抄録
Improvement of surface transferability of an injection-molded polymer was examined by using heat insulating layer-coated molds. Polyimide (PI) and HIPS were used as the heat insulating material and molding polymer, respectively. Although the transferability of the mirror-finished mold surface, i. e. gloss of the molded surface, depends both on the thickness of PI layer and on the molding conditions, e. g. injection velocity and temperature of polymer, the PI layer of about 0.1mm thickness enables us to obtain sufficient transferability with negligible elongation of the cycle-time.
In order to examine the relation between the thickness of thermal insulation layer and the improvement of surface transferability due to the PI layer, time-dependent temperature distribution near the mold-polymer boundary was numerically studied. The results showed that the temperature of PI layer coated on the mold surface rises immediately after the layer is in contact with the polymer melt, and that the period during which the mold surface temperature is kept at high increases with increasing the PI layer thickness. We therefore concluded that the molding pressure applied during the period is effective for improving the surface transferability of the molded.
From the practical viewpoint, the PI layer covered with nickel surface layer was also tested, and the improvement of surface transferability was correlated with the integration value ΔH of the excess temperature of the mold surface over the glass-transition temperature of polymer to the time elapsed. The result clearly showed that the significant improvement of surface transferability can be obtained only under the condition where the ΔH value is higher than 2s·°C.
