Meaning
Statistical calculations compare the actual distribution of part measurements against the specific width of the engineering tolerance limits to determine process stability. A formal cpk capability study evaluates how closely the process mean sits to the center of the design window and whether the spread remains narrow enough to prevent defects. Results indicate the percentage of output likely to fall outside specified upper and lower boundaries during steady state manufacturing.
Centering Metric
Calculations assess the distance between the process average and the nearest limit. In injection moulding, a cpk capability study helps technicians adjust gate timing or pressure settings to move the whole population closer to the nominal dimension. Shifting the mean identifies sources of systematic error in the tool or material feed.
Variation Spread
Standard deviation provides the denominator for this performance index. Wide spreads usually signal inconsistent resin moisture or erratic heater bands during the cycle. A higher value suggests the process is capable of repeating its success without constant manual interventions.
Control Limit
Manufacturers use this index to sign off on tool acceptance before mass production begins. Achieving a index of 1.33 suggests high confidence in the output. Monitoring cpk capability study results over several months identifies when a mould begins to wear or a hydraulic system starts to fail.
