Single-screw And Twin-screw Extruders in Terms of Their Structural Characteristics And Typical Application Scenarios?

The single-screw extruder is by far the most versatile and popular in use.

The single-screw extruder consists essentially of a screw that rotates in an axially fixed position within the close-fitting bore of a barrel. Extruder sizes are identified by the inside diameter of their barrel. Size range from ¼ to 24 in. diameter with the usual from 1 to 6 in. (Europe and Asia sizes range from 20 to 600 mm with the usual from 25 to 159 mm.). The screw is electrically motor driven through different devices such as a gear reduction train or belt to meet different performance and cost requirements. These gear reducers are rated in mechanical horsepower and thermal horsepower as defined by the American Gear Manufacturers (AGMA). The AGMA rating system is based on the understanding that not all gear reducers are used the same way. There are also gearless drive systems such as those using Siemens high-torque motor with an unusual low-inertia hollow shaft.476

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is largely a function of die resistance and dependent on die geometry and melt viscosity. Extrusion pressures are lower than those encountered in injection molding. They are typically 500 to 5000 psi (3.5 to 35 MPa). In extreme cases, extrusion pressures may rise as high as 10,000 psi (69 MPa). Variants on the single screw include the barrier or melt extraction screw and the vented screw (Chapter 3).

The twin-screw extruder may have parallel or conical screws, and these screws may rotate in the same direction (co-rotating) or in opposite directions (contra-rotating). Extruders with more than two screws are known as the multiple-screw extruder. These extruders are normally used when mixing and homogenization of the melt is very important, in particular where additives, fillers, and/or reinforcements are to be included in the plastic.

They are extensively used for plastic compounding that includes heat-sensitive materials such as PVC, processing of difficult-to-feed materials (such as certain powders), reactive processing,197 and for plastic devolatilization. Twin-screw extruders particularly offer a wide processing variability. They can be starve-fed so that residence time, amount of shear, and control of melt temperature can be controlled by means of their segmental modular designs.