LOW-PRESSURE ATOMIZING SPRAY GUN

Abstract: (57) Abstract: A low-pressure atomizing spray gun, wherein a plurality of V-shaped air grooves (1a) are drilled at the center of a paint nozzle spray port located at the tip of a paint nozzle (1) of a low-pressure spray gun so that these grooves are converged in the direction of spraying, and guide walls (1b) which are opened divergently in conical shape are formed in front of a position where the groove bottom parts of the air grooves (1a) cross the inner diameter of the paint nozzle, whereby an air flow sprayed from a center air port of an air cap (2) is collided crossingly with a paint flow sprayed from the paint nozzle spray port for atomization; the guide walls (1b) located at the tip of a paint nozzle (11) being formed projectedly from the front end surface of the air cap (2).

Description: TECHNICAL FIELD The present invention relates to a spray gun for atomization of a 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 a compressed air and a paint are mixed outside a spray head and which is capable of providing a spray pattern which assures an improved atomization of the paint. BACKGROUND ART The 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 a 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 high speed than the sound velocity 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 a widest 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. In this situation, 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 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 with 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 coarse paint particles will fly around the spray pattern when the width of the elliptic 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 solve. According to the invention disclosed in the above Japanese Patent Application, compressed air will flow into a flow of paint yet 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 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 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. DISCLOSURE 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 a paint is partially insufficient due to a low pressure of an 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, 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 pain flow, to thereby provide a uniform spray pattern without reduction in amount of the paint spray due to the attraction by the air jet. 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 that 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 the first aspect of the present invention, there is provided a low-pressure atomising spray gun as set out in Claim 1. Preferred features are set out in Claims 2 to 6. Thus, owing to the air jets from the plurality of air grooves, the compressed air is mixed with the paint flow deep to the centre of the latter, thereby permitting to atomize the paint completely and uniformly. In addition, the atomized paint flow is controlled against any further flying, 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, whereby assuring a stable spraying of the paint. The relatively thick air flow jet from the annular slit defined around the paint nozzle assures to uniformly atomize 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, and addition of the atomization mechanism according to the present invention to the conventional low-pressure air spray gun based only on the increase of the central air flow, assures to atomize the paint with a reduced amount of air and a highly improved efficiency. 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. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a conventional low-pressure atomizing spray gun as a whole; FIG. 2 is a sectional view, enlarged in scale, of the front end portion of the low-pressure atomizing spray gun according to the present invention; FIG. 3A is an explanatory drawing, enlarged in scale, of the paint nozzle tip and air cap; FIG. 3B is a projection view of the paint nozzle tip and air cap from the delivery port; FIG. 4A is a sectional view, enlarged in scale, of the paint nozzle tip and air cap; FIG. 4B is a projection view of the paint nozzle front end portion and air cap from the delivery port; FIG. 5 is a perspective view of the air grooves in the paint nozzle tip; FIG. 6 is an explanatory drawing, enlarged in scale, of the paint nozzle tip; FIG. 7 is a sectional view of the paint nozzle tip and air cap when paint and air flows crossingly collide with each other and the paint is atomized; FIG. 8 is also a sectional view of the paint nozzle tip and air cap when paint and air flows crossingly collide with each other and the paint is atomized; and FIG. 9 is a projection view of the paint nozzle from the delivery port in FIGS. 7 and 8. BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, there is schematically illustrated the construction of a conventional low-pressure atomizing spray gun which will be illustrated and described by way of example herein for the better understanding of the present invention. The body of the spray gun is generally indicated with a reference 10. As shown, the spray gun body 10 includes a barrel 20 and grip 30. An air inlet fitting 31 is provided in the lower portion of the grip 30. The air inlet fitting communicates with an air passage 32. It is to be connected to a compressed air source. Compressed air supplied from the source through the air inlet fitting 31 is fed to the tip of the spray gun body 10 through an air valve chest 16 provided above the air passage 32. The air valve chest 16 includes an air valve seat 14, air valve 15 and packing set 19. The air valve chest 16 has also a coil spring 17 by which the air valve 15 is pressed to the air valve seat 14 of the air valve chest 16, whereby the air valve is sealed. There is also provided a cap screw 18 to adjust and set them. The air valve 15 has a rod 15a extending to a trigger 13. When the trigger 13 is pulled, a needle valve guide 5a is slid back to pull a needle valve 5 and the air valve 15 is opened slightly earlier than the needle valve 5 thus pulled, so that the compressed air will be fed slightly earlier than the paint is delivered from a paint nozzle 1. There are provided on a rearward extension line of the center of the paint nozzle 1 screwed to the barrel 20 the needle valve guide 5a to pull the needle valve 5 linearly and also a guide chamber 23 to guide the needle valve guide 5a. Compressed air is fed around the guide chamber 23. The needle valve 5 is pressed by a coil spring 22 provided behind the needle valve guide 5a to the inner surface of a seat in a delivery port of the paint nozzle 1, whereby the needle valve 5 is sealed. The coil spring 22 is retained by a paint delivery control knob 21. As the paint delivery control knob 21 is screwed, a guide rod provided behind the needle valve guide 5a abuts the paint delivery control knob 21 to limit the sliding stroke of the needle valve 5, that is, the clearance between the seat of the delivery port 100 of the paint nozzle 1 and the needle valve 5, whereby the paint delivery can be controlled. Also, there is provided above the paint delivery control knob 21 of the barrel 20 a pattern divergence adjuster 24 to shunt the compressed air to the center and front end 2a of an air cap 2 and adjust the amount of air to the front end 2a. The pattern divergence adjuster 24 is screwed along with a pattern divergence adjusting valve 27 and pattern divergence control knob 25 to a pattern divergence adjustment guide 26. By turning the pattern divergence control knob 25, the amount of compressed air to the lateral air holes 2c in the front end 2a of the air cap 2 screwed with an air cap cover 3 to the barrel 20 is adjusted in accordance with the clearance between the pattern divergence adjusting valve 27 and a valve seat 28 provided in the air passage and thus the divergence of a sector-like spray pattern is adjusted. A passage (not shown) of the compressed air fed from the air passage 32 formed in the grip 30 is formed in parallel to and alongside an air passage 9 provided in the seat 28 of the pattern divergence adjusting valve 27. Therefore, the air passage is branched out at the seat 28 into two of which one supplies the air to the center of the air cap 2 while the other supplies the air to the front end 2a. There is no control in the passage for the air supplied to the center of the air cap 2, and so an air pressure from the air passage 32 will be supplied at it is to the center of the air cap 2. In the above spray gun, when the trigger 13 is pulled about a trigger pivot 13a like a pendulum, the air valve 15, and then the needle valve 5, is pulled. On the other hand, paint is supplied to the paint nozzle 1 from a paint source (container or hose; not illustrated) connected to a paint joint 8. There is provided a needle valve packing 11 to provide a sealing against paint leak from the seat of the delivery port of the paint nozzle 1 and needle valve 5 behind the paint nozzle 1. The needle valve packing 11 is retained by a packing adjusting screw 12. In case of a suction or gravity type spray gun adapted to suck and spray a paint with the attraction by the compressed air jet from the central opening of the air cap, the needle valve packing 11 functions also to prevent the compressed air from entering the paint passage. The packing adjusting screw 12 is screwed with an appropriate tightness to tighten the needle valve packing 11, prevent paint leak or suction of outside air, and for the needle valve 5 to be able to smoothly work. Referring now to FIG.2, there is illustrated in the form of a sectional view, enlarged in scale, the front end portion of the low-pressure spray gun according to the present invention. In FIG. 2, the same or similar elements as in FIG. 1 will be indicated with the same or similar references as in FIG. 1. As shown, the paint nozzle 1 is screwed with a paint nozzle screw If to the barrel 20 of the spray gun body 10 and lower tapers le and 1g are connected to each other so that supply of the paint from the paint joint 8 is shut off and supply of the compressed air to the annular slit 4 and front end 2a is also shut off. The air cap 2 is installed to the barrel 20 with the air cap cover 3 under which a cover ring 7 is placed, to thereby cover the paint nozzle 1. An upper taper 1d is provided to shut off supply of the compressed air to the center and front end of the air cap as with the lower taper 1e. The inside of the paint nozzle 1 is sealed since an inner taper of the paint nozzle 1 is pressed by an end taper of the needle valve 5. The compressed air under a low pressure, shunted inside the barrel 20 and supplied through the air passage 9, is passed via a lateral air passage 2b in the front end 2a of the air cap 2 through a collar 6 and jetted out of symmetrically formed lateral air holes 2c to form an elliptic spray pattern. One or more lateral air hole 2c is provided in each of symmetrical positions in the front end of the air cap 2. By adjusting the amount of air jet from the lateral air holes 2c by means of the pattern divergence adjuster 24, the sector-like divergence of the spray pattern is adjusted. The other air flow shunted inside the barrel 20 is passed from the air passage 32a to the center of the air cap 2 through a central air hole 1c formed in the paint nozzle 1. There are provided in the center of the air cap 2 the annular slit 4 defined between the front end circumference of the paint nozzle 1 and the central portion of the air cap 2, and auxiliary air holes 2d crossing the extension line of the air jet from the lateral air holes 2c. The auxiliary air holes 2d are destined to balance the spray pattern correspondingly to the force of the air jet from the lateral air holes 2c. One or more such hole 2c is provided. The tip of the paint nozzle 1 is located in the central opening of the air cap 2 to define the annular slit 4, and the plurality of air grooves la formed in the tip of the paint nozzle 1 supplies radial air flows to the annular slit 4. As shown in FIGS. 3 to 5, the air grooves la are formed convergent from the outside diameter of the tip of the paint nozzle 1 towards the center of the delivery port 100 of the paint nozzle 1 and the bottom of each air groove 1 a is at the inside diameter of the paint nozzle 1. The angle of the convergence is within a range of 45 to 90 deg. whereby the air flows coming into the air grooves 1a collide with the paint flow from the paint nozzle so that even a low-pressure air flow can mix with the paint flow deep to the central of the latter, thus providing a complete atomization of the paint. Note that the number of the air grooves 1a is not limited but when the difference (thickness) between the outside and inside diameters of the paint nozzle 1 is 0.5 to 2 times of the inside diameter, four air grooves la should suitably be provided. Normally, to form a spray pattern, the compressed air jets from both sides are directed towards the center of the central spray flow to spread the spray perpendicularly to the compressed air. To balance the air flow, the air grooves should preferably be provided at 6 to 8 places. Each of the plurality of air grooves la is formed from a V-shaped groove 101 starting at a point 102 which is inside an inlet end 201 of the central opening of the air cap 2. See FIG. 4. Normally, the inlet end 201 of the central opening 200 in the air cap 2 adjoins a large angle-tapered surface 202 of the air cap 2. The junction of the inlet end 201 and taper surface 202 is rounded in some cases. In such a case, the compressed air flows into the central opening 200 substantially at the inlet end 201. Therefore, when the compressed air supplied into the air cap 2 flows into the central opening 200, portions thereof passing through the air grooves 1a will flow into the central hole 201 and thus effectively collide with the paint flow while increasing the area of gas-liquid contact. Divergently tapered guide walls 1b are provided at positions outer than the intersection of the air grooves la and inside diameter of the delivery port 100 of the paint nozzle tip. The angle of the divergence of the guide walls 1b is about 90 deg. The guide wall 1b extends from the delivery hole 100 to near the outside diameter of the paint nozzle tip to guide the paint flow at the same angle for a divergently wide spreading. Thus the paint flow will crossingly collide with the air flow jet from the annular slit 4. Further, the starting points 102 of the plurality of air grooves 1a are positioned at or upstream of the inlet end 201 of the center hole 200 of the air cap 2, so that the compressed air under a relatively low pressure can cut into the paint flow deep to the center of the latter and disperse the paint, thereby assuring an improved atomization of the paint. Also, since the air grooves 1a extends to the inside diameter of the delivery hole 100 of the paint nozzle 1 and the guide walls 1b extend divergently in a conical form forward from the other end of the delivery hole 100, the paint flows colliding with each other and thus dispersed inside the delivery hole 100 of the paint nozzle 1 can be prevented from being diffused more than necessary and hence the paint flow can positively collide with the compressed air jet from the annular slit 4 and be atomized with a higher efficiency. FIG. 4A is an explanatory drawing, enlarged in scale, of the paint nozzle front end portion, and FIG. 4B is a projection view of the paint nozzle front end portion from the delivery port. FIG. 4A is a sectional view taken along the line A-O-B in FIG. 5B. FIG. 6 is an explanatory drawing, enlarged in scale, of the tip of the paint nozzle 1. As shown, the air grooves 1a each being a V-shaped one opened at an angle β of more than 45 deg. are formed to extend towards the center of the paint nozzle 1. The divergent angle β of the V-shaped groove is 90 deg. to which however the present invention is not limited. The reason why the air groove 1 a is formed as a V-shaped one is that the paint flow has to be split by the air flow directed to the center of the paint flow. The radius of curvature of the groove bottom should be smaller than the inside diameter of the delivery hole 100 of the paint nozzle 1, and preferably be smaller than a half of the inside diameter. The conical guide walls 1b are provided at the front end of the air grooves 1a. The guide wall 1b is divergent at an angle α. As shown, this angle α of the guide walls 1b is within 90 deg. since the guide walls 1b extend in a cylindrical form from the delivery hole 100 of the paint nozzle 1. Namely, the divergently conical spreading of the guide walls 1b makes more effective the paint atomization by the crossing collision of the paint flow with the compressed air jetted forward. Owing to the aforementioned construction, the compressed air will mix with the point flow deep to the center of the latter and the paint will be dispersed. The thus dispersed and diverged flow of paint particles is controlled by the guide walls 1b in the flowing direction of the paint flow, the compressed air flow layer supplied from the annular slit 4 around the paint nozzle 1 and having a relatively large thickness will atomize the paint flow uniformly deep to the center of the latter. Thus, the paint will be atomized with a uniform distribution of the paint particles and a high efficiency. Furthermore, by projecting the paint nozzle 1 more forward, it is possible to prevent the paint from adhering to the air cap 2 and thus assure a stable spraying of the paint. FIGS. 7 and 8 show the flows of paint from the paint nozzle 1 and compressed air jet from the annular slit 4, and FIG. 9 is a projection view of the paint nozzle from the delivery port 100 in the paint nozzle 1. FIG. 7 is a sectional view taken along the line D-O-B in FIG. 9, and FIG. 8 is a sectional view taken along the line A-O-C in FIG. 9. It should be noted that the white arrow indicates the air flow while the black arrow indicates the paint flow. As will be seen from these drawings, both the air flow from the annular slit 4 and those from the air grooves 1a around the paint nozzle 1 will cut into die paint flow deep to the center of the latter and contribute to the paint atomization. The air flows from the air grooves 1a (V-shaped) can effectively cut into the paint flow. However, such air grooves, if applied, in the conventional gravity type or suction type spray gun will limit the delivery of the compressed air, leading to a reduced delivery of paint spray. To assure a paint spray delivery of 100 to 200 ml/M which will not influence the paint coating, the convergent angle β of the air grooves 1a should desirably be as small as possible within a range of about 45 to 90 deg., and the geometrical relation between the paint nozzle 1 and air cap 2 is such that the tip of the paint nozzle 1 projects 0.3 to 0.8 mm from the central opening of the air cap 2. However, it should be noted that if the starting point of the V-shaped groove 101 of the air groove 1a is located beyond the central opening 200 of the air cap 2, the paint flow will not effectively be atomized. Namely, a smaller the convergent angle β of the V-shaped groove 101 will provide a longer guiding by the groove, namely, a more effective delivery of compressed air and will reduce the influence of the air grooves on the delivery of paint spray. Also, by projecting the tip of the paint nozzle 1 forward from the front end of the air cap 2, it is possible to effectively prevent the delivered paint particles from adhering to the air cap 2. INDUTRIAL APPLICABILITY As having been described in the foregoing, the present invention provides a low-pressure atomization spray gun with which a paint flow under a pressure of less than 0.07 MPa can be atomized, the paint mist can be prevented from being easily airborne and the paint can be sprayed with an improved efficiency. Therefore, the spray gun according to the present invention can overcome the drawbacks such as mist scattering and loss of the paint due to an overspray with the non-premixing type air spray gun and will contribute very much to the improvement of working environment and prevention of air pollution.

Claims: A low-pressure atomising spray gun including an air spray gun body (10), a paint nozzle (1) screwed to the spray gun body (10), and an air cap (2) installed with a cover (3) thereof in the spray gun body (10) so as to cover the paint nozzle (1); the paint nozzle (1) and air cap (2) between them defining an annular slit (4) formed between a top portion of the nozzle (1) and a wall of a central opening formed in the air cap (2) and working cooperatively with each other to mix, in the atmosphere, compressed air and a paint just delivered from the nozzle (1) to atomize the paint; the said spray gun further comprising a plurality of air grooves (1a) formed on the tip of the portion of the paint nozzle (1) convergently towards the centre of a delivery port (100) of the paint nozzle (1) so that the intersection of the bottom of the air grooves (1a) with the inside diameter of the paint nozzle (1) approximately coincides with the front end of the central opening in the air cap, and each of the air grooves (1a) starts at or upstream of the inlet end of the annular slit (4), and being characterised in that : the front end of the paint nozzle tip projects 03 to 0.8 mm from the front end of the central opening in the air cap. A low-pressure atomising spray gun according to Claim 1, wherein the plurality of air grooves (1a) are formed such that the cross section of each groove (1a) progressively increases towards the paint nozzle delivery port end and the bottoms of the plurality of air grooves (1a) extend from the outside diameter to the inside diameter of the paint nozzle (1). A low-pressure atomising spray gun according to Claim 1 or 2, wherein the air grooves (1a) converge at an angle of 45 to 90 degrees and each groove (1a) has a V-shaped section. A low-pressure atomising spray gun according to any preceding claim, wherein fowardly diverging conical guide walls (1b) are provided at the intersection of the air groove bottoms with the inside diameter of the paint nozzle (1). A low-pressure atomising spray gun according to Claim 4, wherein the front ends of the guide walls (1b) project from the front end of the central opening in the air cap (3). A low-pressure atomising spray gun according to any preceding claim, wherein the wall of the central opening in the air cap (3) is slightly tapered outwardly from the inlet side to outlet side.

Inventors: KANEKO MASARU, KANEKO, MASARU
Applicants: ANEST IWATA CORP, ANEST IWATA CORPORATION
Application year: 1999
Main classification: B05B7
IPC codes: B05B7