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New progress in thermoplastic prepreg processing has led to the eco-friendly production of complex, safety-relevant and high-performance structural components by laser-assisted tape laying.
1- PROF. DR.-ING. CHRISTIAN BRECHER,
2- DIPL.-ING. ALEXANDER KERMER-MEYER,
3- DIPL.-ING. MARTIN STEYER,
4- DIPL.-ING. MARKUS DUBRATZ,
5- DIPL.-ING. MICHAEL EMONTS, FRAUNHOFER INSTITUTE FOR PRODUCTION TECHNOLOGY IPT
The substitution of thermosetting resins by thermoplastic resins offers the potential to increase impact resistance, to reduce weight and to decrease the cost of composite components (Fig. 1).
Continuous fibre-reinforced thermoplastic components can be produced by laying up prepregs by laser-assisted tape laying. Laser-assisted tape laying is an out-of-autoclave process that allows the automated, eco-friendly production of lightweight, high-performance structural components for a variety of industries such as aerospace and automotive. Modular tape laying and tape winding units developed by Fraunhofer IPT were used to produce high-quality flat, contoured and tubular multilayered structures made of carbon-reinforced PA 12 and PEEK. The Institute has experience in developing automated processes and production machines for thermoplastic fibre-reinforced plastics (FRP). However, its latest breakthroughs resulted from the in-depth investigation of the laser-assisted tape laying process, including statistical sensitivity analysis and process modelling. As a result, the Institute was able to build up a comprehensive process understanding based on process models and empirical data that proved very useful to produce high-quality multilayered structures. These structures exhibited low void content, high interlaminar shear strength, and ensured the shape and dimensional accuracy of the final component.
The superiority of a laser heat source for laying up thermoplastic prepregs in tape laying and tape winding processes – compared with open-flame and IR-assisted processes – was recently evaluated in the Institute’s latest publication by comparing grinding surface patterns and interlaminar shear strength. However, attaching the laser source to the lay-up unit was the least of the difficulties when it came to producing high-quality thermoplastic FRP components. Advanced process control and understanding was the key to success.
A major challenge in tape laying of thermoplastic prepregs is the production of precise parts with a defined geometry. For example, deformation can occur due to shrinking of the thermoplastic matrix materials, especially in unidirectional fibre reinforced parts. This effect may even exist when processing thermoplastic FRP by compression moulding, even though a closed mould and a symmetric temperature profile within the parts are associated with this process. Both these features are lacking in the laser-assisted tape laying process. However, the Institute successfully handled the challenge to produce even, non-deformed plates in multiple ways.
An inadequately adjusted lay-up of thermoplastic prepregs by tape laying results in component deflections (Fig. 2). These deflections can be subdivided into lateral and longitudinal deflection, as well as twisting. Lateral deflection can be easily explained by the deflection model of Stitz [STI73] and has been compensated by accurate temperature control during the process. Among others, the twisting effect is caused by varying degrees of compaction in the laid-up prepregs. Twisting was eliminated by precise force control. Longitudinal deflection is caused by fibre corrugations within the prepreg (Fig. 3). The amount of corrugation is a function of the applied force, force distribution and induced temperature. This effect can be avoided by the correct adjustment of these parameters.
A thorough understanding of the process is important in the laser-assisted tape laying of thermoplastic prepregs. Capitalizing on the gained knowledge, the Institute developed precise temperature and force control systems. Since the use of a stiff roller has certain drawbacks in the production of two- and three-dimensional parts, the Fraunhofer IPT developed novel consolidation systems for gentle material lay-up and stable production of complex parts.
The process investigations were performed on flat and windable parts (Fig. 4).
Based on the acquired process understanding and the ambition to perform the automated production of complex, high-performance composite parts, the Institute developed a modular fibre placement unit with integrated fibre coupling to a diode laser source. The fibre placement system was developed under the 3D-Thermolay research project funded by the German Federal Ministry of Economy and Technology. The 3D Thermolay system is specially designed to lay up complex, three-dimensional structural components (Fig. 5).
The Formula 1 monocoque is a good example for explaining the benefits of the novel system. The good properties of thermoplastic FRPs, such as high impact strength and low density, can be used to produce complex, safety-relevant structural components with reduced weight. Due to the fusibility of thermoplastic FRPs, functional elements can be joined to the main structure without additional adhesive materials. The high energy efficiency of the process allows “green” production due to out-ofautoclave processing, the use of a highly energy-efficient laser system and the good recyclability of thermoplastic FRPs [SCH07].
It is one of the central tasks of the Fraunhofer Institute for Production Technology IPT to transfer the findings of our cutting-edge research projects directly into an industrial environment. This is why the Institute focus the research effort on production technologies with large potentials for industrial applications, and this is why IPT provide a wide range of technological products and services in process technology, production machines, metrology and quality management as well as technology management.
Core activities are bilateral projects with the industry, the additional research projects are funded by the BMBF, the AiF, the Land North Rhine-Westphalia, programmes of the German Research Council (DFG), the publicly funded "Sonderforschungsbereiche" and the European Commission. Their clients and cooperation partners are coming from all parts of the manufacturing industry, particularly from the automotive industry and its suppliers, tool and die making, fine mechanics, the optical industry, aviation and space technology.
Hence the 3D-Thermolay system can be used to produce ecofriendly, highly complex and safety-relevant composite components. The system will start operating in summer 2010 at Fraunhofer IPT.
We would like to thank the German Federal Ministry of Economy and Technology for funding 3D-Thermolay and the participating companies AFPT B.V. & GmbH, CENIT AG Systemhaus, DIAS Infrared GmbH, Eurocopter Deutschland GmbH, INGENERIC GmbH, Robert Timm GmbH and the Institute of Plastics Processing IKV for their cooperation. 3D-Thermolay was funded by the BMWi on behalf of the German Bundestag. This publication is based on a project funded by the German Federal Ministry of Economy and Technology under the project funding reference number IN6518. The authors bear the responsibility for the data content of this publication.
[DOM98] Domininghaus, D.: Die Kunststoffe und ihre Eigenschaften, Springer Verlag, 1998
[SCH07] Schürmann,H.: Konstruieren mit Faser- Kunststoff-Verbunden, Springer Verlag, 2007
[STI73] Stitz, S.: Analyse der Formteilbildung beim Spritzgießen von Plastomeren als Grundlage für die Prozesssteuerung, Diss. RWTH Aachen, 1973