How Are Radial Tires Made?
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Mixing Rubber with Chemicals
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Many ingredients are used in the construction of radial tires. In addition to as many as 30 distinct types of rubber, the process also requires metals, fabrics, dyes and other chemicals. The first step in the process involves the blending of several types of rubber with other additives that include pigments, oils and carbon black, a type of amorphous carbon produced by the incomplete combustion of several petroleum products. The resulting compound is put into an industrial mixer called a Banbury, which blends it into a thick goop. Once dried into shapes, the rubber compound is sent to several destinations in a factory complex. Some of the compound is made into slabs that are pressed, together with additional materials, between a series of rollers in an area called the breakdown mill. The slabs are next sent to the feed mill, cut into tire-wide strips and made into treads or sidewalls. Another portion of the original rubber-and-chemical compound is used to coat the synthetic fabric (typically nylon, rayon or polyester) that comprises the body of the tire. The fabric is fitted to the "bead" or "bead bundle," a circle of monofilament steel wires matched to the size of the wheel rim. The bead is also coated with the rubber compound.
The Tire-Building Machine
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Up until this point, none of the tire's components has been solidly formed into a recognizable shape. The tire-building machine, a cylindrical drum, serves as the template around which to build the familiar tire profile. The first material to be put onto the tire-building machine is the inner liner, made of a separately produced extruded halobutyl rubber that is applied in two layers. The inner liner's function, like the inner tube on a bicycle tire, is to create an airtight seal for inflation. Over the inner liner are added strengthening layers called "cords" (a double fabric layer), "apexes" (triangular extruded metal strips) and double "chafer strips" made of tough, hard rubber that are designed to protect the bead from damage. The steel belts are added next, followed by the tread. The entire tire is squeezed through rollers to mash all of the components together before it is sent to be cured.
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The Curing Process
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Before heat is applied, the tire is simultaneously inflated from within by a rubber bladder (to a pressure of about 350 PSI) and pressed inward from the outside. The outside mold delineates the treads, sidewalls if any and all markings on the sidewalls. The bladder is heated with hot gas, hot water or steam to approximately 350 degrees F. Curing times vary widely, with passenger tires being heated for as little as 15 minutes, while heavy equipment tires can take up to 24 hours. After release from the mold and a cooling-down period, the tire is held rigid and kept inflated to maintain its shape. It is eventually put onto a conveyor belt to be sent for final inspection.
Testing, Finishing and Inspection
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A variety of tests are performed on the tires. Quality control personnel choose tires at random from the line, X-raying them and cutting them open to monitor the integrity of their inner parts. The tires are spun against hard surfaces as a simulated road test, during which stability measurements are made. Tires are given a visual inspection for flaws or blemishes. Any sidewall coloration is also applied at this time. While most of the processes described here are common to all tire manufacturing, the materials and the specific techniques of manufacturing vary widely between manufacturers and models.
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