Structure of the extruder screw
Usually 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.
Most screws have these three zones:
1, Feed zone. Also called solids conveying. This zone feeds the resin into the extruder, and the channel depth is usually the same throughout the zone.
2, Melting zone. Also called the transition or compression zone. Most of the resin is melted in this section, and the channel depth gets progressively smaller.
3, Metering zone. Also called melt conveying. This zone, in which channel depth is again the same throughout the zone, melts the last particles and mixes to a uniform temperature and composition.
In addition, a vented (two-stage) screw will have:
4, Decompression zone. In this zone, about two-thirds down the screw, the channel suddenly gets deeper, which relieves the pressure and allows any trapped gases (usually moisture or air) to be drawn out by vacuum.
5, Second metering zone. This zone is like the first metering zone, but with greater channel depth, and repressurizes the melt to get it through the resistance of the screens and the die.
The screw length is referenced to its diameter as L:D ratio. For instance, a 6-inch (150 mm) diameter screw at 24:1 will be 144 inches (12 ft) long, and at 32:1 it is 192 inches (16 ft) long. An L:D ratio of 24:1 is common, but some machines go up to 32:1 for more mixing and more output at the same screw diameter. Two-stage (vented) screws are typically 36:1 to account for the two extra zones.
And Each zone is equipped with one or more thermocouples or RTDs in the barrel wall for temperature control.
Structure of the extruder screw