The flawless finish you set out to achieve is now an orange peel disaster. Consider the following: Figure 1 shows areas where pressure loss can occur in a system. It should not be one of these little dinky things where you can't distinguish anything finer than 5 The air pressure that starts out as a shout at the air source is barely a whisper by the time it enters the spray gun, usually resulting in a poor finish. and reduced coating-part interactions make pretreatment a must. If the air hose is not removed from the gun more than once or twice per day, these devices should probably be avoided. (Volume of air pressure per cubic foot). * Inlet air pressures are 85 to 90 psi (586 kPa to 620 kPa). non-HVLP a 0-100 psig gage would be appropriate. Fifty psig 2 psi 4 psi 6 psi 8 psi 10 psi * Adjusting the pressure this way can make a significant difference in how well the paint is atomized by the gun and consequently how well it flows out on the surface. When the maximum spray pattern (widest fan) is achieved, additional turns will not affect the pattern. Air pressure controlled with an air-adjustment valve-type restrictor will first exit the air cap with an initial burst of pressure, then level off to the adjusted pressure and may fluctuate with changes in system pressure. D. Atomization Air Input This is where the atomization airline is attached. Knowing the amount of air pressure at the air cap allows you to determine whether there is enough air to atomize the material being sprayed and remain compliant with environmental regulations. A poor tech sheet will just say 10 psig and leave it up to the reader to know what it means. In a conventional air spray gun, air cap pressure is restricted only by compressor capacity, regulator pressure and normal pressure drop conditions. Use one QD per gun located at the gun inlet. For example, I typically use about 50 feet of 3/8" ID air hose supplied from a wall-mounted regulator. These devices are convenient, but depending on design, can be a major source of pressure drop. With the volume requirements of HVLP spray guns, avoid using 5/16 inch hose to ensure maximum air available to the spray gun. at the inlet to the gun, with the trigger pulled, the cap pressure will be no more than 10 psig and the legal requirement for HVLP will be met. In a typical HVLP design, the regulated 50 psi (345 kPa) of air supplied to an HVLP spray gun will be reduced to a pressure of no more than 10 psi (68 kPa) as measured at the air cap. This does Unlike conventional air spray guns, modern HVLP spray guns atomize coatings by using a high volume of air at low air cap pressure, generally in the 1 - 10 psi range. A good tech sheet will tie it down to 10 psig "at the cap". gun, with the trigger pulled. Modern spray guns, particularly HVLP, require greater volumes of air to atomize today’s high solids coatings. All of the above sources of pressure loss can prevent enough air from getting to the spray gun to generate the necessary air cap pressure. HVLP gun inlet pressures required to keep the cap pressure at a maximum of 10 psig are different, depending on gun manufacturers. HVLP, in order to comply with legal requirements. This is volume of air pressure per cubic foot. For A quick disconnect at the wall may only be used once a week, a steep price to pay for the loss of pressure. Depending on the pressure and volume requirements, it is possible to have a pressure drop as little as 1 psi or as great as 25 psi for one quick disconnect. Set the air pressure at the inlet to the gun to the manufactures specs. That is On an HVLP gun this spec is usually found on the gun and is the maximum PSI it can have while still maintaining the maximum 10 lb at the cap for legal HVLP transfer efficiency (68 %). Regulator capacity specifications should exceed spray gun demand. Impact 1140. There does not have to be any paint in the gun at the time HVLP spray depends on sufficient air pressure and volume at the air cap for optimum performance. It must be measured and adjusted while the trigger is pulled. not include pressure drops for equipment upstream of the regulator, such as my air filters, will require 67 psig at the regulator to achieve 50 psig at the gun inlet. Air regulator design can also restrict air flow and can create excessive pressure drop. For HVLP guns, an equivalent pressure "at the gun inlet" “Quick disconnect” fittings are part of the sample system in Figure 1. • Air hose that is too small in diameter requirement is 13 cfm when there is 50 psig at the gun inlet. It drops again to 28 psi at the point it enters the spray gun after going through another quick disconnect. Ask yourself, does the application require them? A final suggestion is to use an air cap test kit with HVLP spray guns. However, it is recommended that a wall regulator be installed for each spray gun to limit the maximum air pressure at the spray gun. For non-HVLP guns, that pressure is usually between 40 and 60 psig. right at the gun rather than back at some wall-mounted or compressor-mounted regulator. The 10 psig maximum for HVLP guns is an environmental legal requirement in order to comply with required HVLP transfer efficiency. Examine the small opening in the female portion of the QD. High Volume Low Pressure (HVLP) It does not mean that the gun will be harmed if you increase the pressure (within reason of course)...only that you may be breaking some law, depending on your situation and location. using a gage "at the gun inlet", you will be compensating for whatever drops you may have in your own system. Here's what my HVLP spot gun looks like with gage attached: It makes a difference, sometimes a big difference to measure this pressure I can't say that I have ever found the need to increase air pressure beyond recommended ranges, although I do prefer to use the middle to upper end if a range is given. As air is forced through the line, pressure decreases the farther the air has to travel. 1/4" quick couplers between the wall regulator and the gun. Such pressure drops are caused by frictional effects in the hose and resistances in fittings, particularly quick coupler assemblies. I do not use the small 1/4" whip hose that I use with my In this configuration there are two 3/8" quick couplers and two Personally I spray at about 17 psi when spraying base coat and 20-25 for clear coat using the low volume low low pressure spray guns. I have no experience with turbine-supplied guns which are very different. tests but I haven't found that extra effort to be necessary to get results I Selecting the proper hose diameter is an important step in maintaining air pressure. No. For HVLP guns it is always 10 psig or less. •Restrictive quick disconnect designs If there is a pressure drop reading difference of more than 5 psi, the regulator may be too restrictive. With my full size non-HVLP gun, the difference is a lot greater. A 5/16 inch (7.9 mm) hose delivers less pressure than a 3/8 inch (9.5 mm) hose. That's a 17 psig drop from regulator to gun. Tips to Avoid Pressure Drop in Air Spraying. and is the pressure existing just behind the air cap and immediately prior to the small orifices in the cap. Air hose with 1/4 inch inside diameter (ID) is not recommended for use with any air spray guns. •Inadequate capacity air compressors. Remember that it does not matter what a gage back at your regulator says. The following factors contribute to pressure drop in conventional air spray and HVLP (High Volume Low Pressure) systems. I think about 30 psig is typical but I have read about pressures as low as 15 and as high as 50 psig. It will always be less than at the For my spot gun I also use an 11 foot length of 1/4" ID whip hose just prior to the gun to permit easier handling. Adjustable Air Cap. 4 Ref. Volume of air used at 40 PSI. but the air adjustment on the gun should be wide open for a maximum pattern. My HVLP spot gun has this inlet pressure stamped on the handle as 29 psig max. It is the maximum pressure that can legally be applied "at the air cap" and is the pressure existing just behind the air cap and immediately prior to the small orifices in the cap. Too often, only atomization pressure requirements are considered when both pressure and volume are critical to the proper operation of a spray gun. In a typical HVLP design, the regulated 50 psi (345 kPa) of air supplied to an HVLP spray gun will be reduced to a pressure of no more than 10 psi (68 kPa) as measured at the air cap. Perhaps the least understood of all spray system components, air hose diameter plays a significant role in proper spray gun operation.

the maximum spray gun air cap pressure is generally:

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