L
acado and more efficient coating of plastic parts quality, costs and sustainability in the spotlight.
by Doris Schulz*
The number of plastic pieces that are coated and lacquered grows non-stop, and also with a practically unlimited variety. The quality and individuality requirements of lacquered surfaces are also constantly increasing. At the same time, increasing pressure for costs and the requirement for organic production make more efficient, flexible and resource-efficient lacquering processes necessary.
Manufacturers are increasingly betting on plastic components, not only in the automotive industry. Today, there is virtually no sector that has nothing to do with these components made up mainly of macromolecules. When it comes to providing products with an attractive look and feel, an individual appearance, resistance to mechanical, chemical and physical attacks as well as special functional properties, lacquering comes into play. But it faces some challenges. These include optimized quality with increased profitability and sustainability. Add to this smaller and smaller batch sizes and a growing variety of colors.
Recognition of optimization potentials
Dirt deposits and other errors in lacquering lead to high costs of successive work and debris. Companies engaged in the lacquering of plastic components are particularly affected by this. On the one hand, this is often due to the large amount of dirt, starting with injection molding and going through deburring until reaching the strong presence of the top spray. On the other hand, most plastics used today tend strongly to form surface electrostatic charges that attract dust directly. To avoid errors in lacquering and optimize quality, it is worth looking at the entire process chain with a magnifying glass.
This begins, as far as possible, with a construction of the pieces that adapt to the lacquer. In this way, for example, a small number of perforations helps to optimize the lacquering capacity. Rounded and non-sharp edges of the components can also be better covered with lacquer. Deburring processes reduced to a minimum also contribute to optimizing quality, not only because the remains of the deburring or the particles of the removal of the overcollate can produce dirt that can reach the lacquering booth. The cleaning of tools also has its role: Due to the deposited components of plastics, errors can occur in the lacquering and penetration of dirt.
Optimization of cleaning and activation
These formula components, such as separators, additives, filling substances and fibers, frequently provide plastics with their good processing and use properties. However, if they reach the surface, these components can damage the lacquer. Another challenge is the low surface energy of many plastics. In addition, the trend towards water-based lacquers and reduced process coatings places higher requirements for substrate surfaces.
Reliable cleaning or pre-treatment of the surface is therefore essential. The trend in cleaning plastic parts is moving towards dry processes, such as cleaning with CO2 snow jets, plasma cleaning and steam cleaning with superheated steam. These often allow cleaning processes with reduced costs and space as well as integrated into the lacquer line and are also ecologically suitable. The electrostatic charge of the plastic surface can be combated by ionization.
To avoid adhesion problems in hard-to-lacquer and non-polar plastics, such as polypropylene (PP) and polyethylene (PE), their surfaces are activated before lacquering. Conventional processes for this are gas phase fluoridation, atmospheric pressure plasma activation, corona treatment and flaring.
Reduced lacquering processes
The reduction of the steps required for lacquering makes an important contribution to achieving an increase in efficiency and a reduction in the costs of the parts. The trend, therefore, moves towards lacquering processes that achieve the desired result by applying a single layer. The corresponding lacquering systems are available as solvent lacquers and as water-based formulas. Its use requires a corresponding pre-treatment of the surfaces of the pieces.
Approaches to reducing lacquering processes also include the elimination of potential sources of losses, such as superior spray sprays, as well as losses from changing the lacquer in the application and supply of lacquer. This can be achieved, among other ways, by using lacquer guns and high-rationalization atomizers with electrostatic support, which achieve a degree of application efficiency of more than 90%. Application systems that allow for minimized use of cleaning products and reduced color change times also contribute to savings.
Likewise, the trend towards automated application with robots continues uninterruptedly. It is not surprising that this is obtained, in addition to savings in materials, a great reproducibility of the result of lacquering and a reduction of the remains. A positive saving effect is obtained from the automated application of lacquer thanks to the facilitation of the change from fresh air-exhaust systems to surrounding air systems for the conditioning of the lacquer cabin. Between 60% and 70% of energy can be saved.
Smart lacquer logistics also contributes to reducing material consumption. This stands out, among other things, for the fact that the unprocessed lacquer from the ducts is recovered or transported to the application technology for special lacquers with precisely defined quantities of lacquer. This guarantees savings in the treatment of the lacquer also in the lacquering of small batch sizes.
In addition to dryers, top spray separators are among the most energy-consuming elements in the lacquering process. In the conventional lacquer drying sector, optimized air direction through lock and tunnel areas, as well as improved heating aggregates and exhaust heat harnessing systems, save you on managing expensive energy. An alternative to this is infrared lacquering, which can also be used on temperature-sensitive parts. The radiation penetrates the material and dries the lacquer film from the inside out. The formation of layers or bubbles on the surface is thus avoided and the drying of the lacquer is accelerated.
The classic top spray wet erosion requires a lot of energy and water. To achieve savings here, dry separator systems are available. For this, different solutions are available that work with different separation concepts, for example brushes, electrostatic energy, dust or special cardboard constructions.
Lacquering systems - adapted to individual requirements
In principle, a quality and efficient plastic lacquer requires a lacquering system ideally adapted to the substrate, the technology of the installation and the requirements. In the case of solvent lacquers, which are still widely used for the coating of plastics, solvent emissions are considerably reduced thanks to the so-called systems with high concentrations of solids (Very-High-Solid and Ultra-High-Solid). Water-based lacquering systems remain at the forefront. New advances whose formulas are based on innovative raw materials achieve the level of quality and strength of conventional solvent systems. They are available, depending on the degree of surface stress, as multi-layer systems composed of primer, base layer and light layer or coverage lacquers and light lacquers or as single-layer surface finishes with 1K or 2K systems in a wide range of colors and effects.
The systems are individually adapted to the different requirements and conditions of the application process. Among the novelties in the lacquering sector, among other things, a system designed especially for the coating of plastics with carbon fiber reinforcement. It stands out for its good adhesion, its high stability to UVA rays as well as its resistance to scratches and chemicals in accordance with the automotive standard. The base of the coating system is a transparent base layer with high strength. This makes it possible to achieve large layer thicknesses for each lacquering process and thus effectively and economically cover defects in the surface of composite materials. The clear lacquer to the base layer and the support material protects the material.
* Doris Schulz is a journalist and represents the organization of the PaintExpo fair – international leader fair in the field of industrial lacquering techniques, whose next edition will be held from April 17 to 20, 2018 in Messe Karlsruhe – Germany. www.paintexpo.com
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