Products Finishing

JAN 2013

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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PAR TS CLEANI NG CL INIC n Cleaning Die Cast aluminum Parts Q. What is the simplest way to clean a die cast aluminum part one week out of the mold with surface contamination from graphite and grease? I do not wish to shot blast, as it is important to keep the finish as natural as possible and maintain good corrosion resistance. The part is a 7.5-kg exterior luminaire housing measuring 600 �� 600 �� 270 mm. Is there a cleaning process that also can leave a protection film and be the finish, such as phosphating or passivation? P.G. a. If the graphite and grease is diffcult to remove, it is unlikely a simple degrease and conversion coating system would work. That contamination may require an aqueous cleaning tank with a moderate-to-higher (and inhibited) pH cleaner. To improve graphite removal, physical action such as ultrasonics may be necessary to assist in loosening the soil and removing it. There are some simpler three-stage phosphate systems that can do a cleaning and coating step in the frst phase, but those will only work with a soil that is relatively easily removed. If the graphite and grease is heavy and irregular, it will result in a coating that is similarly irregular, since the coating will form frst on the areas where the contamination was frst removed. The more heavily soiled areas may never be fully cleaned, making coating diffcult or impossible in those regions. n sPray ZinC PhosPhate Q. Is it possible to spray calcium-modified zinc phosphate at a coverage of 1,000-1,500 mg/sq ft? S.N. a. Most zinc phosphates are immersion-applied. I am not sure if there is something that would specifcally inhibit the ability to apply a calcium modifed formulation by spray application. In general, the calcium modifed phosphates have fner crystal structures, so this morphology pfonline.com/experts may be more amenable to a spray application. However, I think it may be diffcult or impossible to develop a coating weight as heavy as you are requiring with a spray coating. I would discuss this specifcally with your supplier, but I do not think you will be able to achieve these coating weights. n reworking e-CoateD Parts Q. I currently have sheet metal parts e-coated in epoxy. They originally come to me with a bright gloss, but when I send some parts back for rework, they return to me with a dull finish. The supplier can only shot blast the parts. Is this causing the dull finish? Should the parts be acid stripped instead and recoated? P.R. a. You did not mention why you are sending the parts out for coating and what your agreement is with the job shop. Presumably you are in need of a complete and corrosion-resistant fnish if you are utilizing an epoxy e-coat. If your coating shop is shot blasting parts to remove the coating, the dulled areas you are referring to are from the unintended overspray you may get on otherwise acceptable surfaces. While the coating is not completely degraded, it would be considered compromised in those areas, due to the roughness of the shot blasting removing some of the coating. Since the coating would be applied to maximize corrosion resistance, it would generally be better to strip the part and recoat, if that is possible. There are several alternatives besides an acid stripping. There are solvent-based and caustic strippers, as well as more controlled abrasive blasting processes like plastic media and baking soda. To understand the degree of loss, it may be possible to have the coating shop���s paint supplier provide a simple salt spray comparison of an otherwise good ecoating compared with one that has been degraded by some of the blast process. That way you can judge for yourself how compromised your coating and your sheet metal products are. For instance, do you have 20 percent or 90 percent of the coating���s original corrosion resistance? n Cleaning the Cleaner Q. I use a cleaner to clean flat rolled steel. I have an 800gal tank and would like to talk to someone about equipment to clean and re-use the aqueous cleaner. A.F. a. There are three general methods for removing lubricants from aqueous cleaners. The least expensive is an oil skimmer. It utilizes a wheel and plastic disc or belt to which the lubricant adheres. As this rotates, the DAVID S. PETERSON, Consultant, partscleaning@pfonline.com 46 JANUARY 2013

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