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What should a buyer approve before using a plastic-part measurement fixture to accept an injection mold?
Approve the drawing revision, measurement state, datum alignment, support and restraint locations, loading instructions, and reload evidence. Then compare identified samples using the agreed method at both locations. A green dimensional report alone does not prove that the fixture is repeatable or that the part was measured in the required state.
Decision checklist
- Assign each critical characteristic to the drawing-defined free-state, restrained or assembled condition.
- Record the fixture revision, contact locations and loading sequence before collecting acceptance data.
- Separate repeat measurements without moving the part from trials that unload and reload it.
- Resolve supplier-to-buyer measurement differences before authorizing fixture or mold acceptance.
- Include the fixture, inspection program and unresolved exceptions in the handover scope.
Start with the condition on the drawing, not the inspection machine
Consider a thin-walled electronics housing with a perimeter sealing surface. This is an illustrative procurement scenario, not a GEERPOWER case study. One inspection team supports the housing at its datum features; another presses its perimeter against a nest. Those reports may answer different questions even when both use the same nominal CAD model.
Ask the design owner which condition applies to each characteristic. A free-state requirement must not be replaced by an assembly-simulating restraint simply because the constrained result looks better. Conversely, an explicitly restrained or assembled requirement needs the specified conditions, not an improvised free-state test. If the drawing is ambiguous, record a clarification before commissioning the fixture.
Hexagon's plastic-lens application describes deformation risks when fragile parts are positioned for inspection. That example supports treating the holding method as part of measurement planning; its lens-specific equipment capabilities are not a tolerance promise for other molded products.
Put the fixture acceptance scope into a one-page agreement
The following is a proposed buyer-side scope table, not a universal metrology standard. Have the responsible quality engineer fill in the drawing-specific method, limits and approval authority. Avoid writing only 'fixture included' on the purchase order.
| Decision | Evidence to request | Hold point |
|---|---|---|
| Part and revision | Ballooned drawing, CAD revision, cavity identification and sample labels | The fixture references a superseded drawing. |
| Measurement state | Required condition for each critical feature and any specified restraint | Free-state and constrained results are mixed without identification. |
| Location and support | Datum mapping, contact diagram, fixture ID and orientation photos | Extra supports or contacts have no documented purpose. |
| Loading method | Operator instructions, restraint sequence and specified settings where applicable | Operators must improvise to obtain a passing result. |
| Instrument access | Probe or optical access plan and inspection-program revision | A contact hides a feature or requires an undocumented setup change. |
| Acceptance evidence | Reload study, comparison samples and agreed decision rules | Only a final pass/fail screenshot is supplied. |
Ask for reload evidence, not just repeated scans
A useful procurement trial distinguishes the instrument repeating a measurement from the complete setup being repeated. First obtain measurements with the part left in place. Separately, ask the supplier to unload and reload identified parts according to the proposed work instruction. Where multiple operators will use the fixture, include that operating condition in the agreed study.
Have the quality team choose representative parts, repetitions, operators and acceptance limits appropriate to the application. Keep the raw results by part and characteristic, including failed or excluded observations with explanations. Do not adopt an arbitrary percentage from an unrelated project, and do not call a small demonstration a completed measurement-system study.
Renishaw describes documented fixture configurations and reproducible setups as features of its metrology-fixturing approach. Our purchasing recommendation is to require a usable setup record, rather than assume that buying modular hardware makes the full measurement process acceptable.
Correlate the supplier and receiving-site methods
Before shipment, agree on a set of identified samples that both teams can measure. Preserve cavity identity, molding date and the applicable conditioning record. Record measurement time and relevant environmental conditions so that disagreements are not silently attributed to the mold.
Compare the drawing interpretation, alignment, fixture revision, software program and measurement method before changing tooling. A color-map image alone is insufficient if one program uses a different alignment or evaluates a different surface region. Ask the two quality teams to document what changed and which dataset is authoritative.
The dimensional-report guide covers drawing-to-result traceability. Fixture acceptance adds a narrower question: can the agreed physical setup and evaluation method be reproduced at the receiving site?
Keep fixture acceptance separate from part and mold acceptance
An accepted fixture is not proof that every part passes. A passing part is not proof that the fixture is suitable. Maintain separate decisions for the measurement setup, the inspected samples and any mold-release milestone. This avoids accepting an unresolved measurement disagreement as a tooling defect, or overlooking a genuine part issue because the fixture is new.
Request an exception list naming the affected characteristic, evidence gap, action owner and release condition. If a deviation is accepted, obtain the authorized concession and its scope rather than silently changing the tolerance or inspection procedure.
- Fixture package: identification, revision, contact diagram, parts list and setup instructions.
- Inspection package: program revision, measurement-state definition, raw results and comparison report.
- Handover package: ownership, shipping protection, replacement-contact requirements and change-control responsibility.
What to send for a GEERPOWER project review
Share the part drawing, resin grade, cavity plan, critical assembly interfaces, intended inspection equipment and destination. Identify whether the quotation must include fixture design, fabrication, correlation support or only mold-trial samples. Ask for each responsibility to be confirmed in writing; those services should not be assumed from an equipment quotation.
This guide provides a procurement framework. It does not certify a measurement system, establish a universal restraint force or replace the drawing authority's inspection requirements.
Common buyer questions
Should every molded plastic part be measured without restraint?
No. Use the condition required by the approved drawing and inspection plan. Free-state, restrained and assembled measurements answer different questions. Obtain a design-owner clarification when the required condition is not explicit.
Is repeated scanning of one mounted part enough to accept a fixture?
No. It does not show what happens when the part is removed and loaded again. Agree on reload and operator trials with the quality team, with sample selection and decision limits suited to the application.
Can a supplier's passing report replace receiving-site correlation?
It can provide evidence, but it does not resolve differences in fixture setup, alignment or evaluation methods. Agree on shared samples and a correlation plan when the receiving site must reproduce the result.
Is a custom fixture always necessary?
Not necessarily. A documented modular setup may be suitable. Select the approach around the required measurement state, critical features, access and operating conditions rather than assuming custom hardware is automatically more accurate.
Sources and references used for this guide
This guide supports early project planning. Final machine selection and commercial configuration must be confirmed from the actual part, mold, material, factory utilities and production target.
