5 Geometry Tips for a Successful Plastic Part Design
Designers will often change metal components to plastic to achieve quality improvements and/or cost savings for their products. One of the most important steps in this process is making subtle changes to the part geometry to make the design more compatible with plastic materials. Failure to adjust the part geometry for the use of plastics may result in premature failure. This article provides some tips about plastic part geometry for a more successful metal to plastic conversion.
1. Consider opening up dimensional tolerances
Plastic materials generally require more open tolerances than metals. Engineering plastics typically have 6 to 10 times the rate of thermal expansion compared with metals, which results in significant dimensional changes for plastic parts when the ambient temperature changes. Figure 1 shows the coefficient of thermal expansion for some common plastics and metals. Plastics may also change size due to the absorption of atmospheric moisture or due to the relaxation of internal stresses. These inherent dimensional stability challenges often necessitate wider tolerances when using plastics.
2. Add radii to sharp internal corners
Many plastic materials are notch-sensitive, and plastic parts with stress concentrations due to sharp internal corners may exhibit brittle behavior. For example, Figure 2 shows the impact strength of polycarbonate as a function of notch radius. Although polycarbonate is widely used because of its toughness and ductility, the graph clearly illustrates how sharp internal corners can detract from these properties. Adding even a small radius at an internal corner can greatly enhance the durability of a plastic component.
3. Select mechanical fasteners carefully
4. Be careful when locating assembly holes for fasteners
5. Design for manufacturing
It is essential that the geometry of a plastic part be compatible with the intended manufacturing process. For example, machined plastic parts require features that will allow the workpiece to be held as material is machined away.
Thermoformed components require relatively even walls, limited undercuts, and draft angles so that parts will easily release from the mold. Injection molded plastic parts require careful considerations for gate locations, wall thickness, parting lines, weld lines, and draft angle. The thermoformed plastic container on the right has generous draft angles on the vertical surfaces to ensure ease of manufacturing.
About the author
Dr. Keith Hechtel is Senior Director of Business Development for Curbell Plastics, Inc., based in Orchard Park, NY. Dr. Hechtel has a Bachelor of Science degree in Geology, a Master of Science degree in Industrial Technology, a Doctor of Business Administration degree, and over 30 years of plastics industry experience.
Much of his work involves helping companies to identify plastic materials that can be used to replace metal components in order to achieve quality improvements and cost savings. Dr. Hechtel is a recognized speaker on plastic materials and plastic part design. He has conducted numerous presentations for engineers, designers, and fabricators in both industrial and academic settings. Contact Keith.
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