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-degrees-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/cm2 then seems to be sufficient. A minimum power density of 10 W/cm2 is needed for the relaxation to occur in this specific system.