Painting Q&A: Air Atomized Applicators versus Rotary Atomizers
The major differences between a gun and a bell.
Q. What are the major differences between a gun (air atomized applicator) and a bell (rotary atomizer), and what improvements in application and efficiency can be expected?
A. The major differences between a gun and a bell are the atomization technology, pattern shape, finish quality and transfer efficiency. The combination of these differences can often provide desirable outcomes for a manufacturer.
A gun, or air atomized applicator, uses air at a high pressure or high volume (depending on the technology) impinging on a fluid column to create atomized droplets. These droplets form a cone-shaped spray pattern that is then influenced by a second air source (“fan air”) to create an elliptically shaped spray pattern. As the fan air is increased, the size of the spray pattern increases. Through the development of the droplet and resulting spray pattern, a significant amount of energy is used resulting in a high particle velocity. Although high velocity can be used to drive coating into recessed areas, it also creates a considerable amount of overspray.
A bell, or rotary atomizer, uses mechanical shearing action to create the droplets which put less energy into the particle resulting in lower velocity. Generally, mechanics of the atomizer include a “bell cup” between 30–70 mm in diameter mounted on an air bearing turbine. Material is fed into the center of the rotating bell cup through a fluid tube and makes contact with the splash plate. The material is transferred from the splash plate to the face of the bell cup. The rotational force draws the material across the face of the cup to the edge and shears off creating particles. In some versions, the edge of the cup has serrations that causes the material to form into ligaments before breaking up into fine particles. Typical turbine rotation speeds are 20k-45k rpm for industrial coatings, and 35k-70k for automotive type coating materials. Around the perimeter of the bell cup, shaping air reduces the size of the spray pattern and provides forward direction. Increasing the volume of the shape air reduces the spray pattern size. A single or dual shape air function is provided, depending on desired pattern size.
A gun produces an elliptically shaped spray pattern and a bell produces a round spray pattern. When spraying with a gun, more articulation is required to normalize the spray pattern to the surface or edges being coated. With the round spray pattern of the bell, less articulation is needed. Studies conducted by robot manufactures have indicated that there is a 10 percent or greater improvement in programming efficiency when using a bell as opposed to a gun.
A bell typically produces droplets of similar size, resulting in a more uniform finish quality. I often use the analogy that a gun produces droplets varied in size like marbles and BBs. Imagine these on a flat surface, where there will be quite a bit of texture. A bell based on turbine rotational speed can produce all BBs or all marbles, resulting in a smoother surface.
The round spray pattern combined with more consistent droplet size from the bell will also produce a more uniform finish relative to film thickness variability, especially desirable for use with metallic coating materials to minimize mottling (light/dark streaks) in the finish.
Transfer efficiency is a measurement expressed as a percentage of the coating sprayed or how much actually goes on the part to form the film. Due to the lower particle velocity, a bell will be 15-35 percent more efficient than a gun. We have run many spray trials in our applications lab and we consistently see this result.
Originally published in the October 2015 issue.
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