The influence of surface heating on the birefringence distribution in injection molded parts

In order to reduce the frozen‐in orientation, which is usually present in injection molded products, the effect of high‐performance mold surface heaters was investigated. The heaters are capable of changing their surface temperature by 70°C within a few tenths of a second typically, which minimizes their effect on the cooling time. The effects of the heating time, the instant of heating, and the heating power on the birefringence distribution of a polystyrene resin were studied. Reductions of the birefringence peak by a factor 4 to 7 were observed. The birefringence is removed most effectively by heating briefly before and during the injection stage. A heating pulse of about one second and with a power density of 20 W /cm 2 then seems to be sufficient. A minimum power density of 10 W /cm 2 is needed for the relaxation to occur in this specific system.