As metal parts transition to plastics, we are having more conversations with customers about tolerances. Plastics can be more difficult to achieve tight tolerances than steel. Certain materials are more stable and can achieve tighter tolerances while environmental concerns, such as temperature and moisture, also play a factor in holding tight tolerances.
Acetal materials are fairly stable and +/-.001 tolerances can be held on most dimensions. However, UHMW, being a softer material and greatly affected by heat, require that the tolerances be opened up. UHMW has a coefficient of thermal expansion of .0001/inch/degree which does not seem like much but on a 10’ piece of material that means movement of .012”/ degree of temperature change.
Nylon is fairly stable in a dry environment but in a wet/humid environment it absorbs the moisture and starts to swell. Nylons can hold +/-.002 while machining. Nylon is commonly used in roller applications due to the durability and load bearing capabilities that it has. Many of those rollers have bearings installed in the ends to roll more efficiently. It is important to consider press fit and slip fit tolerances on drawings as parts are converted to plastics from steel. Tolerances for installing bearings in nylon materials should be -.003 to -.01 depending on the size of the bearings. A press fit of -0.01” in UHMW will allow for some material expansion due to heat and still maintain the fit required. However, if you go with too light of a press fit, the part can grow or expand with heat and the pins can start to fall out in service.
Below is a general list of tolerances for different materials that can be held on most dimensions.
|UHMW/ HDPE||+/-.005” (remember temperature change)|
|Teflon||+/-.003” (very soft)|
For more information on achievable tolerances in plastic materials and specific part design, please contact one of our Customer Account Specialists at [email protected].
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