NOZZLE ASSEMBLY FOR AN ELECTROSTATIC SPRAY COATING APPARATUS

Abstract: A nozzle assembly for spray coating apparatus such as an electrostatic spray gun includes an air cap (50) having a central bore (52) which engages a nozzle member (26). The bore has a plurality of uniformly dimensioned axially aligned gas flow passages (62) evenly spaced about its circumfer­ ence and a plurality of radially inwardly extending ribs (64) therebetween. In order to obtain uniform atomization of coating mate­ rial and prevent the spray pattern to be badly distorted the side of a bore engages the outside surface of the nozzle member to positively align its centre axis on the axis of the central bore. The air cap and nozzle member thus cooperate to form a plurality of uniform gas flow passages around the nozzle (38) thereby producing a finely atomized uniform spray pattern of coating material emitted from the nozzle member.

Description: NOZZLE ASSEMBLY FOR SPRAY COATING APPARATUS The invention relates to spray coating systems and in particular to an improved nozzle assembly for spray coating guns. Specifically, the invention relates to an external air atomizing nozzle assembly for use on electrostatic spray guns such as that disclosed in United States Patent No. 3,747,850 assigned to the applicant of this invention. In conventional electrostatic spray systems, a fluid coating material such as paint, varnish, lacquer or the like is passed through the barrel of a spray gun, into a fluid tip which is threaded at its rear into a counterbore in the forward end of the barrel, and through and out of a small diameter nozzle at the forward end of the fluid tip. An air cap surrounds the forward end of the fluid tip and includes a central bore surrounding the nozzle so as to define an annular air passage around the fluid nozzle. Air issuing from this annular passage impacts with the stream of material issuing from the material orifice of the nozzle to at least coarsely atomize the material stream. There may be additional openings or ports in the air cap to further atomize or control the material stream as well as a pair of fan-shaping ports located in a pair of opposed horns of the air cap. A trigger operated valve controls the flow of air through the atomizing air passage, and a manually adjustable valve controls the mount of air issuing from the horn of the nozzle and thus the degree of "fan" formed by the atomized spray. Patents generally illustrating such systems are United States Patent Nos. 1,655,254; 2,101,175; 2,138,300; 3,672,569; and 3,747,850. In such systems, it is of utmost importance that the annular air passage defined by the wall of the central bore in the air cap and the outside diameter of the fluid tip nozzle be accurately concentric with the material orifice of the nozzle. If this concentricity deviates by as little as one or two one-thousandths of an inch, atomization of the material becomes non-uniform and the shape of the spray emitted from the gun becomes badly distorted. Because the fluid tip is supported at its rearward end or at an intermediate position still removed from the nozzle, it is extremely difficult to obtain the accurate alignment of the nozzle in the central bore. This is particularly true when the nozzle assembly is formed of a nonconductive material such as plastic since it is particularly difficult to manufacture plastic parts in the tolerances required to achieve concentricity. The problem of controlling the atomization of the fluid material and the shape of the spray emitted from the gun increases as the flow rate of material through the gun decreases. Therefore very small variations in the annular air passage surrounding the fluid tip nozzle have been found to have very drastic effects on the shape of the spray pattern emitted from the gun. It is an object of the invention to provide an atomizing nozzle for a spray coating gun having improved relative concentricity between the material orifice at the forwardmost end of the fluid tip and the atomizing opening in the centre of the air cap to obtain improved control and uniformity of the material spray pattern. It is a further object of the invention to obtain such improved control and uniformity of material spray pattern particularly in a nozzle assembly formed of a non-conductive material for an electrostatic spray gun. Another object of the invention is to obtain exceptionally wide fan patterns from an electrostatic spray gun at low flow rates, i.e., fans up to 20 inches in width at a leo inch nozzle-to-workpiece distance with flow rates in the range of 1 1/2 to 6 ounces of coating material per minute. Furthermore the invention intends to provide a nozzle assembly for an electrostatic spray gun which is rugged in construction and relatively simple to manufacture but which accurately aligns the nozzle in the central bore of the air cap to achieve uniformity in spray pattern and fine atomization. These objects and intentions of the invention are achieved by providing an improved nozzle assembly for a spray coating gun including a fluid tip and an air cap which cooperate to form a multiplicity of uniformly dimensioned, evenly spaced atomizing -gas flow channels. Gas passing through these channels or passages converges symmetrically against the material emitted from the fluid tip nozzle to transform the material stream into a uniform and finely atomized pattern. The nozzle assembly of this invention provides a uniform spray pattern even when the nozzle is formed of a plastic material and when the material flow rates are low. In accordance with one embodiment of the invention, the fluid tip is threaded at its rear into a counterbore in the forward end of the barrel of the electrostatic spray gun and includes a nozzle portion through which the coating material passes. The air cap has a central bore through which a gas, e.g. air, is ejected for atomizing the coating material. The air cap further includes a number of uniformly dimensioned, axially aligned gas flow passages which are evenly spaced about the circumference of the bore. Spaced, radially inwardly extending ribs are located between the gas flow passages. The ribs engage the outside surface of the liquid coating tip nozzle to thereby positively align the centre axis of the material orifice of the nozzle on the axis of the central bore. The fluid tip is thereby supported at its rear end by the barrel and at its forward or nozzle end by the ribs in the central bore of the air cap. The air cap and nozzle tip thus cooperate to form a plurality of air flow passages of uniform dimension around the nozzle to thereby produce a uniform atomizing air flow pattern around the nozzle. In a preferred embodiment of the invention, the nozzle assembly is made of an electrically non-conductive plastic material and contains a central opening of a size to receive a relatively small metal insert, e.g., an insert formed of pressed and sintered brass or aluminum. In this embodiment, the metal insert is formed with the central bore having the circumferentially spaced gas flow passages and radially inwardly extending ribs for engaging the outside surface of the fluid tip nozzle. Preferably, the ribs are so dimensioned that a press fit occurs between the metal ribs and the plastic nozzle when the nozzle is inserted into the centre bore of the insert. Use of the metal insert provides for resistance against wear and deformation of the ribs as the nozzle assembly is repeatedly disassembled and reassembled on cleaning. It has been found further that the amount of metal used in the insert is so small that it does not present any problems in use with an electrostatic spray gun. Another aspect of this invention is predicated upon sealing the circumference of the air cap to prevent excessive leakage of gas to the atmosphere. That is, in prior art nozzles, the air cap was sealed by a series of washers and a relatively flimsy ring. These washers were easily deformed by pressure and temperature and thus failed to effectively seal around the air cap. This invention includes as part of the nozzle assembly a retaining ring having a rigid annular sealing lip. The air cap includes an annular groove on the outer surface thereof which receives the annu#lar lip by snapping the air cap into position over the annular lip. The air cap and ring thus forms a seal which prevents excessive air from escaping air to the atmosphere. This aspect of the invention thus eliminates the need for washers making the nozzle assembly less expensive to manufacture and assemble, more compact and more effectively sealed. The invention will now be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a side elevational view showing in phantom a manually operated electrostatic air spray gun incorporating a nozzle assembly in accordance with the invention (shown in solid), Figure 2 is an exploded perspective view with part broken away of the nozzle assembly shown in Figure 1, Figure 3A is a partial exploded perspective view of a prior art nozzle, Figure 3B is an end view of the prior art nozzle shown in Figure 3A, Figure 4 is an axial cross sectional view of the nozzle assembly shown in Figure 1, Figure 5 is an end elevational view taken on line 5-5 of Figure 4, Figure 6 is a cross sectional view taken on line 6-6 of Figure 4, Figure 7 is a cross sectional view of second nozzle assembly in accordance with the invention, Figure 8 is an end elevational view of the nozzle assembly shown in Figure 7, Figure 9 is a cross sectional view of third nozzle assembly in accordance with the invention, Figure 10 is an end elevational view of the nozzle assembly shown in Figure 9, Figure 11 is a cross sectional view of fourth nozzle assembly in accordance with the invention, and Figure 12 is an end elevational view of the nozzle assembly shown in Figure 11. Referring to the drawings, the gun 10 shown in Figure 1 comprises an electrically conductive metal handle assembly 11, an electrically insulative barrel assembly 12, and an insulative nozzle assembly 13. Paint or other spray material which may be in the nature of a coating, varnish or lacquer (referred to in regard to this invention generically as paint or coating material) is supplied to the gun from an external reservoir or tank (not shown) through a material passage 14. A high voltage source of electrical energy is supplied to the gun by a cable 15 from an external electrical power pack (not shown). The handle assembly 11 is generally made from a metal casting and includes an air inlet 16, a trigger actuated internal air flow control valve 17 and a trigger 18 for controlling the flow of air through the valve 17. There is also an adjustable air valve 20 in the gun handle for controlling the shape or "fan" of the spray emitted from the gun. The air inlet 16 opens into a generally vertical air passage in the handle 11 which communicates through the air flow control valve 17 with a pair of internal passages 22, 24 (see Figure 4) passing through the barrel 12 of the gun and terminating at the forward end of the barrel 12. The passage 22 provides atomizing air while passage 24 provides the fan-shaping air. The flow of air through passages 22, 24 is controlled by the trigger operated air control valve 17 while the flow of fan air through the passage 24 is further controlled by the fan control valve 20. Referring now to Figures 2 and 4, the nozzle assembly 13 is made from an electrically non-conductive material. It has a nozzle member or fluid tip 26 which is threaded at its rear 28 into a counterbore 30 in the forward end of the barrel 12. The fluid tip 26 has six circumferentially spaced axial passages 32 which open into the rear of the counterbore 30 which in turn communicate with the air passage 22 such that atomizing air passing through the passage 22 may enter and pass through the axial passages 32 in the fluid tip and into an internal chamber 33 surrounding the forward end 34 of the fluid tip. The fluid tip 26 also has a central axial passage 35 communicating with a material flow passage 36 in the gun 10 for supply of liquid or fluid via the inclined passage 14 (see Figure 1) from the tank or reservoir. The forward end 34 of the fluid tip or nozzle member 26 terminates in a nozzle 38 having a small diameter orifice 40 through which the coating material is emitted. A material charging electrode or antenna 42 is mounted on the centre axis of the fluid tip and is held in place in the passage 35 by means of a non-conductive holder 44 (see Figure 6). Electrical power is supplied to the electrode 42 which protrudes from the orifice 40 of the nozzle 38. This power is supplied generally from the electrical power pack which is connected to the gun via a cable 15 which is connected to the electrode 42 via an insulated cable 46 and spring 48. The air cap 50 surrounds the forward end 34 of the fluid tip 26. It includes a central bore 52 through which the nozzle 38 extends, two pairs of fan control ports 54 located on either side of the bore 52, two pairs of recessed fineatomizing ports 56, and a pair of ports 58 in each air horn 60. Referring also to Figure 5, the air cap 50 further has a number of uniformly dimensioned, circumferentially spaced gas flow passages 62 whose axes are aligned with the axis of the central bore 52. These passages 62 define a series of circumferentially spaced axial gas flow passages with spaced, radially extending ribs 64 therebetween. The nozzle portion 38 of the fluid tip 26 extends through the central bore 52 and the ribs 64 engage its outside surface. The ribs thereby positively align the nozzle such that the centre axis of the material orifice 40 is on the centre axis of the central bore 52. The co-action of the fluid tip nozzle with the air cap thus provides a plurality of uniformly dimensioned air flow passages uniformly spaced around the fluid tip nozzle thereby producing a uniform atomizing air flow pattern. This co-action may be further understood and appreciated by referring to Figures 3A and 3B wherein a prior art nozzle assembly is illustrated. Referring first to Figure 3A the nozzle end 70 of the fluid tip 72 extends through a central bore 74 in the air cap 76 which has a diameter greater than the outside diameter of the nozzle 70 to form an annular air passage around the nozzle. However, in the prior art, the fluid tip 72 is supported at points removed from the nozzle end 70 and because of inaccuracies in manufacture and dimensional instability of materials, it is rarely aligned in the central bore to provide a uniform annular air passage. Rather the misalignment of the nozzle in the central bore, as illustrated in Figure 3B, results in a lack of concentricity of the air passage 74 about the nozzle, and therefore resulting in non-uniform atomization of the material exiting the nozzle. As may be best seen in Figure 5, the nozzle assembly of the present invention by virture of the cooperation of the air cap 50 with the fluid tip nozzle 38 provides uniformly dimensioned, spaced air flow of passages 62 around the nozzle. The air cap 50 is mounted to the gun 10 by means of an annular retaining ring 80. The retaining ring 80 is also made from an electrically non-conductive plastic material. It is threaded over a threaded section of the barrel 12 at one end and at its other end has an annular lip 82. The retaining ring 80 although rigid is sufficiently flexible at the lip 82 to permit the air cap 50 to be snapped into position with the lip 82 engaging a wall 84 in an annular groove 86 in the outside surface of the air cap 50 such that the air cap is securely retained and sealed against escape of air to the atmosphere. The air cap 50 and fluid tip 26 include mating frustoconical surfaces 88 and 90, respectively, which seal the atomizing air in chamber 33 from the fan-shaping air in an annular chamber 92 when the retaining ring 80 is securely tightened on the barrel. The chamber 92 communicates with the air passage 24 and with passages 93 in the air horns 60 in turn communicating with ports 58. Referring now to Figures 7 to 10, there are shown embodiments of the present invention wherein the air cap 50 includes a ceramic insert 94 which is mounted in the centre of the air cap. Ceramic inserts are advantageously used from a manufacturing standpoint. In the embodiment shown in Figures 7 and 8, the ceramic insert 94 includes a central bore 96, a plurality of uniformly dimensioned, circumferentially spaced gas flow passages 98 axially aligned with the central bore, and radially inwardly extending radial ribs 100 therebetween, as heretofore described. The axial length L of the ribs 100 are about 0.060 inch and the nozzle extends approximately 0.025 inch past the outer surface of the air cap. Eight holes of about 0.031 inch in diameter are equally spaced on a 0.103 inch diameter. The diameter of the central bore is also about 0.103 inch. The ribs have a width of about 0.008 inch. In the embodiment shown in Figures 9 and 10, a similar ceramic insert 102 is shown but differing in that the axial length L' of the ribs 104 are smaller and in the range of about 0.020 to 0.040 inch. These shorter axial length ribs are advantageous when heavier viscosity materials are employed or better air contact is desired. In the further embodiment shown in Figures 11 and 12, the air cap 50 is provided with a central bore 106 in which is mounted a circular insert 108. The insert 108 is preferably formed of a pressed and sintered metal powder such as brass and aluminum and is so dimensioned with respect to the central bore 106 that when inserted therein, the plastic material of the air cap 50 is slightly compressed to form a tight press fit therebetween. The insert 108 has four like gas flow passages 110 spaced about the circumference of a central bore 112 therein with radially inwardly extending ribs 114 therebetween. As in the prior embodiments, the gas flow passages 110 are axially aligned with the axis of the small diameter orifice 40 of the nozzle portion 38 of the fluid tip 26. The ribs 114 include lands 116 which lie on the circumference of the bore 112 and which engage the outside of the nozzle 38. Preferably, the diameter of the central bore 112 is smaller by a few thousandths of an inch than the outside diameter of the nozzle 38. Accordingly, when the nozzle 38 is inserted in the bore 112, a press fit is achieved between the outside surface of the nozzle 38 and the lands 116 of the ribs 114. The nozzle 38 is thereby positively aligned in the bore 112. Referring to Figure 11, it may be seen that the axial length of the ribs 114 is approximately half the thickness of the insert 108. This provides less resistance to flow of the atomizing gas through the passages 110. By forming the insert from pressed and sintered powdered metal, the tolerances on the order of 0.001 inch can be maintained. Moreover, the nozzle assembly can be disassembled and reassembled, for example, for cleaning, and the nozzle 38 inserted and removed from between the ribs 114 without wearing or deforming the ribs since they are harder than the nozzle material. Accordingly, the insert will maintain its dimensions over a relatively long period of use. Further, it has been found that the amount of metal needed in the insert is relatively small and does not adversely affect the operation of an electrostatic spray gun. The invention has been described with reference to an air operated electrostatic spray gun which relies upon the impact of an air stream with a liquid stream to effect atomization of the liquid stream, however, the invention is equally applicable to all electrostatic spray guns or to spray coating systems in general. The gun 10 shown in phantom in Figure 1 is described in detail in United States Patent No. 3,747,840.

Claims: CLAIMS: 1. A nozzle assembly for a spray coating apparatus comprising a nozzle member (26) through which liquid coating material is emitted through a nozzle (38), an air cap (50) through which gas is ejected for impinging and atomizing the liquid coating material emitted by the nozzle member to form a spray, characterized in that the air cap has a central bore (52) the side of which engages the nozzle member to align the central axis of the nozzle with the central axis of the bore to provide uniform atomizing gas flow around the nozzle member from the air cap. 2. A nozzle assembly as claimed in Claim 1 wherein the air cap is positionably supported by the spray coating apparatus. 3. A nozzle assembly as claimed in either Claim 1 or Claim 2 wherein the side of the bore defines a plurality of circumferentially spaced axial gas flow passages (62) with spaced radially inwardly extending ribs (64) therebetween, the ribs engaging the nozzle member. 4. A nozzle assembly as claimed in any preceding claim substantially constructed of non-conductive plastic material. 5. The assembly as claimed in any preceding claim wherein the spray apparatus is an electrostatic spray gun and the nozzle assembly includes a charging electrode (42) protruding from the nozzle. 6. The assembly as claimed in any preceding claim further comprising an annular retaining ring (80) receivable on the spray apparatus having an annular lip (82) engageable with an annular groove (86) in the outer surface of the air cap to seat and retain the air cap on the nozzle member. 7. The assembly as claimed in any preceding claim wherein the forward end of the nozzle member protrudes forwardly of the outer surface of the air cap. 8. The assembly as claimed in any one of Claims 3 to 7 wherein the air cap includes an insert (94) in the centre thereof, the insert having the central bore, the plurality of axial gas flow passages spaced, and the radially inwardly extending ribs therebetween. 9. The assembly as claimed in Claim 8 wherein the insert is formed of pressed metal powder. 10. The assembly as claimed in any of Claims 3 to 9 wherein the diameter of the central bore is smaller than the outside diameter of the nozzle such that the nozzle member is pressed between the ribs. 11. The assembly as claimed in any preceding claim wherein the air cap further includes fan-shaping ports communicating by means of a chamber (92) with a source of gas, the chamber being sealed from the impinging and atomizing gas flow in the nozzle assembly by mating annular surfaces of the air cap and the nozzle member, and means (42) mounted in the nozzle member and protruding from the nozzle for charging the liquid coating material. 12. A system for the coating of articles with a liquid coating material supplied from a pressurized bulk coating source wherein the liquid coating material is emitted from a coating material spray device in the form of an atomized spray produced by impacting a central stream of liquid coating material under pressure with a pressurized gas stream encircling the central liquid stream and wherein the articles to be coated are spaced from the spray device, the system comprising, a source of liquid coating material under pressure, a source of pressurized atomizing gas, a material spray device having a liquid conduit with flow control means therein adapted to be connected to the source of pressurized liquid coating material for providing relatively low liquid coating material flow rates in the approximate range of 1 1/2 - 6 fluid ounces of material per minute, and having a gas conduit therein adapted to be connected to the source of pressurized atomizing gas, and a spray coating nozzle assembly made which is substantially constructed of non-conductive plastic material comprising a liquid tip communicating with the liquid conduit and having a nozzle portion through which the liquid coating material is emitted in a central stream at the relatively low flow rate, and an air cap communicating with the atomizing gas conduit through which gas is ejected for impinging and atomizing the central stream of liquid coating material emitted from the nozzle portion of the liquid tip, the air cap being positionably supported by the spray device only in the rear region of the air cap to effectively leave the forward region thereof positionably unsupported by the spray device, the air cap having a central bore and a plurality of circumferentially spaced axial gas flow passages (62) with spaced, radially inwardly extending ribs therebetween, said ribs (64) engaging the outside surface of said liquid coating tip nozzle portion to positively align the centre axis of said liquid coating nozzle portion on said axis of said central bore to provide uniform atomizing gas flow around said liquid coating tip nozzle portion for producing at said relatively low flow rate a finely atomized uniform spray pattern of said coating material emitted from said liquid tip.

Inventors: HASTINGS, DONALD R.
Applicants: NORDSON CORPORATION
Application year: 1981
Main classification: B05B5
IPC codes: B05B7, B05B5