Injection Moulding Quality: Tolerances, Inspection and Acceptance

Define the critical dimensions, inspection method and acceptance criteria before ordering. Nordmould assesses the proposed production route for your drawing; one tolerance or certificate does not apply to every part. Agree samples, reports and correction scope in the quotation.

Start with function, then define the drawing

List what the part must do in its assembly: locate, seal, slide, snap, carry load or protect another component. Identify the surfaces and dimensions that control those functions and define a datum system that reproduces the intended assembly. Applying a tight general tolerance to every dimension can add inspection and tooling work without protecting the actual fit.

Provide a released drawing alongside the 3D model. Resolve conflicts between drawing and CAD before quoting, and distinguish an actual acceptance limit from a reference dimension. State the exact resin grade, reinforcement, colour and any material evidence needed. A polymer-family name cannot establish dimensional behaviour, chemical resistance or regulatory suitability for a finished application.

Agree how each characteristic will be measured

For a critical dimension, record the instrument or method, fixture/restraint, datum alignment, units and conditioning state. Agree the time and environment between moulding and measurement where these affect the result. Flexible features can change under measuring force; moisture-sensitive materials can change after conditioning. Comparing reports without matching conditions may create an apparent defect or hide a real difference.

Ask whether a full drawing report or selected-feature inspection is required. Define sample quantity, cavity coverage and when samples are taken. A first-article report, an ongoing production sample and a capability study answer different questions; ordering one does not automatically include the others.

An illustrative inspection decision

This fictional example uses feature 17: diameter 10.00 ± 0.05 mm, acceptable range 9.95–10.05 mm. One reading from cavities 1, 2, 3 and 4 is respectively 10.01, 9.99, 10.08 and 10.00 mm. Cavities 1, 2 and 4 pass this characteristic; cavity 3 fails. Although the average is 10.02 mm, approval based on that average would hide the failing cavity.

Record the actual readings, cavity and sample identifiers, then hold the affected release for review. Four readings are a teaching example, not a production sampling plan, proof of process capability or an instruction to release the other cavities without investigation.

Judge appearance and assembly against agreed evidence

Mark cosmetic faces, gate and ejector locations, permitted witness marks and any texture or gloss reference. Agree lighting, viewing distance and orientation when judging appearance. Retain an approved sample or controlled photographs with a clear revision; an unlabelled image is a weak acceptance standard.

Test the interfaces that dimensions alone do not establish, using representative mating parts and defined conditions. Record the method and result for fit, movement, retention or sealing where required. DFM helps identify manufacturing concerns but does not replace product testing, validation or certification.

Release, hold and repeat orders

The release record should identify drawing revision, material, sample report, open issues, decision-maker and date. Missing evidence or a failed requirement remains open until corrected or explicitly accepted by an authorised customer decision. A concession needs a defined quantity or expiry and follow-up; it must not quietly become a permanent tolerance change.

For repeat orders, agree the control plan, traceability, retained reference samples and change-notification process. Tool repairs, a different material grade, a moved process or changed equipment may require renewed assessment. State the triggers and evidence rather than assuming the first approval covers every later configuration.

Prepare a reviewable quality request

Use the sample-approval pack to record cavity measurements, cosmetic and assembly checks, corrections and the release decision. Read T1 sampling, FAI/PPAP and DFM, then attach your drawing and required inspection scope to the project enquiry.

References and limits

Protolabs: injection moulding tolerances discusses dimensional influences; its published tolerances are specific to its service. Protolabs inspection options distinguishes report types. Neither establishes Nordmould capability or an applicable certificate. Request the proposed site's certificate, scope and validity when your project requires certification.