Visualization of Extra Melt Motion Induced in the Packing Stage and Its Effect on the Residual Stress Frozen in an Injection-Molded Polymer Product

Abstract

This paper deals with the extra melt motion induced in the packing (or packing-holding) stage and its effect on the residual stress distribution frozen in an injection-molded polymer product. The packing-holding process is a technique commonly applied for avoiding shape deformation of the injection-molded products. Indeed this process is effective to obtain precise products, but, at the same time, it has been pointed out that the packing process increases the residual stress frozen in the final products. However, details of the phenomena occurring in the packing-holding process have not been made clear. In this paper, relations between the residual stress and the packing-holding process conditions were studied by visualizing the extra melt motion and the time-dependent birefringence distribution in the molded polymer during the packing-holding process.The visualized results clearly showed that the packing-holding process induces extra melt motion and that this melt motion results in the increase of residual stress (or birefringence) frozen near the gate of the final product. The extra melt motion observed in the packing process can be separated into two stages; “packing” flow during the packing-holding process and rapid inverse flow at the end of the holding process. The former melt motion is induced both by the compressibility of polymer melt and by the compensation of the polymer shrinkage due to solidification during the packing-holding stage. On the other hand, the later melt motion results from the compressed melt release from the mold gate. Therefore, the rapid inverse flow due to melt leakage should be avoided by a proper “gate-seal” technique, while the “packing” flow during the packing-holding process is essential for improving the quality of injection-molded products.