Talc-filled polypropylene injection moldings show a peculiar crystal orientation, in which the planes of the talc particle plates are aligned parallel to the molding surface, the c- and a*-axes of polypropylene crystals are bimodally oriented in the flow direction, and the b-axes are oriented to the thickness direction. If the samples are melt-recrystallized, although the orientation of talc and the b-axis orientation of the poly-propylene crystals are restored, the c- and a*-axes become unoriented in a plane parallel to the molding surface. This peculiar crystal orientation cannot be explained by any effect of an epitaxy caused by lattice matching between the cleavage plane of the talc and the (040) plane of the polypropylene crystal, a crystallization nucleation effect, a concentration of stress or melt orientation, or their synergisms. From the facts that, even if talc-filled polypropylene injection moldings are melt-recrystallized, the b-axis orientation to the thickness direction is restored and that an injection molding of polypropylene filled with only 0.5 wt% talc already shows the peculiar crystal orientation, it is assumed that talc has a remote influence on the crystallization of polypropylene.

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A correlation between the peak cavity pressure and filling time was found empirically during the injection molding process. Based on this correlation, a holding pressure control method to improve the stability of peak cavity pressure was proposed.
Fluctuations in peak cavity pressure were compared among with the typical holding pressure control, cavity pressure feedback control, and the control proposed in this study for GPPS, PMMA, PP, and POM. In the case of GPPS and PMMA, the fluctuations in peak cavity pressure for the control proposed in this study were smaller than those for control holding pressure control, and almost equal to those for cavity pressure feedback control.
Under stable mold temperature and melt temperature conditions, negative correlations between peak cavity pressure and filling time were found for GPPS and PMMA which have meet viscosities with relatively large temperature dependences. When mold temperature and melt temperature were varied widely, correlations were found for all the materials used in this study.
In conclusion, it was confirmed that the control proposed in this study can be effective to control even small fluctuation of peak cavity pressure when using materials with relatively large temperature dependent melt viscosities such as GPPS and PMMA. Further more simple control systems to improve reproducibility in product quality can be developed by the approach developed in this study without using cavity pressure feedback control.

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