Color Shift and Discoloration: Mold Trial Review for Buyers

Color problems split two ways: aesthetic mismatch and material degradation. A part that is consistently the wrong shade needs a formulation change or a new color standard. Yellowed zones, brown streaks, or shot-to-shot drift point to heat damage. Degraded polymer is weaker polymer. Separate those two states before you approve.

Identifying the Source of Color Issues

Typical observations: off-color lots, streaks from the gate, yellow or brownish tint, pearlescent marbling, or drift during a run.

Approving “slightly off color” without classifying the cause risks approving degraded material. Yellowing and burn-tinted streaks often mean the resin was overheated or sat in the barrel too long, with hidden embrittlement. Discoloration can also be the start of black specks; both come from thermal degradation.

Thermal degradation: Yellow or brown tint, or burnt streaks, points to high melt temperature or long residence time. Check process settings against the resin manufacturer’s guidelines.

Moisture and splay: Silvery or off-color streaks often overlap with splay from under-dried material.

Mixing and formulation: Uneven color or marbling suggests masterbatch ratio, mixing, or carrier compatibility problems. If the base shade is consistently wrong without streaks, the issue is the color standard or formulation.

Regrind contamination: Off-shade lots often track regrind ratio changes or a contaminated material path.

Hot runner degradation: On hot-runner tools, polymer degrading in a dead spot can cause brown streaks. Eastman’s mold design guidelines warn that molten polymer filling the insulating gap around a hot drop will degrade and show as streaks or specks.

Read the Axis, Not the Adjective

“The color is off” is not a diagnosis. Instrument measurement gives direction. Color difference is measured in CIELAB: L* (lightness), a* (green–red), and b* (blue–yellow). A single ΔE number shows magnitude. The axes show the type of defect:

• ΔL* shift: Lighter or darker. Points to pigment loading or a surface-finish change.
• Δa* shift: Greener or redder. Usually a red pigment dosage or grade issue.
• Δb* shift: Bluer or yellower. A positive b* shift is the classic signature of thermal degradation or long residence time.

A yellowing part and a mismatched part read differently on a spectrophotometer. Asking which axis moved routes the problem to the process engineer or the colorant supplier. Konica Minolta notes ΔE perception thresholds vary: below 1 is generally imperceptible, 1–2 is barely noticeable, and 3 or above is clearly visible in most industries.

Questions to Ask the Supplier

Ask for the process record: melt temperatures, cycle time, drying data, regrind ratio, and colorant lot. If degradation is suspected, request a melt-flow or simple mechanical comparison between affected and good lots.

Ask whether the deviation is uniform across the part and the run, or localized and drifting. Uniform deviation suggests formulation. Drifting or streaked color points to process instability or degradation.

Compare melt temperatures and residence time to the resin maker’s guidance.

Clarify the colorant system (pre-colored resin vs. masterbatch), let-down ratio, and any changes. Confirm an agreed color standard exists (physical chip or instrumented values) and which light source is used for visual judgment.

When to Withhold Approval

Do not concede on streaked or yellowed parts until degradation is ruled out. If color varies within a run and the supplier cannot name the cause, hold approval. Reject remedies that re-color around an uncontrolled process instead of stabilizing the process.

Sourcing and RFQ Guidelines

Define the color standard in the RFQ: reference chips, judging light source, and instrumented tolerances if used. State adjacent-part matching requirements so bidders quote for color control. For color-critical and heat-sensitive resins, ask how residence time and temperature will be controlled.

Use the T1 Sample Review Checklist at trial. Log color events in the Defect Log Template.

Material-side behavior (drying, heat sensitivity) is covered per resin in the materials section. Process-discipline signals are in scientific molding.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, perform material analysis, or certify suppliers. Color and degradation behavior are resin- and process-specific. Verify against the material datasheet and your supplier’s process records.