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The air spray gun (SYNO: spray gun, paint gun) uses compressed air to atomize the paint and to apply it to the surface. Compressed air and paint enter the air spray gun through separate passages and are brought together (atomization) at the air cap when leaving the spray gun. The resulting spray pattern (shape and size) depends mainly on the design of the air cap (SYNO: air nozzle).

Guide to Air Spray Guns: How to choose a spray gun?

Gravity, Suction and Pressure feed

Air Spray Guns are usually classified by the location of the material container (SYNO paint container).

Gravity feed spray guns have the material container on top (SYNO: paint cup) and the material travels down into the spray gun carried by its own weight (gravity). Here only atmospheric pressure is present. Gravity feed spray guns are suitable for a wide range of applications and materials:

  • Low to high viscosity materials (as long as the material can flow by itself from cup to gun it can be sprayed)
  • Automotive refinishing (primer, base coat, clear coat), touch-up finishing
  • General industry, incl. Glue and adhesives

Suction feed (SYNO: Siphon feed) spray guns have the material container at the bottom and material is drawn to the gun by suction. Compressed air from the spray gun creates a vacuum at the air cap which initiates the suction/siphoning action. Suction feed spray guns are only suitable for low to medium viscosity materials.

Pressure feed spray guns are connected via a fluid hose to a separate, pressurized paint container (SYNO: paint pressure pot, pressure tank). The material is pressurized in the paint container using external compressed air from a compressor. The pressure forces the material through the hose, fluid nozzle (SYNO: fluid tip) to the air cap for atomization. Pressure feed spray guns are suitable for low to high viscosity materials and are normally used when large quantities of material are to be applied in one batch, or when the material has extremely high viscosity. Pressure feed is also the fastest application method. Note: In some cases the pressurized material container is replaced by a pump system.

Fluid Nozzle: How to select the right fluid tip?

Fluid nozzles are available in a variety of sizes (Inner Diameter) to handle fluids of various types, flow rates and viscosity. The lower the viscosity of the material, the smaller the I.D. of the fluid tip, and vice versa. Fluid Nozzles are usually made from stainless steel for both non-corrosive and corrosive materials. Tungsten carbide or Carboloy inserts are used for extremely abrasive materials. The type and viscosity of the material being sprayed is the first and most important factor to consider. The viscosity of the material can be determined using a viscosity cup, i.e. the flow rate is measured in seconds (most common in finishing are Zahn and Ford viscosity cups). The measured viscosity can then be converted into another unit with the help of viscosity conversion charts, if necessary. The viscosity of the material is the most basic factor to choose the right nozzle size. But also the employed material feed system, whether pressure, suction or gravity feed, are important. For example: If a certain material works fine with a 1.3 mm nozzle on a gravity-feed spray gun, then the adequate nozzle size for a pressure feed spray gun would be usually one (or even two steps) smaller for the same material.

In general, the nozzle selection is not a stand-alone task, but must be done in close consideration with the air cap as most air caps work best with a certain fluid nozzle size.

How to select the Air Cap?

The air cap (SYNO: air nozzle) directs compressed air into the fluid stream to atomize the paint and form a specific spray pattern. As different air caps deliver various pattern characteristics, the selection of the right air cap (SYNO: air nozzle) is quite complex and many factors have to be considered:

– viscosity and volume of material to be sprayed

  • Physical size of object to be sprayed (and herewith the spray fan pattern size)
  • Desired finishing speed vs. quality (for speed nozzle combinations are used that produce a pattern as wide as possible while for better quality nozzle combinations are used that produce finer atomization and smaller pattern size)
  • Size of fluid tip to be used
  • Gun model being used
  • Available air volume and air pressure from the compressor
  • Material feed system to be used (whether pressure, suction or gravity). Often there are dedicated air caps for the different material feed systems, for example an air cap for a suction feed spray gun will most probably not work for pressure feed.

What is a HVLP Spray Gun?

HVLP stands for High-Volume Low-Pressure and uses a high volume of air (typically 280-700 lpm/10-25 cfm) delivered at low pressure at the air cap (about 0.7 bar/10 psi) in order to atomize the paint. As a result overspray and bounceback are reduced. HVLP can be used with a wide range of materials, including lacquers, stains, primers, adhesives, urethanes, acrylics, epoxies, enamels, and two-component coatings. The HVLP atomization technology was introduced in the US many years ago as a concept to provide a better painting environment for the operators.Transfer efficiency shall achieve at least 65% as required by the VOC directives. There are still certain states in the US who prescribe and control the usage of HVLP. This is usually tested with a so-called air cap testing kit.

Summarized: HVLP stands for high air consumption, low pressure at the air cap and transfer efficiency of at least 65%. It does not, as often falsely interpreted, say anything about the quality of the spray gun or the final finishing.

Does HVLP slows down the paint job?

The air cap pressure of HVLP spray guns is lower than the pressure of standard air caps, therefore the paint flow is lower (i.e. spray gun delivers slower) than that of non-HVLP air spray guns.