The project encompassed replacement of a three step solvent based painting process with a more efficient and environmentally sound single step powder paint application for a military project. The new equipment included a powder paint booth, powder cure oven, and an overhead conveyor system. The solvent painting method included application of a wash primer, primer and top coat on passivated stainless steel parts in a waterfall paint booth. These painting components contained various hazardous solvents including MEK, toluene, xylene, and butanol, as well as, heavy metals like chrome, lead, and zinc. The paint process exhausted 31.5 tons of solvents per year to the atmosphere. The heavy metals were trapped by the waterfall and formed 35,000 lb/yr of sludge with 72,000 gal/yr of process water requiring disposal.
The purpose of solvents in the painting process is to provide a transfer medium for the solids in the paint to deposit on the work piece. Powder paint contains no hazardous solvents. Compressed air propels the solids to the work piece. The powder starts out in a supply hopper where compressed air diffuses it into a fluidized state. Next, air pumps to move it through flexible tubing to the spray guns. The spray guns create a cloud of electrostatically charged powder particles directed toward the grounded work piece. The powder adheres to the work piece due to the electrostatic charge. The over spray powder not adhering to the work piece is pumped back to the supply hopper ready for reuse. The coated parts are transferred, usually by conveyor, into an oven where the powder melts, flows, and fuses to the part's surface. Besides the environmental impact, powder paint also has a positive affect on quality. Powder is used as received. No mixing is required. No primer coasts are required, replacing a three coat operation with one. There are no intercoat adhesion failures as associated with multiple coating operations. Powder paint finishes have no runs, drips, bubbles, or blisters. Powder coatings can be formulated for increased hardness, flexibility, weatherability, abrasion resistance, impact strength, and corrosion protection. Powder paint also has its limitations including; the initial cost of a complete new system, cure temperatures above 300 degrees F, powder is based on electrostatics eliminating most non-metallics, color changes make system cleanliness extremely critical and, part configurations can be troublesome due to the difficulty in curing parts with large variances in thickness.
The powder paint system eliminated solid hazardous waste disposal problems for the project it supports. It eliminated the 35,000 lb/yr of chrome bearing sludge and 72,000 gal/yr of chrome bearing process water, as well as, fouled spray booth filters. Also eliminated was the 31.5 tons/year of solvent emissions from the paint mixing, painting, and cure processes. This also removed the safety hazard of having paint operators exposed to a variety of chemicals.
Details of Reductions
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The switch to powder paint provided savings in many areas. Powder transfer efficiencies and the ability to collect and reuse it provided an 80% savings in material costs. The switch from a three coat process that required mixing and extensive clean up reduced labor by 73%. The ease of care of the powder paint booth translated into a 98% savings in booth maintenance. The cost to dispose of the solid hazardous wastes was eliminated. Another major savings was realized when a follow on operation was eliminated due to the hardness, and abrasion resistance of the powder paint saving an additional $90,000. System pay back was calculated to be 1.5 years. Additional items that also should be considered savings are; reduced environmental liability, less rework and a higher quality finish, eliminated waterfall booth process water spills and clean up, reduced health and fire risks with elimination of solvents, and no requirement to condition make up air for the waterfall paint booth.
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