EN 
简体中文Twin-screw extruders have two identical screws that intermesh, housed in a matching barrel. Screws and barrels can be parallel or conical In the twin screw extrusion process – PVC / C-PVC is transported, compressed, degassed, plasticized, sheared, kneaded, and fused at an optimum level before entering the die and homogenize. If the "screw flight paths" intersect, the screws are said to intermesh.
Screws are said to be fully engaged if the threads intersect in such a way that the tip of the thread of one screw nearly touches the bottom of the channel or root of the other screw. This clearance is called the "milling clearance". Fully intermeshing, counter-rotating twin-screw extruders are essentially friction-independent positive displacement pumps designed for heat-sensitive polymers such as PVC or C-PVC.
Due to the clearance between the screw flight and the barrel, full positive displacement cannot be achieved. Therefore, as the clearance increases due to wear, the output also decreases. Both parallel and conical screws are used for processing U PVC.
General Considerations for Conical vs. Parallel Screw Barrels:
1. Parallel screws are too close together to provide sufficient thrust bearing. Obviously, one screw shaft must extend beyond the other to accommodate the appropriate thrust bearings. In the case of tapered screws, this problem is solved due to the larger diameter at the feed end and the greater spacing between the screws. This allows the use of standard thrust bearings and a simple, robust distribution gear system.
2. The counter-rotating conical extruder has a large capacity feed section to accommodate compounds with lower bulk densities. This provides flexibility in handling various compounds.
3. Over time, the output of the parallel screw gradually decreases due to wear. Whereas in the case of conical screws, the output can be maintained by periodically forwarding the screw.
4. As the polymer is plasticized, an outward force is applied to the screw. These forces increase with increasing polymer viscosity. As a result, uneven forces are applied to the screw shaft and bearings. Unbalanced forces acting on the screw can really create deflection issues as the screw gets longer. There is an obvious advantage to conical twin screws, a more efficient cantilever, and better resistance to bending.
5. The compression zone provides a seal to the exhaust zone so that powder cannot be drawn out of the feed.
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