After some experience inspection of any article will show how it was moulded. The fin will indicate the joint in the mould, and with this as a clew the mould can be almost certainly constructed like the original.
India Rubber Bulbs.—Bulbs and hollow articles generally, such as dolls, toys and the like, cannot be made without special high pressure hollow moulds. The general process consists in cutting out gores from mixed sheet as for a balloon. The edges are coated with cement (thick benzole or carbon disulphide india rubber solution) and while the rubber is warm the seams are pressed and knitted together with the fingers. A hole is left in one place through which some pure water or water of ammonia is introduced. The bulb is now blown up with the mouth or otherwise, and while inflated the hole is pressed shut. This is often done with the teeth. Any projections around the seams are cut off with curved scissors. The mould is of iron and in two halves. Powdered talc is applied, and the bulb is placed in and shut up in the mould which it should exactly fill. The mould is clamped together and the whole is put into a vulcanizer, and the rubber is cured. The steam and vapor formed by its liquid contents expand it and press it with great force against the sides of the mould. After curing the mould and bulb are removed from the vulcanizer, cooled by a shower bath of cold water, the mould is opened and the bulb is removed. Often an iron pin is left projecting through the side during the vulcanizing, which pin, when withdrawn, leaves the necessary aperture, or it is perforated. The bulbs are polished by tumbling in a revolving cylinder. Considerable skill and practice are needed to succeed in making hollow bulbs. Great accuracy is needed in cutting out the gores and in joining the seams.
CHAPTER X.
THE MANIPULATION OF MASTICATED SHEET RUBBER.
The manipulation of pure sheet rubber is simple, yet is liable to lead to disappointment. When two pieces are laid face to face and cut across with a sharp knife, or scissors, the edges will adhere with considerable tenacity. This may be increased by applying some thick solution of india rubber in a volatile solvent, and by manipulating the sheets so as to bring the entire surfaces of the cuts together. Finally the material may be charged with sulphur by absorption or by Parkes’ process, and cured in a glycerine or calcium chloride bath, all of which are described in chapter XI. The same treatment will affect the cement used in making the joint also, bringing about its vulcanization.
Such in a few words is the main process in the treatment of this class of goods. Where it is desired to prevent adherence, soapy water or powdered talc is used.
Adherence may be produced between the surfaces of the sheets if they are clean, by pressure and a little warmth. The method of making toy balloons will give an example of how the article is dealt with by the manufacturer.
A pile of pieces of masticated sheet rubber is made. Every piece has one side coated with powdered talc, and two talc-coated sides are placed in contact in each pair. As they are piled up, the outer surfaces of each pair are moistened with water. A steel punch or die, pear shaped in outline, is used to cut down through the pile, cutting all the pieces into that shape.
The pile is then taken apart in pairs. The separation takes place between the wet surfaces, the edges of each pair adhering slightly so as to enclose the talc-coated surfaces. The neck is opened if necessary. A rather weak or thin solution of india rubber in benzole is now brushed over the freshly cut edges. By pulling out the centre of each piece the edges are brought into contact, and adherence is produced.
If the Parkes process of vulcanizing, chapter XI., is employed they are cured to the slight extent necessary upon a tray coated with talc. The balloons are then ready for inflation.