Short answer: treat colour masterbatch as a dispersion problem, not a conveying problem. You need conveying elements to feed and build pressure, 45° kneading blocks to rupture pigment agglomerates, mixing elements to even out the colour, and wear-resistant steel because TiO2 and carbon black are abrasive.
In a filled compound, the filler is usually already coarse and well characterised. In a colour masterbatch, the pigment arrives as agglomerates of primary particles, often below one micrometre each, bound together by the pigment manufacturer's own treatment. The job of the screw is to wet those agglomerates with carrier resin and then break them down far enough that no speck is visible in the finished part.
That changes the priorities. Throughput is secondary. What matters is stress intensity in the dispersion zone, enough residence time for wetting, and a screw geometry that does not re-agglomerate the pigment after it has been separated.
| Zone | Element | Purpose |
|---|---|---|
| Feed | Wide-pitch conveying, 1.5D to 2D | Volumetric capacity for low bulk density pigment and carrier |
| Melt | Conveying, then a short kneading block | Complete melting before any pigment is compressed |
| Melt seal | Reverse-flight or 90° block | Prevents unmelted polymer reaching the dispersion zone |
| Dispersion | 45°/5/45 kneading blocks, two or three in series | Ruptures pigment agglomerates under high stress |
| Distribution | SME or gear mixing element | Evens out colour without adding heat |
| Degassing | Conveying either side of the vacuum port | Removes moisture and volatiles released by wetting |
| Discharge | Narrow-pitch conveying | Builds die pressure with a stable, full channel |
The single most common mistake is placing the mixing element directly after the feed. It looks like the screw is working hard, but no agglomerate has been ruptured yet, so the mixing element only redistributes large particles evenly — which is exactly the defect you are trying to avoid.
| Pigment | Dispersion difficulty | Recommended approach |
|---|---|---|
| Organic pigments | Hard to wet, moderate agglomerate strength | Longer wetting zone, 45° kneading, avoid excessive shear that shifts shade |
| Inorganic pigments (iron oxide, chrome) | Easy to wet, abrasive | Standard 45° kneading, wear-resistant steel is the priority |
| Titanium dioxide | Easy to disperse, extremely abrasive | Moderate kneading, high-hardness or coated elements, expect wear |
| Carbon black | Very hard to wet, high structure | Strong dispersive kneading, high torque capacity, wear protection |
| Effect and pearlescent pigments | Platelets fracture easily | Minimal kneading, distributive mixing only, short residence |
In a white or black masterbatch line, screw elements are a consumable. TiO2 at high loading and carbon black both remove metal steadily, and a worn flight profile silently destroys dispersion quality long before anyone notices the parts are finished.
For these duties we normally recommend hardened and tempered tool steel or a powder-metallurgy grade at 58–62 HRC, with a surface hardness in the 900–1100 HV range after treatment. Where the budget allows, a wear-resistant coating pays back fastest on the dispersion-zone kneading blocks, because those see the highest stress and the most abrasive contact.
| Defect | Likely cause | Fix |
|---|---|---|
| Visible specks or gel | Insufficient dispersion | Add a kneading block, increase stagger to 60°, check melt seal |
| Colour streaks between pellets | Insufficient distribution | Add SME or gear mixing element after dispersion |
| Shade drift over a run | Worn flights, unstable fill | Measure flight outside diameter; replace worn elements |
| Degradation, burnt smell | Over-shearing, long residence | Reduce 90° blocks, shorten dispersion zone |
| High torque alarms | Over-filled zone, too many neutral blocks | Open the pitch before the kneading stack |
When writing the enquiry, screw element vs screw segment vs kneading block helps avoid the wrong part name. Masterbatch duty is hard on the bore as well — see barrel and liner wear. If the masterbatch carries mineral filler, CaCO3 and talc filled compounding covers the abrasive side.
Typically two to three 45° blocks in series for the dispersion zone, depending on pigment hardness and the required dispersion quality. More blocks increase dispersion but also raise melt temperature and torque, so the limit is usually set by the polymer's thermal stability rather than by the pigment.
A hardened tool steel or powder-metallurgy grade in the 58–62 HRC range, ideally with a wear-resistant coating on the dispersion-zone elements. TiO2 is one of the most abrasive pigments in common use, and standard nitrided elements wear quickly at high loading.
You can, but dispersion quality will be inconsistent. A general-purpose screw usually lacks a melt seal before the dispersion zone, so unmelted polymer reaches the kneading blocks and shields pigment particles from the shear they need.
Most often it is unstable fill in the dispersion zone, or progressive wear on the conveying flights upstream. Both change the stress history the pigment sees. Check flight wear first, then verify that the feed zone is consistently full.
Imported-machine elements take 30-45 days with material available and drawings confirmed; standard Chinese machines are usually ex stock. Keep one set of dispersion-zone kneading blocks as a spare — those wear fastest and a line-down situation is far more expensive than the parts.