Strand drawing
The extrusion process (also known as pultrusion or pultrusion ) is a continuous manufacturing process for the manufacture of fiber-reinforced plastic profiles .
history
At the beginning of 1954, Ernst Kühne developed the pultrusion process in a technological development laboratory of an international electrical company. He managed to independently manufacture the first drawn products with glass rovings - impregnated with epoxy resin . In addition to the actual production process, Ernst Kühne previously developed the necessary machines and the precise drawing tools. The first electrotechnical, chemical and mechanical parameters of an extruded profile were determined during this time. Then the first European pultrusion company, the Ernst Kühne Kunststoffwerk, was founded .
At the same time, WB Goldworthy, as one of the pioneers of fiber composite technology, developed the strand drawing process in the USA . However, the main focus was on the production of profiles on polyester resin basis , while Ernst Kühne mainly with profiles on epoxy base employed.
At the beginning of the 1960s there were around 20 manufacturers - mainly in the USA - while today around 90 manufacturers worldwide serve the main markets in the USA, Europe and the Far East. In Europe alone, the market volume is estimated at 16,000 tons per year.
Raw materials
A fiber composite material consists of a reinforcing material that is embedded in a matrix. Both thermosetting and thermoplastic polymers can be used as the matrix. The relatively inexpensive polyester resins, vinyl ester resins and epoxy resins are used as thermoset matrix systems .
In order to improve special properties such as sliding properties, subsequent deformation under heat and abrasion resistance, thermoplastic fiber composite materials can also be produced. Mainly polyamides, polypropylenes and polyethylenes are used as matrix systems.
The reinforcement material used is mainly synthetic fibers made of glass, carbon and aramid, which can be used as rovings, scrims, fabrics or fleeces. This means that the properties can be varied over a wide range, both absolutely and in terms of their relationship between the longitudinal and transverse directions.
Procedure
The basic structure of a pultrusion system consists of the following elements:
- Fiber shelf
- Fiber guides
- Waterproofing device
- Forming or hardening tool
- Pulling device
- Cutting unit
In an open pultrusion process, the fiber rovings are guided over fiber guides from a multi-storey spool store into the resin bath, the impregnation device. The fibers pass through several preforming stations so that they are brought into the desired profile shape. At the fiber guides, mats, fabrics, nonwovens or nonwovens can be integrated into the process in order to adapt / optimize the mechanical properties compared to those of a purely unidirectional reinforcement , such as that achieved by fibers.
In the so-called closed process, all of the reinforcing fibers only come into contact with the uncured monomer in the molding tool - but then with increased pressure for better impregnation. In the mold, the monomer reacts to form the polymer. The cured semi-finished product is pulled through a caterpillar haul-off and thus also pulls the fibers together with the matrix.
Basically, a distinction can be made between the following three methods of resin impregnation:
- Tub method
This process is the most common for the production of pultruded profiles, especially with simple cross-sections. The impregnation and soaking takes place in an open resin bath through which the dry fibers are drawn. The fibers are deflected into and out of the resin bath by means of guide panels.
- Pull-through process
This process is used in particular in the manufacture of profiles with geometrically complex cross-sections. The reinforcement fibers are guided through the resin bath without deflection, so that the impregnation unit is passed through horizontally. At the inlet and outlet side of the resin bath there are preforming stations that are similar to the later profile shape. The matrix scraped off by the fiber guides is collected under the impregnation unit with the help of a trough.
- Injection process
In the injection process, the reinforcing fibers are guided through the impregnation tool without being deflected. This tool has the shape of the profile to be produced and expands inside. The resin is injected into this cavity from both sides across the grain. This method is used in the production of simple profiles with high production quantities.
The final shaping of the profile and hot curing then take place in a temperature-controlled mold with a length of 0.5 m to 2.5 m at temperatures between 100 ° C and 200 ° C. The finished profile and thus the fibers together with the matrix system and the reinforcing material are drawn through a subsequent drawing device, e.g. B. in the form of a caterpillar haul-off or pneumatic grippers, continuously conveyed and pulled out of the tool at a constant speed (hence the English name pultrusion from to pull - pulling and extrusion - pushing through ). The grippers work according to the so-called alternating principle: while one gripper clamps the profile under pressure, the other returns to the initial situation. At the end of the gripping path, the second gripper takes over the pulling without interrupting the conveyance. The drawing speed of the process is adapted to the wall thickness, the complexity of the profile cross-section and the matrix system. A process speed of 0.1 m / min to 1.2 m / min is usual.
scope of application
The extrusion process is suitable for producing (relatively) inexpensive fiber-reinforced plastic profiles. Due to the high hardening temperature and the constant conditions, the quality of the composite material obtained in this way is well above the quality that can be achieved with cold-hardened manual processes. Higher qualities can be achieved through complex prepreg autoclaving processes.
A special problem is the control of the hardening reaction of the resin with large profile cross-sections, which can lead to shrinkage cracks - so-called "center cracks".
Above all, the speed of production, combined with the high degree of automation and the associated lower costs, open up new fields of application for fiber-reinforced plastic profiles manufactured using the pultrusion process, for example as a structural replacement for steel profiles in building construction, lightweight construction or in areas with severe corrosion . These profiles are regulated in EN 13706-1 ... 3.
The same applies to the manufacture of GRP reinforcement. The application and properties of these products are regulated by approval of the respective building product at DIBt .
Until recently, the use of pultruded GRP profiles was restricted by the fact that only straight profiles could be produced in this process. The figure "Various straight pultrusion profiles" shows some examples of profiles.
Recent modifications to the procedure
While in the standard process only straight profiles can be produced due to the process sequence in which the profile is drawn through a stationary mold, this problem is solved by reversing the process.
This reverse process, known as radius pultrusion, was developed in 2008 by Thomas GmbH + Co. Technik + Innovation KG . In contrast to the standard process, with this process the mold, the cavity of which corresponds to the course of the desired profile, is moved step by step over the resulting profile. The gripper that is only present in this process holds the profile while the mold is moving forward and releases the profile when the mold moves backwards, as shown in the illustration of the process sequence of the radius pultrusion.
With this method and its variants, it is possible to create almost any curved profile and z. B. also produce helical profiles.
Web links
- EPTA-European Pultrusion Technology Association
- Thomas Isenburg: It's all about the mix . In: machine market . No. 4 . Vogel Verlag, February 19, 2018, ISSN 0341-5775 , p. 25 .
Individual evidence
- ↑ Christian Bonten: Plastics Technology Introduction and Basics , Hanser Verlag, 2014. ISBN 978-3-446-44093-7 .