There are five possible zones in a thermoplastic screw. Since terminology is not standardized in the industry, different names may refer to these zones. Different types of polymer will have differing screw designs, some not incorporating all of the possible zones.
Feed zone. Also called solids conveying. This zone feeds the resin into the extruder.
Melt zone. Also called the transition zone. The resin is melted in this section.
Pressurizing zone. Also called metering or melt conveying. This zone gives the plastic uniform pressure and flow characteristics.
Decompression zone. In this zone, the melt is unpressurized, allowing trapped gases (hydrochloride) to escape and be vented out.
Mixing zone. There are two types of mixing zone. They either distribute small particles evenly, or break large particles into small ones which can then be mixed.
Often screw length is referenced to its diameter in terms of an L:D ratio. For instance, a 6-inch (150 mm) diameter screw at 24:1 will be 144 inches (12 ft) long, and 192 inches (16 ft) at 32:1. In years past screw ratios of 24:1 were fairly common, but modern machines use 32:1, or higher ratios, which allow better mixing at higher throughput.
Each zone will be equipped with one or more thermocouples or RTDs for temperature control.
The basic extrusion screw has three distinct parts, each engineered to do a specific task. The feed section is in the rear of the screw, where plastic pellets are gravity fed from above and conveyed forward. The length to diameter ratio of the feed section is typically four or five to one, which is sufficient to build up the pressure needed to transport the plastic. However, the friction between the barrel wall and the plastic must be greater than that between the screw and the plastic in order for lateral movement to occur.
A barrel heater helps the plastic develop tack and stick to a wall. Sometimes, the screw is also chilled to free it of clinging pellets. Feed section length ratios can be increased to eight or ten to one for plastics with a low coefficient of friction. This extra length gives the plastic more time to heat up to a higher temperature, creating more friction at the barrel. From here, the plastic is channeled into the transition section of the screw.
In the transition section, the plastic is transformed into its liquid state through two concurrent methods. Barrel heaters provide some initial melting, while the shear caused by the motion of the plastic against the barrel completes the process. In this stage, the root of the screw increases while the flutes, or “flights,” subsequently decrease in size. This leaves less space for the plastic mass that has been compacted in the feed section. As pressure, shear and friction increases, the plastic begins to melt and flow. The transition section typically occupies five to ten diameters of the cycle.
In the metering or pumping section, the molten plastic is guided into a die. The root diameter of the screw and the size of the flights remain constant in this stage, and its length varies from four to eight diameters, depending on the application.
The screw is an essential component of a plastic extrusion machine. Through its turning motion inside a tight fitting barrel, the screw conveys the plastic, melts it and forces it through a die. These three steps are carried out in a continuous process capable of producing extrusions in a variety of lengths. The screw’s design is similar in principle to Archimedes’ screw of 250 BC, which was used to raise water to higher levels via axial rotation.
Modern engineering has expanded upon the screw of antiquity by redeveloping its geometry and applying it to plastic extrusion. Today’s extrusion screw can have varying pitch, helix angle, length to diameter ratio and length of processing stages. These variables influence the different pressure, shear and feed rates that are effective for a given plastic. Some extruders also use double screws to provide better additive mixing.
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