Measurement of effective cure shrinkage of epoxy-based molding compound by fiber Bragg grating sensor using two-stage curing process

Cure shrinkage accumulated only after the gel point is known as effective cure shrinkage (ECS), which produces residual stresses inside molded components. The ECS of an epoxy-based molding compound (EMC) is measured by an embedded fiber Bragg grating (FBG) sensor. Under a typical molding condition, a high mold pressure inherently produces large friction between EMC and mold inner surfaces, which hinders EMC from contracting freely during curing. A two-stage curing process is developed to cope with the problem. In the first stage, an FBG sensor is embedded in EMC by a molding process, and the FBG-EMC assembly is separated from the mold at room temperature. The molded specimen is heated to a cure temperature rapidly in the second stage using a constraint-free curing fixture. Several technical issues have to be taken into considerations to ensure that (1) EMC does not pass the gel point before it reaches the cure temperature, and (2) EMC cures uniformly around the FBG during measurements. The ECS of an EMC with a filler content of 88 wt% is measured by the proposed method, and its value is 0.077%. The repeatability of the proposed method is corroborated by the results of a duplicate test.