Keeping It Clear: Scratch-ResistantCoatings for Optical Parts

Injection moulded parts are often scratch sensitive, but many transparent parts need to be scratch resistant. A Netherlands-based company offers injection moulders a new coating technique that provides a silicone-based scratch-resistant finish to moulded parts, selectively. GE Eurogard in Geldrop was established in 1992 as a 50/50 joint venture of the two companies, GE Plastics of Bergen op Zoom and Knaapen Industrial Paint Applications, Geldrop. The company now does US$ 10 million a year applying coatings either manually or with an automatic coating system. Here's how it works.

The moulded parts are laid on specially shaped carriers to align them for the coating process. These carriers pass into a coating line, where jets of ionized air first remove dust particles. The first coat is a primer, which is followed by a drying stage and, finally, the scratch-resistant coating, which is cured in a second drying zone.

The coatings are not atomized or sprayed on, but instead emerge from a fishtail nozzle without pressure or bubbles. They flow down the part and form an overall layer of only 7 µm thickness. Correct alignment and matching of the nozzles and mouldings are essential for uniform wetting of the surface. As a last step, the parts are manually packaged, usually in individual bags, and then stacked in cartons.

This flow-coating process prevents adhesion of dust particles and fluff to the part to be coated. Very little coating material accumulates in the bed of the painting line. What does accumulate is continuously recirculated and cleaned. By contrast, dip coating, because of the large bath volume, is more likely to leave defects, and a dip-coating bath wets the parts all over. With flow coating, the underside can remain uncoated if necessary.

Even manual coating of extremely small runs by this process can be economical. The components are laid on temporary carriers and coated with a jet from a circular nozzle guided by hand across each part. The drying and the two coating operations are carried out consecutively in adjoining cleanrooms. The batch size is not important. Even individual parts can be coated if necessary.

For an efficient overall coating process, the primer and coating material must be optimally matched to the particular substrate. New developments in primers and pretreatment systems allow a wide range of plastics to be coated.

GE Eurogard won't release details on the chemistry of the coating material, but this is an attractive process for enhancing such products as domestic appliances and other consumer goods, where outstanding surface quality must be protected.