Virtual Paint Shop (VPS) - Towards a Process Chain for Virtual Car Body Painting -

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1 Virtual Paint Shop (VPS) - Towards a Process Chain for Virtual Car Body Painting - Günter Müller Klemens Rother Cord Steinbeck-Behrens Christoph Müller CAD-FEM GmbH, Grafing bei München Deleted: ) Deleted: - Deleted: - Formatted Formatted Formatted Formatted Abstract Today numerical simulation based on the Finite Element Method (FEM) is an established tool in product development to evaluate the behavior of car body designs. For example, in crashworthiness analysis of cars reliable results can be achieved. This allows designers to reduce the number of prototypes, and benefit from a considerable reduction in time and cost. In the future, computer based simulations will also be used much more in process simulation. Metal forming simulations are already commonly done. In this paper, simulation tools for the painting process will be presented. In cooperation with the BMW AG, CAD-FEM GmbH has developed a series of simulation tools called VirtualPaintShop (VPS) that allow the designer to simulate the painting process in advance on the computer. VirtualPaintShop reduces the need for prototypes in the development phase and helps the engineer to recognize problems at an early design stage. VPS is designed in such a way that it can be used by paint specialists, shop floor experts or car body designers with only little or no knowledge in FEM.

2 1. Introduction The BMW Group has now adopted an innovative method of optimizing and speeding up vehicle development in the paint shop for car bodies. They have commissioned CAD-FEM GmbH to produce a modular software suite that allows the painting process to be simulated in advance on the computer screen. Although commercial software is readily available for crash simulation, no such software is available for paint shop processes. But precisely such computer aided tools are particularly important when it comes to shortening the development cycles for new vehicles and reduce risks of late changes due to manufacturability issues and lacking paint quality in the daily series production. These tools can reduce the need for prototypes in the development phase and can help engineers to recognize design problems at an early stage, implement the necessity of modifications to the paint shop in sufficient time and to optimize the process parameters on the computer. The software package VPS is built on top of the ANSYS program, a standard Finite Element Analysis System. Analytical algorithms, extensive routines based on empirical and knowledge based rules are added to account for the specific needs to simulate the painting process. VPS is designed as a modular software suite currently consisting of the following modules: VPS/DRY (Drying) VPS/EDC (Electro Deposition Coating) VPS/DIP (Dipping in and out) CATIA-Toolkit (Modeling tool, specifically suited for VPS/EDC and VPS/DIP) With the help of the program system VPS the user can analyze complete car bodies (or sections) in a virtual drying process or painting process. In most cases the software VPS will be used in departments, where use of Finite Element Simulation is not common practice. Therefore, ease of use was one of the basic requirements for the architecture of this program. Another primary goal was speed. For example, results of a drying process simulation are needed within a day. Therefore, a simple but sufficiently accurate approach was used. 2. VPS/DRY Numerous trials are performed to arrive at the target cure schedule for each vehicle and an associated coating. Time and temperature data is collected (Datapaq Logger and software) by running an uncoated body through the oven with attached temperature

3 probes in specific locations. The data analyzed and the process is iterated until desired heating is obtained. The vision of VPS/DRY is twofold. First to reduce the numerous trials required to arrive at the proper schedule for a vehicle, and second, to better understand the physical heat transfer phenomena that occurs as a vehicle passes through a paint shop oven. Important time and money savings gains can such be derived. For paint curing of base coat, top coat or clear coat, the local characteristics of the temperature history drives the process. The software VPS/DRY computes the temperature history and distribution on a body in white as it passes through a drying oven. In the simulation process, thermal boundary conditions as a function of time and location, free and forced convection and thermal radiation are applied on the car body. It is necessary to analyze all of these influences in combination, and therefore, a coupled fluid flow heat transfer analysis would be required. But doing so would also result in extensive amount of computing time and resources. VPS/DRY overcomes this problem by doing just a simple and quick heat transfer analysis with ANSYS using approximations for convection and radiation effects. Special routines have been developed by CAD-FEM to take into account these effects according to the data sets of the specific oven. These data sets have to be calculated and calibrated with the oven only once and can be reused in further applications. An already existing FE-model from a crash or strength / fatigue analysis can be used. This FE-model is then modified for use in the thermal analysis. The user needs no specific knowledge to use the program. He only needs to read in the FE-model, read in the oven data set, start the solution and post process the results.

4 Bypass to VPS/DRY Standard ANSYS Inputs Fig. 1: The user interface of VPS/DRY is integrated inside the ANSYS program. Plot shows a text example With the computation performed by VPS/DRY, the temperature as a function of time is available at every location on the body in white. For a given time, contour plots of temperatures are available or for a given location a graph of temperature over time can be shown. Also the distribution of dwell above a specified temperature level can be visualized. The results can be transferred to other software tools for the analysis of the chemical behavior of the lacquer, for stress prediction or for three-dimensional visualization by Virtual Reality programs. Fig 2: Dwell time in seconds at 130 degrees as contour plot and as graph with distribution over the surface. Plot shows a test example To simulate the thermal driven chemical reaction of the paint, CAD-FEM offers, in cooperation with NETZSCH-Gerätebau GmbH, an add-on solution for VPS/DRY. This add-on solution consists of measuring devices and software to identify the behavior of

5 lacquer using small test specimen. The result of this solution in conjunction with the temperature history calculated by VPS/DRY allows to predict paint quality measures such as viscosity and setting degree which are particularly important for powder coating. Knowing the history and distribution of temperature a thermal stress analysis can be easily performed to determine the stresses and deformation of the body. These are particularly of interest for luxury cars which often are manufactured using different materials (steel, aluminum, magnesium or polymers) with different thermal expansion values, Thermal stresses might cause failure of spot welds but also might lead to buckling of parts of the body. 3. VPS/EDC During the EDC (Electro Deposition Coating) procedure the car body is dipped into a bath of lacquer and an electric potential between the car (cathode) and an anode is applied. Due to the local electric current density on the car body structure the lacquer adheres on the surface of the body. The objective of VPS/EDC is the simulation of the EDC process and to obtain the time dependent distribution of the lacquer film thickness. Sometimes, different geometric locations, such as the surface inside a cavity or the cavity between two flanges, do not get enough or even no lacquer coated on certain areas. The EDC simulation is performed using an electrostatic analysis solver from ANSYS. In the simulation the conductivity of the film is described by a function of film thickness. The film thickness is an integral function of the current density over time. This function is described by specific parameters. For the current density, the voltage distribution in the lacquer bath is desired. As input values to the program, the user must declare the material properties of the lacquer bath, electrical material properties for the car and parameters as described above. For this simulation, a more complex FE-mesh model is needed. It must take into account the car-body, the lacquer bath (fluid) but also the cavities between two flanges must be represented. With common tools the meshing would require a tremendous effort. Therefore it is recommended that the user creates his FE-model with the help of the software CATIA-Toolkit. This tool is specially developed for these applications and can significantly reduce modeling time.

6 Fig 3:Test example of side frame. Colors show thickness of paint film A test simulation of a partial car model showed very good results. A model with elements was calculated for a process time of 200 seconds. The simulation took 40 hours of CPU time. 4. VPS/DIP Sheet metal structures are commonly dipped into liquids as part of the manufacturing process. Examples of this include: cleaning baths, pickling processes (as pretreatment for painting), and the dipping painting procedure. In structures with open cavities, bubble inclusions may occur when the structures are immersed in liquids. These bubbles of trapped gas prevent a complete wetting of the surfaces. Furthermore, during retraction of the car body from the dip, the liquid draining may be delayed or fluid pits remain, which is undesirable for following processes. Also, high buoyancy forces may occur which can exceed the structural strength causing failure (e.g. dents) of bowl structures. The aim of VPS/DIP is to estimate location and size of possible air bubbles, fluid pits and buoyancy forces. Furthermore it helps to optimize the spatial motion of the car body in the lacquer bath to omit air bubbles and pits and speed-up the process. The simulation software program VPS/DIP uses sound engineering methods rather than a highly complex CFD (Computational Fluid Dynamics) simulation. For a FE-model, a reduced model of the VPS/EDC simulation can be used. Here the fluid bath surrounding the model is not needed, but all the cavities and drainage have to be considered. CATIA- Toolkit can help to modify the simulation model VPS/DIP is still under development. The first release is expected by mid 2003.

7 5. CATIA-Toolkit CATIA-Toolkit is a software which generates volumes for the surrounding air and the cavities of the car body. From these volumes meshes for VPS/EDC and VPS/DIP can easily be generated. During the coating process, each part of the car body is surrounded by paint; some parts are also filled with paint. For the analysis, this paint must be represented as volume within the CAD-System. To replace the open faces of a sheet metal part by a closed volume, artificial faces are created. All faces are marked, providing an indicator to the FE-tool whether they represent actual sheet metal or the boundaries of a paint volume. Additional work is required for closed volumes. Flanged sections where parts will be welded together, are not accessed by paint, therefore they are deleted. This deletion leads to gaps in the geometry that must be filled by tying the remaining faces together. Another cause for gaps in the geometry is the fact, that one side only within the CAD-System represents sheet metal. If one or more parts are in contact, gaps may occur, because one or more faces are representing the outer side of the sheet metal. The complete process, from the sheet metal representation within the CAD-System to volume meshes needed for the analysis is very time consuming and includes many repetitive tasks. To speed up these tasks, CAD-FEM has developed a collection of tools, of C-Routines, using the CATIA V4-API. These routines are accessed through the CATIA- Menu and support the designer in creating the required volumes and meshes. Preliminary tests have shown a significant reduction in time due to the use of the Toolkit routines. 6. Implementation of VPS To implement the VPS simulation tools we recommend the following 3-step procedure: 1) Definition of a budget and a team for a pilot project. 2) Experts from CAD-FEM perform the pilot project in cooperation with the customer. The customer must supply data needed for simulation and must do all necessary testing for validation. 3) Installation of VPS software on customers site and training of users. Customer gets continuous support from CAD-FEM and / or CAD-FEM s local partner. For VPS/DRY a specific car body to be analyzed has to be selected and an existing finite element model has to be provided. For calibration of the drying kiln a test with about 25 temperature sensors has to be done.

8 The customer gets from the first project the temperature distribution for a car body as well as the data set for his drying kiln, which can be used for further analysis of other car bodies. For VPS/EDC we recommend a pilot project using a part of a car body structure to reduce the effort. A door structure, for example, represents sufficiently a complex geometry with nested cavities to capture specific effects such as clasps in slender cavities. A CATIA model of the structure has to be supplied to CAD-FEM as input for model generation using CATIA-Toolkit. To identify the material data of the paint, a test using a test box has to be performed either by CAD-FEM or by the customer. 7. Conclusion The software VPS provides powerful and validated tools to simulate the painting process. VPS helps to reduce cycle time, resources, pollution, and helps to improve the quality of the paint. This results in considerable time and money savings. The software is designed such that users with no specific FEM-background can use it and get results in a reasonable time. Before the tools can be used in production for analyses, several variant calibrations with the oven data and paint material have to be done. In the first analysis where calibration is needed, experts from CAD-FEM should be involved. The most mature module is VPS/DRY. It provides a method to simulate the curing process of base or top coat for a car body structure in the drying kiln. This technology is now applied at BMW Group for each new type of car. The application of VPS/DRY allowed BMW to postpone the purchase of a new dryer for a new type of car. Besides BMW a number of OEMs have ordered pilot projects in VPS/EDC provides a tool to simulate the thickness distribution of base coat by electro coating. A full scale model for a BMW car body has been generated and analyzed using CATIA-Toolkit and VPS/EDC successfully. The first release is available early VPS/DIP provides a method to simulate flooding of cavities. It predicts possible air bubbles in cavities or remaining liquid in pits after emerging the structure from the lacquer basin. It also determines forces due to buoyancy on the conveying system and deformations of the car body due to air bubbles. This tool is currently still under development. The first release is expected in mid CATIA-Toolkit is a software which generates volumes for the surrounding air and the cavities of the car body. From these volumes meshes for VPS/EDC and VPS/DIP can easily be generated. But they can also be used to generate meshes for electromagnetic or acoustic finite element analysis. Due to the modern knowledge based approach time for

9 generating a mesh can be reduced to one fourth of the time using modern commercial meshing tools. CATIA-Toolkit is available now. CAD-FEM will continuously improve the existing tools but will also explore new solutions for other problems like cavity sealing. 8. References Deleted: /1/ Groth, C.; Steinbeck-Behrens, C.; Klocke, C.: Schneller per Rechner, Automobil Produktion, Juni 2000 /2/ Klocke, C.: Die Berechnung des Aufheizprozesses bei der Lacktrocknung und die Auswertung durch Virtual-Reality-Methoden, VDI Berichte Nr 1559, 2000, VDI Verlag GmbH, Düsseldorf 2000, ISBN /3/ Klocke, C.; Böhm,O.; Groth,C.; Steinbeck-Behrens,C.: Virtuelle Lacktrocknung bei BMW, Journal für Oberflächentechnik Nr. 9, September 2000 /4/ Kern, S., Klocke, C., Opfermann, J., Steinbeck-Behrens, C.: Simulation of Paint Drying in Technical Every Day Life, NAFEMS Seminar Proceedings, Numerical Simulation of Heat Transfer, May 9-10, 2000, Wiesbaden, Germany /5/ Rother, K.; Steinbeck-Behrens, C.; Kern, S.: Towards a Virtual Process Chain for Body Painting: Modular Developments and Implementation Strategies,., 3 rd Intl. Strategy Conference Core Issues for Future Car Body Painting, 19./ , Berlin, Automotive Circle International (ACI)