Page 321 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

0 Chapter 12 Metal Casting: Design, Materials, and Economics
Blind risers are good design features and maintain heat longer than open risers do.
Risers are designed according to six basic rules:
l. The riser must not solidify before the casting. This rule usually is satisfied by
avoiding the use of small risers and by using cylindrical risers with small aspect
ratios (small ratios of height to cross section). Spherical risers are the most
efficient shape, but are difficult to work with.
2. The riser volume must be large enough to provide a sufficient amount of liquid
metal to compensate for shrinkage in the casting.
3. junctions between the casting and the riser should not develop a hot spot
where shrinkage porosity can occur.
4. Risers must be placed so that the liquid metal can be delivered to locations
where it is most needed.
5. There must be sufficient pressure to drive the liquid metal into locations in the
mold where it is needed. Risers therefore are not as useful for metals with low
density (such as aluminum alloys) as they are for those with a higher density
(such as steel and cast iron).
6. The pressure head from the riser should suppress cavity formation and encour-
age complete cavity filling.

Machining Allowance. Because most expendable-mold castings require some addi-
tional finishing operations, such as machining and grinding, allowances should be
made in casting design for these operations. Machining allowances, which are in-
cluded in pattern dimensions, depend on the type of casting and increase with the size
and section thickness of castings. Allowances usually range from about 2 to 5 mm
for small castings to more than 25 mm for large castings.

l2.2.3 Design for Permanent-mold Casting
Typical design guidelines and examples for permanent-mold casting are discussed in
Example 12.1. Special considerations are involved in designing tooling for die cast-
ing. Although designs may be modified to eliminate the draft for better dimensional
accuracy, a draft angle of 0.5° or even O.25° usually is required; otherwise, galling
(localized seizure or sticking of material) may take place between the part and the
dies and cause distortion.
Die-cast parts are nearly net shaped, requiring only the removal of gates and
minor trimming to remove flashing and other minor defects. The surface finish and
dimensional accuracy of die-cast parts are very good (see Table 11.2), and in general,
they do not require a machining allowance.

EXAMPLE l2.| Illustrations of Poor and Good Casting Designs

Several examples of poor and good designs in design eliminates this problem and also may
permanent-mold and die casting are shown in simplify die and mold manufacturing.
Fig. 12.3. The significant differences in design are Large flat surfaces always present difficulties in
outlined here for each example: casting metals (as well as nonmetallic materi-
a. The lower portion of the design on the left has als), as they tend to warp and develop uneven
a thin wall with no apparent function. This surfaces. A common practice to avoid this situ-
location of the part thus may fracture if sub- ation is to break up the surface with ribs and
jected to high forces or impact. The good serrations on the reverse side of the casting.