Shaping processes for the rubber products can be divided into four basic categories. They are:
In shaping process, some products require several fundamental processes plus assembly work in their manufacture, for example, vehicle tires.
Figure 1: Mixers used in rubber processing: (a) two-roll mill and (b) Banbury-type internal mixer.
Two machines shown in above ‘figure 1’ can also be used for mastication of natural rubber.
1. Extrusion:
Screw extruders are used for extrusion of rubber. As with extrusion of thermosetting plastics, the L/D (length to diameter) ratio of the extruder barrels is less than for thermoplastics, usually in the range between 10 to 15, to reduce the risk of early cross–linking. Die swell occurs in rubber extrudates because the polymer is in a highly plastic condition and exhibits the memory property. It has not yet been vulcanized.
2. Calendering:
Figure 2: Roller dies process rubber extrusion followed by rolling.
The calendering 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 a heavier type of construction compared to used in thermoplastics industry, because rubber is more viscous and harder to form compared to others. The output of the calendering process is a rubber sheet of thickness determined by the final roll gap; again, swelling occurs in the sheet, causing its thickness to be bit greater than the gap size. The calendering process can also be used to coat or impregnate textile fabrics to produce rubberized materials.
There are few problems in producing thick sheet by either extrusion or calendering process. Thickness control is difficult in the former process, and air entrapment occurs in the latter. These problems are largely solved when extrusion and calendering processes are combined in the roller die process (Figure 2). The extruder die is a slit that feeds the calender rolls.
3. Coating:
Coating or impregnating fabrics with rubber is an important process in the rubber industry. These composite materials are used in automobiles tires, conveyor belts, inflatable rafts and waterproof cloth for tarpaulins, tents and raincoats. The coating of rubber onto substrate fabrics includes a variety of processes. ‘Figure 3’ illustrates one possible way in which the fabric is fed into the calendering rolls to obtain a reinforced rubber sheet.
4. Molding and Casting:
Molded articles include shoe soles and heels, gaskets and seals, suction cups, and bottle stops. Molding produces many foamed rubber parts. The molding process is important in tire production.
Principal molding processes for rubber are
- Compression molding,
- Transfer molding, and
- Injection molding.
Figure 3: Coating of fabric with rubber using a calendering process.
Compression molding is the most important technique because of its use in tire manufacture. Curing is accomplished in the mold in all the above three principal molding processes, this representing a departure from the shaping methods, 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.
A form of casting, called dip casting, is used for producing rubber gloves and overshoes. It involves submersion of a positive mold in a liquid polymer for a specified duration to form the required thickness. The coating is then stripped from the form and cured to cross–link the rubber.
