Keeping Spray Masks Clean

Thomson Consumer Electronics is the world's fourth largest consumer electronics company. It pioneered color television, and today it manufactures RCA, General Electric and other top-of-the-market sets for distribution worldwide.

The company's Indianapolis facility molds and decorates more than 2.5 million polystyrene cabinets per year. The sets are assembled in Bloomington, Indiana, the largest of the company's two manufacturing plants. Cabinets for sets up to 32 inches diagonal are molded in black and then masked and painted on the front side with a suede-like finish.

Paint masks have long been a critical component in repetitive decorative painting. Keeping masks clean is one of the most important factors in achieving a crisp paint line.

In the past, when solvent-borne paints were used more often, the most common method of mask cleaning used volatile solvents with low vapor pressures. This resulted in significant loss through VOC evaporation. Generally, this was 50 to 60 pct of the total volume after two average production shifts.

If the system started with 100 gal of reclaimed solvent, 40 to 50 dilute gal of paint and solvent would have to be removed from the mask washer, stored and distilled. After distillation and paint separation, 25 to 30 gal of reclaimed solvent would be returned to the system. After reclaiming the solvent several times, virtually all of it was lost to the environment.

In 1989, Thomson made a company-wide commitment to switch to 100 pct solvent-free finishing. As a first step, the company sought a solventless mask washing system that would perform well in a short processing cycle and be compatible with an existing solvent-borne paint system.

In 1993, Thomson took a second step in its transition to solvent-free finishing by replacing its solvent-borne paint with a water-borne epoxy hybrid enamel.

This required a change in mask-washing chemistry. A team from Thomson, headed by process engineer Ervin Simons, contacted Challenge, Inc., Indianapolis, Indiana, for assistance. The Thomson team and Challenge chemists developed a mask-washing chemistry that allowed the company to complete the third and final phase of its transition.

633-S is a one-step process based on a cationic polymer. This water-borne barrier coat system is pH stable, non-foaming and biodegradable. With the system, an aqueous solution of film-forming material is applied at 180F to the mask using pressure spray and then dried. Following spray painting, the mask is sprayed again with a film of the same aqueous solution, removing the paint and simultaneously redepositing a new film onto the mask.

The system is based on a proprietary polymer molecule and combines the environmental safety and material cost advantages of water-based chemistry with fast cycling. A typical wash and dry cycle is three min or less.

Mr. Simons was involved in the decision to implement the original solvent-free mask washer and to upgrade it later. Mr. Simons explained, "The original system allowed effective barrier coating and shielding, but when the water-borne paints were introduced it caused the paints to dissolve into their natural components of paint and water. This created a maintenance headache. Also, it was time consuming and costly to manage.

"Because it was created specifically for the water-borne paint," said Mr. Simons, "the newer system converts water-borne paint into a semi-solid that is easily filtered."

When using the water-borne mask washing on solvent-borne paint, mask washers were cleaned out every six weeks. Using the new product, the mask washers are cleaned once a year. Debris does not accumulate in the bath and the bath chemistry does not demand routine dumping and recharging, only filtration.

However, there were some bugs to work out at the beginning. The semi-solid condition of the paint is actually a crystallized form. Because of this, Thomson found that it needed a basket to capture and filter the crystallized material as well as a stainless steel screen. Valves and joints also had to be stainless, because the water-borne material would adhere to galvanized surfaces and harden.

In his 18 years of service at Thomson, Mr. Simons has earned a reputation as an innovator. "Ninety pct of the improvements we have made are things I have instigated," he admitted.

Thomson is satisfied with both the product performance and the cost benefits. Mr. Simons noted that material costs in most parts of the country are similar; the difference is in the number of bath changes.

Mask washing improvements, such as pressure-flood mask washing and high-energy spraying, provide for the removal of two-component coatings and other tenacious finishing systems from the mask washer as well as from tooling and machinery.

Paint solids accumulated after filtration can be disposed of directly in the sanitary sewers in most communities with no adverse environment consequences or fees.