A large variety of articles are made directly from sheet rubber. The highest class of cut sheet rubber is used in the manufacture of such articles as tobacco pouches. Permanent joints can be made by simply pressing together the cleanly cut edges of the sheet. In order to prevent adhesion at other points, French chalk is dusted over the surface. If lower grade sheet is used, it may be necessary to moisten the edges with solvent or with rubber solution in order to make a permanent joint
Block rubber is reconstructed from sheet for the manufacture of such articles as railway buffers. The sheet is rolled up into a cylinder and firmly pressed together. Greater strength is thus obtained than by simply blocking the mixed rubber. The nature of the increased strength may be understood from the analogy of heavy guns wound from wire, which are much stronger than the old fashioned guns cast in one piece.
Large tubes, such as the inner tubes for motor tyres, are made by rolling the sheet rubber round an internal mould known as a mandrel. The edges of the sheet are joined together with solution. The ends of the tube are cemented up after vulcanisation and removal from the mandrel. The removal of the tube is effected by the aid of compressed air. Smaller tubes of a simple kind, and such articles as solid rubber tyres, are made by "squirting" the rubber mixture through a die by means of a forcing machine. Such a machine consists of a large screw working inside a cylinder, and so forcing the rubber forward after it has been fed in through a hopper at the top of the machine. In making tubes a rod or mandrel is made to pass through the centre of the screw and of the die, and the rubber is forced round the mandrel, which is moved forward at a suitable rate. The mandrel may be first coated with canvas and the rubber forced over it Another layer of canvas can then be applied, and the forcing process repeated, until a large tube has been built up of alternate layers of canvas and rubber. Other tubes may be built up by hand on a mandrel with alternate layers of sheet rubber and cloth. The covers of electric cables are similarly constructed, or they may be wound with thin ribbons of rubber in a special machine. A large variety of other articles are prepared by moulding or pressing the rubber whilst in the plastic condition.
In making india-rubber balls and other hollow objects, a small blob of india-rubber containing no sulphur is placed against one side internally. When the ball cools after vulcanisation the walls collapse The ball is then inflated through a fine-pointed syringe pushed through the lump of soft rubber. When the syringe is withdrawn the hole is closed airtight by the unvulcanised rubber, and the inflated condition is thus preserved.
Various other simple articles are cut from rubber which has already been vulcanised. Rubber rings, for example, are sliced off the ends of vulcanised tubes.