Raw Materials
The most important raw material in an eraser is rubber. The rubber may be natural or synthetic. Natural rubber is obtained from latex produced by the rubber tree (Hevea brasilienesis). Synthetic rubber exists in a wide variety of forms. The most common synthetic rubber is derived from the chemicals styrene and butadiene. Styrene is a liquid derived from ethylbenzene. Ethylbenzene is usually made from ethylene and benzene, both of which are derived from petroleum. Butadiene is a gas, derived either directly from petroleum or from substances known as butanes and butenes, which are derived from petroleum.
Other ingredients added to rubber include pigments that change the color of the eraser. White can be produced with zinc oxide and titanium oxide. Red can be produced by iron oxide. Many other colors can be produced with various organic dyes.
An important ingredient added to almost all rubber is sulfur. Sulfur allows rubber to be vulcanized. This process was invented by Charles Goodyear in 1839. It uses heat and sulfur to make rubber more durable and resistant to heat.
Various other ingredients may be added to rubber. These include vegetable oil, to make the rubber softer and easier to shape, and pumice, a natural mineral which makes the eraser more abrasive.
Making erasers
Rubber arrives at the eraser factory and is mixed with pigments, vegetable oil, pumice, sulfur, and other ingredients that modify the properties of the final product. Synthetic rubber is easier to mix because it usually arrives as a powder or a liquid. Natural rubber usually arrives in bales and must be pulverized into powder or dissolved in a solvent before it can be mixed.
The mixture is heated, causing the sulfur to vulcanize it, making it more stable. To make plugs, which will be attached to pencils, an extrusion process is usually used. The mixture, in the form of a soft solid, is forced through a die to form a long cylinder. The cylinder is repeatedly cut as it emerges, forming plugs.
To make flats, which are not attached to pencils, an injection molding process is usually used. The mixture, in the form of a warm liquid, is forced into molds and allowed to cool into a solid. The flats are then removed from the molds.
Plugs are shipped directly to pencil manufacturers. They are attached to pencils by small, cylindrical, metal cases known as ferrules. Ferrules are made of plain aluminum for less expensive pencils, or painted brass for more expensive pencils. The ferrule is attached to the pencil with glue or with small metal prongs. The plug is inserted into the ferrule and clamped around it.
Flats may be marked with the name of the manufacturer or other markings. This may be done by stamping—pressing an inked stamp on the eraser. It may also be done by screen printing—moving an inked roller over a patterned sheet of silk or another material which covers the eraser. Three-dimensional markings can be made by embossing—cutting into the eraser with a sharp die. The completed flats are packed into cardboard boxes and shipped to retailers.