injection molding tolerances

[wattsteam]

SPI's AQ102 guide is an excellent, practical-test-derived reference for plastics tolerances although some more modern resins and blends are not included due to the document's age. The tolerances are indexed to parts size, features layout and in some cases gate placement / flow direction. I have found the tolerance data predictions quite accurate compared to real-life findings on actual moulded parts over many years' use.

Best regards Jeff Dayman
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[dgschaefer]

A good rule of thumb estimate I've used in the past was 20% of the shrinkage for a given dimension. ABS is about 0.005" of shrink per inch, so if your part is 10" long that's 0.05" of shrinkage. Your tolerance would be +/-0.01". Polypropylene can shrink about 0.015" (it actually varies a lot), so the same dim in PP would be +/- 0.03". Your molder will be able to tell you what they might be able do to improve on that based on their processes.

Some resins, particularly glass filled, have more shrink variance with the flow direction, which you'll need to account for.

Not precise by any means, but should get you in the ball park.

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