Extruders

Extruders are used to produce plastic products such as flexible pipes and plastic sheets. They can also be used to mix materials. For more information on mixing applications, see the Mixers section of the encyclopedia.

(Courtesy of Welex, Inc., Blue Bell, PA)

Single Screw

Material flow in single screw extruders is driven by the properties of the material. Depending on the material being fed, a different screw can be used to handle the new material. Single screw extruders are really popular and can handle some of the toughtest materials. The single screw extruder pictured below comes with a flame-hardened screw for handling abrasion and corrosion.

(Courtesy of Milacron, Batavia, OH)

General Information

In single screw extruders, a screw mixer melts the entering plastic materials, pushes the material through the extruder, and forms the plastic into the desired shape.

Equipment Design

Extruders have three key components:

  • The hopper, where material is fed to the extruder.
  • The screw, which runs along the length of the machine.
  • The optional static mixers at the end.

Single screw extruders rely on the friction between the material and the barrel for flow: the greater the friction, the slower the flow. One way to increase friction is to increase the length of the barrel, but this could result in a change in viscosity with position. The extruder temperature can be adjusted to control the viscosity.

Usage Examples

Typical extruder products such as plastic panels, plastic molding, pipes, and tubes are shown below.

(Courtesy of Milacron, Batavia, OH)

Advantages

Disadvantages

  • Less expensive than twin screw extruders
  • Less shear than twin screw extruders
  • High thru put
  • Handles a variety of materials
  • Poor mixing compared to twin screw extruders
  • Relies upon the physical properties of the material to drive the material through
  • Temperature regulation necessary

Twin Screw

Not all extruders with two screws are twin screw extruders. Screw placement and type are also important determinants.

(Courtesy of Baker Perkins Group Ltd., Grand Rapids, MI)

General Information

Twin screw extruders have two intermeshing screws and operate in the same manner as a single screw extruder. The picture below shows the barrels of the extruder exposed. Twin screw extruders depend minimally on the friction of the material against the barrel to move forward. They rely instead on the properties of the extruder and the screws. The two most important features are the meshing characteristics and the type of rotation.

(Courtesy of Baker Perkins Group Ltd., Grand Rapids, MI)

Equipment Design

Meshing characteristics and type of rotation are the two most important features of twin screw extruders.

Intermeshing screws can engage each other fully, partially, or not at all. Non-intermeshing twin screws would operate as parallel single screw extruders, and the flow would be governed by material properties.

Nonconjugated screws are those in which there is ample clearance between the two screws. Conjugated screws result in minimum clearance and narrow flights .

(Courtesy of Baker Perkins Group Ltd., Grand Rapids, MI)

The rotation of the screws affects the mixing properties, as seen in the animations below.

Counterrotating Screws

Corotating Screw

In counterrotating screws, a single stream travels between screws rotating in opposite directions. In corotating screws, two streams of material travel in parallel along two screws rotating in the same direction.

Usage Examples

One interesting product of twin screw extrusion is the production of medicine capsules, as shown below, by the process of extrusion spheronisation. The use of a twin screw extruder allows for the production of uniformly sized spheroids, which aids in the controlled release of the medicine inside.

(Courtesy of Baker Perkins Group Ltd., Grand Rapids, MI)

Advantages

Disadvantages

  • Good, thorough mixing.
  • Material propelled forward by properties of screw, not properties of material.
  • No temperature regulation needed.
  • More expensive than single screw extruders.

Multi Screw

General Information

Multi screw extruders are used to create multilayer products. The multi screw extruder on the left has eight screws and is capable of making a sheet with up to eight different layers of plastic and other materials. In the product described below layers of glue are intermixed with plastic layers.

(Pictures courtesy of Welex, Inc., Blue Bell, PA)

Usage Examples

The picture below shows some of the large variety of multilayer products that can be manufactured using a multi screw extruder. These products include plastic milk containers, multicolor sheets for plastic cups, and fast food service folding boxes.

Mul-use.JPG

(Courtesy of Welex, Inc., Blue Bell, PA)

Sheet

General Information

Sheet extruders are specifically designed to produce sheets of plastic. Plastic enters through the feed hopper, is processed, and travels between three rollers along the flowpath shown below. The center roller is fixed and the height of the upper and lower rollers can be adjusted to vary the sheet thickness.

(Pictures courtesy of Welex,Inc., Blue Bell, PA)

Usage Examples

Sheet extruders are used exclusively to produce sheets of plastic such as those shown below. The sheets can then be thermoformed into applications from cups and luggage to bathtubs.

(Courtesy of Welex, Inc., Blue Bell, PA)

Acknowledgements

Baker Perkins Group Ltd. , Grand Rapids, MI

Milacron , Batavia, OH

Welex, Inc. , Blue Bell, PA

References

Christensen, R. E. and C. Y. Cheng. "Processing Polyolefins on Single-Screw Extruders." Plastics Engineering June 1991: 31-4.

Lawal, Adeniyi and Dilhan M. Kalyon. "Mechanisms of Mixing in Single and Co-rotating Twin      Screw Extruders." Polymer Engineering and Science October 1995: 1325-38.

Martelli, Fabrizio G. Twin-Screw Extruders . New York: Van Nostrand Reinhold Co., 1983. 7-91,      11-3 - 11-21.

Scenkel, Gerhard. Plastics Extrusion Technology and Theory . London: Iliffe Books Ltd., 1966.      13-48, 53-67, 243-259.

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