Laser Curing of Powder Coatings

Recently, precisely this final heat treatment has increasingly emerged as the Achilles' heel of the entire process. Until now, it has usually been carried out in a gas-fired convection oven, which raises questions in view of the development of gas prices and the CO2 balance of fossil fuels alone. The fact that the oven process consumes as much energy as all the other steps of the coating process combined is an even more significant factor. Moreover, the ratio between throughput and energy consumption is so unfavorable that it is difficult to call it an economically viable process, not to mention the environmental concerns. This economic-ecological process deficit has several reasons: Since gelling in the convection oven requires thorough heating of the part to be coated, particularly with thick metal sheets, a long heating-up time must be accepted. As a result, the parts remain in the oven for up to 40 minutes, with corresponding negative effects on the energy balance. In addition, the long oven time reduces the throughput rate, which is also affected by the oven's air flow. Since this can stir up some of the electrostatically adhering particles, causing a powder flight, large distances between the components must be maintained during loading. Otherwise, there is a risk of critical contamination by different colored particles, especially with color coatings. Even this measure only partially protects against the risk of defects: If the oven process needs to be stopped unplanned – for example, due to a technical malfunction — the thermal inertia of the oven can lead to overheating of the coatings, potentially rendering the parts unusable.

The major advantage of curing with diode lasers is the precise and disruption-free energy input: While the entire component is heated during the oven process – and this is done by an air flow that proves to be disadvantageous for high throughput rates – a comparatively “cold” process is realized during laser curing: The precisely sized Ultra Wide Spot ensures that only the area that is actually to be coated is heated, without stirring up particles. The full output power of the laser is available immediately and is adjusted within milliseconds during the process: a closed-loop control system using a pyrometer that continuously measures the surface temperature of the treated area and adjusts the power of the laser accordingly ensures both seamless process control and the correct energy input. In the heating phase, this process design allows a 90 percent increase in the heating rate, which significantly reduces the component exposure time. Since the large-area laser beam does not cause powder flight, a more tightly controlled heat treatment and thus a significant increase in throughput rates is possible. Finally, the constant temperature control prevents the coating from being over-fired. If the component throughput must be stopped completely, the laser can be switched off in milliseconds and the energy input reduced to zero – except for an uncritical residual heat in the surrounding air, which is due to the radiation of process heat.