THE TECHNOLOGY FOR LOW-VOLUME MANUFACTURING OF FENDERS FOR AN ADVANCED LIGHT ELECTRIC VEHICLE

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1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia THE TECHNOLOGY FOR LOW-VOLUME MANUFACTURING OF FENDERS FOR AN ADVANCED LIGHT ELECTRIC VEHICLE Pääsuke, K; Pohlak, M. Abstract: Modern manufacturing industry is bound to meet the customers requests for increasing customization of products. With conventional technologies, being suited for mass production and using expensive tooling, it is often complicated to fulfill these demands with reasonable price. Several low-volume manufacturing technologies and also Additive Manufacturing technologies will be analyzed and compared in present paper. In the end of the paper a case-study of determining the most-suitable technology for manufacturing fenders for an advanced light plug-in electric vehicle will be presented. Keywords: Electric vehicle, Additive Manufacturing, Rapid Prototyping, Vacuum Forming. 1. INTRODUCTION In the increasing need for customizing products, technologies which are suitable for flexible and low-volume production gain importance. Rapid Prototyping (RP) technologies were first used for prototyping, but several of these have proved to be ready for real production and constituted a new group of manufacturing technologies - Rapid Manufacturing (RM) [1, 2, 3]. According to Eyers these technologies are: Stereolithography (SLA), Fused Deposition Modelling (FDM) and Selective Laser Sintering (SLS) [4]. The selection of a RP process is mainly based on the speed and costs involved; the quality and accuracy are a secondary priority. If the application is an end-use product, then the requirements are much more stringent: in product design, some of the most important criteria are the product functionality, appearance, shape or geometry and most relate to the material selected by the designer and material properties [4]. In the present paper the potential manufacturing technologies for manufacturing of the fenders of an advanced light plug-in electric vehicle - EXO bike - are analyzed. The manufacturing of the scooters is in the transition-phase from prototype to low-volume production, hence the quantities are small and the developed solutions can change to some degree. As different technologies have different advantages and disadvantages, the selection of the technologies will have a strong effect on the final outcome is it the mechanical properties or the look itself. In addition, the costs of different technologies can vary greatly. 210

2 2. EXO BIKE AND ITS FENDERS EXO bike (Fig. 1) is a state-of-the-art electric scooter. The frame is cut by laser from aluminum sheet and bent afterwards to achieve the structural strength and low weight. The energy is stored in lithium-ion batteries. The scooter weights altogether approximately 40 kg and can drive roughly 60 km. As the design of the scooter is unique there was a need to develop and manufacture inter alia its own fenders. The design of the fenders (Figure 2 and 3) is similar to the scooter itself: edgy, facetted and futuristic. The overall dimensions of the front fender are 470 x 150 x 200 mm and rear fender 430 x 200 x 200 mm. The requirements for the material of the fenders are mostly associated with mechanical properties: it has to withstand vibration and slight impacts, but also atmospheric conditions. Figure 2. Front fender. Figure 3. Rear fender. 3. POTENTIAL TECHNOLOGIES In the present chapter the selection of suitable Additive Manufacturing (AM) and low-volume manufacturing technologies will be presented. Figure 1. EXO bike RP/RM TECHNOLOGIES The RP technologies are manufacturing techniques for building objects from a threedimensional Computer Aided Design (CAD) models. The product to be fabricated is presented as a multilayered CAD model, where successively each layer is converted to a physical layer, then bonded to the preceding layers, until the product is completed [5]. 211

3 The most popular RP technologies for plastics are SLS, FDM and SLA. Additive Manufacturing derives from the principle the parts are built/manufactured by adding material, not by removing it, as in most of the machining technologies Selective Laser Sintering In SLS the parts are made from powder, which is spread layer by layer onto the building platform and each layer is sintered by hatching the present cross-section with laser beam. The unsintered powder will support the parts during manufacturing Stereolithography UV-curable photopolymer resin is used in SLA for making the parts. SLA offers the highest accuracy among the AM technologies [6]. In the manufacturing process the resin is spread layer by layer onto the vat and the cross-sections of the part are cured by laser beam. As the resin is liquid, it does not support the parts and supporting structures must be added to ensure the stability and immobility of the parts during manufacturing. In build-process the resin is only partially cured and postcuring is required Fused Deposition Modelling In FDM thermoplastic filament is used to achieve three dimensional complex parts layer by layer. The filament is melted immediately before depositing onto the model. Soluble supports are required for overhanging sections and fragile structures Low-volume manufacturing technologies There are several technologies, which can be used for production in high- and lowvolume, but also for prototyping. The present paper will focus on three of such technologies, which all are used for manufacturing thin-walled parts from sheetmaterial. These technologies are: vacuum forming (VF), hydroforming (HF) and Incremental Sheet Forming (ISF) Vacuum Forming VF is used for low- to high-volume manufacturing, but also for prototyping depending from machines and also from the durability of the mould [7]. In vacuum forming thin plastic sheet is heated up slightly above glass transition temperature (T g ) or near to melting temperature (T m ) [8], stretched onto a mould and then formed by applying vacuum between the mould and plastic sheet. Vacuum forming itself is relatively fast and simple, but machining the mold is time-consuming. In prototypingphase the expenses of mould should be as low as possible as the mould is used only a few times, therefore the moulds in prototyping-phase are often made from lightly machinable material. The size of the fenders meets the possibilities of vacuum forming. There are several types of materials with different thicknesses, mechanical properties, colours and appearances. If the prototype and mould are suitable, the shift from prototyping to low-volume production takes minimum effort Incremental Sheet Forming (ISF) ISF is considered an emerging sheet metal forming technology [9-12], for prototyping or small- to medium-batch manufacturing. There are two basic types of ISF: forming without support (negative) and forming with 212

4 Figure 4. Honda S800 hood made by Amino in small series [13]. support (positive). The process is numerically controlled and capable of producing complex structures. As ISF is usually carried out in conventional CNC milling machines, the process offers high flexibility. ISF has so far interested mostly car-industries (Honda (Figure 4), BMW, Daimler-Chrysler), who see the potential of ISF by manufacturing large body-panels for prototypes and individualized vehicles [13] Hydroforming Hydroforming has gained interest in latest years due to increasing need of weight reduction of modern vehicles [14]. There are two types of hydroforming: tube hydroforming and sheet hydroforming. The process starts with fastening the sheet-metal between the mould and supporting frame. Figure 5. The tree of handled technologies for manufacturing the fenders. The hydraulic fluid in high pressure is directed between the sheet-metal and the frame. The pressurized fluid causes the sheet to expand and form following the mould surface. Finally, the hydroformed sheet is removed and post-processed (the fastening-edges are cut off etc.) if necessary. 4. THE COMPARISON OF THE TECHNOLOGIES In present chapter the previously described technologies will be compared and analyzed. The diagram of different technologies that were analyzed to produce the fender are depicted in Figure Additive Manufacturing Technologies The AM technologies are suitable for manufacturing a few samples, as it is fast and comfortable. As mentioned earlier, the technology imposes almost no restrictions to the design of the part. Therefore it would be possible to manufacture the fenders, which would be optimized in reference to low weight and high mechanical properties. Such optimizations lead often to complex design and constructions, which would be impossible to achieve with conventional technologies. The hindrance of using AM technologies, in this particular case, are the large dimensions of the fenders. To manufacture the fenders in one piece, the building chamber of the AM system has to be relatively big. This all results in high building-volume and price. The materials offered in AM are usually with limited properties. For example the mechanical properties are slightly lower than 213

5 in case of injection moulded parts [15]. As the parts are made layer by layer in AM technologies, the surfaces of parts can be stepped (depending on the position in manufacturing) and the surfaces might need post-processing. The selection of colours is also limited in AM excluding FDM Low-volume manufacturing technologies The low-volume manufacturing technologies described earlier require greater human labor. As all of the described technologies use mould, the expenses of manufacturing the mould must be added to the costs. ISF is performed mostly on CNC milling machines and as the main technology for manufacturing the mould is also machining, it can be executed on the same machine. Although the mould for HF is probably the most expensive and most time consuming to manufacture, the process of HF itself is times quicker than of ISF. The moulds for VF in prototyping/lowvolume manufacturing phase are usually made from easily machinable materials. In addition, it is possible to manufacture the mould also by AM technologies, for example by 3D Printing or SLS. All of the three technologies require postprocessing to cut out the formed part. To achieve the desired colour by ISF and HF, painting is also needed. In addition, none of the technologies enables negative angles for the parts to ensure the removing of the parts from the mould. 5. THE MANUFACTURING OF THE FENDERS As VF is the cheapest and simplest of the three low-volume manufacturing technologies and as there is first a need for several fenders not just one, VF was chosen for the technology to manufacture the fenders of EXO bike. The mould was milled from MDF-blank (MDF - medium-density fibreboard) by 3- axis CNC milling machine DYNA 3116 by using a round mill. The mould was postprocessed with sandpaper and filler afterwards. VF was performed on Formech 300 XQ Quartz vacuumforming machine. For the material HIPS (high-impact polystyrene) with thickness of 3 mm was selected. Although the design and functionality are often verified by obtaining the physical model by some agile technique, in present case the rationality of using a AM technology is arguable. The costs of fenders obtained by VF are comparable to the costs of for example SLS by the same model. 6. CONCLUSION Several low-volume manufacturing technologies and AM technologies were analyzed regarding the need for manufacturing fenders for an advanced light plug-in electric scooter in low volume. Due to flexibility and low costs vacuum forming was selected for the technology for manufacturing the prototype and the first series. The mould was milled from FDMmaterial by 3-axis CNC milling machine with round mill. Although AM technologies offer even higher flexibility and they are 214

6 used even for real production, these technologies were considered unsuitable for such large parts. 7. REFERENCES 1. Ajoku, U.; Hopkinson, N.; Caine, M. Experimental measurement and finite element modelling of the compressive properties of laser sintered Nylon-12. Materials Science and Engineering: A, 2006, 428, Zarringhalam, H.; Hopkinson, N.; Kamperman, N.F.; de Vlieger, J.J. Effects of processing on microstructure and properties of SLS Nylon 12. Materials Science and Engineering: A, 2006, , Kruf, W.; van de Vorst, B.; Maalderink, H.; Kamperman, N. Design for Rapid Manufacturing functional SLS parts. IPROMS, 2006, Eyers, D. R.; Dotchev, K. D. Rapid manufacturing for mass customization. IPROMS, 2009, Dong, L.; Makradi, A.; Ahzi, S.; Remond, Y. Three-dimensional transient finite element analysis of the selective laser sintering process. Journal of Materials Processing Technology, 2009, 209, Melchels, F.P.W.; Feijen, J.; Grijpma, D.W. A review on stereolithography and its applications in biomedical engineering. Biomaterials, 2010, 31, Harper, C.A. Handbook of Plastic Processes. John Wiley & Sons, Timonium, Schmidt, F.M.; Le Maoult, Y.; Monteix, S. Modelling of infrared heating of thermoplastic sheet used in thermoforming process. Journal of Materials Processing Technology, 2003, , Rauch, M.; Hascoet, J.Y.; Hamann J.C.; Plennel, Y. A new approach for toolpath programming in Incremental Sheet Forming. International Journal of Material Forming, 2008, Pohlak, M.; Küttner, R.; Majak, J. Modelling and optimal design of sheet metal RP&M processes. Rapid Prototyping Journal, 2005, 11(5), Pohlak, M.; Majak, J.; Küttner, R. Manufacturability and limitations in incremental sheet forming. Proceedings of the Estonian Academy of Sciences, Engineering, 2007, 2, Pohlak, M.; Majak, J.; Küttner, R. Incremental sheet forming process modelling-limitation analysis. Journal of Achievements in Materials and Manufacturing Engineering, 2007, 2, Emmens, W.C.; Sebastiani, G.; van den Boogaard A.H. The technology of Incremental Sheet Forming A brief review of the history. Journal of Materials Processing Technology, 2010, 210, Hartl, Ch. Research and advances in fundamentals and industrial applications of hydroforming. Journal of Materials Processing Technology, 2005, 167, Gibson, I.; Rosen, D.W.; Strucker, B. Additive Manufacturing Technologies. Springer, New York,