The constituents are weighed out and combined by a mixing process which must blend the ingredients thoroughly in a repeatable way. This is achieved either by an internal mixer, where the compound is mixed by two meshing rotors in an enclosed case; or by open mill mixing, adding the ingredients carefully into the “nip” between two steel rollers, typically of 30″ diameter.
The result of either process is a batch of uncured rubber compound. This is allowed to settle for a time before undergoing Quality Assurance tests. Once passed, it can be formed into suitable shapes for moulding.
Each moulding process has its own requirements for uncured material. Compression moulding, for example, requires a “blank” of material in a size which will fill the cavity exactly. Direct injection moulding needs relatively large quantities of compound in a continuous strip. Due to the nature of the injection process, material properties must be precisely measured and controlled to achieve the planned flow and cure behaviour, as well as the desired final characteristics of the rubber.
A variety of processes are used to produce material suitable for moulding:
Uncured material is produced in sheets of the desired thickness. Sometimes “blanks” are cut from the sheet, like pastry cutting.
Extruders force warmed compound through a shaped die. Any reasonable length of shaped material can be produced. Once cooled this is fed into the direct injection presses.
Extrusions as above are cut to required lengths as they emerge from the die. This process can be accurately controlled to produce blanks of precise volume for compression moulding.