Extruder Knowledge-Part 5-Single Screw Extruder and Twin Screw Extruder
What kind of extruder is the usual chosen by pipe manufacture? It’s no doubt, they are single screw extruder and twin screw extruder.
Here has talked lots of single screw extruder in passed 4 pages. Today let’s find the structure of twin screw extruder and its principle.
First let’s check the below picture which showing us the overview of twin screw extruder and it’s structure.
We could found that the mainly system is almost the same as single screw extruder, it has driving system, feeding system and heating system, etc. Also here has gear box and screw, The screws we found are 2 sets, that’s simple reason why we call it twin screw.
Conical twin screw
Parallel twin screw
As above drawing shows, here has two different style of twin screw, one is Parallel twin screw, the other one is Conical twin screw.
The working mechanism of the conical twin-screw extruder and the intermeshing parallel counter-rotating twin-screw extruder is basically the same: the spirally advanced passage of the material on one screw is blocked by the other screw, and cannot be made into a "∞"-shaped spiral flow. It can only make a circular motion in the gap between the screw and the simple screw and the gap between the two screws under the push of the thread, and also moves toward the head and produces strong shear. Therefore, the counter-rotating twin-screw extruder has strong conveying capacity and the ability to establish the head pressure, and the plasticizing effect of the material is also very good, and is suitable for the extruded product. The counter-rotating conical twin-screw extruder has a large space between the two screw axes of the feeding end, and can adopt a large thrust bearing and a torque distribution gear, so as to withstand high torque and high axial load, and is suitable for use. At present, most of the U-PVC products are extruded by the conical twin screw extruder. However, when the diameter of the screw is large to a certain extent, the conical twin-screw extruder requires a large thrust bearing and a torque distribution gear, which makes the machine structure too large, so the large-sized twin-screw equipment adopts parallel counter-rotating twin-screw extrusion machine. Since the design and manufacture of tandem thrust bearings are not a problem at present, the difference in the difficulty of manufacturing the transmission system between the conical and parallel twin-screw extruders is much more easier than ever before, and the more prominent problem is the cone. The screw and barrel of the twin-screw extruder are difficult to manufacture and wear more. At present, medium and small-sized counter-rotating twin-screw extruders with screw diameters in the range of 55-80 mm mostly use conical twin-screw extruders, while large units with diameters above 80 mm mostly use parallel counter-rotating twin-screw extruders. Smaller, such as parallel counter-rotating twin-screw extruders with a diameter of about 30mm, are mostly used in laboratories and other occasions.