Rotational bell painting

A rotary atomizer is a paint applicator used in high volume production environments. Also called a 'paint bell', or 'bell applicator', it is preferred for high volume paint application for its superior transfer efficiency, spray pattern consistency, and low compressed air consumption, when compared to a paint spray gun.

Assemblies

The typical bell applicator consists of five major assemblies: the valve module, the bell cup, the turbine, the shaping air shroud, and the electrostatic system.

The valve module is a manifold consisting of passages for paint, solvent, and compressed air, and valves to control the flow of materials for paint delivery, cleaning and purging with solvent, and management of compressed air to the valves, turbine, and shaping air shroud.

The bell cup is a conical or curved disc fixed to the shaft of the turbine. Paint is injected into the center of the rear of the disc, and is atomized by being forced out to the edge of the cup by centrifugal forces. The flow of the paint over the cup and off the edge breaks up the paint into atomized droplets.

The turbine is a high speed, high precision air motor that rotates the bell cup at speeds ranging from 10,000rpm to 70,000rpm, depending on the cup diameter, atomization desired, and physical properties of the paint. Typical turbines for this application use an air bearing, where the spinning shaft is suspended in a cushion of flowing compressed air, with virtually no frictional resistance.

The shaping air shroud, or shaping air ring, is simply a ring with passages for air to flow out the front of the atomizer, outside of the cup diameter, to manage the size of the spray pattern produced. As more air is forced through the shroud, the atomized paint is forced into a smaller pattern.

The electrostatic system can be internal or external (or direct or indirect charge), and supplies high voltage (30,000 to 100,000 volts DC) charge to the applicator, or the air surrounding it. This has the effect of negatively charging the paint, while causing a region of positive charge to form on the workpiece, resulting in electrostatic attraction between the paint and the workpiece. The electrostatic system is visible only on an external (or indirect) charge applicator, where it appears as a series of 4-8 forward-facing electrodes in a circular array around the bell. It is typically called a 'candelabra' for its resemblance to one.

The bell is also used in powder coating. Although the bell looks different from a topcoat bell.

Introduction to the Graco Probell article can be found here.