Innovative manufacturing of 3D-lightweight components New insights in the processing of continuous fibrereinforced thermoplastic prepregs Fibre-reinforced plastics (or: fibrereinforced polymers, FRPs) a group of materials with excellent and largely adjustable properties have found their way into many industries over the past years and have established themselves in various sectors. Continuous FRPs have a wide potential for lightweight designs due to their ability to withstand high mechanical loads despite their tare weight. Today s air and space technologies or the wind power industry are unimaginable without this material. Fibre-reinforced components will form one of the most important key factors for the European industry with its energy-efficient production to fulfil the increasing global demands e.g. energy efficiency, waste avoidance and reduction of CO 2 -emissions. Nowadays, manufacturing equipment for thermosetting composite materials is present, but a competitive system for the manufacturing of continuous carbon fibrereinforced thermoplastic components was not yet available. Within the research project»3d-thermolay«, the Fraunhofer Institute for Production Technology (IPT) has developed a production system for the automated laser-assisted tape placement of carbon fibre-reinforced tapes addressing intersectoral branches, e.g. public and private transportation, mechanical, chemical and civil engineering as well as consumer goods. State of the art The substitution of thermosetting resins by thermoplastic ones offers the potential to increase impact resistance, to downsize production time and costs and to reduce weight and to decrease the costs of composite components. Continuous fibre-reinforced thermoplastic components can be produced by laying up tapes (pre-impregnated ther- Christian Brecher From 1995 to 2001 Prof. Dr.-Ing. Christian Brecher was member of the research staff and chief engineer of the department of machine technology at the Laboratory for Machine Tools and Production Engineering (WZL). After a 3 year employment as head of machine development and head of machine design at DS Technologie Werkzeugmaschinenbau GmbH in 2004 he received a professorship at the Chair of Machine Tools of RWTH Aachen and got a member of the board of directors of the Fraunhofer Institute for Production Technology IPT. the authors Prof. Dr.-Ing. Christian Brecher E-mail: c.brecher@wzl.rwth-aachen.de Phone: +49 (0)241 / 80-27407 Fax: +49 (0)241 / 80-22293 Joffrey Stimpfl Dipl.-Ing. Joffrey Stimpfl studied mechanical engineering focussing on production technology at the RWTH Aachen and completed his diploma thesis in the area of fiberreinforced plastics at the Fraunhofer IPT. From 2008 he worked as research assistant in the department of Fiber-reinforced Plastics and Laser System Technology at the Fraunhofer IPT. Since the beginning of the year 2011 Mr. Stimpfl has also been group leader of the department of Fiber-reinforced Plastics and Laser System Technology. Dipl.-Ing. Joffrey Stimpfl E-mail: joffrey.stimpfl@ipt.fraunhofer.de Phone: +49 (0) 241 / 8904-470 Fax: +49 (0)241 / 8904-6470 Markus Dubratz Dipl.-Ing. Markus Dubratz studied mechanical engineering with the focus on production technology and materials science at the University Siegen and finished his diploma thesis at the Carl Zeiss SMT AG. Since 2007 he is a member of the research staff of the department Fiber-reinforced Plastics and Laser System Technology. As project leader he is especially concerned with the development of laser-assisted fibre-placement systems as well as production processes for the manufacturing of lightweight components. Dipl.-Ing. Markus Dubratz E-mail: markus.dubratz@ipt.fraunhofer.de Phone: +49 (0)241 / 8904-269 Fax: +49 (0)241 / 8904-6269 Michael Emonts Dr.-Ing. Michael Emonts studied mechanical engineering with the emphasis on production technology at the RWTH Aachen. From 2005 to 2009 he was a member of the research staff of the Fraunhofer IPT concerned with the integration of laser technology into production systems. From 2007 to 2009 he lead the group Fiber-reinforced Plastics and Laser System Technology. Since January 2010 Dr. Emonts is chief engineer of the department Fiber-reinforced Plastics and Laser System Technology at the Fraunhofer IPT. Dr.-Ing. Michael Emonts E-mail: michael.emonts@ipt.fraunhofer.de Phone: +49 (0)241 / 8904-150 Fax: +49 (0)241 / 8904-6150 Fraunhofer Institute for Production Technology IPT Steinbachstraße 17 52074 Aachen, Germany 36 LTJ September 2011 No. 5 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1: Laser-assisted tape placement system. moplastic composites with unidirectional fibres) by laser-assisted tape placement. Laser-assisted tape placement 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 placement and tape winding units developed by Fraunhofer IPT were used to produce high-quality flat, contoured and tubular multilayered structures made of carbon-reinforced e.g. PA 12 or PEEK-tapes. This specific thermoplastics such as PEEK and PEKK can provide comparable or improved performance to epoxies [1,2]. Compared with thermoset resins, thermoplastic materials can be easily recycled and have superior fatigue and impact resistance, as well as better damage tolerance and vibration damping behaviour. These features are useful in the production of automotive underbody components and in offshore applications [3, 4]. With the laser-assisted tape placement process pre-impregnated, dimensionally stable and continuously fibre-reinforced tapes with a thermoplastic matrix can be processed. These tapes can have a fibre volume content of more than 60%. During tape placement the laminate stack-up is build up by placement down several tape layers in any desired fibre orientation (Fig. 1). Load optimized 3D-shaped or two-dimensional parts are built up layer by layer automatically by the use of modular applicable tape placement units routed by robotics. The process principle of the laser-assisted process is based on the placement or winding of fibre-reinforced tapes which consist of continuous UD-fibres surrounded by a solidified thermoplastic matrix. By absorption of laser radiation between the tape layers during the placement process the tape material is locally molten a short time before the contact between the individual layers. The in situ consolidation of the molten tapes leads to conjoined layers. In order to produce fibre-reinforced components with a certain wall strength, a defined number of tape layers must be applied. After the tape placement process, the structure can be removed directly from the form and can be processed further. Since the fibre angles of the individual tape layers can be individually set, the component created can be tailored and optimised to the specific application and load. The requested tape material is stored on spools and led by a tape guide system into the process zone. Furthermore the laser-assisted placement process enables tape lay-up rates up to 160 m/min, whereby mechanical properties of pressed and autoclave cured fibre-reinforce plastics (FRP) can be reached [5]. Using laser-assisted thermoplastic tape placement thus allows the cost-efficient production of fully consolidated load-optimised tailored FRP blanks as well as high-complex double curved structure components. Because of possible collisions with the tool the required construction space of the tape placement unit leads to a limitation of the producible part complexity. In case of the fibre-placement head of the Fraunhofer IPT e.g., the limit of concave radii in laying direction is 600 mm. Furthermore, the tape placement process is even more efficient the more extensive the part surface is, because the non-productive time, e.g. for the fixation of the tapes when starting a new path, can be reduced. Moreover, product design can be based on load specifications and is less dependent on manufacturing requirements. This solution reduces material consumption while better utilizing the weight-saving potential of continuous fibre-reinforced thermoplastics. After improving the thermoplastic winding process, we successfully produced Laser-assisted fibre-placement process Figure 2: New Fraunhofer IPT tape placement system. 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.laser-journal.de LTJ 37
Figure 3: Project consortium of the research project 3D-ThermoLay. flat and contoured laminate structures using the automated laser assisted thermoplastic tape placement method. Tape placement system Within the research project 3D-Thermo- Lay, the Fraunhofer IPT has developed a completely new tape placement system especially for the manufacturing of three-dimensional fibre-reinforced components. The tape placement unit is a modular end-effector for robot systems to produce multi-layered parts based on flexible fibre-reinforced thermoplastic tapes. The required main components of a laser-assisted tape placement unit are a laser guidance system with laser source system, fibre optic cable with usually circular cores, laser zoom-optics, thermal camera system for temperature-based control of the laser power and laser power control unit to heat up the thermoplastic tapes to their fusion temperature within their contact zone. During the manufacturing process the tape the company The Fibre-Reinforced Technology and Laser Systems Technology department of the Fraunhofer IPT aims to meet the increasing industrial demand for fibrereinforced lightweight components by developing production processes and systems for the automated manufacture of lightweight fibre-reinforced thermoset and thermoplastic components. The research, development and service activities include the design of fibre-reinforced plastic (FRP) components and the manufacture of prototypes. placement unit is moved above a form to place the tapes and to consolidate the layers in situ to a rigid lightweight component by a consolidation roller (Fig. 2). Because of the high stiffness regarding the tape width of the thermoplastic tape material within the tape placement system six smaller tapes (tape width 6 mm, thickness 0.15 mm) are used for draping on double curved forms which opens a wider range of application. By the application of smaller tapes instead of wider tapes the load of each tape will be reduced during the placement process significantly. Especially the placement of curves across the laying direction allows the decrease of radii laid by the tape placement system. For the first time this effect was implemented in the designed placement head by the utilisation of six tapes with a width of 6mm which can manufactured simultaneously by the laser-assisted tape placement process. For a successful manufacturing, each tape has to be guided and controlled individually by the tape tension control system. By this, a homogeneous and uniform tape tension during the placement process is realized. With regard to the design of different placement strategies high complex fibre-reinforced components can be produced. The tape placement system was developed within the research project 3D-ThermoLay by an interdisciplinary consortium (Fig. 3) with 7 industrial partners (AFPT B.V., Cenit AG Systemhaus, DIAS Infrared GmbH, Eurocopter Deutschland GmbH, Ingeneric GmbH, KOELRIT GmbH and Robert Timm GmbH) and 2 research institutes (Institute of Plastics Processing IKV and the Fraunhofer Institute for Production Technology IPT). The research project was founded by the German Federal Ministry of Economy and Technology under the project funding reference number IN6518. Results of the investigations Using the tape placement process especially large sized structure parts with radii exceeding 600 mm can be produced economically for example aerospace, transportation or sports applications (e.g. aircraft or railway construction). Especially double curved structure components can be produced by the use of optimized system modules of the tape placement system which open the possibility for a flexible processing of the tapes on complex surfaces. Furthermore a major challenge in tape placement of thermoplastic tapes is the production of precise parts Figure 4: Deflection effects on thermoplastic FRPs. 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 placement process. By utilisation of adjusted process equipment and heated tool surfaces, we successfully handled the challenge to produce non-deformed plates. Hence the process system can be used to produce highly complex and safety-relevant composite components. An inadequate adjusted lay-up of thermoplastic tapes during tape placement process causes component deflections. 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 [6] 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 tapes. Twisting was eliminated by implementing precise force control. Longitudinal deflection is caused by fibre corrugations within the tape. 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 (Fig. 4). A thorough understanding of the process is important in the laser-assisted tape placement of thermoplastic tapes. 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, our institute developed novel consolidation systems for gentle material lay-up and stable 38 LTJ September 2011 No. 5 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 5: Comparison of the different placement strategies. Figure 6: Manufactured 3D-demonstrator geometry manufactured by geodetic (left) and curved placement paths (right). production of complex parts. The process investigations were performed on flat and windable parts. For the demonstration and verification of the fibre-placement system the Fraunhofer IPT has derived a complex non-developable tool surface. In cooperation with Cenit AG Systemhaus two tape-laying strategies for the placement on the three-dimensional surface were established and tested. Most of the state-of-the-art algorithms for the tape placement process focus the detecting of ideal placement paths [7,8]. So in many applications local effects like the study of gap width or tape overlap are not in the center of the investigations. The double-curved demonstrator geometry is built up by two Gaussian -functions in x- and y-direction. The Gaussian-peak is centered in the middle of the tool and has a thickness of 50 mm. With different numerical approaches, we have derived two fibreplacement strategies for the application on the Gaussian-tool. The challenge regarding this strategies is the determination of the optimal tape distribution for a largely coverage of the tapes on the adjacent surface areas. The first strategy is based on equal distances of the placement paths regarding to the base of the tool. This strategy accepts gaps between the tapes especially in areas with a high gradient of the form. As already published in [9] by adjusting the process parameters the width of the tapes can be modified. With this effect a closing of the gaps can be achieved by adjusting the process parameters in interaction with the tool geometry [Fig. 5, left]. This key-effect opens up the opportunity for manufacturing double curved structural components because the tape width will be online adjusted along the whole fibre-placement path so that a gap- and overlap free 3D-processing of the tapes can be established. To address a new dimension of structural components, we e.g. will investigate the detected key effect of the tape width variation for establishing new process algorithms, which describe the process behaviour of the tapes during the deposition onto different 3D-surface geom- etries and which can be integrated in existing conventional CAD/CAM-Systems. A second placement strategy is based on a parallel positioning of the tape paths on the tool. For this tool geometry the developed algorithm generates curves which introduces a potential profile into the tapes during the placement of the tape material. This means that the stress within the tape increases by the decrease of the associated placement radius. With this strategy the expected gaps are shifted to the border of the form and not distributed on the form itself [Fig. 5, right]. By a first approach it was shown that both strategies are well suited for the manufacturing of three-dimensional fibre-reinforced components (Fig. 6). During the lay up process both strategies have been carried out in different laying directions (0, 90, 45, -45 ). This had enabled the laser-assisted manufacturing of multidirectional three-dimensional structures by using thermoplastic tapes for the first time. In further process development steps, the process limits have to be evaluated. Compared to metallic parts the FRP components have a reduced weight, higher stiffness, significantly higher strength and threefold higher energy absorption. Due to this characteristics FRP components are suitable for applications in transportation industry (e.g. coverings, roof frames), in case of security relevant parts (e.g. protection systems, coverings), in the sport sector (e.g. sports vehicles) and especially in aerospace applications (e.g. window frames, front wings). Tailored fibre-reinforced blanks The needed base for a successful and reproducible production of high quality fibrereinforced thermoplastic composite (FRTC) parts with UD-fibres is the production of high quality FRTC tapes (pre-impregnated) that can be further processed by tape placement (tapes) or by thermoforming (plates) and by combining both tape placement and thermoforming. Until now, the available production systems and processes for the processing of Figure 7: Tailored fibre-reinforced thermoplastic semifinished sheets, so called tailored FRP blanks, can be processed. 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.laser-journal.de LTJ 39
tapes had to be individually optimized depending on the delivered product qualities of FRTC tapes. The automated processing of thermoplastic FRTC tapes and plates (in order to produce multilayered FRTC parts) is accomplished by laser-assisted tape placement of FRTC tapes and by thermoforming of FRTC plates. Furthermore a new process approach is investigated: The combination of thermoforming and tape placement. To apply the potential of continuous thermoplastic FRPs and the advantages of sheet metal forming in state-of-the-art large-scale production processes, fibre-reinforced thermoplastic semi-finished sheets were developed (so-called tailored FRP blanks, Fig. 7). These tailored FRP blanks can be processed in one or more subsequent process steps. Summary and outlook The utilization of thermoplastic FRP provides high potential to cope with the increasing demands in modern automotive production. Due to the very high specific strength of continuous fibre-reinforced thermoplastic composites significant weight advantages can be achieved which offers huge opportunities to reduce the CO 2 emissions and energy demand (electric mobility). Together with the good recyclability of thermoplastic FRP advantageous life cycle assessment can be achieved. Due to the superior damping as well as force absorption behaviour of thermoplastic matrices, both the driving comfort and the security of modern cars can be increased. Such high-performance lightweight structures can be produced with short cycle times using the possibility of in situ consolidation of thermoplastic FRPs as well as using the synergies due to the combination of laser-assisted tape placement with the thermoforming process. In different research projects (e.g.»fibrechain«) and projects founded by industry the described new technologies of fibre placement and thermoforming will be developed further and transferred into industrial approach by the Fraunhofer IPT. References [1] J. D. Mozzy & A. O. Keys: Thermoplastic vs. Thermosetting structural composites, Polymer Composites 5 (1984) July [2] J.-M. Bai et al.: High Performance Thermoplastic Polymers and Composites, SAMPE 2005, Corina, CA, USA, S. 1391-1405 [3] http://www.autosieger.de/article7985. html, [20.07.2009] [4] A. Mondo et al.: High Speed Processing of Thermoplastic Composites for Oilfield Pipe and Tubular Applications, In: Composite Materials for Offshore Operations, http:// www.automateddynamics.com/tech_papers_final.php, PR 15, 2000 [5] A. Burkart: Feasibility of continuous-fibre reinforced thermoplastic tailored blanks for automotive applications, SPE Automotive Composites Conference & Exposition, 2005 [6] S. Stitz: Analyse der Formteilbildung beim Spritzgießen von Plastomeren als Grundlage für die Prozesssteuerung, Diss. RWTH Aachen, 1973 [7] B. Shirinzadeh et al.: Fabrication process of open surfaces by robotic fibre placement. In: Robotics and Computer-Integrated Manufacturing. 2004, Nr. 20, S. 17-28 [8] W. Xiao et al.: Uniform Coverage of Fibres over Open contoured Freeform Structure Based on Arc-length Parameter. In: Chinese Journal of Aeronautics. 2008, Nr. 21, S. 571-577 [9] M. Steyer et al.: Laser-assisted thermoplastic tape laying system, JEC Composites Magazine, No. 47, March-April 2009 40 LTJ September 2011 No. 5 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim