Page 312 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Qualitative Problems 29|

REVIEW QUESTIONS

I I.I. Describe the differences between expendable and per- I I.7. What are composite molds? Why are they used?
manent molds. I I.8. Describe the features of plaster-mold casting.
II.2. Name the important factors in selecting sand for II.9. Name the type of materials typically used for
molds. permanent-mold casting processes.
II.3. What are the major types of sand molds? What are II.I0. What are the advantages of pressure casting over
their characteristics? other processes?
II.4. List important considerations when selecting pattern I I.I I. List the advantages and limitations of die casting.
materials. I I.I2. What is the purpose of a riser? A vent?
I I.5. What is the function of a core? I I.I3. What is squeeze casting? What are its advantages?
II.6. What is the difference between sand-mold and shell- I I.I4. What are the advantages of the lost-foam casting
mold casting? process?

QUALITATIVE PROBLEMS

I I.I5. What are the reasons for the large variety of casting I I.28. You have seen that, even though die casting produces
processes that have been developed over the years? Explain thin parts, there is a limit to how thin they can be. Why can’t
with specific examples. even thinner parts be made by this process?
I l.I6. Why are risers not as useful in die casting as they are I I.29. How are hollow parts with various cavities made by
in sand casting? die casting? Are cores used? If so, how? Explain.
I I. 7. Describe the drawbacks to having a riser that is (a) too I I.30. It was stated that the strength-to-weight ratio of die-
I
large and (b) too small. cast parts increases with decreasing wall thickness. Explain
I I.I8. Why can blind risers be smaller than open-top risers? why.
II.3I. How are risers and sprues placed in sand molds?
I I.l 9. Why does die casting produce the smallest cast parts?
Explain, with appropriate sketches.
I|.20. Why is the investment-casting process capable of
I I.32. In shell-mold casting, the curing process is critical to
producing fine surface detail on castings?
the quality of the finished mold. In this stage of the process,
II.2I. What differences, if any, would you expect in the
the shell-mold assembly and cores are placed in an oven for a
properties of castings made by permanent-mold versus sand- short period of time to complete the curing of the resin
casting processes? binder. List probable causes of unevenly cured cores or of
I I.22. Would you recommend prehearing the molds used in uneven core thicknesses.
permanent-mold casting? Would you remove the casting soon I I.33. Why does the die-casting machine shown in Fig. 11.20
after it has solidified? Explain your reasons. have such a large mechanism to close the dies? Explain.
I I.23. Give reasons for, and examples of, using die inserts. I I.34. Chocolate forms are available in hollow shapes.
I I.24. Referring to Fig. 11.3, do you think it is necessary to What process should be used to make these chocolates?
weigh down or clamp the two halves of the mold? Explain I |.35. What are the benefits to heating the mold in invest-
your reasons. Do you think that the kind of metal cast, such ment casting before pouring in the molten metal? Are there
as gray cast iron versus aluminum, should make a difference any drawbacks? Explain.
in the clamping force? Explain.
II.36. The “slushy” state of alloys refers to that state be-
I I.25. Explain why squeeze casting produces parts with tween the solidus and liquidus temperatures, as described in
better mechanical properties, dimensional accuracy, and sur- Section 10.22. Pure metals do not have such a slushy state.
face finish than do expendable-mold processes. Does this mean that pure metals cannot be slush cast? Explain.
I I.26. How are the individual wax patterns attached on a I I.37. Can a chaplet also act as a chill? Explain.
“tree” in investment casting? I I.38. Rank the casting processes described in this chapter
I I.27. Describe the measures that you would take to reduce in terms of their solidification rate. (That is, which processes
core shifting in sand casting. extract heat the fastest from a given volume of metal?)