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Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS 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 40 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. Call us TODAY for more information! *resin and bulk density specific 860-496-9603 www.wittmann-group.com info.us@wittmann-group.com Integrated Conveying and Drying Control • Color touch screen display with AmbiLED - Control up to 24 Loaders from Dryer Control! • Integrated Conveying Control • -65°F Dew point capable • Capable of processing up to 200 lb/hr of Plastic Resin* ATON plus G Takemori also pointed out that as the material temperature increases, the dimensions of the sample increase due to thermal expansion. This introduces a countering influence into the mix that actually pushes on the device used to measure the deflec- tion, prolonging the test. It has also been noted that the level of molded-in stress in the specimen influences the temperature at which the DTUL is measured. Compression molded samples contain less internal stress than injection molded ones and therefore register a higher DTUL. Samples that have been annealed will also produce a higher measured DTUL for the same reason. And not all samples are prepared to the same thickness. Thinner samples will deflect at a lower temperature than thicker ones. Given all these variables that are inherent in determining this temperature, how useful can it be as a factor in an equation? Even if we were to assume that there is some solid science behind this, there are some glaring problems. These are most readily observed in soft, flexible materials. Polyethylene, some polypropylenes, most elastomers, and even PTFE do not have a measurable DTUL because they do not exhibit a modulus high enough to bear the specified stresses prescribed by the test method without immediately deflecting to the degree that signals the conclusion of the test. In order to measure the DTUL of these materials it would be necessary to cool the material below room temperature, allowing the material to stiffen to a point where the test could be started. The result would be a "heat" deflection temperature at some point below room temperature. Therefore, if we use the DTUL as the ejection temperature, these materials would have to be cooled to some very low temperature before the mold could be opened and the part ejected. This is just the beginning of the problem associated with the way we propose to "calculate" the cycle time of a part. In the next article we will look at how we treat temperature in our equations. We will also explore further some adjustments made to the use of DTUL and review some alternative approaches. M AT E R I A L S