Filler masterbatch on a twin-screw extruder: how specific torque sets your loading and output

Calcium carbonate filler masterbatch looks like a simple product: a PE or PP carrier and a lot of mineral powder. On the extruder it is one of the hardest jobs there is. At high loadings the melt is stiff, the powder is abrasive and full of air, and the line usually hits its torque limit long before it reaches its maximum screw speed. That is why specific torque is the first number to compare when you buy a twin-screw extruder for filler masterbatch.

What specific torque means

Specific torque is the torque each screw shaft can transmit, divided by the cube of the distance between the two shaft centres. It is written in Nm/cm³. Because it is normalised to the size of the machine, it lets you compare extruders of the same screw diameter from different makers on equal terms.

VJJ co-rotating twin-screw extruders by Kunwei come in two torque classes:

At the same screw diameter and the same screw speed, 14 Nm/cm³ gives about a quarter more torque than 11.3 Nm/cm³.

Why filler masterbatch is torque-limited

The power a gearbox puts into the melt is torque multiplied by screw speed. Each formulation needs a certain amount of energy per kilogram to melt the carrier, wet the filler and disperse it. When the melt is as viscous as a highly filled compound, the extruder reaches full torque at a moderate speed, and output stops rising.

• More torque, more output. On a torque-limited job, output rises roughly in step with the torque available, as long as the feeding and venting keep up.
• More torque, higher loading. With headroom on the gearbox you can push the filler content up without overloading the drive.
• More torque, lower melt temperature. The line can run a little slower at the same output, which helps colour and carrier properties.

Feeding the powder is the second bottleneck

Fine CaCO3 has a low bulk density and carries a lot of air. If it all goes into the main hopper, the screws fill with powder instead of melt, and output drops.

• Side feeding. Melt the carrier first, then add the powder through a twin-screw side feeder into the molten resin.
• Venting before and after the side feeder. An atmospheric vent lets the entrained air escape; vacuum degassing near the die removes moisture and volatiles.
• Gravimetric feeders. Loss-in-weight feeders keep the filler ratio constant, which keeps torque and pellet quality stable.
• Coated filler. Stearic-acid-coated CaCO3 disperses more easily and needs less energy than uncoated powder.

Wear and pelletizing

Mineral filler is abrasive. Ask which screw elements and barrel liners are wear-resistant and what their expected life is on your formulation. Highly filled strands are brittle, so die-face (hot-cut) or water-ring pelletizing is common for filler masterbatch; strand pelletizing works at lower loadings.

What to send for a quotation

• Carrier resin and its melt flow index
• Filler type, particle size and whether it is coated
• Target filler content in percent and target output in kg/h
• Other additives, such as dispersant, wax or colour
• Pelletizing preference and your plant power supply

See the VJJ specifications on the twin-screw extruders page.