Painting Trends in Plastics Processing
by Doris Schulz
Flaming causes the occurrence of polar groups within the surface of the originally non-polar substrate, thus allowing for good paint adhesion.
In order to create a precious-metal look for these cosmetics packages, the plastic surfaces have been metallized by means of a PVD process. Next, they received a pigmented clear paint layer as a top coat, which is resistant to the chemical and mechanical stressing associated with daily use.
Quick curing of UV paints in just a matter of seconds contributes to a significant increase in production rates.
In contrast to conventional wet scrubbing, the E-Cube system allows for effective, dry removal of paint overspray in the recirculating air mode, the supply air mode or the exhaust air mode, along with significantly reduced energy consumption for air conditioning.
An innovative RIM curtain coating system with self-healing effect is opening up new perspectives for the coating of high quality parts made of plastic, wood and metal.
- PaintExpo is an international trade fair for industrial coating technology. It will take place April 8-11 at the exhibition centre in Karlsruhe, Germany. The expo covers the entire process sequence in the field of coating technology and offers a comprehensive overview of the latest developments in the areas of liquid painting, powder coatings and coil coating, from pre-treatment to quality control. The exhibition program covers equipment and application technology, paints, drying and cross-linking systems, conveyor systems, automation solutions and painting robots, pretreatment, measuring and test equipment, quality control, environmental engineering, filtration technology, accessories, consumable materials, services, paint stripping and technical literature. Learn more at www.paintexpo.com.
For plastic parts, painting is more than just a protective skin. It provides the product with optical and haptic characteristics that decisively influence the customer’s perception, as well as his or her buying decision. Because painting also is one of the most energy-intensive processes in the manufacturing sequence, costs must be kept under control. As a consequence, the painting technologies industry is working on solutions that promote efficient use of resources while optimizing quality.
Whether the automotive industry, household appliances, consumer electronics, the cosmetics industry or toys are concerned, painting plastic parts is becoming more and more common. Optimization of the qualitative characteristics is one of the challenges in this respect. On one hand, this involves improving functional characteristics such as durability and resistance to physical, chemical and mechanical influences. On the other, emphasis is placed upon improved optics and haptics that serve the purpose of, amongst other things, individualization. Rising cost pressure from global competition and demand for environmentally sound production are making it necessary to lay out the painting process in a more efficient, resource-conserving manner. As a result of decreasing lot quantities and an increasing variety of colors, enhanced flexibility is a further issue that concerns companies with in-house painting operations.
Surface cleanliness has a decisive influence on painting results. In order to conduct cleaning as efficiently as possible, more and more companies are making use of alternatives to conventional power washing systems. Amongst others, these include CO2 snow blasting and plasma technology. Particulates, as well as film-like contamination, can be removed with the help of CO2 snow. Plasma cleaning, using low pressure plasmas and plasmas at atmospheric pressure, is suitable for removing thin layers of organic contamination. Both processes allow for dry cleaning and can be integrated into the painting process in a space-saving fashion.
For example, pre-treatment by means of a flaming process is necessary in order to avoid adhesion problems from excessively low surface tension, as in the case of non-polar plastics. This involves briefly exposing the plastic surface to a gas flame with excess oxygen. As a result, molecular bonds in the surface of the substrate are broken down and free, active ions in the flame are integrated. Consequently, polar groups occur within the originally non-polar material, which allow for good paint adhesion. Corona treatment can be used as an alternative. It’s conducted with an alternating voltage electrode by means of which atmospheric oxygen is ionized with the help of corona discharge, resulting in oxidation of the plastic surface.
Simpler processes for perfect surfaces
Reducing the number of steps required in the painting process makes a significant contribution to increased efficiency. In addition to reduced energy consumption, this also results in less material usage, lower emissions and a smaller footprint for the painting system. Consequently, the trend is moving toward painting processes that provide the desired results with just a single coat. Painting systems for lean processes of this sort are available with solvent and water-based paints. Use of these systems necessitates corresponding preparation during production of the respective parts – for example, assurance of flawless, glossy surfaces. If this challenge is met successfully, it’s even possible to omit pretreatment in some cases.
Paint solutions for all cases
Solvent-based paints still are widely used for coating plastics. Very-high and ultra-high-solid paints are available in order to further reduce the emissions that result from these paints. These paints are distinguished by a very high solid content that considerably reduces solvent emissions. Utilizing solvent-based paints frequently necessitates only minimal adjustments to existing painting systems. In the case of water-based paint systems, new developments also are contributing to optimized quality and greater efficiency.
UV technology allows for cost-effective, low solvent or even solvent-free coatings are associated with various other advantages as well. These include considerably reduced process times and smaller painting systems, as well as quality aspects such as high degrees of gloss and highly resistant, scratch-proof surfaces. This technology is penetrating more and more applications thanks to new developments in the areas of both paints and painting systems. And thus, UV curing is making it possible to paint more and more complex workpieces in an inert gas atmosphere. The oxygen-reduced atmosphere that results from the use of, for example, CO2 or nitrogen, prevents the radicals required for the polymerisation process from reacting with oxygen in the atmosphere, thus ruling out the possibility of so-called oxygen inhibition. Curing can take place at larger intervals, and areas which are subjected to significantly less UV radiation are better cured as well.
In addition to gas discharge tubes, LED radiators also are used for curing UV paints in the meantime. Amongst others, the advantages of this curing technology include reduced temperature stressing of the substrate and the system due to the use of cold light, immediate readiness for operation without any standby mode and LEDs which do contain any mercury or generate any ozone. On the other hand, UV LED curing systems generate exclusively monochromatic light, which means that only a small wave window is available. Other paint formulations become necessary as a result.
So-called combination surfaces and plastic parts with chrome effect are also in vogue – not only in the automotive industry, but in the household appliances, mobile phones and cosmetics packaging industries as well. Plastic parts are chrome plated by means of either conventional electroplating or PVD coating. Conventional pigmented systems and UV-curing paints are available for protecting the metal layer after the coating process, and for providing it with an individualized appearance.
Regardless of the utilized painting technology, high quality, efficient painting of plastics always necessitates a paint system ideally matched to the substrate, the utilized system technology and the requirements specified for the finished product. This frequently results in paints specially optimized for the respective application, or newly developed.
Resource-conserving, cost-saving removal of paint overspray
In addition to dryers, air conditioning and overspray removal are amongst the hungriest energy consumers in the painting process. One approach to generating savings is based on optimized removal of overspray, which occurs during the painting process, because conventional wet scrubbing consumes enormous amounts of energy and water. This problem is solved by a new mechanical overspray removal system that functions without any water, chemicals or additives at all. The modular system can be operated in the recirculating air mode, the supply air mode or the exhaust air mode and is suitable for painting systems of nearly any size. Retrofitting existing painting systems, which have used other removal systems in the past, is trouble-free.
Innovative curtain coating system
An innovative curtain coating system is opening up new perspectives in the area of reaction injection molding (RIM) for high-quality parts made of plastic, wood and metal produced in large quantities. This modular paint system is based on 2-component, solvent-free, aliphatic polyurethanes that are processed in high pressure systems. Coating to a thickness of 0.1-2mm is accomplished in a single process step. The part to be painted is inserted into an oversized mold to this end, and the remaining space is filled with paint. Depending on part geometry and material type, coating and cross-linking take two to three minutes. The part can then be removed from the mold and further processing is possible after roughly 24 hours, although no grinding is necessary. In contrast to multi-coat painting, this process not only offers time savings, it’s also highly efficient thanks to minimal material loss.
The surface also fulfils the requirements of the automotive industry and is distinguished by good resistance to heat and UV radiation. Surfaces with better scratch resistance and a self-healing effect also can be produced in this way. If the surface is scratched, the paint system’s self-healing function becomes active at room temperature, or it can be triggered by applying a minimal amount of heat.