The effectiveness of cavity pressure control in the injection molding process was investigated with regard to reproducibility in product quality.
Since injection molding is generally carried out under varying mold temperature or melt temperature condition, the effectiveness of a process control should be considered with relation to temperature variations. From the analysis of the relationship among temperature, pressure, and product weight, it was derived that cavity pressure control can be effective in improving the reproducibility in product weight if the product weight fluctuations caused by cavity pressure variations are larger than those caused by material temperature variations in the cavity.
To confirm the analytical results above, experiments were carried out under both varying and constant temperature conditions. When the melt temperature was varied, the product weight fluctuations caused by pressure variations were relatively large, and the control was effective. On the other hand, when the mold temperature was varied, the product weight fluctuations caused by the temperature variations were relatively large, and the control was not effective. Under constant mold temperature and melt temperature conditions, differences in product weight fluctuations with and without the control were not observed.
In conclusion, it was confirmed that cavity pressure control can be effective under stable mold temperature conditions. And when the control can not be effective, necessity to develope the control which takes temperature variations into consideration was suggested.

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The transverse bending properties of long fiber reinforced thermoplastics using Commingled Yarn were shadied. The materials used were Glass fiber and PA 6. The transverse bending strength was depended on both molding pressure and time. Microscopic crack and debonding occurred at fiber/matrix interface because of the thermal shrinkage of the matrix. These phenomena could be explained by the microscopic molecular structure in the interface.

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