Shaping processes for rubber products can be divided into four basic categories: (I) extrusion, (2) calendering, (3) coating, and (4) molding and casting. Some products require several basic processes plus assembly work in their manufacture, tires being an example.
Extrusion. Screw extruders are generally used for extrusion of rubber. As with extrusion of thermosetting plastics, the L/D ratio of the extruder barrels is less than for thermoplastics, typically in the range 10 to 15, to reduce the risk of premature cross-linking. Die swell occurs in rubber extrudates, since the polymer is in a highly plastic condition and exhibits the memory property. It has not yet been vulcanized.
Calendering. This process involves passing rubber stock through a series of gaps of decreasing size made by a stand of rotating rolls. The rubber process must be operated at lower temperatures than for thermoplastic polymers, to avoid premature vulcanization. Also, equipment used in the rubber industry is of heavier construction than that used for thermoplastics, since rubber is more viscous and harder to form. The output of the process is a rubber sheet of thickness determined by the final roll gap; again, swelling occurs in the sheet, causing its thickness to be slightly greater than the gap size. Calendering can also be used to coat or impregnate textile fabrics to produce rubberized fabrics.
There are problems in producing thick sheet by either extrusion or calendering. Thickness control is difficult in the former process, and air entrapment occurs in the latter. These problems are largely solved when extrusion and calendering are combined in the roller die process. The extruder die is a slit which feeds the calender rolls.
Coating. Coating or impregnating fabrics with rubber is an important process un the rubber industry. These composite materials are used in automobile tires, conveyor belts, inflatable rafts, and waterproof cloth for tarpaulins, tents, and rain coats. The coating of rubber onto substrate fabrics includes a variety of processes. Calendering is one of the coating methods. Fabric is fed unto the calendering rolls to obtain a reinforced rubber sheet.
Alternatives to calendering include skimming, dipping, and spraying. In the skimming process, a thick solution of rubber compound in an organic solvent is applied to the fabric as it is unreeled from a supply spool. The coated fabric passes under a doctor blade that skims the solvent to the proper thickness, and then moves into a steam chamber where the solvent is driven off by heat. As its name suggests, dipping involves temporary immersion of the fabric into a highly fluid solution of rubber, followed by drying. Likewise, in spraying, a spray gun is used to apply the rubber solution.
Molding and Casting
Molded articles include shoe soles and heals, gaskets and seals, suction cups, and bottle stops. Many foamed rubber parts are produced by molding. In addition, molding is an important process in tire production. Principal molding processes for rubber are (1) compression molding, (2) transfer molding, and (3) injection molding. Compression molding is the most important technique because of its use in tire manufacture. Curing (vulcanizing) is accomplished in the mold in all three processes, this representing a departure from the shaping methods already discussed, which require a separate vulcanizing step. With injection molding of rubber, there are risks of premature curing similar to those faced in the same process when applied to thermosetting plastics. Advantages of injection molding over traditional methods for producing rubber parts include better dimensional control, less scrap, and shorter cycle times. In addition to its use in the molding of conventional rubbers, injection molding is also applied for thermoplastic elastomers. Because of high mold costs, large production quantities are required to justify injection molding.
Dip casting is used for producing rubber gloves and overshoes. It involves submersion of a positive mold in a liquid polymer (or a heated form into plastisol) for a certain duration (the process may involve repeated dippings) to form the desired thickness. The coating is then stripped from the form and cured to cross-link the rubber.
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