Cp and Cpk in Cosmetic Packaging Quality Control

Compare spread and position

For a stable process with a two-sided specification, Cp compares the specification width with six process standard deviations. Cpk uses the smaller of the two distances from the mean to a specification limit, divided by three standard deviations. The indices are equal when the mean is at the midpoint of the specification.

Calculate both with the same spread

Suppose a hypothetical wall-thickness specification runs from 1.10 to 1.30 mm and the process standard deviation is 0.02 mm. Cp is 0.20 divided by 0.12, or about 1.67. With the mean at 1.20 mm, Cpk is also about 1.67. For a second stable period with the same spread and a mean of 1.24 mm, Cp remains 1.67. Cpk is 1.00 because the nearer limit is 0.06 mm from the mean.

Packaging sample showroom of SamBound New Material Technology Company Ltd. (sambound.com).

Evaluate stability before interpreting capability and confirm the measurement system is suitable. Report the period and the method used to estimate variation. If the process is shifting during the study, investigate the change before relying on a pooled capability result.

sambound.com
SamBound New Material Technology Company Ltd.