Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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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 • were discussed in last month's column. It is interesting that the same attention to mold temperature that ensures optimal crystallization in semi-crystalline polymers also produces optimal properties in amorphous polymers. If mold temperature is thought of as the sole factor in determining cycle time, then there is a great reluctance on the part of the processor to increase it. However, if it is understood that mold temperature and melt temperature work together, then it can be seen that an appropriate balance between the two parameters is the key to optimizing material properties while maintaining a competitive cycle time. While the focus of these articles has been on processing, it is important to note that the ability to achieve a uniform cooling rate and an optimal pressure distribution in the cavity also depends upon good part design and mold design. Designing a part with large variations in wall thickness, or selecting a gate location that results in filling a thicker section through a thinner section, will create problems that the processor will have difficulty compensating for. Traditionally, poor design decisions have simply been passed along and it has been left to the processor to figure it out. Frequently the result is a set of process conditions that represent an attempt to correct for the poor design. These often include lower mold temperatures and high packing pressures, the very things that create elevated levels of internal stress. Good concurrent engineering practices call for a thorough discussion between all interested parties during the early stages of product development so that these problems can be solved before the mold is built. sent a source of internal stress. The level of retained orientation, and the difference in cooling rate between the surface of the part and the core can be reduced by operating at a higher mold temperature. This reduces the rate of cooling, allowing the polymer to relax to a greater extent as it develops the structure in the part. Another secondary benefit to running higher mold temperatures is that it slows down the rate at which the frozen layer develops. The instant that the molten polymer enters the mold, the material at the surface begins to freeze. The flow of the polymer through the cavity is continued by molten material moving through the interior layers that have not yet solidified, a phenomenon known as fountain flow. The shearing forces that develop between the exterior layers that have stopped flowing and the interior layers that are still moving are a source of internal stress. So, to the extent that the rate of frozen-layer development can be reduced, the internal stress can be reduced also. Many cosmetic defects, such as tiger stripes, flow lines, and orange peel, are symptoms of a frozen layer that develops too early in the mold-filling process. In addition, recall that the pressure drop is governed to a significant degree by the size of the flow path. In a round cross section, a reduction from 0.050 in. (1.25 mm) to 0.040 in. (1 mm) results in doubling the pressure loss. We tend to think that if a part has a wall thickness of 0.080 in. (2 mm), this represents the available path through which the polymer flows, regard- less of location in the cavity. But in reality, the size of the available flow path becomes smaller as the material moves farther into the cavity, away from the gate or gates. This is another factor in the pressure drop that we observe between the gate and the end of the flow path. Higher mold temperatures reduce the rate of frozen-layer development and allow for achievement of a more even pressure distribution in the cavity. These principles of polymer flow and cooling are behind the observations that Custom Vacuum Solutions for Extrusion Processes 1-800-USA-PUMP I Busch offers the most comprehensive product portfolio and the most economical solution for your specific needs – from a simple vacuum pump to a complex vacuum system complete with a control system. @plastechmag 27 Plastics Technology M AT E R I A L S

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