Plastics Technology

AUG 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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completely unacceptable to the end user. Process development had minimized the problem, however quality checks still involved cutting open the thick section in almost 5% of our production to inspect for voids, since non-destructive imaging techniques were not readily available at that time. And even with all the appropriate process strategies in place, we struggled with certain lots of material. When we analyzed our quality data as a function of lot-to-lot behavior, we noticed that the problem lots were the ones manufac- tured to the high end of the melt-flow-rate range. The nominal MFR specification was 11 g/10 min with a low end of 9 and a high end of 13. Lots that came in below 11.5 g/10 min afforded us the needed processing window without producing voids. Voids occur due to excessive, localized shrinkage; and when the crystallization rate was faster this shrinkage occurred to a greater degree. The higher-viscosity lots shrank less. We may have been the first processor to ever contact a material supplier and ask for a material that did not flow as well. The technical-service represen- tative for the resin company even tried to dissuade us from going in this direction, arguing that we needed an easier-flowing material so that we could pack more effectively. But the experimental data confirmed our approach and by cherry picking lots of higher viscosity we essentially eliminated the defect. We used this approach many times to solve internal void issues in thick walls. We have probably exhausted the crystallinity conversation at this point. Next time we will delve into another defect that can be difficult to detect but is often driven by molding conditions: internal stress. 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 • Other factors are related to the nature of the material that has been selected. This is frequently beyond the control of the processor. However, a processor who knows the role of material selection in the achievement of crystallization can offer its customer a way out of a thorny problem. Nucleation is one modifi- cation of material composition that influences the way a material crystallizes. This is often achieved through additives and is commonly used in PPs, nylons, and polyesters. Nucleation allows crystallization to begin at more sites simulta- neously, producing a larger number of small crystals as opposed to a smaller number of large ones. This will alter the properties of the material, increasing stiffness while typically reducing impact resistance. It will also reduce the amount of shrinkage that the material exhibits as it cools, and it allows for a reduction in cycle time since the crystallization process begins and ends at higher temperatures. While nucleation is often accomplished intentionally with additives, it also can occur unintentionally with the addition of certain fillers and colorants. Another material property that influences the rate of crystallization is molecular weight (MW). This is an often-forgotten relationship. Higher-MW grades of a given polymer will crystallize more slowly than their lower-MW counterparts. Again, if it is understood that crystallization is promoted by chain mobility, this makes perfect sense. In higher-MW systems, chain entanglement occurs to a greater degree and this limits mobility, suppressing the tendency for the material to form crystals. We used this principle many years ago to solve a difficult problem with voids that formed in the thick section of a part molded from a glass-fiber-reinforced PBT polyester. The part had been designed with a nominal wall thickness of 3/16 in. but had an area approximately 3 in. from the gate where the wall thickness increased to almost 1 in. This area was prone to form voids as the part cooled, a defect that was Too much crystallinity will reduce the toughness of the material and can result in brittle failures. 513-619-7415 Your parts, service and calibration experts save you money Precision Solutions. Measurable Results. Control retrofits for all makes of molding machines Value-priced injection molding machines from Asian Plastic Machinery - sold, installed and serviced exclusively by CPT 20 AUGUST 2018 Plastics Technology M AT E R I A L S K now How

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