Product Design for Sand Casting

Module-I of PDPT

1.5 PRODUCT DESIGN FOR SAND CASTING As in all manufacturing operations, certain guidelines and design principles pertaining to casting have been developed over many years. These principles were established primarily through practical experience, but new analytical methods, process modeling, and computer-aided design and manufacturing techniques are now coming into wider use, improving productivity and the quality of castings and resulting in significant cost savings. The design of the casting and the pattern equipment should be such that the cost of all other operations is reduced to the minimum. Such operations may include finishing, or the elimination of finishing where possible, in whole or in part; assembly of one part with another part; and economical servicing during the life of the casting. Accordingly, product design process should be studied under the following categories: 1. Design for economical moulding • Parting line • Bosses and undercuts • Coring • Simplified moulding 2. Design for elimination of defects • Shrinkage defects • Distortions • Hot tears • Escape of gases 3. Design for features to aid handling of castings Design for economical moulding Parting Line The parting line is the boundary where the cope, drag and the part meet. If the surface of the cope and drag are planar, then the parting line is the outline of the cross-section of the part along that plane. It is conventional that the parting line should be planar, if possible. The simplest parting line is that running through the centre line of the casting. Unnecessary complexities in the parting line increase the cost. A very small of metal will always “leak” outside the mold between the cope and the drag in any casting. This is called the “flash”. If the flash is along an external surface, it must be machined away by some finishing operation. If the parting line is along an edge or at the corners of the part, it is less visible – this is preferred.

(a)

(b) Figure 1.5.1: Parting line modification

When an irregular parting results in a deep mould pocket, it may be more economical to redesign the pattern equipment and change over to a mould with a straight parting involving the use of cores. The location of the parting line is important because it influences mould design, ease of moulding, number and shape of cores, method of support, and the gating system. Bosses These are frequently used to increase the sectional thickness of the housing in order to provide longer bolt or tap holes or to improve the strength of certain parts of the casting. This may be cast satisfactorily if the axis of the cylindrical boss is parallel to the direction in which the pattern is drawn out of the mould or if the centre line of the boss is in the parting plane. When this is not the case, the boss on the pattern must be

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Module-I of PDPT loose, and the skilled technique of moulding loose piece pattern must be employed. The figure shows the positioning of a boss well below a flange whose upper surface is chosen as a parting line. To mould this design a core is required to permit removal of the pattern from the mould. In producing such a casting, accurate positioning of the core is difficult, and any shifting of the core results in surface irregularities. A somewhat less complicated design extends the boss to the flange, eliminating the need for a core.

Figure 1.5.2 : Modification of bosses to reduce dry sand core Coring Cores are placed in the mould to provide castings with contours, cavities and passages not possible otherwise to obtain by normal moulding. The Fig1.5.3 shows an original design which required a core to form the interior of the casting. Redesigning the casting as shown in Fig 1.5.4, a green sand core can be substituted for dry sand core, thus achieving the economy.

Figure 1.5.3 : Eliminating dry sand core by modifying the draft angle When the cores cannot be avoided, the designer should strive to make them as simple as possible in the interest of economy, by using simple surfaces which are easier to produce.

Figure 1.5.4 : Simple core shapes

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Module-I of PDPT Also the cores must be capable of being properly supported so that they do not get misplaced during casting. The arrangement shown in Fig.1.5.5 is not recommended because the core is left self supporting. For an item of this kind further support must be provided as shown.

Figure 1.5.5 : Providing proper core support If several cores are needed, they are best positioned on the same parting line as shown in Fig.1.5.6.

Figure 1.5.6 : Providing support for several cores Simplified Moulding Generally a two part moulding is simpler and economical compared to a three part moulding. But when the part configuration, as shown in Fig.1.5.7, necessitates, it may be desirable to modify the moulding procedure by providing an external ring core to avoid the intermediate flask. But this is also an expensive alternative. The best choice would be to redesign by eliminating the bottom flange which completely avoids the external core and thus is economical.

Figure 1.5.7 : Redesign to reduce three flask moulding

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Module-I of PDPT Designing for Eliminating Defects Shrinkage The reasons for these defects are: • Volumetric contraction both in liquid and solid state; • Low strength at high temperature. Cracks often begin at the shrinkage cavities and work their way outward as the casting is stressed in service. It should be noted that solidification progresses fro thin to thick sections and that external angles have a greater cooling rate than internal angles, the reason being that sand around internal angle is surrounded on two sides by the heat sources.

Figure 1.5.8 : External and Internal angles at junction If temperatures were taken simultaneously at various positions on a casting of uniform width and thickness, no temperature variations would exist through out the length of the casting. If the casting had non-uniform cross sectional areas, the temperature would vary considerably depending on the variation in the sectional thicknesses. A high temperature position is called the hot spot. Unless a casting is properly fed, volumetric shrinkage often appears at the hot spots. The best way to avoid volumetric shrinkage is to design a casting that has no isolated hot spot which cannot be properly fed. If good designing is not sufficient to prevent defects, various foundry techniques such as the use of chills, feeders and cores must be resorted to. The designer should try to place and proportion members and their intersections in such a way to establish a positive temperature gradient which is lowest at points farthest away from the feed head and which gradually increases toward the feed head. This is called directional solidification. The shrinkage problem is particularly severe in junctions. The check circle method is used for detecting the concentration of the metal. As shown in Fig.1.5.9, a concentration of metal always occurs at the point where two walls of equal thickness come together. Such a concentration is reduced, if one of the walls is made thinner. That is why for steel castings, ribs are always made thinner than the wall in the ratio Rib thickness / wall thickness = 0.6 to 0.8.

Figure 1.5.9 : Avoiding concentration of metal at joints A rib which enters a wall at an angle gives a higher concentration of metal than one which enters perpendicularly.

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Module-I of PDPT

Figure 1.5.10 : Concentration of metal at angular joints The various designs for junctions and fillets are given here.

Figure 1.5.11 : Reducing concentration of metal at joints

Figure 1.5.12 : Avoiding concentration of metal

Figure 1.5.13 : Concentration of metal in housing

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Module-I of PDPT

Figure 1.5.14 : Design modification to avoid shrinkage cavities

Figure 1.5.15: Maintaining uniform cross-section to avoid hot spots and shrinkage cavities Distortion Internal stresses appear in the casting walls when shrinkage is restricted because of the resistance of the mould elements or the action of the adjacent walls. Increased internal stresses make the casting warp and may lead to the development of cracks. Shrinkage stresses develop during cooling when the metal loses its plasticity (within 500-600 0C for cast iron and 600-7000C for steel). At higher temperatures, the change in dimensions is readily compensated by plastic flow of the metal and thus shrinkage manifests itself only in the thinning of the walls. In the box shaped casting shown in the Fig 1.5.16, the internal partition cools at a slower rate than the horizontal walls because of the core sand being heated from all sides. While cooling below the temperature at which metal passes from plastic to elastic state, the partition material hardens and contracts and as a result, it undergoes tension. If the tension exceeds the strength of vertical walls, then they are likely to warp and introduce distortions in the casting. For example, as shown in Fig.1.5.17, if the casting walls have non-uniform thickness, then the thinner walls would have cooled down very quickly leaving the thicker wall still in the plastic state. When the thinner walls contract due to solid shrinkage, because of the plasticity, the thicker rib would warp.

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Module-I of PDPT

Figure 1.5.16 : Distortion in sand castings

Figure 1.5.17 : Warping due to uneven section thickness

Hot Tears Hot tears are formed in the castings because of the differential cooling rates and low strength of metal at higher temperatures. This is the extension of warping. In the Fig.1.5.18, the horizontal members being thinner cool fast and try to bring the thick vertical members closer which is resisted by the core. This resistance would cause the tearing of the metal to take place at the joint with the thicker rib. In the hand wheels or big wheels where the rims are connected to the hub through spokes, hot tears are likely as shown in the design. But by increasing the ductility of the spokes, it is possible to reduce the formation of hot tears. The ductility can be increased by making the spokes curved and having them in odd numbers. The odd number of spokes ensures that the restraining force acts in only one direction making the wheel more ductile.

Figure 1.5.18 : Hot tear formation (1)

Figure 1.5.18 : Hot tear formation (2)

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Module-I of PDPT

Figure 1.5.19 : Reducing hot tear by making curved spokes in wheels The following rules will promote directional solidification and reduce shrinkage stresses and distortion. • Casting walls should preferably be of uniform thickness. • Casting elements cooling under conditions of reduced heat removal (internal walls) should have smaller cross sections to accelerate their solidification. • Transition between casting walls of different thickness should be smooth. • Casting walls should have no abrupt changes, but be connected by smooth transitions. • Local metal accumulation and massive elements should be avoided, if possible. • Sections where casting walls join massive elements should be gradually thickened towards the latter or reinforced with ribs. Escape of Gases The internal cavities should be so designed as to permit the escape of gases evolving from the cores when the molten metal is poured in. Internal cores, which are small and long, are likely to pose difficulties in cleaning. Sand burned into core holes, and fins or veins, are very difficult to remove when they are hard to reach. Providing access holes or clean out holes, make available additional core prints for support, to vent core gases as well as to permit the core sand to be removed. For example, an unsatisfactory design is given in Fig.1.5.20. The gases accumulating in the upper part of the core form blow holes. This problem may be reduced by making small vent holes (plugged after wards if necessary) for the escape of gases. The redesigned vaulted shape of the upper portion of the casting would be the best way to ensure the escape of gases through the top core print.

Figure 1.5.20 : Escape provision for core gases

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Module-I of PDPT Another way of solving the problem is to change the position of the casting with reference to the parting plane. In Fig 1.5.21(a), the casting being in the cope, the core gases are not vented at all. But by bringing the casting into the drag as in Fig.1.5.21 (b), the core gases are properly vented.

Figure 1.5.21 : Core gases venting Features to Aid Handling If machining is required after casting process, then some provision must be made for mounting the casting in the machine tool during the finishing process. The Fig.1.22 shows a chucking extension on a casting, which will permit the entire casting to be machined on a lathe in one setting, the last operation being the cutoff.

Figure 1.5.22 : Chucking extension Similarly, castings with tapered sides are difficult to chuck in a lathe and if possible, should be provided with pads or flats as shown in Fig.1.5.23

Figure 1.5.23 : Provision of holding surface Summary Casting design for minimum stress concentration, maximum castability and casting consistency has been summarized into the following 14 rules by American Foundrymen’s Society. 1. Round external corners with radii 10 to 20 per cent of section thickness. 2. If at all possible use minimum radii equal to the small section thickness when joining sections of dissimilar size or when L and T sections are used. 3. Use L junctions with four times the section thickness if design considerations permit. 4. Use even larger radii when joining connecting members to sections with much larger section moduli. Radii of 10 or more times the section thickness are beneficial.

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Module-I of PDPT 5.

If small fillet radii must be used in simple T and L junctions because of design considerations, consider increasing the diameter, coring the sections and decreasing the section modulus of the connecting members for better dispersion of stress. 6. If small fillet radii must be used and design cannot be changed at junctions of sections stressed in fatigue, consider strengthening these areas by imposition of surface compressive stress by rolling, coining, shot peening or selective hardening. 7. Simplify complex junctions like X, V, Y and X-T junctions to T, and if possible to L junctions. 8. Eliminate ribs if at all possible, especially those stressed in tension. 9. Consider the use of corrugated sections to replace ribs, T and X-T junctions. 10. Consider the use of sections other than standard I, H, Z and channel sections for more efficient load carrying ability and improved castability. 11. Consider unbalanced sections in grey iron design. 12. Take advantage of flexibility of the casting process to use tapered sections confirming to the stress pattern, particularly bending. 13. Many a times a slight change in design will eliminate cores or result in a simple casting. Such changes will result in substantial decrease in cost. 14. Improved casting consistency and decreased cost can many times be attained by matching the design to the heat flow pattern. The basic relationship is that the thickness of connecting members of multi-walled configurations should be equal or greater than the strength. Reference: 1. Manufacturing Technology by P.N.Rao, TMH, page 200-211 2. Manufacturing Engineering & Technology by Serope Kalpakjian, Pearson Education

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