Extrusion – a production method for producing molded polymer products with a constant cross-section. The method is based on the continuous extrusion of molten polymer (melt) through a forming element (die), followed by cooling and fixing the dimensions of the polymer product. Extrusion is the most productive method of processing of polymer materials into products and, along with injection molding, the most common. By this method can be processed thermoplastic polymers, composites based on them and reaction polymeric materials. Our company offers the production of plastic products by extrusion and co-extrusion.
|The principle of the extrusion process on a single screw extruder|
Depending on the mechanism of creating pressure on the die, extruders are divided into screw, plunger, disk, etc. Today, screw extruders are most widely used, which in turn are divided into single screw and multi-screw, among which the most common are twin-screw.
The principle of operation of a single-screw extruder is quite simple: the polymer material is fed under the action of gravity down from the loading hopper into the working chamber of the extruder (barrel) – feed section. The main characteristic of the screw is the ratio of its length L to diameter D (L/D). In the barrel the polymer is in a closed space between the wall of barrel and the rotating screw.
Due to the supply of heat to the barrel and, as a result, heating of the polymer material, as well as friction, the plastic moves along the cylinder and melts – this happens in the compression section. Subsequently, the polymer melt is fed into the forming head (die) – metering section.
With the development of technological progress, a new type of extruder for the processing of polymers appears – multi-screw, in particular, the twin-screw extruder has become most widespread. In the construction of twin-screw extruder a new mechanism for transporting polymer to the forming head was laid – forced.
The main advantage of a twin-screw extruder is a good capture of the polymer material in the feed zone, regardless of the aggregation state and particle shape, as well as the forced transportation of the material to the forming head.
|Screw and barrel of single screw extruder||Screw and barrel of twin-screw extruder|
The difference in the principles of operation of twin- and single-screw extruders is also associated with different mechanisms of creating pressure in the material. In a single-screw extruder, pressure develops due to the viscous flow of the polymeric material caused by the relative movement of the screw and cylinder, and in twin-screw extruders, the extrusion action of the turns of the conjugated screws is added to this factor.
In addition, twin-screw extruders provide a short time of polymer processing as well as a narrow distribution of residence time. At the same time, improved mixing and heat transfer characteristics allow for good temperature and processing time control, which is very important for the extrusion of products from materials with limited thermal stability, in particular polyvinyl chloride (PVC).
|The classic look of an industrial twin-screw extruder|
for processing of polymer and composite materials
There are a lot of varieties of twin-screw extruders. The most common are:
а) Extruders of extruding actions with screws with meshing. Depending on the direction of rotation of the screws, they can be with different directional of screws rotation (low-speed extruders for the production of profiles, high-speed for composing raw materials) and with unidirectional rotation of the screws (extruders with conical screws, extruders with parallel screws).
б) Extruders with screws without meshing, which are divided into extruders with different directional of screws rotation (with the same screw length or different). Coaxial screw extruders (extruders with internal movement of the polymer melt in the direction of extrusion; extruders with internal movement of the polymer melt in the opposite direction of extrusion, extruders with internal plasticization).
|Screws with meshing||Screws without meshing|
There are given the comparative characteristics of twin-screw extruders depending on the screw configuration.
|Twin screw extruders with screws with meshing||Twin-screw extruders with screws without meshing|
|Self-cleaning process possible||Self-cleaning is not possible|
|Good melting characteristics||Satisfactory melting characteristics|
|Good distribution mixing||Good distribution mixing|
|Good dispersion mixing||Poor dispersion mixing|
|Good degassing||Good degassing|
|L/D ratio ≤ 60||L/D ratio ≤ 60|
The proportion of twin-screw extruders with screws with meshing in the polymer and composite materials processing industry is much larger than extruders with screws that operate without meshing.
A comparative characteristic of twin-screw extruders depending on the direction of rotation of the screws is given.
Different directional rotation
Screw cutting step
High up to 1400 rpm
Fairly high 500 rpm
The main direction of use
Twin-screw extruders are also used in special operations of chemistry and technology of polymer and composite materials, in particular, extrusion of reactive polymers, polymerization of oligomeric products, degassing and modification of polymer raw materials.
The company Replast has experience in the production of plastic products by extrusion and co-extrusion on both twin-screw and single-screw extruders.
Author – chief technologist of Replast LLC, Ph.D. Masyuk Andrii
Chief Process Engineer at Replast-ltd