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Parallel Versus Conical Twin Screw Extruders for PVC

Two identical screws that interlock and are housed in matching barrels make up twin screw extruders. Screw and barrel might be conical or parallel. Before entering the die during twin screw extrusion, PVC and C-PVC are transported, compressed, degassed, plasticized, sheared, Kneaded, fused at the ideal level, and homogenized.

The screws are said to be intermeshing if the “screw flight path” overlaps. The screws are referred to as fully intermeshing if the flights meet in a way that the tips of one screw’s flights almost touch the channel bottom or root of the other screw. The void is known as a “milling gap.”

Thermostatically sensitive polymers like PVC or C-PVC are designed for fully intermeshing, counter rotating twin screw extruders, which are effectively positive displacement pumps independent of frictional criteria.

Due to the distance between the screw flights and the barrel, full positive displacement cannot be achieved. Therefore, as a result of wear and tear, as the gap widens, so does the output. For processing U PVC, parallel and conical screws are both employed. Many manufacturers tout the benefits of their extruders.

The diameter of the screw at the end of the metering section identifies the extruder in conical extruders. Extruder size and flight diameter are related, however. It is asserted that the output of a 65 mm parallel screw extruder can be matched by a 50 mm conical extruder. The feed section screw diameter for such an extruder is 100 mm. While the friction and shear in the metering portion correspond to 50 mm parallel screws, the plasticizing in the feed section is equivalent to 100 mm parallel screws. Lower axial force on the screws is the result of the metering section’s smaller diameter.

Instead of shear, the temperature of the screws and barrels in conical extruders is primarily used to control the plasticizing rate of PVC, which helps

In conical extruders, plasticizing rate of PVC is mainly controlled by controlling temperature of screws and barrels, rather than by shear, which contributes to significantly lower amperage and higher power economy at higher rpm. It is claimed that by lengthening the processing unit and reducing the taper, in comparison to earlier version, in the newer designs, it is possible to achieve screw surface area similar to parallel extruders having L / D ratio of 28:1. This lengthening also provides pre-heating effect.

In the plasticizing section, the intermeshing surface in case of conical screws is larger than in parallel screws. This results in higher but controlled energy inputs through shear. On the other hand in the metering section, the intermeshing surface of the conical screws is less than the parallel screws. This provides lower shear energy inputs.

Smaller diameter at the metering section further reduces the shear rate. Thus, unwanted energy input into melt is avoided and pressure build up is achieved with less stress on the material. Lower shear rate means lower temp rise, lower degradation and lower die swell. This arrangement facilitates processing of C – PVC. To minimize torque and wearing of screw and barrel, fusion of PVC / C – PVC is moved forward towards the metering zone. This is where the back pressure build-up takes place. As expected, the back pressure is directly related to the cross sectional area of the metering zone. The ratio of external / internal lubricant is adjusted with more concentration of external lubricant, to achieve later fusion.

General guidelines for parallel and conical screws:

  • Due to their near proximity, parallel screws make it difficult to provide enough thrust bearing. It goes without saying that one screw shaft must be longer than the other to accommodate the proper thrust bearing.
  • Conical screws have greater diameters at the feed end, which allows for a wide gap between the screws, which solves this issue. This enables the use of simple, reliable distribution gear systems and conventional thrust bearings.
  • Conical extruders that rotate anticlockwise have a large volume feed section that can handle materials with lower bulk densities. This offers versatility to manage different chemicals.
  • The output of a parallel screw steadily diminishes over time as a result of wear and tear. Conversely, the output of conical screws can be maintained by periodically forwarding the screws.
  • The screws are subjected to external stresses as the polymer is plasticized.As the viscosity of the polymer increases, these forces grow. The result is that the screw shafts and bearings are subjected to unequal forces. As the screws increase longer, unbalanced forces exerted on them do cause deflection issues. Conical twin screws are advantageous because they provide a stronger cantilever beam and are less prone to bending.
  • The compression zone seals off the vent zone to prevent powder from being pulled from the feed.