TYPICAL EXTRUSION MOULD: Most extrusion moulds are simply one round piece of steel with the profile of the intended extrusion wire cut into them. Allowances are made for the shrinkage / expansion of the intended compound. Extrusion dies are by far the least complex of the moulds.
MOULDING PROCEDURE: Although extrusion moulds are quite simple the extrusion moulding process requires great care in the set up and manufacture and final processing to ensure consistency of product during the run and over a period of time.
The beginning of a continuous strip of rubber compound is fed into the extruder. The material is heated and under pressure is forced through the die plate that has the correct profile cut into it. Variations in feed rate, temperature and pressure need to be controlled.
Unlike compression or injection moulding the rubber is not cured when it comes out of the mould. The raw rubber is laid out on circular or long trays (depending on the profile) and loaded into an autoclave for curing under heat and pressure.
For long continuous lengths a salt bath or microwave curing system may be used and for silicone extrusion a continuous heating line is used. The curing process used is dependent upon the quantity and profile of the extrusion required.
After curing the strip is further processed - cut to lengths etc - and rolled or packaged for dispatch.
GENERAL COMMENTS: These moulds are relatively cheap to build but because of the processing involved minimum run quantities will vary. Cut extrusion is often used for packing pieces rather than moulded parts as the unit cost is not as high, however, as cutting is not entirely automatic process, tolerances can't be controlled as easily as they can be with moulded parts.