Compounding is the operation of bringing together all the ingredients required to mix a batch of rubber compound. Each component has a different mix of ingredients according to the properties required for that component.
Rubber compounding is generally carried out on open mills or internal mixers.
An open mill consists of twin counter-rotating rolls, one serrated, that provide additional mechanical working to the rubber. The rolls can be heated or cooled as necessary. The rubber is placed on the rolls and mixing is achieved by the shearing action induced at the “nip” between the rolls. Additives are added in carefully weighed quantities during the mixing process. After the mixing operation is complete, the compound is removed from the mill in the form of sheet.
Internal mixers are often equipped with two counter-rotating rotors in a large housing that shear the rubber charge along with the additives. The mixing can be done in three or four stages to incorporate the ingredients in the desired order. The shearing action generates considerable heat, so both rotors and housing are water-cooled to maintain a temperature low enough to assure that vulcanization does not begin.
