The article presents how a marked increase in the quality, efficiency and flexibility of paints for the automotive sector has been generated.
by Doris Schulz*
Varied customer demands, increasingly stringent quality requirements , ever-shorter model life cycles, the increased use of lightweight design technologies, the advancement of digitalization and the interconnection of manufacturing processes are all trends that keep the automotive industry (including vehicle painting) and its suppliers busy. Companies in the industrial paint technology sector are working on developing innovative solutions for every imaginable application that will allow them to rise to the challenges posed by these trends.
Car paint is traditionally expected to have an attractive appearance and provide ideal protection to the metal exterior. However, the demands have changed radically. For example, increasing individualisation is required for both the exterior and interior of cars, a requirement that has ultimately resulted in a single-unit manufacturing batch. It is increasingly common for vehicle bodies and components to be produced using combinations of materials that contribute to reducing the weight of the vehicle and therefore generate lower fuel consumption.
Trends in car sharing, electromobility and autonomous driving, as well as the advancement of digitalization and interconnection of production processes, are also giving rise to new tasks and requirements in the field of paint technology. Factors such as increasing the share of the first batch and improving system availability, as well as energy and material efficiency, play an important role in implementing these tasks in global markets. Companies in the paint industry are working to improve existing solutions and develop new ones, at all stages of the process, in order to achieve these goals.
Pretreatment adjustment
Composite designs, specifically, the use of carbon fiber reinforced polymer (CFRP) plastics, in combination with metal components, remains a challenge. On the one hand, numerous intermediate steps, such as sanding and putty application, are required to produce a class A surface. On the other hand, it is necessary to avoid electrochemical corrosion at the edges of CFRP components, which can occur after processing if loose carbon fibers come into contact with metal components. Different projects are currently being developed that address these problems, with the aim, among others, of developing a process for the proper sealing of the edges.
In the case of lightweight temperature-sensitive materials such as plastics and composites, high furnace temperatures, which range from 100 to 200 C after cathode coating by immersion, can also be problematic. Paint suppliers are working to solve this problem, for which it is enough to have considerably lower oven temperatures. However, it must also be ensured that other stages of the process, such as curing adhesives, sealants and alloys that harden by baking, can also be reliably performed at these temperatures. Quality inspection of cathode coating by immersion, a time-consuming process that is still performed with destructive methods, can be completely reduced or replaced with the help of proper simulations.
The traditional downstream water dryer pressure washing system is increasingly being replaced by alternative technologies such as CO2 snowblasting, plasma processes and steam cleaning techniques for the pretreatment of components made of plastic and CFRP. In addition to a considerable reduction in costs, space required and energy consumption, another reason to adopt this trend is the possibility of easily integrating and controlling these alternatives in an interconnected manufacturing environment.
Highly efficient painting processes
Paint without filling is not completely new, but it still has great relevance. The function of the filler layer is usually achieved by means of a water-based paint system for this integrated concept. Depending on the appearance, tone and quality levels that can be achieved with the no-fill process, the conventional process with filler, base layer and transparent layer is the most efficient alternative for cool (wet on wet) applications without intermediate drying.
Speaking of appearance, the current trend is towards greater polychromatism and a more individualized painting. The goal is to be able to paint without masking and significantly reduce the time and effort required. Unmasked paint has already been implemented in the aviation industry to coat the tail fins of airplanes, to which has been added the decoration without any excess spraying, thanks to the use of individual drops. The aerosol film is another method that can be applied with a very sharp edge and that is enough to remove after a while.
A new conveyor system that moves the bodies through the paint shop without using steel skids reduces energy costs, since it is not necessary to heat or cool the skids. In addition, this system makes the painting process more flexible because the speed can be adapted to the respective step of the process. In this way, it is possible to organize and optimize cycle times as well as performance with greater flexibility.
Decorative and functional parts for the vehicle interior and moldings, such as the moldings of the parales that are around the body, are usually manufactured by plastic injection molding. The surfaces of these components must meet very specific requirements in terms of optical and haptic characteristics as well as durability. These requirements can be met with the help of advanced solutions based on the RIM curtain coating process (reactive injection molding). The two paint components are injected into the mold during the mold or in-mold decoration process. If color is added as a third component, the 3-component mixing heads allow it to be injected directly into the part, and color changes are as quick and easy as with spray paint. Therefore, coated components can be manufactured by means of a single process.
In the case of conventional spray painting of plastic parts for the interior of the vehicle, single-layer systems reduce the number of process steps, resulting in higher performance and time savings and reduced material consumption. Newly developed painting systems can also be employed for the protection of fine wood decorations. Therefore, the top coating can be done in about 3 hours, either by a manual or automatic spray painting process. In contrast to the systems that were used before, this represents a time saving of 5 hours, which generates a considerable increase in productivity.
Better efficient application technology
As far as the application is concerned, the objective is to implement processes that allow coating, eliminating, or significantly reducing, excess spraying. Until solutions of this type are actually available on the market, the highest priority is given to increasing efficiency in the application of paint. Companies use various optimization possibilities to achieve this end, for example, the modification of air control units and rotating hoods, as well as the adjustability of the spray jet.
Thanks to their flexibility, atomizers for water-based paints can be used to paint various plastic components, as well as for combined areas on the inside and outside of the vehicle. The compact size makes access easy, even in geometrically complex areas. In addition to this, the new atomizers are distinguished by robust high-voltage technology and by generating minimal contamination, which reduces cleaning costs. When it comes to cleaning paint application systems, a new volatile organic compound (VOC)-free rinse technology allows for rapid color and paint system changes, and thus increases the flexibility and productivity of the painting process.
There is a marked trend towards dry cleaning systems to remove excess spraying. In this way, the air in the paint application booth can be made recirculating. In addition, the air dehumidification required for wet scrubbing systems is eliminated and the energy consumed for this purpose can be saved.
Innovative drying technology
The new dryers also save considerable amounts of energy and space in the car body painting process.
For example, an oven was created in which the burner and fans were integrated into the side panels of the dryer tunnel, which not only saves space, but also reduces flow loss. Temperature and airflow can be adapted to various types and areas of the body with the use of rotating nozzles. In the case of electric vehicles, for example, this ensures that the threshold panel, which has thicker materials, is heated to the required temperature, to the same extent as areas where substrates with materials of a minimum thickness are used. An additional feature of the new dryer is that its operation depends on the load, which also clearly contributes to the reduction of operating costs.
The inspection of the coating film in car bodies, which is still usually performed visually by production employees, is a source of fatigue and, therefore, errors. New solutions, for example, based on light tunnels, equipped with LED technology for energy savings, in which the bodies are illuminated homogeneously, and largely without glare, allow ergonomic workstations and a lower error rate. At the same time, the data obtained in the inspection can be obtained and transferred to the production control automatically.
Painting Workshop 4.0 and Machine Learning
A high-performance MES (manufacturing execution system) that monitors and controls production is the key to efficient manufacturing. The integration of the data obtained by the sensors of the various areas of the paint shop allows to configure the process more effectively.
If essential data about the state of the system, processes and products are analyzed and linked together, systems can also become intelligent and self-regulating using knowledge-based algorithms. Corresponding solutions are already available for various functions within the paint shop, for example, air management for paint dryers and preventive maintenance.
And the self-programming paint cell for coating single-unit production batches is no longer fiction. It was developed during a research project and allows an automatic painting process to be carried out in five steps: first, the object to be painted undergoes a three-dimensional scan and a dynamic fluid simulation is created based on the scanned data. The trajectories of the paint droplets and the ideal amounts of paint and air for the desired thickness of the layers are determined. As a third step, the system conceives the best possible path of the robot for the painting process from this data. Next, the effective painting process occurs, after which the quality inspection is carried out, the fifth step, by using terahertz technology.
* Doris Schulz is a journalist and represents the organization of the PaintExpo fair – international leader fair in the field of industrial lacquering techniques, whose last edition was held from April 17 to 20, 2018 in Messe Karlsruhe – Germany. www.paintexpo.com
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