Extrusion, Education, Plastics

Extrusion Processes

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Extrusion Processes
Extrusion is the process where a solid plastic (also called a resin),
usually in the form of beads or pellets, is continuously fed to a
heated chamber and carried along by a feedscrew within. The
feedscrew is driven via drive/motor and tight speed and torque
control is critical to product quality. As it is conveyed it is com-
pressed, melted, and forced out of the chamber at a steady rate
through a die. The immediate cooling of the melt results in re-
solidification of that plastic into a continually drawn piece whose
cross section matches the die pattern. This die has been engi-
neered and machined to ensure that the melt flows in a precise
desired shape.
Examples of extruders products are blown film, pipe, coated paper,
plastic filaments for brush bristles, carpet fibers, vinyl siding, just
about any lineal shape, plus many, many more. There is almost
always downstream processing equipment that is fed by the
extruder. Depending on the end product, the extrusion may be
blown into film, wound, spun, folded, and rolled, plus a number of
other possibilities. This article limits any equipment discussion to
the extruder itself.
Plastics are very common substances for extrusion. Rubber and
foodstuffs are also quite often processed via extrusion.
Occasionally, metals such as aluminum are extruded plus trends
and new technologies are allowing an ever-widening variety of
materials and composites to be extruded at continually increasing
throughput rates. This article will focus only on the extrusion of
plastics.
Features and Properties of Plastics
To understand how to optimally process plastics, it is essential to
understand some physical and chemical properties.
1. All plastics are composed of long chain molecules (extremely
high molecular weights) based on simple "building blocks"
called monomers. Each polymer molecule typically contains
several thousand monomer blocks and the reaction that creates
polymers by monomer linking is called polymerization. Monomer
units can either be all the same (vinyl chloride monomer or
VCM polymerizes to make PVC) or two or more different
monomers can polymerize in a repeating or random pattern
(acrylonitrile, butadiene, and styrene polymerize to form ABS
copolymer).
2. These long chain molecules vary widely in type as well. Two
distinct classifications of plastics, which exhibit highly different
physical behavior, are directly related to the degree that the
polymer molecules interact or cross-link with each other.
a. Thermoplastics typically have little cross-linking. These
materials are easily deformed, flexed, and can be repeatedly
melted and re-solidified (barring some off-reactions due to
excessive thermal degradation). Examples are polyethylene
(plastic milk bottle material) and polypropylene (used as
insulation in a spun fiber form, for example.).
b. Thermosets are highly cross-linked polymers. They tend to be
hard and brittle, and typically are cured either chemically or by
heat. Once formed they are infusible, and will thermally
degrade before a melting temperature can be reached.
Examples are polyurethane (insulation) and bakelite.
Thermosets are typically poor extrusion candidates and will not
be discussed any further here.
Introduction
Application Solution
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3. Plastics conduct heat inefficiently. This means that heating
(and cooling as well) is a slow process. Extruder designers must
take this into account so complete melting of the plastic is
considered for the desired production rates. Otherwise the
extruded product will be unevenly formed and of inferior quality.
However, providing excessive heat to simply assure that melting
is complete also has its own set of negatives.
a. Since plastics are inefficient conductors, the excess heating
is inefficient and the extra energy involved is costly.
b. Overheated melts require extra time to re-solidify, increasing
the likelihood that the extruded product becomes deformed or
be misshapen upon hardening.
c. Excessive temperatures can also promote off-reactions of the
plastic or between any of the additives. This may result in
thermal degradation, off-color/off-spec products, or toxic by-
products.
4. Extruded resins are highly likely to contain other compounds
and chemicals in varying amounts. These are blended by a
process called compounding prior to formation of the pellets or
beads. Ranging from trace amounts of property-enhancing
additives to bulk filler material, various types and their
purposes are mentioned here.
a. Stabilizers are used to block formation of harmful
off-products (example: additives in PVC neutralize or absorb
hydrochloric acid formed at elevated temperatures.)
b. Lubricants make product more pliable and reduce adherence
to the extruder walls. This saves energy and eliminates
potential hot spots that could be sites for thermal degradation.
c. Dies and colorants give extruded materials their desired color
or tone.
d. Plasticisers reduce brittle behavior, making processing easier
and less costly.
e. Fillers are typically inorganic compounds (talc, graphite,
chalk, etc.) that are cheap and do not affect the integrity of the
resin matrix. This makes the material less expensive on a
weight basis than pure plastic. New developments and
engineering efforts have utilized fillers to achieve targeted
properties as well.
f. Alloying polymers (similar to metal alloys) can take advantage
of desirable properties of either polymer.
g. Other additives give plastics their glossy look, feel, flame
retarding characteristics, and other specified properties.
5. Shear, on a microscopic level, is defined as layers or planes of
molecules sliding across one another. The measurement of
force applied to move these planes is the shear stress and the
amount of shear over time is the shear rate. Viscosity is an
important fluid property and is defined as the shear stress/
shear rate. Molten plastic is subject to shearing as it moves in
an extruder, and the lower the melt's viscosity, the less applied
torque is required to extrude it.
Extrusion Equipment
Overview
Figure 1 shows a basic extruder machine. Plastic pellets or beads
(also referred to as resin) are fed from the hopper along a feed
screw through a barrel chamber. As the resin travels along the
barrel, it is subject to friction, compression, and heated zones.
The result is that the resin melts and further travel at the exit end
of the screw serves to mix the melt homogeneously. The melt
enters a chamber designed to ensure an evenly distributed flow
to the die. In many machines, a melt pump is used to prevent any
pressure surges. Also, breaker plates serve to prevent any solid
particles or foreign objects from passing through the die.
The die is a precisely machined part with a patterned opening
such that the extruded plastic takes that die pattern for its cross
sectional area. With products such as extruded sheet, there are
adjustments to the die to allow for a variety of sheet thicknesses
with one die. Shapes are varied, and typically are holes for fila-
ment, annular rings for pipe and tube, or geometric patterned
shapes for items such as vinyl siding and window frame stock. All
die surfaces must be free from defects otherwise unwanted pat-
terns will appear on the extruded product.
Product from the die solidifies quickly. Depending on the end
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product, this may be achieved by immersion in cooling water, air-
cooling, or contact with chill rolls. As mentioned above, overheat-
ing the melt is to be avoided at all costs, or the product will not
form properly on solidification. Once solid, the product material
can be wound, spun, or cut in defined lengths depending upon its
intended end-use.
Figure 1
Basic Extruder Machine
The feedscrew, barrel, and temperature controller form a section
of the extruder called the plastication unit. Plastication is defined
as the conversion of a thermoplastic to a melt. As mentioned
before, this is critical to successful extrusion processes.
The major components in an extruder are discussed here.
Feedscrew
As the only moving part in many extruders, feed-screws must do
the job of moving the resins through the barrel chamber in a
steady and predictable manner. As a result, and the feed-screw is
critical to the design
Figure 2 shows examples of feedscrews. There are at least three
defined sections in a basic feedscrew, and if specifically
engineered to accomplish a definite purpose, they can have
additional sections.
1. The feed zone takes resin from the hopper and conveys it
along. During the journey, resin pellets encounter friction from
feedscrew surfaces, barrel surfaces, and each other. This
mechanical friction is about 85% of the required heat, so it is
critical that the drive equipment to turn the screw have the HP
capabilities to overcome friction AND turn the feedscrew at a
steady and controlled rate. Some extruders can continue to
plasticate materials long after their external heat sources are
shut down.
2. The compression zone is next. Here, the channel depth
between screw flights diminishes and the result is to pressurize
the now melting resin. Friction, barrel heating, and compression
in this stage should complete the melting process. Two
important design parameters are associated with this zone.
a. The compression ratio is measured as the channel depth at
the end of this zone divided by the channel depth in the feed
zone. Different compounds or operating pressures require
different compression ratios.
b. The length of the compression zone affects the rate of
compression. These two parameters will be different for
different compounds.
3. The metering zone has a constant channel depth and primarily
exists to further mix molten resin. The end result is a smooth
consistent melt with uniform temperature.
4. In some processes, a de-gassing or devolatizing section is
required. This is a shorter zone that immediately follows the
compression zone (See figure 2) . Channel depth is suddenly
increased, and the resulting pressure drop causes a release of
any gas, which can be vented or drawn off via vacuum pump.
The remaining melt is re-compressed and metered.
Figure 2 Typical Feedscrews
Plastic
Hopper
Drive
Industrial Control
Screw
Extruder
Die
Extruder
Plastic
Material
Downstream
Equipment
AC Pump Motor
Basic Extruder
Extruder Duty
DC or AC Motor
Power
Transmission
Components
Force Fed
Motor
Breaker Plate
Fee section
Compression
Metering
section
Helix Angle
Screw diameter
Channel = depth feed section
Lead
Channel = depth
metering section
Flight
Devolatizing (two-stage) screw
Three-zone screw
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Mechanical screw design also requires the selection of high-
grade materials and precision machining. The screw must fit
tightly in the barrel to prevent excessive back-flow or drag flow of
resin due to excessive gaps between the screw flights and the
barrel surface. It must not be so tight that it contacts the barrel
surface itself, causing grooves and other damaging effects.
As if the tight tolerances were not enough of a challenge, some
materials require extra processing and are best handled in a
twin-screw extruder. Here, two screws are tightly mounted in a
"figure 8 " type barrel, and the screw flights are designed such
that they avoid grinding each other during rotation. The screws
can be designed to operate co- or counter-currently.
Co-current operation adds a degree of mixing to the process and
would be advantageous where, for example, green and blue
pellets need to be mixed as extrusion occurs to get a melt that
has an aqua hue. The resin is carried from the first screw to the
second between each flight.
Counter-current operation serves to convey the melt in a smooth
predictable manner and helps eliminate pressure pulsing. Due to
machining and operation demands, this equipment is more
expensive to build and maintain than single screw extruders, so it
is reserved for special extruding needs.
Barrel Chamber
This thick-walled steel chamber that is expected to withstand
high pressures ( 20,000 psig), is precisely machined for a tight fit
with the feedscrew, and has a hardened steel alloy on its inside
wall to prevent wear and corrosion. Some barrels will also have a
grooved feed zone to increase the frictional forces on the resin.
The barrel also is heated to facilitate melting of the resin.
Although the major contributor to melting is friction, the heat as
conducted through the barrel can serve as a "fine adjust" or
vernier in temperature control and energy input. Electrical
resistance heating is a common method employed. Advantages
are that several temperature zones can be set up with multiple
elements, and temperature profiles can be created as material
requirements vary. When thermal needs are not so complex,
steam heating via a jacketed barrel chamber. A jacketed chamber
uses cooling water to prevent overheating of the melt in the
vicinity of the die as well.
Dies
The opening that allows plasticated material to form particular
shapes is also a highly engineered part. Dies are designed to
compensate for effects of shrinkage when a melt re-solidifies,
two dimensioned size adjustments, and varying rates of solidifica-
tion. Dies must be free from defects and scratches, otherwise the
melt could show the defect's pattern. The flow of melt to the die
typically follows a tapered path, with the die having a thickness
associated with it. (See figure 3) This results in the melt
undergoing a pressure drop as it exits the die, and this prevents
unwanted build-up at irregular places along the die, which would
spoil the product.
Dies can take on a variety of shapes and have adjustable
openings. In the case of filament extrusion and others, multiple
duplicate die patterns to extrude many strands in parallel can be
found on a single die.
Figure 3
Other Equipment
There are other parts of the extruder that deserve a brief mention.
Different hoppers are used for different purposes. Feed hoppers
hold and supply resins to the feedscrews. Motor driven helical
screws or vibrators help eliminate any bridging or arching of the
resins that prevent the smooth flow from the hopper to the feed
zone.
Mixing hoppers upstream of the feed hoppers compound any
Adapter
Die for square �U� shape
Die opening
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needed plasticisers and fillers to the required specifications.
Melt pumps can smooth the effects of pressure fluctuations that
otherwise would result in uneven extrusions and resulting off-
spec products. These help out in cases where multiple dies are on
a machine, and can be individually closed off on the fly. The
downside of melt pumps is their expense, plus they are extra
moving parts that must be maintained in good condition.
As an alternative to a melt pump, there is a feedscrew design
variation that adds an additional zone with screw flights with a
reverse pitch from the other sections. This serves to act as a
surge suppressor and is illustrated in figure 4.
Figure 4
Power Transmission Equipment
As mentioned before, the feedscrew is the moving part and it
must be driven. Operation in a steady and predictable manner is
vital to making quality extrusions. As friction represents about
85% of the energy used in heating resins, this also means that
the power transmission equipment must be capable of supplying
the energy to overcome this friction, particularly if starting from
rest or recovery from a maintenance outage.
Good speed control is extremely important to assure that ade-
quate resin is being fed to the process. However the ability to
maintain even pressures to get consistent flow is equally impor-
tant. Good response to torque changes as well as steady speed
control of high friction loads is the challenge.
Historically, DC drives and motors have been the ideal drives for
extrusion. Their relative advantages are listed here.
�DC drives and motors offer wide constant torque speed ranges
(20:1).
�DC has been the simplest choice of design when considering
choices between AC, DC or servos.
�They offer smaller sizes at larger horsepower ratings (>60HP).
�DC drives are easily retrofitted to existing DC motors.
On the technology front, AC drives/motors are coming into their
own as good extruder candidates. With the continual develop-
ment of PWM technology and more rugged AC motor designs,
more and more extruder manufacturers are looking for AC solu-
tions. AC drives/motors offer the following advantages.
�Dynamic response with vector operation. Recent designs
employ sensorless vector operation and give high speed
response yet require no feedback.
�AC motors require minimal maintenance (no brushes or
commutators) and are suitable to harsh environments.
(Elevated temperatures, dust, volatiles, etc.)
�Motor designs for extruder duty units feature high overload
capabilities and very wide constant torque speed ranges.
Regardless of the choice between AC or DC for an extruder,
Reliance Electric has the right products and technologies to pro-
vide good solutions.
� Microprocessor-based regulators;
� Easy-to-configure drives with quick-start capabilities;
�Control from any number of sources: local, remote, network,
serially to a PC;
� AC and DC motors that are specifically designed as extruder
duty; and
�Easily modified with a wide variety of optional kits available for
those extra special applications.
When it comes to extruders and their application technology,
Reliance Electric has the answers.
The supressor minimizes pressure surges by accepting or rejecting
excess resin from metering section.
Hopper
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Publication D-7741 � March 2000
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