Manufacturing Processes: Why Forged Parts Outlast Cast Ones
How everyday objects get their shape, from sand molds and drink cans to cutting tools, welds, plastic bottles and 3D printers.
Try 3 questions
1. Why is a casting pattern made slightly larger than the finished part?
- To leave room for a thick layer of paint
- To allow for shrinkage as the metal cools
- Because molten metal expands inside the mold
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Answer: To allow for shrinkage as the metal cools
Metal contracts as it solidifies and cools to room temperature, so patterns carry a shrink allowance, around 1 to 2 percent for many metals.
2. What is the job of a riser in a sand casting mold?
- To hold the core in its exact position
- To feed molten metal as the casting shrinks
- To let steam escape from the damp sand
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Answer: To feed molten metal as the casting shrinks
A riser is a reservoir that stays liquid longer than the casting, so it can fill the voids left as the casting solidifies and shrinks.
3. Investment casting, or lost-wax casting, now makes jet engine turbine blades. Roughly how old is the technique?
- About 200 years
- Over 5,000 years
- About 80 years
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Answer: Over 5,000 years
Lost-wax castings date to around the fourth millennium BCE, and pre-Columbian goldsmiths in the Americas also mastered it.
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