Page 28 - Plastics News June 2021

FeAtures

Glass-Fiber Compounding on Single Screws

Jim Frankland

Uniform distribution of the fibers into the pellet or flake polymer such as breaking up fiber bundles is important
so that fiber agglomerates do not form early and then cannot be broken up.

owadays, most compounding of short-glass fiber into extreme shear rates; the fibers will break unless they
Npolymers is done on corotating twin-screw extruders. are aligned with the shear (see Fig. 1). Most preventable
But many still compound glass fibers on a single screw. fiber breakage occurs when fibers are forced to make
Unfortunately, this process is still a bit of an “art.” To severe bends or sustain high shear forces while locked in
avoid breaking the fibers there are numerous restrictions a solid polymer/fiber matrix. In the corotating twin screws
on screw design and operating conditions. Maximization the fibers are typically introduced at an intermediate
of fiber length has a significant effect on the final physical downstream location where the polymer is all melted,
properties of the compounded polymer, so the value of avoiding this issue.
the final product is largely determined by the amount of I n order to reduce fiber breakage, the single-screw
fiber breakage.
design should have a more gradual compression section
to minimize plugging of the screw channels with solids.
In addition, the compression ratio from the feed to
metering should be kept on the low side for any specific
polymer to prevent such plugging. Feed sections are also
typically several flights longer to develop some melting
before compression starts. Higher barrel temperatures in
the rear zones can also help to initiate melting as quickly
as possible.

Screw speed is also a factor with any particular screw
design. Screw speed multiplies the shear forces on the
fibers so that output may have to be substantially reduced
For a single screw, the blend of the polymer/fiber mix into to minimize the forces applied to the fibers. Head pressure
the extruder feed throat must be closely controlled. The should be kept as low as possible; it’s possible that
old saying is, “Better mix in, better mix out.” Although redesign of the die head will be necessary, as increased
there is localized mixing in single screws, there is little pressure causes backflow in the screw, thereby subjecting
axial redistribution, so if the mix entering is not uniform the fibers to redundant shearing. Greater L/D can be used
it will exit in much the same way. Uniform distribution of as an advantage or a disadvantage. Greater L/D will assist
the fibers into the pellet or flake polymer such as breaking in obtaining a complete melt without high-shear mixers
up fiber bundles—is important so that fiber agglomerates
do not form early and then cannot be broken up. Although
the screw does not have to be starve fed, its typically
better to minimize fill in the hopper and throat so that
column pressure does not segregate the mixture due to
random solids particle flow. Probably the most critical
area for fiber breakage is before any significant melting
has formed to fluidize the mass and give the fibers some
degree of movement.

Some fiber breakage is unavoidable; when fiber-filled
polymer passes over the screw flights it is subjected to

Plastics News June 2021 28