How To Solve Carbon Black Dispersion Problems in Black Masterbatch Production?

When producing black masterbatch, one of the most common challenges is uneven dispersion of carbon black, which can lead to strand breakage, difficulty in pelletizing, and poor processing stability.

In one case, a manufacturer used 40% carbon black with EVA wax as a dispersant. During strand pelletizing, some strands became brittle or broke easily, while others could not be pulled at all. The root cause was determined to be poor carbon black dispersion. The production line used a twin screw extruder, with materials pre-mixed by a high-intensity mixer and fed into the hopper. The temperature settings were 160℃, 165℃, 170℃, 180℃, and die head 175℃.

On-Site Observations

• Materials in the hopper often failed to flow, causing strand breakage. Initially mistaken as material shortage, manual pushing revealed plenty of material was stuck inside.

• At 20–30% carbon black, no such issue occurred.

• During extrusion, strand output speed was uneven: two die holes extruded faster than the others (4-hole die).

• No reliable quality testing method was available; production went directly to customers without dispersion evaluation.

Practical Solutions

1. Increase EVA wax dosage to improve carbon black wetting and dispersion.

2. Lower processing temperature by 10℃ to avoid premature melting and bridging at the hopper.

3. Adjust screw speed (increase or decrease depending on dispersion and feeding behavior).

Root Cause: Hopper Bridging and Poor Feeding

The hopper stores plastic raw materials and should allow smooth feeding. Poor hopper design or material flow properties often cause bridging (arch formation) or funnel flow, leading to unstable feeding.

Definition of Bridging

When the hopper’s conical section narrows too quickly near the extruder feed port, materials compact and form a solid arch, preventing flow into the screw.

Causes of Bridging

• Large, irregular recycled granules increase bridging risk.

• Fine powders or lightweight fillers (e.g., carbon black, calcium carbonate) can form funnel flow and block feeding.

• Air trapped in powder increases bulk volume and prevents smooth conveying.

Solutions to Bridging

• Install a vibration device on the hopper.

• Optimize hopper cone angle for specific particle sizes.

• Avoid excessive hopper heating, which can soften pellets and cause sticking.

• Improve formulation:

• Mix light and heavy calcium carbonate evenly.

• Add internal lubricants to improve powder flowability.

• Pre-mix powders with white oil (1–2%) to remove entrapped air and increase bulk density.

Extruder Feeding Zone Considerations

The feeding zone of a twin screw extruder is crucial:

• If temperature is too low (below 120℃), resin will not adhere to the barrel, causing poor conveying.

• If temperature is too high (above 160℃), premature melting creates a lubrication layer, leading to starvation.

Key Improvements

• Extend L/D ratio of the feeding zone.

• Apply forced cooling or vibrating feeders for stable feeding.

• Use segmented screw elements with optimized clearance.

• Widen or modify the feed throat design for better flow.

Pigment Dispersion in Masterbatch

In black masterbatch, carbon black dispersion quality directly affects strand strength and color uniformity.

• Poor pre-dispersion leads to color streaks (“flowering”).

• Carrier resin and masterbatch MFR mismatch can also cause uneven color distribution.

• Overly tough carrier resins may resist shearing, preventing fine dispersion.

Experience Summary

1. Use dried materials – Moisture causes poor feeding and partial melting at the inlet.

2. Lower hopper temperature – Set as low as 100℃ to prevent bridging.

3. Optimize screw design – Use smaller-diameter elements in the feeding zone.

4. Enlarge feed port and install mixing or stirring devices for continuous feeding.

5. Increase screw speed carefully – Faster mixing improves dispersion but may cause higher torque load.

Conclusion

For black masterbatch production, stable carbon black dispersion is key to achieving strong strands, uniform pellets, and high-quality final products. By optimizing dispersant dosage, hopper design, screw configuration, and processing parameters, manufacturers can avoid strand breakage, improve extrusion efficiency, and ensure consistent pigment performance.

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