When designing parts with tight tolerances it is important to consider the tolerance of the plastic being used. This is one mistake many design engineers make when designing a part for the injection molding process. For example, if the shrink tolerance on a materiel is between .004 and .008 per inch. Then it would make no sence to require your injection molder to hold less than .004 as the range is .004. When tighter tolerances or repeatability is required try choosing a material with low stable shrinkage.
If your project has tight tolerances usually less than .004 per inch or .1 mm. It is advised to leave key dimensions stock safe and later groom those dimensions into tolerance. This will save a lot of money in tooling cost as stock safe changes are much cheaper than adding stock.From Website
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