Black masterbatch defects: gels, streaks and specks — causes and fixes
Most black part defects get blamed on the masterbatch. Sometimes that is right. Often the root cause sits in the carrier match, the let-down, or a temperature window nobody wrote down. This is the order in which we look.
By Zoe Lin, Export Manager, E-LUCK (Guangdong Yijuyuan Plastic Industry Technology Co., Ltd.)
1. Start with the symptom, not the grade
Different defects have different causes, and "the black is bad" is not a diagnosis. Before you change supplier, note four things: what the defect looks like, where it appears (random, at the die, at the gate, at a weld line), when it started (new lot, new resin, new settings), and whether it is on one machine or all of them. Those four answers usually narrow it to one or two causes immediately.
2. Gels and fisheyes in film
A gel is a small, hard, usually translucent lump in the film. In black film it shows as a hard spot that will not stretch with the surrounding material.
- Carbon agglomerates. Undispersed carbon clumps. This is a masterbatch dispersion issue and the most common cause.
- Carrier mismatch. A PE-carrier masterbatch in a PP line, or a high-melting carrier running below its melting point, leaves unmelted particles.
- Degraded resin. Oxidised material from an overheated barrel or a dead spot in the die.
Fixes, in order: confirm the carrier matches your polymer; raise the melt temperature into the grade's window; check the screen pack; then review carbon loading — a very high loading is harder to disperse and may be more than your shear can handle.
3. Streaks and flow lines in mouldings
Streaks are directional lines of colour difference. On a black part they read as grey or dull bands.
- Let-down too low. Not enough masterbatch to distribute evenly through the shot.
- Poor distributive mixing. Screw design or back pressure not moving the colour through the melt.
- Melt flow mismatch. The masterbatch flows differently from the base polymer, so it separates at the flow front.
Fixes: raise the let-down slightly; increase back pressure; check that the masterbatch MFR is matched to your base polymer. For visible parts, a high-gloss grade formulated for that polymer matters more than a general-purpose black.
4. Specks and black spots
A speck is a discrete dark particle — different from a gel, which is a hardened lump of the same material.
- Burnt material from a dead spot, a worn screw or an overheated zone.
- Contamination carried in from regrind, a previous colour, or a dirty hopper.
- Carbon contamination in the masterbatch itself — rare with filtered production, common with unfiltered.
Fixes: purge and inspect the barrel; check regrind cleanliness; then look at the masterbatch. We filter-pressure test each lot precisely because specks are usually the first sign that dispersion was not controlled.
5. Uneven colour between lots
The part was right last month and darker or duller this month. Assuming nothing else changed, this is a lot-to-lot consistency problem — and it is the one that should never happen.
What should be controlled: carbon content per lot, dispersion, MFR, and the carrier. We keep lot records so a reorder matches the previous lot, and we issue a certificate of analysis with each shipment. If your supplier cannot tell you what lot you bought last time, that is the answer.
6. When the masterbatch is not the problem
Being honest about this saves everyone time. If the defect appears on one machine only, look at that machine. If it started with a new resin batch, look at the resin. If it appears after a long shutdown, look at purge and barrel condition. Replacing good masterbatch will not fix any of those.
Where we can help regardless: send us the defect description, a photo, your polymer, your machine settings and the lot number. If it is ours, we will say so and replace it. If it is not, we will tell you where to look.
Questions buyers ask about defects on the line
Most black masterbatch defects are not a supplier problem. Here is how to tell the difference before you blame the material.
Are gels in a black masterbatch a dispersion problem?
Sometimes, and sometimes not. A gel that appears consistently from the first lot is usually dispersion. A gel that appears after a storage period, or only after the summer, is more often moisture or thermal history. Establish when it started: change the lot and run again, and change nothing else. If the defect follows the lot it is a material question; if it follows the line it is a process question. Doing this one test before contacting a supplier saves most of the argument that follows.
Why does black masterbatch streak when the carrier is not a good match?
Because dispersion and compatibility are different properties. A pigment can be well dispersed and still not wet out properly in a carrier it has no affinity for. In practice you see it as pigment at the die, pressure variation, or a surface that looks marbled rather than uniformly black. Black is unusually good at hiding small dispersion problems and unusually revealing about compatibility problems, because the loadings are high and the colour is absolute.
Can a screen pack upgrade fix a dispersion problem?
It can push the material through the machine at higher pressure, which sometimes recovers a marginally wet compound, and it can protect the die. It cannot fix pigment that was never properly incorporated. If a finer pack is the only change that works, you have bought yourself time rather than a solution. The question worth asking is whether the grade was specified with your carrier and your process in mind; a grade chosen for a different application is the usual root cause.
How do I tell whether a defect came from the material or from my line?
Run the same lot and a known-good lot back to back on the same line with no setting changes. If the defect follows the lot, it is material. If it follows time on the line, it is the machine — most often residue building in a dead spot, or a heater reading differently than the setpoint. Keep a sample from every incoming lot and every suspect batch. That habit turns a recurring argument into a five-minute comparison, and it is the single cheapest quality practice available to a compounding buyer.
Continue reading
- How to calculate let-down ratio for black masterbatch — why a cheaper per-kg grade can blow the budget on the line.
- 2026 black masterbatch market outlook — what is moving carbon-black feedstock and resin pricing this quarter.
Related pages
- PET black masterbatch: gels and high filter pressure — when the defect is specific to PET and not to the grade
- Film grades — no gels at thin gauge
- Injection moulding grades — high-gloss and matte options, no flow lines
- Pipe grades — UV-stable, clean extrusion
- Buyer FAQ — MOQ, lead time, confidentiality, sampling