Spray gun
A spray gun is a tool for the application of paints and emulsion paints , which, after its use as Lackierspritzpistole or paint spray gun (colloquially injection part without the addition ... ...) is referred to. The material is atomized with the aid of compressed air from a compressor and / or a pump and applied to the object to be coated. Compared to manual paint application with a brush , the effectiveness can be increased by using a spray gun. By atomizing the paint, a significantly better surface quality is achieved.
The term spray gun is mainly used for devices that are used for technical and industrial paint application. The English term airbrush is used for paint spray guns for the smallest amounts of paint that are used by artists .
Basic principle
With the help of the pressure difference , a liquid substance is atomized into small drops . The droplets are deposited on the sprayed object and form a confluent surface film if they have been applied in sufficient quantities. The generation of the pressure difference and also the supply of the substance to be atomized can take place in different ways; the individual spraying processes are differentiated according to the type of pressure build-up.
Material feed
The material feed to the spray gun can be pressureless or pressurized, depending on the system.
The simplest type of feed is the gravity feed cup, which is located above the gun. Here the paint material to be applied is conveyed to the gun by gravity . This system is very simple, but has the disadvantage that the pistol cannot be tilted sharply. This material feed is used especially for airbrush work, since such airbrush guns can work with even the smallest amounts of paint.
Spray guns with suction cups have a similarly simple structure. With these, the paint to be applied is sucked out of the container with the help of a Venturi tube . The disadvantage is that the material to be applied must not be too thick in the suction cup. This disadvantage can be eliminated by pressurizing the container in a spray system with a pressure tank.
In large industrial plants, the paint is stored in a central tank and conveyed to the individual spray guns via a ring system with a circulating principle.
Types
Conventional paint spray gun
With conventional painting with compressed air, the paint is atomized with an air pressure of 2.5 to 4 bar. The air inlet pressure into the spray gun is around 2.5 to 5 bar, measured under the air cap. This spray technique achieves the finest atomization and the best spray result. The paint is broken up into droplets by the speed difference between the flow of material and air. The higher the speed difference and the negative pressure, the smaller the droplet size and the finer the atomization and spray result. However, smaller droplets produce more spray mist.
Compressed air spray gun / low pressure spray gun
In the case of compressed air-operated spray guns, the paint is atomized by the air escaping in a ring around the nozzle and carried onto the workpiece. The paint either enters the nozzle via a flow cup or suction cup attached to the spray gun or is supplied through a hose from a pressure container or a pump. This process atomizes the paint jet into very fine droplets, which results in a very good spray pattern, but also creates a lot of paint mist.
HVLP spray gun
With HVLP technology ( High Volume Low Pressure ), the air inlet pressure in the spray gun is reduced to a fraction, for example to a ratio of 1: 6 to 1:10. With an air inlet pressure of 5 bar, the air pressure under the air cap is 0.5 to 0.7 bar. The air pressure under the air cap must not exceed 0.7 bar here. This reduces the speed difference between the material and air flow, which results in larger droplets and less spray mist. Up to 80% of the paint is applied to the surface. The HVLP process was developed in California (USA) under the pressure of strict environmental laws.
LVLP spray gun
The LVLP process (Low Volume Low Pressure) is a further development of the HVLP process with significantly reduced air consumption. Good LVLP spray guns have an air consumption of only 230 to 260 Nl / min. You need up to 40% or 150 Nl / min less air compared to the consumption of conventional HVLP spray guns of approx. 380 Nl / min. The LVLP process has the advantage over HVLP processes that the reduced air flow also reduces paint mist and paint rebound. In addition, the costs for paint and filters can be reduced by up to 40% and a better spray pattern can be achieved.
Airless spray gun
With the airless spray process (painting without compressed air atomization), the paint is atomized with hydraulic pressure at 50 to 250 bar. The paint is put under pressure by a pump and pressed through a nozzle so that it is also finely atomized. The color dissolves into droplets approx. 2-3 mm behind the nozzle. Since atomization takes place without air, there is less paint mist formation.
The advantages of the airless process compared to atomization with compressed air are no bubble formation on the surface to be painted, since spraying is carried out without air supply, lower paint consumption due to less spray mist and faster painting of large areas.
The disadvantages are the less fine atomization of the paint with the formation of edge zones, which can lead to the formation of streaks when surfaces are coated where they overlap. This process cannot be used to stain or patina translucently. The application quantity cannot be regulated during spraying and the high pressure increases the risk of accidents.
Air-assisted airless spray gun / AirCombi spray gun
This is air-assisted airless spraying, i.e. a combination of airless atomization and air atomization. This allows better effectiveness and finer atomization in edge areas to be achieved. With air-assisted airless spraying, the material inlet pressure is usually between 30 and 80 bar. The speed of the paint in the nozzle is around 50 to 100 m / s.
With the air-assisted airless process, the spray of paint created by the material pressure is formed with two additional air currents, which flatten it and mix with the paint. This creates smaller droplets and a better surface. The edge zones are softer, which means that the transitions are smoother. The air pressure under the air cap is 0.8 to 1.5 bar and the exit speed is around 150 to 200 m / s.
literature
- European reference book series Holztechnik Fachkunde 20th edition Verlag Europa-Lehrmittel 2005, ISBN 3-8085-4039-7 , pp. 268-271
- Training documents for paint spray guns, Bersch & Fratscher GmbH, 3rd edition, March 1st, 2009 - Explanation of the different spraying methods. Optima paint spray gun product range.
Individual evidence
- ↑ H. Römpp; Römpp Lexikon Lacke und Druckfarben , p. 537, Thieme, Stuttgart, 1998, ISBN 978-3-13-776001-6 .