Automated Direct Fibre Placement with Online Binder Application
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KeywordsFibre Reinforce Polymer Binder Content Binder Material Single Process Step Consolidation Pressure
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 .
Process Chain to Achieve Online Binder Application
Liquid Composite Molding processes offer a huge potential for industrial mass production.
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.
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
During further research work  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.
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