Making Rubber Products

In the manufacture of most rubber products, the rubber is kneaded, mixed with various substances, formed, and then vulcanized. The vulcanization process usually involves heating rubber to which a vulcanizing agent such as sulfur has been added.

Masticating

Rubber is first cut into small pieces and then masticated (kneaded to make it workable) in a rubber mill, a Ban-bury mixer, or a Gordon plasticizer.

The Rubber Mill

consists of two rollers that rotate at different speeds. The rollers thoroughly knead the rubber and make it plastic.

The Banbury Mixer

is a water-cooled chamber within which two rollers fitted with blades rotate in opposite directions. The Banbury mixer is the most commonly used masticating device.

The Gordon Plasticizer

is a long barrel within which a screw rotates. As the rubber is kneaded by the screw, its temperature is raised as high as 360°F. (182° C.). It is this heat, rather than mechanical kneading, that plasticizes the rubber.

Mixing

Various substances are added to produce rubber with desired properties. Sulfur is mixed with rubber to be vulcanized. Carbon black is added to tire-tread rubber and other kinds of rubber that must resist abrasion and exposure to light. Zinc oxide gives rubber a light color. Selenium makes rubber less likely to crack when repeatedly stretched. Paraffin helps protect rubber from deterioration by ozone. Other substances preserve rubber from oxidation, give it various colors, make it harder or softer, or give it other properties.

Extrusion

Many rubber products, including gaskets and sheeting, are made by extrusion—by being squeezed out of machines just as toothpaste is squeezed out of a tube.

Molding

Rubber heels, hot water bags, and a number of other products are shaped by molding. In compression molding, the proper quantity of rubber is placed in the lower part of a mold and the upper part is squeezed over it by a press. In transfer molding, fluid rubber is forced into hollow molds by hydraulic pressure. In injection molding, a screw mechanism forces fluid rubber into hollow molds.

Electrodeposition

Rubber gloves, overshoes, rubber-coated metals, and a number of other products are made in the same way that metals are plated with silver. A metal mold is connected to the anode, the electrode is connected to the positive pole of a source of direct current, and the mold is submerged in latex. When the current is turned on, the negatively charged latex forms a coating on the mold. When the coating reaches the proper thickness, it is stripped from the mold and vulcanized.

Dipping

Some articles, such as toys, balloons, and rubber gloves, are made by dipping a mold in liquid latex. The latex is then dried and stripped from the mold.

Other Processes

Tires are manufactured by the series of processes shown in the illustrations accompanying the article TIRE. Sponge rubber is produced by bubbles formed by gases released from certain ingredients by the heat of vulcanization. Hard rubber, known as ebonite and vulcanite, is made by adding 30 to 50 per cent sulfur to rubber before vulcanizing it. Hard rubber was once widely used for such products as bowling balls, telephone receivers, and storage battery cases, but in many instances it has been replaced by plastics.