Electro Deposit Coating for Honda - Acura TL

Coating of Car Bodies

Durable Paint Simulation with Virtual Paint Shop (VPS) for Honda - Acura TL
Avoiding damage to the car due to corrosion is understandably a high priority in the automotive industry. Honda relies on simulation to achieve a high quality of protective paint for the Acura TL.

Electrostatic painting is a method of painting used in the automotive industry to apply paint to the metal surfaces of the automobile. The electrostatic painting process starts with a bare bone car frame known as a body-inwhite or BIW. The BIW has been stripped of all its moving components leaving only the sheet metal parts that are welded together forming the general shape of the car. This BIW proceeds through multiple steps in the factory including electrodeposition coating (ED-coat) and several spray applications.
Most car drivers are unaware that their vehicle has been through a five-step paint process that results different layers of paint. Starting from the surface of the car body we have:

  • Pre-Treatment: Providing surface adhesion for the following layers
  • ED-Coat: Offers protection from corrosion
  • Primer Surface: Prepares the surface for the adhesion of the color
  • Base Coat: The color layer
  • Clear Coat: Gives a glossy finish and protects from weather
Coating of Car Bodies

The ED-coat is of particular importance. This layer provides shielding from moisture and other corrosive elements that can eventually damage the automobile. The process of applying the ED-coat requires dipping a negatively charged BIW into a positively charged bath of paint. This electrochemical process should result in the paint covering the entire car body, inside and out.

But what if it doesn’t?

An insufficient ED-coat can lead to poor corrosion protection during the lifetime of the car. Depending on company warranties, this could lead to a massive recall by the auto manufacturers. In 2001 alone, three auto companies were forced to recall nearly 1 Million cars due to issues related to corrosion1. Along with fuel efficiency, crash safety and the overall aesthetic appearance of the car, designers must take into consideration the BIW’s design with regard to coating.

Visual inspection on the processing line only tells half the story, it does not show missed areas. Common areas of concern for manufacturers are the enclosed volumes and cavities within the BIW. The latter are the most challenging areas where insufficient ED-coating often occurs due to the Faraday Effect (isolated regions with no electric current and therefore no paint deposition). These areas can’t be visualized during the process. Holes in the sheet metal will reduce the Faraday effect; however, they will have a significant influence on the structural behavior of the car body. Therefore, the concern for many automotive companies is developing a way to look inside those hidden regions to ensure sufficient coating without paint pooling on the material within the pocket.

Working together with BMW, CADFEM combined decades of knowledge in the simulation business with the automakers knowledge to develop tools for simulating the painting procedure.

Virtual Paint Shop (VPS®) developed by CADFEM GmbH consists of two main applications, one to analyze the ED-coat behavior in the paint bath (VPS/EDC) and the other to simulate the BIW in the drying oven (VPS/DRY). By utilizing simulation, engineers can determine the film thickness growth of the ED-coat as well as temperature profile and mechanical deformations due to high temperature loads during the drying process.

It is this film thickness analysis that was of interest to Honda R&D America’s, Inc. Working with CADFEM, Honda was able to confirm the ED-coat simulation with VPS/EDC for their development of new car bodies and achieve a virtual prototype in the painting area. This was validated by a stepwise approach and a detailed comparison between simulation results and measurements on real structures.

Because the processing line must maintain a certain speed, simply leaving the body in the paint longer is not a viable option. However, with the results from the ED-coating simulation engineers can change the geometry of the BIW to allow for a better coating in all areas of the body. At this point the model can be ‘cut’ within the virtual environment to observe the cavities and volumes that are typically problematic for the manufacturer during the painting simulation. In addition, simulation also has the advantage of easily showing the effects of changing voltages in the ED bath at different zones along the line.

In order to implement the Virtual Paint Shop simulation, an auto manufacturer must provide three things. The first and most fundamental is the car body CAD data. Honda provided a reduced body-inwhite that consisted of the most crucial sections of the car body. Since this is the focus of the analysis, it is the most essential part required for the simulation.

The second requirement is the general paint parameters. A calibration test must be performed from the paint used on the factory line. A simple, but essential, test involving two plates and an anode is performed and the characteristic such as the conductivity, electric current and throw power (paint’s ability to grow between two adjacent metal sheets) is measured. Temperature and other conditions can be varied during the calibration as well. Based on these measurement results, a parameter extraction is done to obtain material parameters for the specific paint line.

The third and final group of required input is the data pertaining to the processing and equipment. Aspects such as line speed, tank geometry, distribution of the anodes and voltage program are considered when setting up the simulation.

To truly appreciate the complexity of this simulation one must understand the deposition process. As mentioned earlier, the electrons flow from the anode to the car body (cathode) and a thin layer of coating builds up on the car surface. The rate of deposition decreases as time goes on. This is because the deposition rate depends on the current density which is a function of the surface resistance of the car body. This resistance depends heavily on the thickness of the ED-coat which is getting thicker every moment. By calculating the integral of this deposition rate CADFEM engineers are able to find the precise thickness of the coat at any point on the body and at any time step.

Those familiar with simulation and finite element procedures are well aware that a majority of the preprocessing of CAE models is spent creating a proper mesh. For paint simulation there is an additional effort in creating all required volumes besides the meshing process. The timeframe of meshing a full body-in-white model takes around five weeks to complete. However, by focusing on the most critical regions of the body to be simulated Honda was able reduce this to about three weeks.

Honda created a two step procedure for validation. The first step involved a rectangular tube positioned in the center of the vehicle as the initial test for the capabilities of the VPS tool. After the simulation confirmed the results from this test, a full body-in-white was simulated. VPS successfully predicted 100 % accuracy within the requisite tolerance range. There were over fifty measurement points compared with simulation results, distributed all over the car model.

Due to the importance placed on preventing corrosion automotive manufacturers are looking to simulation methods to predict the ED-coat thickness for their BIWs. The results of the simulations performed by CADFEM demonstrated to Honda that the VPS software was capable of predicting the ED-coat paint thickness in body-in-white structures. By integrating VPS in their product development, Honda was able to balance the performance of Corrosion, NVH (noise, vibration & harshness) and Crash prior to establishing specifications for their BIW structures.

read more (PDF 230 kB)