Page 37 - Plastics News June 2018
P. 37
FeAtures
The other location where a vent can be installed is
in the twin- screw, upstream of the feed barrel. The
entrained air is easily able to flow backwards in the
twin-screw to the upstream vent barrel, while the
solid powder is conveyed by the screws downstream.
These three strategies can be employed individually
or in parallel with each other.
Another highly effective technique for getting
powders into a twin-screw extruder is with the
use of Feed Enhancement Technology (FET). In this
technology (Fig. 4), a vacuum is applied through
a porous filter in the barrel wall. Air is removed,
while the powders are kept in the barrel. It works
primarily by forming a cake of material on the barrel,
increasing the coefficient of friction on the barrel
wall. This increases the conveying efficiency. The For this process, the throughput is limited by the
secondary effect is to remove some of the entrained volume of talc that can be fed. At rates higher
air from the powder, reducing its volume. than 600 kg/hr, the side feeder starts to back up,
and the extruder is able to use only 54% of its
total motor power. By installing FET, the conveying
efficiency of the side feeder is greatly increased,
and the throughput can be increased until the
motor power becomes the limitation at same screw
speed, the specific mechanical energy input to the
material also decreases, and less power is used per
kilogram of material. With a profit margin of 20¢/
kg, the 500 kg/hr increase in rate could mean an
increased profit of $600,000 annually, offsetting the
The magnitude of the effect of FET is not the cost of FET several times over. The specific energy
same for all powders, and depends on attributes requirements are reduced by 4% or 33 MWh annually.
such as bulk density and particle size. For talc,
it has been proven to increase throughput rate The examples offered highlight just a few common
by over 250%, while for calcium carbonate, the limitations in feeding twin-screw extrusion
increase in throughput is mostly negligible. The processes. There are many more areas where
limitation is that FET can only be used in zones limitations can occur. The principle of first
where only dry, unmelted powders are present. Wet identifying the limitation, understanding it, and
or molten materials will plug the filter; and large, implementing a solution, then repeating for the
solid particles such as pellets will damage it. For next limitation, can be applied for those limitations
compounding processes, this technology is typically not discussed in this article. Factors such as energy
applied to the side- feeder barrel, though it is also cost, profit, and ability to sell excess production
available in the main extruder barrels as well. should be evaluated for your specific business.
These calculations can provide the framework
To calculate the profitability of installing FET, to justify taking on these optimization projects.
consider an example of a 45-mm twin-screw As always, your twin-screw extruder manufacturer
extruder compounding polypropylene with 40% talc is a good first point of contact for process-
for an automotive application (Table 2). optimization advice.
37 June 2018 Plastics News
