When a coating is applied to the surface of a workpiece of various materials, the coating to obtain a good atomization depends on many factors. Coatings, spray gun selection and spray techniques are undoubtedly important. A thorough understanding of the spray tool, the air atomizing spray gun, is the best way to achieve high quality finishes.
Three main types of spray gun feeding
The main ways in which the spray gun delivers paint include
Gravity (upper pot), siphon (lower pot) and pressure feed three ways:
1. The paint pot of the gravity spray gun is designed on the upper part of the spray gun. The paint relies on its own gravity plus the compressed air to generate a vacuum in the venturi effect formed by the nozzle and the hood to spray the paint;
2. The siphon type (lower pot) spray gun mainly relies on the venturi effect to extract the paint from the siphon cup (the lower pot). Therefore, under the same conditions and paint flow requirements, the nozzle diameter of the siphon type spray gun is higher than that of the gravity type spray gun. Large
3. The paint delivery of the pressure-feeding spray gun is carried out by the pressurization of the paint conveying equipment, generally by the paint pressure tank or the diaphragm pump, because the paint is sent out by pressure, and the paint flow can be adjusted by applying different pressures. Generally, the nozzle diameter is smaller than the above two types of spray guns.
The basic structure of the spray gun consists of the gun body, nozzle set (hood, nozzle and needle, referred to as three-piece), control components and other accessories. In order to achieve a perfect spray effect, it is inseparable from the core components of the spray gun. The nozzle set of the spray gun is usually produced by computer inspection and monitoring. Some responsible and professional manufacturers will manually adjust each set of nozzle sets, and combine them after the actual test. Therefore, when replacing the nozzle set, the three-piece set is strictly prohibited from being randomly combined, otherwise it will affect the perfect coating effect.
Key to air atomization: hood, nozzle and needle
The nozzles and needles of the world-branded spray guns on the market are precisely machined from the durable wear-resistant and corrosion-resistant V4A steel. The inside of the nozzle is a tapered coating passage that matches the front tip of the needle. Acting as a paint valve.
The main functions of the air spray gun nozzle are reflected in the following three aspects:
1. Provide a valve seat for the needle valve (the tip end of the needle) to restrict or cut the passage of the paint;
2. Control the flow of paint;
2. Divide the paint into the atomizing gas stream. The selection of a nozzle of the appropriate diameter is usually determined by parameters such as the viscosity of the coating, the properties of the applied coating, the thickness of the coating required, and the size of the area to be sprayed. The basic function of the spray gun is to use compressed air to blow the paint into droplets and blow it onto the object to be coated. At present, air atomizing spray guns mostly use external mixed atomization. Air nozzles provide the primary method of atomization. The primary atomization is accomplished by the space between the paint nozzle and the hood (i.e., the central atomization orifice), and the air column and paint stream from the space cooperate to produce a first level of atomization of the coating. The auxiliary atomizing holes are identical to the angle of the air nozzle and are located at the end faces of the hood. These holes serve two purposes:
1. Keep the spray pattern from expanding too fast and keep the end face of the nozzle hood clean. If these holes are blocked or not present, it will cause eddy current or backflow;
2. Particularly effective for atomization of high flow rate coatings, which produces a second level of atomization of the coating; the fan control holes of the hood provide a fan-shaped pattern of airflow, without the air jets of these fan control orifices, the atomized airflow will be circular Not oval. The number and size of the air holes on the hood of the spray gun are matched with the overall design of the spray gun. Therefore, the atomization of the spray gun cannot be judged according to the number and size of the air holes.
Characteristics of three atomization technologies
(1) Conventional high-pressure atomization method: the spray pressure of a conventional high-pressure spray gun is generally 4.0 bar, and the paint is atomized by the pressure of compressed air. Therefore, the lance pressure of the traditional spray gun must be high enough to have enough energy to atomize the paint, but the high atomization pressure will cause the sprayed material to produce a large flow rate, when the paint flow rate is too fast. When the ground is sprayed on the surface of the workpiece, a certain rebound will occur to form an overspray, which greatly reduces the transfer efficiency of the paint (ie, the paint rate). Its transfer efficiency is generally only about 35%-45%, except that it is unnecessary. Waste is also wasted in the paint.
(2) HVLP high-flow low-pressure atomization method: HVLP (High Volume Low Pressure) high-flow low-pressure spray technology is a modern advanced spraying technology in the coating industry. What is high flow and low pressure? It atomizes the coating with a very low hood atomization pressure (0.7 bar, inlet pressure 2.0 bar), which reduces the coating loss and environmental pollution caused by over-spraying, with a relatively high compressed air flow (approximately gas consumption) 430 liters per minute to replenish energy and complete atomization; HVLP was first proposed in 1988 by the South Coast Air Quality Administration of California, USA, and formulated the relevant 1151 environmental management regulations. At that time, only the air hood at the hood of the spray gun was specified. A spray gun with a pressure lower than 0.7 bar (≤ 0.7 bar) and a paint transfer efficiency higher than 65%, that is, a HVLP spray gun can be used in the area. The HVLP environmentally friendly paint series spray gun features a saving of paint (actually saving 15-30% of the paint compared to conventional spray guns) and reducing pollution. However, its air consumption of about 430 liters per minute is slightly higher than that of a conventional spray gun, so it needs to be stronger. A stable compressed air supply system and a slightly different spray method than conventional spray guns.
(3) RP low flow medium pressure atomization mode: RP (Reduced = reduced; Pressure = hood pressure) paint efficient series of spray guns is the most advanced atomization technology, it combines the advantages of traditional spray guns and HVLP spray guns, The hood air pressure is between 1.2-1.3 bar; the paint transfer efficiency is above 65%, but the air consumption is lower than the traditional spray gun (295 liters per minute, inlet pressure 2.5 bar), and the operation is very close to the traditional spray.
HVLP environmentally friendly paint spray gun and RP paint efficient spray gun have a common feature, that is, the atomization pressure is lower, the transfer efficiency of the paint is higher than 65%, and there is a common feature in the nozzle structure, that is, they are all adopted. One of the most advanced designs of today, the air spoiler design, ensures that the atomizing air is evenly ejected from the inside of the hood. When the atomizing air is equalized, the atomized coating is very uniform, and thus The coating color is guaranteed to be evenly sprayed.
Because the paint transfer efficiency of RP paint efficient spray gun is very close to that of HVLP spray gun, HVLP spray gun can only be used in the United States. However, since last year, RP paint efficient spray gun has been approved and approved by more than 20 regions in the United States.