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Parallel Versus Conical Twin Screw Extruders for Pvc
Abstract: Quality of UPVC pipes is mainly checked by testing internal hydrostatic pressure test at 27 °C, drop impact test at 0°C and reversion at 150 °C as per IS 4985:2000 std or as per various standards and accepted if it meets acceptance limit. Once the formulation is fixed in terms of filler and other additives content, these properties are process related in terms of extent of fusion and orientation.
Introduction:
Twin screw extruders have two intermeshing identical screws encased in a matching barrel. Screw & barrel can be parallel or conical.
During twin screw extrusion – PVC / C-PVC is conveyed, compressed, de gassed, plasticated, sheared, Kneaded, fused at optimum level and Homogenized before it enters the die.
If the “screw flight path” intersects, the screws are called intermeshing.
If the flights intersect in such a manner that the tip of flights of one screw nearly contacts the channel bottom or root of the other screw, the screws are called, fully intermeshing.
The gap is called “milling gap”.
Fully intermeshing, counter rotating twin screw extruders are essentially positive displacement pumps independent of frictional criterion and are designed for thermally sensitive polymers such as PVC or C- PVC.
Full positive displacement cannot be achieved, due to clearance between screw flights and barrel.
Hence, as the gap increases due to wear & tear, the output also decreases.
Both parallel and conical screws are used for processing U PVC.
Various manufacturers claim advantages of their extruders.
Conical extruders are pioneered by Cincinnati while host of manufacturers like Battenfeld, had introduced parallel screw extruders.
In conical extruders, the extruder is designated by the screw diameter at the end of the metering section. While, in parallel extruder size is the diameter of the flight.
It is claimed that 50 mm conical extruder matches the output of 65 mm parallel screw extruder.
For such an extruder, the diameter of screw in feed section is 100 mm.
In feed section, the plasticizing is equal to 100 mm parallel screws, while in the metering section, the friction and shear matches 50 mm parallel screws.
The lower diameter in the metering section results in lower axial force on the screws.
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 considerations for Conical Vs, Parallel screws:
- Parallel screws lie so close that providing adequate thrust bearing is a problem. Obviously one screw shaft has to extend beyond the other to accommodate appropriate thrust bearing. In case of conical screws, this problem is solved due to more diameters at the feed end that provides wide spacing between the screws. This allows use of standard thrust bearings and simple, rugged distribution gear system.
- Counter rotating conical extruders have large volume feed section that accommodates lower bulk density compound. This provides flexibility to handle variety of compounds.
- Over a period of time, due to wear and tear, the output in parallel screw decreases gradually. Whereas, in case of conical screws, the output can be maintained by periodically forwarding the screws.
- As the polymer is plasticated, outward forces are applied to the screws. These forces increase with increasing polymer viscosity. The effect is that, uneven forces are exerted on the screw shafts and bearings. Unbalanced forces acting on the screws do present a problem of deflection as the screws get longer. Conical twin screws have a distinct advantage as, a conical screw a more effective cantilever beam and resist bending better.
- Compression zone provides a seal for the vent zone so that the powder is not drawn from the feed.
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