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OVERVIEW

Uniformity in the booth air flow is one key piece of the application variables chart. Previously we discussed that “more powder flow is not better when dealing with transfer efficiency.” “More air flow” will also affect your first-pass transfer efficiency. As this section of the paper deals with application transfer efficiency variables, powder booths are designed with two requirements; 1) powder containment and 2) safety. We will focus on powder containment.

The first requirement in any powder system is powder containment. To contain oversprayed powder particles, the powder booth is designed to provide average face velocity of 100 lineal feet per minute (lfpm) air flow across all openings.

Following are some exceptions:

  • Tall parts greater than 6'0".
  • Heated parts greater than 1200 F.
  • Crossdrafts in excess of 60 FPM in area of the booth.
  • Short hooks up to 18".

Again, we are looking for powder containment. Too much air flow will draw the powder away from the parts being coated. Many try to overcompensate with a higher powder flow rate and less first-pass transfer efficiency.

Booth Canopy Design

The design of a powder coating booth begins with the canopy or enclosure that contains the oversprayed powder.

The canopy is best described as a small room with four walls, a floor and a ceiling. The ceiling has a slot running lengthwise through which hangers from the conveyor protrude to support the part to be coated. Because powder is applied via an electrostatic charge and our goal is high first-pass transfer efficiency, we want the powder to be attracted to the part and not to the booth. To achieve this, the booth canopy or the area around the automatic powder guns should be constructed of a low, non-conductive material. This will allow the electrostatic field emitted by the guns to attract the powder to the part and not the booth wall.

  • Gun-to-Booth Wall Distance
    One canopy issue that will affect first-pass transfer efficiency and good housekeeping is the distance of the tip of the gun to the booth canopy behind the gun. The powder gun should be at least 12 inches inside the boot so the electrostatic field is attracted to the part and not to the booth canopy. If the base of the powder booth is not wide enough to place the gun inside the booth, you can add a booth wall extender at the gun opening for added space between the gun tip and the wall. As a result, more powder is attracted to the part, increasing operating efficiency. This also speeds up booth cleaning during color changes.
     
  • Gun-to-Part Distance
    The distance between the parts you are coating and the tip of the gun will affect your transfer efficiency. There are many variables with gun placement that depend on line speed and part specification. Generally, start at 8 to 12 inches away from the part.

Humidity

Both humidity and temperature can affect the performance of a powder coating system. The powder system should be installed in an environmental room. The value of an environmental room is consistency as any change in temperature and humidity may affect fluidization, filter efficiency, filter life, and charging capabilities of the powder. Temperature of the environmental room should remain at 680 F to 800 F. Relative humidity should remain at 47 to 55 percent.

Ground

Part grounding is extremely important. An ungrounded substrate will attract the charged powder to a certain point. After which, it begins to repel the charged material. It will also affect your first-pass transfer efficiency. An ungrounded part has the potential to ignite the powder and create a fire.

Ground is measured as one megohm resistance.

Powder

There are physical and chemical properties that can affect first-pass transfer efficiency. Not all powders are the same, so I would recommend you work with your powder supplier.

 

PF Zone: Booth Air Flow

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