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.

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An apparatus for measuring the PVT properties of polymers using a metal bellows was developed for pressures up to 200MPa and temperatures from 313K to 473K. The relationship between the volume of the sample cell and the displacement of the bellows was determined measuring the PVT properties of water for temperatures up to 473K and pressures up to 200MPa with an accuracy of ±0.1%. The PVT data for mercury measured with the apparatus showed good agreement with literature values. The PVT relationship for polystyrene was measured in the same ranges of temperature and pressure. The estimated uncertainty in data was less than ±0.0053cm³/g. The PVT data in the rubbery state were correlated with three equations of state. The Simha-Somcynsky equation of state showed the best agreement with experimental values.

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