Authors: Ch. Hopmann, R. Riedel, Ch. Karatzias
Continuous fibre reinforced plastics with high fibre volume content (FVC) are materials with high lightweight potential. For their use in extensive applications, such as boot floors or panelling of railway vehicles , a high stiffness, excellent bending and flexural properties as well as high kink resistance are required in addition to high strength . Regarding these properties monolithic components encounter their limits. Sandwich components however show particular advantages for such applications and are therefore well suited for lightweight components of that kind. Because the distance of the two surface layers is included in the bending stiffness to the second power sandwich parts have high lightweight potential. The impact is shown in figure 1 by the component weight and the component thickness as well as the stiffness of the component. The depicted sandwich variant has a reduced component weight by 82 % compared to the monolithic component with an equal level of bending stiffness . Therefore the use of sandwich panels ensures a significant reduction in weight while providing the same stiffness. However, the manufacturing procedures of sandwich panels are characterised by many process steps and are often considered to be not economically viable, particularly for fibre reinforced plastics (FRP) with continuous fibre reinforcement.
Bild 1: Leichtbaupotenzial von Sandwichbauteilen
Bild 2: Möglichkeiten und Herausforderungen von Sandwichbauteilen mit Wabenkern
Potential and limits of existing sandwich applications
In the automotive industry sandwich components are often manufactured based on long glass fibre reinforced polyurethane (PU). Conventional applications are extensive components, which are not strained with large loads, such as boot covers and parcel shelves . As core materials, paper honeycombs with very low densities are most commonly used in combination with a PU spraying process. The PU is sprayed either as a pure matrix system on a fibre mat (Baypreg process ) or as a long glass fibre and matrix mixture directly onto a sandwich core (Long Fibre Injection, Composite-Spray-Moulding, Fibre Composite Spraying ). The open honeycombs are closed by the PU foam which fills them up to a certain penetration depth. Paper honeycomb cores have good formability and can map variations in thickness to a certain level in a pressing process (figure 2, left). However, negative effects are caused by the honeycomb structure as well. At high degrees of deformation, the so-called “saddle-effect” occurs (figure 2, right), meaning in the direction perpendicular to an applied bending an additional bending is induced, which has to be taken into account throughout the design of the component . Another effect is the “telegraphing effect”, describing the sign-off of the honeycomb structure on the surface of the sandwich component, which leads to a challenge for the use of honeycomb cores in components with a visible surface .
Bild 3: Weiterentwicklung des PUR-Sprüh-Nasspress-Prozesses für die Herstellung von Sandwichplatten
As an alternative material for paper honeycomb cores closed-cell rigid foams with excellent insulation properties and high compressive strengths can be used . The use of closed-cell foams is an advantage because moisture cannot permeate into the core. This benefits the functionalisation with inserts or the subsequent placement of screw connections. Furthermore good component surface qualities can be achieved without sign-offs, due to a smaller pore size compared to honeycomb cores . Closed-cell foams can either be processed as pre-cut, 3-d milled cores (for example AIREX or ROHACELL)  or get formed (PU) between two textile surface layers in the mould during the manufacturing process . When using pre-cut foam cores the material of the surface layer only minimally permeates into the foam, due to the closed-cell structure of rigid foams. Thus they are, in contrast to honeycomb cores, suitable for processing in wet impregnation processes with non-foaming, low-viscosity matrix systems, which achieve significantly higher fibre volume contents (FVC), compared to the conventional PU spraying processes with long-fibre reinforcement. Therefore, sandwich components with compact continuous fibre-reinforced surface layers and a high FVC are also suitable for highly stressed structural applications. But using pre-cut foams for complex geometries has a negative impact on the efficiency of the process. The manufacturing of complex, extensive foam cores includes a preceding time consuming milling process, which in some cases result in a high amount of scrap (up to 95 % for extensive, thin-walled components). For the foam core, this leads to uneconomically high costs for materials. Regarding the direct foaming of the core in the manufacturing process, so far only applications do exist, in which the foaming system also forms the matrix for the surface layers. Despite continuous fibre reinforcement, it is not possible to reach the mechanical properties of sandwich structures with compact matrix systems for the surface layers in such processes.
Structural sandwich components need to withstand high loads. The combination of continuous fibre-reinforced compact surface layers with high FVC and a closed-cell rigid foam core, which is foamed directly during the manufacturing process, provides the best results in terms of mechanical and optical properties as well as cost-effectiveness. A manufacturing technology for the production of such components is currently developed at the Institute of Plastics Processing (IKV) in Industry and the Skilled Crafts at RWTH Aachen University. An efficient and economic production of these components, which can also be applied to complex geometries, is realized by parallelizing the foaming of the core with the forming and curing of the surface layers in one process step. Process steps such as the pre-cut of the foam cores or the gluing of the layers to the foam are thus eliminated.
Bild 4: Querschnitt von Sandwichproben mit geringer und hoher Schaummenge
The process technologyTo realize the foaming of the core parallel to the forming and curing of high reinforced surface layers (FVC of up to 50 Vol.-%) in a single process step, a combined PU spraying and wet-pressing process  for the manufacture of sandwich components with continuous fibre-reinforced surface layers is currently developed at the IKV. First, a dry preform layup is impregnated with compact matrix material in a vacuum-assisted PU spraying process. The resulting “prepreg” is subsequently compacted and cured in a wet-pressing process. Due to the separation of the process steps “impregnation” and “curing” these steps can be parallelized and thus the overall cycle time of the wet-pressing process is significantly reduced. In this process chain the impregnation of the fibres takes place before the actual wet-pressing process. This fact entails the possibility to achieve high impregnation quality and low porosity in the surface layers at relatively low pressures (10–25 bars) in the pressing process. These process characteristics predestine the combined PU spraying and wet-pressing process for manufacturing structural sandwich components with process integrated foamed PU cores and continuous fibre-reinforced compact PU surface layers. The process sequence of the combined PU spraying and wet-pressing process for the production of sandwich components is shown in figure 3. During the spraying process the two preforms for the surface layers are impregnated. For this purpose matrix material is applied to the preforms by PU spraying. The preforms are positioned on top of each other on a perforated vacuum tool. The vacuum provides the permeation of the matrix system in the preform layup. To achieve a homogenous FVC in the component, the vacuum has to be adjusted, so that equal amounts of PU penetrate into both preforms. Afterwards the impregnation process, the upper preform is first placed in the wet-pressing tool. After that the PU foam system is dispensed. Thereafter, the second preform is placed on top of the foam system and the press is closed. During the closing, the cavity is evacuated to reduce the air content inside the cavity in the course of the pressing process. The press then closes onto spacers, which adjust the corresponding target thickness of the component. The foaming reaction starts thus with atmospheric pressure in the mould until the cavity is filled. Only then a homogenisation of the surface layers and the foam core takes place due to the internal cavity pressure generated by the foaming reaction.
Tabelle 1: Verwendete Materialien und Lagenaufbau
Analysis of the manufacturing process
In the process the internal cavity pressure used for homogenisation of the surface layers and the core directly results from the foaming pressure. At constant component thickness the component quality thus directly depends on the dispensed amount of foam system. Therefore, the influence of the amount of foam material on the quality of the core and the surface layers was investigated. For the experimental results presented below, the used materials and the surface layup are shown in table 1.
The experiments were carried out on a high pressure mixing and metering machine, type RIM-Star MiniDos 8/8, KraussMaffei Technologies GmbH, Munich, as well as a pressing machine, Müller Weingarten AG, Weingarten, with a maximum pressing force of 2000 kN. Based on previous studies with monolithic components , the FVC of the surface layers was adjusted to 38 Vol.-% in the impregnation process. With this FVC a homogenous impregnation over the complete “prepreg” is achieved in the spraying process, hence a good quality of the surface layers (high impregnation of the glass fibres and low porosity) is achieved even at relatively low pressures (10–25 bars). All experiments were carried out at a pressing tool temperature of 70 °C. During the experiments, the amount of foam system for the core was varied. Iteratively 452 g of foam was defined as the minimum amount of foam system that permits a homogenous manual distribution of the foam system onto the “prepreg”. In order to identify the precise influence of the dispensed amount of foam system on the internal cavity pressure or the resulting core and surface layer quality, twice the amount of foam system, 904 g, was also examined. In figure 4, the cross sections of two sandwich components are shown. The figure shows the variation with a low amount of foam system on the left and the variation with a large quantity of foam system on the right. With regard to the quality of the surface layers, significant differences between the two variations can be detected. In the component that was manufactured with a high amount of foam system, a good impregnation quality is obtained in the surface layer. On the other hand the component that was manufactured with a low amount of foam system shows distinct impregnation defects within the fibre bundles. Moreover, in the component with a low amount of foam system a large number of foam pores from the core permeated into the surface layer and partially advanced to the surface. Thus, for the low amount of foam system, the internal cavity pressure generated by the foam reaction is not sufficient in order to achieve adequate compacting and homogenisation of the surface layers. With regard to the foam morphology, the component with a low amount of foam system shows a more homogenous structure in terms of pore size as well as pore dispersion. In the core of the component with a large quantity of foam system, significantly larger foam pores can be detected towards the surface layers than in the centre of the core. In addition, between the pores there are some areas where no pores can be detected. These areas of pure resin influence the foam density, which is 645 kg/m³. This is more than twice as high as for the component with a low amount of foam system, which is 295 kg/m³. In conclusion it can be determined that with the low amount of foam system, a good foam morphology can be produced. Nevertheless the impregnation of the surface layers has various defects and thus the mechanical properties of the component are expected to be lower than the common properties for this high FVC. The component with the higher amount of foam system shows a very good impregnation quality in the surface layers. However, the component has a significantly higher density, which is in conflict with the lightweight aspect of sandwich panels.
Conclusion and outlook
The development of the combined PU spraying and wet-pressing process for the production of sandwich components with a process integrated foamed PU core and compact continuous fibre reinforced PU surface layers initially enables the formation of the foam core in the forming and curing step. This eliminates the need for expensive pre-cut foam cores. To further improve the lightweight potential both the manufacture of lower densities in the foam core, as they are common in pre-cut foams (about 100 kg/m³), as well as a higher FVC in the surface layers are targeted. Monolithic plates manufactured in the combined PU spraying and wet-pressing process with a FVC of 50 Vol.-% achieve good component qualities at relatively low pressures (10 - 25 bars). This is why a FVC of 50 Vol.-% for the sandwich components should be reached in further investigations. This could be achieved by providing a process-specific adaptation of the PU material systems for the foam core and the surface layers as well as by the further development of comprehensive process knowledge.
The depicted research is funded by the Deutsche Forschungsgemeinschaft (DFG). We would like to extend our thanks to the DFG.
Above we thank Rühl Puromer GmbH for providing PU systems and KraussMaffei Technologies GmbH for providing a mixing and metering machine.
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Regina Riedel, M.Sc., studied mechanical engineering at the Technical University RWTH Aachen, specialization in plastics processing. Since September 2013 she is working as a scientific assistant in the field of “Preforming and PU spraying processes” in the composite department of the Institute of Plastics Processing (IKV) at RWTH Aachen University.