Guest Writer: John Owed, ITW Ransburg The object of this paper is to identify and discuss the parameters related to the application equipment, define their relative purpose and discuss methods for controlling, optimizing and/or monitoring them. Once identified, the “normal” operating parameters used for day to day operations of a finishing system can serve as a barometer of the system performance. When given parameter changes, effort should be taken to immediately identify why it changed.
Atomization Air \ Pattern Air: Atomization Air is the air pressure used to break up a fluid stream into small particles. The atomization air holes on the air cap are holes drilled in the face of the air cap surrounding the center where the fluid exists.
Pattern (also referred as Horn or Fan) Air is used to shape the atomized particles into a elliptical shape. This elliptical shape is often referred to as the spray pattern. The pattern air exits through the raised portion (horn) or the air cap.
These terms are generically associated with all types of spray applicators including, manual, automatic, electrostatic, conventional, HVLP and LVMP. The most common mistake made with these types of applicators is the failure to obtain a uniform elliptical spray pattern. All too often the applicator is adjusted by ear or feel. The operator turns up the air pressure until they can’t hear the supervisor or until the gun kicks when the trigger is pulled.
In the case of a manual application a poorly shaped spray pattern is compensated for by the operator. The operator will make adjustments in their technique based on the visual feed back they receive while spraying.
With a machine or robot mounted applicator, it is imperative that a uniform spray pattern always be maintained. Since the machine or robot program is made based on a specific spray pattern size and index, a misshapen spray pattern will cause rejects. In effort to obtain optimal results with an air atomized applicator, the atomization air and pattern air must be able to be adjusted independently.
Testing has proven that in most cases the pattern air pressure should be 5 to 10 psi higher than the atomization air pressure. The following procedure should be used to obtain an optimal spray pattern: 1. Adjust the fluid flow rate to the volume required to obtain the desired coating thickness. This volume can be calculated based on the surface area, volume solids, desired coating thickness, and approximate transfer efficiency. In the case of an existing system, start at the rate that is presently being used. 2. Adjust atomization air pressure to the lowest level at which acceptable atomization is obtained. Atomization quality can be visually evaluated by passing a metal test panel through the atomized particles at the proper target distances. Start at 15 psi of air pressure and adjusted in increments of 5 psi. Label the panels and visually compare the size of the atomized particles. Poor atomization is indicatd by large droplets nestled close together. Over atomization will yield very small dry looking particles. With good atomization the particles will have a wet appearance that are some what uniform in size throughout. 3. Adjust the pattern air by starting at the air pressure setting used for the atomization. Using a sturdy panel (that won’t move) located at the proper target distance (10-14 inches), spray the applicator for several seconds or long enough so that the pattern will sag. The air cap should be positioned so that the pattern is horizontal.
Because several spray outs are typically required, it is best to cover the panel with pieces of foil that can easily be removed and compared. Examine the spray pattern; look for the paint to sag uniformly along the length. Adjust the pattern air pressure up or down as required in 5- psi increments until a cigar shaped pattern with tapered or blunt edges is obtained. If the spray pattern has the appearance of a “dog Bone”, heavy on the ends and light in the middle, then the pattern air is too high. If the pattern looks like a “football” heavy in the center and light on the ends, then the pattern air is not adjusted high enough. If a teardrop shaped pattern is obtained (heavy on one side only), the air cap should be rotated 180 degrees. If the heavy end switches sides, then there is a good change that some of the air holes in the cap are plugged or damaged. Clean the cap and repeat the test.
If the heavy end does not move when the air cap is rotated, the fluid tip may be damaged. Inspect the air cap and fluid tip alignment. The fluid tip and air cap should be concentric. If the fluid tip is damaged and touches the air cap, this can block air flow and cause the abnormality. Once a uniform pattern is obtained, the length of it should be measured and documented.
Although the above procedure appears to look long and drawn out on paper,the actual process can be accomplished in several minutes. To eliminate the need for repeating the above procedure on multiple applicators, test air caps are available. The test cap consists of an air cap fitted with hoses and gauges. They allow the operator to check the air pressure settings directly at the applicator. Once the above settings are documented on the first applicator, the others can be quickly adjusted by using the test cap. The operator should not depend solely on the reading of a gauge at a pneumatic panel. In effort to ensure that all the spray patterns will be the same, the air pressure at the applicators must be checked. The fluid flow rate at each applicator should also be adjusted so that they are all equal. If the above procedure is followed, all applicators will have consistent spray patterns.
Air atomized electrostatic guns are typically mounted on a long stroke reciprocator. A long stroke reciprocator is generally classified as having a stroke of three feet or greater. When adjusting the cycle rate of a long stroke reciprocator the pattern width and conveyor speed must be taken into consideration. There are several overlaps that will produce a uniform film. The two most common and easily calculated are a 50 and 75% overlap. With a 50% overlap, the conveyor advances one pattern width per cycle of the machine (a cycle is made up of two strokes). In effort to calculate a 50% overlap, divide the conveyor speed in inches per minute by the pattern width in inches (the pattern width should be measured using the method outline above). The answer will be the number of cycles per minute the machine should run. Divide this number into 60 to obtain seconds per cycle and adjust the machine with a stopwatch. The maximum speed of the machine should not exceed 240 feet per minute. At speed over this, the pattern of the guns will “broom” and the atomized particles will be redirected. This speed should be checked by multiplying the cycles per minute by the total length of chain in the machine.
Please note the following example: Example: Calculating a 50% Overlap, assume the following Conveyor Speed: 15 feet x 12 in. = 180 in. Min. 1 ft. Min. Pattern Width: 14 in. Machine Data: Sprocket Centers = 6 ft. Chain Length = 12 ft. Calculating Cycles per Minute: Conveyor Speed in/min = Cycles per minute (cpm) Pattern Width 180 in/min = 12.86 cyc/min 14 in/cycle Calculating chain speed: Cycles per minute x chain length in feet = Chain Speed 12.86 cyc x 14 ft. = 180 ft. min. cyc min. In the above example, the machine speed was calculated at 180 feet per minute. This is an acceptable speed well within the range of an efficient application based on a 50% overlap. With a 75 overlap, the conveyor advances half of the pattern width per cycle. A 75% overlap is obtainable with regards to machine speed is considered more desirable. A 75% overlap can be easily calculated by doubling the numbers calculated in the above example. In the above example the machine speed of 180 ft/min @ a 50% overlap would become 360 ft/min. @ a 75% overlap. This number far exceeds the recommended maximum of 24 ft/min. In this example, a 75% overlap could not be used.
When the multiple applicators are used on a long stroke reciprocator which is typically the case, this in no way impacts the above calculations. Each applicator should be visualized putting down a uniform coat of material based on the overlap selected. The spacing of the applicators on the traveling gun bar should be such that the patterns do not interfere with each other. If 14 inch spray patterns are used, the applicators should be spaced a minimum of 16 inches apart.