Page 329 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 12 Metal Casting: Design, Materials, and Economics

TABLE 2.6
I
General Cust Characteristics of Casting Processes

Cost* Production rate
Casting process Die Equipment Labor (pieces/hr)
Sand L L L-M < 20
Shell mold L-M M-H L-M < 5 0
Plaster L-M M M-H < 1 O
Investment M-H L-M H < 1 OOO
Permanent mold M M L-M <60
Die H H L-M <2O0
Centrifugal M H L-M < 5 O
*L I low; M I medium; H = high.

expensive materials and a great deal of preparation. There are also major costs in-
volved in making patterns for casting, although (as stated in Section 11.2.1) much
progress is being made in utilizing rapid prototyping techniques to reduce costs and
time.
Costs also are involved in melting and pouring the molten metal into molds
and in heat treating, cleaning, and inspecting the castings. Heat treating is an impor-
tant part of the production of many alloy groups (especially ferrous castings) and
may be necessary to produce improved mechanical properties. However, heat treat-
ing also introduces another set of production problems (such as scale formation on
casting surfaces and Warpage of the part) that can be a significant aspect of produc-
tion costs. The labor and skills required for these operations can vary considerably,
depending on the particular process and level of automation in the foundry.
Investment casting, for example, requires much labor because of the many steps in-
volved in the operation, although some automation is possible, such as in the use of
robots (Fig. 11.16c), whereas operations such as a highly automated die-casting
process can maintain high production rates with little labor required.
Note that the equipment cost per casting will decrease as the number of parts
cast increases. Sustained high production rates, therefore, can justify the high cost of
dies and machinery. However, if demand is relatively small, the cost per casting in-
creases rapidly, and it then becomes more economical to manufacture the parts by
sand casting or other casting processes described in this chapter or by other manu-
facturing processes described in detail in Parts III and IV.

SUMMARY

° General guidelines have been established to aid in the production of castings that
are free from defects and that meet dimensional tolerances, service requirements,
and various specifications and standards. These guidelines concern the shape of
the casting and various techniques to minimize hot spots that could lead to
shrinkage cavities. Because of the large number of variables involved, close
control of all parameters is essential, particularly those related to the nature of
liquid-metal flow into the molds and dies and the rate of cooling in different
regions of the mold.
° Numerous nonferrous and ferrous casting alloys are available With a wide range
of properties, casting characteristics, and applications. Because many castings are
designed and produced to be assembled with other mechanical components and