Color Matching and Masterbatch Control in Plastic Packaging Production

Abstract

Color is the first property a consumer judges and one of the hardest to control. Resin, pigment, processing temperature and part geometry all shift the final appearance, and two parts that match under one light can diverge under another. This article explains how color is specified for molded packaging, how masterbatch dosing works, and what controls keep successive production batches visually consistent.

Specifying a Color

A color specification should include a physical standard, a measurement geometry and a tolerance. A printed color reference alone is insufficient because it is defined for ink on paper, not for a molded plastic surface. The practical approach is to approve a molded plaque or an actual production part as the master, record its spectral curve, and define an acceptable color difference in a recognized unit. The tolerance must be realistic for the process: injection molding with regrind and variable cycle times cannot hold the same tolerance as a printed label, and demanding one produces rejections without improving appearance.

Masterbatch and Dosing

Masterbatch is a concentrated pigment dispersion in a carrier resin, dosed into the molding machine at a defined ratio, commonly between one and four percent. Consistency depends on the accuracy of the dosing unit and on the dispersion quality of the masterbatch itself. Poorly dispersed pigment produces streaks and color drift between parts from the same shot. The carrier resin must be compatible with the base resin; a carrier that softens at a different temperature creates uneven distribution and can also affect mechanical properties. Where the color is critical, pre-compounded resin removes dosing variation entirely at a higher material cost, which is often justified for a flagship pack.

Metamerism and Light Sources

Two colored objects can match under daylight and differ under tungsten light. This is metamerism, and it arises when the pigments used have different spectral curves even though their tristimulus values coincide. For packaging it matters because retail lighting is often warm white while photography uses daylight, and an e-commerce image shot under one condition may not match the product the customer receives. The remedy is to measure under at least two defined illuminants and to prefer pigment combinations with similar spectral behaviour rather than relying on a single-condition match.

Process Variables That Shift Color

Melt temperature changes pigment dispersion and can degrade heat-sensitive colorants, shifting hue toward yellow. Longer residence time in the barrel produces the same effect. Mold temperature affects surface gloss, which changes perceived color even when the pigment loading is identical. Regrind reduces opacity and shifts light-colored parts toward gray. Screw wear changes shear history and therefore dispersion. Because these variables interact, a color match approved on one machine is not automatically valid on another, and a production transfer should trigger a re-approval rather than an assumption of equivalence.

Control Checklist

• Approve a molded master part, not a printed reference alone.
• Record spectral data for the master and keep it light-protected.
• Define color difference tolerance appropriate to the process.
• Verify masterbatch dosing accuracy at each start-up.
• Measure under two illuminants to detect metamerism.
• Limit regrind percentage and document it.
• Re-approve color after any machine or resin change.
• Retain a production sample per batch for later comparison.

Approval Workflow From Brief to Production

A workable approval workflow has four gates. First, the brief fixes the target color and the substrate, since the same reference looks different on PP and on PET. Second, a color chip or trial part is produced and measured, with the spectral data recorded. Third, a production-scale trial confirms that the color holds at running temperature and cycle time, which is where most matches fail. Fourth, the first mass-production batch is compared against the approved part under two illuminants, and a retained sample is archived. Each gate is cheap; skipping the third is the most common cause of a batch that matched in the laboratory and does not match on the shelf.

Conclusion

Color control is a chain from master approval through masterbatch dosing to process stability. A molded master, a realistic tolerance, two-illuminant measurement and disciplined regrind control cover most of the risk. The remaining variability is usually traceable to melt temperature or dosing accuracy, both of which can be measured and corrected.

References

• ISO 105-A02:1993, Textiles — Tests for colour fastness — Grey scale for assessing change in colour.
• ISO 7724-1:1984, Paints and varnishes — Colorimetry — Principles.
• ASTM E313-20, Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates.
• ASTM D2244-23, Standard Practice for Calculation of Color Tolerances and Color Differences.
• ISO 9001:2015, Quality management systems — Requirements.
• ISO 178:2019, Plastics — Determination of flexural properties.