Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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Understanding the concepts of pack and hold and applying them during process development is critical for molders to achieve consistent part quality. Process Capability and the 'Hesitation Effect' A two-part series on process capability by this author was pub- lished in Plastics Technology in May 2018 ("Improving Molding Process Capability: Understanding the PVT Graph") and August 2018 ("Improving Molding Process Capability: The Role of the Five Essential Pillars"). The articles were based on studies conducted by the author at his firm's lab in Carlsbad, Calif. Generating the data for these two stories revealed an intriguing phenomenon: The injection molding machine was very consistent in delivery of the melt, but the calculated variation for individual cavities showed large differences. It was puzzling at first, but further investigation uncovered the reason. This article describes the findings and rea- sons for the process-capability numbers, called the hesitation effect. What is the hesitation effect? Figure 1 is a schematic of a family mold with two non-identical parts. Part A has a rectangular cross- section and Part B has a cross-section with a thin section. They both have one gate. Figure 1 shows the initial fill pattern of the plastic, where the flow is even. As the flow advances, the plastic reaches the thin section in Part B and therefore the required pressure to push past the thin section increases. At the same time, the plastic in Part A does not need high pressure to continue filling the part, since the cross-section is thicker than that of the thin section in Part B. Since the required fill pressure is low in Part A, the melt front preferentially flows in part A and the flow front in Part B slows down, increasing the viscosity of the melt and cooling the melt. Once Part A is filled, the plastic now tries to By Suhas Kulkarni Fimmtech QUESTIONS ABOUT INJECTION MOLDING? Visit the Injection Molding and Injection Molding Know-How Zones. In this experiment, 30 shots were collected, and the weights were recorded for the individual cavities and the runner. The combined range for both cavities was low, but the range for the individual cavities was higher. The low combined range suggests that the machine was consistently delivering an accurate amount of melt from shot to shot. However, if the individual cavities are showing variation, it suggests that there is a hesitation effect in the flow into the cavities, where one is filling preferentially over the other. Consistent Machine, Yet Inconsistent Fill Cavity 1 Cavity 2 Cavities 1 & 2 Part Weight, g FIG 2 0.14 0.13 0.12 0.11 0.10 0.09 0.08 0.07 Range = 0.113 g Range = 0.114 Range = 0.086 Suhas Kulkarni, president and founder of injection molding consultancy FimmTech, instructs participants during a Design of Experiments (DOE) workshop at custom injection molder Comar's Garden Grove, Calif. plant, held in conjunction with the Molding 2018 Conference. 42 JANUARY 2019 Plastics Technology PTonline.com Tips and Techniques

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