CHAPTER IV.
THE MANUFACTURE OF MASTICATED, MIXED SHEET, AND VULCANIZED INDIA RUBBER.
The manufacture of india rubber relates to the production of two principal products. One is masticated unvulcanized sheet and thread rubber; the other is unmasticated mixed and cured rubber, otherwise vulcanized rubber. For the purposes of the rubber-stamp maker an intermediate product is required, namely, unmasticated mixed sheet which is uncured. This is really incompletely vulcanized india rubber.
It will be evident from the description to come that it is not advisable for any one without considerable apparatus to attempt to clean and to wash (“to sheet”), to masticate, or to mix india rubber. These operations are best accomplished in the factories. The partially vulcanized (“mixed sheet”) or the pure masticated article are regular articles of commerce. Yet a full insight into the manipulation of india rubber can only be obtained by understanding its treatment from the gum up to the two separate lines of products we have indicated.
A third type of product is coated tissue, such as Mackintosh. This really is a sequence of one of the other two processes and a few words will be said of it in concluding the chapter.
As the caoutchouc is received by the manufacturer it appears an utterly intractable mass. It occurs in lumps of every size, varying in color and odor, and very tough but elastic. In virtue however of the properties already described, its power of cohering when cut, and its softening when heated, it becomes amenable to treatment.
It is to some extent received in such assorted condition as to secure even grades, and then each grade may be washed by itself. It is thrown into water which is in many cases kept at the boiling point by steam-heat and left there for some hours. It absorbs some water and also softens. Some gum is so soft that it will not stand hot water. For such the water is kept cold. The purer gum floats; such pieces as have stones, dirt, iron, etc. in them, perhaps placed there purposely from fraudulent motives, sink and can be picked out for separate treatment.
The lumps are next cut up. A revolving circular knife driven by power is often used, and sometimes an ordinary knife is adopted. At this part of the operation there is frequently need for sorting, as the grades received may have inferior pieces mixed with the good. The cutting is mainly to secure good grading, and to remove concealed impurities. The gum then goes to the washing rollers, called the washer and sheeter. (See cut, [p. 37].)
Washer and Sheeter.
These are heavy corrugated rolls made very short, 9–18 inches in length, to prevent springing. They are grooved or corrugated and have a screw adjustment for regulating their distance apart. They are geared together so as to work in corresponding directions, like a clothes wringer or a rolling mill of any kind. The pieces of gum are fed into the rolls and are drawn between and through them. The friction tends to heat the gum. To prevent this and also to effect the washing, a supply of water, either hot or cold, is kept playing upon the mass. This dissolves out all soluble matter and washes away mechanically the chips, dirt, etc. which may be present. The whole operation is one of main force. The caoutchouc is torn and distended and delivered as a rough perforated sheet. It is passed repeatedly through the machine, the rollers being gradually brought closer together, or else different sets of rolls are used, set to different degrees of fineness. The wash water passes through a screen which catches any small detached fragments of gum.