Injection molding shrinkage variation is a difference in measured part dimensions across shots, cavities, production runs or material lots. A nominal mold dimension alone cannot explain that variation. Buyers and suppliers need a consistent measurement method and production records that show which material, cavity, tool revision and process produced each sample.
Define the dimension and measurement method
Start with the drawing revision, critical dimension, datum scheme, gauge or measurement system, units, conditioning requirements and inspection timing. Use the same method for the comparison samples. Record the part identification and cavity number. If the measurement system, part conditioning or inspection method changes, note that change before interpreting a dimensional trend.
Separate the sources of variation
Compare results by cavity, by shot within a run, and by production lot or date. Then check whether the samples share the same resin grade and lot, color or pigment, material preparation, machine, mold revision and approved process. Dimensional drift that follows one cavity points to a different investigation than drift that follows a material lot or appears across all cavities.
Autodesk identifies material variation, moisture, pigmentation, inconsistent packing, mold or melt temperature variation, machine condition and a narrow molding window as possible contributors to dimensional variation. These are investigation paths, not a universal diagnosis. A worn check ring or an unstable controller, for example, should be confirmed from the machine and process evidence.
Compare records before changing the tool
Review the approved setup and available records for fill and packing, mold-temperature distribution, cycle stability, material changes, maintenance and tool condition. Check the locations of the measured features against the part geometry and gate or cavity layout. Use a project-specific sampling plan that represents the production conditions under review rather than treating one sample as proof of long-term capability.
Validate corrective action against the drawing
When evidence supports a change, document the condition changed, the sample group, the measured result and any side effects. Recheck all agreed critical dimensions and functional requirements, not only the dimension that triggered the investigation. Keep the measurement equipment and calibration records appropriate to the inspection decision; NIST’s guidance explains the role and limits of metrological traceability.
There is no single cavity arrangement, temperature or pressure setting that controls shrinkage variation for every resin, part and mold. For dimensional requirements, see DAYIN’s injection molding tolerances guide. For measurement and process records, see the supplier process-control guide and the custom injection molding service.
Technical references: Autodesk’s dimensional-variation troubleshooting guidance and NIST’s metrological traceability policy and FAQ.