Rapid prototyping of control device for preventive control of children plaything.

Vaupotic, Bostjan ; Pahole, Ivo

1. INTRODUCTION

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 (Urad RS, 1999). 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 71.1. 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. MANUFACTURE OF THE TEST DEVICES ACCORDING TO EUROPEAN STANDARD 71.1

The first stage covered the manufacture of 3D CAD (Jaballi et al., 2007) models according to the requirements of standard EN 71.1.

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 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 assuring simple adaptation and corrections of 3D models (Fig. 1).

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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 (Pham & Dimov, 2002). 3D models in stereo lithographic format (STL file) were used as the basis for printing. During the export from SolidWorks into the STL file the proper size of triangles must be set so that the made surface is flawless (Drstvensek & Balc, 2004). In the graphic environment attention is paid to optimal setting of the model on the working tray (Fig. 2).

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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 (Drstvensek, 2004).

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. 3). 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.

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.

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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.

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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. 4). 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 sharpness) made by layer technology were used as final products (Fig. 5).

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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.

3. 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 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.

4. REFERENCES

Drstvensek, I. & Balc, N. (2004). Layered Technologies, Faculty of mechanical engineering, University of Maribor, Maribor

Drstvensek, I. (2004). Rapid prototyping, Available from: http://maja.uni-mb.si /slo/index.htm, Accessed: 2008-04-13

Jaballi, K.; Bellacicco, A.; Louati, J.; Riviere, A. & Haddar, M. (2007). A computer aided toleracncing: algorithm for 3D manufacturing tolerancing, APEM journal, Production Engineering Institute (PEI), University of Maribor, ISSN 1854-6250, Maribor

Pham, D.T. & Dimov, S.S. (2002). Rapid manufacturing: The technologies and applications of rapid prototyping and rapid tooling, Springer, London

Urad RS: za standardizacijo in meroslovje, Ministrstvu za visoko solstvo, znanost in tehnologijo (1999). Slovenian standard DSIST EN 71-1: Safety of Toy-Part1: Mechanical and physical properties, Slovenia