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Designing A Plastic Part Is More Than Skin Deep

This article originally appeared in Plastics Today March 20, 2019.

Design is an all-encompassing term that involves nearly every facet of product success. Its preponderance in product development is often overlooked, however, and given neither the proper time nor effort that is required.

While design involves the traditional definition of form, function and cosmetic appearance, it embodies more than just how the part looks: Design involves many steps, along with cross-functional collaboration, to be successful. It includes the resin that will be used, as well as the additives to increase longevity and safety, and reinforcements to enhance mechanical properties. Design comprises the geometric features that provide support, cover, feel and shield. It is intimately involved in the ability to manufacture the part and assemble it with other parts to create an assembly or product, referred to as design for manufacturing and assembly (DFMA) or design for processing (DFP). How the part functions, behaves, lives and interacts with its surroundings is directly related to design.

Not being the least important, design plays an exceedingly important role on cost. Design establishes the complexity of the manufacturing process, the elaborateness of shape and size, and multiplicity of the materials to be used, which all obviously heavily influence cost. At the same time, design is highly associated with the inherent and perceived value of the part. Design is so intertwined with the many aspects of successful part development that without a keen focus on it and the understanding of its importance at every step, the final part is likely to fail at some point along the development process, during manufacturing or while in the field. Early and repeated attention to the details of all aspects of part design can play a crucial role in manufacturing success, true and perceived quality, appearance, costs, prevention of failure and part longevity.

Nearly everyone who has been involved in the product development process can attest to the fact that cost can make or break the existence of a part. It is common for a superior design to be subjected to value engineering to investigate where costs can be trimmed to create savings. Sometimes these cost savings are real, with little bearing on quality, appearance and function. At other times, these changes are made without proper knowledge of their implications. This happened several years ago on a project involving a large metal part that was being converted into a plastic. Using plastic would allow the company to design the part with a relatively complex geometry, which, in turn, would make the end-use device more efficient and quieter. It was revolutionary and was going to be a slam-dunk success. The original plastic part, which had a number of unavoidable knit lines, was to be made out of 30% glass-reinforced polyamide 6/6. The part was well-designed, with most aspects of proper design addressed. Unfortunately, the decision makers determined that switching to a glass-reinforced polypropylene was going to save money. Not a difficult calculation to make, and they were correct in determining that material costs would be reduced. The short-term properties and finite element analysis indicated that the part would perform satisfactorily with this new material; thus, it appeared to be a no-brainer. Fortunately, the engineers wanted to confirm the long-term performance of this new material and put into place a series of test modules to accelerate lifetime behavior of the part. During this testing, the product made of polypropylene began to fail much earlier than expected, while the polyamide 6/6 product exhibited no such behavior. To confirm the mechanism of failure and predict longevity of the part, it was requested that lifetime analysis be done.

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Paul Gramann, PhD, PE

Paul Gramann is one of the founders and current President of The Madison Group. Dr. Gramann received his Ph.D. from the University of Wisconsin – Madison where he served as an adjunct professor within the Mechanical Engineering Department. He is dedicated to design verifications and failure analysis of thermoplastic, elastomeric and thermoset parts. His expert testimony has been invaluable in numerous depositions and trials for both defense and plaintiff counsels.