Black specks are the most-argued-about defect in plastics processing, because everyone in the chain has a plausible reason to blame someone else.
The processor suspects the material. The material supplier suspects the machine. The masterbatch supplier suspects the regrind. Everyone is sometimes right.
The way out is not opinion. It is a diagnostic sequence that costs almost nothing and settles the question in an afternoon.
First, look at the speck properly
Before changing anything, examine the defect under magnification. The specks themselves tell you where they came from.
| What you see | What it usually is |
| Hard, angular, brittle, carbonised – crushes to black powder | Degraded polymer. Long residence time or a dead spot |
| Soft, rounded, colour-tinted rather than truly black, sometimes with a halo | Pigment agglomerate. Undispersed masterbatch |
| Fibrous, or with a distinct non-polymer core | Foreign contamination. Dust, jute, packaging, metal |
| Flat metallic flake | Wear. Screw, barrel or screen pack |
| Translucent, lens-shaped, unmelted core | Not a speck – a gel or fish-eye. Different problem, see the film article |
This step alone resolves a surprising number of disputes, and it takes a hand lens.
The diagnostic sequence
Run these in order. Each isolates one variable, and you stop as soon as the specks disappear.
Test 1 – Natural resin alone. Purge thoroughly, then run virgin natural resin with no masterbatch, no regrind, no additive. Run long enough for a genuine sample.
Specks still present → the problem is your machine or your resin. The masterbatch is exonerated. Go to Test 2. Specks gone → something you add is implicated. Go to Test 4.
Test 2 – Extended purge. Purge with a proper purging compound, or with high-viscosity natural resin, and inspect what comes out. If the purge itself is full of black debris, you are carrying carbon in the barrel.
Purge is dirty, and the part cleans up afterwards → carbon build-up. The material sat somewhere it should not have. Look for dead spots: a worn non-return valve, a damaged screw flight, a nozzle with a step in it, an oversized barrel for the shot size.
Test 3 – Residence time. If a clean purge fixes it and it comes back over hours, the material is degrading in normal running. Check barrel temperatures against the resin’s real limit, not the setpoint you inherited. Check shot-size-to-barrel-capacity ratio – a small shot in a large barrel means material sits for many cycles. Check for hot spots from a failed thermocouple or a stuck heater band.
Test 4 – Regrind out. Remove all regrind and run virgin plus masterbatch.
Specks gone → your regrind is the source. Contamination in reground material is extremely common: it collects dust, it picks up whatever else was granulated on that machine, and it has already been through one full heat history so it degrades sooner on the second pass.
Test 5 – Masterbatch alone. Run natural plus masterbatch, no regrind, on a clean machine. If specks appear only here, the masterbatch is implicated – and now you can prove it rather than assert it.
When it genuinely is the masterbatch
Three distinct mechanisms, and they need different fixes.
Pigment agglomerates. Carbon black in particular is extremely difficult to disperse – it has enormous surface area and a strong tendency to re-agglomerate. A masterbatch that was not properly compounded carries clumps that survive into your part as visible dark specks. These look soft and rounded under magnification, not carbonised.
Contamination in the concentrate. The masterbatch itself has been made with contaminated feedstock, or ground in a dirty environment, or contains recycled carrier of unknown history.
Degraded carrier. A masterbatch whose carrier is over-processed or under-stabilised arrives already partly degraded, and it degrades further in your machine.
The test that settles it: filter pressure value
Filter pressure value (FPV) is the objective, numerical measure of how clean a masterbatch is.
The concentrate is extruded through a fine screen under controlled conditions and the pressure rise is recorded, normalised against the mass of pigment that passed through. Fully dispersed, clean material passes through with little pressure rise. Agglomerates and hard contaminants blind the screen and the pressure climbs.
FPV converts “your masterbatch is dirty” from an argument into a number.
Ask for it on the technical data sheet, along with the mesh size and test conditions – an FPV without conditions cannot be compared to anything. A supplier who measures FPV routinely is running a real quality system. A supplier who has not heard of the test is selling on price.
This is also why FPV matters most in thin film, where a speck that would be invisible in a 3 mm part is a visible defect and a potential web break.
Prevention: the things that actually reduce speck rates
Housekeeping around the hopper. Open hoppers in a dusty plant collect exactly what you would expect. Cover them, and keep the area around granulators clean.
Segregate regrind properly. Label it, keep grades and colours apart, and set a maximum number of passes. Regrind that has been through three heat histories is a speck source no matter how clean your machine is.
Match shot size to barrel. Running a small part on a large machine means long residence time, and long residence time degrades material. This is a machine selection decision, not a process setting.
Purge on colour change, properly. A quick flush that gets the colour out has not necessarily got the material out of the dead spots, and the residue shows up hours later.
Screen packs and pressure logs. A rising pressure trend at the screen is telling you something is accumulating. Log it.
Buy masterbatch on FPV, not on price per kilo. The cost of a speck problem – scrap, sorting, a customer rejection, a line stopped for a purge – dwarfs the few rupees per kilo saved on the concentrate.
The conversation to have with your supplier
If you have run the sequence and the masterbatch is implicated, you are in a much stronger position than most complaints ever get to. Ask for:
- The FPV for the batch you received, with mesh and test conditions
- The dispersion rating and how it was assessed
- Whether the carrier contains any recycled material
- The batch and production date, so it can be traced
- A retained sample from the same lot, tested in front of you
A supplier who can answer all five has a quality system. A supplier who offers a credit note and changes the subject does not.
FAQs
What causes black specks in injection moulded parts? Most commonly degraded polymer from long residence time or a dead spot in the barrel, carbon build-up on the screw, contaminated regrind, or undispersed pigment agglomerates from the masterbatch. The shape of the speck under magnification tells you which.
How can I tell if black specks come from the masterbatch or the machine? Run virgin natural resin alone on a purged machine. If specks still appear, the masterbatch is not the cause. If they disappear and return when masterbatch is added – with regrind removed – the masterbatch is implicated.
What is filter pressure value? FPV measures the pressure rise when a masterbatch is forced through a fine screen, normalised for pigment mass. It is the objective test of dispersion quality and cleanliness, and it is the number to ask for when specks are disputed.
Are black specks and gels the same thing? No. A gel or fish-eye is a translucent lens-shaped defect with an unmelted or incompatible core, usually a carrier resin problem. A black speck is opaque and is usually degraded polymer, carbon, or a pigment agglomerate.
Does regrind cause black specks? Frequently. Reground material has already been through one heat history so it degrades sooner on the next pass, and it collects dust and cross-contamination during handling. Removing regrind is one of the fastest diagnostic tests.
Why is carbon black harder to disperse than other pigments? Because of its very high surface area and strong tendency to re-agglomerate. Proper dispersion requires twin-screw compounding with a suitable dispersing package, which is exactly what distinguishes a well-made black masterbatch from a cheap one.