Extrusion is one of the most widely used
processing methods in the plastics industry. The development of extrusion
technology is therefore key to the advancement of the entire plastics industry,
where higher precision and more efficient production are needed to push the
limits. The article analyzes the major advancements of extrusion technology,
including higher efficiency, new extruder design, more precise production as
well as virtual production simulation.
Extrusion is one of the most widely used
processing methods in the plastics industry. Sheets, films, wire & cables
and, profiles are typical extruded products. Compounding is another important
segment in extrusion which forms the basis for injection molding, blow molding,
The performance of extrusion is directly
associated with product quality, energy consumption and production efficiency.
The development of extrusion technology is therefore key to the advancement of
the entire plastics industry, where higher precision and more efficient
production are needed to push the limits.
Extensive use of plastic products in the
worldresults in higher requirements on the extrusion process. Improving output
and efficiency of extruders is crucial to the sector's development. Major
advancements come in the following two areas.
High L/D ratio
The KraussMaffei Group launched the
MNE60-41 singlescrew extruder with L/D ratio up to 41, substantially improving
the production capacity. The challenge of processing precision using high L/D
screws is successfully dealt with by its patented technology. In addition, it
reduces melt temperature, improves product quality and reduces per-unit energy
High screw speed
Increased screw speed will improve the
output of extruders. In the 1990s, The US company HPM launched a single screw
extruder with 50-115mm screw diameter and 50:1 L/D ratio for production of
high-concentration masterbatches. Its patented technology reaches maximum screw
rotating speed of 800-1000rpm. For instance, a single screw extruder with 65mm
screw produces1,090kg/h of high density polyethylene (HDPE) at 850rpm. The Monos series single screw extruder from
Cincinnati Group released in 2006 can reach 1,100kg/h maximum output with 90mm
screw and 37:1 L/D ratio. It can be used to produce PE80 and PE100 pressure
In 2010, the Battenfeld and Cincinnati
companies combined to form battenfeld-cincinnati. The monos extruders have been
replaced by new single screw extruder series that offer even higher outputs and
better melt homogeneity. For example, the solEX single screw extruder series
for high performance HDPE and PP pipe extrusion.
At K 2016, the company introduced the solEX
NG extruder series with outputs range up to 2,750kg/h.
New structural design
To meet the requirements of new processing
technology, new extruders have been redesigned structurally to achieve high
efficiency and low energy consumption.
High performance screw
Adopting polygon channel structure, the
single screw design for chaos trigger extrusion from the South China University
of Technology creates a chaotic flow in the extrusion process, and through this
discrete fusion and efficient chaotic mixing status, it improves melting,
plasticizing and compounding efficiency, thereby reducing energy consumption
enhancing production capacity. Extruders adopting such technology reach
0.17kw/kg in nominal power density in addition to other advantages such as
lower melt temperature, extensive material adaptability, lower energy
consumption and better product quality. It ranked second in China’s award for
science and technology advancement.
New cylinder structure
Grooved-feed barrel is an old technology
appeared several decades ago. However, an optimized structure and screw design
allow it to be used increasingly in high-speed, high-output extruders. The
German Extrudex has developed a new extruder in collaboration with Paderborn
University. It adopts a fully grooved barrel structure, with a deep channel for
the feeding section and a shallow channel for other parts in combination with
barrier screws. As a result, this extruder far out performs general single
screw extruders and has been successfully used for pipe extrusion and hollow
Extruder based on elongation flow behavior
Based on the rheological principle,
Professor Qu Jinping of South China University of Technology advanced
roto-extruder with a completely new topological structure. This extruder
leverages the topological structure between the rotor and cylinder in
combination with unique rotation to accomplish positive displacement of polymer
melts in the extruder. The polymer melt subjects only to tensile strain in the
extrusion process, while the impact of shear action on polymer molecular chains
is eliminated. At the same time, the high efficiency of tensile strain achieves
relatively efficient dispersion and distribution during the dispersing phases.
This unit offers strong potentials with several advantages such as high
efficiency, short heating cycle for materials and low processing temperature.
Direct electric drive
The use of direct electric drive is a major
trend in high performance single screw extruders. Screw rotation is driven
directly by the electric drive instead of a gearbox, resulting in improved
efficiency and noise, weight and maintenance cost. For the direct drive, both
high torque AC and DC motor, permanent magnet synchronous motor, or even high
performance servo motor are applicable. The rotation speed is directly
controlled by the electric drive.
Germany’s K&A knoedler and other
companies have been using this technology for polypropylene (PP) and
polystyrene (PS) sheet extrusion, as well as high density polyethylene (HDPE)
pipe extrusion. Following suit, many Chinese enterprises have launched similar
extruders with a simplified structure that reduces energy consumption and
The extrusion power, limited by the
characteristics of screws, unavoidably fluctuates. Other problems include high
resistance correlation between output and extruder head,which substantially
affects the process’ precision and product dimension. To improve the
dimensional precision and accuracy of extruded products that meet the
requirements of high quality products such as optic fiber, medical catheter and
artificial blood vessels, has become a major challenge both home and abroad.
Gear pump assisted extrusion
Leading high precision single screw
extruders in the world often use gear pumps in tandem. The melt gear pump,
generally installed between the extruder and the extruder head, acts as a
pressure stabilizing unit. The gear pump consists of two engaged gears. This
design can separate pressure fluctuation generated by the extruding system from
the extruder head and downstream equipment, thus eliminating the impact.
This is because the gear pump has an
independent motor drive, making it a positive displacement transmission device.
Meanwhile, it is also a “melt booster”. No matter how much the temperature
fluctuates inside the extruder, it can convey materials to the extruder head at
stable pressure and flow rate as long as melt at the inlet can fill up the gear
pump inlet, ensuring fluctuation-free extrusion and thus producing products
with high dimensional precision.
Generally, when used in tandem with
extruder, the melt gear pump has the following advantages: ① Improves melt pressure stability, thus effectively improving
product accuracy and reducing scrap rate; ② Improves output and
reduces energy consumption; ③ Reduces wear and
tear, extending mechanical lifecycle; ④ Reduces melt
temperature for molding. However, what is most important is that the melt gear
pump offers remarkable pressure stabilizing effect for precision extrusion.
As shown by the research of US’ Normag Corp,
pressure change in the extruder head remains within 0.7%while the pressure of
melt entering the gear pump increased by 43%. Leveraging this technology, the
company is able to produce PVC thin-wall medical hose within 0.37% in thickness
tolerance. Another US company, Luwa Corp can reduce PET film thickness
tolerance from 3% to 0.5%.
Strict control of extrusion parameters is
crucial to the stability of extrusion flow rate, which is extremely sensitive
to temperature. To ensure precision extrusion, the fluctuation of extrusion
temperature should be kept within 0.5°C. Electric drive stability is another
important factor to ensure a stable process. The use of variable frequency
motor and brushless DC motor can reduce the fluctuation of screw speed to
0.01%. However, current PLC control technology is based often on a static
control model to maintain open loop control for the extrusion process. There is
no closed loop control between gear pump outlet/inlet pressure and screw speed,
which affects the precision to some extent.
On the other hand, control format based on
static control model can only detect errors after the extrusion process. It is
not able to predict errors and take subsequent correction measures,therefore not
the ideal tool for high-precision extrusion.
SPC (Statistical Process Control) is a
technology using statistical technology to compile data into information and
documents for correction and improvement of the extrusion process. It ensures
that the whole production process remain under statistical control status and
thus achieving high product quality. This is because any fluctuation in process
temperature, pressure and polymer viscosity may cause instability and
eventually affects product quality and precision.
Using SPC control to monitor the extrusion
process offers statistical analysis for various quality data, providing e.g.
graphs and curves on real-time basis, and proposes remedy measures to ensure
process stability, high product quality and dimensional accuracy of the
In the global market, SPC system has been
used on extrusion lines for precision products such as medical catheters to
effectively ensure high product quality. The highprecision extruder unit,
developed by the research group headed by Wu Daming of Beijing University of
Chemical Technology, has been successfully used for the production of optic fiber
and medical catheters.
Simulation of extrusion process
A single screw extruder is often connected
with other equipment to form a complete processing line for production of, e.g.
pipe, panel and sheet. Therefore, the coordination between the main unit (extruder)
and the auxiliary equipment is fundamental to product quality. Particularly, a
closed loop control for the entire production system is extremely important to
ensure high speed, high efficiency and high quality. It is a major point of
focus for equipment development.
Traditional analyzing method cannot meet
the requirements of precision molding. To accurately simulate various molding
processes, industrial experts around the world are constantly developing new
models, new algorithms and molding simulation systems, and then coordinate
software with product design and manufacture for an integrated solution. The
trend of using computer simulation to develop integrated and intelligent
production line is apparent.
Currently, commercially available extrusion
simulation software include US’EXTRUD, PASS; PEX from German; France’s
CHEMEX-TRUD and EXTRUCAD in Canada. These systems not only can simulate the
extrusion and plasticizing process for conventional extruders, but also
simulate complicated process, such as screws with blending units, multistage
screws and screws with exhaust vents.
viscosity models include Newton Model, Power Law Model, Carreau Model and
Polynomial Logarithmic Model. Typical analytic tools for flow process inside
the extrusion die includes FI-DAP, POLYFLOW FLATCD, PROFILECAD and LAYERCAD.
Typical analytic software for extrusion swelling analysis includes POLYCAD-2D
and POLYCAD-3D. This numerical analytic software products provide useful tools
for analysis of screw, extrusion die design and flow process. This is of great
instructive significance for the development of new screw structure and runner
structural design for precise extruder die.
Xie Linsheng, East China University of
Science and Technology