Painting plastics efficiently

 In many industries, plastics make it possible to produce parts which weight less, are less expensive and/or include integrated functions. Painting is taken advantage of when the product needs to be provided with an attractive appearance, resistance against mechanical, chemical and physical stressing or special functional characteristics. The recently concluded PaintExpo covered the entire process sequence in the field of coating technology and offered a comprehensive overview of the latest developments in the areas of liquid painting, powder coatings and coil coating, from pre-treatment right on up to quality control.

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. Furthermore, 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. However, this system is being replaced by CO2 snow-jet cleaning to an ever greater extent. On the one hand, this is due to savings amounting to as much as 50% for investment costs, 20% for operating costs. and up to 80% for floor space requirements. On the other hand, the 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 above 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 atmospheric-pressure 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, thus 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
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