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-projections

-discontinuities

-defective surfaces

-incomplete

-incorrect dimensions

-inclusions

•Typical inspection methods are,

-polishers & microscopes to look at microscopic structures

-metal analyzer to determine chemical composition

-X-rays are used to examine hidden cracks and blowholes

45.5 Design of Castings

•When designing casting the most important consideration is the effects of shrinkage during cooling. Other important factors include metal flow, and porosity.

•Some general rules of thumb are,

-Avoid sharp corners - they can lead to hot tearing during cooling.

-Use fillets cautiously - they lead to stresses as they shrink a radius of 1/8” to 1” are acceptable.

-Avoid large masses - they will cool more slowly, and can lead to pores and cavities in the final part. Cores can be used to hollow out these large volumes. Metal padding ‘chills’ can also be placed inside the mold near large masses to help increase cooling rates.

-Use uniform cross sections -this will keep the cooling rate relatively uniform and avoid stresses.

-Avoid large flats - large flat areas tend to warp.

-Allow some give as the part cools - by allowing the shrinkage of one part to deform another slightly, the internal stresses will be reduced. Figures of 1-2% shrinkage are common.

-Put parting lines near corners - this will hide the flash.

-Straight Parting Lines - where possible a straight parting line will allow easier mold making.

-Use a Draft angle - A small angle of 0.5-2° on the vertical walls will make the pattern easier to remove.

-Machining Allowances - allow excess material for later machining of critical dimensions

-Wide Tolerances - because shrinkage occurs as the part cools it will be very hard to keep tight tolerances.

-Stress Relieve When Needed - Stress relief can reduce the effects of non-uniform cooling.

-Avoid thin sections - These will be very hard to fill, and will tend to harden quickly.

-Avoid internal features - These will require extra steps in mold making, and may create

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metal flow problems.

45.6 REFERENECES

Lewis, R., His previous course notes for MEC015 have basically been adapted to what is shown here.

Kalpakjian,

Krar,

45.7 PRACTICE PROBLEMS

1.TRUE / FALSE - Investment casting is well suited to producing many parts at once.

2.The part below will be referred to in a number of questions. The drawings are not to scale but they do show an axisymmetric part (i.e., round) with a hollow internal core. The drawing is not to scale, but the rough dimensions are given. You are free to make assumptions (they must be stated) where necessary.

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a)Describe in details the steps required to make this part using sand casting.

b)List the steps in detailed to make this part with investment casting.

c)List other casting processes that could be used to make this part. Provide your opinion of relative ranking (e.g., 1 to 5) with a general reason for each.

d)List other casting processes that should not be used to cast this part. Provide a reason why not.

e)If the part is to be made with injection molding, what special considerations would be required?

f)List appropriate techniques for making this part using thermoplastics. Give a relative ranking (e.g., 1 to 5) with reasons.

g)List types of composite manufacturing techniques that are, and are not suitable for making this part. Give reasons why.

3. a) Design a sand casting mold for the jar shaped part below. Include risers, gates, runners, etc., Indicate the parting line between the cope and drag. It will be filled from the side as drawn.

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b) List and explain why 2 features of this part would be hard to cast.

4.Suggest parts that are best suited to produce with the following casting techniques. You must briefly state why is best suited to the method.

a) Centrifugal lost wax investment casting b) floor/pit casting

c) die casting

d) true centrifugal casting

5.Describe the procedures that would be involved in making a bronze statue. Which casting processes would be suitable? Why?

6.Why are castings normally cooled slowly?

7.How does the microstructure of a casting relate to the cooling rate?

8.What factors will result in a cast part not matching the pre casting mold shape?

9.Suggest two casting methods that would be suitable for making small toy cars? Indicate which would be better and why.

10.Suggest two casting processes would be well suited to making a large casting of a 4 foot tall ornament? Indicate the benefits and limitations of each.

11.How can hot tearing be avoided in castings?

12.Why should risers be located near large masses in cast parts?

13.How can chills help deal with large masses in a mold?

14.List and describe 8 different casting applications.

15.Identify design features that will cause problems when casting.

16.What are the major advantages and disadvantages of casting over other manufacturing pro-

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cesses.

17.Why is a complex runner/gate system used in sand casting. Why is it important to pour slowly and continuously?

18.Why is it important to allow gases to escape? Are there any processes where this would be more important? Which processes eliminate this problem.

19.Why is moisture such a significant problem in casting?

20.