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PFonline.com/experts A. I will look at it as though you have a pretty old system. Newer gun technology uses better pumps, digital controls and more precise design to give you better efficiency than what was available 5 to 10 years ago. Some offer control functions like factory presets and can dial in precise and repeatable recipes for accuracy and consistency. They can be equipped with a variety of features for gun movement, flow control and electrostatic functions. There is no doubt that new guns can save money in compressed air, powder material, gun maintenance and lower reject rates. The other question is how to measure and understand those savings. Start by generating a baseline for where you are now. Determine how much powder you use for a starting point. Measure film thickness on your parts and calculate your transfer efficiency (TE). Compare your actual current film build to your target film build and see if you are reasonably accurate with your current equipment. Chances are that your film thickness varies quite a bit. TE can be measured by setting up a controlled run of parts. Weigh the parts before and after coating and weigh the powder before and after coating. This will tell you how much powder was used to coat the parts. Next, visit the lab of a vendor you are considering for new application equipment. Run the same parts and see how they come out. How well can you control film thickness and how much powder do you use to coat the same parts? You should find better film build control and lower powder use at a lower flow rate when compared to your current guns. Compare the flow rate, powder use and film build control. The savings will probably be in the area of 10 - 30 percent when compared to your current guns. Considering the value of the powder, you can develop an expected return on investment (ROI) and then see if the purchase price makes sense. Sometimes the cost is hard to understand because the lack of precision in the older equipment means you spend more time adjusting and maintaining the guns and you generate more rejects from the inconsistency. The spray equipment supplier can help you compile infor- mation on potential savings. The savings can be substantial in less than two years. The Value of New Application Equipment Q. We have an old set of automatic spray guns. The controllers are an analog design with none of the more modern fea- tures we see at shows and online. How can we save money on gun operation and how do we evaluate the savings? What is Back-Ionization? Q. We spray parts using a silver metallic powder by hand. The surface has a lot of texture and small stars in certain areas and our powder supplier says it is "back-ionization." What is back-ionization and how do we overcome the problem? A. Let's start with corona charging. The way a corona gun charges is by discharging high voltage from an electrode at the gun tip. The voltage may have the potential to reach 100 kV while the micro-amps remain under 100. The high voltage collides with air molecules in the atmosphere and splits them into a negative component (electrons or ions) and a positive component (proton). The ions collect on the powder or flow to the grounded part. The proton is attracted back to the negative electrode. The charge from the free ions attracts the powder to to the part and sticks to the surface. Voltage and amperage can change based on resistance between the gun electrode and the part. Move the gun closer and resistance goes down. This creates a higher current flow (9 microamps) to the part and reduction in voltage. Move the gun farther away and the opposite occurs; current flow is reduced due to greater resistance and voltage goes up. Back-ionization is caused by a radical jump on current at the part surface. If the gun gets so close that the current jumps up, it can cause some problems with orientation of the powder particles at the surface. In some cases, the break- down of air molecules can take place under the film because of the extreme amount of energy present from the radical jump in current. The proton is pulled back to the gun elec- trode and it breaks up the film as it goes, causing a rough, textured film in that area. This is back-ionization; too much current (micro-amperage) in a small space. You can cut down on voltage, which reduces the current but also may reduce charging efficiency. A better approach is to reduce micro-amps. Most newer gun technology has a current limiting adjustment so you can reduce to a maximum level. Experiment with different current levels to see if you can find the best level for that powder and your parts. If too close to the part, you can cause back-ionization even at a lower current level. Make sure operators are The potential savings with new equipment can be substantial in less than two years. 44 JUNE 2017 — PFonline.com POWDER COATING C L I N I C RODGER TALBERT Consultant Rodger has more than 30 years of experience in the powder coating industry.