468 



and depths according to the nature of the work. Cold water is 

 always playing over the surfaces of the rollers in use, and it is 

 directed usually above the rollers on to the rubber in the hopper, 

 but in some cases at the sides, with the object of washing the dirt 

 away from the rubber as the rubber emerges from between the 

 rollers. The rubber usually goes through two sets of rolls, the first 

 two break down the big lumps and roughly sheet the rubber, the 

 second pair of rolls is smoother and in closer contact, and the final 

 washing and working into a fine-grain uniform sheet is performed 

 on these. In preparing rubber for the very finest work, such as 

 cut sheet, a third set of rollers with smooth chilled steel surfaces 

 is used. The object of these is to crush any particles of sand or 

 grit which might otherwise be left in the rubber and damage the 

 knives and spoil the sheet rubber when being cut. The rollers are 

 always provided with metallic guides to keep the rubber away 

 from the ends of the rolls and prevent contamination with grease 

 and oil from the bearings. There is great uniformity of pattern 

 and general arrangemeut of these washing mills in all the factories, 

 the differences are in the form of grooving and roughening of the 

 surfaces. The rollers which produce the smoothest and most uni- 

 form sheets are those in which the grooves are nearly obliterated, 

 and in which the surface has become rough with the natural wear 

 of the metal : rolls in this condition w ould, I think, be most 

 effective with freshly coagulated latex on an estate. 



DRYING. 



19. The rubber in the washed or crepe form is wet not only 

 with surface moisture but with water held in the substance of the 

 rubber itself. It is usually dried by hanging up the strips in dark 

 rooms warmed to about 90 F., an operation taking about a week 

 or ten days. In no case did I notice any artificial circulation of 

 the air to accelerate the drying. A few manufactories have adopt- 

 ed vacuum drying, which I have already described and discussed. 

 There is no sign, however, of this process ousting the older fa- 

 shioned method of simple air drying. 



MASTICATING RUBBER. 



20. The next process through which the washed and dryed rub- 

 ber passes is that of mastication, during which the rubber is torn, 

 stretched, heated, and generally kneaded about until the toughness 

 and elasticity, so characteristic of it hitherto, are destroyed, and 

 the rubber becomes plastic. 



The masticating machine consists of two steel rollers with 

 smooth polished faces, which revolve on horizontal axes in the same 

 horizontal plane. The distance between the two rolls can be adjust- 

 ed until they are brought into contact with one another. The rolls 

 may be of any convenient size, and are usually about 3 feet in 

 length and 12 to 18 inches in diameter. They are hollow and 

 heated by injected steam, and may be driven at even or differential 

 speeds. The machine, in fact, is in many respects similar to a 

 rubber-washing machine, but differs in the rollers being smooth 



