Color Masterbatch and Additive Selection Guide

Coloring a molded part looks like the simplest decision in the whole project: pick a color, add a pellet, done. In practice, the masterbatch choice touches mechanical properties, cycle time, warpage, surface finish, food-contact compliance, and how repeatable the color is from lot to lot.

Figure 1. Carrier compatibility table and let-down ratio chart for color masterbatch.

This guide explains how masterbatch is specified, what the carrier and let-down ratio actually control, and how coloring affects the mechanical properties you have already designed around.

The three ways to color a part

Masterbatch dominates because it gives reasonable cost, good dispersion, and a dosing ratio you can adjust at the press. Everything below assumes masterbatch.

What is in a masterbatch

A masterbatch pellet contains four things:

1. Pigment or dye — the colorant itself.
2. Carrier resin — the polymer the colorant is dispersed in.
3. Dispersing agent / wax — helps wet out and separate pigment particles.
4. Additives — UV stabilizers, antioxidants, flame retardants, or processing aids that are co-supplied.

The carrier is the part that causes most field problems, because it must be compatible with the base resin.

Carrier compatibility

Two consequences follow. First, always specify the carrier resin explicitly on the masterbatch datasheet, not just "universal carrier". A universal carrier is a compromise that will be worse than a matched one. Second, visual acceptance is not compatibility. A part can look perfect at the press and fail a drop test three weeks later because the carrier created weak weld lines.

Special case: POM and PVC are the two families where carrier mismatch is genuinely dangerous rather than merely detrimental. POM with an incompatible carrier can degrade and release formaldehyde; PVC with the wrong carrier can dehydrochlorinate.

Let-down ratio

Let-down ratio is the ratio of masterbatch to natural resin, usually expressed as a percentage by weight.

Rules that matter:

• Higher let-down is not simply "more color". Above a certain loading, pigment agglomerates form, dispersion worsens, and mechanical properties degrade.
• Dosing accuracy is a real variable. Gravimetric dosing holds ±1% of setpoint; volumetric dosing drifts with pellet shape and settles over a shift. For color-critical work, use gravimetric.
• Verify the let-down ratio against the masterbatch supplier's dilution curve, which relates loading to the achieved color and to opacity. Opacity in particular is not linear.
• Keep the total additive loading in mind. If your masterbatch carries 2% of a UV stabilizer package at 4% let-down, you are adding 0.08% stabilizer — check that against the stabilizer's own requirement.

Dispersion: where quality is won or lost

Dispersion is how finely and evenly the pigment is distributed in the matrix. Poor dispersion is the root cause of:

• Color streaks and flow lines in the part
• Specks and agglomerates visible on the surface
• Reduced impact strength, because agglomerates act as crack initiators
• Inconsistent color between shots and between machines
• Filter pack blocking in downstream processes

Dispersion depends on shear history. That means it depends on screw design, back pressure, melt temperature, and residence time — not only on the masterbatch. If the color is inconsistent, check the press before blaming the masterbatch:

1. Increase back pressure modestly and check whether dispersion improves.
2. Confirm melt temperature is within the resin's range (a cold melt will not disperse).
3. Reduce excessive screw speed — very high screw speed can shear-degrade the carrier.
4. Check for dead spots in the nozzle, hot runner or sprue.
5. For very fine color, use a masterbatch with a lower-melting carrier wax.

Effect on mechanical properties

Coloring is never mechanically neutral. The size of the effect depends on pigment type and loading:

Test the actual color at the actual loading on a real part. A drop test on natural resin is not evidence that the colored part will pass.

Special caution — laser welding. Carbon black and many dark pigments transmit or absorb laser energy differently. If the part will be laser welded, the colorant package must be specified for laser transmission on the transmissive side and absorption on the absorbing side. This is a joint design decision, not a purchasing decision.

Special caution — food contact and medical. Only pigments and carriers with the relevant regulatory listing may be used. Ask for the migration and compliance documentation for the specific masterbatch, not the resin.

Color matching and control

• Match under the right illuminant. Specify D65 for daylight matching and check metamerism — the risk that a match under one light looks wrong under another.
• Customers see parts, not plaques. Approve the color on a production part with the production texture, not on a flat molded chip.
• Set a color tolerance before production, with a number, not a verbal description. Spectrophotometric ΔE limits are the practical tool.
• Define the acceptance procedure: instrument, illuminant, aperture, and the number of parts measured.
• Freeze the masterbatch source. Changing masterbatch supplier mid-program almost always changes the color, even if the pigment code matches.

Additive masterbatch worth considering

Masterbatch is also a convenient way to dose functional additives:

Dosing additives through the same masterbatch as the color is convenient but locks the two together. If the color and the additive need independent adjustment, use separate masterbatches.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Base resin and masterbatch carrier explicitly matched and stated on the datasheet
• Let-down ratio taken from the supplier's dilution curve, not assumed
• Gravimetric dosing used for color-critical parts
• Color approved on a production part, under a defined illuminant
• Color tolerance expressed numerically before production starts
• Mechanical properties verified on colored, conditioned samples
• Laser welding, food contact, medical or flame requirements addressed in the colorant package
• Regrind percentage and its color effect accounted for
• Masterbatch source frozen for the life of the program
• Dispersion troubleshooting sequence defined at the press

Masterbatch is a small percentage of the part and a large percentage of its perceived quality. Match the carrier, control the let-down, verify the color numerically, and re-test the mechanical properties that the color quietly changed.