Statistical Process Control Subgrouping Strategies for Multi Cavity Injection Molding
Rational subgrouping in multi-cavity molding must isolate between-cavity mold offsets from shot-to-shot press drift to prevent control chart desensitization.
Cavity-Difference Chart
Meaning
Statistical process control tools that track the relative variation between individual impressions in multi-cavity tooling isolate systematic mold imbalances from common-cause machine instability. A cavity-difference chart plots the dimensional or mass deviation of each cavity against the subgroup mean across sequential production shots. The mechanism isolates runner imbalances, thermal gradients, and gate erosion across the physical mold layout.
Tooling-level variation marks the operational boundary of the method, leaving machine-wide pressure drops and barrel temperature shifts to standard subgroup control charts.
Variation Isolation
Measurement protocols record key characteristics across every impression during prototype validation runs. By subtracting the shot average from each impression measurement, the cavity-difference chart filters out overall machine drift such as hydraulic pressure fluctuations or resin viscosity changes. The resulting traces display pure cavity-to-cavity offsets that reveal machining discrepancies or unbalanced gate diameters.
Toolmakers inspect these offsets to adjust steel dimensions before authorizing hardened production steel.
Readiness Audit
Multi-cavity validation requires proof that every impression produces parts within engineering tolerances before ramp-up. During formal qualification runs, the chart exposes whether an individual cavity produces out-of-spec parts while the overall shot average remains centered. Approving multi-cavity tooling based strictly on shot-level averages hides severe dimensional spread among individual impressions.
When parts enter downstream automated assembly, this hidden spread causes intermittent feeding jams and high scrap rates.
Corrective Steelwork
Persistent offsets across production runs direct localized electrical discharge machining or gate sizing modifications on specific tool inserts.