Electrostatic fluidized bed technology
The Electrostatic Fluidisierbetttechnik (TransApp) is a gun-free coating technology , offering combined with fast curing processes new possibilities for the production of compact and economical powder coating systems primarily for flat substrates , but also for suitable three-dimensional parts.
The classic powder coating systems, which work with electrostatic spray elements, can be used in many ways, but they require voluminous and cost-intensive booths and suction devices, with a high level of cleaning effort when changing colors and with a process speed limited to around 20 m / min.
Against this background, a powder coating technology was developed at the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) in Stuttgart, which allows process speeds of over 150 m / min and thus enables high-speed coating of two-dimensional substrates such as coils and wood-based panels. In these applications, which have so far been dominated by highly efficient liquid coating processes such as roller application techniques or casting processes, one of the greatest potentials for powder coatings lies in the surfaces to be coated.
principle
The basic principle of the coating technology is based on the electrostatic fluidization bed technology, which has been known for decades: The powder is fluidized in a basin with a floor through which air flows, i.e. it is brought into a liquid-like state. High-voltage electrodes arranged in the area of the fluidizing base ensure that the particles are charged and thereby transported upward to the earthed substrate moving past by means of electrical field forces and deposited there. However, the uneven coatings often occurring in earlier electrostatic fluidized bed systems have long prevented this technique from being widely used. The complex electrostatic and aerodynamic processes were therefore analyzed by means of computer simulations and approaches for redesigning the electrostatic fluidized bed technology were derived from them. The focus was on a high degree of flexibility in terms of substrate geometry and material, throughput speed, layer thickness and powder coating properties. The decisive improvements in this regard lie in new possibilities for controlling the electric field and the air flow. This means that all common powder coating systems, including fine thin-layer powders, can be applied evenly.
The extremely compact system dimensions are characteristic of the new fluidized bed technology, even with a high area throughput. With a 1.4 meter long and 0.8 meter wide system, horizontal surfaces up to 0.4 meters wide can be coated at speeds of over 150 m / min. Only a few percent of the powder throughput occurs in the form of "overspray". This can be recovered via the edge suction and fed back into the fluidizing basin.
With a conventional electrostatic powder coating system, the coating performance of the test system would theoretically require around 50 spray guns, but the process would no longer be manageable due to the strong air currents and layer thickness fluctuations.
With the new fluidized bed technology, suitable three-dimensional parts can also be coated, for example profiled wood-based panels, trapezoidal and corrugated sheets as well as rotationally symmetrical parts such as cans , bottles , spiral springs and the like. The electrostatic charge surrounding the workpiece facilitates complete coating of the parts. The considerably shorter coating times are a major advantage compared to traditional powder coating technology. In connection with the process speeds in the range below 15 m / min, which are usual for three-dimensional parts, this allows the design of compact and cost-effective powder coating systems.
Transfer application
A variant of the electrostatic fluidized bed technology was developed especially for lateral coating, in which the powder is first applied to a transfer belt and from there is transferred laterally to the substrate moving past by means of a high-voltage electrode. This principle was implemented for the powder coating of the outer surface or the bottom of cans and bottles made of aluminum at cycle rates in the range of three workpieces per second. The solvent-free powder coating is more environmentally friendly than the previous liquid paint coating. The better mechanical strength of the powder layer is z. B. used in floor coating to prevent damage to the floor during the production and filling process as well as scratches on sensitive furniture surfaces when placing the cans or bottles.
Fast burn-in technique
The rapid application of the powder coating usually also requires rapid melting and crosslinking of the powder layer. Conventional continuous air circulation ovens can only be used at very low process speeds due to the long dwell time of the workpieces in the oven. Medium- and short-wave electric or gas-powered infrared emitters with a high power density are used, which enable burn-in times of less than 10 seconds. The required high output is achieved either through very high radiator temperatures, such as with the electric near-infrared radiator, or through a very large radiant surface at lower radiator temperatures, such as with gas-powered bright radiators and electric carbon radiators. At lower radiator temperatures, the emitted power is distributed over a broader wavelength spectrum . IR emitters of this type can be used more universally with regard to different substrates and powder coating colors. In the case of three-dimensional substrates in particular, the higher proportion of thermal convection inherent in gas infrared emitters is advantageous.