Knowledge Center Article

Physical Properties and Tolerance Capabilities of Plastics

The physical properties and tolerance capabilities of plastics and metals are very different.  You should keep in mind some of the following advice when you are designing plastic parts.

When designing components that are manufactured in a rigid material it is important to understand that rigid plastics are more brittle than metal.  So, where stress risers occur from sharp inside corners, the addition of generous radii to inside corners will help to distribute the load and help to minimize breakage. 

Although with compression molding and thermoset materials, we do not have to be very concerned about dissimilar cross sections. However, most people are familiar with thermoplastic injection molding. When manufacturing these components, maintaining a relatively thin uniform wall thickness is important.  Injection molding plastic is a hot process and once molded, the material begins to cool and shrink.  Since, injection molded parts cool from the outside-in. If a wall is too thick, the outer skin hardens but the interior mass of the part continues to cool and shrink.  This causes surface distortion called “sink”.  Thin walls promote uniform cooling and minimize distortion (warpage and sink) and help you to make a good part the first time. 

Another consideration when designing elastomeric components is that they cannot be held to the same tolerance levels that metals are capable of holding.  Most metal parts are capable of being manufactured to high precision levels and tight tolerances in elastomers are achievable. Elastomers, both rigid and soft, have a much greater thermal expansion and contraction due to heat than do metals. This drastically affects the achievable tolerance levels of the finished good. If components need tighter tolerances, filled materials tend to be much more dimensionally stable.  Since many elastomers are hygroscopic, as they absorb or dissipate moisture. This causes them to grow or shrink depending on the humidity level in the atmosphere. 

All of these things are taken into consideration when developing the right material and design for any given application. In what environment will this be exposed? Is it wet, hot, or both? Does it see harsh chemicals that could cause swelling? Achieving the proper design starts with the environment and the proper material, to ensure that both dimensional and physical requirements are achieved.

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