For automotive parts, a specification related to the thermal cycling (thermal shock) is usually required with a minimum number of cycles before getting a failure/crack and a temperature range (typically -40°C to +160°C). When an insert (metal, glass, plastic...) is overmolded by a plastic, the thermal strain (shrinkage) of the plastic when the assembly is cooled down out of the mold is constrained somehow by the metal leading to (mainly tensile) residual stresses in the plastic. Since we often have stress concentrations (sharp edges of the metal insert), a failure (in fatigue) in these areas is expected after a given number of thermal cycles. Some internal molding tools have been developed to compare the grades, the designs, the processing conditions… An extensive study - see document and raw data - made with the Fuji’s mold shows the performances of many of our grades and has already been used to promote some of them for real customer applications.
The influence of the other parameters (weld line, insert temperature, design…) is described in the following document (still to be performed).
The prediction of failure when such a structure is submitted to a thermal cycling s more than challenging and is discussed here.
