Direct extrusion with twin-screw extruders


In direct extrusion, raw ingredients such as polymers, fillers, and additives are mixed, reacted, and devolatilized in a high-speed, twin-screw extruder and directly made into a final product, bypassing pelletizing. Although high-speed, twin-screw extruders have been used for many years to produce sheet, film, profile, and fibers, only in the past five years has there been a concerted effort to use them to perform direct extrusion.

Direct extrusion using twin-screw technology was initially mandated, in desperation,
by the need to produce formulations that were adversely affected by the second
heat and shear history inherent in the single-screw extrusion step. Once the technical
viability of direct extrusion was demonstrated, it became apparent to the marketplace
that substantial cost savings were also possible using this technology.

Additional benefits include the ability to adjust formulations inline to accelerate
development efforts, and to maintain a proprietary in-house manufacturing process.
Materials that benefit from direct extrusion include filled olefins, TPE/TPO/TPVs,
polyesters, PVB, wood-fiber composites, adhesives, foamed polymers, nylons, and
degradable polymers.

Corotating twin-screw extruders allow screw rotation up to 1200 rpm, and dominate
the high-speed, twin-screw market for applications that require intensive mass
transfer. Counter-rotating intermeshing and nonintermeshing twin-screw extruders
are also being used for direct extrusion applications. Typical processes performed
in the twin-screw extruder, regardless of the mode of operation, include reactive
processing, devolatilization, alloying, and compounding particulates into plastics.

High-speed, twin-screw extruders are starve-fed devices in which the output rate
is determined by the feeder(s), and screw rpm is used to optimize compounding
efficiencies. Feeders maintain consistency of the formulation, introduce ingredients
in the proper order along the length of the process section, and regulate the
extent of mixing. For direct extrusion, the feeding and materials handling system
to the twin-screw extruder is critical to maintain front-end pressure stability.


Loss-in-weight feeders are typically specified for direct extrusion where the
auger speed is modulated up or down to maintain a consistent mass flow to the
extruder, based on the rate of weight changes in the hopper situated on a load
cell. Twin-screw systems may use up to eight or more feed streams.

Volumetrically controlled feeders are generally not acceptable, even for premixes,
due to the inherent fluctuations in feedrate that result in pressure fluctuations
at the die inlet. If pellets were the end product this would be a nonissue, since
±20 percent dimensional stability for a pellet is often acceptable.

Almost all high-speed, twin-screw extruders use segmented screws that are assembled
on high-torque hammered and splined shafts. Barrels are also modular and can be
configured from feed, plain, vent, side stuff, and liquid addition sections. Each
barrel section is electrically heated, and is internally cored for high-intensity
cooling near the process melt.


This transparent view of a parallel twin-screw extruder illustrates the structure of the basic hardware that accomplishes compounding and end product extrusion in one step without subjecting the material to interim pelletizing and multiple heat histories. Virtually infinite variations of the hardware can be specified, depending on the desired product.

The modular nature of twin-screw extruders offers extreme process flexibility.
Barrels can be rearranged, the L/D can be increased or decreased, and screws can
be modified. For direct extrusion applications, the machine is usually somewhat
longer so that the latter part of the process can be dedicated to pumping with
a more stable pressure than is mandated for pelletizing (see photo at right).

Twin-screw Extruder Function and Design

Twin-screw compounding extruders perform these basic functions: feeding, melting,
mixing, venting, and developing die/localized pressure. The segmented nature of
the twin-screw extruder in combination with the controlled pumping and wiping
characteristics allows specific screw and barrel geometries to be matched to the
required process tasks. This enables the same machine to perform both dispersive
and distributive mixing, which is a major benefit for certain products that are
fabricated by direct extrusion. One example of this scenario is the mixing of
glass microspheres into an extruded part so that the spheres never experience
the high-shear stress associated with plastication in the single-screw extruder.

Screw design is the heart of any twin-screw compounding extruder. An infinite
number of screw design variations are possible. There are, however, only three
basic screw elements: flighted, mixing, and zoning. Flighted elements move material
past barrel ports, through mixers, and out of the extruder to the die. Zoning
elements isolate two operations within the extruder. Screws can be made shear
intensive or passive, based on the elements used in the design. (It is interesting
to note that almost all materials that are processed in single-screw machines
were compounded on a high-speed, twin-screw extruder.)

Mixing in the screws may be dispersive or distributive. The wider a mixing element,
the more dispersive it becomes. Elongational and planar shear effects occur as
materials are forced up and over the land, and more energy is imparted into the
process. Narrower mixing elements are distributive in nature with high melt division
rates and significantly less elongational and planar shear (see diagram below).

The pressure gradient in the twin-screw extruder is determined by the selection
of screws. Since the twin-screw extruder is a starve-fed device, flighted elements
are placed strategically so that the screw channels are not filled and there is
no pressure underneath downstream vent/feed sections. This facilitates downstream
feeding of fillers (e.g., calcium, flame retardants, talc, titanium dioxide, and
so on). The zero-pressure feature also facilitates single or multistage devolatilization.

The viscosity of melting resin is high, so in the early stages of the extruder
the strain rates can produce high stress rates. These may be critical to attain
dispersive mixing, but can also cause degradation of shear-sensitive materials.
In the latter stages of the extruder the viscosities fall such that high strain
rates factored against a decreased viscosity produce comparatively low stress
rates that enable heat- and shear-sensitive materials to be mixed with a minimal
peak shear.


Above is a ZSE-50 twin-screw extruder with gear pump front end and Vulcan downstream profile system. Gear pumps are typically protected by an upstream screenchanger that filters out contaminants. Alternatively, a single-screw pump front end attachment can be specified in place of a gear pump. At left is the ZSE-40 twin-screw extruder with single-screw pump from Merritt. Both the ZSE-50 and ZSE-40 overall extrusion systems are designed and supplied by Leistritz. Some components come from other suppliers, including those noted.

Process Design

Combining compounding/devolatilizing with direct extrusion in a high-speed, twin-screw
extruder presents significant process design challenges. The system requires high
mass transfer in combination with consistent pumping. The selection of screw elements
must take into account the mixing requirements, and also provide stable pumping
to the die or front-end device.

For direct extrusion the machine is longer than a standard compounding extruder.
The last sections of the screws are dedicated to building and stabilizing pressure.
The vent is normally approximately 10D back from the end of the machine to allow
enough process length to build a steady state pressure, as compared to 6D for
a standard compounder. Distributive mixers are often used towards the end of the
screws for thermal homogenization of the melt stream, which would normally not
be required for pelletizing.

To maintain dimensional tolerances, a gear pump front-end attachment may be used
to build and stabilize pressure to the die. A gear pump dampens out pressure fluctuations
by approximately a factor of 10 (i.e. ±200 psi on the inlet of the pump
equals ±20 psi on the outlet of the pump). Gear pumps typically have upstream
protection provided by a screenchanger. This prevents damage by filtering off-spec
substances away from gear teeth (see photo at right).

Alternatively, a single-screw pump front-end attachment may be specified in place
of a gear pump. The length is approximately 10D, essentially the length of the
metering section of a single-screw extruder. This device is more stable than a
standard single-screw extruder since plastication and compression are not present,
which is the main cause for pressure instability in a pellet-fed, single-screw
extruder (see photo below).

The twin-screw compounding system for direct extrusion is significantly more complex
than a single-screw extrusion line. The feeding system to a twin-screw compounder
sets the formulation tolerance and also plays a major role in pressure stability.
There may be as many as eight or more feed streams into the high-speed, twin-screw
compounding extruder. Typically, a PLC-based master control system is required
to manage the system as well as facilitate recipe retrieval and data archiving.


The residence time of 15 seconds to 2 minutes for materials in a twin-screw extruder
must be taken into account for the pressure control algorithm. A possible control
scenario is for the gear pump rpm to be locked to set a constant volumetric rate
for the process melt stream to the die. Feeders are adjusted in very small increments
every few residence times to maintain a stable front-end pressure over the long
term. Screw rpm is continually adjusted, within limits, to maintain front-end
pressure over the short term.

If a screw’s rpms are allowed to adjust without limit, widely varying degrees
of imparted shear might adversely affect the quality of the final product. When
sequential feed streams are introduced into the twin-screw extruder at various
points in the process, the closed loop control obviously becomes more complicated,
as various residence times must be managed.

The control software uses an algorithm program to analyze the inputs from key
points in the system, make numerical calculations, and apply corrections to the
screw rpm and feedrate. The objective of the algorithm is to maintain the gear
pump inlet pressure at its setpoint. If this is successfully accomplished, the
backpressure in the twin-screw extruder is held constant, which provides for consistent
shear and mixing. With a stable melt delivery to the gear pump, the discharge
flow and pressure to the die is uniform—the ultimate objective of the entire
system.


Mixing can be primarily dispersive, as with the wider element, upper left, or distributive, as with the narrower element, at right.Various elements can be combined in screw design to meet whatever mixing challenge is at hand.

Delivering a Consistent End Product

Providing a usable melt to the die and downstream system is only half the battle.
To deliver a high-quality end product, the appropriate die and downstream equipment
is required, whether the product is a film, sheet, fiber, or profile. The following
examples identify products successfully made by direct extrusion:

  • Battery separator/sheet. A PE/silica formulation is premixed and fed into the
    extruder feedthroat and oil is injected into a barrel section in the early stages
    of the process. The materials are mixed and devolatilized in the twin-screw extruder
    process section, which is directly coupled to a sheet die. After the die, a high-pressure
    calendar “squeezes” the extrudate and sets the final dimension, eliminating
    the necessity for a gear pump and closed loop pressure control.

  • Foamed profiles. The polymer is fed into the twin-screw extruder and melted
    prior to gas injection. The polymer is intimately mixed at high pressures with
    high-division distributive mixers to minimize viscous heating. The latter part
    of the process section uses low-energy-input pumping elements so that the barrel
    sections serve as heat exchange devices to cool the process melt. A screw or gear
    pump front-end attachment may be used, depending on the particular application.
    Performing this process in a twin-screw extruder provides an alternative to the
    tandem single-screw systems traditionally used.

  • Filled film/sheeting. The polymer(s)/rubber/additives are fed into the main
    feedthroat and melted prior to downstream introduction of fillers (carbon black,
    talc, calcium carbonate) into the process melt stream via a side stuffer. The
    materials are mixed/devolatilized in the twin-screw extruder, which is typically
    mated to a screenchanger and gear pump front-end assembly. Olefins, TPOs, and
    fluoropolymers all use this system configuration.

  • Adhesive compounding. Rubbers, tackifiers, fillers, and oils are mixed and devolatilized
    in the twin-screw extruder with a gear pump front end upstream from a film/lamination
    die, or a rod die for direct glue stick profile extrusion. In some instances,
    the twin-screw process has proven superior to separate twin-screw compounding
    and single-screw extrusion operations, as a demixing effect often occurs as the
    materials coalesce in the single-screw process.

  • Wood-fiber/composite products. The primary functions of the twin-screw extruder
    in this application are to remove water and distributively mix the natural fibers.
    Because this type of product typically cannot be passed through a screenchanger,
    a single-screw pump front-end attachment is used. Products include profiles for
    decking and sheeting.

Direct extrusion from a high-speed, twin-screw extrusion system can improve quality
and reduce costs when compared to a two-step compounding/production operation.
The downside of this emerging technology is that the intricacies of the overall
system increase as the complexity of the upstream materials handling/feeding equipment
are now combined with the nuances associated with sizing and cooling an extruded
part. In spite of the associated challenges, there are many successful direct
extrusion installations of a high-speed, twin-screw extruder, many of which are
highly proprietary and confidential. The product mix and anticipated volumes need
to be carefully assessed to determine whether direct extrusion is the preferred
manufacturing method, based on the prevailing economic and product performance
issues.


CONTACT INFORMATION

Leistritz, Somerville, NJ

Charlie Martin; (908) 685-2333
[email protected]

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