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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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