In the injection molded parts of glass fiber reinforced PP (PPG), the increase of fiber content to improve the mechanical properties, results in protruding the surface at weld-line region, where melt flows unite in at injection mold, and deteriorates the surface appearance.
In this study, the optimum material design were discussed to improve both surface appearance quality and mechanical property of PPG.
The weld-line protrusion height as the surface appearance quality and izod impact strength as the mechanical property were measured for PPG's with various fiber contents. The influences of fiber content, short-fiber/long-fiber ratio and flowability of matrix PP were discussed. It can be found that the low-flowability PP with a proper quantity of long fiber is effective for solving the weld-line protrusion without reducing the impact strength.

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As a fundamental to develop a process control system based on the prediction of polymer behavior in a mold cavity, the dynamics of cavity pressure during the pressure holding stage was analysed. Based on the analysis, a new procedure to predict the variation of cavity pressure during the production caused by some kinds of environmental disturbances was proposed.
By using a simplified lumped model, it was analytically shown that the cavity pressure during the pressure holding stage is determined by the polymer flow into the cavity caused by the pressure gradient from the reservoir of the barrel to the cavity, the volumetric shrinkage of the polymer induced by cooling, and the additional polymer flow into the cavity induced by the pressure drop due to the volumetric shrinkage. Furthermore, the relationship between the cavity pressure and its fluctuation causes such as mold temperature, polymer temperture in the barrel and the holding pressure was clarified.
Based on these analytical results, a new procedure to predict cavity pressure variation was proposed and verified experimentally. As a result, a close agreement between measured and calculated cavity pressures was obtained.
In conclusion, it was confirmed that the procedure for prediction presented in this study can be applied effectively to adaptive process control based on the theoretical prediction of polymer behavior in the cavity.

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