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Lightweight Design worldwide

, Volume 10, Issue 2, pp 48–53 | Cite as

Automated Direct Fibre Placement with Online Binder Application

  • Oliver Rimmel
  • Jens Mack
  • David Becker
  • Peter Mitschang
Production Preforming 2.0
  • 288 Downloads

Keywords

Fibre Reinforce Polymer Binder Content Binder Material Single Process Step Consolidation Pressure 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Dry Fibre Placement (DFP) is an automated preforming method for load-related alignment and positioning of fibre materials in preforms for Liquid Composite Moulding (LCM) processes. Thus, it enables an optimal utilisation of fibre properties and is an interesting addition to established preforming technologies. Furthermore, DFP provides the major advantage of low scrap rates and the possibility to omit cost-intensive textile processes. At IVW, initially a system for online binder application has been integrated in a DFP process. This allows a large variety of combinations of fibre and binder types as well as local adjustment of binder amount.

In times of increasing energy costs and decreasing emission limit values, there is a rapidly growing demand in materials with high lightweight potential. Particularly in mobility and energy sectors, such as automotive and aviation as well as wind energy industry, a reduced part mass can contribute to a significant increase of total efficiency. Due to achievable weight savings of up to 75 % compared to steel parts, fibre reinforced polymers (FRP) offer a huge potential. However, their use is usually accompanied with higher part costs in most cases as the raw materials (for example carbon fibres) are expensive and most state of the art processes induce high scrap rates. Furthermore, many process types are rather suitable for smaller batch sizes and therefore inefficient compared to traditional metal manufacturing, when applied for large batch manufacturing [1].

Liquid Composite Molding (LCM) processes offer a huge potential for industrial mass production, since they can be automated to a great extent and short cycle times can be achieved at high process stability. For current LCM applications, the dry fibre reinforcement structure is manufactured in several process steps to achieve the so-called preform. Therefore, at first the fibre material is processed to textile semi-finished products such as woven fabrics or non-crimp fabrics. Due to their structure, which is usually at least bi-axial, the manufacturing of preforms with highly load-related fibre orientation is very difficult. Consequently, the lightweight potential of the expensive fibre material is not fully exploited. What is more, the large scrap rates which especially occur in case of complex parts often make the use of textile semi-finished products inefficient. For these reasons, a wide-spread intention is to omit these semi-finished products and produce preforms directly from rovings. As shown in Figure 1, the process chain can then be remarkably shortened and a high amount of scrap can be saved. This method is also referred to as direct preforming.
Figure 1

Schematical depiction of state of the art LCM process chain and comparison with Dry Fibre Placement respectively online binder application (© IVW)

For the manufacturing of preforms via DFP, automated layup heads are used which can be for example mounted to an industrial robot, Figure 2, or a gantry system for movement. To enable an adhesion of rovings on the tool surface respectively previously laid layers, a fixation of fibres on this surface has to take place. This can for example be achieved by using a polymer binder, for example in form of a binder fleece or powdery binder material. Such binder materials are usually applied onto the roving within a separate process step for semi-finished products and allow their processing using a consolidation pressure and activation heat. Besides additional costs for manufacturing of the semi-finished product, this also causes a low flexibility of the process, as available combinations of fibre and binder material in a preset amount have to be used. Thus, a local adaptation of binder amount as it would be useful for critical spots as edges with small radii is not feasible. For this reason, it is expedient to achieve a local adaption of binder content by adapting an online binder application into the process.
Figure 2

Preform layout using an industrial robot for dry fibre placement (© IVW)

Process Chain to Achieve Online Binder Application

To be able to achieve an online binder application during layup, a detailed examination of the single process steps is necessary. For this reason, the entire process chain starting from the roving and ending with the finished part has been built up and analysed at IVW GmbH. First, the roving delivered by the manufacturer is spread to achieve a homogeneous distribution of fibres and a constant width during layup. Subsequently, the powdery binder material is applied and the roving is processed to a preform in the layup process. The steps of spreading and binder application have been combined in a binder-roving test rig, Figure 3, to allow efficient investigation. It allows testing and optimising parameters of the single process steps offline for better process understanding. Due to modular design, single parts can easily be exchanged, for example if different spreading methods have to be examined. The used measurement systems allow documentation of input and output width of the roving during spreading and images taken by an industrial camera can be evaluated to measure spreading quality. For example, gaps and inhomogenities are common flaws to be expected. Specific parameter studies help to minimise such flaws.
Figure 3

Schematic depiction of offline binder roving rig for optimisation of spreading and binder application process (© IVW)

Liquid Composite Molding processes offer a huge potential for industrial mass production.

Besides spreading of the roving, binder material can be applied using the test rig. In the developed process, a powdery binder material is used, which can for example be based on thermoset or thermoplastic. For this reason, fibres and binder material can be freely combined during the process without being dependent on the semi-finished products obtainable by a manufacturer. In principle, all endless fibre materials such as carbon, glass, aramid, or steel fibres can be processed as long as they possess sufficient temperature stability during binder activation. In consequence, a large variety of combinations of fibre and binder materials can be achieved, which also enables the production of hybrid-preforms. Application of binder material onto the roving is done by an air stream which is led through a binder-conveying unit, Figure 4. The powder inside the storage container is dosed volumetrically by a rotating ratchet wheel and the passing air flow transports it to the nozzle at the layup head using pipes. The subsequent activation of binder particles is conducted with a hot air fan and thus the particles stick to the roving surface. By changing the rotational speed of the ratchet wheel, the output rate of binder powder and thus the binder content of the produced binder roving can be changed.
Figure 4

Binder supply unit (© IVW)

The influence of the layup parameters consolidation pressure and activation temperature for binder material on the preform stability has been examined by manufacturing sample preforms and measuring their storage modulus via Dynamical Mechanical Analysis (DMA). Based on the results shown in Figure 5 a consolidation pressure of 3 bar and a hot gas flow of 4 nl/min have been chosen for subsequent examinations.
Figure 5

Dependency of storage modulus (DMA measurement) on consolidation pressure and amount of hot gas supplied as indicator for prefom stability (© IVW)

An explanation for the initially increasing preform stability with increasing activation temperature is given by a closer look at the binder. By changing activation temperature, the softening degree of the powder particles can be changed. At higher temperatures, the binder is increasingly softened, which allows migration of binder particles into the single rovings, Figure 6. However, this generally positive effect is reduced again at excessively high temperatures because the binder penetrates further into the rovings, which increases the cohesion of the roving itself, but at the same time reduces the adhesion between the individual rovings.
Figure 6

Increasing softening of binder material at increasing temperature (from left to right), transport of specimen with 15 m/min passing a Leister Triac S hot air fan with temperature settings 130 °C, 190 °C and 240 °C (© IVW)

The bindered roving is subsequently stored on a spool for further processing. Applying the binder material in an earlier process step already bears the potential to flexibly combine fibre and binder material, but a local adaption is not possible using this method. Furthermore, the storability of this semi-finished product is restricted, as the roving will stick together over time. To address these disadvantages, a method for online binder application has been developed at IVW GmbH, the so-called online binder application. With the previously gained understanding of single process steps, the shown components have been further developed to allow application of the binder powder inside the nip point, Figure 7. In sum, the binder content can variably be adjusted and therefore only the amount of binder necessary for cohesion in critical areas has to be added, while the rest of the preform stays unaffected.
Figure 7

Schematical depiction of online binder application process (left), online binder application with hot gas torch during layup (right) (© IVW)

The use of binder material becomes even more important if curved parts or preforms have to be manufactured, since in this case not only the rovings have to be bonded but also the 3D-shape has to be fixed. When using an industrial robot, also concavely and convexly curved preforms can be obtained. As an alternative, a flat preform can be formed into the desired shape after layup. If this can be combined with closing the tool for a later injection step, this method is even more efficient. In Figure 8, local reinforcement of a glass fibre non-crimp fabric with a carbon fibre roving is shown. The whole preform is subsequently formed to an S-wave contour.
Figure 8

Manufacturing and forming of a locally reinforced preform (© IVW)

Permeability

In Liquid Composite Molding processes, a dry fibre structure is impregnated with a thermoset resin system consisting of resin and curing agent. Every fibre has to be completely enclosed by the resin system and no air inclusions shall occur. In LCM, the impelling force is always a pressure difference between dry reinforcement structure and fluid resin system, whereas this difference is applied by an overpressure of the fluid, an applied vacuum onto the cavity or a combination of both.

Permeability describes the ability of the porous media to conduct the fluid and is thus determinative for process speed. For the saturated case, it can be calculated by few variables: Volume flow q, fluid viscosity η, flown-through length Δx, flown-through area A and pressure drop Δp, as shown in Eq 1.

According to this equation, it can also be derived that already for the one-dimensional case, a halved permeability leads to a doubled infiltration time. In mass production, cycle times plays a crucial role, thus a lower permeability does not only bear the risk to decrease part quality, but also to severely decrease economic efficiency of the process.

Further Processing of the Preforms via Liquid Composite Molding

High achievable fibre volume contents in interaction with outstanding fibre orientations of DFP bear the possibility to fully use the advantageous fibre properties efficiently. Still, these advantages also bear difficulties for later impregnation steps. Woven materials possess a certain grade of macro flow channels due to their textile structure and the woven fibre bundles. Thus, the resin flow inside and through the preform is significantly enhanced. Compared to this, DFP preforms are very dense and only possess little porosity between single fibres and not between fibre bundles as can be seen in Figure 9. Due to these very different microstructures, a difference in permeability in the range of several decades occurs between preforms manufactured by textiles and DFP, even if the overall fibre volume content is equal. This causes a severe prolongation of the injection time, as it gets clear when considering the law of Darcy (see Box “Permeability”). In worst case, this can lead to an incompleteness of the injections process due to a premature curing of the resin system or to a deficient micro impregnation.
Figure 9

Comparison of plate microstructure via microsections, manufactured by DFP (left), manufactured with woven fabric textile (right) at comparable fibre volume content (© IVW)

During further research work [2] it has been examined how these drawbacks can be diminished or even compensated by a targeted optimisation of process parameters. Possible measures range from enlarging micro flow channels by increasing binder content or binder particle size-up to insertion of macro flow channels into the preform structure. This can for example be achieved by sewing, tufting, or changing the lay-up pattern. On the one hand, this can possibly reduce mechanical properties of the part due to the inhered undulations, but on the other hand workability in serial processes can be achieved at all. In order to compare the mentioned possibilities for permeability enhancement, preforms with a layup of (0/90°)4 with a total thickness of 2 mm have been produced with a 50K carbon fibre roving. Permeability measurement was done using a saturated measurement principle in thickness-direction at a fibre volume content of about 55 %. By varying binder particle size, a slight enhancement of permeability could be observed. Significant enhancement was given when the macro structure of the preforms was changed by insertion of undulations with a changed layup or by inserting flow channels by application of a tufting process.

The conducted research work concerning DFP with online binder application has already shown promising results and a huge potential for industrial applications. Currently, the researchers aim to expand the technological possibilities, for example by further investigations concerning preform permeability and implementation of an online spreading.

References

  1. [1]
    Lässig, R.; Eisenhut, M.; Mathias, A.; Schulte, R. T.; Peters, F.; Kühmann T.; Waldmann, T.: Serienproduktion von hochfesten Faserverbundbauteilen — Perspektiven für den deutschen Maschinen- und Anlagenbau. Roland Berger Strategy Consultants, 2012Google Scholar
  2. [2]
    Rimmel, O.; Becker, D.; Mitschang, P.: Maximizing the out-of-plane permeability of preforms manufactured by dry fiber placement. In: Advanced Manufacturing: Polymer & Composites Science, Volume 2 (2016), No. 3–4, pp. 93–102Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Oliver Rimmel
    • 1
  • Jens Mack
    • 1
  • David Becker
    • 1
  • Peter Mitschang
    • 1
    • 2
  1. 1.Institut für Verbundwerkstoffe GmbHKaiserslauternGermany
  2. 2.Technical University KaiserslauternKaiserslauternGermany

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