Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Imagine : Processing Activity-driven instruction increases the ability to troubleshoot problems and develop standardized processes Designing Design for manufacturability offers solutions to keep bad part and mold designs from getting into production Empowerment Provide knowledge and skills to make data driven decisions, which benefits both the employee and employer ROI Decreased cycle times, scrap reduction, risk management, and upfront analysis methods create higher quality parts and molds Training transforming your employees into problem solvers ABOUT THE AUTHOR Mike Sepe is an inde- pendent, global materials and processing consultant whose company, Michael P. Sepe, LLC, is based in Sedona, Ariz. He has more than 40 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 • cooling rate between the surface material and the core material, coupled with a reduction in the thickness of the oriented layer of material. Both factors reduce the level of molded- in stress in the part, making it more capable of managing an externally applied stress. The natural response of most processors when presented with these data is to protest that the cycle time will be longer if the mold temperature is increased. However, this objec- tion can be answered by observing what occurs when the melt temperature is reduced. While lowering the melt temperature had little to no effect on part performance when the mold was cold, the same adjustment produced an additional improvement in impact resis- tance when the mold temperature was increased. The best results, an impact resistance of 48 N-m (35 ft-lbs), were achieved with the lowest melt temperature and the highest mold temperature. By reducing the melt tempera- ture, the increased mold temperature can be managed without changing the cycle time. The best way to remove heat quickly is not to introduce it in the first place. The lower melt temperature also reduces the risk of polymer degradation. This behavior can be verified by using another measure of molded-in stress, resis- tance to stress cracking. When molding some clear PC parts using a relatively high melt temperature of 321 C (610 F) and a low mold temperature of 24 C (75 F), it was observed that the parts developed cracks when exposed to olive oil. In fact, parts molded at these conditions spontaneously developed cracks even without exposure to the olive oil if enough time was allowed to pass after the parts were produced. When the mold temperature was raised to 105 C (221 F) and the melt temperature was reduced to 277 C (530 F) the stress cracking was elimi- nated without lengthening the cycle time. The part surface also had a better appear- ance because the higher mold tempera- ture delayed the rate at which the surface material solidified. These are anecdotal results, and in the world of plastics everyone has a good story or two. But this does not pass for science. In our next article we will elaborate on the foundation principles behind these improvements and translate them to the process conditions that ensure optimal performance for the molded part. Any fabricated part will contain some level of internal stress that arises naturally from melt processing. 32 SEPTEMBER 2018 Plastics Technology M AT E R I A L S K now How

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