Self-reinforced polymer composites are thermoplastic materials for design of recyclable lightweight components. The combination of sheet forming and subsequent overmolding from these materials allows an efficient manufacturing process with function integration. This article investigates how temperature exposure during an overmolding cycle influences the tensile modulus, shrinkage, and warpage for an insert made from self-reinforced poly(ethylene terephthalate) (PET) when it is overmolded with polycarbonate (PC)/PET material. The temperature gradient that is induced by overmolding is simulated, and the results are validated through experiments. The results show that the insert reaches a temperature above the glass-transition temperature for the matrix material in the main part of the insert, which leads to relaxation of residual stresses and thereby a reduction of the tensile modulus up to 18% for the insert after overmolding. Even though overmolding is an efficient process, it requires thorough knowledge of how temperature influences the material when applied to self-reinforced composite materials. The study shows very interesting results, which can lead to new areas of applications for self-reinforced PET.