The internal stresses produced in three cylinder models simulating the structures of many epoxy resin encapsulated components were measured by strain gauge. The mechanism of stress generation is very different between an open type structure that has one surface of the resin open, and a closed type that does not. Internal stresses of an open type structure are essentially produced by the thermal contraction below the Tg of the epoxy resin, which agrees with other workers. 1) 3) However, stresses of a closed type structure are produced by various volume changes after gelation of the epoxy resin. In particular, large stresses are produced by chemical contraction after gelation and thermal contraction above the Tg of the epoxy resin. These stresses cause the generation of cracks and peeling in epoxy casting components.
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