Page 316 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 12.2 Design Considerations in Casting 2

modeling, and computer-aided design and manufacturing techniques have all come
into wide use as well, thus improving productivity and the quality of castings and
resulting in significant cost savings.
All casting operations share some characteristics, such as phase change and
thermal shrinkage during the casting cycle. Consequently, a number of design consid-
erations apply equally to, for example, sand casting and die casting. However, each
process will have its own particular design considerations: Sand casting will require
the consideration of mold erosion and associated sand inclusions in the casting,
whereas die casting will not have this concern (although it has its own problems, such
as heat checking of dies which reduces die life).
Troubleshooting the causes of defects is often complicated, and the considera-
tions presented in this chapter are by no means an exhaustive list. Also, defects
frequently are random and difficult to reproduce, further complicating the implemen-
tation of corrective measures. In most cases, a given mold design will produce mostly
good parts and some defective ones. It is very difficult for a mold to produce no defec-
tive parts. For these reasons, quality control procedures must be implemented for
critical applications of castings (see Chapter 36).

l2.2.l General Design Considerations for Castings
There are two types of design issues in casting: (a) geometric features, tolerances,
etc., that should be incorporated into the part and (b) mold features that are needed
to produce the desired casting. Robust design of castings usually involves the follow-
ing steps:
I. Design the part so that the shape is cast easily. A number of important design
considerations are given in this chapter to assist in such efforts.
2. Select a casting process and a material suitable for the part, size, required
production volume, mechanical properties, and so on. Often, steps 1 and 2 in
this list have to be specified simultaneously and can be a demanding design
challenge.
3. Locate the parting line of the mold in the part.
4. Locate and design the gates to allow uniform feeding of the mold cavity with
molten metal.
5. Select an appropriate runner geometry for the system.
6. Locate mold features, such as sprue, screens, and risers, as appropriate.
7. Make sure proper controls and good practices are in place.
We will now examine these general rules regarding casting conditions and then
discuss specific rules applicable to particular casting operations.
Design of Cast Parts. The following considerations are important in designing
castings, as outlined in Fig. 12.1:

° Corners, angles, and section thickness. Sharp corners, angles, and fillets should
be avoided as much as possible, because they act as stress raisers and may cause
cracking and tearing of the metal (as well as of the dies) during solidification.
Fillet radii should be selected to reduce stress concentrations and to ensure
proper liquid-metal flow during pouring. Fillet radii usually range from 3 to
25 mm, although smaller radii may be permissible in small castings and in spe-
cific applications. However, if the fillet radii are too large, the volume of the
material in those regions also is large, and consequently, the rate of cooling is
lower.