Abstract:
A spray gun includes a main body and a spray head. The spray head has an air cap, a hollow nozzle member having a nozzle tip for dispensing a liquid to be sprayed. The hollow nozzle member has a radially extending weir which together with a surface of the main body defines an air distribution chamber for receiving and distributing compressed air from a supply inlet. An annular sealing member is sealingly disposed between the radially extending weir and the surface of the main body, dividing the chamber, so that air passing into the chamber through an opening to one side of the annular wall is directed to a central aperture of the air cap and air passing into the chamber through an opening to the other side of the annular wall is directed to lateral apertures of the air cap.

Description:
FIELD OF THE INVENTION 
     This invention relates to a spray gun for applying coatings such as paint in which compressed air is used both to atomise the paint to form the spray and to control the shape of the spray pattern emerging from the nozzle of the spray gun. 
     BACKGROUND OF THE INVENTION 
     Spray guns for spraying coatings such as paint, using compressed air to atomise the paint and control the shape of the resulting spray pattern, are well known. Examples of such spray guns are disclosed in, for example, U.S. Pat. Nos. 5,803,367 and 5,209,405. 
     Spray guns of this type typically have a spray head comprising a nozzle assembly and an air cap. The air cap has a central aperture which surrounds the paint-dispensing tip of the nozzle, such that air passing through the central aperture past the nozzle tip serves to atomise the paint to form a spray. The air cap is also provided with forwardly extending formations known generally as “horns” which direct compressed air inwardly towards the plume of atomised paint emerging from the central aperture so as to shape the plume of atomised paint to a desired pattern, such as an elliptical pattern. Usually a pair of horns are provided and these are generally arranged symmetrically with respect to the central aperture. Air used to shape the spray pattern is often referred to as “fan air” whereas air used to atomise the paint is often referred to as “atomisation air” or “atomising air”. 
     In order to provide a supply of compressed air to both the central aperture and the horns, the incoming air must be partitioned, and in known types of spray gun, complex baffle arrangements are often employed in order to divide the incoming air stream in a consistent and controlled manner and ensure that the required pressures of atomisation air and fan air are delivered to the central aperture and horns respectively. The complexity of the baffle arrangements, and the number of parts required, can considerably complicate the manufacture of the spray guns. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a simplified means of controlling the partitioning of compressed air between fan air and atomisation air and to simplify the manufacture of the spray guns, for example by reducing the number and complexity of the parts required in the spray head. 
     The present invention provides an improved spray gun in which a simple annular sealing member is used to partition incoming air between atomisation air and fan air. The interior of the main body of the spray gun is configured to provide a plurality (e.g. two) of air inlet passages leading from the compressed air source (usually via an air control valve) to an air distribution chamber within the spray head, and air pathways are provided within the spray head for directing air onwards to both the horns and the central aperture. The annular sealing member serves to divide the air distribution chamber such that air passing into the chamber to one side of the sealing member is directed to the horns whilst air passing into the chamber to the other side of the sealing member is directed to the central aperture. 
     Accordingly, in a first aspect, the invention provides a spray gun comprising a main body, a spray head, an inlet for a liquid to be sprayed, and an inlet for a supply of compressed air; 
     the spray head comprising an air cap, a hollow nozzle member having a nozzle tip for dispensing the liquid, and means for securing the air cap and nozzle member to the main body; 
     the air cap having a central aperture through which atomising air is directed to atomise liquid dispensed through the nozzle tip to form a spray, and one or more lateral apertures through which fan air is directed for shaping the spray to a desired pattern; 
     the hollow nozzle member having a radially extending weir which together with a surface of the main body defines an air distribution chamber for receiving and distributing compressed air from the supply inlet; 
     wherein an annular sealing member having an annular wall is sealingly disposed between the radially extending weir and the said surface of the main body so as to divide the air distribution chamber, whereby air passing into the chamber thorough an opening to one side of the annular wall is directed to the central aperture of the air cap and air passing into the chamber through an opening to the other side of the annular wall is directed to the lateral apertures of the air cap. 
     The annular wall can be circular or non-circular, for example ovoidal or polygonal (e.g. octagonal) but preferably it is circular. The annular wall is preferably set into a recess in the said surface of the main body. This provides a simple means of locating the sealing member and ensuring that it remains in place without the need to provide additional securing means. The recess is advantageously formed by a simple machining operation into a leading surface of the main body of the spray gun. Alternatively, the recess can be formed in the surface during casting of the gun body. 
     For ease of manufacture, the main body of the spray gun is formed (e.g. machined) to provide a plurality (e.g. two) of air passages opening into the air distribution chamber wherein the openings are arranged symmetrically with regard to a main axis of the spray gun. In such a case, the annular wall of the sealing member may advantageously be arranged eccentrically with regard to the main axis, such that one of the air passages through the main body opens into the air distribution chamber within a region bounded by the annular wall, whilst the other symmetrically disposed air passage opens into the air distribution chamber outside the region bounded by the annular wall of the sealing member. Air from within the region bounded by the annular wall of the sealing member can then be directed onwards to the central aperture whilst air outside the region bounded by the annular wall of the sealing member can be directed onwards to the horns, or vice versa. 
     Typically, air from the air distribution chamber is directed to the central aperture via a path extending through the weir, whereas air is directed to the lateral apertures (horns) via a path extending around the edge of the weir. Thus the weir may be provided with one or more holes to allow movement of air between the air distribution chamber and the central aperture of the air cap. 
     The air cap preferably has a rearwardly extending cylindrical formation which, together with a forward surface of the radially extending weir defines an annular chamber surrounding the hollow nozzle member for receiving atomising gas from the air distribution chamber. The central aperture is typically defined by an opening in a front wall of the annular chamber, and the nozzle tip will usually protrude into the central aperture so as to define an annular flow path around the nozzle tip for the atomising gas. Air passing along the annular flow path past the nozzle tip will bring about atomisation of the paint or other coating liquid as it emerges from the nozzle tip. 
     The sealing member is typically formed from a plastics material, the plastics material being such that at least the edges of the annular wall are crushable or deformable to allow a seal to be created between the main body of the spray gun and the weir of the nozzle assembly. However, the sealing member should also be sufficiently rigid to be able to retain its shape and not collapse or deform to any extent under the pressure of air entering the air distribution chamber. Thus preferred plastics materials are relatively rigid plastics such as polyacetal or polyamides, or polyolefins such as polyethylene and polypropylene. A currently most preferred plastics material is high density polyethylene. 
     A major advantage of the sealing member is that not only does it provide a means of partitioning the air distribution chamber and dividing the flow of compressed air between atomisation and fan air, but it also provides a seal between the main body of the spray gun and the nozzle assembly thereby avoiding the requirement for a separate sealing member. The number of parts required in the spray head is thereby reduced and hence manufacture is considerably simplified. 
     The invention has been defined and described so far with reference to the distribution of air within the spray head. However, for the avoidance of doubt, the term “air” as used herein includes not only air as such but also any other gas that might conceivably be used for atomisation and spray pattern control in a spray gun. 
     The invention will now be illustrated and described in more detail, but not limited in any way, with reference to the particular embodiment shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of a spray gun according to one embodiment of the invention. 
     FIG. 2 is a view from direction D of the spray gun of FIG.  1 . 
     FIG. 3 is a partial sectional elevation along line I—I in FIG.  1 . 
     FIG. 4 is a sectional elevation along line II—II in FIG.  2 . 
     FIG. 5 is an expanded side sectional elevation through the spray head of the gun shown in FIGS. 1 to  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a gun according to one embodiment of the invention comprises a main body  2 , and a spray head  6  extending from the forward end of the body. A trigger  8  pivotably mounted on the main body  2 , is provided for controlling the initiation and stopping of the paint spraying operation in standard fashion. 
     At the lower end of handle  4  is an inlet  10  for connection to a compressed air supply. On the upper surface of the gun body  2 , an inlet formation  12  is provided for connection to a gravity feed cup containing a coating composition such as a paint or a primer. 
     At the rear end of the body of the spray gun, are provided control knobs  14  and  16  for controlling air flow rate and paint flow rate respectively. 
     The main body  2  of the spray gun is formed from forged aluminium which has been machined and drilled out to provide the necessary passageways for conveying compressed air and paint to the nozzle. With reference to FIG. 4, air passing through inlet  10  (FIG. 1) and up through the handle is channelled into air chamber  20  which is created by machining a bore in the rear surface of the body  2 . Air passing through chamber  20  is divided into atomisation air and fan air. A threaded gland  22  is seated in the mouth of chamber  20  and a control valve  24  extends through the threaded gland  22 , a gas-tight seal being provided between the control valve  24  and gland  22  to prevent the escape of compressed air. At the distal end of chamber  20  is a counterbore  26  containing an opening which leads into a gas passage (not shown) that channels compressed air (fan air) towards the nozzle assembly  6  (FIGS.  1  &amp;  4 ). A further opening (also not shown) channelling compressed air (atomisation air) towards the nozzle assembly is located in the wall of the chamber  20 . The distal end  28  of control valve  24  is seated in counterbore  26  and serves to control the flow of air towards the nozzle assembly. 
     Paint inlet formation  12  (FIG. 1) takes the form of a bore  30  and counterbore  32  set into the top surface of the main body  2 . The counterbore at region  32  is threaded to receive the correspondingly threaded end of a gravity feed cup (not shown) containing a reservoir of paint. Bore  30  communicates with paint dispensing chamber  34 . 
     The nozzle assembly  6  (FIG. 1) comprises a nozzle body  36 , a cylindrical sealing member  38  and an air cap  40 . The nozzle body  36  is held in place by means of a threaded connection  42  into the main body  2 . The nozzle body  36  has a cylindrical hollow interior  44  leading to a spray aperture in nozzle tip  46  at its forward end. Nozzle body  36  has a radially extending flange or weir  48  which has an opening  50  through which compressed air may pass in a manner described below. The radially extending weir  48  and the confronting surface  51  of the main body together form an air distribution chamber  53  into which air is channelled from the air chamber  20  and its counterbore region  26 . 
     Clamped between the radial flange or weir  48  and the surface of the main body  2  is the cylindrical sealing member  38  which is seated in a recess  51   a  machined into the surface  51  of the main body  2 . Importantly, the recess  51   a  is disposed eccentrically with regard to the axis of the spray head, as can be seen more clearly in FIG.  3 . It will be appreciated that by virtue of its disposition, the cylindrical sealing member  38  divides the air distribution chamber into a region  55  bounded by the annular wall of the sealing member, and a region  57  outside the annular wall. 
     Arranged forwardly of the nozzle body  36  is air cap  40  that has a central aperture  52 , which aperture forms an annular passage with nozzle body  36 , and a pair of horns  54 , one of which is shown in FIG.  4 . Horns  54  each have a pair of small apertures  56  through which compressed air is directed so as to shape the pattern of the paint spray emerging when paint from the nozzle tip  46  combines with air from aperture  52 . Either side of the central aperture  52  are disposed small openings  59 , air passing through these holes serving to prevent paint from fouling the horns. 
     A retaining ring  58  is threaded onto the end of the main body  2  thereby securing the air cap  40  and locating it on to the nozzle body  36 . 
     To the rear of the main body  2 , just above the handle, is provided a paint supply adjustment screw  6 . Paint supply adjustment screw is screwed by means of a thread within bearing sleeve  60  set into a bore through the main body portion. The screw  6  is hollowed out to receive a return spring  62  and shaft  64  of needle valve  66 . The shaft of the needle valve passes through a sealed gland  68  and thence through the interior  44  of the nozzle body, the tip  70  of the needle valve being seated in the spray aperture of the nozzle tip  46 . 
     Needle valve  66  has an enlarged mid portion  74  that engages a rear surface of the trigger  78 . In use, movement of the trigger forces the shaft of the needle valve  66  rearwardly against the force of the return spring  64  thereby opening the aperture  46  of the paint spray nozzle tip  36  to allow paint to flow through the nozzle. The volume of paint flowing through the nozzle tip can be controlled by adjusting the control screw  6 . 
     The passages (not shown) conveying air from the air chamber  20  and the counterbore region  26  through the main body  2  of the gun to the spray head terminate in openings  94  (FIG. 3) and  92  (FIG. 3) respectively in the front surface  51  of the main body  2 . One opening ( 94 ) is set into the eccentrically machined recess  51   a  in the front surface  51  of the main body  2  and is located within the region  55  of the air distribution chamber enclosed by the annular wall of the cylindrical sealing member  38 . The other hole  92  is located in the region  57  of the air distribution chamber lying outside the annular wall of the cylindrical sealing member  38 . 
     In use, the direction of flow of air passing from the air chamber  20  and its counterbore region  26  into the nozzle is controlled by the cylindrical sealing member  38 , the air emerging from hole  94  within region  55  of the air distribution chamber  53  being directed through holes  50  in weir  48  and thence into an annular chamber  96  defined by the outer surface of the nozzle  36  and the inner surface of the air cap  40 . The air passing along this route serves as the atomising air, the air stream rushing past the nozzle tip serving to draw paint out through the nozzle tip whilst at the same time atomising the paint to form a spray in conventional fashion. 
     Air passing out through hole  92  into the region  57  of the air distribution chamber  53  is diverted around the edges of the radially extending flange or weir  48  and through flow passages (not shown) to the openings  56  in the horns  54 . The air emerging from the horns serves to shape the pattern of the atomised paint emerging from the central nozzles. 
     The advantages of the eccentrically cyclindrical sealing member is that it allows the compressed air stream to be partitioned and channelled to either the horns or the central aperture without the need for complex labyrinthine baffles of the type found in many prior art devices. The cylindrical sealing member is formed from a material (e.g., a plastics material such as polyacetal) such that the edges of the cylinder are crushable to form gas tight seals against the weir or radial flange  48  and the surface of the main body  2  respectively. By providing an annular member that not only serves to divide the air distribution chamber but also functions as a seal, the number of parts required in the spray head is substantially reduced compared to known spray gun configurations, and manufacture is made considerably easier. 
     Control of the fan air volume and/or pressure, i.e. the volume and/or pressure of air passing to the horns, is effected by means of the control valve  24  which regulates the flow of air from the counterbore region  26  of chamber  20  along the passage leading to opening  92  into the air distribution chamber  53 . Thus, by means of the control valve  24 , fine control can be exerted over the fan air pressure and hence the shape of the paint spray pattern. However, the pressure and volume of the atomisation air is determined by the pressure input to the spray gun, and the inherent restrictions to air flow within the gun and spray head. 
     It will be appreciated that numerous modifications and alterations could be made to the spray gun shown in the accompanying drawings without departing from the principles underlying the invention. For example, although the sealing member is shown as being circular in plan, it could be differently shaped, for example, it could be ovoidal or polygonal, or for example hexagonal, although circular plan is preferred in order to provide ease of manufacture. 
     All such modifications and alterations are intended to be embraced by this application.