One of the most asked questions during my training sessions is,” How do I keep powder from surging out of the gun?”
First, let’s talk about how the two most common types of manual powder guns work and what you should be looking for to prevent powder surges.
Box-Fed Powder Guns
How it works: When you use a box-fed powder gun, you set your box of powder onto the vibrating pad and insert the pick-up tube into the box. The pick-up tube releases a small amount of air near the tip to help fluff the powder so it can be easily transferred to the gun. The pick-up tube works with the vibrating pad to aerate the powder and keep it from clumping together.
What to look for: When properly adjusted, the powder should have a small volcano appearance around the spot where the pick-up tube goes into the box. There should not be enough pressure to shoot powder out of the box, but enough to keep the powder in a loose state and provide a consistent, smooth flow of powder to the powder gun.
Hopper-Fed Powder Guns
How it works: Hopper feed systems typically use a cylindrical stand-alone hopper container that you pour the powder into and close the lid. Air is supplied to the hopper, and the compressed air bubbles up through very small holes in a membrane located at the bottom of the hopper.
What to look for: When properly adjusted, the powder should look like boiling water when you peek in the container. If you stick your fingers into the powder, it should feel like silky smooth baby powder.
Top Five Reasons for Powder Gun Surging
1) Too Little Air
It’s possible that not enough compressed air is being supplied to the hopper bottom or the pick-up tube tip. The air supply may have been disconnected or the line may be obstructed, but usually the problem is due to the gun not being properly adjusted.
How to Troubleshoot: There is a small adjustment nut on the side of the unit that regulates the air flow going to the hopper bottom or tube tip. Adjust the nut up and down so that you get the effect described above (boiling water for hopper-fed or small volcano effect for box-fed). Remember, different powders have different densities, so you may have to turn it up a bit when using white primer powders or down a little when spraying something like red gloss topcoat powders.
Also, check your air supply and make sure you have a consistent 60-80 pounds of dry shop air going to the powder unit. If the air pressure is jumping up and down – perhaps because you have multiple pieces of equipment using the same compressor – consider installing a dedicated compressor for your finishing process.
Moisture may be preventing the powder from flowing smoothly. If you have a box-feed system, bringing a fresh box of powder from an air-conditioned storage room to the hot, humid, plant environment can cause problems. You will get humidity condensation on the inside of the box. This makes the powder very difficult to fluff and it won’t be properly picked up by the tube, making it much more likely to surge. You can also have problems if you warehouse your powder in a damp or hot/humid environment (like the corner of the shop). Over time, exposure to moisture will cause the powder to clump up into large chunks.
How to Troubleshoot: First, if you are storing your powder in the shop, move it. Use a clean, air-conditioned storage room to warehouse your powder. Bring a box of powder out about 2 hours before you need it, open the plastic bag, and let it acclimatize to the shop air before using.
3) Water or Oil In The Air Lines
Moisture in your compressed air lines can cause powder to clump. In addition to the problem described above, the situation gets even worse when the air supplied to the gun unit has oil or water in the lines. Since the air is going through the gun and touching the powder, water propelled through the gun will cause the gun to clog, sputter, and surge powder on to your parts. The compressed air can also contaminate the finish by bringing along any oil or contaminants it encounters as it travels through the air lines.
How to Troubleshoot: Be sure to have a good air dryer or a multi-stage air filtration system installed in the system prior to the gun, especially if your air lines usually carry moisture or oil.
4) Worn or Clogged Equipment
Parts of the powder unit can wear down due to the abrasive nature of powder and cause the gun to perform poorly. The first place to check is the venturi nozzle sleeve. This is a plastic sleeve that periodically needs replacement.
How to Troubleshoot: Check the venturi nozzle sleeve to see if the powder has cut grooves into it, or if the orifice has changed from round to oval. If either is true, replace the sleeve.
Powder can also start to stick to wear surfaces in the gun system, especially in hot environments. When powder sticks and hardens, it is due to something known as impact fusion. Normally, compressed air will clean out most areas in the powder unit, but not when impact fusion occurs.
How to Troubleshoot: Q-tips soaked in isopropyl or rubbing alcohol are the best tools for removing powder residue that has partially gelled due to impact fusion.
5) Improper gun settings for the hose or particle size of powder
An improperly adjusted gun can cause surging. Powder gun units have an adjustable ratio of powder to air that is supplied to the gun through the powder hose. If this ratio is off, powder surging can occur.
How to Troubleshoot: The first thing to try is to increase the amount of powder to air, which fill the powder hose and keep continuous powder moving. This will help with intermittent surging. Different powder systems have different adjustments for this, so consult your gun manual or talk to your powder supplier’s technical representative.
Some powders are heavier in density or larger in particle size. For example, white primer is more dense than a red gloss topcoat powder. If you get a big burp of powder when you first trigger the gun after switching colors but no more surging, you may need to reduce the amount of powder to air.
Remember to Maintain Your Change Logs
Every time you make a change to your powder settings, make sure you update your log book. When troubleshooting your process, only change one thing at a time and record the results. This can help you isolate the cause of the problem and more quickly diagnose problems in the future.