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Jumping in to add that the manufacturer makes adjustments for shrinkage based on material properties, thermal expansion effects, and moisture sensitivity. Shrinkage also depends on the geometric shape of a particular part: thin-walled sections usually shrink more than thick ones, and the presence of undercuts leads to non-uniform shrinkage. Furthermore, resin compositions can affect how much a part shrinks; hence, it is important to consult with your manufacturer regarding material choices. Given these factors, and by delineating explicit tolerances that have been set, one ensures that part dimensions are maintained according to specifications. Always reconfirm how shrinkage is handled with your manufacturer beforehand.

[John O’Sullivan]

Hi,  I am concerned about shrinkage in polyurethane vacuum-cast parts during the curing process. These parts have tight tolerances, such as ±0.2 mm, and some features are designed carefully to fit together, such as snap joints or press-fit inserts. Should I make adjustments to my CAD model sizes to account for shrinkage, or is this something that the mold manufacturer automatically takes care of?  

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[Dimitris]

Shrinkage occurs frequently in vacuum casting, but that is not your problem as a designer—rather, it’s something for manufacturers to deal with. Your CAD model should be submitted at nominal dimensions when you send it to the manufacturers. They will then adjust the mold-making process to compensate for any shrinkage that may occur during the curing cycle of the urethane resin. This adjustment is typically built into the mold itself, so your CAD model needs no changes whatsoever. You should only need to indicate clearly defined tolerances within your model. If you’re concerned, you could always ask the manufacturer how they deal with shrinkage compensation in their system before finalizing anything.

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[David Y]

Jumping in to add that the manufacturer makes adjustments for shrinkage based on material properties, thermal expansion effects, and moisture sensitivity. Shrinkage also depends on the geometric shape of a particular part: thin-walled sections usually shrink more than thick ones, and the presence of undercuts leads to non-uniform shrinkage. Furthermore, resin compositions can affect how much a part shrinks; hence, it is important to consult with your manufacturer regarding material choices. Given these factors, and by delineating explicit tolerances that have been set, one ensures that part dimensions are maintained according to specifications. Always reconfirm how shrinkage is handled with your manufacturer beforehand.

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