Simulation on Car Body Painting Processes Gerhard Zelder, Dr. Cord Steinbeck-Behrens CADFEM GmbH, Grafing b. München, Germany 1 INTRODUCTION For car body development processes the time frames for new models become shorter and requirements on reliability and quality are increasing. This situation also changes the requirements in the planning of car body painting processes. Quality and durability against corrosion have to be assured without trials on car body prototypes. Therefore simulation of car body painting processes becomes more and more important for evaluation of the paint processes and car body design. The effect that design changes or process changes has on the quality must be predicted. To reach these requirements it is very helpful to use modern numerical methods complemented with special features for painting processes. Sometimes it is the only way to avoid costly and time consuming tests. In recent years some methods were developed to do this simulation efficiently, even on a full car body. Simulated processes in the focus of this article are: Dipping process (dip into a liquid and evaluate remaining air while the structure in the liquid and remaining fluid after the dipping process) Film build while cataphoretic painting / electro deposition painting Temperature while drying in an oven Painting with spraying, also spraying in combination with an electro static field / Electro Static Coating UV-Curing The description is based on the solutions offered by the CADFEM GmbH. Most of the methods are being developed by CADFEM, with continuous improvements and maintenance of the methods over the last 10 years. 2 DIPPING SIMULATION WITH VPS/DIP The dipping simulation provides the virtual behaviour of a car body while the dipping process. While dipping, the air regions captured by the car body below the fluid level is of the most interest. This air is preventing either a painting, cleaning or surface treatment process. After dipping, the next area of concern is the remaining fluid. These fluids can pollute successive bathes or disturb a following drying process. 2.1 Simulation Method With the principles of conservation of mass for air and fluid and pressure dependant compression of air an equilibrium state of all pressures is found. In this state, where each individual region is continuously filled with air, the pressure is assumed to be constant. The
pressure in the Fluid is a function of vertical distance to the fluid surfaces. For the virtual dipping process the volume surrounding the car body is meshed with a Finite-Element mesh. This mesh includes all details of the car body. For the simulation software an algorithm sequentially identifies the flow of fluid and air until the equilibrium is reached. The algorithm is developed solely for this task. Figure 1: Areas of fluid (blue) and air (red) on a dipped structure similar to a siphon trap 2.2 Requirements and Prerequisites The shape of the car body is represented by a tetrahedral mesh. All the volumes (inner and outer regions) have to be considered in a volume mesh, connected to each other at the numerous holes and connections. The whole model is included in a bounding box. The surface of the bath is defined with respect to a car body coordinate system. A dipping curve defines the transition and rotation of the car body while moving through the bath. 2.3 Results and Findings During the dipping process, the trapped air can be visualized, where no paint on the surface will be deposited. After dipping out of the paint bath the remaining fluid in the car body can be shown. The amount of paint volume can be calculated, in every part of the structure, where fluid remains. Modified dipping curves can be examined easily. Also, variations of the holes for the inlet or outlet of the fluid can be compared.
3 FILM BUILD WHILE CATAPHORETIC PAINTING WITH VPS/EDC The car body is painted with widely-used, but special technique, the Cataphoretic Painting (or: electro deposition coating, EDC). Here, inner surfaces or regions that are difficult to access, e.g. A-/B-/C-pillars) are painted. The goal is, to get a minimum thickness for optimized corrosion protection. 3.1 Simulation Method With the finite element method the electric potential surrounding the car body is evaluated. The resulting current density causes the growth of the film thickness. The electrical resistance on the car body surface is dependant on the paint thickness. This is changing the electric field and therefore also changing the film thickness growth. With growing resistance on outer surfaces a higher current density on unpainted and initially shielded inner regions occurs. Figure 2: Characteristics of the wrap-around effect 3.2 Requirements and Prerequisites As described for the dipping simulation, a mesh surrounding the car body in all details is required. The mesh must include the shape of the bath and the anodes. The mesh must be fine enough to evaluate the electric potential also in small holes or gaps with the required precision. The time dependant voltage at the anodes describes the process. Material properties of the paint in the bath and in the film must be found for each paint compound. Usually this is unique for each paint shop. Therefore a workflow has been developed to get the required parameters out of measurements on a small test geometry by comparison of the results of measurement and simulation. Once the required material properties are available they can be used on all different car body designs painted in this bath. 3.3 Results and Findings One direct result of the software is the film thickness on the surface of the car body. Effects of different designs and processes to apply paint on inner surfaces can be evaluated. Critical regions can be highlighted with the contour plot.
4 TEMPERATURE WHILE DRYING IN AN OVEN WITH VPS/DRY During the painting process a car body is passing through several drying stages. Besides drying and hardening of paint the oven process has to fulfil requirements for bake-hardening effects, curing of adhesives or expanding of foam parts. Because of additional requirements such as limited overall process time, fast heating and cooling, the mechanical load due to temperature expansion can be critical. 4.1 Simulation Method For the temperature simulation in a structure, boundary conditions for convection and radiation must be defined. Evaluation of these boundary conditions at each surface of the car body is a complex task. For this purpose the geometry of the car body is analysed automatically. Results of the geometrical analysis include: the covering by additional sheets, influence of nozzles and radiation effects of heated oven walls. Properties which depend on the time or the locations, like air temperature or convection conditions, define the simulation model of the oven. 4.2 Requirements and Prerequisites A shell mesh of the car body describes the car body. One benefit is that a mesh generated for crash or fatigue analysis can be reused for this simulation. The oven is divided into zones to define time dependant temperatures and properties of nozzles and radiation walls. Usually the required information is not available with satisfying amount. Therefore a workflow has been developed to get this information in a process of comparing measurement results with simulation results. An oven description verified by this process can be reused for all new car bodies in this particular oven. 4.3 Results and Findings The basic result of the drying simulation is the time dependant temperature of the car body. This result can be used to plot an oven curve for each location or to generate contour plots on the car body. Often the requirements are given in times above temperature levels. Also the time above a required temperature level can be shown. The temperature can be used as an input value for subsequent simulations such as the mechanical deformations or stresses, or the viscous behaviour of the paint or adhesives. A link to formal kinetic algorithms allows the simulation of temperature dependant chemical processes like the hardening process.
EASC 2009 Figure 3: Temperature distribution on a car body 5 PAINTING BY SPRAYING WITH VPS/ESC Several layers of the paint like basecoat, topcoat or clear coat are applied by spraying processes. Sometimes an electric field is used to reduce overspray. Geometrical issues a sink at the door grip or edges are a reason for non constant thickness of applied paint. 5.1 Simulation Method Starting at an atomizer the movement of paint droplets in the influence of air flow and electrical field is evaluated. Electrically charged droplets influence the electrical field. The film build is computed from the number of droplets per unit of area on the car body surface. In the first step of the simulation the film build is simulated at several representative locations along the path of the atomizer. In the second step a special solver is creating a time dependent result out of the previous result based on the movement of the atomizer. 5.2 Requirements and Prerequisites Droplet size and droplet density are measured in laboratory experiments and transferred as initial conditions very close to the atomizer. A simulation mesh must be generated surrounding the region near the atomizer. This must be done for each type of atomizer. The region between the mesh surrounding the atomizer and the car body is meshed automatically for the representative locations during the simulation. For this workflow the required surfaces of the car body must be inserted in this simulation by a surface mesh.
5.3 Results and Findings The primary result of the simulation is the distribution of the thickness on the surface of the car body. It is possible to modify process parameters like the environmental air flow, air flow rate of the shaping air, voltages or the path of the atomizer virtually. 6 UV-CURING BY VPS/UV In UV-curing processes the chemical reaction used for hardening is activated by UV light. With the simulation, the intensity and the exposure of UV light can be computed without an experiment. Therefore a cost efficient development of a process plan is also possible for complex geometries. 6.1 Simulation Method The intensity of the UV light in the influence region of a UV lamp is described by a lamp model. This lamp is moved virtually as defined in a process definition. Taking into account shadowing and reflection the light intensity absorbed on the surface is evaluated. 6.2 Requirements and Prerequisites A lamp model required for the simulation usually is defined out of measurements done in a laboratory. Some often used lamps are already available in the standard version of the lamp library within the software. For surface description of the parts the required mesh can be reused from other simulations or generated out of CAD geometry. 6.3 Results and Findings The intensity and the exposure/dose of UV-light can be shown. Upper or lower limits for exposure or intensity can be used for specific highlighting of the results. Figure 4: UV Dose rate on a wheel rim 7 FURTHER INFORMATION Please find further information on the webpage www.virtualpaintshop.com.