Calculating Shrinkage for Cast Parts
- Category: Tech Tips
Before casting a urethane part, an accurate assessment of shrinkage is a key to obtaining dimensionally accurate results. Shrink in urethane systems, and other thermoset plastics, is affected by several factors:
- Casting size and wall thickness
- Shop temperature
- Peak exotherm of the polyurethane
- Mold material and temperature
As a starting point, the manufacturer-supplied shrink characteristics of the polyurethane being used should be considered. Shrinkage is measured using ASTM Test D-2566. This test typically involves use of a 9.75 in. long x 1 in. deep rectangular steel mold. The test is conducted under set conditions in which the room, casting material and mold are maintained at temperature of between 72ºF and 77ºF.
After the mass of polyurethane is mixed and poured into the mold, it is allowed to cure overnight before being demolded. The resulting test piece is measured for shrink. The polyurethane sample then cures at room temperature for seven days and is measured again to determine the amount of dimensional change from initial demolding until full cure.
While the ASTM D-2566 shrink results are helpful in estimating how a particular polyurethane will perform in a real-world application, the values cannot be fully relied upon because the tests were conducted in the controlled laboratory setting with mold dimensions that are probably different than that of the customers parts.
In addition, the physical characteristics of a resin system will affect its shrinkage. For example, materials with high exotherm can be expected to shrink more than low exotherm products. Similarly, when resin systems are cast to form thick-walled or large parts, they will generate more heat during curing and exhibit greater shrinkage than when used for thin-walled parts.