A customer approves a color sample, then rejects the first production run because it “doesn’t look the same.” The color is within specification. The problem is perception, lighting, texture, and all the variables that specifications don’t capture. Color matching in injection molding involves science, measurement, and process control, but it also requires managing human perception and expectations. Understanding both dimensions prevents the disputes that damage customer relationships.
How Plastic Gets Its Color
Color in plastic comes from additives that selectively absorb and reflect light. Two categories of colorants create different effects.
Pigments are insoluble particles dispersed throughout the polymer matrix. They scatter light, creating opacity. Organic pigments (carbon-based compounds) provide bright, clean colors but may have limited heat stability. Inorganic pigments (metal oxides and other mineral compounds) offer excellent stability but sometimes duller colors. Pigment particle size affects both color intensity and dispersion difficulty.
Dyes dissolve into the polymer, becoming part of the matrix rather than dispersed particles. They allow light to pass through, creating transparency or translucency with color. Dyes typically provide brighter, more vivid colors than pigments but may migrate over time or have reduced lightfastness.
The polymer matrix itself affects perceived color. The same red pigment looks different in polypropylene versus polycarbonate because the resins have different optical properties. Refractive index, natural color (some polymers have inherent yellow tint), and crystallinity all influence how colorants appear in the final part.
Colorant loading affects both color intensity and cost. Higher pigment concentrations produce stronger colors but increase material cost and may affect processing or mechanical properties. Finding the minimum effective loading balances color requirements against economics.
Masterbatch Approach
Masterbatch is concentrated colorant in a carrier resin, typically loaded at 20 to 40 percent pigment. This concentrate blends with natural (uncolored) resin at the press, typically at 2 to 4 percent let-down ratio, producing the final color.
Advantages of masterbatch:
Inventory flexibility allows maintaining stock of natural resin and multiple masterbatch colors rather than pre-colored versions of each color. This reduces inventory investment and obsolescence risk. Color changes happen at the press by switching masterbatch, often without full purging between colors.
Shorter lead times result from masterbatch availability. Stock colors ship quickly; custom colors require compounding but typically faster than pre-colored resin. Small batches are economical because minimum orders are smaller than pre-colored alternatives.
Disadvantages of masterbatch:
Dispersion challenges arise because mixing occurs in the injection machine’s plasticating unit, which isn’t optimized for distributive mixing. Inadequate dispersion creates color streaks, specks, or inconsistency. Screw design affects mixing quality; general-purpose screws may not disperse masterbatch as well as mixing screws.
Metering accuracy requirements increase because color consistency depends on precise let-down ratios. Volumetric feeders estimate masterbatch addition by volume, but bulk density varies between masterbatch pellets and natural resin. Gravimetric feeders provide weight-based metering for better consistency but add equipment cost.
Carrier resin compatibility matters because the masterbatch carrier should match the base resin. Polypropylene masterbatch in polystyrene creates compatibility issues. Universal carriers exist but may compromise properties or introduce contamination concerns.
Pre-Colored Resin Approach
Pre-colored resin has colorant compounded into the polymer by the resin supplier or a custom compounder. The material arrives ready to mold with no at-press mixing required.
Advantages of pre-colored resin:
Consistent dispersion results from compounding equipment designed for mixing. Twin-screw compounders distribute colorant uniformly throughout the melt. Color consistency within lots is typically superior to masterbatch approaches.
Simplified processing eliminates dosing equipment, metering errors, and dispersion concerns. The operator loads colored pellets like any other material. Process troubleshooting involves fewer variables.
Property optimization allows the compounder to adjust formulation for the specific application. Colorant selection considers the processing conditions, UV exposure requirements, and any special needs the application presents.
Disadvantages of pre-colored resin:
Minimum order quantities typically start at 500 kg or more, sometimes much higher for cost-effective pricing. This requires volume commitment and creates obsolescence risk if color requirements change or demand doesn’t materialize.
Longer lead times for custom colors require planning. Standard colors may ship quickly, but custom matches require compounding time, often 2 to 4 weeks.
Inventory carrying costs increase when multiple colors require separate stock. Each color SKU requires storage space and ties up working capital.
Color changes require purging between different pre-colored materials, consuming time and material compared to masterbatch color changes within the same base resin.
Achieving Color Match
Getting colors to match between samples and production involves multiple variables beyond colorant selection.
Lab matching versus production matching creates a common frustration. Colors matched on small samples in controlled lab conditions may not reproduce exactly in production parts. Differences in processing temperature, cooling rate, and part geometry affect final appearance. Production qualification samples should represent actual molding conditions.
Metamerism causes colors that match under one light source to differ under another. Two samples can appear identical under fluorescent lighting but noticeably different in daylight. This phenomenon occurs when different pigment combinations produce the same perceived color through different spectral reflectance curves. Specifying viewing conditions (light source, typically D65 daylight) in color standards reduces metameric disputes.
Texture impact changes perceived color significantly. The same colorant looks different on polished, matte, and textured surfaces because surface structure affects how light reflects. Color matching must specify the surface finish; matching to a polished chip then molding textured parts guarantees disappointment.
Part thickness affects color, especially with translucent or semi-transparent formulations. Thicker sections appear darker because light travels through more colored material. Wall thickness variation within a part can create visible color differences even with perfectly uniform colorant dispersion.
Specification and Tolerance
Color specifications attempt to quantify what humans perceive qualitatively. This translation creates both clarity and confusion.
Delta E (ΔE) measures total color difference in the CIELab color space, combining differences in lightness, red-green, and yellow-blue axes. Lower values indicate closer matches.
| ΔE Value | Interpretation |
|---|---|
| < 0.5 | Imperceptible difference |
| 0.5 to 1.0 | Very slight, trained observer may notice |
| 1.0 to 2.0 | Noticeable to trained observer, acceptable for most applications |
| 2.0 to 3.5 | Noticeable to average observer |
| > 3.5 | Obvious difference |
Typical commercial specifications require ΔE below 1.0 for tight matches (automotive, consumer electronics) or below 2.0 for standard commercial applications. These tolerances represent good color control but do not guarantee visual acceptance in all viewing conditions.
Visual standards supplement numerical specifications. A physical reference sample, stored properly to prevent fading or yellowing, provides the ultimate visual comparison. Matching to the standard under specified lighting (light booth with controlled illumination) adds another layer of agreement.
Why numerical specs don’t eliminate disputes: Human color perception is complex and varies between individuals. Acceptable ΔE values in measurement may still produce “doesn’t match” reactions from customers who see parts in different lighting or compare to different references. Context affects perception: a color that looks right alone may look wrong next to a slightly different component.
Practical tolerance setting balances capability against requirements. Tighter tolerances cost more (better process control, more inspection, higher reject rates). Understanding what the application actually requires prevents over-specification that adds cost without value.
Troubleshooting Color Issues
Systematic diagnosis identifies color problem sources faster than random process adjustments.
Batch variation in masterbatch or pre-colored resin appears as shift between production lots. Compare incoming material to retained standards. If variation exists in material, work with the supplier. Document lot numbers to track correlations.
Inadequate mixing manifests as color streaks, inconsistent intensity, or specks of concentrated colorant. Increasing back pressure, adjusting screw speed, or extending residence time improves mixing. Screw design changes may help persistent problems. Masterbatch applications are more susceptible; pre-colored resin rarely shows mixing issues.
Temperature effects change color because some pigments are temperature-sensitive. Over-processing (excessive temperature or residence time) can cause yellowing, color shift, or pigment degradation. Compare parts made at different temperature settings to identify sensitivity.
UV degradation causes color change during use rather than manufacturing. If parts fade or shift during outdoor exposure or under fluorescent lighting, the colorant system needs UV stabilization. This is a formulation issue requiring colorant supplier involvement.
Contamination from previous colors creates streaks or off-color tones. Thorough purging between colors, especially going from darker to lighter colors, prevents this. Some color sequences are particularly problematic: dark blue preceding white, red preceding yellow.
Color matching seems simple until the first dispute. Clear specifications, realistic tolerances, and consistent processes prevent the arguments that strain customer relationships. Investing in proper measurement equipment, controlled viewing conditions, and documented standards pays back through reduced rejections and faster problem resolution.
Sources
- Society of Plastics Engineers. “Color Technology for Plastics Applications.”
- Plastics Decorating Magazine. “Color Matching Fundamentals.”
- ColorMatrix. “Color Measurement and Specification Guidelines.”
- Plastics Technology. “Troubleshooting Color Problems.” https://www.ptonline.com/
- ASTM E308. “Standard Practice for Computing Colors of Objects.”