As some of the india rubber is sure to protrude, the progress of the work can be watched from its action. By pressing the point of a knife against it the period of vulcanization can be told. Before the material is heated it is elastic and resists the pressure of the knife; as heat is applied it becomes soft like putty; as the heat increases it again stiffens and becomes quite elastic. At this point the press can be opened and the sheet and matrix can be taken out or the platen swung aside. On pulling or stripping the sheet from the matrix it will be found to reproduce the model in elastic india rubber to the minutest detail.
Oil Stove for Heating Vulcanizers.
As regards the minor details there is something to be said. Distance pieces to gauge the thickness have been recommended for the home-made press, [page 48]. Care must be taken to have these low enough to provide for enough excess of material to produce a good impression. For ordinary stamp work they should allow about one-sixteenth of an inch for the “squeeze.” It will be seen that by using the distance or gauge pieces both for making the matrix and for moulding and curing the stamp, absolute parallelism of surfaces will be secured.
The reader will have noticed in the description and will find at once in practice that the press has to be screwed up as the rubber softens. Where heavy iron presses are used the large mass of heated iron comprised in the platen of the press instantly heats the upper surface of the india rubber sheet and the heat immediately penetrates into it, while the heated matrix heats it from below. Thus it softens at once, and the press is directly turned down and the india rubber is driven into the mould and curing at once begins. But where small presses are used this manipulation is not so easy. For such the springs mentioned on [page 51], are highly to be recommended. The matrix and india rubber can be put into the cold press, and the tympan with intervening springs can be screwed down so as to compress them. Then on applying heat the moulding takes place automatically.
With a hot press and good sheet a period of three to ten minutes is ample for moulding and curing.
Instead of sprinkling with talc the matrix may be oiled and sprinkled with plumbago and afterwards polished with a brush. This is not so clean a material as talc and is not to be recommended for general use, especially as oil is a bad substance to bring in contact with rubber.
The distance or gauge pieces whose use has been recommended are not necessary where presses working truly parallel as regards their opposing faces are used. But where home-made apparatus is used they will be found a valuable addition.
In describing the simple press it was said that it could be made of wood. It is evident that a wooden press could not be used for direct heating. Such a press must be used in a hot chamber or vulcanizer, properly so called. Originally rubber stamps were generally made in chamber vulcanizers.
The next [cut] shows a combined matrix making, moulding and vulcanizing apparatus of very convenient and compact form and adapted for rapid work. As the press stands in the cut the matrix press is seen in front. A box or chase is carried under its platen by two trunnions, so as to be free to oscillate to a limited extent. The type model is secured in this box. Above this box or chase is a cross-bar with screw and platen attached, connected at will to two standards or pillars, so as to constitute the matrix press.