Masterbatch Dosing: Correct Calculation for Color Consistency in Production

The question of how much masterbatch to add seems simple — until a batch fails color inspection and the investigation reveals the dosing was never formally calculated, just estimated by feel. Dosing is the single most controllable variable in plastic coloring, and it is the one most often managed empirically.

The let-down ratio: where every dosing calculation starts

The let-down ratio is the proportion of masterbatch mixed with base resin to achieve the target color. It is expressed as a percentage by weight and depends on two variables: the pigment concentration in the masterbatch (declared on the technical datasheet) and the color intensity required in the finished part.

A masterbatch with 40% pigment concentration requires half the dosing of one with 20% concentration to achieve the same color depth, all other process variables equal. But declared concentration alone does not predict actual tinting strength in production — dispersion quality inside the concentrate determines how efficiently that pigment concentration translates into color in the finished part.

Why 0.3% dosing variation can fail a color inspection

Pigments do not have a linear relationship between concentration and perceived color. In high-saturation colors, a 0.3% dosing variation can produce a ΔE greater than 2.0 units — clearly visible to the naked eye under standard illumination. In pastels and whites, the same variation may produce a ΔE below 0.5, effectively invisible.

This means the required level of dosing control depends on the target color, not a fixed universal tolerance. A red at full saturation demands much tighter dosing control than the same red diluted to a pastel. Define your ΔE acceptance criterion first, then work backward to specify the dosing system precision required to meet it.

Gravimetric vs. volumetric dosing: precision where it matters

Volumetric dosing systems (auger feeders) deliver a fixed volume of masterbatch per cycle. Their precision is ±5% to ±10%, acceptable for low-specification applications but insufficient for color-critical production. Gravimetric systems measure the mass of masterbatch delivered per cycle and adjust feeder speed in real time to compensate for density variations. They achieve precision of ±0.5% or better.

For any application with contractual color consistency requirements, gravimetric dosing is the correct specification. The best-formulated masterbatch on the market cannot compensate for an inaccurate dosing system.

Establish base dosing by machine trial: Produce control plaques at the datasheet-recommended dosing, then adjust in ±0.5% steps until ΔE vs. approved standard is below acceptance threshold. Document this as the controlled process parameter.

Revalidate when material changes: A new resin lot, a different supplier, or a changed virgin/regrind ratio all require dosing revalidation before volume production — even if the masterbatch lot is unchanged.

Never adjust dosing empirically during a run: In-run color variation is rarely caused by masterbatch dosing if the system is gravimetric and calibrated. Investigate temperature profile, residence time, and regrind proportion before touching dosing.

Account for regrind pigment contribution: Regrind already contains masterbatch from the original run. Adding it to virgin resin plus fresh masterbatch increases effective pigment concentration above the calculated dosing. Fix regrind percentage, keep it constant, and adjust fresh masterbatch dosing to compensate.