Page 35 - Plastics News June 2018

FeAtures

In addition to selecting feeding equipment to best suit
the mate- rials, take time to consider feeder control
systems. One example is the automatic hopper-refill
control. If the hopper is refilled too frequently, the
controller spends less time measuring the loss in weight
over time, which can reduce the accuracy of the feeder.
On the other hand, if the level in the hopper is allowed
to drop too low, the sudden pressure of material dropping
into the hopper during a refill may cause a surge of
material to slip through the screw, especially for low-
bulk-density and easily fluidizing materials. When this
surge of material falls into the extruder, it will cause a
momentary increase in load on the motor, manifesting
itself as an increase in torque on the extruder control
screen. These torque spikes cannot exceed 100% of the
extruder’s available torque or else safety interlocks will
shut down the extruder to avoid an overload condition. As
a result, the normal operating torque of the process must
be kept lower to create room for these potential spikes.
The increase in profitability with feed-system optimization
feed to the extruder. To do this, the feed system must can be calculated by the increased productivity. An
be designed according to the materials and throughput example of his calculation is shown in Table 1 for a 70-mm
rates being fed (see Fig. 1). Easily fluidizing powders twin-screw extruder processing a polyolefin masterbatch
are handled well by a feeder with twin, narrow-pitch, with a profit margin of 10¢/kg. In this example, a feeder
con- cave augers, as the smaller clearances and that was experiencing surging during refill was replaced
longer path to the discharge abate the fluidization with one better suited for the material, allowing the
behavior. On the other hand, solid polymer pellets throughput to be increased from 2000 kg/hr to 2300 kg/hr
may become impinged in the tight clearances between (4409 to 5070 lb/hr). This results in a possible $180,000
these screws, so pellets benefit from being fed with in additional annual profit, more than four times the cost
a coarse-pitch, single auger. Other materials, such as of the new feeder. Additionally, since the degree of fill in
wet or cohesive aggregates, tend to be best suited the twin-screw extruder increases, specific mechanical
for a belt feeder rather than a screw feeder. Consult energy input to the material decreases. This results in
your feeder OEMs for their specific recommendations. an energy savings of 5% or 138 MWh/yr
For materials that tend to bridge, rat hole, or stick to FEED INTAKE
the hopper, agitation is needed to keep the product After the feed system delivers material to the extruder, it
flowing consistently. Classically this has been done by must be conveyed downstream in the feed intake zone. A
some type of a mechanical stirrer. One downside of limit to the throughput rate occurs when the intake zone
this method is that it can compact sticky materials, has insufficient capacity to convey the materials. The
and increase the cleaning time when changing over conveying capacity of the feed intake zone is determined
to different materials. An alternative to mechanical by the free area of the twin screw and the pitch of the
stirring is the ActiFlow system, which works through screw elements in this zone, along with screw speed and
an intelligent vibratory drive attached to the a few product-related parameters.
outside of the feed hopper. It continuously optimizes
frequency and amplitude based on changing material The free cross-sectional area of the extruder is fixed
flow conditions and provides feedback to the feeder in a standard twin-screw extruder. Extruders with a
controller to remove the vibratory noise from the larger outer-diameter to inner-diameter ratio will have
load-cell signal. more free area. It is important to consider this when

35 June 2018 Plastics News