The key to reducing the unit costs of coating and painting metal parts is the optimisation of all steps along the process sequence. Article by Doris Schulz.
Coating embodies a key technology which is indispensable for product manufacturing in practically all industry sectors. Coating of metal parts and substrates is essential for meeting a great variety of functional, decorative and haptic requirements with specified quality characteristics.
Where coating and painting processes are concerned, the goal is the same around the world: reduce unit costs per part. This is only possible where the specified surface quality and high levels of productivity are achieved with as little material and energy consumption as possible. Key to this is the optimisation of all steps along the process sequence.
Increased individuality and sustainability are further challenges which have to be mastered by job-shop coaters and companies with in-house operations. At the same time, growing competitive pressure necessitates the reduction of unit costs per part. An essential factor in reaching this goal involves use of the required resources in the form of materials and energy with the highest possible levels of efficiency. Points of departure for conserving resources can be found throughout the entire process sequence.
Simulation From Pre-Treatment To Final Inspection
The quality and economy of coating processes depend decisively on optimised interaction amongst various system components and the coating itself. Today, new and further developments in the field of numeric simulation make it possible to virtually replicate all of the steps of the painting process.
For example, inflow and outflow performance in pre-treatment, application with pneumatic, electrostatic, airless and high-speed rotation systems, airflow within the painting booth, film formation, development of paint films and layer thickness, overspray, the spreading of solvents and drying can all be represented within the model. If the right simulation instruments are used, the process pays off for smaller companies too for the efficient layout of painting processes and the optimisation of existing systems.
Optimising New Systems
The use of a new system often results in reduced material consumption. For example, liquid paints as well as powdered enamels are available, by means of which the specified quality of the coating can be achieved with fewer applications.
In the case of powdered enamels, these include, for example, so-called powder-on-powder solutions which permit coating without energy-intensive cross-linking. New systems are also being offered for wet-on-wet processes. In addition to reducing process cycles by roughly 20 percent, these also make it possible to reduce overall process costs and increase productivity without compromising quality. Paints which dry or cross-link at low temperatures also contribute to considerable cost reductions.
Increasing Transfer Efficiency and Reducing Loss
In order to realise potential savings within the painting process, possible causes of loss must be eradicated such as atomiser overspray, as well as losses due to colour changeovers, during application and in the paint supply lines.
Savings of this sort can often be implemented by modifying specific system characteristics. One approach involves increasing transfer efficiency by using a spray pattern which is matched to the geometry of the part to be painted, as well as electrostatically supported paint guns and spray guns with pre-atomiser technology and slot nozzle. Where automated application is involved, there is a trend towards high-speed rotary atomisers, with which transfer efficiencies of greater than 90 percent can be achieved.
More and more importance is being attributed to paint supply in the field of application technology. Ideally matched equipment makes it possible to avoid coating errors, simplify and optimise processes, and reduce production costs.
For example, the respective industry sector offers mechanical and electronic mixing systems for two-component plaints which are used to an ever greater extent for coating metal, plastic and wood substrates. They ensure precise dosing of the components and homogenous mixing of the paint. Beyond this, only the actual required amount of material is mixed. After painting or a colour changeover, only those system components have to be rinsed which are actually covered with mixed material. This results in significantly reduced paint loss and rinsing agent consumption.
Booths and Dryers
The trend towards automated application with robots remains unbroken as well. And it is no wonder because in addition to reduced material consumption, it also results in better reproducibility of the painting results and lower scrap rates.
Automated application has a positive effect expressed in terms of savings because it is easier to switch from fresh exhaust air systems to recirculating air for conditioning the paint booth. Energy savings of 60 percent to 70 percent can be achieved. For example, energy consumption can be reduced in painting booths for manual application by optimising air flow and precisely adjusting air volume, as well as by reclaiming heat from exhaust air.
Conventional wet scrubbing for overspray also consumes a great deal of energy and water. Dry scrubbing systems can be used to obtain savings in this area. Various solutions are available. They make use of different scrubbing concepts such as brushes, electrostatics, powder and special carton constructions.
In addition to dryers, conditioning and overspray removal are amongst the largest energy consumers in the painting process. Where conventional paint drying is concerned, optimised air routing through sluice and tunnel areas, as well as improved heating units and waste-heat utilisation systems assure pennywise consumption of costly energy. Infrared drying can be used as an alternative, or as a supplement, and is also suitable for temperature-sensitive workpieces. The radiation penetrates the material and dries the paint layer from the inside out. This prevents the occurrence of a skin or bubbles on the surface, and paint drying is accelerated. This frequently leads to shorter drying tracts and faster painting processes.
Knowing The Options
Selecting the right process is a prerequisite for a material and energy-efficient coating process with lowest possible unit costs: whether liquid painting, powder coating, a UV painting process or a combination is best depends on, amongst other factors, the goods to be painted, the required quality and the degree of flexibility.
And it pays off to closely scrutinise processes which have been in use for years or even decades, and to compare them with alternatives.
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