Manufacturing Process - Forward & Backward Extrusion Process
Extrusion of Polymers
Mixing is generally done with a twin-screw extruder, and the single-screw extruder, which is used in product production, is generally overlooked as a device for mixing.
More than 15 years have passed since the first edition of Polymer Extrusion was published - making it one of the most enduring (and best-selling) books on the subject.
Covers the fundamental aspects of polymer mixing as well as modern mixing equipment, emphasizing the basic mechanisms involved in the different mixing processes with a quantitative description of each process.
Extrusion fits under the general category of forming in manufacturing processes. The term applies to a variety of processes that involve confining a material in a container and applying a force to push the material through an orifice to produce a required shape.
There are several different types of extrusion:
- Direct Extrusion of Metals, where a heated billet is forced through a die to produce long continuous lengths limited by the amount of material held in the container. The die shape can be simple or complex, a good example of a simple die is for the production of extruded bar.
- Direct Extrusion of Polymers, this is where polymer granules are fed into a heated barrel with a screw feed. The polymer melts and is compacted before being forced through a shaped die.
- Forward Extrusion, where metal is forced to flow in the same direction as the ram being used to apply pressure. The ram is close fitting to the die cavity consequently preventing material extruding past it and controlling the flow of the material in the same direction as the ram. A loose fitting pin can be used to create a cavity in the material as it is forced past.
- Backward Extrusion, as the description suggests is the opposite of forward extrusion and is where metal is forced to flow in the opposite direction to the ram. The ram can be recessed to create a mirror image of the recess in the material as required.
- Indirect Extrusion, this is where a heated billet is backward extruded by a smaller diameter punch containing a die e.g. a washer shaped die can be forced into the billet with the result that the material is extruded back through the centre of the washer. This method produces shorter lengths than the forward extrusion process.
The direct and indirect extrusion processes produce continuous shapes where re-entrant angles can be generated on 2D profiles. Froward and backward extrusion processes are used to produce small 3D components.
Ductile metals, rubber, thermoplastics and some thermosets can be produced in this way. The quality of the components produced is generally good but with metals the forming temperature will cause the surface texture to deteriorate as it increases. Tooling is normally dedicated and quite expensive, but production is rapid.
Direct Extrusion of Polymers
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