Page 36 - Plastics News June 2018

FeAtures

opening (Fig. 2). As a result, there is a competing flow of
the raw materials entering and the entrained air exiting.

procuring a new extruder. The product parameters are
tied to the selection of raw materials. Changes are often One way to reduce the amount of entrained air is to
complicated for purchasing or product- quality reasons. place the powder feeder on the same vertical level as
the extruder, as close to the feed hopper as possible.
Therefore, the parameters that can be optimized on an This minimizes the drop and reduces the amount of air
existing machine are the pitch of the screw elements and that gets entrained.
the screw speed. Increasing screw speed will typically Another strategy is to design vents into the system to give
result in a higher energy input to the material. This
increase in energy can be partially offset by an increase the air alternate escape routes. A vent can be installed
in the feed hopper itself. To increase the effect, the
in feed rate. However, this increase tends to trail off at
increasing screw speeds. This results in a higher melt hopper should be designed so that the feeder discharges
material down the side of the hopper on the same side as
temperature, which among other downsides could cause
issues in pelletizing or in product quality. the down-turning screw in the twin-screw pair, with the
vent stack on the other side of the hopper. This allows
Increasing the pitch of the screw elements in the feed the air to flow away from the polymer on its way down
intake zone is the most innocuous change. As a rule to the extruder (Fig. 3).
of thumb, it is recommended to always start with the
widest-pitch screw elements in the feed intake zone.
Feed intake limitations can result when feeding powders,
especially low-bulk-density or easily fluidizing ones, due
to air becoming entrained as the powders drop from the
feeder into the extruder.
In typical polymer processes, there is a melt zone
comprised of a section of kneading blocks fully filled by
polymer. Since this section of the extruder is fully filled,
the entrained air cannot move down-stream past the
melt seal with the molten polymer. In typical polymer
processes, there is a melt zone comprised of a section
of kneading blocks fully filled by polymer. Since this
section of the extruder is fully filled, the entrained air
cannot move downstream past the melt seal with the
molten polymer. Instead, it is forced to escape through
the nearest upstream opening, which is usually the feed

Plastics News June 2018 36