January 2003 Q&A Columns

Ensure Compressed Air Quality

I have been told that compressed air quality is important for a powder coating system. What kind of air quality should I have? What kind of problems can I expect if the air supply is not maintained properly?

For a quality compressed air supply, maintain the correct volume of air available for the powder paint system. Clean, dry air is important. The compressed air supply feeding the powder system needs to be conditioned. This is accomplished with a properly sized air dryer. Either a refrigerant or regenerative air dryer is acceptable. When the air volume required is 500 scfm or greater, a regenerative unit may be preferred. Other items required with the air dryer are a particle filter and a coalescing filter. It is best that the filters in the system be set up with automatic drains or flags that indicate when service or replacement is required.

Air quality requirements are three-fold: maximum oil content of 0.1 ppm, dew point of 35°F or lower, and particulate matter no larger than 10 microns. By keeping the powder free of moisture, oil and other dirt, powder fluidizes more evenly, and the pump can lift the powder through the suction tube easier and transport it to the gun more efficiently.

Powder Pump Control Critical

My operators spray more powder than necessary and complain about "puffing." What role does the pump perform in the powder process and how can I avoid the "puffing" that occurs.

Control of the powder delivery is critical to the performance of the application. Charging efficiency and applied film thickness uniformity are dependent upon consistent powder delivery.

Many users do not understand the importance of powder pump control. A pump is used to get powder to the spray gun. This pump, called a venturi or injector pump, is designed to pneumatically convey the powder material from the fluidized feed hopper to the spray gun.

Control of the powder delivery is important to maintaining good film control. If too much powder is sprayed, either more powder is applied to the part than is required or more reclaim powder is created. Too much powder delivery may also decrease charging efficiency by diluting the ability of the applicator to completely charge the material being applied. This leads to poor transfer efficiency and reduced material utilization. Operators often times over-compensate on powder delivery by increasing the powder output to avoid undercoated or "light" parts. This leads to an increase in powder consumption. Other factors affecting the powder delivery rate are as follows:

  • Inconsistent compressed air supply or quality from the plant air source can cause pump delivery rates to increase or decrease.
  • Tubing and powder hose lengths can affect the ability to provide consistent delivery rates.
  • Wear of the powder pump parts can cause poor powder delivery.
  • Maintain a good blend of reclaim and new powder to help control the powder particle size distribution.
Another issue associated with the powder pump is "surging." This typically is the result of an inconsistent compressed air supply or blockage in the powder pump or feed line, causing the powder to puff, or surge, out of the powder feed line and cause a reject. When powder feed surging occurs:
  • Check powder feed hoses to determine if they are pinched, routed correctly, excessively long or kinked.
  • Check for impact fusion inside the hoses or on the gun components that can impair flow and cause powder surging. Performing periodic maintenance of the equipment can avoid this problem. Follow the manufacturers' guidelines for regular checks and cleaning of the components in the powder path. This includes the gun tips, electrodes and pump inserts. To prevent additional build up, never leave the powder hoses full of powder at the end of shifts or overnight. Before shutting down the system, use compressed air to blow the lines and guns free of residual powder. This should also be done at the end of each shift and before breaks.
  • Check fluidization that may cause powder surging. Uneven fluidization can cause too little or too much air in the mixture at the suction tube. Fluidization should be an even and uniform soft boiling in the feed hopper.