Type and design of extrusion screw

Type and design of extrusion screw

Screw is an important part of plastic extruder. By its rotational motion in a tightly fitting drum, the screw delivers the plastic, melting it and forcing it through the mold. These three steps take place in a continuous process capable of producing extruded parts of various lengths. The screw was designed in principle similar to the Archimedes screw of 250 BC and was used to lift water to a higher level by axial rotation.

Modern engineering has extended the ancient screw by redeveloping its geometry and applying it to plastic extrusion. Today's extrusion screws are available in different pitch, helical Angle, aspect ratio, and length of processing stage. These variables influence the different pressures, shear, and feed rates that are effective for a given plastic. Some extruders also use twin screws to provide better additive mixing.

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The basic extrusion screw has three distinct parts, each designed to perform a specific task. The feed section is located at the rear of the screw, and the plastic particles are gravity fed from above and transported forward. The aspect ratio of the feed section is usually four to one or five to one, which is sufficient to establish the pressure required to transport the plastic. However, the friction between the barrel wall and the plastic must be greater than the friction between the screw and the plastic for lateral motion to occur.
Barrel heaters help the plastic become sticky and stick to the wall. Sometimes, the screw is also cooled to remove adhering particles. For plastics with low friction coefficients, the feed section length ratio can be increased to eight or ten to one. This extra length gives the plastic more time to heat up to a higher temperature, which creates more friction on the barrel. From here, the plastic is guided to the transition part of the screw.
In the ITD, plastic is converted to a liquid state by two parallel methods. The cartridge heater provides some initial melting, and the shear caused by the movement of the plastic against the cartridge completes the process. At this stage, the root of the screw increases and the size of the groove or "thread" subsequently decreases. This leaves less room for compacted plastic blocks in the feed section. As pressure, shear and friction increase, the plastic begins to melt and flow. The transition portion usually occupies five to ten diameters of the cycle.
In the metering or pumping part, the molten plastic is guided into the mold. At this stage, the root diameter and thread size of the screw remain the same, and its length varies from 4 to 8 diameters, depending on the application.