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.