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The Banbury® Mixer was introduced to the rubber manufacturing industry in 1917. The machine is called a compounder, mixer, or blender. It mixes components together that do not readily blend but require a lot of energy to become homogeneous. The mixer is a batch operation and is used to blend rubber and plastics or many different materials with additives.
The mixer was invented by Fernley H. Banbury. When his employer declined to patent the invention, Banbury left the company and filed the patent himself. He subsequently sold the design to the Birmingham Iron Foundry, which manufactured the equipment under the Banbury® trademark. During the early years of the mixer, many designs competed with the Banbury® mixer, including single rotor mixers, but they were commercially unsuccessful. In Europe and Asia, machines of similar design have been manufactured in conflict with the American patent rights.
In 1917, Goodyear was the first rubber manufacturer to take advantage of the superior blending performance of the Banbury® mixer. The mixer was a breakthrough event, as the automobile industry was growing rapidly, resulting in a new demand for rubber tires. While the equipment has continued to be refined through many feature enhancements, the basic mechanism has remained unchanged. The Banbury® Mixer has been specified through the development of many new synthetic materials, including high-viscosity polymers.
The materials to be blended are introduced through a hopper. A ram pushes the materials into the mixing chamber. Two counter-rotating rotors provide the mixing action, much like double dough hooks. The blended batch is discharged through an opening in the bottom of the mixing chamber to the next processing step. Improvements made over the years include many design changes to enable easier operation and maintenance.
Heat transfer is closely controlled in the modern Banbury® Mixer. The rotors generate a lot of friction with the material, so mixing is generally an exothermic process, giving off heat to the environment. Some mixes absorb heat and become cooler, however. In many processing operations, heat may be added to the system.
Internal pressure is also controllable, and many mixing operations occur under increased pressures and temperatures. This capability is necessary for the mixer to act as a reactor chamber in the manufacture of many polymers. Compounding of rubbers or rubber recycling is often spoken of as mastication and softening. While those terms imply the first steps of human digestion, the rubber is usually not chemically changed.