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.
Issue link: https://pf.epubxp.com/i/620919
pfonline.com/experts 40 JANUARY 2016 — pfonline.com Two-Color Powder Coating Q. Can a wheel be powder coated with two different colors? A. Yes, a wheel can be coated with two colors. You would apply the lighter of the two colors, cure the coating and then mask off the area of the first coat. There are high temperature tapes that you can use to cover the area with the first coat. Apply and then cure the second color in the unmasked area. There are some challenges. The parting line where the mask is taped to the wheel may come out a little rough due to the coating thickness and the tendency of the second coat to "bridge" to the masking material. You can remove the mask before curing the second coat and thus reduce the roughness at the parting line. However, there is a risk of having a little dry powder drift onto the first coat or messing up the line when you remove the mask. If you have a very clean line in the design of the part you can use it to separate the two coats (like a sharp crease in the wheel); it will be easier to separate the two colors. Regardless of what masking material you use or whether you remove the mask prior to cure or after, I would recommend some trials to perfect the process before you do it on your wheels. With some trial to develop your technique you should be able to pull it off. Hanging Parts Q. We are having a lot of trouble with rejects. We can do a good job some days and get out a lot of good parts, but we are not as consistent as we need to be. One of my concerns is the way we rack our parts and take care of our hooks and racks. Some of our racks have a lot of powder buildup on the contacts, and rack maintenance is spotty. We have some racks that are banged up and we use some hooks that do not hold as many parts as we could fit on the line if we had a better racking scheme. Is there any way to qualify the impact of racking on the powder coating process? A. This is an interesting question. If I were to single out the biggest and most obvious loss of revenue in a coating shop I would say it is poor racking. It is very common to see large line gaps, bent and broken racks, twisted hooks, too much powder buildup on racks, racks that are only partially full, different hooks or racks being used for the same part or just bad rack designs. The cost of poor racking is ridicu- lously high. Yes, the costs can be evaluated to determine the differ- ence between a good racking arrangement and a poor racking arrangement. First, you need to understand the principles of good racking. Good density is a key. Parts should be grouped close enough together to reduce the space around them so the ratio of metal to air is high. There is a limit to density; there must be enough space around a part to make it easy to cover. But within reason, more is better. Second, the parts should be positioned so that they can be easily accessed and easy to see. They should be held in a stable and consistent position. There should not be large gaps between racks as long as there is sufficient time to coat the parts based on the line speed and number of guns. Contacts must always be clean so that there is no way a part can be insulated from earth ground by coating build- up. Good racking is an important key to profit- able coating. To determine the cost associated with the racking, you need to evaluate what your throughput could be if you had proper racks and compare it to how you run it now. You can build samples to test theories and compare the defect rates to any other rack method. If you can quantify the current rack capacity and quality you can compare them to any other rack option. It is simple math to evaluate the number of parts per foot and the number of good parts versus bad. Many companies waste a lot of dollars by using sub-optimum racks because they do not want to invest in tooling, they do not want too many different rack designs around or they just do not even evaluate it at all. Evaluate the options and there is a good chance you can improve the throughput and reduce costs by using the right racking scheme. Consider a rack that holds one part like a simple hook. You know you can hold two parts if you build a special tool. The tool cost will be $600 for a full set of two piece racks. If you divide your fixed cost by your production throughput you can deter- mine the savings. Then you can see how long it will take to recover the $600 investment. If you run the part a lot your return on investment will be fast. If you run it infrequently the ROI may be too slow. If you improve your first pass yield the ROI could be very fast. Always evaluate options and chose the smartest racking method, even when it means investing in a new tool. Pretreatment Methods Q. We are just learning the powder coating business. We have been told that we must get the part clean before we powder coat it, and we are learning about our options. We are also looking at chemical treatments to provide better adhesion and rust protection. Are you aware of any simple pretreatment options that will work with powder coating and help to limit rust on steel surfaces with a lot of welds and other complex issues? p f o n l i n e . c o m / e x p e r t s POWDER COATING C L I N I C RODGER TALBERT Technical Contributor powdercoating@pfonline.com