Meaning
A statistical metric measures the ability of a manufacturing process to produce parts within specified dimensional or weight limits while accounting for process centering. This dimensionless value, known as process capability index Cpk, governs quality assurance approvals in injection molding by showing how close the process distribution lies to the tolerance limits. The index applies to high-volume stable production runs and cannot be calculated for erratic or out-of-control processes.
Statistical Measure
The index is calculated by dividing the distance from the process average to the nearest specification limit by three times the standard deviation of the process. A high process capability index Cpk indicates that the process variation is small relative to the tolerance width and that the average is centered. Most automotive and medical molding programs require a minimum value of 1.33.
Process Alignment
When the average shifts away from the nominal dimension toward either the upper or lower limit, the index value decreases. For example, if a molded part dimension drifts due to tool wear or resin batch variation, the capability index drops even if the overall variation remains small. Molders must adjust process parameters or modify tooling to restore centering.
Sourcing Impact
Sourcing departments use this index to qualify molding vendors and release tool retention payments. A low index indicates a high risk of producing defective parts that will require manual sorting or return shipments. Achieving high process capability allows molders to operate with reduced inspection frequencies.
