Issue link: https://www.e-digitaleditions.com/i/941907
Shortening dwell times to increase line speed Q We are designing a powder coating line. We will be coat- ing flat sheets of metal, and line speed is our major concern. We will choose the powder based on the end use, exterior, interior, and post forming. Most powders are formulated for slow dwell times. We assume heating the metal will allow us shorter dwell times. Also, we figure IR gas fired ovens will also help with shorter dwell times. Do you have any other suggestions to shorten dwell times? E.A., Elgin, Ill. A It sounds like you are looking to install a Flat, Coil, or Blank coating line. These designs can be very efficient at coating a lot of metal very quickly at line speeds up to 150 fpm. This makes curing the powder coating the equipment component that takes the most room in your design, especially when most powders cure at 400°F for 10 minutes or 375°F for 20 minutes. These types of coating lines normally try to cure the powder with high efficiency, fast reacting, electric IR oven systems. These devices can achieve a metal surface temperature within the coating of 600°F to affect full cure in 30 seconds. Al- though they can be very expensive, they are the key to having a robust system de- sign that provides excellent control of in- tensity and have fast reacting elements for zoning and triggering control. These systems are easily automated using PLC controls and optical pyrometers. If you elect to use these types of IR ovens, you should review powder formulation issues with your supplier to procure ones that are formulated to be fast reacting and compatible with this type of oven. Some powders will not be as forgiving using this type of cure system design. I do not recommend you preheat your parts to reduce cure time, as that may cause film thickness control issues. It is a known fact that part surfaces above 100°F will attract more powder than cooler surfaces. Even if you reduce gun outputs to compensate for this pre- heated surface temperature, it may pre- sent issues as the ambient plant temper- ature affects the part temperature at time of spray throughout the seasons. Good luck, and feel free to contact us if you need help. —N.L. Coating a tricky disk with a Faraday cage area Q My company powder coats auto parts and domestic appli- ances. We have a new client who wants us to powder coat an auto part that looks like a disk. The main problem is that this disk has a 90-degree edge of 1 inch with a Faraday cage in the inside and an overspray problem on the outside. This particular edge is then folded on a machine (post-painted) that usually takes off the powder coating on the outside of this edge. How can I de- sign a rack or achieve a very consistent low film thickness on this area? E.R., Apaseo el Grande, Guanajuato, Mexico A Thanks for sending a photo of the part. The heavy coating on the outside of the part at the break, as I understand it, is likely the cause of the powder cracking or break- ing away when the part is bent. So, somehow you need to reduce the amount of powder in that area. May I assume that the heavy film is caused by attempts to overcome the Faraday area on the inside of the bend? If you haven't tried already, reduce the volt- age when spraying in this area, and be careful of atomizing air. You may be able to achieve good coating this way. Using tribocharging application could be a problem with other things you spray be- cause powder designed for tribocharging isn't very compatible with other coatings. You may have to create a masking device to deflect the excess powder from the bend area so that you won't have heavy film. Can you spray the Faraday section of the part as a final application as the part is processed? This applied coating will help insulate and reduce the Faraday effect. Hey, I've seen it work, but maybe we just got lucky. —G.T. Powder coating hammertones Q I've been asked by a customer to find out if we can do a hammertone powder coating process. I was told there is a two-step process for this. However, I haven't been able to find out how this process is done. They have provided me with two powders that are to be used. They are a silver vein and a clear gloss. Any help or George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers POWDER COATING, February 2018 31