requirements:

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Design Considerations

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For large castings which are difficult to mould, which are heavy and large are made into or more castings and are joined after the castings are made. These components are called as cast-weld components. This very large core eliminated and casting simplify process.

Criteria and methods for cast weld components:

Several features of the cast-weld construction method play an important role in its selection. The most prominent feature of the cast-weld construction method is that it makes possible the production of components that are too large to cast in one piece; it makes feasible the production of a part configuration that would be difficult or even impossible to cast as a single high quality casting because of the laws of molten metal feeding; in foundries with limited casting capacity, the designer can make cast weld assemblies of castings that are within the foundry casting capabilities; finally , materials of different compositions can be assembled in a onepiece component. For example see fig 17.

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Fig.17 Cast weld component. Upper fig. of 17 shows the exaggerated view of the bottom cast-weld component . For successful cast-weld construction the methods of welding must be examined for their capability of producing the desired chemical composition, physical and mechanical properties, as well as ease and rate of welding. The weld engineer has several processes that can be considered. These include: 1. 2. 3. 4. 5.

shielded metal-arc welding(SMAW) submerged arc welding(SAW) Gas metal-arc welding(GMAW) Gas tungsten-arc welding(GTAW) Electro slag welding(ESW)

The rate of welding as related to the size of the weld to be made, determines to a great degree the economics of the process. The manual arc process is most versatile, but for large cast-weld construction it is limited in application because changing electrodes and other interruptions typically limit the weld deposit rate to an average of 0.9kg/h. the submerged arc process, on the other hand, due to its continuous operation with wire electrodes, deposits metal at much higher rates, as do gas shielded methods. Electroslag welding is accomplished at rates of approximately 14(kg/h) per electrode.

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Design for Manufacture and Assembly Form Design of Castings and Weldments When joining components of different composition, choosing the correct method is very important. Example: stainless steel joined to carbon or low alloy steel is desired in certain applications to achieve specific properties. The manual arc welding process is often used in such instances with electrodes of the 300 series. However care must be taken to have a minimum of penetration as carbide formation with mild or low alloy steels will give an excessively brittle layer, which can result in early failure. Distortion of an assembly is another major consideration in selecting a welding process. Single pass procedures, such as electroslag welding , produce less distortion than multipass processes in which each pass results in warpage. Unless the setup compensates for distortion or a constraint is applied, multipass welds will pull a weldment out of alignment. Considerable care must be used in the setup and in the sequence of welding. Stress relief heat treatments after a number of weld passes help avoid problems. Consequently, many weldments are never allowed to cool to room temperature before being stress relieved.

Cast Weld Components

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Use Of Weld Symbols: Welding cannot completed as an engineering tool for conveying information from the designer to the workman. These symbols provide the means of placing complete welding information in drawings. Olden ways of representing is confusion because of “far side” and “rear side” becomes same in designer point of view. So, in present systems the is taken as basis of reference. Any joining process the symbol contains “arrow side”, “other side” and “both sides. T these are used herein to locate weld with respect to the joint. Appropriate finish marks have been found to be necessary. However, recommendations as to what finish marks have to be used are not strictly within the province of this standard. When the American standards association adopts a system of finish symbols, it will be desirable for all concerned to use the same system. The tail – designating the welding specifications, procedure or other supplementary information. If welding operator knows the size and shape of the weld, he requires less information. He know about process, identification of filler metal, whether or not peening or root chipping is required, and other pertinent data must be known. If nothing is specified at end of the tail then take depending on user requirements. If notations are not used, the tail is omitted.

Elements of welding symbols: This standard makes a distinction between the terms weld symbol and welding symbol. The weld symbol is the ideograph Used to indicate the desired type of weld. The assembled welding symbol consists of following eight elements, or such of these elements as are necessary: 1. Reference line 2. Arrow 3. Basic weld symbols 4. Dimensions and other data 5. Supplementary symbols 6. Finish symbols 7. Tail 8. Specification, process, or other references

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Basic Weld Symbols: Arc and gas weld symbols:

Resistance weld symbols:

Supplementary symbols:

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Design for Manufacture and Assembly Form Design of Castings and Weldments See next page for more symbols and information on uses of weld symbols.

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Form Design Of Injection Moulded Components: Introduction

Thermoplastic materials are synthetic organic chemical compounds, which soften or liquefy when they are heated and solidify when they are cooled. When cooled, they are relatively tough and durable and suitable for a wide variety of product applications.

The process:

These materials are formed to specific shape by injecting them when into a mould from they their final shape as they cooled and solidify. The plastics normally are received by the molder in granular form. They are placed in a hopper of an injection-molding machine; from they are fed to a heated cylinder. As they heated in a cylinder, they melt, plasticize. Atypical melting temperature is about 180C, although this varies with different materials and molding conditions. The mold, usually of steel, is clamped in the machine and water-cooled. A plunger force plasticized material from into the mould. There it cools and solidifies the mold is opened ,and the molded part with its attached runners is removed the process, with the usual exception of part removal ,is automatic It requires about 45s/cycle,more or less, with most of that time being devoted to the cooling of the material in the mould. Very high pressure on the order of 70000 k pas. or more or require during injection.

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Design recommendations: Gate and ejector pin location: The designer should consider the location of these elements since they can impair surface finish. Ejector pins can usually be located on the under side of a part if it has an outside and an under side. Gates can be located in a number of locations as illustratedFig.18.1. Center gating of round and cylindrical parts and near center gating of other large area parts or desirable for trouble free mould filling.

Suggested wall thickness:

The recommended normal and minimum wall thickness for common thermoplastics when injection molded. Generally, thinner walls are more feasible with small parts rather than with large ones. The limiting factor in wall thinness is the tendency for the plastic material in thin walls to cool and solidify before the mould is filled. The shorter the material flow, the thinner the wall can be. Wall should also be as uniform in thickness as possible. When changes in wall thickness are unavoidable, the transition should be gradual, not abrupt.

Holes:

1. Holes are feasible in injection-molded parts but are a complicating factor in mould construction. They also tend to cause flashing at the edge of the hole and to cause “knit” or “weld” lines adjacent to it. Fig.18.2 2. The minimum spacing between two holes or between a hole and sidewall should be one diameter. See fig 18.3. 3. Holes should be located three diameters or more from the edge of the part to avoid excessive stresses. See fig 18.4. 4. A through hole is preferred to a blind hole because the core pin which produces the hole can then be supported at the both ends, resulting in better dimensional location of hole and avoiding a bent or broken pin. 5. Holes in the bottom of the part are preferable to those in the side since the latter required retractable core pins. 6. Blind holes should not be more than two times diameters deep. If the diameter is 1.5mm or less, one diameter is the minimum practical depth. See fig 18.5

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Design for Manufacture and Assembly Form Design of Castings and Weldments 7. To increase the depth of a deep blind hole, use steps. This enables a stronger core pin to be employed. see fig 18.6 8. Similarly, for through holes, cutout sections in the parts can shorten the length of a small diameter pin. See fig 18.7

Ribs:

1. Reinforcing ribs should be thinner than the wall they are reinforcing to prevent sink marks in the wall. A good rule of thumb is to keep rib width to one half or wall thickness. 2. Rib should not be more than one and half wall thickness high, again to avoid sink marks. 3. Rib should be perpendicular to the parting line to permit removal of the part from the mould. 4. Rib should have a generous draft. 5. Methods for disguising sink marks. See fig 18.8

Bosses:

Bosses are protruding pads, which are used to provide mounting surfaces or reinforcement around holes. 1. They would have generous radii and fillets. 2. The rules indicated apply as well to boss. See fig 18.9 3. Bosses in the upper portion of a die can trap gas and should be avoided if possible. 4. Use a five degree taper for bosses, the same as with ribs. 5. If large boss are needed they should be hollow for uniformity of wall thickness.

Undercuts:

Under cuts are possible with injection molded thermo plastic parts, but they may require sliding cores or split moulds. External under cuts can be placed at the parting line or extended to obviate the need for core pulls. See fig 18.10

Screw threads: It is feasible, though a complicating factor, to mould screw threads in thermo plastic parts. 1. Use a core, which is rotated after the molding cycle has been complicated. This unscrews the part and unable it to be removed from the mould. 2. Put the axes of the screw at the parting line of the mould. This avoids a rotating core but necessitates a very good fit between mould halves to avoid flash across the threads. This suitable for external threads and higher cost and feasible. 3. Make the threads few, shallow, and of rounded form so that the part can be stripped from the mould with out unscrewing. A coarse thread with a somewhat rounded form is preferred for all screw threads because of ease of filling and avoidance of farther edges even if it is removed by unscrewing. See figures 18.11,12,13.

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Inserts:

Inserts are useful and practical to provide reinforcement where stress exceeds the strength of the plastics material. Although they are economical, they are not without cost and should be used only when necessary for reinforcement, anchoring, or support .see. fig. 18.14,15,16,17

Lettering and surface decoration: Lettering and other raised or depressed surface decorations and textures are easily incorporated into plastic parts. Once the lettering has been incorporated in the mould, each part will automatically show the lettering with few or no extra steps. see fig 18.18,19.

Draft:

It is highly desirable to incorporate some draft or taper in sidewalls of the injection molded parts to facilitate removal of the part from the mould. The following are recommended minimum drafts for some common materials. Polyethylane0.25degrees Polystyrene 0.5degrees Nylon 0to0.12degrees

Corners: radii and fillets: Sharp corners should be avoided except at the parting line. They inter fear with the smooth flow of material and create possibilities for turbulence with attendant surface defects fillets and radii should be as generous as possible.

Surface finish:

High gloss finishes are feasible if the mould is highly polished and if molding conditions are correct. Painting of most thermoplastics is feasible but is not recommended if the color can be molded in to the part.

Flat surfaces: Flat surfaces, although feasible are some what more prone to show irregularities than gently curved surfaces. Since later also produce more rigid parts they are preferable.

Mould parting line:

Even injunction molding shows the effect of the mould parting line, the junction of the two halves of the mould .The part and the mould should be designed go so that the parting occurs in an area where it does not adversely affect the appearance or function o the part . Parting lines should be straight the two mould halves should meet in one plane only this obviously provides more economical mould construction, but it may not be possible if the part design is irregular. If it is not possible to put the parting line at the edge of the part, cleaning parting line flash is facilitated by having a bead or other raised surface at the parting line. See fig 18.20

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Dimensional factors and tolerance recommendation: Though surprisingly tight tolerance can be held when molding thermoplastic parts, dimensions can not be held with the precession obtainable with close tolerance machined metal parts, the reasons for these are 1. Material shrinkage including, variation and unpredictability in the shrinkage. 2. Plastics exhibits high thermal coefficient of expansion .As result if tolerances are extreme designers should specify the temperature at which the measurements should be taken. See fig 18.21,22

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Problems:

1. A cast iron bearing bracket is shown in figure 18.23 indicate preferred parting line and any necessary sand cores. Offer a design modification that will reduce or eliminate the need for sand cores. 2. Indicate the parting line for steel forked leaver casting seen in figure 18.24 and also the necessary sand cores. Maintaining as nearly as possible, the existing weight of casting , offer a design modification that will alleviate the sand core requirements. 3. For the pedestal housing shown in figure 18.25 indicate the probable parting line and any necessary sand cores, accepting that the probable parting line is the one involving the minimum sand cores. Maintaining as nearly as possible, the existing weight of casting, offer a design modification that will alleviate the sand core requirements. 4. There are two possible parting lines for v belt pulley. Figure 18.26 indicate both of this with the appropriate sand cores . Accept that v grooves are machined from a solid rim. Maintaining as nearly as possible , the existing weight of casting , offer a design modification that will alleviate the sand core requirements

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Fig 18.26

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Design for Manufacture and Assembly Form Design of Castings and Weldments References: 1. 2. 3. 4.

Casting Design Hand Book –American Society For Metals Hand Book For Product Design For Manufacturing –James G. Bralla Welding Codes , Specifications And Standards –Jeffery D. Mouser Steel Castings Hand Book – Steel Founders : Society Of America

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Form Design of Castings and Weldments

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