TWIN SCREW EXTRUSION SYSTEMS TO PROCESS BIO- BASED ...

TWIN SCREW EXTRUSION SYSTEMS TO PROCESS BIO-
BASED RESIN FORMULATIONS
Prepared for
Bioplastics Processing Conference
Sponsored by Plastics Technology Magazine
December 5-6, 2006
Charlotte, NC, USA
Charlie Martin
Leistritz
169 Meister Ave., Somerville, NJ, 08876, USA
Phone: 908/685-2333 x616, E-mail: cmartin@alec-usa.com
High speed, energy input (HSEI) twin screw extruders are emerging as the
manufacturing methodology of choice to process innovative compounds that are derived
from or utilize biobased renewable materials. Some examples of materials derived from
renewable resources include PLA, PHA, TPS, natural fibers, starch, oils, and many other
feedstocks. Applications include compounding, reactive processing, devolatilization and
foaming. The final product is often a pellet to facilitate accurate/consistent feeding into
an injection molding machine or single screw extruder. There is also a trend to bypass
the pelletization step and to extrude a film/fiber/sheet/profile in one-step…this is referred
to as direct extrusion. Although similar to traditional plastic processes, there are subtle
differences when processing heat/shear sensitive, biobased compounds.
HSEI twin screw extruder with provision for downstream feeding and venting

HSEI Twin screw extruders process materials bounded by screw flights and barrel walls.
Screws are segmented and assembled on high torque splined shafts. Barrels are also
modular and utilize liquid cooling, which is particularly beneficial for heat sensitive
formulations. A typical process length to diameter ratio (L/D) is 32 to 48 to 1 L/D, with
up to 60 to 1 L/D’s being possible to facilitate reactive processing and/or multi-stage
devolatilization. The motor inputs energy into the process via rotating screws that impart
shear into the materials.
HSEI twin screw extruder process section with multiple vents & downstream side stuffing
Segmented screws/barrels, in combination with the controlled pumping and wiping
characteristics of the screws, allows screw/barrel geometries to be matched to the
process tasks. Solids conveying and plastication occurs in the first part of the process
section. Screw elements for mixing and devolatilization are then utilized as dictated by
the process. Discharge elements finally build and stabilize pressure to a die. Screw
designs can be shear intensive or passive.
Co-rotating intermeshing twin screw extruder screw set
HSEI twin screw extruders are starve fed, which means the output rate is determined by
the feeder(s), which meter pellets, liquids and fibers into the process section. The
extruder screw RPM is independent from the feed rate and is used to optimize
compounding efficiencies. Because the pressure gradient is controlled, and zero for
much of the process, materials can be introduced into downstream barrels sections. For
instance, a twin screw side stuffer can “push” high filler loadings, such as a starch or
natural fiber, into the extruder screws after melting has already occurred. Downstream

side stuffing can be beneficial to obtain high filler loadings, to decrease the abrasive
wear in the extruder process section, and when processing shear sensitive materials.
The controlled pressure profile also facilitates venting. Factors that effect devolatilization
efficiencies include residence time of the melt under the vent(s) (longer RT = better
devol), the surface area of the melt pool (smaller melt pool = higher surface area = better
devol) and the surface renewal of the melt pool (higher renewal = better devol).
Vacuum pump system that interfaces to HSEI twin screw extruder for devolatilization
Front end design: The front-end design of the HSEI twin screw extruder is critical to the
overall success or failure when processing many bio-based products, as this is where
degradation often occurs. Adapters and front-end attachments should be designed to
minimize dead areas that can/will result in high pressures and/or flow stagnation.
If filtration is a requirement, the adapter should facilitate a low volume, streamlined
transition to the breaker plate. In many cases an oval breaker plate is used and the
screws are extended from the extruder to minimize the distance from the screw tips to
the breaker plate. Options include a simple breaker plate that holds screens in the
front-end adapter, slide-plate (discontinuous) screen changers and continuous screen
changers of various designs. The throughput of the system, level of contamination and
degree of filtration are all selection factors. It is worth noting that high-volume, nonstreamlined
melt flow paths associated with some of the continuous model screen
changers will preclude usage with some bio-based formulations.

Gear pumps may also be attached to the front-end of a HSEI twin screw extruder to
build pressures, which can be useful in a number of scenarios. HSEI twin screw
extruders operate at comparatively low pressures, typically below 1500 PSI. At higher
pressures the materials tend to “back up” and discharge up the vent. Gear pumps also
facilitate a low front-end pressure (below 500 PS) for the HSEI twin screw extruder to
minimize the viscous heating associated with the discharge screw elements prior to exit.
Gear pump and screen changer attached to HSEI twin screw extruder
Feed system design: Feeders are a critical component in a twin screw extrusion
system to maintain formulation consistency/rate. Various delivery mechanisms are used,
including: vibratory (for pellets), single screw (for pellets, regrind and some powders),
and twin screw (for powders). Volumetric feeders maintain a constant rpm and are
acceptable for pre-mixes that do not segregate, but for multiple feed streams loss-inweight
feeders are preferred. Loss-in-weight feeders modulate the auger speed to
maintain a consistent mass flow to the extruder, using an algorithm that is based on
materials usage from the hopper situated on a load cell, that compensates for feed
density changes relating to the level of hopper fill. The material handling system, which
moves raw materials from storage to the feeder(s), is also critical to the system design.

Pelletization: Pelletizing systems, as the name implies, are used to produce pellets for
feeding into another device. Cooling may be either in air, on cooling belts or water.
Typically, pellets do not have tight dimensional tolerances as compared to final products.
Strand pelletizing is the most widely utilized and versatile pelletizing technology. An
extruder forces the melt through a strand die which is then pulled through a cooling
medium by feedrolls and cut by a bed knife/rotor assembly. Strand pellets are typically
in the 3 to 4 mm range, but can be as short as 1 mm and as long as 25 mm based upon
the comparative speeds of the feed rolls and rotor in combination with the number of
rotor teeth. The strands are typically cooled in a water bath, or sometimes on a
waterslide. Prior to entering the pelletizer, an air stripper device removes moisture from
the surface of the strands. The end product is a cylinder.
Strand pelletizer with 2-roll puller and cutting head assembly
For products that cannot contact water, the cooling bath is replaced with a cooling belt or
static cooling table. Cooling belts can be stainless steel with liquid “spray” cooling
underneath the belt to facilitate additional heat dissipation. Auxiliary cooling blowers
with ambient or refrigerated air streams can be specified.
Another alternative is referred to as “die face” pelletizing, where the extrudate is cut at
the die face while still molten and then cooled via air or liquid. The advantage of this
system, if workable, is that strand breakage during the cooling phase is eliminated. If

the process is amenable, die face pelletizing is often preferred for high volume and
reclaim applications.
Air quench die-face pelletizing is used for highly filled or viscous formulations, such as a
fractional melt HDPE with a natural fiber, such as rice hull. After the cut, pellets are
pneumatically conveyed to either a vibratory cooler/separator or fluidized vibratory unit,
where cooling is finished. Additional product drying is possible via a heated air stream
in the fluidized bed. Most formulations cannot be processed in this method, as many
materials tend to smear at the die face. Vortec tubes to direct chilled air at the die
face and/or atomizers to mist the cutting chamber can be used for borderline
formulations. Pellets produced via the air quench method are typically slightly deformed
in cooling, and are generally deemed not as aesthetically appealing as others.
Underwater pelletizing is exactly what the name describes. The molten product is fed
through a die with a series of holes in a circular pattern. When the product emerges from
the die, it is cut into pellets by the rotating blades and immediately quenched in water.
The water/pellet slurry is then pumped to a spin dryer for dewatering and surface drying.
This system produces a unique spherical pellet resulting from the product being cut in a
submerged environment. Underwater pelletization is often used for PLA formulations.
HSEI twin screw extruder with underwater pelletizing system

Direct extrusion: Most HSEI twin screw extruders produce pellets, however
compounding/devolatilizing combined with direct extrusion of sheet, film, fiber or profile
is now common. To maintain dimensional tolerances a gear pump front-end attachment,
as described above, is often used. Alternatively, a single screw pump may also be
specified. The length is approximately 10 to 1 L/D, essentially the metering section of a
single screw extruder. For many foaming applications the single screw pump provides
additional cooling of the melt prior to the die.
Gear pump and sheet die attached to HSEI twin screw extruder
The HSEI twin screw extruder performing direct extrusion is more complex as compared
to a single screw extrusion line. The feeding system sets the formulation tolerance and
also plays a major role in pressure stability. Typically, a PLC based master control
system is required to “manage” the system, as well as to facilitate recipe retrieval and
data acquisition/archiving.
The residence time inherent with a HSEI twin screw extruder (5 seconds to 2 minutes+)
must be taken into account for the pressure control algorithm. A possible control

scenario is for the gear pump rpm (or screw pump) to be locked to set a constant rate to
the die. The control software uses an algorithm program to analyze the inputs from key
points in the system, makes numerical calculations, and applies corrections to the screw
RPM and feed rate to maintain the gear pump inlet pressure at its set point.
Providing a usable melt to the die and downstream system is only half of the battle. Now
the appropriate die and downstream equipment is required size and cool the extrudate,
such as a film, sheet, fiber, or profile.
Summary: Many processors have struggled to process bio-based formulations because
most of their experience has been with traditional plastics. Once it is understood that
renewable resources tend to be more heat and shear sensitive (and sometimes
increased devolatilization requirements) as compared to standard polymer compounds
then the HSEI twin screw extruder and system can be configured and operated
accordingly.
This is not virgin territory, in that food extrusion has been utilized for decades. Rigid
PVC and other plastics products, such as glass filled compounds, have similar process
requirements. Now that significant development and marketing efforts are being targeted
at “bioplastics” the learning curve will accelerate, and in the foreseeable future
processing of these materials will become commonplace. The flexible and sophisticated
design of the HSEI twin screw extruder makes it the ideal device to process these
materials to make pellets and/or parts.