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A Single Screw Extruder has Three Sections

Views: 1     Author: Site Editor     Publish Time: 2024-02-29      Origin: Site

Referring to Figure 15.1, it is customary to divide a single-screw extruder into three sections:

Figure 15.1

  • Feed section: the main function of this section is to act as a screw conveyor, transporting the material from the feed entrance to the subsequent sections. Almost no compression or modification of the mass occurs in this section

  • Transition section: this is the section where the material is compressed and heated

  • Metering section: this is the section where most of the objectives of the extrusion process (melting, texturization, kneading, chemical reactions etc.) occur through shear and mixing.

Through the friction, most of the power used for turning the screw shaft is dissipated into the material as heat. Thus, a portion of the heat delivered to the product is generated in situ. Additional heat is transferred from the externally heated barrel surface and supplied by direct injection of live steam. In single-screw extruders, the internally generated heat constitutes the major part of the energy input. Consequently, heating in a cooking extruder is extremely rapid.

Because of the high pressure in the extruder, the moist material can be heated to temperatures well above 100°C (sometimes up to 180–200°C). When the pressure is suddenly released at the exit from the die, some of the water in the product is flash evaporated and, as a result, the product is puffed. The degree of puffing can be controlled by releasing some of the pressure and cooling the mass at the last section of the extruder, before the die. Figure 15.3 shows an example of pressure–temperature profile along the extruder, in a process where puffing has to be avoided (e.g. in the production of pellets).

Figure 15.3

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