There is a wide variety of extruders
, produced today in the world; in general, they can be classified into two main families, that is the single screw extruder
and twin screw extruder
The corotating twin screw extruders manufactured today very little heat through mechanical work, far below the amount needed for the material to reach the temperature required for gas mixing; additional heat is applied to reach it. After the mixing, no more heat is applied, and due to the low shear developed, the temperature of the melt will decrease; because of the relatively slow movement down the barrel, with a reasonable little cooling, the temperature will easily reach the one required for the extrusion. The principle of operation that characterizes corotating intermeshing twin-screws extruders can be summarized as follows:
-The screws can be starve-fed by an independent mechanism so that screws speed setting does not affect output.
-Positive forwarding action eliminates problems like the bridging or slipping of solid material in the feed section, allowing the feed of pellets
, beads, or powder.
-Shear rate is very low, and additional external heat, much easier to control, is added; the shear stress distribution can be adjusted by changing screw speed and/or output.
-Stock temperature can be raised to the desired level without exceeding it, and low-temperature extrusion is easily obtainable.
-Positive conveying of the melt eliminates surging and promotes better dimensional control of the extrudate.
-Mixing is also positive and accomplished without overheating the material.
-Better dispersing and homogenization.
-Self-cleaning is fast and complete and accomplished without overheating the material.
-Lower HP required per rated output, resulting in a lesser stress on the machine.
-Energy-saving, as the actual energy consumption is very close to the theoretical value needed.