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How to select a spray gun
FAQ

Spray Gun Q&A

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Presented here are points to note when selecting a spray gun and the answers to frequently asked questions.

Spray Gun FAQ

Q1: What is a round spray and what is fan spray?
Q2: Does it matter whether the spray gun is mounted facing up or facing down?
Q3: How should the spray data in your catalog be interpreted?
Q4: What unit of measure is used for spray guns?
Q5: How should spray guns be controlled?
Q6: What is the difference in how a system that separates atomized air and piston air and a system that combines them are used?
Q7: Can spray guns suction up liquid?
Q8: What cautions apply to using a pressurized tank?
Q9: The same φ0.5 nozzle is used for the ST-5 (which handles both round and fan spraying), the ST-5R (which is for round spraying only) and the ST-5S (which is for fine cone spraying). How do they all differ?
Q10: I'd like to change the nozzle orifice. Can I do that by just replacing the nozzle?
Q11: What is the best way to select a compressor?
Q12: How do I spray a liquid that contains sediment?
Q13: What is the difference between the PR-20 and the ST-5?
Q14: Nozzle orifice is the same for the PR and ST series. How do they differ?
Q15: Can spraying be done using nitrogen gas?
Q16: How heat-resistant are spray guns?

 

What is a round spray and what is a fan spray?

A round spray is when the spray emitted from the nozzle forms a round of about 15ー and a round shape forms when spraying is done level. In this state, if the air that is emitted from the pair of horns on both sides of the air cap is used to squash the pattern, it will take on a fan shape. (The result will be an ellipse with little distension.) This is known as a fan spray. Moving the item being sprayed enables a wide band pattern to be formed, making this type of spray appropriate for covering large areas. Most spray guns have a pattern adjustment knob for switching between round spraying and fan spraying, while others use special caps for one or the other that are selected from the outset. Guns with a fan-spray cap can also be used for round spraying, thus making a gun exclusively for roound spraying unnecessary. Guns equipped with a cap for round spraying, however, have the following advantages: the cap does not easily get dirty and does not easily clog, and a sharper pattern can be obtained for fine spraying.

In addition, when little liquid is used, the spread of a fan spray will decrease. However, we have spray guns that provide a wide pattern spread even when little liquid is used (the ST-6W, HM-1W and others).

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Does it matter whether the spray gun is mounted facing up or facing down?

No. There are no restrictions on this.

For nearly all types, the piston is built in. When spraying is stopped, the liquid will automatically be stopped. Thus, no matter what direction the spray gun is mounted in, the liquid will not dribble out. In addition, in terms of the spray itself, when a long distance is involved, gravity will cause mist particles to come down, but for usage at a regular distance (within tens of centimeters), this phenomenon is not pronounced.

One caution is that when spraying with the gun mounted facing up, the mist sprayed will alight over time and the air cap will be easily dirtied. Thus, in some cases it might be better to mounting the gun at an angle and keep it covered, if possible. Also, please give consideration to operational aspects, such as how to ensure dedicated cleaning.

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How should the spray data in your catalog be interpreted?

Spray guns spray a variety of liquids, including paint, oil, adhesives and seasonings. Depending on viscosity, surface tension and other factors, spray condition will also differ. However, because of the infeasibility of preparing data on every type of liquid used, the data in our catalog is for water sprays.

Using water as a gauge makes it possible to grasp differences in models and differences in nozzle orifice for the same model. In general, when liquids more viscous than water are used, pattern spread will be narrower and particle diameter bigger than in our catalog data, making it possible to generally infer what the condition of the spray would be.

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What unit of measure is used for spray guns?

The unit of measure for pressure used to be kgf/cm2 (kilogram force per square centimeter). Recently, MPa (Megapascals) has been used. Such usage is based on international agreement. The familiar way that air pressure was referred to in weather forecasts, for example, has been changed from millibars to hectopascals. The pascal is used as a unit of pressure. The conventional 1kgf/cm2 is equivalent to 0.1MPa.

Piping screw size is generally expressed in inches, such as 1/4h (nibu) or 1/8h (ichibu).
Screws are differentiated as male or female, tapered or straight, with letters used at the beginning of codes.
For example, a male straight screw would be indicated as G1/4, a tapered male screw as R1/8, and a tapered female screw as Rc1/4. ISO revision has rendered into disuse PT, PF, PS and other indications formerly used.

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How should spray guns be controlled?

Lumina automatic spray guns (except for some, like the TB-2 and HM-9) have built-in piston valves (or needle valves linked to pistons). When air over the prescribed pressure (usually 0.3~0.4MPa) enters, the valve opens and liquid is emitted. When the air stops, the valve is closed by spring force and the liquid is stopped. It is sufficient to use a solenoid valve to control airflow. No valve is necessary in the liquid path.

When using a gun with a piston-air separation system, please make sure to use a three-way solenoid valve to control the airflow for piston operation. Use of a two-way solenoid valve would prevent the air from escaping when operations are stopped. The air would then return to the piston, and the liquid would not be stopped.

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What is the difference in how a system that separates atomized air and piston air and a system that combines them are used?

Some spray guns share the air for atomization and piston operation in one pipe (the ST-5, PR-20, HM-1 and others) while others separate them (the ST-6, PR-30, HM-6, etc.).

While the shared type requires just one air pipe, they are restricted to the minimum air pressure for piston operation and thus cannot be used for blowing a soft mist with weak air. (The PR-20 enables a soft mist to be blown easily with the finger grip on the body of the gun but does not allow precise control.)

In contrast, with separation systems that use two pipes for air connection, air pressure for atomization can be set freely, irrespective of the air for piston operation, using an external decompression valve. Thus, when applying oil or the like, by atomizing at a very weak pressure of 0.02~0.05MPa, a soft mist can be blown with little upward drift. In addition, using separate solenoid valves for the air for atomization and the air for piston operation, airflow can be controlled with the timing staggered.
This makes fine control possible, such as only releasing air, releasing air first and then releasing the liquid after a slight delay, or, when stopping, stopping the liquid first.

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Can spray guns suction up liquid?

Although spray guns have very slight suction, pressure would generally need to be applied to the liquid. Applying pressure would not necessarily mean that a pump or pressurized tank would be needed. For other than highly viscous liquids, all that would be needed would be to place a container (a gravity-type container with a capacity of about 500cc to 4 liters, made of polyethylene or the like) of the liquid either in the same place or a slightly higher place than the spray gun. For water, for example, for a 1m head, 0.01MPa (0.1Kgf/cm2) of pressure is applied.

When seeking to use a large volume of a high-viscosity liquid or when wishing to place the apparatus on the floor, use a pressurized tank.

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What cautions apply to using a pressurized tank?

A pressurized tank is made to feed air regulated with a decompression valve into a sealed container of liquid and to push out the liquid at that pressure. As it is often used when viscosity is concentrated, the tendency is to want to raise fluid pressure, but if fluid pressure above the necessary level is applied, too great a volume will be emitted, thus making it necessary to restrict volume at the mouth of the spray gun. The spray gun's volume adjuster uses the gap between the nozzle and the needle for adjustment. This requires that gap to be narrowed in order to restrict the volume. Attempting to pass a highly viscous liquid through a gap on the order of 0.1mm will make the flow unstable. Thus, when using a pressure-feeding tank, take care not to apply too much fluid pressure. Although it will depend on the viscosity of the liquid and the volume you want to emit, this would usually be about 0.01MPa (0.1kgf/cm2) to 0.05MPa.

Leave the spray gun's liquid volume adjuster open at the highest position. Gradually increase fluid pressure, and fixate the decompression valve used for pressurization at a place where slightly more than the desired fluid volume would be emitted. Use the spray gun's liquid volume adjuster to make fine adjustments to obtain the desired volume.

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The same φ0.5 nozzle is used for the ST-5 (which handles both round and fan spraying), the ST-5R (which is for round spraying only) and the ST-5S (which is for fine round spraying). How do they all differ?

The ST-5 and ST-5R have the same body, nozzle and needle but different air caps. The ST-5R's air cap does not have the horns for emitting air for fan spraying that the ST-5 has. It also does not have a knob for adjusting the air from the horns. Because it lacks these extra protrusions, it is easy to mount and does not take up as much space. In addition, because the cap for fan spraying gives priority to fan-spray patterns, it has an auxiliary air hole near the center of the cap so that an attractive fan spray can be achieved. If this cap is used for round spraying, the air that is emitted through this hole will create a somewhat larger pattern; thus, the cap specially made for round spraying enables spraying to be done in a small, clear circle.

The ST-5S is for extra-small round patterns, and the shape of its nozzle, needle and cap are thus different. Specifically, the end of the needle is tapered and projects markedly from the nozzle. If the end of the needle is bent, a sharp line cannot be drawn; thus, a protective cover is provided.
Although a sharper pattern can be achieved if the cover is removed, for safety's sake, please leave the cover on.
The ST-5S should be handled delicately. Unless you need it for spraying particularly small circles or fine lines, we recommend the ST-5R, which is ease to handle and highly durable.

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I'd like to change the nozzle orifice. Can I do that by just replacing the nozzle?

In addition to nozzles, air caps and needles also come in different sizes to accommodate the orifice of liquid nozzles. Thus, please replace the entire set. (The HM series contains no needles, so please just replace the nozzle and cap.) In addition, if a nozzle is worn and you want to replace it with the same hole diameter, please also replace the three aforementioned items as a set. (The fact that the nozzle has become worn means the needle is also worn. Although it might not be necessary to replace the air cap, please do so if patterns seem narrower or somehow slack.)

For nearly all models, the needle is assembled to the piston. Although the piston can be taken apart and just the needle replaced, this is time-consuming. It is far easier to use a piston set, in which the piston and needle have been assembled ahead of time.

For the ST and PR series, a special wrench is necessary to replace the nozzle. (When you purchase spray guns in these series, the wrench will be included.) Please note that use of a monkey wrench or the like could damage the nozzle on its circumference, rendering it unusable.

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What is the best way to select a compressor?

Our catalog specifies the volume of air used for 0.3MPa. The catalogs issued by compressor manufacturers provide data on the volume of air discharged. We recommend consulting the catalogs of different manufacturers for details. Very broadly speaking, it will often be the volume of air supplied at 0.3MPa. Thus, based on this figure, please select a compressor that can supply the volume of air for the number of spray guns you will be using.

However, compressor efficiency may be deteriorated over time or by air temperature. It is also necessary to consider the loss during piping. If you select a compressor that is too near the limit of what you require, an air shortage might result, causing trouble. We thus strongly recommend that you select a compressor with a higher capacity that you actually think you need.

The aforementioned applies to continuous spray-gun operation. If spraying is done intermittently, the compressor can store air in the tank during breaks, which would make it possible to get by with a compressor with horsepower lowered by that amount. In an extreme case, even a compressor that discharges 50 L/min of air (1/2 horsepower), if the tank was completely filled with air, it would allow a spray gun that consumed 100 L/min of air to operate for about 15 sec. Thus, if there will be a break of several minutes until the next round of spraying is done, a small compressor can be used in some cases. However, if operation is done at a greater frequency than planned at the outset, a shortage of air could prevent atomization from occurring. Please consider this risk thoroughly before selecting a compressor.
To repeat, trouble that results from insufficient compressor capacity is extremely common. We thus advise avoiding making a selection that comes too close to your minimum requirements.

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How do I spray a liquid that contains sediment?

With liquids that contain sediment, if that sediment precipitates inside the gun (particularly in the nozzle) or in the piping, it could clog, preventing the liquid from being emitted. To prevent this, it is necessary to continue to keep the liquid moving, even when you have stopped spraying.

In addition to placing the liquid in a container equipped with a stirrer and providing a path for supplying the liquid to the gun, a return path from the gun to the container could be established and a pump used to keep the liquid constantly circulating. Some of our spray guns also have specifications for circulation and are equipped with an exit for the liquid that enables it to return to a tank when spraying is stopped. (Model nos. for such types have a gCh at the end, such as the PR-30C).

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What is the difference between the PR-20 and the ST-5?

Both the PR-20 and the ST-5 have one air pipe. They differ in that the PR-20 has an adjuster for atomized air and can spray easily even at low pressures. (Low-pressure spraying enables scattering to be prevented with a coarse mist and coating efficiency to be increased.)

However, the PR-20's low-pressure atomization differs from the use of a decompression valve. There is no way of knowing the pressure at which one is spraying, and fluctuations in the pressure of input air will change mist pressure. It should not thus be used when seeking to give spray particles a fixed diameter. Please keep in mind that it just a simple method of a low-pressure atomization.

In contrast, in the ST series, the ST-6 must be used for low-pressure spraying, and it is equipped with two air pipes.

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Nozzle orifice is the same for the PR and ST series. How do they differ?

In the PR series, body size is larger. Thus, the liquid path in the main unit and the size of screws for piping are also larger. As a result, even if nozzle orifice is the same, the PR series features little liquid path resistance during spraying and good emission of highly viscous liquids when spraying.

In comparison, for liquids of particularly high viscosity, such as adhesives, there is also the MS-8, which has been designed with a liquid path that is even wider than it is in the PR series.

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Can spraying be done using nitrogen gas?

As nitrogen gas constitutes 80% of the atmosphere, when used in the physical state of compressed air, it essentially is no different. However, atomizing large volumes of nitrogen gas in small rooms will result in reduced oxygen concentration, posing the danger of asphyxiation to workers. Please be very cautious about this.
In addition, make sure never to use hydrogen, oxygen or the like for atomization, as explosions or fire could result.

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How heat-resistant are spray guns?

Of the materials used in spray guns, those with low resistance to high temperatures include O-rings, packing and grease, among others. As the heat resistance of O-rings used in places that are in contact with liquid to be atomized may change due to the effect of said liquid, no generalizations can be made. Broadly speaking, even standard spray guns can be used between 80°C and 100°C.

By replacing O-rings and grease with those that have high heat resistance, it is possible to achieve heat resistance up to 290°C. Please consult us for details.

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