Meaning
Statistical indices measure the relationship between the natural variation of a process and the specified tolerance limits. A cpk capability report provides a numerical value that shows how well an injection moulding process stays within the required dimensional boundaries. High values indicate that the part dimensions are centered and far from the upper and lower limits.
Statistical Index
Calculations for this metric require a stable process where the data follows a normal distribution. The cpk capability report uses the mean and the standard deviation of a sample set to predict the likelihood of producing defective parts. Process engineers use the resulting figure to determine if the machinery can reliably hold a one percent tolerance over a long production run.
Dimensional Stability
Variations in resin viscosity or barrel temperature can shift the mean of the process. If the distribution shifts toward one of the limits, the value in the cpk capability report decreases even if the total spread remains the same. Maintaining a centered process is as necessary as reducing the overall variation.
Quality Evidence
Buyers of high precision components often demand these documents as proof of production readiness. A report showing a value below 1.33 suggests that the process is not sufficiently stable for high volume manufacturing.