Troubleshooting · Defect guide
Colour streaks, mottling and specks
Is it the masterbatch or the machine?
Colour streaks are the defect where masterbatch quality matters directly, and also the one most often wrongly blamed on the masterbatch. This guide covers how to tell streaks apart from lot-to-lot colour shift, how colour disperses in the screw, four groups of causes, checks from cheapest to most expensive, and which parts are fixed in the formulation versus on the machine.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is a colour streak
A colour streak is uneven colour within a single part: darker or lighter lines along the flow direction, blotchy patches, or scattered small colour specks. It differs from defects that look similar:
- Colour streaks / mottling: colour varies within one part, usually most visible near the gate and along the flow.
- Lot-to-lot colour shift: each part is evenly coloured, but today's parts differ from yesterday's. That is a dosing and raw-material issue; see the colour shift row in the troubleshooting table.
- Splay (silver streaks): shiny silver lines along the flow, caused by moisture or gas, not pigment.
- Black specks: black or brown spots from degraded or burnt polymer in the barrel, even with a different colour masterbatch.
Mechanism: how colour disperses in the screw
A masterbatch is concentrated pigment dispersed in a carrier polymer. The screw has to melt that carrier together with the base resin, then spread the concentrated colour evenly through the whole shot. If the carrier melts later, has a very different viscosity from the base resin, or the screw mixes too little, the concentrated colour enters the mould as strands before it is diluted, and shows as streaks in the part.
The practical takeaway: streaks come from three things that must all be right: a steady actual masterbatch feed, a masterbatch designed to match your base resin, and a machine that mixes enough. Miss any one and streaks can appear.
Four groups of causes
1. Masterbatch
A carrier whose viscosity or MFI is far from the base resin, or a different polymer that blends poorly. Pigment that was poorly dispersed in the carrier when the masterbatch was made. A concentration so high that the let-down becomes very low and hard to spread evenly.
2. Dosing and premixing
A dosing unit that feeds unevenly or has not been calibrated, and masterbatch pellets that differ in size or density from the base resin and segregate in the tumbler or hopper, so colour goes light and dark in waves.
3. Process and machine
Low back pressure, an unsuitable screw speed, rear-zone temperatures too low for the carrier to melt in time, a general-purpose screw with no mixing section, and a previous colour not fully purged from the barrel.
4. Part and base resin
Thin or transparent parts show unevenness far more than thick opaque ones. Base resin lots can differ in natural colour, and a varying share of already-coloured regrind adds its own colour.
Step-by-step checks, cheapest first
- Weigh the actual masterbatch feed. Collect what the dosing unit delivers over a fixed time and weigh it against the base resin used in the same period. A set point is not proof of what is fed. Weigh before you argue.
- Mould the base resin alone for 20–30 shots. If specks or streaks are already there, the masterbatch is not the cause.
- Step up back pressure. If streaks drop clearly, mixing was insufficient. Watch cycle time and melt temperature at the same time.
- Mould a flat plaque. A thin, light-coloured plaque held against the light shows streaks and specks far better than the real part. Use it to compare before and after every change.
- Compare premix with the dosing unit, and sample pellets from the top and bottom of the hopper. If the ratios differ, it is segregation.
- Ask the masterbatch maker for carrier data: polymer type and MFI, compared with your base resin. If they are far apart, trial a masterbatch with a closer carrier side by side.
- Consider the machine. If none of the above works, a mixing nozzle or a screw with a mixing section is the most expensive route.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Actual masterbatch feed drifts | Weigh feed per time interval against the set point | Calibrate the dosing unit; switch to gravimetric if drift is large |
| Segregation in tumbler or hopper | Compare pellets from the top and bottom of the hopper | Dose at the hopper throat instead of premixing; keep the hopper level low |
| Too little mixing in the screw | Step up back pressure and inspect a test plaque | Reset back pressure and screw speed, or add a mixing nozzle |
| Carrier does not melt in time | Streaks drop when rear-zone temperature rises | Adjust the temperature profile, or use a carrier that melts like the base resin |
| Carrier mismatched with base resin | Compare carrier type and MFI with the base resin | Choose a masterbatch with the same polymer carrier and a similar viscosity |
| Poor pigment dispersion in the masterbatch | Specks vanish with base resin alone and return with that masterbatch lot | Send samples and the lot number to the masterbatch maker |
| Let-down too low for the part | Streaks drop when a higher let-down is trialled | Use a lower-concentration masterbatch at a higher let-down; check cost per good part first |
| Previous colour left in the barrel | Streaks fade gradually after a colour change | Standardise the colour-change procedure; use a purging compound for frequent changes |
Formulation fixes versus machine fixes
Unlike sink marks, here masterbatch design really matters, but not in every case. Separate the two before changing supplier.
- Fixable in the masterbatch: a carrier of the same polymer as your base resin with a similar viscosity, good pigment dispersion in the carrier, and a concentration that lets your feeder dose accurately. If you run several masterbatches together, such as colour plus UV or antistatic, every carrier should match the base resin. See the product groups ACS supports on Products & TDS.
- Fixable only on the machine: an inaccurate dosing unit, hopper segregation, low back pressure and a screw without a mixing section. No masterbatch can compensate for these.
- Judge on cost per good part. A masterbatch that is cheaper per kilogram but needs a higher let-down or adds scrap can cost more overall. See Formulation economics.
For the basics of screw mixing and back pressure, see Injection molding.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Colour streak questions we hear often
Raising back pressure removed the streaks. Does that mean the masterbatch is poor?
Not necessarily. More back pressure means more mixing work in the screw. If the streaks disappear, the melt needed more mixing than you were giving it, perhaps because the carrier viscosity differs from the base resin, the let-down is low, or the screw has no mixing section. If the setting stays within what the machine handles and cycle time does not suffer, keep it. If you have to push so high that the melt overheats or the cycle gets longer, talk to the masterbatch maker about the carrier and dispersion.
A cheaper masterbatch needs a higher let-down. Is it worth it?
Judge it on cost per good part, not price per kilogram. A masterbatch that is cheaper per kilogram but needs a higher let-down, or causes more scrap from streaks, can cost more overall. Put the actual let-down, price and scrap rate of both options into the same calculation. Our formulation economics article shows how.
Small dark specks in a clear part: is that undispersed pigment?
It may be pigment agglomerates, but rule out black specks from degraded or burnt polymer in the barrel and contamination from a previous colour. The first check is to mould the base resin alone without masterbatch. If the specks remain, the masterbatch is not the cause. If they disappear and return when the masterbatch goes back in, send part samples and the masterbatch lot number to the maker to check dispersion.
Is it better to premix masterbatch in a tumbler or use a dosing unit at the hopper throat?
Premixing costs little, but masterbatch pellets that differ in size or density from the base resin can segregate during conveying or in the hopper, so colour goes light and dark in waves. A gravimetric dosing unit holds the ratio more steadily but needs regular calibration. If colour drifts over time, sample pellets from the top and bottom of the hopper and count or weigh them against each other.
Streaks still there after tuning the machine?
Send photos of the part and a test plaque, the masterbatch name and lot, and your machine settings on LINE. Our technical team will help you separate masterbatch, dosing and mixing causes.