Plastics Technology

APR 2017

Plastics Technology - Dedicated to improving Plastics Processing.

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There is a saying that has been attributed at various times to Mark Twain, Abraham Maslow, and many others: "If all you have is a hammer, everything looks like a nail." Warren Buffet, although not the originator of the maxim in anyone's account, cited it in the 1980s when critiquing academic studies of financial markets based on what he deemed inappropriate math- ematical techniques. It also has application in the field of failure analysis. There are perhaps as many as 60 different analytical tech- niques that can be used to analyze a polymer problem. But like it or not, most of us in the analytical services field have become comfortable with six to eight of these, and we tend to rely on them almost exclusively. Which tech- niques we select depend greatly upon our training, our profes- sional path, and what is available to us. Unfortunately, this often results in a mismatch of tech- niques to the problem. And when the data does not fit the emphasis offered by the method, this simple fact is often ignored or an attempt is made to shoehorn the data into a rather tortured scheme. Several years ago, we worked on a problem that followed such a path. It involved a very low failure rate on an LDPE part. A solution to the problem had previously been sought by two facilities with well-defined specialties. The first group focused on internal stress in the parts and used instrumentation The Need for Generalists designed to make precise measurements of these stress levels. They had taken numerous data points; however, when the numbers were crunched, the correlation between the measured stress and the likelihood of failure was poor. The next group had focused on composition. This facility was very strong in high-end techniques such as chromatography and nuclear magnetic resonance and liter- ally pulled the PE molecule apart while at the same time analyzing the additives in the material. This approach also uncovered nothing that distinguished a good part from one that failed. Problems with very low failure rates are the most challenging because they inevitably are due to multiple factors. A strategy that looks for one major contributor may work in a case where 10-20% or more of the parts are failing. But when the occurrence is in the range of 10-20 ppm, experience has shown that one influ- ence significant enough to cause product failure simply does not come and go with this frequency. Instead, three or four factors will be involved and only when these multiple factors combine in a particular way will the part fail. In failure analysis, there is a tendency to gravitate to a few common test protocols. But this approach can result in a mismatch of techniques to the problem. PART 3 Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS By Mike Sepe DSC tests performed on good and failed parts showed that 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. Problems with very low failure rates are the most challenging because they inevitably are due to multiple factors. DSC Results Showing Difference Between Failed and Good Parts Temperature, C Heat Flow, W/g 20 40 60 80 100 120 140 160 180 0.0 -0.5 -1.0 -1.5 -2.0 22 APRIL 2017 Plastics Technology PTonline.com Step Change Prior to Melting Point in Cracked Parts K now How MATERIALS

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