Twin screw extruder

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Twin screw extruders are machines from the plastics technology and food industry and belong to the multi- screw extruders or to the upper group of extruders . In plastics technology, they are used to prepare and shape plastic melts. For this purpose, the polymer is conveyed through a heated cylinder by two rotating screw shafts and melted in the process.

Construction of twin screw extruders in plastics technology

Co-rotating twin screw extruder gear unit with attached lubricating oil unit

The most important procedural element are two screw shafts , which are interlocking in a cylinder with a figure eight-shaped bore (see picture below). This cylinder, like the screw, can be made up of individual modules for flexible conversion, mainly for laboratory extruders. The worms are mounted on one side and are connected to the gear stage via splined shaft connections or feather key connections . They are usually driven by electric motors via an extruder gearbox . In order to melt the thermoplastic material , the shots are usually brought to temperatures that are approx. 20 ° C above the melting point of the material using electric heating cartridges or heating shells. Each shot is individually tempered and can be regulated according to the specifications of the process. Water circuits that can be individually controlled via relay circuits are used to cool the housing. In this way, overheating of the material with the associated harmful material degradation can be avoided. The heating and cooling are not used to melt the plastic, but to control the temperature of the process. Melting takes place via the introduction of energy through the rotation of the screw, thus largely through the power of the drive.

Types of twin screw extruders

Depending on the center distance between the worm shafts and the direction of rotation, a distinction is made between:

  • tangential co-twist twin-screw extruder
  • closely intermeshing co-twist twin-screw extruder
  • tangential counter-twist twin screw extruder
  • closely intermeshing counter-twist twin screw extruder

Counter-twist twin-screw extruders introduce little shear into the material and therefore put little stress on it. You are therefore z. B. used in the processing of temperature-sensitive materials such as PVC. The most important type, however, is the closely intermeshing co-twist twin screw extruder, which will be discussed below.

Snail design

The screw design is done in close connection with the planning of the necessary process steps. A large number of different design options are available for the implementation of certain tasks.

Process zones of twin screw extruders

In principle, the following process engineering areas can be characterized over the length of twin screws:

  • Feed zone: This is where the material is drawn in and conveyed
  • Melting zone: Here the material is melted and fillers etc. are predispersed
  • Compression zones: The material is compressed by a decreasing gradient, which increases the degree of filling in the screw flight and the pressure rises once it is full
  • Mixing zones: The mixing performance is increased by specially integrated mixing elements in the screw. This is particularly important for the homogeneous incorporation of additives such as carbon black, colorants, etc.
  • Zone degassing : In order to draw volatile constituents from the melt, the filling level in the screw cylinder must be below 100%, as otherwise the melt would be forced into the degassing nozzle.
  • Discharge zone: The discharge zone creates the necessary pressure build-up to overcome the flow resistance of the tool.

Geometric relationships

Cross-sectional drawing through the barrel of a twin screw extruder - Geometric relationships

The funding effect is based on the principle of forced funding. The drifting flank of one screw transfers the melt at the crotch point of the drifting flank of the other screw. As a result, flow rearrangements and shearing of the material occur again and again.

In the intake area, the screw elements are used to increase the conveying capacity on the effective conveying flank, i.e. H. the "pushing" flank is flattened. These elements are also known as push-edge elements. Toothed mixing and kneading elements are used to achieve an additional mixing effect. They may or may not be eligible for funding.

particularities

Twin screw extruders are much more expensive to manufacture than single screw extruders . This is due on the one hand to the smaller area of ​​application and the associated lower number of pieces, as well as to the more expensive machine technology. Due to the high loads in the gusset area, the screws are pushed outwards and must therefore be resistant to bending. In addition, they have to be made harder and more wear-resistant than screws for single-screw extruders. The bearing and the figure-eight hole are also more complex to design and manufacture. Particularly noteworthy are the significantly better mixing effect compared to single-screw extruders, the self-cleaning properties of twin-screw extruders (constant twist, closely intermeshing) and the very good degassing performance.

The process parts in twin-screw extruders are sometimes subject to extremely high corrosive and / or abrasive wear. With the right choice of material for the screw elements, housings, bushings or screw core shafts, the service life can be increased considerably and costs can be reduced.

Areas of application

Due to their good mixing effect, twin screw extruders are used in the plastics industry for the following purposes:

In the chemical industry they are also used for the crystallization and degassing of polymers.

They are also used for shaping in the food industry and, to a limited extent, for the production of animal feed for dogs, cats and fish.

Manufacturer

Manufacturers of twin screw extruders in Europe include a .:

Manufacturers of process components for twin screw extruders are z. B .:

  • Clextral

Selected literature

  • W. Meskat, R. Erdmenger : Device for kneading, gelatinizing and pressing plastic masses . German patent specification DBP 862 668, 1944. The basic patent for the tightly intermeshing twin-screw extruder.
  • JL White: Twin Screw Extrusion. Technology and Principles . Carl Hanser Verlag, Munich 1990
  • K. Kohlgrüber: The co-rotating twin screw extruder. Basics, technology, applications . Carl Hanser Verlag, Munich 2007, ISBN 978-3-446-41252-1