Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/864540
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. This graph shows the PVT relationship between pressure, temperature, and volumetric change in the polymer. Notice that as the pressure applied to the polymer increases, the glass- transition temperature also increases. FIG 2 PVT Diagram for an Amorphous Polymer Temperature→ Room Temperature Specific Volume→ Atmospheric Temperature Processing Pressure Pressure Increases Slope Change Represents Tg www.AIM.Institute or 1-866-344-9694 Learn from the best A d v a n c e d P l a s t i c s E d u c a t i o n & T r a i n i n g John Bozzelli Michael Sepe John Beaumont Processing Temperature Another aspect of the heat-removal equation that is often neglected is the amount of energy we impart to the material in the barrel. The person running the cold mold was running a melt temperature of 321 C (610 F). I opted for a much lower melt temperature of 260 C (500 F) and took advantage of a frozen layer that developed much more slowly at the higher mold temperature, allowing the mold to fill with the cooler melt. Two years after the molding date, the parts produced in the cold mold with the hot melt began to spontaneously develop stress cracks that continued to grow larger and more numerous, while the parts produced with the opposite set of conditions continued to perform well. The other factor that is not considered in the discussion about cycle time is the effect of pressure on the relationship between volume and temperature. Figure 2 shows a PVT diagram for an amorphous polymer. This shows the relationship between pressure, temperature, and the volumetric change in the polymer. The sudden change in the slope of the specific-volume plots represents the glass transition. Notice that as the pressure applied to the polymer increases, the glass-transition temperature also increases. So, the pressures used during the pack-and-hold phase of the injection process can actually influence the temperature at which this very important transition occurs. Higher pressures should result in a higher glass- transition temperature and therefore an earlier attainment of an ejectable modulus. An examination of Fig. 1 should make it quite evident that the rules for semi-crystalline polymers are different. In our next installment, we will examine these materials. @plastechmag 29 Plastics Technology M AT E R I A L S