Acceptable Tolerances in Injection Molding

Injection molding is a widely used manufacturing tool for producing plastic parts. It is often used for parts consisting of multiple steps or assembly during the final production phase, which requires perfect alignment between each part. This alignment is controlled by dimensional tolerances. Failure to meet required tolerances can result in defects, lost material, and assembly failure.

While dimensional tolerances are critical in production, they are scientific in nature. A tolerance must be assigned based on standard guidelines such as the type of resin, the size of the part, the nature of the dimension (concentricity vs flatness, for example), and the quality of the mold. A part can not meet a requirement beyond what is possible based on the design of the part or the resin selected.

A higher-class mold or a different resin may be required if a tighter acceptable tolerance is necessary than would generally be achievable. Some tolerances are not reasonable, no matter what approach is taken. These limitations will typically be identified during the Design For Manufacturability (DFM) review.

The two most significant drivers of acceptable tolerances are the resin’s shrink rate and the part’s size. Resins are chemical products; the lower the shrink, the more stable the process and measurements. However, as the part’s size increases, stability decreases, making the tolerance range more open.

Using the charts below, let’s compare the acceptable tolerance of a part made of the same resin but vastly different in size.