Extrusion (process engineering)

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During extrusion (from Latin extrudere , `` push out '', `` drive ''), solid to viscous hardenable materials are continuously pressed out of a shaping opening (also known as a nozzle , die or mouthpiece) under pressure . This creates bodies with the cross-section of the opening, called extrudate , in theoretically any length.

The extrusion of ceramic masses is also called extrusion . A similar process is the extrusion of metals.


Extrusion technology products can be found in numerous areas of application. Different processes can be distinguished depending on the properties of the material to be processed, the pressure generation and the desired result. Cold extrusion works with low pressures, temperatures and shear forces, hot extrusion with higher temperatures and hot extrusion with both high pressures and high temperatures. The merging of identical or foreign materials before leaving the profile die is also known as coextrusion .


The extruder was already used in the rubber industry before 1900 . After a while, the food industry also discovered a variety of uses. Due to the great boom in the plastics industry in the middle of the last century, the extruder has also established itself as an important procedural element in the continuous production of thermoplastic articles.

Basically, extruders can be divided into two process principles: processing and preparation extruders. Processing extruders are mainly used for shaping (usually single-screw extruders), while processing extruders are used for chemical and / or physical modification (reacting, mixing, degassing, etc.) of substances (concurrent, tightly meshing twin-screw extruders, Buss-Ko-kneaders, etc.).

Depending on the product, pressures of 10 to 1500 bar and temperatures of 60 to 300 ° C are used. The length designations of the extruders are given in 25D, 30D, 40D etc., which means that 25 times (30 times, 40 times etc.) the screw diameter results in the length. To increase output, single-screw extruders, so-called high-speed machines, are manufactured with speeds of up to 1500 revolutions per minute.

Piston extruder

Piston extrusion of plasticine with a children's toy.

With this design, the pressure is generated by means of a piston (ram extrusion). Piston extruders are mainly used if the material to be processed cannot be processed using screw extruders or if a frequent product change is to be carried out.

Piston extruders are widely used for ceramic materials. In ceramic production, screw extruders have the serious disadvantage that the screws wear out quickly due to the abrasive ceramic mass. Other materials that are sensitive to high shear forces can also be better produced by piston extrusion. Piston extruders can be cleaned quickly and have little contact surface with the extrusion mass.

By using large piston diameters and cylinder lengths, semicontinuous production can also be operated. The parallel connection of two piston extruders can also optimize this.

Screw extruder

Schematic representation of a screw extruder.
Vacuum extruder screw
3-zone screw:
metering zone - compression zone - feed zone
P = pitch, h = thread depth, D = outer diameter

With this design, the pressure is generated by means of an extruder screw, also known as a screw. It is located in what is known as the screw cylinder, the nominal diameter of which is almost the same as the outer diameter of the screw. The shaping outlet opening is located at the front end of the screw cylinder. At the rear end of the cylinder is the drive, in most cases an electric motor with a gear unit, which ensures the rotation of the screw.

The materials to be processed are usually fed continuously to the screw from above via a funnel. When there are several components, the exact and reproducible dosage is crucial. A distinction is made between gravimetric and volumetric dosing, depending on the control mechanism. Additional components can be introduced into the screw cylinder via side feeders , needle valves, etc. In plastics processing, these are z. B. so-called masterbatches or liquid colors (colorants), reinforcing fibers, anti-aging agents or plasticizers. With the appropriate dosing technology, it is possible to inject liquid colors downstream into the plastic melt. This means that when changing colors, only a part of the production line needs to be rinsed, which means considerable material and time savings.

The extruder screw itself is generally divided into three zones that perform different tasks. The so-called feed zone is located in the rear area of ​​the screw cylinder. In this, the material to be extruded, the z. B. is present as a solid in the form of granules or powder, fed in via a funnel ("feeding") and, if necessary, melted (in the case of thermoplastics). The rotation of the screw feeds the material. This is followed by the compression zone, in which the material is further compressed by the reduced thread depth of the screw and thus the pressure required for discharge in the tool is built up. Finally, the discharge zone or metering zone ensures a homogeneous flow of material to the tool.

There are extruders with one, two or more screw shafts . In single-screw and (co-rotating) twin-screw extruders, the conveyance and pressure build-up are brought about by the friction of the mass rotating with the screw on the stationary housing wall (cylinder) - in this context one speaks of friction conveyance. The mass that remains in the rotation is pushed by the helical screw flights to the outlet nozzle. In the case of counter-rotating twin-screw extruders, the principle of forced conveyance predominates.

The shape or type of screw determines the properties.

Section through the melting zone of a standard screw. The designations are: active edge - melting pool - solid bed - passive edge
Barrier screw: melt channel - solids channel
Section through the melting zone of a barrier screw. Designations as above, with a middle barrier bar .
  • Single-screw extruders are inexpensive, but do not mix as well. When designing a screw, the probable degree of filling over the process length is important. It is significantly influenced by the pitch P of the screw and the flight depth h. As the slope increases, the degree of filling continues to increase, so that the pressure continues to increase after the fully filled state has been reached. In this way, the pressure required to flow through the tool is built up.
    • Barrier screw with an additional web at the beginning of the barrier zone, the height of which is lower than that of the main web. This web is called the barrier web. The screw channel is divided into melt and solids channel by the barrier web. The gap width is so small that no solid particles can get over the web into the melt channel behind it. The separation of the melt and solids ensures a better melting process, since no solid islands can swim around in the melt.
      In the discharge zone of a barrier screw there are homogenizing elements such as shearing and mixing parts.
      Towards the end of the barrier zone, the solids channel becomes smaller and the melt channel larger. As a result, the remaining solids are forced by dissipation to melt.
  • Twin-screw extruder : In the case of the co-rotating twin-screw extruder, the screws rotate in the same direction of rotation, in the counter-rotating in the opposite direction of rotation. The screws can be arranged parallel or conically. Since the play between the screws is particularly tight and the tendency to wear is correspondingly higher, they must meet the highest quality criteria in terms of strength and hardness.
  • Multi-screw extruder

Other designs

Advantages of the extrusion process

The advantages of the process are the ability to create complex shapes and process brittle and soft material.

Process measurement technology in extrusion

Since very similar machine concepts ( extruders ) are used in extrusion and compounding , the same methods are available for this as in process measurement technology in compounding .

After extrusion, further quality parameters can be determined on the component in the production line:

  • Measurement of the dimensions of the component z. B. with light curtains
  • Color measurement on the extrudate
  • Quality control with camera systems

Applications and Products

Extrusion technology products can be found in numerous areas of application, e.g. B. in construction industry (civil engineering), automotive industry, aviation industry, medical technology, furniture industry (especially kitchen or office furniture), trade fair construction, packaging, agricultural use, hose applications and granulation.

Examples of products made by extrusion are:

Plastics industry

A drainage pipe made by extrusion.

Plastic and plastic granulates , including compound granulates

For example, seamless sheets , tubes and profiles with a constant cross-section over their length can be produced by extrusion . Plastic containers such as bottles, cans or fuel tanks are also manufactured by extrusion. An extruded hose is enclosed in a mold and blown onto the tool wall using compressed air. In this case one speaks of extrusion blow molding .

Extruder for plastic baseboards
Extruder for sheathing lines

In addition to the food industry, the plastics industry is a major customer for extruders. Here, in addition to the structure mentioned above, heating bands are attached to the screw cylinder, which melt ( plasticize ) the plastic granulate or powder that has been filled in . Extruders don't just have to be heated. Some procedural tasks also make it necessary to cool the plasticized plastic. This is done either by cooling fans or by cooling channels incorporated into the cylinder of the extruder, which are filled with a cooling medium, e.g. B. pressurized water are flowed through. Depending on the type of plastic used, additional equipment is required, for example for pumping out gases that would leave bubbles in the end product. The material used is also decisive for the shape of the screw. The pitch, the number of turns and the width of the screw threads can be completely different depending on the plastic.

In addition to the production of extruded semi-finished products shown here, the main area of ​​application for extruders is as a preliminary stage in the injection molding process .

For this purpose, the plastic or the extrudate is first melted and homogenized by an extruder (also known as a screw press ) using heating and internal friction . The pressure required to flow through the nozzle is also built up in the extruder . After exiting the nozzle, the plastic usually solidifies in a water-cooled calibration . The application of a vacuum causes the profile to press against the caliber wall, thus completing the shaping. This is often followed by a cooling section in the form of a cooled water bath. The cross-section of the resulting geometric body corresponds to the nozzle or calibration used. The possible manufacturing tolerances are in the range of ± 0.05 mm. An analogous process based on the same primary forming principle that is used for the hot forming of metals is known as continuous casting .

Pipe and profile extrusion

In the case of pipe or profile extrusion , the extruded material must then be held in shape in a calibration table until it has cooled down. Otherwise it will collapse and deform. The heart of the calibration table is the calibration sleeve, which is primarily used to maintain the shape of the pipe or profile. It is attached directly behind the nozzle of the extruder. It consists of a metal block that has the exact cross-section of the pipe or profile on the inside . The extruded pipe or profile runs through the calibration sleeve. Support air or vacuum is used inside the sleeve to maintain the shape of the profile. Vacuum openings at different points in the sleeve along the cross-section suck the profile onto the inner walls, while water flows around and through the profile until it has cooled down. The type and shape of the calibration table depends very much on the plastic product to be extruded. A lot of know-how is required here, especially with complicated profiles such as window frames, so that the profile does not collapse and deform when it cools down.

A fume cupboard behind the calibration table ensures that the pipe or profile is transported. These are two or more caterpillar chains , the running surfaces of which are pressed against each other. The extruded pipe or profile is then placed between these running surfaces and put under tension by the movement of the chains. The trigger pulls the extruded plastic product out of the extruder and through the calibration table. The speed of the take-off, together with the speed of the extruder, determines the running speed of the profile or pipe.

Behind the trigger there is usually a cutting system that cuts the previously endless pipe or profile to length. A collecting table collects the cut tubes or profiles. The machine operator can bundle and pack the finished tubes or profiles here.

The combination of extruder, calibration table, haul-off, cutting system and collecting table is called an "extrusion line" because the individual machines are positioned one behind the other in one axis according to the production process.

Film extrusion

Extruders are also part of blown film lines . Here the plasticized material is not brought into the shape of a profile, but is blown into a wafer-thin film bubble. For this purpose, the melt is deflected in a vertical direction and brought into the shape of a hose by means of spiral distribution tools . By arranging up to nine extruders in a ring and combining the individual melt streams in the spiral distributor tool, multilayer films can be produced in this way. In order to bring the emerging melt hose to the thickness of several hundredths of a millimeter that is customary today, the hoses are then stretched lengthways and crossways. This is done by centrally blowing in air, which not only mechanically stretches the hose, but also cools it down and thus fixes it in its shape. The film tube is calibrated through a basket, folded over and wound up.

The materials that can be processed on extruders belong to the group of thermoplastics , such as:

and innovative biomaterials like

See also wood extrusion .

Feed and food industry

Peanut flips
Millet balls

In the feed and food industry, as part of the preconditioning z. B. integrated heating and steaming processes, the z. B. break down cereal components and thus make them digestible.

  • Chewing gum and chewing candies
  • Pasta ( noodles )
  • Many snack products:
  • Rice crispies
  • Chocolate flakes
  • caramel
  • Bread (so-called crispy bread)
  • Some breakfast cereals
  • Textured Vegetable Proteins (TVP) (e.g. textured soy , see also: meat substitutes )
  • Dry and semi-dry pieces of food, so-called kibbles, as food etc. a. for dogs, cats, fish and pigs. Processing by the expander makes the feed easier for the animals to digest and increases feed conversion. In addition, the feed is sterilized by the high temperatures.

During hot extrusion, a dough ( mass ) containing water is heated under enormous pressure in a compression screw (similar to a meat grinder ). The pressure drops at the outlet nozzle, so that the water evaporates and the product is loosened considerably. The already gelatinized starch can support the gas retention (similar to an incendiary compound) and hold firm pores. A drying or roasting station is often added downstream. Typical products are peanut flips , onion rings and other snacks. But some types of bread (e.g. flat bread or bread chips ) and pastries are also produced using this process.

See also


  • Hans-Josef Endres, Andrea Siebert-Raths: Technical biopolymers. Hanser-Verlag, Munich 2009, ISBN 978-3-446-41683-3
  • Helmut Greif, Andreas Limper, Gordon Fattmann, Stefan Seibel: Technology of extrusion . Carl Hanser Verlag, Munich 2004
  • Johannes Wortberg, Dirk Kaczmarek: Plastic machine guide . 4th edition. Carl Hanser Verlag, Munich 2004
  • T. Hochrein, I. Alig: Process measurement technology in plastics processing . Vogel, Würzburg 2011, ISBN 978-3-8343-3117-5

Web links

Wiktionary: Bioplastic  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Liquid colors and additives - advantages - ROWASOL. Retrieved January 18, 2018 .
  2. T. Hochrein, J. Botos, K. Kretschmer, P. Heidemeyer, B. Ulmer, T. Zentgraf, M. Bastian: Faster and closer to the process . In: Kunststoffe , 2012, p. 9
  3. Guidelines for bread and biscuits, as of 2005 (PDF; 42 kB) BMELV
  4. Videos of extruders in use. Schaaf Technologie GmbH