Last updated: 2026-09-04 · 5 min read

Color Streaks in Injection Molding: Why Masterbatch Dispersion Decides Your Surface

Color streaks are pigment that never became color: masterbatch that did not melt into the host resin, agglomerates the screw could not break apart, or colorant burnt by excess heat. A masterbatch pellet is mostly carrier with pigment at a few percent, and inside the barrel that carrier must melt, wet every particle, and distribute uniformly. Every streak is a place where that failed — and the streak’s own geometry is diagnostic.

Key Facts

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  • Four failure clusters: poor dispersion (agglomerates surviving screw shear), carrier mismatch (carrier never merging with host), dosing drift (feeder wandering), and thermal degradation (pigment burnt at hot spots).
  • The streak’s geometry decides which failure: flow-aligned streaks implicate melt homogeneity; random specks implicate agglomerates; uniform shade shift implicates dosing; brown-black from the gate implicates heat.
  • The universal first response — adding more masterbatch — fixes only the dosing row and usually deepens the degradation one.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

    What Actually Causes Color Streaks?

    Five causes, ordered by how often each explains real production streaking:

    Cause Mechanism What You See
    Poor dispersion Agglomerates survive screw shear Dark streaks and specks at the gate
    Carrier mismatch Carrier never melts into host Hazy wipeable streaks; gloss mismatch
    Dosing drift Feeder wanders below set point Shade varies purge to purge
    Thermal degradation Pigment burnt at hot spots Brown-to-black streaks from gate
    Moisture carryover Steam disrupts melt skin Splay mixed with pale streaks

    Dispersion is the classic: pigment is manufactured in agglomerates, and high-load masterbatches demand more shear work than many processes give them. Surviving agglomerates appear as streaks aligned with flow, concentrating near the gate. Carrier mismatch is the sneaky one: color gets in, so the mix looks successful, but the phases never merge — the streak is carrier-rich material that wipes off, clouds the gloss, and paints differently.
    Carriers are chosen per resin family for melt-temperature overlap and viscosity match; a “universal” carrier is a compromise excelling in none.

    Dosing drift is the operational one: volumetric dosers meter by volume, so a bulk-density shift, a bridging hopper, or a worn auger quietly changes the let-down. Thermal degradation is the process one: pigments have temperature ceilings, and the barrel adds hot spots — a heater band out of control, a worn check ring, extended residence — burnt colorant enters as brown-black streaks. Moisture is the mixed one: steam shows as splay interleaved with pale streaks — a drying problem wearing a color problem’s clothes.

    How Do You Eliminate Color Streaks for Good?

    Four steps, in order:

    1. Match the carrier to the host resin before touching the process. The carrier should belong to the host’s family — polyolefin carrier in polyolefins, styrenic in styrenics — its melt flow should sit near the host’s so both melt in the same window, and its pigment load should suit the application’s shear budget. Get the supplier’s written let-down ratio and processing window for the actual host grade. Where the part is color-critical, ask for dispersion performance measured in that host rather than a general claim. Record masterbatch lot, carrier type, let-down and host grade together, so when a future run streaks, “what changed” has an answer in a file instead of in memory.
    2. Give the melt the shear and mixing work the pigment demands. Dispersion happens in the screw. The first lever is back pressure: raising it adds mixing work per revolution — a surprising share of streak cases end at a documented back-pressure increase. Behind it: screw design, where a mixing element does what the barrel cannot; and residence discipline, where a small shot in a large barrel degrades colorant. Nozzle static mixers distribute without dispersing — they fix banding but leave agglomerate specks untouched, so knowing which failure you have decides whether the mixer is the tool or a delay. Record the working combination in the setup sheet.
    3. Dose by weight, and audit the feeder as a process variable. Gravimetric, loss-in-weight dosing meters actual mass and alarms on drift; for color-critical parts it is the standard. Where not installed, volumetric dosers need calibration against weighed samples on a schedule. Audit points: let-down math re-checked against actual concentration; regrind percentage held steady since regrind dilutes color; hopper kept free of bridging; doser settings written into the setup sheet. The tell of a dosing problem is temporal: shade varying purge to purge while formulation stays constant. The fix is measurement, not more masterbatch.
    4. Verify at the purge, and hold the shade with a retained standard. A purge showing streaks, specks or shade drift has already told you what parts will do. Inspect every purge against a retained standard under the same light, and require purge-to-purge consistency. Part sampling under the lighting the market actually sees, and a shade record per lot covering masterbatch lot, host resin lot, regrind percentage and doser settings, so drift traces to its variable. When a purge fails, work the fingerprint: wipeable haze says carrier, specks say dispersion, temporal drift says dosing, brown-black says degradation — and escalate to the supplier with purges kept as evidence.

    Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    Color streaks are not a color problem — they are a dispersion problem wearing color’s clothes. Read the streak’s geometry before changing anything: the fingerprint decides which row you are in, and each row has a different fix.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Why are streaks worse in some cavities than others?

    Cavity-to-cavity differences are shear and residence differences, and color shows them honestly. In an unbalanced runner, some cavities fill through long, high-shear paths while others fill gently: high-shear cavities get more dispersion work — often better color, occasionally more degradation — while gentle ones display whatever agglomerates the screw left behind. Gate geometry multiplies the effect.

    The diagnostic use is the pattern: map which cavities streak and how. Every cavity streaking similarly points upstream of the runner; a systematic pattern matching flow balance points into the runner; cavities sharing one gate style differing from the rest point at the gates. Fixes follow the map: runner balancing, gate review, or per-cavity shade verification as part of qualification.

    Would liquid color or precolored resin end the streak problem?

    They change which variable dominates; they do not remove the physics. Liquid color disperses pigment in a liquid carrier: its dispersion advantage is real because pigment arrives pre-wetted, and dosing can be gravimetric-accurate; the trade is new equipment — pump calibration, feed-line hygiene, carrier compatibility — and its own failure mode, pump drift.

    Precolored resin removes the dosing variable entirely; its trade is inventory and lot management, because color matches per production lot and natural-base shifts still arrive with each resin lot. All three routes still need the screw, the heat, and the verification discipline — the streak physics does not care which form the pigment traveled in.

    Why does shade drift between runs when the recipe never changed?

    Because the recipe is only one of the inputs the eye receives. The base resin’s natural color varies lot to lot — a natural polyolefin shifts between faint blue and yellow casts — and the same masterbatch dose over a shifted base lands on a shifted final shade. Pigment lots vary too, organics more than inorganics.

    Regrind is the quiet drift: it carries its own heat history and slightly degraded colorant, so a run moving from 15 to 25 percent regrind shifts shade while nobody touches the doser. Machine condition drifts as well: a wearing screw changes shear history, a replaced heater band changes the temperature map. The control that ends the argument: retained standards from each approved run, and an input record covering resin lot, masterbatch lot, regrind percentage and machine.

    Key Takeaways

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  • Color streaks are a dispersion diagnostic: the streak’s geometry, wipeability and position decide which failure mode you are in — each has a different fix.
  • Carrier matched to host resin, dosing by weight, shear engineered via back pressure, heat audited — treat color as a materials-and-process system.
  • Verify at the purge against a retained standard; the purge predicts the parts, and the record traces drift to its variable instead of starting an argument.

    • Color streaks end when you treat color as a system — carrier matched to host, dosing measured by weight, shear engineered, heat audited — and verify the result at the purge before a single part is packed.
    The streak’s fingerprint is the diagnostic; the retained standard is the evidence.

    Sources

    1. Plastics Technology — Color Dispersion in Injection Molding
    2. Society of Plastics Engineers (SPE)
    3. Plastics Today — Masterbatch Selection Guide
    4. AMPIM — Injection Molding Defects Guide
    5. Clariant — Masterbatch Technical Data

    Sources: Plastics Technology · Manufacturing.net · Society of Plastics Engineers (SPE) · ThomasNet