Influence of low heat conductive inserts on morphology of foam injection molded parts

Foam injection molded parts exhibit a so called integral foam morphology which is represented by a cellular core structure surrounded by compact skin layers, thus leading to a high weight specific bending stiffness. One major disadvantage of these parts is their inadequate surface quality, due to the presence of silver streaks, which limits the use of foam injection molded parts for visible applications. To overcome this hindrance, foam injection and insert molding can be combined. To additionally improve the haptics of such parts, soft-touch inserts can be used. Generally, the heat conductivity of polymer inserts is low, hence, the thermal conditions while formation of the integral structure are changed. The aim of this study is to systematically evaluate the influence of low heat conductive inserts on the morphology of foam injection molded parts. A physically (N₂) foamed polypropylene (PP) is injected in-between two inserts by foam injection molding. The thickness of the inserts is varied between 0.5 mm to 2 mm. Foam morphology is investigated. By combining insert with foam injection molding, a higher average cell size with smaller deviation across part’s thickness has been observed while compact layer thickness has not been affected. Morphology phenomena have been illuminated by foam injection molding simulation. Bending stiffness of PP foam is increased up to 20 % compared to conventional foam injection molding.