Influence of Cooling Conditions at the Cavity Walls on the Sink Location in Injection Molded Polymers

This paper deals with an experimental study on the influence of cooling conditions at the cavity walls on the sink generation appearing in an injection molded polymer product. By employing a test mold which has two transparent windows, the processes of sink generation and solidification of the injection molded polymer were observed under various molding conditions. The observed results showed that sinks appear on the surface just before the whole polymer solidifies. This means that the development of sinks must be influenced by the rigidity of the solidified layer generated at the beginning of the cooling stage. This implies that the sink generation can be controlled by the temperature of the cavity walls. In order to confirm whether the sink location can be controlled by the cooling rate of the cavity walls, sink generation was examined in tests using asymmetrical cooling of the cavity walls. The results are summarized as follows: (1) When the temperature of one cavity wall is 10-20K higher than that of opposite wall, sinks appear only on the surface adjacent to the hotter wall. (2) On the contrary, if the hot wall temperature exceeds the glass-transition temperature of the polymer, the sinks appear on the opposite cold surface. It was thus concluded that the location of sink can be controlled by the cooling rate of the cavity walls.