MODEL OF RADIATION HEATING OF SLUSH MOULDING TECHNOLOGY ING MARTIN HUŠEK Technical university of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies Abstract: The technology for the manufacturing of softened automotive accessories used in company Magna Exteriors & Interiors Bohemia, s.r.o is called Slush moulding. The technology is mainly suitable for the manufactoring of artificial leather, for example for the dashboards. It is a method of radiation heating of shell moulds witch supplies the suitable shape, thickness and design relief to the final product. Infrared emitters are placed around the mould by the numerical model, simulating the whole process of radiation heating. For the purposes of the preparation of the model and the application of boundary conditions is IREviewBlender application suggested. The final temperatures are solved in the software ANSYS. The linkage of these information tools enables simulation of the whole process of transient heating with the application of temperature regulation. Keywords: Slush moulding, shell mould, infrared emitter, transient heating, heat flux 1 Introduction The company Magna Exteriors & Interiors Bohemia, s.r.o has been dealing with the Slush moulding tenology used in the manufacturing of softened automotive accessories for several years. This technology is less energetically demanding and takes up much less space on the manufacturing floor in comparison with other technologies used in the same manufacturing processes. It was clear from the very beginning that this unique technology cannot be implemented without some necessary software which enables the simulation of technological processes in the manufacturing. The present article deals with one of these technological processes and mentions a possibility of simulation of heating of shell moulds. In this case the link betwen commercial software ANSYS and freeware software Blender with addition of new customised functions suitable for the preparation of the model proved to be effective. 2 Slush moulding technology and used simulation method The base of this method of manufacturing is a shell mould. Thermoplastic powder based on PVC or PU is applied on inner surface of the shell mould. Its outer surface is heated by infrared emitters up to the temperature of approximately 220 C. The powder melts and when the mould is cooled down by water, it becomes a compact mass representing artificial leather. The inner face of the mould has to be formed so that it gives the corresponding shape and suitable surface relief to the final product. The final leather is subjected to the strict quality control when removed from the mould. In case of the artificial leather thickness check-up, for example the airbag area is particularly watched, bound to fulfill the prescribed dimensions. Infrared emitters allowing heating of the mould are clamped into a special structure located above the mould. The quantity of the used emitters is usualy counted in tens and is defined by the size and compoundness of the surface of the mould. If a two-cavity mouid is used for simultaneous manufacturing of two artificial skins for a dashboard, the number of applied emitters reached almost two hundred pieces. 1
The whole process of heating is carried out under the temperature regulation. There are thermocouples placed on the outer surface of the heated mould. Each thermocouple regulates one emitter or a group of emitters by two-state control. Due to the complexity of the used moulds it is not possible to achieve even temperature distribution in the range of 20 C without any teperature regulation. 2.1 Process of design of transient heating of moulds The process of design of heating shown in the fig. 1 is divided into two main parts. It is the virtual heating and the heating itself on a test production line or a production line. The virtual heating phase begins in IREviewBlender. With the use of the appropriate functions, the emitters are placed around a model of the mould and heat flux loading of the surface is simulated. After that the FEM analysis in ANSYS follows in order to calculate the final temperature dependant on time. In case of a positive result, the necessary data are exported, mainly the transformation matrix determining the positions of the emitters in relation to the given coordinate point for physical installation into the production line. The phase of the real heating process begins with the installation of the emitters holder in the production line by a robotic arm. Correctness of the installed heating is tested on the first samples of the artificial leather. The samples are then checked for burns or unsufficiently heated patches. If the defects are small, they are corrected on the spot by redirection of a specific emitter, otherwise the design process goes back to the phase of the virtual heating. If everything is in order, the heating is released into batch production [1, 2, 3]. Fig. 1 Scheme of the design process of heating 2.2 Software IREviewBlender as a pre-processor The application can simulate heat flux loading of the mould by individual emitters or by the whole groups, see fig. 2. However, the problem of radiation is not solved by this aplication, but the corresponding heat flux density obtained from the database of the characteristics of different types of emitters is assigned to the individual elements of the mesh. This database was created by means of experimental measurements. Heat flux characteristics consider both the distance of an element center from an emitter and their mutual rotation. For continuous distribution of the heat flux boundary condition it is possibile to interpolate the values. The required types of emitters are selected by the user from the database of emitters, see fig. 3. The figure shows various types of emitters. There are single emitters or double emitters with or without reflectors. The most used emitters are those of 1.6 kw and 2kW. For assembly of the model of heating in the IREviewBlender aplication it is possible to import some important parts for heating, e.g. the frame of the mould or the 2
frame structure for placing the emitters. The user can thus verify the feasibility of placing the emitters in the production line and non-collision of the emitters with other objects. The user can work in several work scenes and thus compare individual variants of the emitters placing. Having finished the model preprocessing, the placing of the thermocouples and their pairing with the emitters follows. For thermal analysis in ANSYS the user does the appropriate settings and data exports [1, 2, 3]. Fig. 2 - Heat flux - IREviewBlender Fig. 3 - Database of emitters IREviewBlender 2.3 Calculation of the temperature in the sw ANSYS It is possible to calculate the final temperatures on the mould surface in the sw ANSYS by means of special exports from the sw IREviewBlender. The user has the possibility to calculate the final temperature on the basis of the heat flux density loading given by the simulation of the IREviewBlender, see fig.4, or wheter he wants to solve the task by the radiation computation, see fig.5. It is clear from the figures, that the temperature field in the first case reaches higher local maxima, as the thermal radiation is more oriented in the space under the emitter. In the second case the thermal radiation is more dispersed and reaches lower values. The real behaviour of the radiation heating in the Slush moulding technology corresponds better to the fig.4, where the most thermal radiation is oriented into the space under the emitter. This behaviour is caused by the characteristics of the sheet metal reflector and characteristics of the reflection layer of the glass tube of the emitter. 3
Fig. 4 - Resulting temperature, loading from IREviewBlender Fig. 5 Resulting temperature, radiation computation 2.4 Possibility of simulation of regulated heating The real heating in the line works with the temperature regulation, otherwise it would not be possible to reach uniform temperature distribution. Suitable combination of sw ANSYS and IREviewBlender allows to realize fully automatic regulated heating. For this purpose it is necessary to place thermocouples in the model. They are most offen placed perpendicularly to the emitter tube or the mould edge closer to the emitter. In the model they are represented by the balls placed on the mould surface, see fig.2. The regulation setup is executed in the application IREviewBlende where the user enters the simulation time, time steps of temperature scan, required temperature and other setup such as regulation deviations from the required temperature. Having started the simulation in the application IREviewBlender, the sw ANSYS starts in the batch mode. It waits for getting the text file with the values of the heat flux density and then computes temperatures. ANSYS then gives back to IREviewBlender the valueas of temperatures on the thermocouples in the textfile. On the basis of these data IREviewBlender switches on and off the emitters. It is two-step regulation where the emitter is either on or off. The following figure 6 shows the regulated temperature behaviour on several selected thermocouples. The regulation was done around the slush temperature 4
corresponding to the value of 220 C. Fig.7 illustrates the same heating without applied temperature regulation. This state would not be suitable for the production of the artificial leather. Fig. 6 - Regulated heating 3 Conclusion Fig. 7 Unregulated heating The method os simulation of the non-stationary heating of the shell moulds for production of artificial leathers presented in the present paper takes advantage of design of heating in the test and batch production lines. It is the method making use of combination of the softwares ANSYS and IREviewBlender. For simulation it is necessary to have data acquired from the experimental of production devices. They are, for example, measured power characteristics of the applied infrared emitters or of test measurements of the slush mould to get temperature dependencies at individual thermocouples. The data acquired from these measurements allwo calibration of the model and specification of the boundary conditions. 5
The necessitiy of this modelling proved in a number of practical tasks solved in the company Magna Exteriors & Interiors Bohemia, s.r.o in collaboration with the company LENAM, s.r.o. References: [1] Hušek M. and Potěšil A., Software Prediction of Non-stationary Heating of Shell Moulds for Manufacture of Artificial Leathers in Proceedings of 18th International Conference ENGINEERING MECHANICS 2012, Svratka - Žďár nad Sázavou, Czech Republic, 14 17 May 2012 (Náprstek J., Institute of Theoretical and Applied Mechanics, AS CR, Prague, 2012), pp. 120-121. ISBN 978-80-86246-39-0. [2] Potěšil A., Non-stationary heating of shell moulds in the process of manufacture of artificial leathers in Proceedings of 17th International Conference ENGINEERING MECHANICS 2011, Svratka - Žďár nad Sázavou, Czech Republic, 9 12 May 2011 (Fuis V., Ústav termomechaniky AV ČR, v. v. i. - pobočka Brno, 2011), pp. 487-490. ISBN 978-80-87012-33-8. [3] Potěšil A., Experimental Measurement of Performance Characteristics of Infra-Red Emitters in Proceedings of 49th International Conference EAN 2011, Znojmo, Czech Republic, 6 9 June 2011 (Návrat T., Ústav mechaniky těles, mechatroniky a biomechaniky, Fakulta strojního inženýrství, Vysoké učení technické v Brně, 2011), pp. 349-356. ISBN 978-80-214-4275-7. Acknowledgements This project was supported by the Ministry of Education of the Czech Republic within the SGS project no. 7822/115 at the Technical University of Liberec. Contact: Ing. Martin Hušek, martin.husek@tul.cz Technical university of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies Studentská 2, 461 17 Liberec 6