Color Spots and Streaks: A Technical Guide to Diagnosing Poor Dispersion

In plastics processing, few problems are as costly and frustrating as part rejection due to visual defects. Color spots (non-dispersed pigments) and streaks (veining or marbling) not only affect aesthetics but can compromise the mechanical properties of the final product. Is the problem in the masterbatch, the resin, or the process? Learning to diagnose the source is the key to reducing scrap.

1. Color spots: the challenge of agglomerates

When small intense spots or 'granules' of color appear, we are facing a dispersion failure. This occurs when the pigments were not adequately separated during masterbatch manufacturing or failed to 'wet' and distribute in the processing machine.

Material cause: Pigments with irregular particle size or a formulation with insufficient dispersing agents.

Process cause: A temperature that is too low in the feed zone, preventing the masterbatch carrier from melting at the same time as the base resin.

2. Streaks and veining: distribution problems

Unlike spots, streaks or the marbling effect usually indicate poor distribution. The pigment is there, but it has not been uniformly mixed into the melt.

Viscosity incompatibility: If the melt flow index (MFI) of the masterbatch is very different from that of the base resin, the color will 'float' or group into threads instead of integrating.

Screw design: In injection or extrusion processes, a screw with a low L/D ratio (Length/Diameter) or without intensive mixing zones will make chromatic homogeneity difficult.

3. Plant diagnostic checklist

Before making drastic changes, check these three critical points:

Back pressure (in Injection): Slightly increasing back pressure can improve the mechanical mixing of masterbatch with virgin resin.

Temperature profile: Verify that the temperature in the transition zone is adequate to melt the masterbatch. If too high, the concentrate may 'slip' without mixing; if too low, it will not disperse.

Moisture: In certain polymers, residual moisture can generate bubbles that are confused with pigmentation defects (vapor streaks).

4. The importance of technical compatibility

To avoid these defects, masterbatch selection must go beyond color. It is vital to ensure that the carrier (vehicle) of the concentrate is chemically and mechanically compatible with the final polymer. A masterbatch designed specifically for the process (injection, extrusion, blow molding) will drastically reduce machine adjustment time.