How does a Banbury mixer combine plastic ingredients?

The Banbury mixer is a specialized internal mixer used in the plastics and rubber industries to combine ingredients through the simultaneous application of heat and pressure.1

The machine functions similarly to a robust dough mixer, featuring an hourglass-shaped chamber that contains two interrupted spiral rotors (or heavy steel paddles) that rotate in opposite directions. As these rotors move—typically at 30 to 40 rotations per minute—they create an intense shearing action.12

Ingredients such as resins, pigments, and other additives are incorporated into the mixture as they are sheared and softened within the narrow gaps between the rotating paddles and the chamber walls. This process is highly efficient, capable of handling large batches—up to one-half ton—and requiring significant power input to process the molten material.132