TREAD, SIDEWALL AND INNERLINER
Tire tread, or the portion of the tire that comes in contact with the road, consists of tread itself, tread shoulder, and tread base. Since there are at least three different rubber compounds used in forming this complex tread profile, the extruder system consists of three different extruders sharing an extruder head. Three rubber compounds are extruded simultaneously from different extruders and are then merged into a shared extruder head. The next move is to a die plate where the shape and dimensions are formed, and then through a long cooling line—from 100 to 200 feet long—to further control and stabilize the dimensions. At the end of the line, the tread is cut according to a specific length and weight for the tire being built.
THE TREAD AND SIDEWALL
Tire components such as tread, sidewall, and apex are prepared by forcing uncured rubber compound through an extruder to shape the tire tread or sidewall profiles. Extrusion is one of the most important operations in the tire manufacturing process because it processes most of the rubber compounds produced from the mixing operation and then prepares various components for the ultimate tire building operation.
The extruder in a tire manufacturing process is a screw-type system, consisting primarily of an extruder barrel and extruder head. First, the rubber compound is fed into the extruder barrel where it goes through a heating, blending, and pressurizing process. Then, the rubber compound flows to the extruder head where it is shaped under pressure. The modern cold-feed extruder is computer-controlled for accuracy.
The tire sidewall is extruded in a way similar to the tire tread component; however, its structure and the compound used are quite different from tread. Sometimes the sidewall extrusion process can be more complicated, and four extruders may be needed; for example, when building a tire with white sidewalls or with white lettering on the sidewalls.
The innerliner is just what it sounds like -- the inner-most layer of the tire. Its main functions are to retain the compressed air inside the tire and maintain tire pressure. Due to its low air permeability, butyl rubber—or halogenated butyl rubber compound—is the primary rubber compound used. Because this is a thin layer, it is also produced using the calender. The gauge control and no-defect surface finish are critical to retaining air pressure. Innerliner calendering is also a continuous operation. The proper length of innerliner sheet is pre-cut to be ready for the tire building process.