By Doris Schulz
The variety of ways in which plastic parts are painted is nearly unlimited today – colourful, matte or glossy, with effects or functional characteristics.
The rules for painting plastics are stricter than ever before.
Good quality and economy, material efficiency, low energy consumption and flexibility are becoming more and more important.
As a result, opportunities for process optimisation must be fully exploited.
In many industries, plastics make it possible to produce parts which weigh less, are less expansive and include integrated functions.
Painting is used when the product needs to be provided with an attractive appearance, resistance against mechanical, chemical and physical stressing or special functional characteristics such as pleasant haptics and resistance to creams.
Demands placed on coating quality have increased enormously in this respect in recent years.
At the same time, global competition and stricter environmental regulations necessitate painting processes which are more and more efficient and sustainable.
Increased flexibility is an additional issue which concerns companies with in-house painting operations.
Increasing effectiveness in pre-treatment
The minimal levels of surface energy demonstrated by plastics are a significant challenge for painting. Residues from the manufacturing process, for example release agents, wax, additives and contamination resulting from transport and storage-like dust, can also impair coating quality.
The trend towards water-based paints and process-reduced coatings is placing greater demands on substrate surfaces. This makes reliable cleaning or pre-treatment of substrate surfaces absolutely imperative.
Traditionally, a power washing system with an aqueous cleaning agent and a downstream, retained water dryer is used. This system is being replaced by CO2 snow-jet cleaning to an ever greater extent.
This is due in part to savings amounting to as much as 50 percent for investment costs, 20 percent for operating costs and up to 80 percent for floor space requirements.
Compact systems are easy to integrate into the painting line, can be used to automate cleaning processes and are capable of removing contamination from very small gaps.
A further advantage of this dry process involves increased freedom in designing plastic parts, because there’s no more need to assure that they don’t include any water retaining geometries.
The same applies to plasma processes, by means of which all thin layers of organic contamination can be removed.
Two different technologies are available to this end.
In the case of low-pressure plasma, treatment is carried out in closed chambers in a partial vacuum.
This makes it possible to process workpieces with complicated shapes as bulk goods or individual parts, and allows for the use of a great variety of process gases.
Direct and indirect corona discharge – dielectric barrier discharge – functions at ambient pressure.
With the first variant, the discharge (plasma) strikes the workpiece directly.
In the case of indirect atmosphericpressure plasma, which makes use of so-called plasma heads (nozzles), discharge takes place at the plasma head and is directed to the surface to be processed by means of compressed air.
The surface is simultaneously cleaned and activated during plasma treatment.
This dual function is based on the physical and chemical characteristics of the process.
The atoms released in the low-pressure plasma bombard the surface of the component to be cleaned.
This functions like a miniature sand-jet in the nano-range, thus removing organic contamination which adheres to the surface such as oil and grease, and to some extent inorganic contamination as well.
At the same time, free ions and electrons react with the surface, forming polar groups.
Consequently, surface tension is adjusted to an ideal value for the subsequent painting process.
The surface becomes highly wettable as a result, which assures ideal painting conditions when water-based paints are used on metals, as well as when coating difficult-to-paint plastics, thus contributing to reduced scrap rates.
Achieving an ideal surface with fewer process steps
Filler-free painting systems are on the advance all over the world in the field of vehicle painting. The function of the filler coat is usually fulfilled by a water-based paint system. In comparison with conventional painting processes, this makes it possible to reduce energy consumption by 15 to 20 percent by eliminating the intermediate drying process. Processing time is also reduced accordingly.
At the same time, the filler-free painting process reduces solvent emissions and eliminates the need for the system’s filler section.
In the meantime, these economic and ecological advantages have triggered a trend towards a two-coat finish without primer for coating plastic parts – not only in the automotive industry and amongst its suppliers.
Solutions which make use of only a single-coat painting process for injection moulded parts are also already being used in mass production.
UV paints on the advance
Fundamentally, high quality, efficient painting of plastics necessitates a paint system which is ideally matched to the substrate, the utilised system technology and the requirements specified for the finished product.
Not infrequently, this leads to specially developed paints. Hue and gloss level are usually individually adjusted, and adaptation to the wet-on-wet painting process is carried out as well.
UV technology has made considerable gains in significance – especially where the painting of large surfaces is involved.
This is due to cost savings made possible by very short drying times without heating and cooling processes, as well as the quality improvements made possible by this method.
The levels of hardness which can be attained with UV paints are especially noteworthy in this respect – with greatly reduced VOC emissions to boot, or even none at all.
Where larger, more complex workpieces have to be coated, UV curing in an inert gas atmosphere is opening up new possibilities.
The gas atmosphere results in greater radiation intensity which means that the radiator can be located farther away from the parts, making it possible, for example, to uniformly cure parts with undercuts.
Appropriate retrofitting or expansion with UV equipment is required in order to take advantage of UV technology in existing painting systems.
Painting with reduced material consumption
Significantly higher paint costs due to the rising prices of raw materials and provision for environmental aspects are making it necessary to exploit potential savings in the painting process.
One of the approaches to this problem is the elimination of possible causes of loss such as atomiser over-spray, as well as losses due to colour changeovers, during application and in the paint supply lines.
Savings of this sort can often be implemented quickly and in a targeted fashion by modifying specific system characteristics.
One possibility involves increasing transfer efficiency by using a spray pattern which is matched to the geometry of the part to be painted, as well as the use of electrostatically supported paint guns and high-speed rotary atomisers.
Transfer efficiencies of greater than 90 percent are achieved with the latter.
Where hydro paints are concerned, new electrical isolation systems which are located directly on the robot arm are also making a contribution in this respect.
They also allow for minimised use of rinsing agents and shortened colour changeover times.
Beyond this, the fine atomisation patterns achieved with electrostatic high-speed rotary atomizers open up the possibility of reducing air sinking speed in the booth, which results in energy savings.
The increasing degree of automation through the use of robots is also leading to reduced paint consumption.
Further advantages include improved reproducibility of painting results and less scrap as well.
Another positive side effect of using painting robots is the fact that it is easier to switch from fresh exhaust air systems to recirculating air for conditioning the paint booth.
This can result in energy savings amounting to as much as 60 to 70 percent.
Intelligent paint logistics also result in reduced material consumption. For example, pipe-clearing technology makes it possible to recover unused paint from the feed lines and to dose defined quantities of paint to the application equipment for special paint coatings. This assures very frugal paint use, even when coating small manufacturing lots.