Image credit: CJ Spray; Northern Tool; Powder Gun Supplier
Spray guns and applicators are responsible for the aim and spray pattern of a spray media, such as paint, chemicals, or powder-based surface coatings. These system components typically have a trigger or button to initiate the spray when they are manually operated, or they reserve a mount for automated spray operations.
Spray Gun and Applicator Operation
Spray guns and applicators are generally available in two control options: manual or automatic. Manual designs are held by the operator, who physically articulates the spray direction and activation. Often, the operator can adjust the nozzle's spray pattern and intensity. Automatic designs are affixed to a robotic arm or a stationary mount where parts feeders or conveyors process finished and unfinished substrates.
Some nozzles are relied upon to provide the atomization of the spray media, and others serve as the point where two separate agents are mixed for the first time. Applicators vary considerably from application to application, as well as by the means of how the coating material is driven.
Hydraulic spray applicators rely upon the turbine, compressor, or pump to impart kinetic force on the surface coating. The nozzle determines spray pattern, and may be integral to the spray media's atomization.
Rotary atomizers utilize a spinning disc or bell to discharge particles away from the applicator.
Ultrasonic applicators use a piezoelectric crystal to separate liquids or powders into particulate form by high frequency vibration.
Electrostatic applicators incorporate an electrode into the applicator to charge the powder or liquid. At the nozzle, the spray agent repels from itself, while being attracted to a negatively charged or grounded surface.
Hydraulic Nozzles—Single Fluid
||Minimal atomization with a direct, concentrated stream.||
||A V-shaped notch fans the spray into a cone spray pattern.||
||An impinged orifice forces the spray into a variety of shapes, dependent upon the geometry of the impingement. In the example at right, a flat surface results in a flattened, one-direction spray. It can produce a hollow or full cone spray.||
||A hallow cone is produced by the use of a spiraled nozzle chamber. A compressed gas is forced through the center of the chamber to form a perimeter-only spray.||
||A swirling motion is induced within the nozzle, but gas pressure is reduced to produce a solid cone spray pattern.||
||An assembly in which more than one outlet is incorporated into the applicator.||
Images credits: Wikimedia
Hydraulic Nozzle—Two Fluid
Internal and external two-fluid nozzles Image credit: Wikimedia
Two-fluid nozzles combine pressurized gas and the spray agent within the nozzle to initiate atomization. They are classified either as internal mix (if the two fluids combine within the nozzle) or external mix (if the two fluids first interact at the orifice). External mix nozzles are preferred for solids. Two fluid nozzles are prone to failure if droplet size is excessive.
Video credit: Ledebuhr Industries via YouTube
The rotational speed of the rotary atomizer determines droplet size. These devices are available in a variety of sizes and configurations, and their main advantages include low levels of paint rebound and overspray. Rotary spray guns are usually automated, and the media is released from orifices around the perimeter of the atomizer.
Video credit: Ledebuhr Industries via YouTube
Ultrasonic applicators rely on vibrational energy (20-50 kHz) to produces a gentle, low-velocity spray. It is frequently used to apply a liquid coating to fragile materials. However, the pzieolastic crystal does not propel the liquid coating, and the applicator is typically fed by gravity or a low pressure pump. As such, ultrasonic nozzles are best utilized to create a downward spray in a minimal-turbulence environment. Drop size and flow rate are determine by the frequency of operation, while patterns are determined by geometry of the exit orifice. These devices are efficient and reliable, and find employment in laboratories and commodities manufacturing.
Electrostatic spray painting
Video credit: Highams Park Science via YouTube
This method is efficient for applying liquid coatings and lubricants, and in this instance the nozzle imparts a high voltage, low current charge to the spray media. Hydraulic pressure feeds the applicator, and a liquid jet disperses via the means of the electric field established by a charge imparted to the workpiece or by a counter electrode. The spray media is uniformly deposited around the workpiece and there is minimal overspray. Electrostatic nozzles are hard to clog and offer great control. Water-based liquids are difficult to spray by this method.
Materials of applicator construction largely depend upon the application and environment intended for its use. Many handheld applicators utilize rubber and plastic housings to protect components and make the design ergonomically functional. Nozzles and internal components are frequently metal, such as brass, stainless steel, and nickel alloys, but other PTFE, PVC, and ceramic materials are also used.
Spray applicators rely upon nozzle geometry to determine spray shape and intensity. The resultant spray patterns resemble one of the following.
Stream: A spray agent is propelled in a homogeneous liquid to provide a surface coating. Almost no atomization occurs at low pressures. The nozzles used for this type of spray are exceptionally basic.
Hollow cone: The spray is distributed from the nozzle in a ring. Very small droplet sizes can be achieved, and there is little chance of providing an uneven coating to the surface.
Full cone: The spray is applied in a round dimension with no coverage absence. Medium to large-sized droplets are produced. This is the most widely used spray pattern.
Flat spray: A thin, long spray outline distributes a coating across the surface. The pattern is uniform across its application and it is helpful for large, flat surface areas.
Images credit: Ikeushi USA
The size of individual particles of the spray is affected by nozzle design, pressure intensity, spray capacity and angle, and the surface tension, flow rate, specific gravity, and viscosity of the spray agent. There is a fine balance between the pressure needed for the spray to reach the component and the droplet size and spray volume required to ensure coverage.
An increase in spray capacity and the surface tension and viscosity of the media will result in larger droplet sizes, while an increase in pressure, spray angle, and flow rate will reduce droplet size.
Image credit: Ikeushi USA
Spray Angle and Coverage
While the spray angle for the nozzle is not affected, the coverage of the spray is partially determined by the distance from the substrate. Spray angle is important to have efficient application of the coating, while covering a larger area helps accelerate the spray process. Liquids more viscous than water typically form smaller spray angles, and vice-versa.
Image credit: Wikimedia
Several properties affect the ability of a surface coating to be sprayed.
Temperature: Temperature affects coating viscosity and surface tension. Manufacturers frequently measure droplet sizes in laboratory settings at 70° F, a temperature considered optimal for liquid spray agents.
Viscosity: Liquids with high viscosity require a higher minimal pressure to initiate atomization. High viscosity also creates higher droplet sizes and smaller spray angles.
Surface tension: Lower surface tensions for can be operated at lower pressures, and higher surface tensions are adverse to hollow cone spray patterns.
Spray Gun and Applicators Standards
The following standards may be useful when considering spray guns.
-BS 6356 P1 Testing agricultural spray nozzles
-A-A-50310 Heavy duty spray paint guns
-BS 6742 P1 General specifications for electrostatic spray guns
-BS EN 50580 Handheld, electric spray guns
-SAE AMS-G-952 Oil and solvent spray guns
-A-A-55748 Manual insecticide sprayers
-BS EN 13966-1 Transfer efficiency of atomized and sprayed liquids
-BS EN 1953 Spray equipment safety
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