Air spray is a widely used painting technology in current paint construction, especially in furniture products painting. It uses compressed airflow to flow past spray gun nozzle hole to form negative pressure which can make paint inhale from tubularis, spray from nozzle, form paint mist which spray on the parts surface to form even paint film.
1. Air spray features
Air spray can produce even paint and paint layer is fine& smooth; for some covert parts (such as gap, concave-convex) it can also paint evenly; paint production is efficient and paint range is wide; paint can realize mechanization, automation, intelligence robot painting; for painting large size parts or products (such as assembled American furniture), it can show more advantages on efficient painting. However, paint loss is very big; usually use rate is 50-60%, other paint and solvents are spread into air to pollute environment. Spray construction must use specialized painting equipment and specific workplace; it requires highly to painting construction environment.
2. Basic technique requirements on air spray
When air spraying polyester or polyurethane resin, it needs guarantee spray system cleanliness that means there mustn’t be any water& oil contamination etc in whole system from air compressor to spray gun. Otherwise it will affect painting quality if oil or water mixed with paint film. During painting process, air compressor pressure can’t fluctuate and it must be controlled in a certain range; usually pressure range is 0.3-0.5MPa.
When painting air pressure is too low (lower than 0.2MPa), it will cause paint liquor atomize insufficiently and paint film is uneven; if pressure is too high, paint liquor atomizes very well; however, paint mist speed is too fast and it enlarges paint loss; also high pressure airflow sprays on un-solidified paint film which will easily cause defects such as scallops and tiny bubbles etc. At different climate and painting temperature condition, paint liquor viscosity for air spray is usually controlled 14-25s (paint—4 cups, 25℃).
In summer solvent is easy to volatilize at room temperature above 35℃, so viscosity should be lower such as 14-18s is ok; otherwise, paint film had bad leveling property; in winter at room temperature around 17℃, solvent volatilizes slowly so viscosity can be controlled between 18-25s; otherwise it is easy to cause sagging. During painting, wind speed must also be controlled as 0.3-0.5m/s; air flow direction should be from the top down; that is fresh and clean airflow flow past painting area and then removed from bottom along with paint mist; if airflow speed is too high, paint mist attachment is reduced on working and paint liquor loss is enlarged; also high air speed will speed up solvent volatilization in paint film and cause bad film leveling property and it is easy to cause painting defects such as orange peel etc.; if airflow speed is too low; it can’t function as ventilation and will affect solvent volatilization and cause environment pollution.
3. Air spray methods and skills
Air spray quality is mainly depended upon following aspects such as painting viscosity, working pressure, distance between nozzle and product and operator’s how skilled etc. To get smooth, flattening and even painting layer, one must know well proper operation method during painting and prepare well before painting: adjust paint viscosity, choose spray gun nozzle caliber, spray distance, confirm painting pressure etc. Air spray methods and skills are as follows:
（1）Use the 4th and 5th finger to wind grips slightly, index and middle finger to buckle trigger, grips is within thumb and index finger; during painting, eyesight moves along with spray gun movement, to notify paint film forming status and paint mist drop point at any time. Spray distance between nozzle& product and verticality is controlled by body itself, use body to match arm’s movement at same time with spray gun moving; wrist can’t be moved but it needs keeping flexibly.
（2）During spray gun working, it should keep right angle with painted material surface and always keeping parallel operation. Spray gun moving speed is always controlled within 0.3-0.6m/s and it would be better to keep uniform motion; otherwise it will cause paint film such phenomenon as uneven etc. the distance between spray gun and painted material should be controlled within 0.2-0.3m. If too near, it will cause paint film uneven and dry speed even, surface sagging; if too far, solvent will be volatilized too much and cause orange peel phenomenon.
（3）During painting, each paint range edge should be re-painted 1/3-1/2 area on already painted edge (that is section width or area between 2 paint trace lap joint), joint width should keep the same; otherwise paint film thickness is uneven and sometimes it will generate stripe or black spot. During multi-repeated painting, spray gun moving direction should be vertical mutually with last painting direction to make painting layer more even.
（4）Open spray gun trigger when it moves each time, also close it during its moving; otherwise it is easy to missing paint or too much paint on parts surface.
4. Air spray common trouble shooting
In actual production, air spray always shows some painting defects; if it can’t be corrected in time, it will affect painting quality.
（1）During painting, if paint atomizes too much, it will lower parts surface painting liquor attachment amount. This results from too much air pressure and paint liquor output is insufficient. It should reduce air pressure properly and enlarge paint input amount.
（2）Spraying is insufficient during painting and spray gun stops working. This is because of too low air pressure so it should increase air pressure.
（3）Atomizing is uneven or spraying is intermittent. This is mainly because nozzle air vent or painting hole is blocked or paint liquor is run out. It just needs simply clean nozzle or adding paint liquor is ok for normal painting.
（4）There is droplet phenomenon when begin painting. Main reason is spray centre doesn’t cross the open air channel. Solution is to adjust centre terminal nut and make it cross the air channel.
（5）If spray gun can only spray round paint mist flow but can’t adjust to flat flow, it is caused by nozzle vent hole is blocked. To clean vent hole can adjust it to flat paint mist flow.
（6）Paint liquor exudes from nozzle end. It is caused by spray gun block or centre sealed not well, nozzle is abraded or isn’t tightening. Clean or tighten nozzle, if it’s damaged then should replace with new one to remove the exudation.
In a word, as long as painting environment is well controlled and one can grasp air spray methods and skills, ideal painting effect can be get; also furniture products surface can be painted delectably.
