Single Screw Extruder Screw Design

extruder screw

The single-screw extruder is mainly used for extruding soft PVC, hard PVC, polyethylene, and other thermoplastics. It can be used with the corresponding auxiliary machines (including the forming head) to process a variety of plastic products, such as films, tubes, boards, ribbons, etc. It can also be used for pelletizing.

The extruder is mainly composed of a screw, a barrel, a heating and cooling system, a transmission system, and a control device. The screw is the most critical functional component of the extruder. The extruder screw design has three sections, namely the charging, compression, and extrusion sections. The main parameters of the single-screw extruder screw design are screw diameter D, screw length-to-diameter ratio L/D, and compression ratio.

Extruder screw
Extruder screw

Single-screw extruder screw design principle

The first section of the material mouth begins to be called the conveying section: here the material is required not to be plasticized, but to be preheated, pressurized, and squeezed solid, here the material is actually a solid plug, as long as the task of conveying is completed.   

The second section is called the compression section, the volume of the screw trough from large gradually become smaller, and the temperature to reach the degree of plasticization of the material, here to produce compression, the corresponding screw compression ratio – 3:1 (variable), complete plasticization of the material into the third section. 

The third section is the metering section, where the material maintains the plasticizing temperature, accurate and quantitative transport of molten material to supply the head, at this time the temperature cannot be lower than the plasticizing temperature, generally a little higher.

Single-screw extruder screw design materials

The performance requirements of the commonly used materials for screws are: good mechanical strength, good corrosion resistance and anti-wear properties, good processing properties and easy access to materials. Generally choose 38CrMoAl nitride steel.

Diameter of the extruder screw design

Extruder screws are designed in the following diameter series: 30, 45, 65, 90, 120, 150, 200. The size of the screw diameter is generally determined by the cross-sectional size of the product being processed, the type of plastic being processed, and the production rate is processed.

There is an appropriate relationship between the size of the product cross-section and the size of the screw diameter. As a rule: the screw diameter chosen for large cross-sectional products is larger, which is more favorable for the quality of the product, the utilization of the equipment, and the operation.

Therefore, an increase in the length of the working part of the screw L screw L/D ratio has the following benefits.

  1. The screw is fully pressurized to improve the physical and mechanical properties of plastic products.
  2. can improve the quality of plasticization, so that the appearance of the product quality is better.
  3. The slope of the screw characteristic curve is small, the extrusion volume is stable, and the cut extrusion volume can be increased by 20%-40%.
  4. It is conducive to the forming of extruded pipes similar to PVC powder.

But the screw length-diameter ratio is too large, which will make the screw processing and assembly difficult, power consumption will also increase, increase the cantilever end of the self-weight bending, to the screw head and barrel wall clearance is not uniform, and even cause grinding scraping barrel wall, reduce the life of the screw and barrel.

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