Design Considerations of Casting | Machine Design

Complex parts, which are otherwise difficult to machine, are made by the casting process using a sand mold.
Casting is an engineering manufacturing process which is used for mass production purpose. In casting, liquid materials or materials in the molten state are poured into a mold where it gets solidified. Mold contains a hollow cavity of the desired shape of the parts to be cast.
Almost any metal can be melted and cast. Most of the sand cast parts are made of cast iron, aluminum alloys, and brass. The size of the sand casting can be as small as 10 grams to as large as 200 x 103 kg. Sand castings have irregular and grainy surface and machining is required if the part is moving with respect to some other part or structure. Cast components are stable, rigid and strong compared to machined or forged parts. A typical example of cast components is machine tool bed and structure, cylinder block of the internal combustion engine, pump and gearbox housing.
Poor shaping of a cast iron component can adversely affect its strength more than the composition of the material. Before designing casting, the designer should consult the foundryman and the pattern maker, whose cooperation is essential for a successful design.

Types of Casting Processes

Different types of casting processes widely used in industries are:
  • Sand Casting
  • Die Casting
  • Investment Casting
  • Centrifugal Casting
For successful casting, proper control of a large number of variables such as properties of metals or alloys casts, casting methods and the various process parameters is required.

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The general principles for the design of casting are as follows:

1. Always keep the stressed areas of the part in compression.

Cast iron has more compressive strength than its tensile strength. The balanced sections with equal areas in tension and compression are not suitable for cast iron components. The casting should be placed in such a way that they are subjected to compressive rather than tensile stresses as shown in the below figure 1.
Always keep the stressed areas of the casting part in compression
Fig. 1 Always keep the stressed areas in compression.

When tensile stresses are unavoidable, a clamping device such as a tie rod or a bearing cap as shown in figure 2 should be considered. The clamping device relieves the cast iron components from tensile stresses.
Clamping to relieve cast iron part from tensile stresses
Fig. 2 Clamping to relieve cast iron part from tensile stresses.

2. Round all external corners

Rounding all external corners has two advantages - it increases the endurance limit of the component and reduces the formation of brittle chilled edges. When the metal in the corner cools faster than the metal adjacent to the corner, brittle chilled edges are formed due to iron carbide.

Appropriately filled radius reduces the concentration. The values if the corner radii for different section thickness are given in the table below.
Wall Thickness
(mm)
Inside corner radius (minimum)
(mm)
0-30
10
30-50
15
50-80
20
80-120
30

3. Wherever possible section thickness throughout should be held as uniform as compatible with overall design considerations.

Abrupt changes in the cross-section result in a high stress concentration. If the thickness is to be varied at all, the change should be gradual as shown in figure 3.
Design considerations of casting process
Fig. 3 Keep section thickness uniform

4. Avoid concentration of metal at the junctions.

At the junction, as shown in figure 4 (a), there is a concentration of metal. 
Fig. 4 Concentration of metal at junctions.

Even after the metal on the surface solidifies, the central portion still remains in the molten stage, with the result that a shrinkage cavity or blowhole may appear at the center. There are two ways to avoid the concentration of metal. One is to provide cored opening the webs and ribs, as shown in figure 4 (b). 
Alternatively, one can stagger the rubs and webs, as shown in figure 4 (c).

5. Avoid very thin sections.

In general, if the thickness of a cast iron component is calculated from strength considerations, it is often too small. In such cases, the thickness should be increased to certain practical proportions. The minimum section thickness depends upon the process of casting, such as sand casting, permanent mold casting, or die casting. The minimum thickness of the grey cast iron component is about 7 mm for parts up to 500 mm long, which gradually increases to 20 mm for large and heavy castings.

6. Shot blast the parts wherever possible.

The shot blasting process improves the endurance limit of the component, particularly in the case of thin sections.

Design Considerations of Casting | Machine Design Design Considerations of Casting | Machine Design Reviewed by Admin on 17:06 Rating: 5

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