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Eliminating Flow Induced Birefringence and Minimizing Thermally Induced 

Residual Stress in Injection Molded Parts

Abstract -- Effects on molecular orientations in terms of birefringence and residual stress profile using a rapid thermal response (RTR) molding technique were evaluated. Experiments showed that, the birefringence level in both RTR molded Polystyrene parts and Polycarbonate parts kept decreasing as the RTR temperature approached and exceeded the glass transition temperature.  When the RTR temperature approached the melt temperature, a dark region was observed in the center of the RTR molded part when the specimen was placed between two cross polarized plates, indicating relaxation of the flow induced molecular orientation. Also reduction of magnitude was observed in the gapwise residual stress profile for RTR molded specimen.

Results -- The testing RTR mold has a rectangle mold cavity, 7.2 cm long x 2.54 cm wide, with the thickness adjustable. A typical temperature response of the RTR mold surface with soaking time needed for molecular relaxation is shown in Figure 1. Figure 2 demonstrates the comparison of color fringe patterns between conventionally and RTR molded 3mm-thick polystyrene parts indicating that 150C mold temperature is effective in reducing the flow induced birefringence.  Figure 3 shows the comparison of the gapwise residual stress profile between these 3mm-thick polystyrene parts.  The compressive stress peak was found to shift toward the surface in the RTR molded parts. Also the overall magnitude of the stress level has been decreased. The influence of RTR molding on birefringence and flow length of 0.5mm-thick polycarbonate parts is shown in Figure 4. Besides the reduction in birefringence, it can also be seen that such thin parts can be completely filled with RTR molding while it cannot with conventional molding on a 30-ton injection molding machine.

Acknowledgement -- We gratefully acknowledge financial support from the National Science Foundation under Grant No. DMI-9713519. Any opinions, findings, and conclusions or recommendations expressed in this website are those of the authors and do not necessarily reflect the views of the National Science Foundation.

Figure 1: RTR mold temperature with

temperature soaking.

Figure 2. Comparison of color fringes of

3mm-thick PS parts.

Figure 3: Comparison of the gapwise residual

stress profile of 3mm-thick PS parts.  

Figure 4: Comparison of color fringes of RTR

 molded (bottom) and conventionally

molded (top) 0.5mm-thick PC parts.