Block Rubber.—After the block is taken from the masticator, it is forced into strong cast-iron moulds, which are first moistened inside with a little soap and water, to act as a lubricant. The blocks are produced in two forms, depending upon the means at hand for cutting them up :— (1) They are forced into long iron boxes, fitted with covers, which are forced down by a screw-press ; as the mass yields, an extra turn is given to the screw. The block remains in the press for a few days or a week, when it is taken out, and placed in an icehouse or cool cellar. (2) Cylindrical moulds are now most generally employed, since from them are obtained blocks which can be out up into continuous lengths, whereas the other method will only yield sheets about 6 ft. long. These moulds, Fig. 846, consist of a cast-iron cylinder A, carefully turned inside, and sometimes enamelled, with a recess or projection at the bottom, upon which rests a strong circular plate or disc B, with a 4-in. hole in its centre. This is first fitted into the mould, then the rubber is forced in by sledge-hammers, and removed to a powerful screw or hydraulic press, where it is forced fairly down into the mould. On the top of the block, fitting evenly into the mould, is placed an iron plate C, similar to the one at the bottom; and through the central hole, is forced a strong bolt or pin P, which passes through the rubber and the hole in the bottom plate of the mould. After the whole is well pressed, the pressure on the blocks is secured by keys or wedges, which pass through a slot cut in the lower part of the bolt, and can then be removed from the press to make room for other blocka During the day, these blocks and moulds are again passed into the press, and forced with the screw, until they yield no more when other wedges are driven in as before. They are then transferred, without removal from the moulds, into a cool cellar or ice-house, until perfectly set and hard, when the moulds are emptied. The blocks remain in ice until required. They are sometimes hardened by being placed in a strong current of cold air, such as a chamber opening into the main shaft of the factory.
Other methods are in use for obtaining blocks of rubber from the raw and washed articles, which consist in forcing it into moulds without masticating, and consolidating it by placing the moulds, keyed or wedged together with their charge, into a heater at about 116° (240° F.) for a few hours. Washed Path yields good sound blocks in this way, but the rubber is deprived of more of its strength than if masticated, and is darker in colour.
Sheet Rubber.—The blocks are next cut up into sheets of different thicknesses. The square blocks are clamped to a plate, which can be raised to any height, according to the thickness of sheet required; this passes forward to an oscillating knife, which slices-up the rubber. The knife can be set in the opposite direction, so as to make another cut as it passes back again, and so on. The cylindrical blocks are forced upon a stout spindle, of the size of the bolts passing through them ; this spindle rotates in front of a similar, though much longer, knife ; the thickness of sheet is regulated by the feed-wheels, which are changed as required ; and when the machine is once started, a block can be out without any further attention, unless demanded by a defect in the machine itself, which occurs generally in the friction arrangement which works the feed-gearing.
These machines are worked at very high speeds, and a good supply of water is kept continually flowing over the knives. The sheets are generally hung up to dry and season, and are soaped, and laid carefully one on the other, or rolled up for stowage. This cutting-up must be done in a cool place, for if the rubber gets soft, it must be again placed in the ice-house to harden ; soft spots or patches in the blocks will lead to inequalities in the thickness. These sheets are manufactured for
tobacco-pouches, tubes, and other articles which are required from masticated rubber ; such have a series of very fine lines or marks, which correspond with the strokes of the knife in cutting. Raw Bard, or just crushed between rollers first, gives a curiously mottled sheet, very strong, and not so easily acted upon by grease or exposure ; it is consequently much used for the backing of wire " cards," and is even said to bo as durable as leather for this purpose. The production of this sheet is a specialty with a few manufacturers, who cut up the raw article, and wash it at the same time in a kind of mincing-machine, in which a series of knives work backward and forward, or transversely, as required.
Tape fillets or tapes are cut from sheets, by winding the quantity required upon a wooden mandril, and securing the last lap, so as to keep the whole firmly together, either by means of a little naphtha or solution of rubber in naphtha. Knives are forced against the rubber as it revolves in an ordinary lathe, whilst a jet of water flows over them. A series of fine cutters, worked by a self-acting slide-rest, may be used in the same way to produce thread.
The rubber, when cut, is very weak, in fact it cannot bear stretching at all. It is run off the rollers or mandrils into a wooden zinc-lined tank, containing water heated to 60° (140° F.); this process seems to anneal or seal the edges, for, on banging up for a few days after this operation, it is found to have acquired extraordinary strength. By carefully stretching it, and keeping it distended for some time, it remains elongated ; when thrown into warm water, it instantly regains its original dimensions, but is much strengthened ; by repeating this, its tensile strength may be increased 5-6 fold.
In passing tapes and threads into hot water, some care is required, as if they are allowed to stick too closely together, they are spoiled. After standing in the warm water for a little time, they are passed through the hands into cold water, to remove the stickiness, and are hung up to dry. Soap lye, alkalies, methylated spirit, and white finish, are sometimes used to prevent this stickiness. In telegraph work, water alone is used, as foreign matters prevent consolidation when heating. Rubber which has been passed only through methylated spirit, much better resists decay ; this may he due to the wood-naphtha present.
Stretched rubber keeps longer than unstretched ; this is not easily explained, although it seems that decay is a matter of simple oxidation. It must be preserved from direct sunlight and heat ; if heated, it rapidly changes into a viscid substance, unless vulcanized. If, at this stage, it be kept in a cool dark place, it undergoes no change ; but if it be heated, and then set by under the same conditions, it will gradually soften, and become rotten, after a time apparently drying up into a brittle resinous-looking substance. These changes are, according to the late Dr. W. A. Miller, due to the direct combination of oxygen with caoutchouc. The liquid condition which Precedes this drying-up is not satisfactorily accounted for.
Dunlop's tape-cutting machine, Fig. 847 is very convenient for producing fillets for insulating telegraph-wire. Discs of rubber, cut from a cylindrical block, are secured on the vertical shaft N, by screwing down the nut 0 ; by turning the handles T, the rubber is moved up to the fluted brass roller M ; the shaft C, worked by a band, gives a uniform motion to M, through the worms F G, and the spur-wheels