Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 124 of 331

ey can buy, but without skilled operators the entire system will fail. Examine the following variables that can lead to prob- lems in the finishing room. Failure to observe these rules may result in finishing problems. These are the five operator variables that must be controlled to im- prove application efficiency. Fig. 9. "C" spray pattern caused by dried material clogging the side port (A). To correct the problem, dis- solve the dired material w ith thinner. Do not use met- al devices to probe into the air nozzle opening. 1.Distance of gun to work. 2.Stroking speed. 3.Pattern overlap. 4.Spray gun attitude (heeling, toeing, fanning). 5.Triggering. Figures 5 through 8 illustrate proper versus improper spraying technique. SPRAY PATTERNS AND PRESSURES The general rule of thumb for setting pressures is to always use the lowest pres- sure that will give a satisfactory pattern. This rule applies to all spraying processes. Failure to observe this rule, or using faulty or dirty nozzles, will usually result in faulty spray patterns. Typical patterns with suggested corrections are shown in Figures 9–13. Keeping the spray gun too far from the substrate will increase the likelihood of fallout. Fan patterns that are excessive in width will increase overspray. Excessive pres- sures will contribute to off-spray and rebound (see Fig.14). MAINTENANCE The spray gun is a precision tool and will perform best if kept clean and lubricated. Siphon Spraying: Wipe off the siphon tube with a solvent rag. Dip the siphon tube into a container of clean solvent and spray. Trig- ger repeatedly to thoroughly flush the passageway and clean the fluid nozzle and needle. (Consult local codes for restrictions on spraying solvent.) Gravity Spraying: Turn off atomization air to spray gun. Remove material from cup. Wipe interior of cup clean with solvent rag. Pour solvent into cup. Pull trigger allowing gravity to flush all fluid pas- sages. Repeat until clean. Pressure Spraying: Substitute clean solvent under pressure for the paint being sprayed, using low flu- id pressure (no atomizing air is necessary) trigger the gun repeatedly to permit the solvent to flush out the passageway. Do this until clean. Wipe off the gun body with a cloth wet with sol- vent and lubricate the spray gun with a drop of light machine oil each day. Fig. 10. Inverted tear-drop spray pattern caused by dried material stuck to the outside of the fluid nozzles tip (B), or by a loose air nozzle or a bent fluid nozzle or needle tip. If dried material is the problem, remove air nozzle and wipe off fluid tip, using a rag wet- ted with thinner. Tightewn the air nozzle and replace the needle, if bent. 123

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