Pacific Plastics - Stable Sizing With an Unstable Resin

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THE CHALLENGE – A prospective medical customer came to us with parts produced by another molder that did not meet tolerance and were continually rejected for processing and quality deficiencies. Since this product was part of the "life blood" of the company and instrumental to their product launch, they needed a competent medical molder who understood these unusual challenges. Referred by another satisfied medical customer, we met to understand their issues of high volume production and resin requirements. Pacific Plastics & Engineering was then instructed to build a new high-volume mold that met the customer's stringent quality requirements.

THE INVESTIGATION – After studying the part design and engineering requirements, we researched the resin characteristics. First, we learned that the resin contains fillers that produce anisotropic shrinkage characteristics. The shrinkage in the resin flow and transverse flow directions were appreciably different. Second, the resin was extremely hygroscopic. It could either absorb water from or lose water to the surrounding environment. With a short time exposure in a humid environment, the part could grow twice its allowable tolerance. Finally, the resin contained an oily additive that could contaminate the part surface and leave residue everywhere inside the tool.

THE SOLUTION – With both processing and tooling concerns well assessed, the first step was to design and fabricate a multi-cavity Class 101 tool that could survive cycling 24 hours a day. This requirement meant that proper steel selections and a robust design would have to be generated. However, before this could start, we needed to assess the differential shrink rates. A quick prototype tool was built using the exact part geometry of a portion of the part. A stable process was established to achieve and hold the dimensions required. After completing the design and building the high-volume tool, the final process could be initiated. To process the part, we implemented several steps: (1) installation of proprietary equipment on the molding machine to remove both moisture and oil; (2) implementing a capable process that includes allowance for moisture absorption to accommodate customer's automation equipment; and (3) adopting stringent quality control procedures over our in-process inspections. With no customer rejects, the tool has run flawlessly since its debut many months ago.

This article was produced by the plastics injection molding and medical device engineering craftsmen engineers at Pacific Plastics & Engineering in Soquel, CA.
For nearly two decades, Pacific Plastics & Engineering has been providing state-of-the-art custom molded plastics and injection molds - for medical device development, biotech products, and more - to customers with tight tolerance requirements as well as tight timelines.