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Precision spray painting technology

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Electrostatic airless rotary atomisers were developed for use in car painting in the 1980s and ‘90s, primarily for their economic use of paint, where 90% of the paint can be applied directly onto the bodywork, improving greatly on earlier spray technology. Applying opposite electrostatic charging to the paint and the receptor body ensures good adhesion and applies a thicker paint coating to edges of panels that are most prone to chipping and abrasion. Rotary atomisers of this kind use a rotating disc, to which the paint is applied, where it forms a thin film on the rapidly spinning disc which becomes increasingly thinner towards the disc edge where it breaks down into droplets whose size can be controlled by the speed of rotation. The paints most commonly used were solvent-based two-packs, usually of a pre-mixed acrylic resin and an isocyanate hardener, the latter having potential significant health hazards for spray operators, requiring complex safety measures. Robotisation of the paint application process, whilst vastly improving spraying accuracy and productivity, have also virtually eliminated those human health hazards. Significant paint technology changes in recent years have seen the progressive switch from solvent-based paints to water-based paints, avoiding hazardous solvents and the necessity to recover such volatile organic compounds (VOCs). Non-isocyanate paints are also now widely used in car production, and the best water-based paints have very few shortcomings, and some clear benefits, over the older solvent-based paints.

Now, though, we are seeing significant developments at the very heart of the vehicle paint spraying process itself, with the introduction of stunning new spraying technology. The Swiss company ABB’s PixelPaint system uses technology similar to that employed in familiar ink-jet office printers. It generates minute individual paint droplets rather than a jet spray, that enables vastly improved spraying precision, although as yet we have no detail of the nature of the paints involved. The inkjet head incorporates over 1,000 tiny nozzles and the size of the paint droplets can be varied from 20µm to 50µm, (1,000µm = 1mm) enabling more precise control of the paint film thickness, and accuracy of application. A PixelPaint assembly can use multiple paint colours to apply both single colour finishes and complex multi-colour distributions to the vehicle body. It does this with unbelievable speed and accuracy compared with the traditional methods of multi-colour spraying. Customised designs that usually involve multiple spray booths and highly labour-intensive masking of panels, are accomplished with ease. ABB is apparently at present mainly targeting the customised body painting market, where today’s consumers are, they say, increasingly demanding such individualised paint schemes. By enabling the multi-tone paint job to be applied on a single run, it can reduce the cost and improve productivity by up to 100%.

Somewhat in parallel with this, the German company Dürr has worked with BMW in developing PaintJetPro, again a fully robotised painting system, with somewhat similar objectives of improving painting speed and accuracy, but with a rather different approach. The robots employ spray equipment using a newly developed nozzle plate with more than 50 tiny precision-machined holes on its surface, which generate precisely metered jets of paint that are projected onto the car body. After consideration and research, Dürr decided themselves not to pursue the inkjet principle, as they established that car paints, being more viscous than ink, are more difficult to use in that way. A single blocked nozzle could cause ugly and expensive mistakes in the painting line. But they involved major car paint manufacturers in their own nozzle plate development work, to get the paint properties and quality right. They also overcame another hurdle, in that, even in top quality cars, two bodies of the same model still have dimensional differences. Discrepancies of up to two millimetres are not uncommon – and that’s too much to guarantee the perfect paint finish. So, prior to painting, a camera scans the entire body, or panel surface, in a few seconds and compares the measurements with the reference data. A software program in the control system then appropriately corrects all the lines of paint due to be applied by the applicator.

So these two items of new painting technology, possibly competing, may well become commonplace, particularly with increasing pressures on waste control and productivity, with the scope for more exciting and complex colour schemes.

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