Colour Matching in Injection Molding: Masterbatch, Drift and Approval

Why an approved plaque fails on the part

Colour approval usually happens on an injection moulded plaque, 2 mm thick, made on a lab machine at moderate speed. The production part is 1.4 mm thick, made on a large machine at high shear, with 15% regrind. The colour is not the same, and nobody is lying — the material simply went through a different process.

Pigments are not inert tracers. They are particles that disperse under shear, and how well they disperse changes the colour. Higher shear breaks up agglomerates, which usually increases colour strength and shifts the hue. Thinner walls scatter less light, which changes the apparent lightness. Regrind carries a heat history that shifts the shade.

What ΔE actually means

Table 1 — ΔE and what it means visually. The scale is not linear in perception: 1.0 is roughly the threshold of a just-noticeable difference for a trained observer.

ΔE is calculated in a colour space (usually CIELAB) as the distance between two points. There are several formulas — ΔE*ab, ΔE CMC, ΔE2000 — and they give different numbers for the same pair of samples. ΔE2000 is the most perceptually uniform and is what most automotive and appliance customers specify. Always state which formula you are using.

Where the colour shift comes from

Table 2 — Sources of colour shift between plaque and part. Thickness and let-down ratio are the two biggest, and both are set before the machine is started.

Wall thickness is the one that catches people most often, because it is not intuitively a colour variable. A 1.4 mm part made from the same compound as a 2 mm plaque will measure lighter, because less material means less absorption and more light scattering back out. On a translucent or light colour the difference can be ΔE 1.5 without anything else changing.

Masterbatch and let-down ratio

Most colour is added as masterbatch, a concentrated pellet let down into natural resin. The let-down ratio is the weight percentage of masterbatch in the final compound, and it is the most direct control on colour strength.

Table 3 — Let-down ratios. Above 4% the masterbatch carrier resin starts to affect mechanical properties and processability, not just colour.

Dispersion quality matters as much as ratio. A poorly dispersed masterbatch produces a colour that is weaker than its nominal loading, with visible streaks and specks. The fix is usually a better dispersing screw section or a masterbatch with finer pigment particles — not more masterbatch, which just adds cost and makes the streaks worse.

The practical test for dispersion is a thin film or a microscope slide: press a small sample thin and look through it against a light. Agglomerates show as specks. If they are visible at 50 µm film thickness, they will be visible as colour variation on the part.

Measuring colour properly

Table 4 — What has to be identical for a colour measurement to be comparable. Any one of these being different invalidates the comparison.

The backing issue is specific to translucent and light colours. A white part measured over a black backing reads darker than the same part over a white backing, because light transmitted through the part is absorbed or reflected by what is behind it. For any part thinner than about 2 mm in a light colour, specify the backing.

Correcting a colour that is out

Table 5 — Correcting colour errors. The yellow shift from overheating is the most common and the most often blamed on the masterbatch.

Overheating produces a yellow shift that is easy to mistake for a pigment problem. The polymer itself yellows as it oxidises, and the effect accumulates with residence time and temperature. If a colour was right last week and is yellow this week on the same masterbatch, check barrel temperatures and residence time before calling the supplier.

Setting up a colour control system

Table 6 — Elements of a colour control system. The production standard — a real part, not a plaque — is the element most often missing.

The production standard is the key document. It should be a part from the first approved production run, kept in a dark drawer, labelled with the date, the machine, the masterbatch lot and the measured values. When a dispute arises eighteen months later, that part is the reference — not a memory of what the colour looked like.