Cpk Capability

Meaning

Statistical evaluation provides a measure of how closely a molding process runs to its specification limits. Within injection molding, cpk capability is the metric that accounts for both the process spread and the centering of the process mean relative to the design tolerances. It gauges whether a dimension like a part diameter or wall thickness can be repeatedly produced without defects.

The calculations cease to be valid if the process is not in statistical control.

Process Centering

Machine settings like hold pressure and melt temperature must remain stable to keep the molded part dimensions centered. When these parameters drift, the process mean shifts closer to one of the tolerance limits, which reduces the cpk capability. This shift decreases the margin of safety even if the overall process variation remains small.

Tolerance Comparison

Narrow tolerance limits require a highly controlled injection process to maintain a high capability index. For example, a tight dimension on a medical syringe demands a higher cpk capability than a domestic container. High-precision tooling helps to achieve this by minimizing the physical variation in cavity dimensions across a multi-cavity mold.

Run Variation

Material consistency across batches affects the process capability. Using virgin resin yields a higher and more stable cpk capability than using regrind material. Regrind increases the variation in viscosity, which causes shot-to-shot weight fluctuations that disrupt the process control.

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