Abstract
The molecular orientation process in the injection molding of polypropylene copolymers with ethylene was theoretically analyzed from a view point of the growth of melt orientation (recoverable shear strain) at the gate and its relaxation in the mold cavity. Regardless of the characteristics of the raw resin (copolymerization type and ethylene content) and molding conditions (cylinder temperature), there exist unitary correlations between the calculated degree of molecular orientation and the experimentally measured degree of molecular orientation such as the thickness of skin layer and the crystalline c-axis orientation function.
According to this analysis, the fact that molecular orientation is decreased by copolymerization with ethylene is due to i) decrease in melt orientation (recoverable shear strain), ii) decrease in the relaxation time and iii) reduction of crystallization temperature.
