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

Many plastic part failures are caused by using incorrect fasteners. For example, flat head screws with wedge-shaped heads create high levels of hoop stress. This increases the likelihood of environmental stress cracking or creep rupture of the plastic part. Round head or pan head screws with flat washers create much lower levels of localized mechanical stress. Screws with standard thread profiles may loosen or pull out from soft ductile plastics such as polyethylene. Screws that are specifically designed for soft ductile plastics, with small minor diameters and high thread profiles have greater resistance to loosening and pullout. As shown in the picture below left, flat head screws created high localized stresses, which resulted in cracking of this floor trim strip on an commercial airplane.

4. Be careful when locating assembly holes for fasteners

The holes used to assemble plastic parts with mechanical fasteners should be set back from the part edges. Fastener holes that are too close to an edge increase the likelihood of environmental stress cracking or creep rupture over time as shown in the image above right where the acrylic cover plate cracked during installation due to the mounting holes being too close to the edge of the part.

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.

The part geometry tips described in this article are intended to help designers avoid some of the most common geometry issues that create headaches during metal to plastic part conversions. For help with a specific plastic part design, Ask a Plastics Expert or call us at 1-800-553-0335.

 

This article provides general guidelines and is intended for informational purposes only. Because every situation is unique, many factors must be considered when selecting a material for something as important as a machine guard. It is the reader’s responsibility to conduct his or her own research and make his or her own determination regarding the suitability of specific products for any given application.