Part tolerances in injection molded parts are one of the most important factors in a finished molded part.

In the plastics industry, there are two levels of part tolerances for injection molding, commercial and fine. The difference between the two is big enough to change the finished part cost. As an example, let’s say the finished part size is a 3″ diameter x 3″ high x 1/8″ wall thickness.  The standard commercial tolerance is +/- .008 for the diameter and the fine tolerance is +/- .004. There are a lot tighter tolerance requirements in injection molding today because of the high-tech machinery that is available.

In the design stages of an injection mold, in order to produce a part, you must note all of the tolerances that are on the part print. The next step is to find out what the material will be for the injection molded part. The shrink rate of the material that will be used varies from plastic to plastic.  For ABS the shrink rate is .005/.007 in. while for Nylon 66 it is .018/.022 in. As you can see, there is a huge difference just between these two materials.  The shrink rate can also be affected by a number of machine process settings that will be discussed in future blogs.

Part design has a lot to do with part shrinkage.  Shrink rates given on injection molding data sheets are given from a molded part usually 6″ x 1″ x .125 thick. Thicker parts will shrink more than thinner parts.  Uneven wall stock also affects part shrinkage. Part designers usually do not take this into account on the initial part design, however, this should be addressed to prevent problems with inconsistent and erratic shrink rates during the injection molding process.

The shrink rate of the plastic must be calculated into the finished steel dimensions.  For example, a part that will have a finished size of 6″ x 4″ x 1/8″ thick injection molded with a plastic that has a shrink rate of .018 in. the steel dimensions for this part will have to be 6.108” x 4.072” x .127” in order for the part to come out to the finished part dimensions noted on the part print. The tolerances on the part come into affect because as mentioned above the factors involved and also the shrink rate given by the plastic manufacturer also has a tolerance on it.

Shrink rates on overmolded parts are sometimes hard to calculate.  The shrink should only be added to the material that is not on the base material. For example, if the base part is 6″ long and the material overmolded is 7″ long with a 1/2″ overhang on each end, then only the 1/2″ should have shrink added to it.