SAND CASTING - INBRIEF NOTES

Sand casting is defined as pouring of molten metal into a sand mold (molds are generally provided with a cavity of the shape to be made) and allowing it to solidify inside the mould. Various patterns are used to create cavity in the molds wherein, pattern can be said as the replica of the final object to be made with some modifications

 Depending on production quantities, different pattern materials namely wood, aluminum, ferrous metals are used in practice. These materials are used for low, moderate and high production quantities respectively. Figure M2.1.1 shows a typical mould arrangement for a sand mold casting. 

Figure M2.1.1: Typical mould arrangement for a sand mold casting

The composition of “sand” refractory is usually a mixture of high purity silica sand, bentonite clay, organic additives, and water. The cavity is formed by packing the moulding sand around a pattern by ramming and squeezing. Holes and internal cavities in the casting are produced by placing an accurate strong component called cores. After the refractory has compacted or chemically hardened, the mould is opened at the parting line and pattern is removed. The two halves of the mould are placed together by using a pin called dowel pins. Metal is poured in to the mould cavity through a previously prepared opening called pouring cup.    

Table M2.1.1: Metal commonly used in sand casting (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) 

Typical characteristics of a sand cast part

•  Complex castings can be produced by the use of sand moulds. For example: Intricate shapes (under cuts, complex contours), both internal and external can be made in the above method which is generally difficult to machine for achieving such shape. 

• The metals those can be melted can be used for casting in this method. Table M2.1.1 shows the list of metal commonly casted in the sand molding process. 

•  Further, casting of any size and weight even as high as 200 Tons can be made in the above method. 

•  Cast components are usually stable, rigid and strong as compared to products which are produced in other manufacturing process. 
•  Generally sand mold casted products are somewhat irregular and grainy surfaces and hence machining is required to get a better surface finish product.   
•  Sand casting processes are used in cylinder blocks, machine tool beds, pistons, water supply pipes, bells etc. 

Design considerations and recommendations

The following important recommendations are need to be considered while designing the sand casted products.

• Shrinkage: As the molten metal cools and solidifies in the mould, the natural shrinkage occurs. The dimension of the casted product gets reduced as compared with the mold cavity. The amount of shrinkage depends upon the type of metal. In order to compensate the shrinkage allowance for outer dimension, the size of the pattern is made over size and for inner dimension like hole; the pattern is made under size. It has been observed that shrinkage happens towards the material side. Table M2.1.2 shows shrinkage of various metals commonly cast in sand mould.

Table M2.1.2: Shrinkage Allowance for Metals used in Sand Moulds (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)

• Parting line: The parting line is a continuous line around a part that separates two halves of the mould. Straight parting lines are more economical than the stepped parting lines as shown in the Figure M2.1.2.

Figure M2.1.2: Recommended straight parting line

•  Draft: For easy removal of pattern from the moulding sand, some degree of taper or drafts are provided. With the provision of little or no draft, there are chances that the pattern may damage the mould rather than slipping out smoothly. Various factors responsible for selecting the proper drafts are: method of moulding and drawing of the pattern, pattern material, surface smoothness and degree of precision. Table M2.1.3 summarizes the recommended draft angles for outside surface of the sand moulded casting. Often risers are provided to compensate the shrinkage. Figure M2.1.3

Figure M2.1.3: (a) Pattern withdrawal problem for no draft (b) smooth withdrawal of pattern from Mould 

Table: M2.1.3 Draft angle for outside surface for sand molded casting (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)  

• Placement of risers: Risers are generally attached to the heaviest section. Heavier sections are closer to the riser and the thinnest sections are farthest from the risers due to faster solidification in thinner section. This minimizes the chances of getting voids. (Refer Figure M2.1.4.) 

                          Figure M2.1.4: Incorrect and correct designs of castings and riser location

•   Ribs and webs: In case of heavier sections, rib intersection with the casting wall can cause hot spot shrinks. The number of intersecting ribs should be minimized to avoid hot spot shrinks. Whenever it is necessary to bring all the ribs to a single point, a cored hole would help in faster solidification, thereby avoiding hot spot shrinks. (Figure M2.1.5. to M2.1.7.)

Figure M2.1.5: Incorrect and correct casting-rib design.         

Figure M2.1.6: Reduce the number of reinforcing ribs that intersect at one point          

     Figure M2.1.7: Design alternatives to prevent hot-spot voids at rib and casting wall intersections.   

• Corners and angles: Hot spot are most common defect in corners and angles of casting design. Use rounded corners having same radius for both internal and external corner. Again too much rounding promote shrink defect in the corner. In particular, in case of T sections, larger inside radius can be used to minimize stress concentration and hot spots. Use of dished contours one on each side of the center legs are also affective. Further, intersection of two walls of the casting should be at right angles to each other if possible to minimize heat concentration. This feature is clearly shown in Figure M2.1.8 & Figure M2.1.9.  

Figure M2.1.8: Sharp corners cause uneven 

This Figure M2.1.9: Avoid sharp-corner and acute angles that cause areas of uneven cooling  

• Wall thickness: If the metal is flowing for a longer distance in the mould, then the section should be heavier. But heavier sections also cause problem with voids and porosity. Keep the wall thickness as uniform as possible (Figure M2.1.10).     

Figure M2.1.10: Keeping wall thicknesses uniform promotes sounder castings 

Table M2.1.4: Recommended wall thickness. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)  

• Section changes: Abrupt changes in the section must be avoided. The relative thickness of the adjoining section should be less than 2:1. If heavy section is unavoidable then a taper of 4:1 is advisable.(Figure M2.1.11)

figure M2.1.11: Design rules for areas where section thickness must change  

•  Interior wall and sections: These members should be 20% thinner than the outside members, since they cool more slowly. ( Refer Figure M2.1.12) 

Figure M2.1.12: Design for interior walls (20 % thinner than exterior walls) 

• Lightener holes: To reduce the weight in low stressed area, lightener holes can be added. 

• Holes and pockets: The draft on the inside of a pocket must be twice as on the surrounding outside surface. The depth of hole or pocket should not be more than 1.5 times its narrowest dimension if it is in the drag half of the mould and this depth should  be no more than the narrowest dimension if the hole or pocket is in the cope half of the mould.(Figure M2.1.13 to M2.1.14)
  
  
  
  Figure M2.1.13: Recommended hole drilling after casting (diameter less than 19 mm)  
  
  
   Figure M2.1.14: Extra material around the hole as reinforcement in a highly stressed section.

•  Bosses and pads Bosses: pads and lugs should be minimized as it creates voids and hot spots.(Figure M2.1.15)

Figure M2.1.15: Design suggestions for minimizing material thickness at bosses 

• Cores: It is recommended to avoid the use of cores as it is expensive to make and handle. Often use of cores are unavoidable and are used to make holes. In such case, the core diameter should have at least equal to the surrounding wall thickness and preferable twice the wall thickness or more. If possible, side bosses and undercuts should be avoided. In case internal cores are used, addition of venting holes are required for removing the gases that are generated while the core comes in contact with the molten metal.(Figure M2.1.16 to Figure M2.1.18) 

Figure M2.1.16: Minimize the need for cores as much as possible by eliminating undercuts. 

Figure M2.1.18: Avoid small cored hole 

 

Figure M2.1.19: Internal pockets in castings to facilitate cleaning after casting. 

• Gears, pulleys, and wheels: To minimize the stress proper balance between the section sizes of the rim, spokes and hub must be attempted. It is recommended to have odd number of spokes with curved in shape. Excessive surface variation is to be avoided.(Figure  M2.1.20 to M2.1.21)   

Figure M2.1.20: Incorrect and correct proportions of elements of pulleys and gear blanks.    

Figure M2.1.21: An odd number of curved wheel spokes to dissipate cast-in stresses.  

•  Lettering and other data: Any lettering should be parallel to the parting plane. These data need to be placed in such a way that these will not interfere with the machining. These can be either sunken or raised above the surface.  Weight reduction: Casting weight is minimized by removing the metal from low stress region and adding to high stress area by the use of simple inexpensive pattern change.(Figure M2.1.21)  Insert of different metals: It is sometime desirable in casting to incorporate a section of  different material either harder or softer than the base metal depending on the purpose and is proves to be economical.(Figure M2.1.22)    

Figure M2.1.22: A cast-iron wear-surface insert in an aluminium aircraft-brake casting.   

•  Design to facilitate machining: Sharp corners and edges are avoided by making sufficiently rounding edges and corners. 

•  Machining allowance: After casting, machining is required to achieve better surface finish. Table M2.1.5 provides the guidelines about the machining allowance.             

Table M2.1.5: Guidelines for machining allowance (Source: Design for Manufacturability    Handbook by James G Bralla, 2nd Ed)

 

•  Dimensional factors and tolerance recommendation: Different factors which influence the variation of dimension of cast pieces are: use of different methods, pattern inaccuracies and difference in mould hardness, internal stress and many more. Table M2.1.6 provides the guidelines about various tolerances.    

Table M2.1.6: Recommended tolerances are provided in under average condition. (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed)