Color Shift and Discoloration: Mold Trial Review for Buyers

Color problems split into two distinct categories: aesthetic mismatches and material degradation. A part that is consistently the wrong shade requires a formulation adjustment or a new color standard. However, yellowed zones, brown streaks, or color drifting shot-to-shot indicate heat damage. Degraded polymer is weaker polymer. Buyers must distinguish between these two states to avoid approving compromised material.

Identifying the Source of Color Issues

Buyers typically observe off-color lots, streaks trailing from the gate, yellow or brownish tints, pearlescent marbling, or drift during a production run.

Approving “slightly off color” without classifying the root cause risks approving degraded material. Yellowing and burn-tinted streaks often indicate the resin was overheated or left in the barrel too long, leading to hidden embrittlement. Discoloration can also signal the beginning of black specks, as both defects stem from thermal degradation.

Thermal degradation: Yellow or brown tinting, or burnt streaks, points to excessive melt temperature or residence time. Verify the supplier’s process settings against the resin manufacturer’s guidelines.

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

Mixing and formulation: Uneven color or marbling suggests issues with the masterbatch ratio, mixing efficiency, or carrier compatibility. If the base shade is consistently wrong without streaks, the problem lies with the color standard or formulation.

Regrind contamination: Off-shade lots frequently correlate with changes in the regrind ratio 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, appearing as streaks or specks.

Read the Axis, Not the Adjective

“The color is off” is not a diagnostic starting point. Instrument measurement provides specific direction. Colour difference is measured in CIELAB, comprising L* (lightness), a* (green–red), and b* (blue–yellow) coordinates. While a single ΔE number indicates the magnitude of the difference, the individual axes reveal the type of defect:

• ΔL* shift: The part is lighter or darker. This points to pigment loading or a change in surface finish.
• Δa* shift: Greener or redder. This usually indicates an issue with red pigment dosage or grade.
• Δb* shift: Bluer or yellower. A positive b* shift is the classic signature of thermal degradation or excessive residence time.

A yellowing part and a mismatched part produce different values on a spectrophotometer. Asking which axis moved routes the problem to the process engineer or the colorant supplier. Konica Minolta notes that 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

When a discoloration issue arises, ask the molder for the process record, including 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 color deviation is uniform across the part and the run, or if it is localized and drifting. Uniform deviation suggests a formulation issue; drifting or streaked color points to process instability or material degradation.

Determine the melt temperatures and residence time the part is experiencing, and compare these to the resin maker’s guidance.

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

When to Withhold Approval

Do not concede on streaked or yellowed parts where degradation has not been ruled out. If the color varies within a run and the supplier cannot identify the root cause, withhold approval. Reject proposed remedies that involve re-coloring around an uncontrolled process rather than stabilizing the process itself.

Sourcing and RFQ Guidelines

Define the color standard in the RFQ. Include reference chips, the judging light source, and instrumented tolerances if applicable. State any adjacent-part matching requirements so bidders quote for color control. For color-critical and heat-sensitive resins, ask suppliers how they intend to control residence time and temperature.

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 covered 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.