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it will be when the material cools to room temperature. Assuming that the innermost layers of the part have reached this temperature, the part has nearly all of the properties it will have when it has cooled completely. One has to wonder then about the previously cited "rule" that the material should be at a temperature that is 80% of its HDT. If we use 135 C (275 F) as our HDT, which is the midpoint between the two values that appear in the graph, and then calculate this 80% value (ignoring for a moment the proper protocol of doing so on the absolute or Kelvin scale, discussed last month), we obtain a value of 108 C (226 F). Over the interval between 135 C and 108 C the modulus has barely changed. So, what is it that we are waiting for? To return for a moment to the point that all sciences treat temperature using an absolute scale, if this 80% value were calcu- lated properly, the value of 135 C would first be converted to 408 K. Taking 80% of this value would give us 326.4 K or a little over 53 C (127 F). This puts us at a somewhat different point on the scale. However, for PC it makes little difference in terms of the modulus of the polymer. This is part of the problem we have as an industry when we espouse these imprecise rules. We are looking at the wrong axis. The vertical axis that plots modulus is the one we need to pay attention to, not the temperature scale. The effect of temper- ature on mechanical properties is not linear. This general lack of understanding of the relationship between modulus and temperature leads to some poor practices when process conditions are established. The recommended mold-temperature range for this polycarbonate is 70-95 C. But it is evident from examining the curve that higher mold temperatures are possible and can be used without any significant penalty in cycle time as long as the wall thickness of the part is accounted for. I have run PC parts with 0.5-in.-thick nominal walls at mold temperatures of 105 C (221 F), and produced parts at the same cycle time as when those parts were run in a mold set at 15 C (59 F). The parts molded at the higher mold temperature looked better, were more dimensionally stable, and contained a lower level of internal stress. This last factor makes the parts more impact resistant and less susceptible to one of the biggest killers of PC parts, environmental stress cracking. When I turned the mold temperature up to 105 C, I was told the material would "never set up." But a quick look at the modulus plot in Fig. 1 shows that it cannot help but do so. An aspect of the heat-removal equation that is often neglected is the amount of energy we impart to the material in the barrel. Simplifying your project needs Integrated Bulk Material Handling Systems Schenck Process Kansas City, MO 262-473-2441 mktg@schenckprocess.com www.schenckprocess.com When it comes to engineered weighing, feeding, pneumatic conveying and air filtration systems, Schenck Process can take you from one end of the plant to the other. Our global project management team works with you to deliver the most ideal system for your specific application needs. To find out more about our products, services and capabilities for plastics processing, call us today! Weighing Feeding Pneumatic conveying Filtration SEPTEMBER 2017 28 Plastics Technology PTonline.com K now How