WO2008015552A1

WO2008015552A1 - Air cap for a paint gun

Info

Publication number: WO2008015552A1
Application number: PCT/IB2007/002220
Authority: WO
Inventors: Paul Lawrence Garlick
Original assignee: Itw Limited
Priority date: 2006-08-02
Filing date: 2007-08-02
Publication date: 2008-02-07
Also published as: GB0615344D0; GB2440517A; TW200821042A

Abstract

An air cap for a paint spray gun includes a centrally positioned, axially extending aperture 21, wherein the aperture 21 (through which, in use, atomising air is passed) has a rotationally symmetrical shape which includes a plurality of radially extending protrusions 22.

Description

AIR CAP FOR A PAINT GUN

This invention relates to air caps for paint spray guns and in particular to the reduction of noise produced by capped spray guns when in use.

Spray heads for paint spray guns can be broadly categorised into two types; round spray and fan spray. In each case, the air cap has a central, typically circular shaped aperture which extends axially; in use atomising air from a paint gun to which the cap is affixed is directed through the aperture. A fan spray air cap differs from a round spray air cap in that it includes lateral apertures, typically provided in "horns" which extend from an external surface of the air cap. The central aperture may be tapered towards the atomising air exit. These designs are well suited to providing an evenly distributed and controllable spray pattern; however, it has been found that atomising air flowing through caps of this design creates a significant level of noise.

It is an object of the present invention to provide an improved air cap design which, when in use on a paint spray gun, produces less noise than prior art air cap designs.

In accordance with the present invention there is provided an air cap for a paint spray gun including a centrally positioned, axially extending aperture, wherein the aperture (through which, in use, atomising air is passed) has a rotationally symmetrical shape which includes a plurality of radially extending protrusions.

The protrusions may take various forms, for example, they may be pointed, square ended, truncated, rounded or otherwise curved. The aperture may be tapered towards the atomising air exit of the air cap. Optionally, in an axial direction, the protrusions extend parallel to the longitudinal axis of the aperture, or at a different angle to the remainder of the aperture.

Desirably, a minimum of 3 protrusions are provided. Practically, up to around 24 protrusions may be provided.

Desirably, the aperture is configured such that, in use, between 20% and 80% inclusive of atomising air passing through the aperture will pass through the protrusions. Preferably, this is closer to between 40% and 60% inclusive.

Example

For a given air cap and nozzle of a paint gun with which the air cap is to be used, the geometry of the aperture can be defined as follows:

[Insert Fig here]

Variables:

Di = outside diameter of fluid tip component D₂ = inner diameter of aperture D₃ = outer diameter of aperture

D₄ = diameter at root of protrusions

Θ = half-angle subtended by inner surface of protrusion

Φ = angular spacing of protrusions Assuming substantially V-shaped protrusions:

Number of protrusions N = 2TT/Φ

Exit area per protrusion; A₉ = ((D₄ - D₂)² - (D₄ - D₃)²)tanΘ/4 Annular exit area A₃ = TT( D₂ ² - Di²)/4

Ratio of areas R = N. A₉/ A₃ Total exit area A_t = N. Ag+ A₃

As previously stated R is desirably between 20% and 80% inclusive, more desirably between 40% and 60% inclusive. For comparable atomisation, A_t is selected to be the same area as has the central aperture of a conventionally designed air cap. Desirably, A_t will fall in the range 1.0 to 20 mm².

The inventors have found that by adapting air caps in accordance with the present invention, the noise made by a working paint spray gun can be reduced by of the order of 3dB. This is equivalent to a reduction in the sound intensity of 50%. It is believed that the novel design reduces the velocity gradient of air as it exits the aperture resulting in a consequent noise reduction. No deterioration in fluid atomisation has been seen.

An embodiment of the invention will now be described in more detail with reference to the following Figures in which:

Figure 1 (a) and (b) shows a conventional air cap design of the fan spray type;

Figure 2 shows in detail, various views of an embodiment of an air cap in accordance with the present invention; - A -

Figure 3 shows perspective views of a round spray and fan spray air cap both being configured in accordance with the present invention.

As can be seen in Figure 1, the nozzle assembly of a paint gun 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 of a paint gun 2. The nozzle body 36 has a cylindrical hollow interior 44 leading to a spray aperture in nozzle tip 46 at its forward end through which fluid is dispensed. Nozzle body 36 has a radially extending flange or weir 48 which has an opening 50 through which compressed air may pass. 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.

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 through which atomising air is dispensed and enabled to atomise fluid exiting from nozzle tip 46. A pair of 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.

Air passing out through hole 51 into the region 57 of the air distribution chamber 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 52. The air emerging from the horns serves to shape the pattern of the atomised paint emerging from the central nozzles.

Figure 2 shows an embodiment of an air cap in accordance with the invention. The air cap has a broadly similar construction to the air cap 40 of Figure 1 but the central aperture is differently shaped.

As can be seen the central aperture 21, when viewed from the atomising air exit surface, is substantially hexagonal shape with a truncated protrusion 22 at each angle. A protrusion is show in enlarged detail at the top left hand corner of the figure. As is visible from the three cross sectional side views of the air cap, the protrusions are essentially axially extending grooves 23 machined into walls of the conventionally circular shaped central aperture 21 of a prior art air cap similar to that of Figure

1. Consequently the resulting protrusions 22 have walls which are substantially parallel to the centre axis of the aperture whereas the inner walls of the original aperture (before the grooves have been machined) subtend an angle to the centre axis.

Figure 3 shows perspective views of a round spray air cap and a fan spray air cap in accordance with the invention. It will be appreciated that the inclusion or otherwise of the fan shaping horns is immaterial to the functioning of the invention.

Claims

1. An air cap for a paint spray gun including a centrally positioned, axially extending aperture, wherein the aperture (through which, in use, atomising air is passed) has a rotationally symmetrical shape which includes a plurality of radially extending protrusions.

2. An air cap as claimed in claim 1 wherein the aperture is tapered towards the atomising air exit of the air cap.

3. An air cap as claimed in claim 1 or claim 2 wherein, in an axial direction, the protrusions extend parallel to the longitudinal axis of the aperture.

4. An air cap as claimed in any preceding claim having at least 3 protrusions.

5. An air cap as claimed in claim 4 having not more than 24 protrusions may be provided.

6. An air cap as claimed in any preceding claim wherein, the aperture is configured such that, in use, between 20% and 80% inclusive of atomising air passing through the aperture will pass through the protrusions.

7. An air cap as claimed in claim 6 wherein between 40% and 60% inclusive of atomising air passing through the aperture will pass through the protrusions.

8. An air cap as claimed in any preceding claim wherein for a given air cap and nozzle of a paint gun with which the air cap is to be used, the geometry of the aperture can is defined as follows:

Di = outside diameter of fluid tip component

D₂ = inner diameter of aperture

D₃ = outer diameter of aperture

D₄ = diameter at root of protrusions

Θ = half-angle subtended by inner surface of protrusion

Φ = angular spacing of protrusions

Assuming substantially V-shaped protrusions:

Number of protrusions N = 2TT/Φ

Exit area per protrusion; A₉ = ((D₄ - D₂)² - (D₄ - D₃)²)tanΘ/4

Annular exit area A₃ = TT( D₂ ² - Di²)/4

Ratio of areas R = N. A₉/ A₃

Total exit area A_t = N. Ag+ A₃

wherein R is in the range 20% and 80% inclusive and A_t is selected to be the same area as has the central aperture of a conventionally designed air cap for use with the same paint gun. A_t is in the range 1.0 to 20 mm² inclusive.

9. An air cap as claimed in any preceding claim wherein the aperture comprises a substantially hexagonal shape having a truncated protrusion at each angle of the hexagon.

10. An air cap as claimed in claim 9 wherein, at the atomising air exit surface of the aperture, each protrusion has two sides which each subtend an angle of about 20 degrees to a radius of the aperture.