Process capability index (Cpk)

Tolerances and Qualification

Often, the largest concern of mold designers or mold makers is to ensure the parts are within specification. However, there is more to the project than just being “in spec”. For products or parts to be validated, they typically must pass a capability study with a process capability index (Cpk) of 1.33 or 1.67. But what does this mean to the mold designer or mold maker?

It means that the higher the Cpk requirement is, the closer to the nominal dimension the part must measure. An example is a nominal dimension of 246.38mm with a ±0.127mm tolerance and a Cpk of 1.33 (typical for automotive). In this case, the total range of samples must measure between 246.34825mm and 246.41175mm (±0.03175mm).

This means we can only use 50% of the total tolerance band for the parts to be capable. In contrast, if the Cpk requirement is 1.67 (typical for medical), all measured parts must measure no less than 246.3546mm and no greater than 246.4054mm (±0.0254mm), which is only 40% of the total tolerance.