This material is incompletely vulcanized. It is in its present condition very amenable to heat and is ready for any moulding process. Generally it is rolled out or “calendered” into sheets of different thickness from which articles are made in moulds by curing.
These sheets are of especial interest to the reader as they are the material from which most small articles are made, including rubber stamps.
This rolling of the mixed india rubber into sheets of definite thickness is done by special calendering rolls. The product is termed “mixed sheet.”
In the mixing rolls the incorporation of other material is often brought about. Zinc white, lead sulphide, antimony sulphide, chalk, clay, talc, barium sulphate, plaster of paris, zinc sulphide, lead sulphate, white lead, oxides of lead, magnesia, silica, form a list of ordinary mixing ingredients. These lower the cost of the finished material and are often serious adulterants. For some cases the addition if not carried too far is not injurious, or even may be beneficial. A proper admixture renders the gum more easily moulded and treated in the shaping processes.
Mixing Rolls.
The next step in the vulcanizing process is the heating of the mass, which step is called “curing.” Up to a temperature in the neighborhood of that of boiling water the mixed rubber can be heated without change except as it is softened. But if the heat is increased it begins to get a little more elastic and less doughy, and eventually becomes “cured” or vulcanized. The temperature for vulcanization is about 284° F. (140° C.). The word “about” is used advisedly, for it is not only a question of heat but of time of exposure. After vulcanizing, including the curing, india rubber cannot be moulded to any great extent. In the manufacturing process, therefore, it is before curing placed in the moulds, heated, shaped by pressure, and by exposure to a higher heat in a steam oven called a vulcanizer, is at once cured.
To prevent adherence to the moulds they are dusted over with ground soapstone, and the rubber itself is often thus coated.
The methods of vulcanization and curing, which may be of special use to the reader, are given in the chapters devoted to that subject (chapter XI.), and in the one devoted to rubber stamps.
Hard rubber, termed ebonite when black, and vulcanite when of other colors, is simply vulcanized rubber containing a large percentage of sulphur added in the mixing process.