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Thermoplastic compounding techniques have seen
several changes over the last decade. Twin screw compounding machinery
with higher speeds (from earlier 400 rpm to as high as 1000rpm)
along with higher torque can enhance output more than 100%. At such
high speeds, the residence time gets drastically reduced. Infact
it almost needs 1-2 seconds for the feed to reach the first melting
zone. Any fluctuations in the feeding could cause the problem of
not only product inconsistency, but could also force the machine
to get jammed or shut down.
The newly designed feeders have the ability to
meet the demands of higher accuracy and shorter response time to
take corrective actions against fluctuations. These feeders for
powder/solid materials are essentially based on 'Loss-in-Weight'
concepts (gravimetric feeders). However accurate they may be, it
is essential that they are evaluated by a long run trial to ascertain
that the short term fluctuations are low (less than 1%). A comparison
of feeder and drive fluctuation accuracy needs to be similar. If
the drive fluctuations are higher than feeders the short term fluctuations
would be quite high compared to the accuracy shown on feeder. This
would be because feeder accuracy is calculated as an average figure
over a longer time span.
There are essentially two types of gravimetric
feeders, vibratory and screw types. Vibratory loss in weight feeders
are most suited to feed heat sensitive materials like chopped glass
fiber. More often vibratory feeders with screws are considered better
options for such products. Single screw feeders are less expensive
than twin screw. They are also quite efficient to feed granules/pellets.
However, twin screws are more efficient for powder feeding. For
liquids, positive displacements pumps (volumetric) can feed uniformly
because their density does not change at a given temperature. What
is important is that more care is required to select an efficient
feeding system for better product consistency.
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