Plastics Technology

APR 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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multiple test techniques that represented these different disci- plines, a careful evaluation of the raw data from each technique, and a practical appreciation of how significant each factor could be. Frequent conversations with the client were also invaluable since each call brought out new observations. Many of the tests used to unravel the problem had not been performed previously. And not every test produced a positive result. One of the tasks of the analyst is to distinguish a significant result from an insignificant one. But one test, known as differential scanning calorimetry (DSC), was of particular importance and had been done previously. But a lack of thorough- ness and a misinterpretation of the results resulted in a missed opportunity. First, these tests had only been performed on failed parts. While most of the tests had included both good and failed parts, the DSC tests had only been run on parts that failed. It was not until the tests were repeated with both good and failed parts that the significance of the results was noted. The accompanying graph shows a comparison between good and failed parts. Every failed part exhibited the unusual secondary step in the thermogram just before the primary melting event. None of the good parts displayed this extra transition. In this case, the failures were caused by a combination of poor color mixing, incidental damage caused by automated handling equipment, a tolerance stack-up between parts in the assembly that did result in elevated stress, and the effects of extended storage conditions on the crystallinity of the material. Therefore, part failure depended on polymer structure combined with dimensional considerations as well as manufacturing issues related to part handling and melt processing. No one of these factors alone was capable of producing a failure—they all had to be in play and to a sufficient magnitude to affect the performance of the polymer. Facilities made up only of polymer chemists or stress analysts are not likely to bring to the table the range of skills needed to properly weigh all of these factors and assemble them into a cohesive picture. To uncover the confluence and the relative importance of each factor required Part failure depended on polymer structure combined with dimensional considerations as well as manufacturing issues related to part handling and melt processing. No one of these factors alone was capable of producing a failure. APRIL 2017 24 Plastics Technology PTonline.com M AT E R I A L S K now How

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