Polymer granules are placed in a feed hopper, heated to a molten state and injected at high pressure through a die with a specific profile to give a uniform cross section and cools as it leaves the die cavity.
The process uses thermoplastics, such as: polyvinyl chloride (PVC), polyethylene (PE) and polypropylene (PP).
One or more injection screws can be used for the process which can be coupled with other processes such as blow moulding so that it is possible to inflate the extruded component. Plastic coatings can be produced with the use of offset dies to combine the extrusion process with metal wire, strips or sections.
Pultrusion process is a means of production for fibre-reinforced rods, tubes and sections.
The process has moderate tooling costs but with very high equipment costs as materials that give off toxic or volatile gases during extrusion demand air extraction or a washing plant. Lead times are usually within weeks depending on die complexity and the process flexibility is limited to the dedicated tooling but with short set-up and changeover times.
The process has extremely high production volumes as continuous lengths are produced at 60m/min and 5m/min for tube profiles and sheet/rod profiles respectively. Multiple holes in the die increase the production rate but to allow smooth running the extrusion is operated below its maximum speed capability. It is economical to run production for 1000kg of profile extrusion and 5000kg for sheet extrusions and costs are dramatically increased with a high output range.Cutting a continuous section to length is the only time there is waste material produced and the only major finishing cost which is relatively inexpensive.
Implications For Design
The final component design always has a uniform cross section which can be highly complex. Open or closed profiles are possible and solid forms can include re-entrant angles. Holes, inserts and grooves are possible but require secondary operations when they are not parallel to the axis of extrusion. Part consolidation features should be integrated into the design for increased ease of assembly
The section range of the component is 0.4150mm for profiles and 0.02150mm for sheets, their size range is 0.4150mm for profiles, 0.02150mm for sheets extruded. The components need no draft angle.
Continuous extrusion produces components that may have complex profiles. Rods, bar, tubing and sheets e.g. pipes, guttering, window sills, insulation on wires, cling-film and tyre reinforcement.
Components made by the process are prone to shrinkage and distortion so that the accurate control of uniformity and cooling rate is important to reach accurate dimensions. The extruded product may increase in size as it leaves the die and may be compensated by decreasing extrusion rate or the melt temperature. One problem that affects the output and quality of extrusion is the possibility of trapped gases in the extrusion screw which is particularly common when using powdered raw materials.