Chipping Edges

Q. Does E-Coat have the same problems with laser cut edges like powder have? I am referring to "chipping" of the coating off of the edges. M.L

 

A. The "chipping" you refer to with powder coating is a loss of adhesion due to little if any pretreatment on the laser cut edge. As steel is cut with a laser, a very thin hard oxide is formed on the sharp edge. When "chipping" was first seen on powder coated parts, many people thought it was because the edge was so sharp. The "chipping" reject was more noticeable on powder coated parts due to greater film thickness of powder on edges compared to liquid or e-coat.

While the "chipping" is more noticeably with powder coated parts, the basic cause is there regardless of the coating being applied. Typical pretreatment cleaners do not normally remove the hard oxide formed during laser fabrication and therefore typical phosphate coatings do not form either. Therefore, the coating is covering the oxide but has little if any adhesion to the oxide.

The only advantage that e-coat has over liquid or powder is the usual uniform thin film thickness applied. Since the phosphate pretreatment is non-existent on the laser edges, adhesion is poor at best. Historically, e-coat has always shown good adhesion to non-pretreated (or poorly pretreated) steel ports. This another reason that e-coated parts show less "chipping" than powder coated parts. The solution to laser oxide adhesion problems is the same for all organic finishes. Oxide removal by mechanical means (media tumbling, sanding, grinding, blasting, wire-brushing, etc.) or chemical means (acidic cleaner or pickling) is the best solution. Since the amount of oxide varies depending on the metal thickness and the equipment used, mechanical removal is the best but very labor intensive.

While chemical removal of the oxide should be less expensive (little, if any labor), the type and strength of acid is important. Contamination of the rest of the pretreatment process can occur and "acid fumes" must be dealt with as far as containment is concerned. Equipment needs to be acid resistant, usually stainless steel or plastic. Most system designs do not allow for varying time in each stage to "soak" laser-fabricated parts in an acidic cleaner or pickle stage. A programmable hoist is the only system that allows for added "soak" time without adding multiple stages on monorail or constant-time square transfer systems.

Work with your pretreatment, coating and equipment suppliers to address chemical removal of laser oxide if parts cannot be supplied "laser oxide free" prior to pretreatment and coating.