By Bongtae Han
The volumetric shrinkage (or chemical shrinkage) of thermoset polymers occurs during curing. As the modulus evolves after the gel point where polymers start to build mechanical strength, the shrinkage starts to develop stresses inside molded parts while curing, which results in “curing-induced” residual stresses. They can cause warpage at the curing temperature, and can affect the reliability of components molded by the thermoset polymers.
Material manufactures often provide a shrinkage quantity called “mold shrinkage”. This value should not be used to predict the residual stresses simply because it contains the effect of the coefficient of thermal expansion (CTE) of the mold as well as the cured thermoset polymers.
The issue of “chemical shrinkage” is not new. Numerous testing methods have been developed for many decades, and some of them are practiced routinely to measure the intrinsic (or total) chemical shrinkage. It is important to note, however, that not all of the chemical shrinkage contributes to the residual stresses simply because the chemical shrinkage occurs even before the gel point. The shrinkage that occurs after the gel point is called “effective chemical shrinkage”.
This seminar first reviews (1) how the mold shrinkage is measured; (2) how the volumetric shrinkage is measured by pressure-volume-temperature-cure (PVTC) tests, and (3) how to measure the effective chemical shrinkage by the embedded fiber Bragg grating technique. Discussion about how these properties are to be used to predict residual stresses for different molding processes are followed.