Sandwich Structures Made of Thermoplastics and Recycled Carbon Fibers
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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.
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 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 % . 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 .
Resulting Properties of Sandwich Structures
Component Design Suitable for Recycling
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.
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. |
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
Krampitz, T.; Lieberwirth, H.; Stegelmann, M.: Werkstoffvielfalt und hohe Komplexität. In: ReSource 04 (2016), pp. 18-24
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
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)
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.
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).