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pfonline.com/experts 38 SEPTEMBER 2015 — pfonline.com 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 Consultant powdercoating@pfonline.com What is the Proper Oven Temperature? Q. Please settle a dispute in our company. When I came to this company a couple of years ago, I noticed they were cooling down the batch ovens before putting powder coated parts in to cure. I have always put coated parts in a preheated oven and believe that is the right thing to do, but I do not have the facts to back it up. Can you help? A. You are correct. There is no value at all in cooling the oven down before putting a load of parts inside. In fact, that will just make the cycle time longer. In the convec- tion oven, the heated air first heats the substrate. When the substrate reaches full temperature it will conduct heat into the coating and the powder will begin to cure. Cooling the oven before putting a load of parts in lengthens the total cycle time without adding any benefit other than the operator does not have to get close to high heat. Normally, you want to lose as little temperature in the oven as possible when the doors are opened. It costs money and adds time to the cure cycle. Airflow in a Powder Booth Q. We are going to purchase a new powder booth for our facility. It will be a batch operation, but we plan to use a hand-push trolley rail to reduce handling and make it easier to move the parts around. A load of parts will be pulled into the booth and then coated in a stationary position. After coating, the load will be pulled to the oven and cured. The booth will be around 10 ft. wide × 20 ft. long and 10 ft. high. One of the potential suppliers has offered a booth with a cartridge collector on one side only and the other supplier recommends we have a collector on both sides of the booth, asserting that the two collec- tors will improve the airflow inside the booth. Can you comment on this and help us understand why one might be better than the other? A. In any spray booth it is desirable to have air flowing away from the operator holding the spray gun. In an inline powder with a motorized conveyor, the booth is usually narrow enough for the hand sprayer to stand outside the booth and work through a window. These booths often have just one collector on one side, even though there are two sprayers—one on each side of the booth. It is an optimum way to move the air, and there are several prac- tical reasons for doing it. The air is pulled across the booth for the one operator and to the side away from the other, so the operator is never standing with his back facing the collector while he sprays. The capture efficiency of the collector in this arrangement is good and the impact on transfer efficiency is modest. In a batch system where the operator is standing inside the booth, you can get away with a collector on one side only, but it is much better to use a collector on both sides. With one collector, the operator will have no choice but to stand inside the booth with his back facing the collector while spraying. The powder that is pulled by the collector will flow back over the operator and the powder that is over-sprayed away from the operator will wind up on the floor on the quiet side of the booth that has very low airflow. The side opposite the collector is low in airflow because the air being drawn to the collector is coming from inlets that are usually in the ends of the booth. By contrast, with a collector on both sides, the airflow is more uniform and less aggres- sive near the collector. The total cubic feet per minute is evenly divided between the two inlet locations and the operator always has air draw away from their face. The powder is contained better and there is less dust in the air. So, either design can work for your operation, but if it was my decision, I would go for the two collectors. Not everyone will suggest it because it costs a little more, but it will be a better atmo- sphere inside the booth. Ovens for a Batch Powder System Q. We are going to install a batch powder coating system and we have been debating the need for two ovens. We have a spray wand system for cleaning and phosphating and we will have to dry the parts after they are treated. Can we dry them in the same oven we will use for curing the powder, or should we use separate ovens? A. I can see why you do not want to buy two ovens, but I think you would be better off if you did. The first consideration is productivity. With only one oven, you will find yourself waiting a lot while parts dry or cure. Consider how long it takes to clean, dry, coat and cure one load. These cycle times will have a profound impact on how much you can get done in a day. With one oven doing double duty, you will be limited in how much you can process. Maintaining uniform cycle times is important to a batch operation. In addition to the obvious logistics of using one oven, you should consider that you have two distinctly different processes. Drying is the removal of moisture, and it can often be done at a much lower temperature than curing. Why dry in a 400°F oven, if the parts will dry very well in a 250°F oven? Lower temperature means lower drying cost and less heat gain in the building. It also means the parts will be cooled off sooner. If you do go with one oven, make sure it is big enough to hold more than one load for curing.