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Extrusion is one of the most widely used processing methods in the plastics industry. Sheets, films, wire & cables and, profiles are typical extruded products.
Extrusion is one of the most widely used processing methods in the plastics industry. Sheets, films, wire & cables and, profiles are typical extruded products.

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, etc.

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.

Higher productivity

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 KraussMaffeiGroup launched the MNE60-41 singlescrew extruder with L/D ratio up to 41, substantially improving the production capacity.
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 consumption.

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 pipes.

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 molding.

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 operating noise.

Precise extrusion

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: Improves melt pressure stability, thus effectively improving product accuracy and reducing scrap rate; Improves output and reduces energy consumption; Reduces wear and tear, extending mechanical lifecycle; 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%.

SPC control

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.
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.
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 products.

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.

Typical 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


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