Issue link: http://www.e-digitaleditions.com/i/862794
POWDER COATING, August 2017 25 to take periodic breathing tests to check on any lung contamination. The most common respirator is the type with a replaceable filter that goes into two round filter units that are af- fixed to an oral breathing unit. And yet, many companies still use a plain paper paint mask. A lot depends on the em- ployees' exposure to dust. Any booth maintenance should be done with a mask on; any spraying should be done with a mask on; and any powder trans- fer should be done with a mask on. For specific requirements, check with your local regulatory agency. —G.T. Shrinking of powder coated parts Q I am currently powder coat- ing some powdered metal parts that are also sintered/in- fused with copper. The parts take the shape of a half circle that would fit around a pipe. Before powder coating, the parts fit around the pipe and after powder coating the parts close up and don't completely fit around the pipe any more. My processing steps are as follows: 1. Preheat the parts in an oven at 600°F to remove any moisture. 2. Sandblast the parts to add texture and make the powder coating stick better. 3. Powder coat the parts at 450°F. After these steps, the parts do not keep their original shape and they shrink up. I am stumped. Let me know if you need any more details. Thanks. N.M., Portland, Ore. A I would check the film thick- ness on the coating, because the parts are probably not shrinking. Powder coatings applied at 450°F will readily build coating thick- ness to more than 10 mils and up to 25 mils. That thick a coating can reduce your assembly tolerance substantially and they will not fit. Try coating the part at less than 100°F to obtain a coating thickness of 2 to 3 mils and you should be ok. —N.L. Powder settling at the bottom of a feed hopper Q What would cause powder to settle at the bottom of a feed h o p p e r ? W e h a v e t w o venders, and we're having a lot of prob- lems with one of them. When we add a bag of powder to our feed hopper, some powder settles to the bottom and then the next bag of powder takes off rolling real heavy. We have to keep adjusting the air pressure to the hopper. Some- times, the hopper overflows when we add a new bag of powder because of the dead powder that was in it. Thanks for any light you can shine on this problem. K.T., Spring Lake, Mich. A If you're adding a 50-pound box or bag of powder to your feed hopper by just dumping it all in at once, you're certainly running the risk of killing the fluidization of the bed. This situation is exacerbated if the powder is finely ground, has a high spe- cific gravity, or is damp. This is why some companies add a stirring blade to the hopper. The blade will help break up the sudden addition of a large amount of powder and will help to maintain decent fluidization, or at least will help the hop- per recover from a "dead bed" to proper fluidization. The powder may also be damp in some boxes. I know you'll probably say that you have done it this way for years without any trouble, but has the operator made a slight change in the way the powder is added? Is there a new hopper? Has there been some change in the compressed air? When you're adding a bag of powder, does someone use an oar or broom han- dle to get the powder flowing again? Maybe the membrane has been damaged as a result. If the powder in the hopper is "dead," why would the operator dump in a fresh bag without stirring the existing powder to some state of fluidization? Naturally, the hopper is going to over- flow if the powder added totals up to more than the hopper can handle. I used to have a customer who had a problem with orange powder getting all over the wall and equipment in a brand new powder system. It took a while to figure out what was going on, but it turned out that the operator was dump- ing two boxes of powder at one time into a 100-pound hopper so that he could walk away from the system and sit in the break room and read the paper and drink coffee. He couldn't be both- ered with taking care of the coating line. Far be it for me to suggest that this is what's happening at your plant, but you never know. —G.T. Powder coating over fiberglass resin Q I have a customer who is trying to fill in some cracks and holes, and wants to use a fiberglass resin. Will the fiberglass hold the pow- der, and will it handle the heat if it has al- ready been cured? A.B., Hugoton, Kans. A I would be concerned about the heat stability and resistance of the fiberglass. I have no idea if what you're using is something from your local home improvement center, autobody supply house, or somewhere else. Moreover, the fiberglass isn't a ground for electrostatics, so you'll have very poor coverage over this type of ma- terial, unless you treat it with an electro- static-enhancing material. Several of the pretreatment chemical supply houses have this material available. You could also preheat the repaired part to maybe 200°F. The heat will help with the electrostatic attraction. There must be an excellent bond of the fiber- glass to the base metal, and the repair needs to be feathered out very carefully. If it isn't, there will likely be a crack in the powder coating where the fiberglass meets the metal. At the risk of sounding like a traitor to the powder coating industry, you could get a color-matched liquid touch-up paint. When these defects occur, do the repair and then use the liquid. This re- pair method is acceptable in the indus- try. The performance specifications of the coating will influence your use of this method. —G.T. Placing the conveyor lube unit at the right place on the line Q This is a two-part question: 1. Is there a recommended oil temperature for an overhead conveyor system in a powder coat shop? 2. What can happen when the oil isn't rated for the right temp? Our e-coat oven is 390°F; and our powder coat oven is 400°F to 420°F. J.G., Ecorse, Mich. A It's a good idea to deal with a large reputable conveyor lube company and tell them what it is that you're doing about conveyor lu- brication. Location of the lube unit is also something that must be considered.