Finding the cure

With ultraviolet-hardened paints becoming increasingly common in the car industry, curing the paint still remains a tricky process that can involve a great deal of trial and error.

In an attempt to address this problem, a team from the Fraunhofer Institute for Manufacturing and Engineering in Stuttgart is using computer simulation to find a way to ensure that the coating is deep, crisp, even — and durable.

Old-style car paints tended to be applied as a liquid spray, with the pigment and film-forming compounds dissolved in an organic solvent. But improvements in coating technology and concerns about the levels of volatile organic compounds in the workplace and environment have led to these substances being replaced with powder coatings.

In these, the pigment is mixed with a polymer matrix and powdered, and it’s this powder which is sprayed on to the car body. It adheres only lightly — often assisted by a static electrical charge on the metal body — and is transformed into the continuous paint film by heating, to melt the individual particles, followed by UV curing.

This process involves intense UV light being shone on to the body, causing changes in the polymer which knits the molecules together with chemical cross-links, creating a dense network. This not only has the prized smooth, shiny appearance of a new car, but also provides far greater levels of protection from scratches and stone-chips than old-style solvent-based coatings.

However, applying the UV light is far from simple. Before the painting process can begin, engineers have to determine three factors. The number of UV lights needed to harden the paint across the whole surface of the car. where they will need to be positioned. and how they will have to be moved.

Successful placement will result in a coating which has hardened evenly everywhere. but if the radiation is too strong, the paint will be brittle, and if it’s too weak it will be soft. Until now, the only way to work out the correct placement has been to put the car in a test facility and try different arrangements until the right one is found.

The Fraunhofer team, led by Andreas Scheibe, is working with a major car manufacturer to develop a simulation process that will predict the best locations and movements for the lights, without needing a test facility. Called DLS-UV, the system determines the level of UV irradiation provided by the lamps at the surface of the car.

‘It uses an algorithm that very quickly calculates the local distribution of radiation on the object,’ said Scheibe. The crucial factor for even hardening is that the correct amount of UV falls on every part of the surface to promote the necessary reaction over the curing time period; the simulation allows engineers to zoom in on particular areas of the car body and simulate time-dependent irradiation. ‘After only 30 minutes, they know whether the chosen arrangement of lamps is really the best, or whether they need to adjust it,’ said Scheibe.

The system is embedded within the programs that control the application of the powder, allowing the whole coating process to be integrated together. ‘Expanding the DLS program to include modules that deal with light and radiation enabled us to complete the virtual process chain for painting 3D workpieces,’ said Scheibe. The system is currently in the test phase, and could be introduced commercially as early as the end of this year.