Rubber Production and Manufacture

Coating.—Coating consists in laying a sheet of rubber com position on bare fabric or on fabric which has been previously frictioned or spread. As in the frictioning operation, the rub ber stock is fed between the upper two rolls and the fabric passes between the lower two. For coating, however, the lower two rolls must run at the same speed. The rubber sheet passing around the centre roll is pressed firmly against the cloth as it passes through the calender but is not forced completely through the meshes of the fabric as in frictioning. By means of four roll calenders two sheets of rubber may be formed simultaneously and applied one on each side of fabric passing between the sec ond and third rolls from the bottom. This is the method used for coating parallel cords fed from a creel in preparing them for use in tire construction. Coating calenders run at speeds of io to 4o yd. per minute. Shoe linings and facing cloth are coated without previous frictioning. Combined operations of frictioning and coating are frequently applied to the same fabric ; i.e., fric tioned two sides and coated one side, frictioned one side and coated two, etc.

Tubing Operations.

Tubing machines, or extruding ma chines, are devices for forcing continuous strips of rubber from a die. These strips may be tubular, rectangular, or any one of a great variety of irregular cross-sectional shapes. The tubing ma chine consists of a horizontal cylinder in which a power-driven screw rotates, forcing the rubber stock through a die inserted at the end of the machine. They are used for the production of tub ing, hose tubes, pneumatic tire treads, solid tire treads, inner tubes for pneumatic tires, channel rubber slides for the windows of automobiles, and many other articles. Specially designed tub ing machines are used for extruding tubes of one colour striped with rubber compositions of another colour. Tubing machines are designed, also, to apply a tube directly on poles fed through a hollow screw, or for covering hose run through the head at right angle to the axis of the screw.

Vulcanization (Curing).

Before vulcanization, rubber is weak, softened by moderate heat, rendered stiff by cold, soluble in gasoline or benzol, and easily plasticized and sheeted between warm rolls. After vulcanizing it is strong, not greatly softened by heat nor stiffened by cold, insoluble in gasoline, and will crumble if run between rolls. Unvulcanized rubber is easily deformed to assume new shapes permanently ; vulcanized rubber returns to its original form after deformation. Hot vulcanization, the process most generally used, is conducted in a number of ways.

Mold Cures.—Many articles are given their final form and vulcanized at the same time by application of heat and pressure to the rubber material in metal molds. Pneumatic and solid tires are placed in engraved steel molds which are then stacked one above the other in a vertical cylindrical heater fitted with a hydraulic ram. When the molds are all loaded on the plate at

the top of the ram a cover is firmly fixed on the vulcanizer and the stack of molds pushed by hydraulic pressure against the cover. Steam is admitted into the vulcanizer around the molds. The heat softens the rubber compositions, which flow into the depressions in the molds to form the tire with its special tread design and its lettering, and, as the temperature rises, causes vulcanization to occur. After vulcanization is complete the steam is blown off, the molds are cooled with water, the ram is lowered, the cover of the vulcanizer removed, and the ram again slowly raised as the molds are removed one by one. Pneumatic tire vulcanizers 54 in. in diameter, 17 ft. high, with a 14 ft. ram stroke, take from 12 to 32 molds in each heat. Pneumatic tires are cured from 5o min. to 225 min., depending upon size. Solid tires are cured for much longer periods,—from three to eight hours. More mod ern equipment for vulcanizing tires consists of two steam jacketed circular molds which close together like a watch-case around a tire, hence the name "watch-case heaters." Inner tubes for tires are vulcanized individually in toroidal shape in somewhat similar equipment. (For rubber tire manufacture, see TYRE.) Press Curing in Molds.—Presses consist of parallel-faced, steam-heated plates between which are placed metal molds in which the articles are formed. Pressure between the plates is secured, as in heater molding, by hydraulic pressure, but the article receives its heat only by transfer through the hollow metal plates and through the metal molds. Steam does not come into direct contact with the molds. The rectangular plates may be cast hollow or may be bored with a series of cylindrical channels through which the steam passes. Originally, presses contained but two steam-heated plates. More recently several plates one above the other, with molds between adjacent plates, are used. Hollow articles, such as tubes for pneumatic tires, syringe bulbs, and tennis or toy balls, are vulcanized in molds. The rubber is forced against the metal mold by internal steam or air pressure or by nitrogen formed by heating pellets of a mixture of sodium nitrite and ammonium chloride inserted inside the article before vulcan ization. Pneumatic tires are expanded against the mold by steam, hot water or air under pressure in expansible rubber bags inside the tires. Sheets of sponge rubber are vulcanized between plates separated to permit the composition to expand, as bicarbonate of soda or other blowing agent incorporated in it forms bubbles of gas on heating. Sponge rubber blown to twice the thickness of the unvulcanized sheet is said to have a "r00% blow." Special presses are made for curing long belts, a section of 20 to 36 ft. being cured at one time, and the belt moved and another section vulcanized.