SFVs to the rescue of co-rotating twin-screw extruders

SFVs to the rescue of co-rotating twin-screw extruders

Conquering feed-limitations

The external profile of intake elements plays an important role in the feed zone of a co-rotating twin-screw extruder. Typical elements used in the feed zone may be normal bi-lobed, SKE or single-lobed elements. Certain low-bulk density materials such as fine powders of talc and carbon are increasingly used as important fillers at high loading levels. When such applications are run in an extruder, there is a limit to the intake capacity after which the hopper gets flooded with material. This condition is called feed-limitation. The single deep-flighted 'V' elements (SFV) are specifically designed to improve the intake capacity of the main extruder and the side feeder, at higher speeds. With increased intake capacity, the specific energy decreases considerably, thereby providing an opportunity to utilise the full speed range of high-speed extruders, Therefore, an extruder with a small diameter of 40 mm can have the production capability of an extruder with 80 mm diameter. To know more, read on ...

The earliest co-rotating twin-screw extruders had shallow flight depths. These were designed to be run with a filled-up hopper, in the same way an i6jection-moulding machine or a single-screw extruder is run. As technology progressed and flight depths increased, it became necessary to adopt a different approach to get the material into the extruder. This approach is now popularly called starve-feeding.
In this approach, an additional feeder is used to provide a steady flow of material
to the extruder. The hopper is no longer a vessel to hold materials but simply a conduit through which material enters the extruder. In many cases, it can be seen that the extruder has a large capacity for accepting such feed. If controls are not exercised, such situations can stall or break the mechanical parts and the electric-drive system.
Therefore, the limiting factor is the torque availability. While feeding polymers devoid of any other filler, the material feed rate is generally a fraction (0.05- 0.20) of the volumetric capacity of the extruder. Since there is limited material in several zones of the extruder, the extruder is considered to be 'starving'.

Determination of starve-feed capacity
During starve-feeding, the intake capacity of the extruder depends on the screw rpm, the geometry of the intake element such as shape, free volume and lead, and the shape and bulk density of the input material.
Furthermore, the efficiency of conveying depends on the degree of fill, the type of input material (frictional coefficient and the degree of compaction), length of the intake zone and the pressure head at the end of the zone.

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