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.
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.
Related pages
- 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