Disclose is a low-pressure atomizing spray gun. The spray gun comprises an air spray gun body, a paint nozzle attached to the spray gun body, an air cap attached to the air spray gun body to cover the paint nozzle, and air groove on a tip portion of the paint nozzle. An annular slit is defined between a central opening of the air cap and the paint nozzle. The air grooves extend from at or upstream of an inlet end of the annular slit towards a discharge end of the nozzle. Also, a discharge end of the paint nozzle projects from 0.3 mm to 0.8 mm from a front end of the central opening in the air caps.
Original Assignee: Anest Iwata Corporation
The present invention relates to a spray gun for atomization of paint under a spraying air pressure of 0.07 MPa or less, and more particularly to a low-pressure air spray gun having an improved atomization mechanism used in a non-premixing type air spray gun in which compressed air and paint are mixed outside a spray head, and which is capable of providing a spray pattern which assures an improved atomization of the paint.
Non-mixing type air spray guns are widely used in the field of general industrial paint coating. They are defined as spray gun in the Japanese Industrial Standard (JIS) as well. According to the definition in JIS, the non-premixing type air spray gun is one adapted to jet compressed air from an annular slit defined between a paint nozzle and air cap and around the paint nozzle at a velocity greater than the speed of sound under a specified spraying air pressure of 0.24 to 0.34 MPa, and thus atomize and spray the paint to an object surface, thereby forming a paint coating on the object surface. This paint coating method has great applicability not depending upon any shape of an object surface to be coated and type of a paint used. However, the coating method is not advantageous in that much atomized paint is easily airborne or scattered and overspray causes a large loss of the paint. Because of the possible environment and air pollution by the airborne atomized paint with the conventional air spray guns, there has been a growing trend over the world to impose limitations on conditions of using the air spray gun.
To accommodate such a trend, various measures have been proposed heretofore. Accordingly, much attention has been focused on a low-pressure spray gun using spraying air pressure limited to less than 0.07 MPa for minimizing the airborne atomized paint and enabling an improved efficiency of paint coating to an object surface.
The low-pressure spray guns include some types based on different principles. One of the principles is to limit the spraying air pressure to less than the standard atmosphere to prevent paint particles from getting airborne or being scattered. With this spray gun, however, the limited spraying air pressure will lead to a reduced air speed, with the result that the paint atomization based on the difference in speed between gas and liquid flows, will be extremely poor. To compensate for the insufficiency of paint atomization, a low-pressure spray gun has been proposed in which the width of an air jet slit formed between a paint nozzle and air cap is increased to atomize the paint with a correspondingly increased amount of air. The mechanism of this low-pressure spray gun is basically the same as that of the conventional high-pressure spray guns. Namely, the air jet slit formed around the paint nozzle is designed wider to provide a larger amount of air jet even under a low pressure.
The poor atomization of the paint due to the reduced spraying air pressure involves some problems which cannot be solved just by increasing the air amount. Namely, it has been pointed out, as such problems, that when the amount of paint flow is increased, the central portion of the paint flow will not sufficiently mix the flow of air, resulting in an incomplete mixing, so that the paint flow in the center of the spray pattern cannot sufficiently be atomized, which is likely to take place with a paint having a slightly higher viscosity, and thus coarse paint particles will fly around the spray pattern when the width of an elliptical spray pattern is adjusted. That is, the;reduction of the spraying air pressure will lead to a non-uniform atomization of the paint.
To solve the above problems, the Applicant of the present invention has proposed to form a plurality of v-shaped air grooves in the tip of the paint nozzle as disclosed in his Japanese Patent Application No. 7-25907 (Japanese unexamined Patent Publication No. 8-196950). However, this method was found practically not satisfactory and involves some problems to be solved.
According to the invention disclosed in the above Japanese Patent Application, compressed air will flow into a flow of paint still in the paint nozzle, thereby improving the efficiency of paint atomization. However, since paint and air flows will prematurely be mixed in the paint nozzle or just before the tip of the paint nozzle, the paint spray will be limited, resulting in a lower efficiency of the paint coating.
That is, when a paint flow from the delivery portion of a paint nozzle is supplied under pressure, the amount of paint spray will depend upon the pressure applied to the paint flow, independently of the pressure and amount of the compressed air jet supplied from the aforementioned annular slit. However, in a gravity or suction type spray gun in which the paint spraying depends upon the attraction by the compressed air jet supplied from the annular slit, the condition of jetting the compressed air to the paint delivery port will not only seriously influence the atomization but also the amount of paint spray; that is, the efficiency of coating and optimization of the spray gun itself.
SUMMARY OF THE INVENTION
To atomize a paint under a low pressure and prevent occurrence of coarse particles in parts of a paint spray and non-uniform paint atomization, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a practically high efficiency spray gun.
More particularly, the present invention has a primary object to overcome the drawbacks, of the low-pressure atomization mechanism used in the conventional non-premixing type air spray gun, that atomization of paint is partially insufficient due to a low pressure of air flow, by providing a low-pressure atomizing spray gun including an atomization mechanism for a non-premixing type air spray gun, adapted to effectively mix low-pressure air jets supplied from an annular slit defined between the tip of a paint nozzle and a central opening in an air cap when the latter is fixed on the paint nozzle, with a paint flow from the paint nozzle and uniformly atomize the central portion of the paint flow. Accordingly, provided is a uniform spray pattern without reduction in the amount of the paint spray due to the attraction by the air jet supplied.
The present invention has another object to overcome the drawbacks of the prior art, that the sprayed paint particles easily adhere to the surface of the air cap depending upon a position where the mixed flow is diffused, the air cap surface has to be cleaned periodically, and it is necessary to prevent a paint coating once formed from being spoiled by the paint particles re-flying from the air cap surface.
According to a first aspect of the present invention, there is provided a low-pressure atomizing spray gun including an air spray gun body, a paint nozzle screwed to the spray gun body, and an air cap installed with a cover thereof to the spray gun body to cover the paint nozzle. The paint nozzle and air cap corporate with each other to mix, in the atmosphere, compressed air and a paint flow just delivered from the paint nozzle and to atomize the paint. The spray gun includes a plurality of air grooves formed in the tip of the paint nozzle convergently towards the center of the delivery port of the paint nozzle. And, each of the air grooves begins at or upstream of the inlet end of a central opening in the air cap.
In the above spray gun, the inner end is the starting point at which the air flow through a central annular slit will substantially form an axial flow. The air grooves are formed such that the cross section of each groove increases towards the paint nozzle delivery port end, and the plurality of air grooves extends toward the center of the delivery port end while converging at an angle of 45 to 90 degrees to the inside diameter from the outside diameter. Thus, owing to the air jets supplied from the plurality of air grooves, the compressed air is mixed with the paint flow deep to the center of the latter, thereby permitting the paint to be completely and uniformly atomized.
Also, according to a second aspect of the present invention, there is provided a low-pressure atomizing spray gun of a suction or gravity type including an air spray gun body, a paint nozzle screwed to the spray gun body, an air cap installed with a cover thereof to the spray gun body to cover the paint nozzle. The paint nozzle and air cap work cooperatively with each other to mix, in the atmosphere, compressed air and a paint just delivered from the nozzle to atomize the paint, wherein an annular-slit is defined between the tip portion of the paint nozzle and the wall of a central hole formed in the air cap. A plurality of air grooves is formed on the tip portion of the paint nozzle convergently at an angle of 45 to 90 degrees from the circumference of the nozzle tip towards the wall of a delivery hole in the nozzle to the inside diameter of the nozzle. The intersection of the bottom of a v-shaped air groove with the inside diameter of the paint nozzle approximately coincides with the front end of the central opening in the air cap, and the front end of the paint nozzle tip projects 0.3 to 0.8 mm from the front end of the central opening in the air cap.
With the above construction, the atomized paint flow is controlled against any further flying, the amount of paint spray is prevented from being reduced-under the action of the air inflow from the air grooves to limit the air jets, and the paint is prevented from adhering to the air cap, thereby assuring a stable spraying of the paint. The relatively thick air flow jet from the annular slit defined around the paint nozzle assures uniform atomization of the paint. Thus, the low-pressure atomizing spray gun according to the present invention can effectively atomize the paint as with a high pressure air spray gun.
The addition of the atomization mechanism of the present invention to the paint atomization mechanism of the conventional low-pressure air spray gun, which is based only on the increase of the central air flow, defines a positional relationship between the paint nozzle and air cap, which assures atomization of the paint with a highly improved efficiency and a reduced amount of air.
These and other objects, features and advantages of the present invention will become more apparent from the ensuing detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.