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- An Innovative One-Step Technology -
Summary
In a joint project with the German automotive industry, the Fraunhofer Institute, material suppliers, component- and mold manufacturers, a thermoplastic sandwich material and its processing technology has been developed. The goal is to offer a cost-effective material with increased mechanical properties to combine the advantages of well-established thermoplastic semi-finished products like GMT and advanced thermoplastic materials. These requirements are fulfilled by a sandwich which consists of outer layers of continuous fibers and a core layer of recycled material mainly of shredded components or production waste of GMT or LFT materials.
The Direct-Compression-Molding process is developed, in which the extruded core is directly inserted between the heated outer layers, followed by a forming process to shape the part. The determination of processing parameters including mold and handling technologies are described. Two different kinds of outer layer fabrics and some different types of recycled material were investigated. The results of the recycling of textile hybrid reinforcements, the developed mold technology and the material handling equipment are discussed.
In order to evaluate this sandwich system the foot support for the smart vehicle has been selected.
Introduction
To meet the demands of the automotive industry with regard to cost reduction and improvement in environmental protection, there is a need for establishing new materials and processing for semistructural components. The content of polymers in passenger cars has reached so far an average value of more than 12%. Polymers are mainly used in the interior, in bumper systems, in body panneling and in engine encapsulations. Further high potential for increase in the use of polymer composites is given for structural and semistructural components. For structural applications in racing cars and small volume models CFRP is used in body structures. Advantages are weight reduction, excellent fatigue behavior, furthermore a high safety standard. The automotive industry focuses on using advanced composites in series production however the cost situation did not allow the introduction of this material group so far.
A problem just being solved is the surface quality (class A) for body components. Meanwhile class A quality is reached in several test components and components for low volume models, where Sheet Molding Compounds (SMC) is in use. There is high interest in the use of composites due to low tooling cost, however the scatter of the trash rate is not acceptable for high volume production at present.
To prove new processing technologies it is favorable first to examine demonstrator components in non visible areas of the car. For semistructural applications higher stiffness values in comparison to conventional GMT are required. After the evaluation of the mechanical test results the optimization of surface quality will be the major aspect in the future.
Sandwich systems offer high mechanical values in bending stiff-ness. Conventional sandwich systems consist of high-quality material combinations - such as Nomex or aluminum honeycombs and Carbon Fiber reinforced polymers (CFRP) - which are too expensive for the application in the automotive industry. Moderately priced sandwich systems are required for future automotive applications.
Project Goals and Partners
In the summer of 2000 the joint project 'Process Development for the Use of Recycled Material of High-quality Thermoplastic Components' which is funded by the German Ministry of Education and Research (BMBF) will be finished. The goal of this project is the availability of cost effective sandwich systems consisting of only one material group which can be processed in one step and recycled without any separation.
The following development goals will be met:
Low cycle times: By use of thermoplastic
polymers cycle times less than one minute are achievable using a
device for mass production.
Weight reduction: By adapting the sandwich thickness to the local load requirements a weight reduction up to 40% in comparison to conventional steels can be achieved. The result is the reduction of fuel consumption and emissions.
Achievement of future recycling rates: A commitment of the German automotive industry states that by the year 2002 only 15% of the weight of a car will be free for deposition (by 2015: 5%). The development and the processing of the new thermoplastic sandwiches offer an important contribution in the use of recycled material.
Easy recycling of the material: For recycling the top and core layers need not to be separated. Common recycling technologies are well established.
The sandwich consists of top layers of woven fabrics and a core layer of recycled material. The top layers preferably consist of the commingled yarn TWINTEX® (PP/Glass; Vetrotex) which offers substantial cost reduction in comparison to impregnated woven fabrics. The core material mainly consists of shredded components or production waste of Glass Mat Thermoplastics (GMT) or Long Fiber Technology (LFT) materials.
For the evaluation of the potential of new material combinations and processes the performance/cost ratio has to be considered. The bending values for strength and Young's modulus for glass fiber reinforced thermoplastics: GMT, LFT, plates of commingled yarn TWINTEX® in comparison to the values of the recyclate sandwich system TP-Rez (0.5mm top layers of TWINTEX® 745A each, 60 weight% glass; 3 mm GMT40 recyclate). The flexural modulus (left) and the flexural strength (right) related to the material cost. Both values are compared to GMT (100%). The flexural modulus / cost ratio of the recyclate sandwich system is 2.5 times better than GMT.
The new technology is successfully under development in close collaboration of material suppliers, component manufacturers, a research institute and the end user. The partners and their tasks are listed below:
DaimlerChrysler
- Material and component selection
- Definition of requirements, design, process development
- Processing of demonstrator elements, material and component tests
Fraunhofer Institut Chemische Technologie (ICT)
- Shredding technology
- Processing and testing of recycled material
- Development of heating and forming devices
- Material testing
Polymer-Chemie ,Kannegiesser
- Recycling technology, compounding and plant technology
- Operation of laboratory device
- Systems Engineering
Deuschle
- Design and fabrication of molds
Vetrotex (associated partner)
- Fabrication of hybrid woven fabrics (TWINTEX®)
Saertex
- Design and fabrication of multiaxial non-crimp fabrics
Design and Calculation of a Demonstrator Component
As a demonstrator component the foot support for the Smart vehicle was selected. It is a flat component with dimensions of 500mm x 400mm in the passenger foot compartment. Design criterion is to withstand a crash load of 5 kN and a low deflection. Thermal loads are limited to 80° C.
The design includes 3 steps:
Step 1: Correlation of fabrication tests and simulation
- First trials with different inserts of the test mold
- Draping simulation with program UFOS-GT
- Correlation of test results and numerical simulation
Step 2: Design variations for the selected component
- Design variations for the foot support
- Draping simulation and stiffness calculation
- Analysis of calculation and derivation of optimization procedures
Step 3: Optimization and determination of final geometries
- Selection of best design in respect to function
- Simulation of draping and stiffness
- Establishing of the CAD- Data set for the mold manufacturer
Technology
In the beginning of the project two different technologies were examined:
Fabrication of a consolidated semi-finished product by a double belt press (like GMT), heating and forming. The complete consolidation is an advantage with regard to low void content, proper fiber/matrix adhesion and handling during heating and processing, however a cost intensive double belt press process for production of sheets and the forming process of the sheets require heating the material twice. The main focus of the project is placed on an alternative processing technique, the Direct-Compression-Molding technology, to minimize energy, logistic and production cost. An additional advantage is the variation of the thickness of the core in order to optimize the component design according to the load requirements.
Material Characterization and Recycling
The materials investigated in this research project are different types of GMT and TWINTEX® (which is a product developed and patented by Vetrotex France). The TWINTEX® fabric used is woven and non crimp.
The fibers commercially available are commingled glass with Polypropylene filaments. The reasons for selecting this material were cost combined with the possibility to achieve short cycle times. This results from the distance between the filaments which does not exceed about 100 microns and which guarantees a quick wetting. One goal of our research project is the determination of recycling parameters for a suitable recycling method for those kinds of materials.
The manufacturing of components using laminated fabrics such as consolidated TWINTEX® sheets needs tracking of this semi-finished sheets while thermoforming to prevent wrinkling. After processing the trimming of the parts leads to production waste. This waste has been characterized in recycling trials for the re-use in a sandwich structure.
On the one hand a sandwich structure with recycled material in the core layer will reduce the costs compared to a homogeneous laminate while on the other hand the weight can be reduced by adding polymer matrix during re-plastification of the recycled material. By adding matrix the glass fiber content can be adjusted and some parameters as for example the impact strength can be influenced. While homogeneous laminates thicken at the radius during thermoforming, the free flowing core layer is able to compensate the variation of thickness of the bottom layers in sections of maximum shear. The intralaminar shear deformation and the friction in the points of intersection of the warp and weft threads reduce significantly the feed rate of homogeneous laminates compared to sandwich sheets. Essential reason for the higher draping quality of sandwich sheets is the increase of tensile force applied per layer of fabric.
The recycling of components manufactured with GMT was investigated in different previous research projects and can therefore be considered as state of the art. Our main focus was the investigation of different recycling technologies especially the comminution of consolidated and heat-set TWINTEX® materials. Therefore technologies with different modes of action were evaluated.
The mechanical size-reduction needed depends on the reprocessing technique for the recycled material. The swing hammer crusher, single screw shredder and cutting mill lead to different particle-size-ranges. The obtained particles were evaluated by a modified sieve analysis and the fractions fed to a plasticising unit. The extrusion compression molded plates were cut into samples and mechanical and morphological tests have been carried out.
Each comminution technology effects the material in a different way. As a function of the sieve whole diameter and the way the energy was introduced, the obtained particle size was detected.
The results have shown that no universal comminution technology for the different fabrics exists. The particle size range is a function of the roving, the matrix, the weaving or the type of fabric and the state of consolidation.
One result is that preserving the fiber length in correlation with an ideal distribution leads to high mechanical properties. Maximum fiber length should be achieved, but there are limitations with regard to the available technologies. The feasibility of reprocessing depends on the type of molding technology, injection or compression molding.
Different materials were comminuted and reprocessed with different technologies. In dependence of the above mentioned parameters for different materials, optimum comminution parameters were determined.
Material recycling of fabric reinforced thermoplastics or sandwich structures, as described, always lead to a more or less Long-Fiber-reinforced Thermoplast. This material can be used as a core layer in sandwich structures up to 100%.
Previous investigations have shown that a PA sandwich structure including 40% of pure recycled woven fabric reinforced PA in the core layer does barely effect the flexural strength compared to a homogenious laminate.
Future Technologies
The goals for future developments are on the one hand to reduce production and logistic cost and on the other hand to increase flexibility in changing or modifying the materials and adjusting the fiber content.
Compared to common sandwich structures the core layer itself has favorable mechanical properties. The idea is to stiffen just regions with high stress load by using continuous fibers such as woven or non crimp fabrics locally while the shape is guaranteed by the long-fiber-reinforced core which is able to flow. New locally reinforced load-optimized designs are possible. The capability to flow facilitates the realization of complex shapes, ribs and the adaptation of inserts.
Beside recycled material, common long-fiber-reinforced semi-finished products such as GMT and LFT can be used. It has to be considered that for large scale production the necessary amount of recycled material available on the market has to be guaranteed. The advantage of the sandwich technology is that depending on the market situation recycled material can be added up to 100% in the core.
The most flexible process is the new LFT-D process which is presently used for the large scale production of frontend carrier structures. In a current research project a further developed version with improved in-line-compounding (ILC) will be evaluated. An extrusion line provides the modified compound while a second double screw device introduces the fibers straight into the melted polymer. The plasticized recycled material is added by a suitable interface.
In addition to fabrics, GMT, LFT, LFT-D, the Tailored-Fiber Placement and the integration of thermoplastic profiles will be evaluated including technologies like GIT and foaming of reinforced structures.
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