Effects of high pressure solidification on the tensile properties of polypropylene
Johnson, Lawrence P.
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Injection pressures available on commercial injection molding machines have increased from 10,000 to 20,000 psi with 40,000 psi in prospect. The pressure during solidification should strongly influence the crystallinity, and hence mechanical properties, of those regular polymers which can crystallize when cooled from the melt. In this study, samples of isotactic polypropylene were solidified under pressures up to 40,000 psi. In addition to tensile properties, the resultant density, microstructure and DTA structure were examined. From all measurements, it was concluded that the degree of crystallinity decreased with increasing solidification pressure. Spherulites formed under higher pressure were smaller in size and less branched than those formed at low pressure. Yield and tensile strengths were reduced as the solidi fication pressure was increased, the reduction being about 10% when solidi fication occurred at 40,000 psi. At first glance, these findings appear to predict strength differences within a particular molded part depending on the molding pressure used. It is concluded, however, that the strength variation would be less in a commercial part. The loss of pressure in the cavity as a result of freezing off at the in-gate should lead to a reduction in cavity pressure early in the solidification process, a lower mean pressure during freezing, and a relatively higher degree of crystallinity.
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