There are two main ways to color a polymer part, and they are not interchangeable. Liquid color is metered into a liquid resin before it cures. Masterbatch is melted into a thermoplastic during molding or extrusion. Pick the one that matches how your process introduces color, and the color goes in cleanly. The format follows the process.
What masterbatch is
Masterbatch is pigment pre-compounded into a solid pellet carrier resin. The carrier is matched to the polymer you are running, so the concentrate melts and disperses with the base resin rather than fighting it. The pellets feed into the press or extruder alongside the natural resin, melt together in the barrel, and color the part as the screw conveys the melt to the cavity or the die.
It is a solid-state operation at the machine. Pellets meter in dry, the part comes out colored, and there are no liquid lines to prime, purge, or clean. For molders and extruders, that means predictable feeding and clean changeovers, with color dispersed in the melt rather than added wet. This is the format for TPU and other thermoplastics, which are colored as a melt during injection molding or extrusion.
What liquid color is
Liquid color is pigment dispersed in a liquid carrier, metered into a liquid resin before it cures. It is built for low viscosity, so it pours, meters, and blends into a thin resin and develops color in the mix. It is the format for cast polyurethane, PU and epoxy resin systems, flexible PVC and plastisol, and PU foam, where color is added in the liquid phase. The carrier ends up in the cured part, so it is matched to the chemistry to keep its effect on the part properties small.
Why the two do not swap
The difference comes down to the state of the resin when color goes in. A cast PU system is a liquid that cures once. You color it before it sets, with a liquid that meters into the pour. A thermoplastic is solid until you melt it, takes shape, and can be melted again. You color it as a melt, with a concentrate that melts alongside it.
Put liquid color into a thermoplastic melt and the carrier flashes off or causes feeding and slippage problems at the screw. Put solid masterbatch pellets into a fast-curing cast pour and they will not melt or disperse in the working window. Each format is built for the phase its process runs in. So the deliverable changes with the process. Cast PU, the heartland of liquid color, takes the liquid dispersion. TPU and thermoplastics take masterbatch. A shop that does both buys both.
How to choose
Work from the process, not the part. If you are casting a two-component PU elastomer, pouring an epoxy floor, running plastisol, or blowing PU foam, color is added to a liquid before cure, so use liquid color. If you are injection molding or extruding TPU or a thermoplastic elastomer, color is added to a melt, so use masterbatch.
A single part design can sometimes be made both ways. A component cast in PU for one program and molded in TPU for another needs the same shade in two formats. That is a matching question, and it is handled by holding one color target across the liquid dispersion and the masterbatch.
Matching across formats
The match is independent of the format. A color reference, whether a PANTONE code, a RAL number, a Federal Standard 595 chip, a hex value, or a physical sample, defines the target. The formulation then carries that target into whichever format the process needs, accounting for the resin, the heat profile, and the exposure the part sees. For a part you make by both casting and molding, the value is one verified shade that reads the same whether it came off the pour or out of the press.
One color partner across both
P.A.T. Products supplies liquid color for cast PU and liquid systems and masterbatch for TPU and thermoplastics, and matches across both in-house. RoHS and REACH compliance is standard, with Proposition 65 and FDA documentation available on request. Tell us whether the part is cast or molded, the resin, and the color target, and we will recommend the format, match the color, and send a sample.
See masterbatch for TPU and thermoplastics