Meaning
Statistical procedure used to confirm that an injection molding process consistently produces parts within specified tolerance limits. In high-volume manufacturing, Cpk verification relies on regular sampling of critical dimensions to calculate the process capability index. This exercise proves that the variance of the process remains narrow enough to avoid producing scrap.
Tool wear or heater band failure will degrade this index over time.
Performance Evaluation
Quality engineers measure a series of molded parts from a production run to assess the stability of the molding press. During Cpk verification, a minimum of thirty consecutive parts are typically measured for critical dimensions such as outer diameter or wall thickness. The resulting data points are plotted to confirm a normal distribution.
If the process shifts, the calculated index drops below the required minimum of one point thirty-three. Technicians monitor these variations closely because even minor deviations in melt temperature can alter the shrinkage rate of the polymer, causing the part dimensions to drift away from the target center-line and threatening the compliance of the entire batch.
Statistical Audit
Mathematical validation of the distribution of measurements ensures that the calculated capability reflects true performance rather than random noise. A thorough Cpk verification requires calculating both the mean and the standard deviation of the sample set against the upper and lower specification limits. Subgroup sizes must be large enough to capture cyclic process changes.
Processors use these audits to approve a mold for production.
Control Limit
Manufacturing contracts often specify the threshold of capability that a mold must maintain throughout its operational life. When Cpk verification reveals a decline in capability, the molding technician must adjust injection pressure or cycle time to restore process control. Uncontrolled thermal fluctuation in the mold steel is a frequent cause of dimensional drift.
Regular validation prevents defective components from reaching the assembly line.
