Issue link: http://www.e-digitaleditions.com/i/653106
Applying adhesives to coated parts Q My foam is not adhering to my powder coated parts. What factors could be causing this issue? Thanks. K.A., Windsor, Ont. A Many powder formulations contain wax additives, Teflon slip agents, and other things that can prevent adhesives from work- ing. This is the very reason you need to work with the supplier of the coating to obtain a coating that provides the per- formance and appearance you need without causing adhesion issues with post applied materials such as tape, la- bels, foam, mastic, etc. However, try wiping the powder coated surface with denatured alcohol to re- move any surface contaminants before applying your adhesives. If this works, the problem may not be in the coating formula but in your process. —N.L. Coating over welds can be tough Q We have problems in weld areas, where pin holes some- times develop and the powder coating breaks through, leaving bare metal. We sandblast our metal in prepa- ration for powder coating and are very satisfied with the results, with the excep- tion of the welded areas. I would appre- ciate any suggestions you may have for correcting this problem. Many thanks. S.C., Simcoe, Ont. A I'll be making some assumtions here because details are lacking, but voids in welds are caused by a less-than-adequate welding job. The voids aren't unusual and from a metal joining effort may not have any negative effects on the product and its perfor- mance. In true critical fashion though, the voids are because of poor welds. OK, so much for the sermon, but as long as there are voids, there will be de- fects in the coating. The powder, or liq- uid for that matter, won't bridge over a void or ding or whatever defect there is in the substrate. Moreover, sandblasting may not be enough in this case for good metal preparation before coating. If the metal cleaning specifications require you to only sandblast the substrate be- fore coating to achieve the proper per- formance specifications, that's okay; however, the coating performance won't be as good as the coating perfor- mance you get when you prepare the substrate with an alkaline cleaner, phos- phate, and seal. Even this method can't prepare a welded surface for good adhe- sion with a coating. Welding requires some special cleaning effort. Sandblasting certainly helps, but if there are voids in the weld, there will be voids in the coating. If you don't have voids in or on the weld, then you need a more concentrated effort in the weld area to present a proper surface for the coating to adhere to. The welding process, such as high concentrated heat, possibly with a flux material and a fillet, will call for spe- cial cleaning efforts. —G.T. Overheating nylon coatings Q We apply a nylon powder over a liquid primer on steel shafts. The parts get a light phos- phate prior to coating. We do not pre- heat the parts. Lately, we have been hav- ing a problem with the powder creeping or pulling away from the edges during its cure time, which is 400°F for 60 minutes. The power is applied at about 8 to 10 mils. Any idea what could cause this problem? J.R., Lombard, Ill. A First of all, a couple of defini- tions need to be reviewed. Coat- ings can be either thermoset or thermoplastic formulations. Thermoset formulas chemically react to cure, sometimes with heat like powder coat- ings. Once fully reacted, themoset coat- ings will not soften when exposed to heat. Thermoplastic formulas do not chemically react and do not cure. These coatings will soften when exposed to heat after they have been initially flowed after application. Nylon coatings are thermoplastic for- mulas and therefore do not cure. Con- sequently, exposing nylon coated prod- ucts to heat for prolonged periods will allow the coating to move by gravity from the place it previously occupied. I expect that you are heating your parts too long, and therefore are allowing gravity to pull the coating from the edges you want to remain coated. Shortening your oven exposure time will eliminate this problem. —N.L. Controlling orange peel and fisheyes Q As a buyer of parts that are powder-coated, can you tell me what causes fisheyes and orange peel finishes? C.Z., New Hamp- ton, Iowa 32 POWDER COATING, March 2016 Questions & Answers George Trigg GRT Engineering Nick Liberto Powder Coating Consultants