Page 35 - Plastics News February 2022

FEATURE

Understanding Melting In Single-Screw Extruders

Jim Frankland

ver the years I have talked to many melt forms on the barrel wall.
Opeople with lots of experience in 3. That thin film gets sheared by the
extrusion who do not understand how relative rotation of the barrel and
single screws melt polymer, or how the screw.
extruder drive supplies that energy. Many
think the barrel heaters supply much of 4. The process proceeds with the help
the energy entering the polymer, which of the screw design as the channel
is mystifying to me … and completely depth is reduced (compression
wrong. section), which forces the unmelt up
against the barrel, maximizing shear
In all fairness, it is difficult to visualize on the unmelt.
melting with the screw turning in the it. In an extruder the distorting force
barrel because of the geometry. But to shear the polymer requires energy to 5. With proper screw design for that
if you flip the observation point— rotate the screw in the layer of viscous polymer, all the unmelt can be
imagine the barrel turning around the polymer. The rotational energy from converted to melt at the proper
screw—you can more clearly visualize the drive is converted from mechanical processing temperature in a
the forces involved. It’s not any different to thermal energy via shear putting heat controlled fashion.
than imagining the sun turning around into the polymer. Even after reaching the melting or
the earth when we know the opposite softening point, the shearing continues
is true. In an actual extruder the screw The shear rate is proportional to the due to the viscosity of the melt. Most of
typically turns counterclockwise, when speed of rotation and the thickness of the drive power in a single-screw extruder
viewed from the drive end (or clockwise the object, achieving the maximum near is used to rotate the screw in the polymer.
from the discharge end), and the barrel the barrel surface and zero at the screw Typical distribution of the drive power is
is stationary. But if we flipped the root, as shown in Fig. 1. 85-90% to melting, with the remainder
observation point so that we’re “sitting” The amount of shear is then proportional to mixing, pressurization and forwarding.
on the screw, the barrel would appear to to the viscosity of the polymer at various The barrel heaters contribute almost
be turning clockwise around a stationary temperatures during shearing. Using a nothing to melting once screw rotation
screw. gradually reducing channel depth in the begins. In fact, for many extrusion
Barrel heating is initially required before operations, the barrel heaters are in
startup to get the screw filled with cooling mode most of the time.
melted polymer and to obtain a surface Each polymer requires a different
temperature where the polymer will stick amount of energy from the drive based
to the barrel. From then on, almost all on the amount of energy required to raise
the energy entering the polymer comes its temperature to the desired processing
from the energy required to turn the temperature. The specific heat of the
screw relative to the barrel—or in our polymer governs that amount of energy.
“flipped” case, the barrel relative to the In screw design, the viscosity of the
screw. Because of the initial heating, “compression section” of the screw polymer while being sheared determines
the polymer is stuck to the barrel and forces any unmelted polymer closer the energy introduced by each revolution
is pushed forward by the angle of the to the barrel wall, where it gets the of the screw. The viscosity decreases as
screw flights. Once screw rotation starts, maximum shear. the polymer progresses from solid to
the polymer is melted almost entirely by In the sequence 1-5 shown in Fig. 2: melt. The initial polymer temperature
shear. “Shearing” is defined as applying governs the total amount of energy
a force that distorts one surface of 1. Screw channel is filled with unmelted required from the screw drive. For
an object relative to another surface. polymer leaving the feed throat. example, preheated polymers require less
Shearing an object introduces heat into 2. Due to barrel heating, a thin film of power from the drive.

34 PLASTICS NEWS FEBRUARY 2022 35 PLASTICS NEWS FEBRUARY 2022