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The Banbury® mixer is a type of industrial machine that’s able to force the cohesion of heavy rubbers and plastics, things that wouldn’t normally blend together without a lot of pressure and energy. Its operation is pretty straightforward. The mixer has a roughly cylindrical shape with a large hopper into which all the ingredients and additives are poured or placed; then, a series of rams and rotators compress the materials while pressure builds and the hopper begins to spin. In most cases heat is also an important element, both through natural friction and external supply. The earliest models were designed for simple manufacturing and debuted in the early 1900s. Many sources also credit this mixer with helping the American automotive industry explode a decade or so later, as it made the mass-production of tires much easier and significantly less expensive. The basic Banbury® concept is still used in many places, though a number of modern tire plants and other rubber manufacturers have transitioned to newer, usually digitally-controlled, alternatives.
The mixer was invented by British entrepreneur Fernley H. Banbury in 1916 while he was living in the United State and working for an American production company that he thought could benefit from the machine’s efficiency and power. 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.
The mixer proved invaluable to many different industries, including those focusing on the production of packaged food and pharmaceuticals. It had perhaps the greatest and most profound impacts on the automotive industry, however. 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. Factories using this mixer were usually able to produce more in less time and with less energy, which created a surge in productivity and also helped bring prices down for consumers.
While the equipment has continued to be refined through many feature enhancements over the years, the basic mechanism has remained unchanged. The Banbury® model 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, and 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 are perhaps most commonly used to describe the first steps of human digestion, the rubber is usually not chemically changed; rather, it’s simply made more malleable and pliable.
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