The effect of holding pressure on the shrinkage and birefringence of injection moulded polypropylene plates

摘要

This paper shows the results of shrinkage and birefringence of injection moulded plates.The study was done with two polypropylenes with different MFI. Focus is made on the effect of theholding pressure on the experimental as-moulded shrinkage and birefringence in the flow direction.The mouldings are edge gated flat rectangular plate. The moulding process was continuouslymonitored with pressure transducers. The results show that the rise of holding pressure determinesthe decrease of the shrinkage. Also, it was observed that the increase of the pressure applied duringthe holding phase and the associated flow induce re-orientation of the melt, especially in the case ofhigher molecular weight PP. Consequently, an increase in the birefringence results.