Page 27 - Plastics News April 2021

FeAtures

Are The General-Purpose Screws on the Comeback

Jim Frankland

GP screws are being specified more often for recycling applications to increase flexibility as recycler processes
several basic kinds of polymers and changes several times a week on an extrusion line.

he “general-purpose (GP) screw” can trace its design • Viscosity at various shear rates and temperatures;
Tback to the 1960s. The idea was to have one screw • Glass-transition temperature;
design capable of handling a range of polymers (except • Specific heat and heat of fusion;
for rigid PVC). Back then, the number of polymers was
limited compared with today and the competition for • Melting point and processing temperature;
production efficiency was not as great. So the concept • Solid, bulk and melt density;
made some sense. With the development of more • Feeding characteristics.
advanced extrusion screw-design technology and new As an example, if we compare a sheet-grade HDPE
types of polymers, this concept had largely disappeared. regrind to a sheet-grade PET regrind at equal volume
Most processors today understand the need to have of output (in.³/sec), they require about the same
specific screw designs for each polymer to maximize energy input to reach a typical processing temperature.
However, the typical viscosity for HDPE in the screw will
be almost double that of the PET. Keep in mind that
the bulk of the energy input is done by shear stress in
the polymer from the rotating screw—not from barrel
heating. At equal screw speed, the resultant energy input
is then largely proportional to the polymer viscosity. As a
result, the HDPE will absorb 50% more heat from viscous
dissipation than the PET as it passes through the screw.
If the screw was an ideal design for the PET, the HDPE
would be well under its ideal melt temperature and
only partially melted. If it was an ideal design for HDPE
the PET would be overheated and possibly degraded.
output and melt quality. Adjustments could be made to the operating conditions
Yet requests for GP screws are being made more to try to better balance the energy input by building
frequently as a result of the rapid growth in recycling. head pressure or reducing the screw speed. But both
The demand for clean post-consumer reclaim is strong, of these adjustments reduce output—an undesirable
and obtaining a steady supply to support a business outcome. In addition to the energy input, the differences
requires constantly searching for new sources, unless in feeding and melting rate could lead to plugging and
one has an in-house source. Often the recycler processes surging because of the differences in the two polymers’
several basic kinds of polymers and changes several viscosity and frictional characteristics.
times a week on an extrusion line, hence the increase A Better Way
in requests for GP screws.
A better solution is to run the same type of polymer on
Still, there is no such thing as a “true” GP screw, an extruder for as long as possible. In most cases, pretty
due to the myriad individual polymer characteristics broad differences in melt flow or melt index can be run
involved for efficient processing. In the design of a high- on one screw effectively as long as it’s the same basic
productivity screw, the following list reveals some but polymer. If changeover between polymers is required
not all the data considered:
for the recycle mix, the extruder should be set up with

27 April 2021 Plastics News