Injection molding does not guarantee that every part will meet a drawing or remain identical across production. Buyers should define the required dimensions and inspection method, then review evidence from identified samples and production conditions. Repeatability is a supplier and process result to verify, not an automatic property of the molding method.
Separate the drawing tolerance from process performance
A drawing tolerance defines the accepted dimensional range for a feature. Repeatability concerns the consistency of results when parts are made and measured under defined conditions. A sample can fall within tolerance without showing whether the process remains stable across cavities, lots or later production runs.
Agree on the measurement plan
Identify the drawing revision, critical features, datum scheme, measurement instrument, conditioning method, units and inspection timing. Use the same method for approval and later comparisons. If a measurement system or setup changes, record the change before interpreting the results.
Identify samples by cavity and production condition
Define a project-specific sampling plan that covers the cavities and production conditions relevant to the release decision. Record the part revision, mold and cavity, resin grade and lot, machine, production date and any approved process change. Compare results by cavity, shot or lot where that distinction can help identify a source of variation.
Review evidence rather than a single capability number
Request the underlying measurements and the method used to summarize them. If a supplier reports a process-capability index, agree the feature, sample period, measurement method, distribution assumptions and acceptance rule before using the number in a decision. A single summary does not replace review of the sample records and production conditions.
Keep corrective actions and approvals traceable
Document the suspected source, the change made, the affected sample group, the resulting measurements and any side effects. Recheck the agreed features before production release. NIST explains that metrological traceability is a property of a measurement result supported by a documented calibration chain, not a label that alone guarantees fitness for purpose.
Autodesk lists material variation, process conditions and machine variation among possible contributors to dimensional variation in its dimensional-variation troubleshooting guidance. For tolerance specification, see DAYIN’s injection molding tolerances guide. For supplier records and project scope, see the process-control guide and the custom injection molding service. NIST’s traceability policy and FAQ explains how traceability relates to measurement results.