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lower mold temperatures do not reduce cycle that much, and the loss in performance does not justify the increased production rate. Also, parts produced with a lower mold temperature often display surface defects such as more pronounced fow lines, gate blush, and incon- sistent gloss. The superior quality of parts produced in a mold set at a higher temperature has been proven repeatedly, but it is still a proposi- tion that is greeted with skepticism by many in the processing commu- nity because the diference in quality is often not apparent unless long-term tests are performed. Once I was involved in sampling a mold producing thick-walled PC parts. The frst run had been done at a mold temperature of 60 F (15 C). The parts had fow lines that reduced the natural transparency of the material. So we did a second run where the mold temperature was increased to 220 F (105 C). To counter the efect of the hotter mold, the melt temperature was reduced from 610 F (321 C) to 500 F (260 C). The cycle time didn't change and the part appearance improved dramatically. But the real beneft to material performance did not become evident until two years after the parts were molded. At that point the parts molded at the low mold temperature began to form cracks at the corners in the absence of any applied load. These cracks continued to grow larger over time, while the parts produced in the hotter mold continued to function and showed no evidence of cracking. In Part 3 we'll discuss the infuence of physical aging on acceler- ated test protocols and why it is important to consider this mecha- nism in interpreting the results of these tests. ABOUT THE AUTHOR Mike Sepe is an independent, global materials and processing consultant whose company, Michael P. Sepe, LLC, is based in Sedona, Ariz. He has more than 35 years of experience in the plastics industry and assists clients with material selection, designing for manu- facturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 • mike@thematerialanalyst.com. shorter chains that exhibit less entanglement and therefore a greater degree of mobility than higher-molecular-weight systems. This has been observed in amorphous PET polyester, where the impact performance of a grade of material with an intrinsic viscosity (IV) of 0.53 dl/g displays a reduction in impact resistance at a rate more than three times faster than a grade with an IV of 0.67. A lower intrinsic viscosity is associated with a lower average molec- ular weight. We will come back to this aspect of physical aging next month in Part 3, as it applies to an accelerated- aging qualifcation process. Finally, the molding conditions have an infuence on how physical aging afects the long-term behavior of a material. No commercial process designed to produce parts at a competi- tive price can achieve a structure that is free of internal stress and achieves the perfect equilibrium state. However, some process con- ditions will produce parts that are closer to this ideal than others. Slow cooling rates provide the time needed to relieve the stresses that are created by the fabrication process. Rapid cooling does not provide for this relaxation process, and the polymer chains will be trapped in a confguration that is farther from the ideal equilibrium state that the material will try to achieve after the part is molded. Cooling rate is controlled primarily by the temperature of the mold, and the relationship between mold temperature and internal stress is quite well established. All things being equal, higher mold tempera- tures produce parts with a lower level of internal stress, which can be verifed by performing tests on molded parts that measure this stress. In situations where the process alone cannot achieve the desired level of molded-in stress, annealing may be performed. But processors tend to run their tooling at temperatures that are lower than optimal for minimizing stress and the subsequent efects of physical aging because they believe that a lower mold temperature will yield a shorter cycle time. But the faster cooling rates aforded by Faster cooling rates afforded by lower mold temperatures do not reduce cycle time as much as is thought, and the loss in performance does not justify the increased production rate. 22 DECEMBER 2014 Plastics Technology PTonline.com M AT E R I A L S K now How