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OVERVIEW

The two major gun categories are manual and automatic.

Manual guns can be used in stand-alone units, applications not requiring automatic guns, or with automatic systems that are generally used for difficult-to-reach and/or complex parts. Since the hand painter uses the spray gun for long periods of time, the flexibility, weight, efficiency and durability of the spray gun are important. When considering the added mass of the hose and cable, keep in mind that most painters will hold the hose and cable with the other hand so that only three or four feet of hose and cable weight are added to the gun. Also, depending on where the hose and cable are attached to the gun, the added weight may improve or degrade the overall balance. (Note: Work with your equipment supplier to help determine the right nozzle type for your application.)

Automatic guns may be fixed, on gun movers, or a combination of both. In many cases, systems may be totally automatic without handguns for touch up.

Here are six things to consider before you purchase your next powder gun:

  • Is the gun easy to disassemble and reassemble?
  • Is there a variety of nozzles?
  • Ease of cleaning for color change?
  • How many wear parts?
  • Where is the manufacturing facility?
  • Test the gun on your parts.

Each type of gun has its place in the market depending on the application. The justification for each has to do with its specific design and overall chargeability. The most important thing to remember is the powder gun is just one piece of the application pie.

Nozzle Design
The gun and nozzle design is a major influence in achieving first-pass transfer efficiency. As previously discussed, charge where air velocities are low and where the powder is well dispersed. This is where the nozzle design comes into place. The two most widely used nozzles are the deflector-pattern nozzles and flat spray nozzles. Both are available in various pattern shapes. The flat spray nozzle is more directional and has a well-defined pattern. The deflector-pattern nozzle has a soft, well-dispersed pattern and has the appearance of a liquid bell. There are many other nozzle types, but these account for 90 percent of the usage in today’s powder coating systems. There are gun extensions that will help you reach deep recessed parts safely and without too much effort (see image below).

Powder Pump and Fluidizing Gun Flow Rate
The purpose of the pump is to supply powder to the gun at a uniform and consistent rate. Powder pumps use a venturi principle to deliver powder from a supply hopper to the powder spray gun. The lowest flow rate possible is the ideal condition for first-pass transfer efficiency while still maintaining coverage through each gun. Consistent with the theory of high-charging efficiency is control of the powder output of the guns. The more powder passing by the gun electrode, the less chance each particle has in picking up the maximum charge. More powder flow is not better when dealing with transfer efficiency. It is better to add additional powder guns to keep outputs low than to reduce the number of guns and increase the output per gun.

A fluidizing hopper/feeder with level control is probably the most overlooked and underrated device of any powder system. Let’s look at some of the critical design criteria of a fluidizing hopper:

  • Level control.
  • Fluidizing plenum.
  • Venting.

A good level control will enable the fluidizing feeder to maintain a consistent powder height and a consistent feed rate.

The fluidizing plenum is designed to distribute the air uniformly across the fluidizing membrane which in turn provides uniform fluidization throughout the interior of the feed hopper. More fluidizing air flow is not better when dealing with transfer efficiency. Too much air or nonuniform distribution of that air will result in extremely violent fluidization in certain areas. As a result, efficiency of the powder pump will be reduced causing the powder pump to draw air and reduce the powder flow rate.

A powder feed hopper is like a pressure vessel in some ways. The feed hopper receives powder through three to four venturi transfer pumps. These pumps are basic and air-type pumps, and the air pressure from the pumps and the fluidizing bed has to be relieved from the hopper feeder. The best way to remove the excess air is through a direct vent design. This design relieves internal pressure back to the collector as shown below.

 

PF Zone: Powder Spray Guns

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