Page 29 - Plastics News June 2021

FeAtures

required to reduce the change in direction of the fibers
as they pass over the moving barrier flight in relation to
the barrel. In general, the screw should be designed so
that such high-shear mixers are not required to complete
melting, which may mean a sacrifice in output rate.

Screw designs that do well processing amorphous polymers
with glass fiber are often ineffective with crystalline
polymers.

A better choice of mixer types is the kind that divides the
melt, minimizing fiber breakage and offering improved
or barrier flights, but also subjects the polymer to more wetting-out of the fibers. These mixers are sometimes
repetitive shearing. For those running glass- or other referred to as Saxton mixers, and consist of multiple flights
fiber-filled compounds on a daily basis, determination of with interrupting cuts (Fig. 2). This design eliminates much
the best L/D can be critical. Selecting the ideal L/D can of the high compressive forces present in shear mixers.
be done by stopping the screw, pushing it out filled, and
having the glass length at various L/Ds measured after A mixer type that I have had success with for low-shear
burning off the polymer. mixing and good fiber wetting is the simple grooved-
root design (Fig. 3). It can be designed to provide any
Crystalline polymers require even more care in order to level of shear and mixing, thereby avoiding the severe
prevent fiber breakage than amorphous polymers, because reorientations that can break fibers. Additionally, designs
they do not soften until reaching their melting point. This like this apply multiple low-shear cross-directional forces
makes channel plugging more difficult to eliminate. That that can help to unbundle fibers as well as reduce the
should be considered along with the compression rate formation of agglomerates.
and feed length. Screw designs that do well processing
amorphous polymers with glass fiber are often ineffective FIG 3 A simple grooved-root mixer design offers low-shear
with crystalline polymers. mixing and good fiber wetting. It can be designed to
provide any level of shear and mixing, thereby avoiding
Mixer Choice is Crucial
the severe reorientations that can break fibers.
Mixers require special consideration as well. High-shear
mixers like the traditional Maddock design can be used, Regardless of the single-screw design, there is always some
but they need to be placed well down the screw where reduction in fiber length during the transition from solid
there is an expectation of complete or nearly complete to melt in the screw. However, with care in the design and
melting, which reduces their basic purpose of melting. operating conditions, suitable product physical property
Even then, a greater clearance over the barrier flight is improvements can be obtained.

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29 June 2021 Plastics News