Stress Whitening on Injection Molded Parts: Buyer Review Checklist

Stress whitening—also known as blush or stress marks—appears as white, cloudy, or pale areas on a molded part. These marks typically show up around ejector pad locations, near the gate, at the base of snap-fits, or where a part was flexed during demolding.

When reviewing samples, buyers often flag these marks, leading to debates with the supplier over whether the issue is purely cosmetic or a structural defect. This guide details how to evaluate stress whitening during sample review and what evidence to demand before approving a tool.

The Mechanics of Stress Whitening

Stress whitening occurs when the polymer matrix is stressed beyond a local yield limit, causing micro-voiding or crazing that alters how light scatters through the material. The mark is internal to the plastic, not a surface residue, meaning it cannot be wiped off.

While sometimes strictly a cosmetic flaw, stress whitening indicates that a specific region of the part has been subjected to high mechanical or shear stress. If this occurs at a loaded feature—such as a living hinge, snap-clip, or structural boss—the whitening may signal compromised mechanical strength. Approving parts with unexplained stress whitening can lead to both cosmetic rejections and field failures.

Identifying the Source of the Stress

When evaluating stress whitening, map the location of the marks to the tool layout and part handling process. The location typically points to the root cause.

Ejection Stress Whitening directly opposite ejector pin locations suggests the part is resisting demolding. This can happen if the part is ejected before it has sufficiently cooled (lacking rigidity), if the ejection force is too high, or if there is insufficient draft on the cavity walls, causing the part to grip the steel.

Gate Blush A hazy halo or pale splay radiating from the gate is often caused by excessive shear stress during the initial fill. This typically points to an undersized gate or an overly aggressive first-stage injection velocity. This is primarily a gate design or process optimization issue.

Demolding Interference Marks at undercuts, clips, or deep ribs may indicate that the feature is deforming over the mold steel during ejection rather than releasing cleanly via a mechanical action (like a slide or lifter).

Assembly and Flexural Stress White lines at the root of a snap-fit that appear after assembly indicate that the feature was flexed past its yield point. This is generally a design-margin issue (the clip is too stiff or the interference is too deep) rather than a molding defect.

Material Sensitivity Some resins (e.g., polypropylene and certain impact-modified grades) exhibit stress whitening much more readily than others. The threshold for whitening depends heavily on the specific polymer blend. Always verify the expected behavior against the material datasheet.

Directing the Supplier Conversation

Rather than accepting “we will adjust the process,” require the supplier to identify the mechanical cause of the whitening.

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

Evidence to Request

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

• Clear photos of the marks, annotated against the tool layout (e.g., identifying which specific ejector pins correspond to the whitened areas).
• The complete process setup sheet (process record) for the sample run, establishing a baseline for any future process adjustments.
• A written cause assessment detailing the supplier’s diagnosis and the intended tooling or process correction.
• If whitening occurs on a functional clip or snap, require mechanical flex testing or assembly trials on the affected samples to verify that structural integrity is maintained.

Approval Gates

Delay tool or production approval if:

• Whitening sits on a designated cosmetic A-surface and the supplier has not demonstrated a physical fix.
• Whitening coincides with a mechanically loaded feature, and no physical testing has been provided to prove the feature retains its required strength.
• The supplier proposes slowing the cycle time or dropping mold temperatures to fix the issue without conducting a root-cause tooling assessment.
• The severity of the marks varies significantly between cavities in a multi-cavity tool.

Identify cosmetic surfaces clearly in your RFQ, and state that stress whitening on these surfaces is a rejectable defect per your inspection standard.

Prepare for sample reviews using the T1 Sample Review Checklist, and track issues using 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 specific 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.