RUBBER PLANTING 229 



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 syringe is withdrawn the hole is closed airtight by the 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. 



