Metal Finishing Guide Book


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Page 187 of 843

A split spray pattern — heavy on each end of a fan pattern and weak in the middle — is usually caused by (1) too much atomizing air pressure; (2) attempting to get too wide a spray with thin material; or (3) too little spray material available to pump through the spray apparatus. This situation can be remedied by (1) reducing the air pressure; (2) opening the fluid control knob to the full posiFig. 14. Improper spraying creates paint losses tion; and (3) turning the spray pattern in the form of overspray, rebound, and fallout. control down to narrow the spray pattern width. This reduces the width of spray but will correct the split pattern (see Fig. 11). Spitting is caused by air entering the fluid supply, and by dried or missing packing around the material needle valve that permits air to get into the fluid passageway. Dirt between the fluid nozzle seat and body, or a loosely installed fluid nozzle, can also contribute to the problem, as well as a loose or defective swivel nut, siphon cup, or material hose. To stop the spitting, be sure all fittings and connections are tight. A fan spray pattern that is heavy in the middle, or a pattern that has an unatomized (salt-and-pepper) effect, indicates that the atomizing air pressure is not sufficiently high or that there is too much material being fed to the gun. This problem can be solved by increasing the pressure from the air supply. Correct air pressures are discussed elsewhere in this article. 182

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