DESIGN CONCEPT AND MANUFACTURE OF AIDS FOR TESTING OF TOY CONFORMITY BY POLYJET PROCESS

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1 5th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING ADDING INNOVATION CAPACITY OF LABOUR FORCE AND ENTREPRENEURS" April 2006, Tallinn, Estonia DESIGN CONCEPT AND MANUFACTURE OF AIDS FOR TESTING OF TOY CONFORMITY BY POLYJET PROCESS Vaupotič, B., Labovič, A., Pahole, I., Drstvenšek, I., Brezočnik, M., Balič, J. Abstract: The market requirements have changed very much. One of the principal reasons is the market globalization and saturation. Thus, now the products have to be adapted to buyers' needs. The products become more and more complex, more beautiful, flawless and safe. Safety is of particular importance for children's toys, where it is in the first place. The European standard for the toy safety EN 71.1 specifies the devices for establishing the proper safety of future toys. After adoption of that law in Slovenia the demand for those devices emerged. The manufacture of the devices was taken up by means of rapid prototyping according to PolyJet process offering verification of functionality, putting together and aesthetic acceptability on a 3D CAD model. The 3D model in hand gives a more complete feeling of size, shape and other aspects of the object than the best 3D graphics. The model made can be used also as the final product provided that the mechanical properties are low. Key words: The European standard for toy safety EN 71.1, Rapid prototyping, PolyJet process, 3D model, 3D CAD model, AutoCAD UVOD The philosophy of the common European market is based, in addition to the free flow of goods, personnel and capital, particularly on assurance of safety of products entering the market. The consumers are more and more self-confident, therefore, when buying a product they want to have the guarantee that it works well and that it is safe. On the admittance of Slovenia to EU the Europe standard for toy safely EN 71.1, specifying the requirements and the test methods for establishing mechanical and physical properties of toys, came into force. That standard is concerned about the children's' toys, the toy meaning any product or material intended to be used in the plays of children younger than 14 years. The standard refers to new toys and considers the period of the anticipated and normal use and the usual behaviour of children provided that the toys are used as planned and in the specified way. It includes special requirements for toys intended to be used by children younger than 36 months and by children, too small to sit independently [ 1 ]. The Health Care Institute in Maribor will implement the test methods for establishing whether the mechanical and physical properties of toy comply with the European standard EN To that end it was necessary to make proper devices, produced at the Faculty of Mechanical Engineering Maribor in the Laboratory for intelligent manufacturing systems, for implementing the test methods. The device for rapid prototyping was of great assistance for that purpose. It helped to make real prototypes in scale. 2. RAPID PROTOTYPING Rapid prototyping is a process introduced some years ago as an important aid for shortening the time from the idea to marketing of the product, with simul- 177

2 taneous reduction of the costs of development and improvement of quality of final products. The basic idea of the product is the fastest possible manufacture of the real prototype on the basis of the CAD model, usually without interference of machining processes. The resulting prototypes have a high geometrical accuracy, whereas the mechanical properties of materials, of which they are made, usually do not satisfy the requirements of the final product. Therefore, such prototypes are intended particularly for: presentations of final products visualization of design concepts shape analyses and conformity analyses making tool die impressions and casting moulds simple functional tests Today, many rapid prototyping technologies are available, using a great variety of processes and adequate materials. The used materials range from paper and various polymers to metallic powders. The number of different technologies increases from day to day; some of the most widespread ones are pointed out herebelow. One of the first technologies to appear was stereolithography (SLA) developed by 3D Systems and today most wide spread. Another popular process is the LOM (Laminated object manufacturing) developed by the company Helisys Inc. Further very popular processes are the FDM (Fused Deposition Modeling) owned by the company Stratasys and SLS (Selective Laser Sintering) developed by DTM (today owned by 3D Systems). There are still many other processes, however, currently the leading rapid prototyping technologies are the Stereo lithography (SLA), the Selective Laser Sintering (SLS) and the so-called PolyJet process, the result of the Israeli company Objet Geometries [ 2 ]. Resolution, accuracy, speed and materials are the basic factors of rapid prototyping, which mutually exclude themselves in most devices. Usually, the systems with high accuracy are very slow or they have a very narrow choice of materials; on the other hand, rapid procedures give very rough results. The PolyJet procedure is a technology successfully solving some at these problems simultaneously. 3. RAPID PROTOTYPING WITH POLYJET PROCESS The PolyJet is one of the most recent rapid prototyping processes appearing on the market. This process is the result of the relatively young Israeli company Objet Geometries. As far as its functionality is concerned the PolyJet process can be grouped into the processes of three dimensional printing. It is a kind of hybrid between selective hardening and drop deposition. The most important component is the printing head as used on large, industrial printers for printing advertising bills. Instead of ink the printing head applies the liquid compound of reactive monomers and oligomers polymerizing due to ultraviolet light. The device prints two-dimensional bit maps (individual layers); the motion of maps into the third direction is effected by the tray. The Laboratory for intelligent manufacturing systems is equipped with the most up-to-date printer, EDEN 330 on which the prototypes of the test devices were made. The thickness of layer on this device is 16µm so that this process can be ranked among the most accurate processes and the device can be considered as on of the fastest devices for rapid prototyping; in addition, the price is favourable. As was said, the process of application of the individual layers is completely identical to the process offered by the so-called inkjet printers, except that here the process is much simpler since practically only twocolour bit maps are printed. Each individual layer represents the crosssection of the model with added supporting material. Thus, the cross-section is a bit 178

3 map where the section plane of the model and supports is located. In fact, the printer applies the model material to the plane of the model ink and the supporting material to the plane of the supporting ink. The application of the material to the tray is effected by the piezo-electric printing head injecting onto the metallic tray which, after injection of each layer, moves downwards for one layer thickness; then the process is repeated with the next layer. On application, each layer applied polymerizes due to ultra-violet light coming from two UV light bulbs fixed to either end of the printing head (Fig. 1). Fig. 1. Diagrammatic representation of PolyJet process Thus, the product is ready for immediate use and does not require any subsequent treatment, except removal of the supporting material. The latter can be removed by means of a high pressure pump and water [ 3 ]. The small layer thickness (16 µm) ensures the manufacture of models with very smooth surface and small details. Therefore, subsequent treatment is not necessary, while the finished products can be sand blasted, polished, ground with emery, painted or treated otherwise. The products are usable as master patterns for making silicone moulds [ 4 ], for vacuum casting in silicone moulds and, if special combustion chambers are used, also for casting processes with lost core. 3.1.Materials At present, the company Objet Geometries offers two kinds of materials for the PolyJet process. One of them is the FullCure 510 which is of translucent yellow colour and can be used on the device QuadraTempo. On the same device it is possible to use also the FullCure 555 which is a material of the same type, except that it is grey and non-transparent. A newer material is the FullCure 700 which can be used only on the device EDEN 330. This is a transparent material having slightly better mechanical properties than the above mentioned material. In particular the toughness and the tensile strength are higher (Table 1). Property Tensile Elongation at Break Modulus of Elasticity Flexural Flexural Modulus Impact (Notched Izod) Heat Distortion Temperature Compression Standar d Value D MPa D % D MPa D790 D790 D256 D648 D MPa 1978 MPa J/m 43 C at 1820 KPa 46 C at 455 KPa 69.4 MPa Table 1: Mechanical properties of material FullCure MANUFACTURE OF THE TEST DEVICES ACCORDING TO EUROPEAN STANDARD 71.1 The first stage covered the manufacture of 3D CAD models according to the requirements of standard EN A roll for small parts (simulating the child's esophagus), two templates A and B (for checking adequacy of geometrical shape of certain toys), two link probes A 179

4 and B (for checking accessibility of part or component), a device for measuring the edge sharpness and a device for measuring the tip sharpness were designed. Designing was effected by programme package SolidWorks 2006 assuring simple adaptation and corrections of 3D models (Fig. 2). Fig. 3. Positioning of 3D models for 3D printing Fig. 2. Three dimensional CAD model The geometrical and functional requirements, specified in standard EN 71.1., were taken into consideration; in cases when products were made in composed form attention was paid to proper clearance on hinges and moving parts. Also the costs, the exactingness and the time periods of delivery were considered. A good compromise was the 3D printing technology as one of the rapid prototyping technologies assuring the manufacture of prototypes and products. 3D models in stereo lithographic format (STL file) were used as the basis for printing. During the export from SolidWorks 2006 into the STL file the proper size of triangles must be set so that the made surface is flawless. In the graphic environment attention is paid to optimal setting of the model on the working tray (Fig. 3). Then the STL file is imported into the specific purpose control program determining the delivery time and the consumption of the bonding agent for full parts and the supporting material for hollow model parts on the basis of the model volume and position. The mass and the time frame serve for the financial estimate. Automatic determination of supports and layering of the model in vertical direction (Z axis) follow. Afterwards, individual layers (bit maps) are sent to the printer which takes them over and deposits them successively into the tray. When the product has been printed by the machine, the product is ready for immediate use and does not require any subsequent treatment, except removal of supporting material. The supporting material looks like "marmalade", although it is not sweet and sticky. It can be removed by means of a high pressure pump and water. The supporting material is simply "washed away" by water jet under the pressure of about 40 bar. The water jet pressure mainly depends on the model to be cleaned. Models with thin walls and very small details were cleaned with lower pressure, while more robust models were cleaned with higher pressure so that the time of cleaning was shortened. The first prototypes ensured the tests of correctness of the product design (Fig. 4). It was found out that on the link probes for testing of accessibility (they were called little finger by us, since they simulate the child's little finger) the fits were slightly too large, therefore they were reduced on the final products. 180

5 Also the design of the box for containing the device for measuring the tip sharpness, printed in assembled form, underwent corrections on the hinges and fixing clamps. Further, the shape of openings for installation of keys and diode lights on the casing for the device for measuring of edge sharpness was slightly changed. sharpness) made by layer technology were used as final products (Fig. 6). Fig. 6. Final products made by RP Fig. 4. Prototypes made according to PolyJet process The 3D CAD models, adapted to new requirements were then printed and functioning, shape and mountability were tested. Prior to manufacture of final products all prototypes were delivered to the client who confirmed correctness and adequacy of prototypes. The last stage comprised the manufacture of final product. Fig. 5. Final products On the basis of findings obtained from prototypes made by rapid prototyping according to the PolyJet process the final products were made and calibrated according to the specified standard (Fig. 5). On the other hand, the models (housing of the device measuring the tip sharpness and the box for its accommodation and housing of the device for measuring the edge The models made by rapid prototyping allow also painting; our models were coated with a black mat paint coat additionally protecting against UV rays and minor mechanical damages. 5. CONCLUSION The manufacture of testing devices for verifying the children's toy safety according to the PolyJet process of rapid prototyping has proved to be a good approach since all requirements were complied with. A minor quantity of products of rather complicated shape was concerned. We must be aware that the price of rapid prototyping is influenced only by the size of the product. The shape complexity is quite unimportant. Due to the high number of different requirements and the introduction of new products the rapid prototyping technology according to the PolyJet process has enabled us to establish by testing the adequacy of shape, functions and mountability of the devices and to adapt it accordingly. As in case of rapid prototyping the data on the shape of the product are in the form of 3D CAD model, the changes on the product are made very simply. On the basis of the experience in rapid prototyping, the applicability of prototype models and the responses from users we conclude that the PolyJet technology for 181

6 rapid manufacture of prototype model is adequate with respect to shape as well as dimensional requirements. With rapid prototyping it is possible to correct the deficiencies of the shape, before the final products are made. The advantages are obvious particularly in case of exacting complicated shapes which would be hardly manufactured by conventional processes or the price would be too high and the manufacturing time too long. In case only a few products, not exposed to major mechanical loadings, are needed, the models made by PolyJet process are usable also as final products. With wider choice of materials the applicability of this technology will increase. REFERENCES 1. Urad Republike Slovenije za standardizacijo in meroslovje pri Ministrstvu za znanost in tehnologijo: Slovenian standard DSIST EN 71-1: Safety of Toy-Part1: Mechanical and physical properties, Slovenia, dec Rapid prototyping, index.htm ( ) 3. Drstvenšek I., Balc N.: Layered Technologies, Univerza v Mariboru, Faculty of mechanical engineering, University of Maribor, Pham D.T., Dimov S.S. Rapid manufacturing: The technologies and applications of rapid prototyping and rapid tooling, Springer, London,