RUBBER PLANTING 223 



Vacuum driers are also under trial in certain 

 factories, but it is said that rubber dried in this way 

 does not recover its " nerve " so completely as it would 

 if the slower process of air drying were adopted. 

 Drying machines of this type are, however, frequently 

 employed for desiccating the materials used for mixing 

 with the rubber, thorough dryness being essential in the 

 latter process. 



Mastication and Mixing. 



Mastication is sometimes carried out in a machine 

 resembling a powerful churn or sausage machine. For 

 most purposes rollers are employed similar to or 

 identical with the mixing rollers. Mixing is always 

 preceded by a masticating process, introduced in order 

 to soften the rubber. The dried rubber is passed 

 repeatedly between a pair of hollow rollers, which can 

 either be heated internally by steam, or cooled by 

 passing cold water through the cavity. The rollers 

 revolve at different rates, and the distance between 

 them is adjustable. The rollers are gradually brought 

 closer together until the soft rubber adheres to the 

 slower moving roller and passes round with it, being 

 subjected to a kneading process as it passes the second 

 roller, which revolves more rapidly. At the beginning 

 of the process the rollers are warmed, but later on 

 friction may give rise to so much heat that it is 

 necessary to pass cold water into the interior of the 



