Appearance, costs, sustainability, and quality in focus

Optimisation efforts implemented by the automobile manufacturers and their suppliers are focused on quality, sustainability, costs and individualisation.

Originally, paint simply served to protect automotive body components from corrosion. In addition to this technical function, which nowadays includes protection against mechanical, chemical and physical stressing, paint also has to fulfil sales promotional aspects: paint finishes for the body, the exterior and the interior of the car make the automobile an emotional experience, offer options for individualisation, increase the value of the vehicle and provide for surfaces which appeal to all of the senses. However, the quality of the coating has to continuously fulfil strict requirements in order to assure that this alluring effect is properly achieved. The painting process is also influenced by an expanding mixture of materials used for auto bodies and other components as a result of lightweight construction strategies. Furthermore, rising cost pressure due to global competition and ever stricter regulations for environmental protection, as well as ecological orientation, necessitate more efficient, resource-conserving painting processes.

Discovering and exploiting potential savings

More than half of the energy required for auto body fabrication is currently consumed by the paining process. Whether or not and which type of potential savings can be taken advantage of in the area of spray painting has been investigated by the joint project known as “Green Carbody Technologies – InnoCaT 5 – Energy-Efficient Painting”. Together with equipment and paint manufacturers, as well as the department for coating systems and painting technology at the Fraunhofer Institute for Production Technology and Automation (IPA), two renowned German automobile manufacturers took a close look at the painting process in 3 subprojects.

The first subproject involved coating systems without any loss of paint. Painting without overspray is considered the most effective solution for minimising energy and material consumption in the spray painting process. The high levels of energy required for spray painting systems result primarily from the need to condition air flow within the spray booth. This is necessary in order to remove up to 40% of the overspray. On the one hand, painting without overspray opens up potential savings where the material itself is concerned. On the other hand, it allows for considerably leaner systems technology. Three different approaches were taken into consideration: an adapted inkjet process, targeted generation of droplets by means of micro-dosing technology and defined droplet size by means of vibration isolation. The trend towards vehicle individualisation through the use of multi-coloured paint finishes provides an additional motive for conducting research on painting process without any loss of paint and selective coating processes. After all, complex processes are currently required to this end with considerable amounts of manual labour for masking body parts, as well as double and multiple runs which involve longer cycle times and reduced throughput.

The second subproject is dedicated to energy-efficient drying. It has brought various measures to light which can be implemented in the short or mid-term, by means of which primary energy consumption at the paint drying system can be reduced by roughly 30%. Amongst others, solutions which can be efficiently implemented in existing painting systems (brownfield) for drying include heat system optimisation, the use of innovative drying nozzles, a reduced exhaust air rate and requirements-oriented control of volumetric flow for recirculating air (energy on demand). When planning new painting systems (greenfield), it’s advisable to examine whether or not the following measures for energy-efficient drying can be implemented: skidless auto body transport, partial exhaust air purification concept, heating units with integrated exhaust air purification and cyclic operation instead of continuous feed.

The third project involved a modular product and painting process. The results of the associated R&D work prove that modular auto body fabrication leads to significantly compacter and more efficient painting of the individual modules. At the same time, this fits in well with the trend towards lightweight construction, for which the use of a great variety of materials is absolutely imperative, for example high tenacity steels, aluminium and magnesium, as well as numerous types of plastic. The fabrication of auto bodies as a framework construction with modular attachments is one possible approach. The various substrates are fed through specific pre-treatment systems. Only the attachment parts are then painted, which frequently have large surface areas. UV technology, for example – with all of its advantages – as well as radiant drying, could gain in significance as a consequence.

Various measures for optimising energy efficiency examined during the course of this project can be implemented on short notice. For others, it will be years before mass produced products are available, and for each of these technologies it must be examined for which applications it will be suitable and economical. Independent of this R&D project, the painting technology industry offers solutions for improving material and energy efficiency, and at the same time for optimising quality in series painting operations for auto bodies and components.

Resource-conserving pre-treatment

More and more frequently, nano-ceramic processes provide for corrosion protection and optimised paint adhesion instead of conventional zinc phosphating when pre-treating auto bodies and metallic components. This trend is based on the ecological and economic advantages of systems which are compatible with numerous metals: products which can be applied by means of dip and spraying processes are free of toxic heavy metals. This reduces the effort and costs involved with wastewater treatment and disposal, as well as system cleaning and maintenance. Beyond this, coating is also possible at room temperature. This further reduces costs and emissions.

In the meantime, environmentally friendly alternatives to cathodic dip painting are also available. These alternatives contain less the 1% solvent and are tin-free, which makes them compliant with future European legislation, as well as other regulations. Dip painting is distinguished by ideal coverage and good corrosion protection.

Destructive testing is still state-of-the-art for quality control of cathodic dip painted components. This costly inspection process can be reduced, or entirely replaced, by means of suitable simulations.

Nor is the traditionally used power washing system for the pre-treatment of plastic parts with downstream retained-water dryer indispensable anymore. This cost, space and energy intensive variant is being replaced more and more frequently with alternative processes such as CO₂ snow-jet cleaning, plasma processes or the steam cleaning method.

Making painting processes leaner and greener

Due to the required drying process, water-based paint systems consume more energy and emit more CO₂, but they nevertheless have a better ecological balance sheet than solvent systems. The reason for this is the minimal VOC emissions associated with water-based paints. Thanks to wet-on-wet application of the primer, the base coat and the clear coat without intermediate drying, economic and ecological advantages can be exploited with conventional painting processes. On the one hand, less energy is consumed and emissions are reduced in this way. On the other hand, less production time and shorter painting lines also speak in favour of the solution without intermediate drying. So-called integrated processes, in which filler coat application and associated drying are eliminated, allow for reductions in energy consumption and CO₂ emissions by up to 20%.

Eliminating possible sources of loss such as overspray and loss of paint at application and feed equipment during colour changeovers also contributes to a more efficient painting process. Modifications at isolated points within the process frequently lead to the desired results – quickly and economically. One approach involves increasing transfer efficiency by using electrostatically supported paint guns and high-speed rotary atomizers. Transfer efficiencies of greater than 90% are achieved with the latter. The increasing degree of automation through the use of robots is also leading to reduced paint consumption, for interior painting of auto bodies as well. The use of painting robots also makes it possible to switch over from fresh air exhaust to recirculating air systems. This can result in energy savings amounting to as much as 60 to 70%.

Dry overspray scrubbing

Conventional wet scrubbing of overspray which occurs during the painting process consumes a great deal of energy and water. Savings in this area are made possible by dry scrubbing. Electrostatic precipitation systems are one possible alternative. With this solution, overspray is fed to the precipitation system over the entire length of the painting area. The air charged with paint particles flows through the alternately arranged active and passive elements of the precipitation system. The paint particles are effectively precipitated and removed from the process. Due to the fact that no mechanical filter systems are required, constant flow conditions prevail in the painting booth without any pressure fluctuation, thus resulting in ideal conditions for good painting quality. At the same time, air conditions are altered only minimally due to precipitation. Consequently, most of the purified air can be fed back to the painting booth. Depending on which type of paint is being used, this allows for up to 95% recirculated air. Furthermore, as compared with conventional wet scrubbing, up to ¾ of the required energy can be saved, and water consumption can be reduced by more than 85%.

A “simpler” option for dry scrubbing is a mechanical overspray separation system of modular design. This solution, which can be operated with recirculating air, as well as with supply and exhaust air, functions entirely without water, chemicals, or any additives. The air charged with paint particles flows through a hybrid filter, i.e. a combination of surface and depth filters. The individual filters are comprised of receptacle and chamber-like structures. This provides for good coarse and fine filtering. A second, separate filter stage for assuring that the desired degree of filtration is reached is located downstream from the separation modules. The modules can be set up and replaced easily and quickly by untrained personnel. This system, whose modules consist primarily of recycled materials, also makes it possible to retrofit existing painting systems.

By: Doris Schulz