Compressed-Air Quality Important

I have been told that compressed air quality is important for a powder coating system. How do I make sure I have good air quality?

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.

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 more easily, transporting the powder to the gun more efficiently.

Powder Pump Control Critical

My operators seem to 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" ?

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 and how it affects a powder coating operation.

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.

By varying the delivery rate, different volumes of powder are sprayed. This allows the user to coat products with different shapes and achieve the same or different film thickness.

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 overcompensate on powder delivery by increasing the powder output to avoid undercoated or "light" parts. This leads to an increase in powder consumption for the same production volume. 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 consistency of delivery rates.
  • Wear of 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 onto the part, causing a reject. Items to check when powder feed surging is taking place would be as follows:
  • Check powder feed hoses to see if they are pinched, routed correctly, too 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 manufacturer's 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 buildup, 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. As a regular rule, this should also be done at the end of each shift and before going on breaks.
  • Check fluidization that may cause powder surging. Uneven fluidization can cause too little or too much air to be in the mixture at the suction tube. Fluidization should be an even and uniform soft boiling in the feed hopper.

Maintain Consistent Level in Powder Hopper

Can you explain the purpose of the powder coating fluidizing hopper and how it can change the film thickness on my parts?

Fluidization of the powder hopper is the process of preparing the powder in the gun feed hopper to become "fluid-like," enabling the powder to become easily transported through the suction tube and to the gun. Other methods for preparing the powder are vibration, agitation (stirring the powder) or combinations of these methods.

However, fluidization is the most frequently used method for material handling and conditioning of the powder. To fluidize the powder, compressed air is brought into the hopper through a plenum chamber and membrane. The air is delivered through the membrane and then through the powder in the hopper. As the air moves up through the powder, it lifts, or "fluffs," the powder, making it fluid-like.

Most users cannot take the time to adjust the hopper fluidization air setting each time the power level changes. As the level raises and lowers, the density of the powder/air mixture in the hopper changes. As a result, the density also changes in the suction tube assembly.

This means that as the powder becomes more or less dense as the level changes, the delivery air remains constant. The result is that the powder pump is actually delivering more or less powder to the gun. Users will notice that the gun outputs vary throughout the day as the powder level in the hopper rises and falls.

The best approach is to maintain a constant level of powder in the hopper. This minimizes the adjustments to the fluidization air and delivery air, keeping a consistent delivery of the powder to the gun and assisting the gun in applying the powder uniformly on the substrate.