Passive mold temperature control by a hybrid filming-microcellular injection molding processing

Microcellular injection molding (also known as Mucell process) with supercritical fluid (SCF) content is capable of producing parts with excellent dimensional stability while using less material, lower injection pressure, and achieving a shorter cycle time. However, most of microcellular processing studies were done on interior morphology, microstructure of the microcellular products, researches and reports on the splay-like appearance, or even a rougher swirl surface which is the restriction for the application of the microcellular injection molding are insufficient. This study investigates the influence of mold temperature on the surface roughness of Polyethylene terephthalate (PET)/Polycarbonate (PC) parts molded by hybrid filming-microcellular injection processing. The PET film is considered as a passive mold temperature controller because of its lower heat transfer coefficient (0.16 J/kg °C) compared with mold base steel of 31.5 J/kg °C. Temperature field changes of mold base caused by different film thickness (0.125 mm, 0.188 mm) were analyzed. The surface roughness of microcellular parts with/without film was measured by a 3D laser microscope. It was found that surface toughness decreases while film thickness increases with minimum surface roughness 1.8 μm. Compared with conventional mold temperature (60 °C), the hybrid molded parts with 0.125 mm film lowered surface roughness from 26 μm to 5.6 μm, to 1.8 μm of 0.188 mm film. And the hybrid processing also improved the uniformity of foamed parts surface quality by eliminating swirl and even surface roughness.