Stress Whitening on Injection Molded Parts: Buyer Review Checklist

Stress whitening (blush or stress marks) shows as white, cloudy, or pale areas on a molded part. Marks usually appear around ejector pads, near the gate, at the base of snap-fits, or where a part was flexed during demolding.

At sample review, buyers flag the marks and then argue with the supplier about cosmetic vs structural. Decide which it is with location, evidence, and testing before you sign the tool off.

The Mechanics of Stress Whitening

Stress whitening happens when the polymer is stressed past a local yield limit. Micro-voiding or crazing changes how light scatters. The mark is inside the plastic, not a surface residue. You cannot wipe it off.

Sometimes it is only cosmetic. It still means that region saw high mechanical or shear stress. On a loaded feature (living hinge, snap-clip, structural boss), whitening may signal compromised strength. Approving unexplained stress whitening can produce both cosmetic rejects and field failures.

Identifying the Source of the Stress

Map marks to the tool layout and part handling. Location usually points to the cause.

Ejection Stress Whitening opposite ejector pins suggests the part resists demolding: ejected before it is rigid, ejection force too high, or insufficient draft so the part grips the steel.

Gate Blush A hazy halo or pale splay from the gate often comes from excessive shear on initial fill: undersized gate or aggressive first-stage injection velocity. That is mainly a gate design or process issue.

Demolding Interference Marks at undercuts, clips, or deep ribs may mean the feature is deforming over mold steel during ejection instead of releasing through a slide or lifter.

Assembly and Flexural Stress White lines at a snap-fit root that appear after assembly mean the feature flexed past yield. That is usually a design-margin issue (clip too stiff or interference too deep), not a molding defect.

Material Sensitivity Some resins (for example polypropylene and certain impact-modified grades) whitening more readily than others. The threshold depends on the polymer blend. Check expected behavior against the material datasheet.

Directing the Supplier Conversation

Do not accept “we will adjust the process” without a named mechanical cause.

• Isolate the timing: Does whitening occur during ejection, or only after post-molding handling and assembly?
• Evaluate the process parameters: What ejection speed, force, and cooling times produced the samples? Is there margin to increase cooling or slow ejection without blowing the quoted cycle?
• Assess the tooling: Would more draft, polished ejector faces, or larger ejector contact area reduce the stress concentration?
• Review the gate: Are gate size and fill speed appropriate for the shear sensitivity of the resin?

Evidence to Request

Do not approve a tool on verbal assurances that marks will disappear in production. Request:

• Clear photos of the marks, annotated against the tool layout (which ejector pins map to which whitened areas).
• The complete process setup sheet for the sample run, as a baseline for later adjustments.
• A written cause assessment with the supplier’s diagnosis and intended tooling or process correction.
• If whitening is on a functional clip or snap, mechanical flex testing or assembly trials on the affected samples to show retained strength.

Approval Gates

Hold tool or production approval if:

• Whitening sits on a designated cosmetic A-surface and the supplier has not shown a physical fix.
• Whitening coincides with a mechanically loaded feature, and no physical testing proves required strength.
• The supplier proposes slowing cycle time or dropping mold temperatures without a root-cause tooling assessment.
• Severity varies significantly between cavities in a multi-cavity tool.

Identify cosmetic surfaces in your RFQ, and state that stress whitening on those surfaces is rejectable per your inspection standard.

Prepare for sample reviews with the T1 Sample Review Checklist, and track issues with the Defect Log Template.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, perform engineering diagnosis, or certify suppliers. Whether a whitening issue is cosmetic or structural depends on the part, material, and tool. Confirm root causes with your supplier and validate structural integrity with appropriate mechanical testing.