ABC’s of Spray Finishing $10.
Forward While this book examines the spray finishing operation and its equipment from many viewpoints, there is still much more to be learned to become truly proficient at spray finishing. The best way to become proficient at spray finishing is to just do it! Many trade technical and community colleges offer courses in spray finishing, a great way to improve your skills. Many of the “tricks” of the professional spray finisher involve paints and coatings.
1. Introduction This book is about the selection, use and maintenance of finishing equipment: spray guns, tanks, cups, hoses, compressors, regulators, spray booths, respirators, etc. It presumes that you are familiar with standard surface preparation techniques that may be required before finishing actually begins. It also presumes a basic knowledge of the many different types of paints and coatings available.
2. Air Atomizing Spray Guns Introduction The spray gun is the key component in a finishing system. It is a precision engineered and manufactured instrument. Each type and size is specifically designed to perform a certain, defined range of tasks. As in most other areas of finishing work, having the right tool for the job goes a long way toward getting professional results.
2. Air Atomizing Spray Guns (Cont’d) A DeVilbiss Pro Tip: When using a gravity feed system, downsize the tip one size from suction. If the suction system calls for a .070", use a .055" or .063" Type Feed Viscosity (#2 Zahn) Fluid Oz/Minute Atomizing Pressure Type Production Suction up to 24 10-12 40-50 Low Gravity up to 24 10-12 30-50 Low Pressure up to 29 30-Oct 50-60 High HVLP up to 29 14-16 10 High Table 1 5.
2. Air Atomizing Spray Guns (Cont’d) PART IDENTIFICATION FUNCTION 9. What are the principal parts of a spray gun? There are various styles of caps to produce different sizes and shapes of patterns for many applications. 12. What are the advantages of the multiple jet cap? This cap design provides better atomization of more viscous materials. It allows higher atomization pressures to be used on more viscous materials with less danger of split spray pattern.
2. Air Atomizing Spray Guns (Cont’d) 15. What is the nozzle combination? Rule of thumb Rule of thumb Optimum fluid pressures are 8-20 psi. Pressures greater than this generally indicate the need for a larger fluid tip size. The lower the viscosity of the material, the smaller the I.D. of the fluid tip. In practice, the air cap, fluid tip, needle and baffle are selected as a unit, since they all work together to produce the quality of the spray pattern and finish.
2. Air Atomizing Spray Guns (Cont’d) have fluid tip openings ranging from .062" to.086", and are designed to handle viscosities up to 28 seconds in a No. 2 Zahn Viscosity Cup. For a DeVilbiss pressure feed gun, the amount of material discharged depends upon material viscosity, inside diameter of the fluid tip, length and size of hose, and pressure on the material container or pump. If the fluid tip opening is too small, the paint stream velocity will be too high.
2. Air Atomizing Spray Guns (Cont’d) the Troubleshooting Section for examples of faulty patterns to help diagnose your problem. (4) If the paint is not atomizing properly, increase the air pressure slightly and make another test pass. Continue this sequence until the paint particle size is relatively uniform. Uneven Distribution Figure 12 – Suction Feed and Gravity Feed System Components OPERATION 26.
2. Air Atomizing Spray Guns (Cont’d) 28. What are the components of a pressure feed system? A pressure feed system consists of: a pressure feed spray gun, a pressure feed tank, cup or pump, an air filter/regulator, appropriate air and fluid hoses, and an air com-pressor (see figure 16). Open fluid adjustment screw until the first thread is visible (see figure 14). 31. How is the pressure feed gun balanced for spraying? 1) Using control knob on fluid regulator, set fluid pressure at 5 to 10 psi.
2. Air Atomizing Spray Guns (Cont’d) 34. What is the proper technique for spray gun stroke and triggering? flat, or nearly flat, surfaces should be sprayed. Remember to overlap the previously sprayed areas by 50% to avoid streaking. The stroke is made with a free arm motion, keeping the gun at a right angle to the surface at all points of the stroke. When painting very narrow surfaces, you can switch to a smaller gun, or cap with a smaller spray pattern, to avoid readjusting the full size gun.
2. Air Atomizing Spray Guns (Cont’d) 38. How should guns be cleaned? A suction or pressure feed gun with attached cup should be cleaned as follows: Turn off the air to the gun, loosen the cup cover and remove the fluid tube from the paint. Holding the tube over the cup, pull the trigger to allow the paint to drain back into the cup. The solvent is contained, and must be disposed of properly. Some states’ codes require the use of a gun cleaner, and it is un-lawful to discharge solvent into the atmosphere.
2. Air Atomizing Spray Guns (Cont’d) Problem Cause Correction Fluid leaking from packing nut 1. Packing nut loose 2. Packing worn or dry 1. Tighten, do not bind needle 2. Replace or lubricate Air leaking from front of gun 1. Sticking air valve stem 2. Foreign matter on air valve or seat 3. Worn or damaged air valve or seat 4. Broken air valve spring 5. Bent valve stem 6. Air valve gasket damaged or missing 1. Lubricate 2. Clean 1. Dry packing 2. Packing nut too tight 3.
2. Air Atomizing Spray Guns (Cont’d) Problem Cause Correction Top or bottom-heavy spray pattern* 1. Horn holes plugged 1. Clean, ream with non-metallic point (ie. toothpick) 2. Clean 2. Obstruction on top or bottom of fluid tip 3. Cap and/or tip seat dirty Right or left-heavy spray pattern* 1. Horn holes plugged 2. Dirt on left or right side of fluid tip 3. Clean 1. Clean, ream with non-metallic point (ie. toothpick) 2. Clean *Remedies for the top, bottom, right, left heavy patterns are: 1.
2. Air Atomizing Spray Guns (Cont’d) Problem Cause Starved spray pattern 1. Inadequate material flow 2. Low atomization air pressure (suction feed) Correction 1. Back fluid adjusting screw out to first thread or increase fluid pressure 2. Increase air pressure and rebalance gun Unable to form round spray pattern 1. Fan adjustment stem not seating properly 1. Clean or replace Dry spray 1. Air pressure too high 2. Material not properly reduced (suction feed) 3. Gun too far from surface 4.
3. Material Containers Introduction The tank is pressurized with clean, regulated, compressed air, which forces the fluid out of the tank through the fluid hose to the gun. All spray painting systems - from the smallest brush to the most sophisticated finishing systemmust have containers to hold the material being applied. The rate of fluid flow is controlled by increasing or decreasing the air pressure in the tank.
3. Material Containers (Cont’d) 8. When is an agitator used in a pressure feed tank? 12. What materials are used to construct pressure feed tanks? When the material being used has filler or pigment that must be kept in motion to keep its particles in proper suspension. An agitator can be hand, air or electrically driven. The smaller, non-code, light-duty tanks are made of plated steel and have lower inlet pressure restrictions.
4. Hoses & Connections Introduction The various types of hose used to carry compressed air and fluid material to the spray gun are important parts of the system. Improperly selected or maintained hose can create a number of problems. This chapter will review the different kinds of hose and fittings in use, provide guidance in selecting the proper types for the job and cover the maintenance of hose. 1.
4. Hoses & Connections (Cont’d) For optimum spray gun results, the following is recommended: up to 20 ft - 5/16" I.D., over 20 ft - 3/8" I.D. 11. What is a threaded- type connection? This is a common swivel-fitting type that is tightened with a wrench.(see figure 4). 8. How are hoses maintained? Hoses will last a long time if they are properly maintained. Be careful when dragging hose across the floor. It should never be pulled around sharp objects, run over by vehicles, kinked or otherwise abused.
5. Air Control Equipment Introduction The control of volume, pressure and cleanliness of the air entering a spray gun are of critical importance to the performance of the system. Following some key installation principles will help decrease the risk of contaminants. For example, it’s important to use the right size air compressor for your application. An overworked air compressor can produce a significant amount of dirt and oil.
5. Air Control Equipment (Cont’d) *Piping should be as direct as possible. If a large number of fittings are used, larger I.D. pipe should be installed to help overcome excessive pressure drop. Check the following: a) Drain transformer, air receiver and air line of accumulated moisture. b) Be sure the transformer is located at least 25 feet from the air source. c) Main air line should not run adjacent to steam or hot water piping.
6. Respirators Introduction Consult with appropriate safety personnel or “Industrial Hygienist” if in doubt as to the suitability of a particular respirator before using it. Respirators may not provide protection against eye and skin absorption of chemicals. Spray finishing creates a certain amount of overspray, hazardous vapors and toxic fumes. This is true, even under ideal conditions. Anyone near a spray finishing operation should use some type of respirator, or breathing apparatus.
7. Air Compressors Introduction All air tools, spray guns, sanders, etc., must be supplied with air which is elevated to higher pressures and delivered in sufficient volume. The air compressor compresses air for use in this equipment and is a major component of a spray painting system. This chapter will examine the various types available.
7. Air Compressors (Cont’d) 9. Is there anything else to know, about air compressors? Because this book mainly focuses on spray guns, it provides only basic coverage of air compressors and how they operate. There is much more to know: How to select the proper equipment in terms of size, delivery, etc. Compressors may be portable or stationary and there are different models to meet a variety of needs.
8. Spray Booths Introduction Containing the overspray and keeping it out of the air and off other objects is an important consideration in a spray finishing operation. This chapter discusses various types of booths and details periodic maintenance. 1. What is a spray booth? A compartment, room or enclosure of fireproof construction; built to confine and exhaust overspray and fumes from the operator and finishing system. 4.
8. Spray Booths (Cont’d) multi-stage filters, designed for spray booth use. Single-stage furnace filters will not do the job. (b) Monitor the manometer reading daily, and know what a normal reading should be. Figure 5 - Manometer 10. What does an air replacement unit do? Figure 4 – Exhaust fan 7. What is air velocity? Air velocity in a finishing operation is the term used to describe the speed of air moving through the empty spray booth. 8.
8. Spray Booths (Cont’d) (e) Keep the booth interior and exhaust stack free from overspray and dirt accumulation. 13. What checks can be used to assure good results from a spray booth? (a) Keep the interior of the booth clean. (b) Maintain and replace intake and exhaust filters when necessary. (c) Caulk all seams and cracks where dirt might enter. (d) Maintain and clean all equipment used in the booth. (e) Keep operators’ clothing clean and lint-free.
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