Electrostatic applicators are commonly used for industrial coating applications because of their high transfer efficiency. Generally paint is atomized either through the use of compressed air or through very high paint pressure or through centrifugal force. Centrifugal atomization is accomplished by supplying a flow of paint to a surface of a device such as a disk or, preferably, a cup or bell shaped device, which is rotated at a very high speed, for example, at from 10,000 to 60,000 revolutions per minute, or more. When the paint is thrown from the edge of the rotating device by centrifugal force, it is atomized or broken up into small particles. As the paint is atomized, an electrostatic charge is imparted to the paint droplets, for example, by maintaining the rotating device at a very high voltage relative to a workpiece which is being coated. The paint particles are charged at a polarity opposite to the workpiece and are drawn through the electrostatic force to the workpiece.
The most commonly used rotary device for atomizing paint is a bell which has an interior conical surface leading to the discharge edge. The bell has an internal web or wall separating a rear chamber from the front conical surface. Paint is initially supplied to the rear chamber and is forced by centrifugal force to flow through a plurality of small circumferentially spaced holes to the conical surface. The holes serve the function of providing a more uniform paint distribution on the conical surface. In the past, the holes had to be drilled through the bell wall with a greater hole spacing than the diameter of the holes. The paint flow from the holes attaches itself to the conical bell surface as ribbons. The ribbons of paint have so much space between them, that they do not join into a uniform continuous sheeting surface at the bell discharge edge. The separate ribbons can result in the paint leaving the discharge edge in relative course, irregular sheets. From the sheets, the paint forms irregular ligaments or filaments which break up into irregular sized paint particles. This effect can be reduced by increasing the size of the bell so that the paint travels further over the bell surface. When the paint film remains on the bell surface longer, it becomes thinner and more uniform at the discharge edge, thus producing finer ligaments and smaller droplets. However, a larger bell produces significantly higher loads on the drive shaft bearings since the bell is rotated at very high speeds.
The design of the rotary atomizing device is an art wherein very small design changes may significantly effect the quality of the coating applied to a workpiece. It is known, for example, that if the paint is discharged from a sharp edge on the bell, air will be entrained in the paint particles and will produce a poor quality finish. One advance in the rotary atomizer art has been the discovery that by rounding at least the outer edge of the bell where the paint is discharged, less air is entrained in the paint and an improved finish is achieved. Another advance was the discovery that by producing a large number of small radially directed grooves in the interior conical bell surface leading to the discharge edge, the paint is forced to flow to the bell edge in a greater number of finer and more uniform streams, rather than in the wider ribbons. The paint is discharged from the bell edge in fine filaments or ligaments rather than in larger, irregular sheets which break up into more coarse and irregular filaments and then into irregular sized atomized paint particles. However, forming the grooves on the inside surface of the bell is expensive and the grooves are limited in their capability of producing uniform small atomized particles, probably due to the limited number of grooves at the bell edge. High paint flow rates and small bell diameter also limit the capability of the grooves in producing fine atomization.