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Lightweight Design worldwide

, Volume 11, Issue 5, pp 20–25 | Cite as

Sandwich Structures Made of Thermoplastics and Recycled Carbon Fibers

  • Jan Luft
  • Juliane Troschitz
  • Michael Krahl
  • Maik Gude
Cover Story
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At TU Dresden's Institute of Lightweight Construction and Polymer Technology (ILK) researchers developed a foam-forming process for the large-scale production of complex shaped thermoplastic sandwich structures, which is particularly suitable for the processing of semi-finished products made of recycled carbon fibers.

All about Recycling

New requirements on mass reduction and cost efficiency in the development of components - for example for mobility applications - have recently led to the success of hybrid lightweight structures. In this context new combination processes have been developed in which flat semi-finished products made of metal or textile-reinforced plastics are formed and functionalized using injection molding technology. The synthesis of different materials with their characteristic property profiles leads to an optimized design for the component under its specific loading conditions. However, recycling of such structures is challenging, since the recycling of the materials generally requires prior separation into homogeneous material fractions. Compared to metal-plastic hybrids, pure plastic- based hybrid structures, consisting for example of organo sheets and injection- molded functional elements, have the advantage of a common matrix system. The individual components differ only in the length of the reinforcing fibers and can therefore be homogenized by mechanical processes like shredding. Especially with Carbon Fiber-Reinforced Plastics (CFRP), however, there is an economic as well as an ecological demand to preserve the value of the cost-intensive reinforcing fibers. Specialized processing methods aiming to return the fibers to the material cycle without reducing the fiber length are addressed in current development projects. Apart from that also the quality of the recycled semi-finished products must be guaranteed and their processability must be proven.

In many cases recycling is still seen as a subsequent processing step after the actual product life cycle, with recycling requirements not being sufficiently taken into account during product development [1, 2]. In contrast, the ReLei research project focuses on an interdisciplinary approach where recycling is regarded as a central point of reference for all development efforts. Therefore the use of recycling materials must be considered in the early design stage of a component development as well as suitable manufacturing processes. Also, aspects of disassembly and material separation at the end of lifecycle must be taken into account from the very beginning. In the project all investigations were carried out based on the material combination of a polyamide (PA) matrix system and reinforcing carbon fibers, as this material system is especially well suited for load-carrying lightweight structures and, in case of recycled carbon fiber reinforcement, offers a considerable cost advantage over virgin material. PA is also established as a typical representative of engineering thermoplastics in a large number of manufacturing technologies in mass production, particularly in the automotive industry. Two different process routes were considered for the provision of semi-finished recycling products. On the one hand recycled Carbon Fibers (rCF) and PA fibers were processed into so-called hybrid nonwovens by Grimm-Schirp Maschinen- und Werkzeugbau as a flat semi-finished product, Figure 1 (left). For this purpose the Airlay process is particularly suitable for fiber lengths between 6 and 40 mm. The fibers can be obtained either from dry cut-offs or composite waste by pyrolysis, solvolysis or physical-mechanical processes (for example electro-hydraulic fragmentation).

Figure 1 Hybrid nonwoven made of rCF and PA fibers (left), recycling granulate made of thermoplastic CFRP waste (right) (© ILK)

On the other hand, at the TU Bergakademie Freiberg PA-based short carbon fiber reinforced recycling granules were produced from thermoplastic composite waste by means of mechanical shredding, Figure 1 (right), which can be injection molded like the virgin material. By combining these two groups of semi-finished products into a sandwich structure, the specific properties of the materials can be used effectively. The hybrid nonwovens are applied as top layers and contribute to a high structural stiffness and strength, especially under bending load, whereas the sandwich core consists of short fiber-reinforced foamed injection molding material.

Sandwich Structures from Recycling Material

For the integral production of complex shaped sandwich structures a new process was developed, the so-called foam-forming process. Thermoplastic Foam Injection Molding (FIM), in this case with the MuCell technology, is combined with the thermoforming of fiber-reinforced top layers in a single process. The thermoplastic CFRP top layers made of hybrid nonwovens are heated by an external infrared heating device, Figure 2 (a) and are subsequently automatically transferred into the open injection mold by a robot-guided handling system, Figure 2 (b). At the same time, the carbon fiber-reinforced PA recycling granules are melted in the plasticizing unit and a blowing agent is introduced into the melt. After closing the mold, it is injected between the hybrid nonwovens, Figure 2 (c), filling the mold cavity completely. The sandwich top layers are pressed against the respective wall on either side of the mold and obtain their final shape. Afterwards the mold cavity is expanded via precision opening, which initiates the foaming of the sandwich core, Figure 2 (d). After cooling, the finished sandwich component can finally be demolded, Figure 2 (e).

Figure 2 Schematic representation of the foam-forming process for the production of complex shaped sandwich structures (© ILK)

For physical foaming with the MuCell technology, PA-based injection molding material with a low content of carbon fibers is particularly suitable, as the fibers support cell nucleation and promote foam formation. A homogeneous and fine foam structure can be maintained up to a cell content of 50 % [3]. The good drapability of rCF-PA hybrid nonwovens in the molten state together with the generally high design flexibility of the injection molding process facilitates the production of geometrically complex sandwich structures. This involves curved surfaces as well as local changes in wall thickness. The process can also be designed in such a way that not the entire component is foamed. In this context a special tool technology was developed by Elring Klinger, where only a section of the mold cavity is expanded and foamed by means of precision opening. In the remaining part of the structure the thermoplastic melt remains almost solid, which results in strength and stiffness properties comparable to conventional injection molding materials. Hence, the foam-forming process leads to a great freedom of design and a high degree of functional integration.

In order to develop such complex structures efficiently and with a short lead time, virtual process design is indispensable. A great challenge in this context was the integral manufacturing process with different sub-process steps interacting with each other in various ways. For this purpose, the project partner Inpro developed finite element modeling methods for the entire process in a continuous simulation chain. The resulting production-related properties can be taken into account in the structural simulation [4].

Resulting Properties of Sandwich Structures

For the component design it is important to have reliable characteristics of the novel recycling material. So the influence of the recycling process had to be analyzed with respect to the mechanical properties of virgin material. For this purpose comprehensive investigations were carried out on foamed panel structures, which were mechanically reprocessed to recycling granules and once again injection-molded into panels. A reduction of the mechanical properties could only be determined in the direction of flow, since the reinforcing fibers are predominantly oriented in this direction during mold filling, Figure 3. The reason for the property reduction lies in the shortening of the fiber length both during mechanical reprocessing and secondary injection molding. However, in the transverse direction the fiber length has only minor influence and the mechanical properties remain at a similar level. Whether virgin or recycled material, the properties of FIM components differ significantly in the two main directions which must be taken into account in the part design. This not only affects the mechanical characteristics but also the shrinkage of the part.

Figure 3 Comparison of the stiffness characteristics of foamed panels made of virgin and recycled material (© ILK)

The integration of almost isotropic hybrid nonwoven top layers in the foam- forming process results in a considerable increase of the material properties on the one hand and significantly smaller differences between the flow and transverse direction on the other, Figure 4. The nonwovens have a higher Young's modulus and higher strength than the core material and are therefore defining the characteristics of the sandwich compound, especially under flexural load. However, the anisotropy resulting from the core material cannot be completely avoided in this case either.

Figure 4 Comparison of the stiffness characteristics of foamed panels with and without non-woven top layers (© ILK)

Component Design Suitable for Recycling

Demonstrating the great potential of these semi-finished recycling products and the foam-forming process for highly loaded structural components, a rear shelf of a car body was selected for a case study, Figure 5. This part is required to meet high demands on overall stiffness and is also subjected to high local mechanical loads. This concerns the fastening of a seat belt retractor and several top tether attachment points, i.e. child safety seat fastenings, in the case of a crash. In addition, acoustic requirements with regard to NVH (Noise, Vibration, Harshness) behavior must be met due to the integration of a subwoofer.

Figure 5 ReLei demonstrator made of semi-finished products based on recycled carbon fibers (© ILK)

With regards to the newly developed manufacturing process, the component can be divided into different functional areas, each designed according to the individual loads applied, Figure 6. In the central part section, the rear shelf comprises a sandwich structure over a large area. The top and bottom layers are rCF-PA hybrid nonwovens with a fiber length of 21 mm, which ensure high bending stiffness. In combination with the foamed core good damping properties can be achieved. The component is attached to the peripheral body structure via the surrounding flanges by means of blind riveting and adhesive bonding. In the flange area, the mold is not opened during the foaming phase. As a result, the injection molding material remains almost solid and provides the necessary strength in the joining zone. Component sections with high local loads are supported with additional layers of carbon fiber-reinforced organo sheets, which have significantly higher strengths than the hybrid nonwovens. However, since these organo sheets are currently only available as virgin material, they are exclusively used in such areas where they are absolutely indispensable. The large segment of organo sheet on the front side is additionally reinforced by solid ribs on the underside, enhancing torsional stiffness. Even though only one matrix system (polyamide) and only one type of fiber (carbon fiber) are used in the demonstrator, each functional area has different properties. This is achieved by combining foamed and non-foamed plastics in one component as well as by using semi-finished products with different fiber lengths (short fiber-reinforced injection molding compound, long fiber- reinforced nonwovens and continuous fiber-reinforced organo sheets). As a result, the use of expensive virgin material is limited to local reinforcements, whereas the low-cost recycled injection molding material or nonwovens, depending on the requirements, can be used in the rest of the component. In total the proportion of recycled material could be extended to around 80 %.

Figure 6 Exploded view of the ReLei demonstrator (© ILK)

In a final component test all relevant load cases were successfully tested. In a direct comparison of components made of virgin and recycled material, the failure strength of the top tether attachment is approximately 10 % lower for the recycled version. This corresponds perfectly well to the tests carried out on plate structures. Nevertheless, the experimentally achieved failure force still far exceeds the required test load.

Summary

Up to now, recycling has mostly only been addressed at the end of the product development process. Consequently, aspects of recycling-friendly design or the use of secondary materials can only be taken into consideration to a very limited extent. For this reason, the joint research project ReLei followed an approach which involves the specific property profiles of various recycled materials, such as recycled carbon fibers and recycled injection molding material, during the early design and development phase. This led to the development of the foam-forming process, which is perfectly suited for the high-volume production of complex sandwich components with both virgin and recycled materials. The successful development, production and testing of the ReLei technology demonstrator highlighted the potential of recycled CFRP for structural parts. |

References

  1. [1]

    Gude, M.; Lieberwirth, H.; Meschut, G.; Zäh, M. F.; et al.: FOREL-Studie 2018 - Ressourceneffizienter Leichtbau für die Mobilität: Wandel - Prognose - Transfer. Plattform FOREL, 2018

     
  2. [2]

    Krampitz, T.; Lieberwirth, H.; Stegelmann, M.: Werkstoffvielfalt und hohe Komplexität. In: ReSource 04 (2016), pp. 18-24

     
  3. [3]

    Gude, M.; Luft, J.; Troschitz, J.; Kupfer, R.; Krahl; M.: One-shot physically foamed sandwich structures with carbon-fibre-reinforced top layers. Proceedings of the 18th SAMPE Europe Conference, 2017

     
  4. [4]

    Luft, J.; Troschitz, J.; Gude, M.; Günzel, S.; Höhne, S.; Gleich, H.: Integrale Fertigung von Leichtbau-Sandwichstrukturen mit kohlenstofffaser-verstärkten Decklagen. In: Konstruktion 01/02 (2018)

     

MuCell process

The MuCell process is a thermoplastic Foam Injection Molding (FIM) process developed by Trexel that enables the formation of a so-called integral foam with microcellular pores. A physical blowing agent (usually N2 or CO2) is injected into the melt as a Supercritical Fluid (SCF) in the plasticizing unit.

Thanks

The research project ReLei was funded by the Federal Ministry of Education and Research (BMBF) (funding ref.: 02PJ2800 - 02PJ2808) and supervised by the Project Management Agency Karlsruhe (PTKA) of the Karlsruhe Institute of Technology (KIT).

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Jan Luft
    • 1
  • Juliane Troschitz
  • Michael Krahl
  • Maik Gude
  1. 1.Institut für Leichtbau und KunststofftechnikDresdenGermany

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