Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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associated with the HDT test increases, the temperature at which failure occurs will decrease. The magnitude of this decrease will depend greatly upon the material being tested, for reasons that will become evident as we fill in the picture with additional data. But as an example, the HDT of a 15% glass-fiber reinforced nylon 6 is measured at approximately 400 F (205 C) under a stress of 264 psi. At 1160 psi the value will decline to approxi- mately 175 F (80 C). There is an obvious reluctance on the part of material suppliers to start publishing the lower values, particularly since they know that many people who read data sheets do not look at the fine print that defines the test conditions. In addition, almost all unfilled materials would fail at room temperature under this higher stress. If a test designed to measure elevated-temperature perfor- mance produces immediate failure at room temperature in thousands of mate- rials when realistic stresses are applied, it raises questions about the utility of the test in general. So why do we continue to use it? The simplest answer is that we have always done it that way. Or perhaps the industry is unsure of what to replace the test with. As is the case with almost all of the values on a data sheet, they represent points on a curve. Tensile yield strength is a point on a tensile stress-strain curve. Impact resis- tance is a point on a plot of collected energy over the duration of the impact test. And the HDT is a point at which the modulus of the material being tested declines to a specific point. In an era when we aspire to replace aluminum and steel, the obvious question is: Why not just provide the curve and let the engineers find the point that is of interest to them for their applications? In our next article we will do just that. 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 • engineer who had written the requirement that the HDT could not be treated as a representation of long-term elevated-temper- ature performance. Like most properties provided on a datasheet, HDT defines what does not work; it tells us nothing about what a material can do. This was stated very plainly by a representative from a material supplier many years ago when he told attendees at a technical session, "Just remember, if there is a number on the data sheet, something bad happened at that point." Tensile strength is quoted at yield or at break, outcomes that we never want to experience in the real world. Impact resistance provides the energy needed to break the test specimen. And HDT defines the temper- ature at which a molded test specimen of a very specific geometry undergoes a certain degree of bending when placed under a given stress. The stresses used under the ASTM protocol are so low as to be laughable: 66 psi (0.455 MPa) and 264 psi (1.82 MPa). A few months ago, I received a call from a university professor who had been asked by a client to perform the HDT test. He was unfamiliar with the procedure and when he calculated the amount of force that would be needed to achieve the required stress, the value he obtained was so low that he was convinced he had made an error. But he had not made a mistake; the stresses associated with the test are ridiculously low. I routinely see finite-element analysis (FEA) plots that show maximum stresses of 3000-5000 psi (20-35 MPa). What possible relevance can measure- ments of elevated- temperature performance have when they are made at stresses that are less than 10% of those at which we plan to use our parts? What would we obtain from an HDT test performed at these higher stress levels? The folks at ISO have attempted to inject a little reality into the test. ISO 75, the method that corresponds to ASTM D 648, calls out a third stress level of 1160 psi (8.00 MPa). However, it has been slow to catch on. It is not hard to see that as the stress level Almost all the information available on the effects of elevated temperatures remains limited to a measurement of heat deflection temperature. Dynamic mechanical analysis (DMA) has been proposed as a more valid alternative to HDT for at least 20 years. Today's more critical engineering applications for plastics make a change all the more urgent. (Photo: DMA 850 from TA Instruments) 24 JANUARY 2019 Plastics Technology M AT E R I A L S K now How

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