Tolerance

The specific dimensions that can be obtained on a finished, processed plastic product basically depend on the performance and control of the plastic material, the fabrication process and, in many cases, upon properly integrating the materials with the process. In turn, a number of variable characteristics exist with the material itself. Unfortunately, some designers tend to consider dimensional tolerances on plastic products to be complex, unpredictable, and not susceptible to control.

If steel, aluminum, and ceramics were to be made into complex shapes but no prior history on their behavior during processing existed, a period of trial and error would be required to ensure their meeting the required measurements. If relevant processing information or experience did exist, it would be possible for these metallic products to meet the requirements with the first part produced. This same situation exists with plastics. To be successful with this material requires experience with their melt behavior, melt-flow behavior during processing, and the process controls needed to ensure meeting the dimensions that can be achieved in a complete processing operation. Based on the plastic to be used and the equipment available for processing, certain combinations will make it possible to meet extremely tight tolerances, but others will perform with no tight tolerances or any degree of repeatability.

Fortunately, there are many different types of plastics that can provide all kinds of properties, including specific dimensional tolerances. It can thus be said that the real problem is not with the different plastics or processes but rather with the designer, who requires knowledge and experience to create products to meet the desired requirements. The designer with no knowledge or experience has to become familiar with the plastic-design concepts expressed throughout this book and work with capable people such as the suppliers of plastic materials.

Some plastics, such as the TSs and in particular the TS-RP composites, can produce parts with exceptionally tight tolerances. In the compression molding of relatively thin to thick and complex shapes, tolerances can be held to less than 0.001 in. or to even zero, as can also be done using hand layup fabricating techniques. At the other extreme are the unfilled, unreinforced extruded TPs. Generally, unless a very thin uniform wall is to be extruded, it is impossible to hold to such tight tolerances as just given. The thicker and more complex an extruded shape is, the more difficult it becomes to meet tight tolerances without experience or trial and error. What is important is to determine the tolerances that can be met and then design around them.

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