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Color Masterbatch and Dispersion: The Real Cause of Streaks and Specks

When streaks appear in a molded part or specks begin showing up in an extrusion line, the first reaction is often to adjust the machine. Barrel temperature is increased. Back pressure is changed. Screw speed is modified. Sometimes pigment loading is increased to “overpower” the defect. If the defect persists across presses, across shifts, or across process adjustments, the issue is rarely at the molding machine. It is almost always structural. And in many cases, that structure was set during masterbatch compounding.

Color masterbatch is not simply pigment blended into plastic. It is a concentrated dispersion of pigment within a carrier resin, designed to transfer color into a base polymer at a defined let-down ratio. Its success depends on whether pigment agglomerates were reduced to stable primary particles during compounding. If they were not, those agglomerates would become visible under shear during processing. Streaks and specks are often dispersion failures that originated upstream.

What Dispersion Actually Means

Pigment particles are not delivered to a compounder as isolated units. They exist as agglomerates, clusters of smaller primary particles held together by surface forces. The goal of compounding is not merely to mix pigment into resin, but to mechanically reduce those agglomerates and stabilize the resulting particles within the carrier.

Two terms must be distinguished:

Distributionrefers to the spatial uniformity of pigment throughout the melt at a macroscopic scale. It answers the question: Is the color present everywhere in roughly the same concentration? Distribution is achieved through distributive mixing, in which the melt is folded and blended so that pigment-containing regions are evenly distributed throughout the material. A melt stream can appear visually uniform and still contain structural defects if dispersion is incomplete.
Dispersion refers to the degree to which pigment agglomerates have been reduced to their primary particle size and stabilized within the carrier resin at the microscopic scale. It answers the question: Have clustered pigment particles been mechanically deagglomerated and properly wetted by the polymer?Dispersion requires sufficient shear energy to fracture agglomerates and adequate wetting to prevent re-agglomeration. Poor dispersion means pigment exists as clusters rather than as discrete, stabilized particles, even if those clusters are evenly distributed throughout the melt.

Why does this Matter?

A melt can appear evenly colored while still containing intact clusters. Those clusters are not cosmetic imperfections waiting to happen. They are structural discontinuities. When shear is applied during molding or extrusion, these discontinuities interact with flow fields differently than the surrounding matrix. That is when they become visible.

Why Agglomerates Become Streaks

Streaking is often shear dependent. It becomes more pronounced at higher screw speeds or in thinner sections. That behavior is a strong indicator of dispersion-related instability.

When a pigment agglomerate survives compounding:

It may not fully wet with polymer.
It may have a different effective viscosity than the surrounding matrix.
It may orient or elongate differently under shear.

As the melt flows through gates, runners, or dies, shear gradients develop. Poorly dispersed clusters can align within these gradients, creating localized differences in pigment concentration. The result appears as streaks that follow the flow direction.

If the agglomerate is large enough, it may not fracture at all. Instead, it appears as a speck or “fish eye” in the finished part. These are not color selection problems. They are mechanical breakdown problems.

Why Molding Conditions Cannot Fully Correct It

Injection molding screws and extrusion screws are designed primarily for melting and conveying. They provide some distributive mixing, but they are not high-energy dispersive systems.

Breaking down pigment agglomerates requires sustained shear under proper wetting conditions. That typically occurs during masterbatch compounding, where screw configuration, temperature profile, and residence time are designed specifically for dispersion.

By the time the concentrate is pelletized, the dispersion state is largely fixed. Downstream processing can redistribute pigment. It cannot reliably fracture agglomerates that survived compounding.

This explains a common production pattern:

Specks persist across multiple machines.
Streaks remain even when process parameters are adjusted.
Increasing pigment loading reduces contrast but does not eliminate the defect.

In these cases, the issue is not at the press. It is embedded in the pellet.

How Dispersion Quality Is Evaluated

Dispersion should be treated as a measurable variable. One practical method is filtration-based testing. In this approach, molten polymer containing the masterbatch is pushed through a defined screen pack under controlled flow conditions. As poorly dispersed particles accumulate on the screen, pressure rises. The rate and magnitude of pressure increase provide insight into particle cleanliness and dispersion quality.

While this method does not measure color directly, it correlates with how cleanly the material will run during extended production cycles.

Additional indicators of dispersion-related issues include:

Elevated or unstable screen pack pressure during extrusion
Persistent die build-up
Specks that appear regardless of machine settings
Streaking that correlates with shear rate rather than dosage variation

When these signals are present, adjusting pigment chemistry should not be the first step. Evaluating the mechanical history of the masterbatch should be.

The Role of the Plastic Resin Supplier

As leading plastic resin suppliers, we understand that dispersion is not a cosmetic detail. It is a structural performance parameter.

At Marval Industries, we approach color masterbatch from the standpoint that particle breakdown, wetting behavior, and shear history determine cosmetic stability. Troubleshooting begins with dispersion quality and carrier compatibility before recommending formulation changes. Color consistency is engineered upstream.

Working With a Plastic Resin Supplier

Streaks and specks are not random defects. They are visible evidence of how pigment particles were treated during compounding. When agglomerates are properly fractured and stabilized, pigment integrates into the base polymer as a uniform microstructure. When they are not, shear during molding reveals the defect.

Adjusting machine settings may temporarily change how the defect appears. It does not eliminate the structural cause. If cosmetic instability persists across machines or process windows, the investigation should move upstream. In most cases, the real cause of streaks and specks is incomplete dispersion within the masterbatch.