Which Factors Determine Molded Part Accuracy?
- December 27, 2021
- Protoshop
You may have wondered which factors determine molded part accuracy and therefore the tolerances of the injection molded parts from both prototype and production molders. It’s a complex question that I will try to explain. Some factors that are within the molder’s control and some are not. The following factors determine molded part accuracy during mold fabrication and molding:

Material selection and shrinkage rate
Molding machine
Molding environment
Molding environment such as the temperature and humidity around the molding machine during molding. If the environment is not controlled part geometry can vary by up to 0.003” using the same molding process compared to a day running with ambient temperate of 75 deg F and low humidity. Therefore, it’s critical to select a molder that has a controlled environment around their molding machines. There are many molders that do not have a controlled
Mold fabrication equipment
Cutting tool

Discussion and Conclusions
Since material selection and shrinkage are the most significant factor towards determining molded part accuracy and tolerance, they deserve further discussion. The details of material selection are the subject of a separate article since there are many factors that need to be considered. Injection molded plastics fall within two categories: amorphous and semi-crystalline. Amorphous materials include polycarbonate, ABS, HIPS, etc.
Semi-crystalline materials include polypropylene, polyethylene, etc. One of the key differences between amorphous and semi-crystalline materials is the shrinkage rate. Semi-crystalline materials typically have shrinkage rates that are 2-3 times greater than amorphous materials. We have already examined how semi-crystalline materials such as polypropylene have a relatively large range of shrinkage and how it doesn’t take a large difference between the estimated and actual shrinkage to create an inaccuracy that creates a bias in the part being either too large or too small compared to nominal that can easily exceed the tolerance. Therefore, the most part accuracy occurs by selecting an amorphous material which allows for tighter tolerances to be defined.
The estimation of shrinkage rate for the mold design can be further complicated if the material to be used contains an additive. Many times, the additive will alter the shrinkage, sometimes significantly such that it can be difficult to estimate accurately. In that case, the approach would be to run the material in an existing mold that has similar size and geometry. Measurements of the molded parts can then be used to determine the shrink rate of the material in that mold. Although the shrink rate would not be an exact estimation of material shrink rate, it is close enough for use in designing a new mold.
As we’ve discussed there are several factors which determine molded part accuracy. However, material selection along with the estimation of the shrinkage rate during mold design are by far the most significant and require the most attention.