Electrostatic high voltage, low pressure paint spray gun

Abstract: Easily replaceable pressure reducers (190, 200) are located at the front of the spray gun (20) behind the air (180) and fluid (178) nozzles. These reducers (190, 200) are calibrated and marked for use with a particular air (180)/fluid (178) nozzle combination and maintain ≦ 10 psig exit air pressure. If the air (180)/fluid (178) nozzle combination is changed, the pressure reducers (190, 200) matched to the new combination (180, 178) are installed at the same time, illustratively using a screwdriver. By designing the air (180)/fluid (178) nozzle combination and pressure reducers (190, 200) as matched sets, the spray gun (20) can always inexpensively be kept in compliance with any requirements under which an HVLP device must operate. "Fan-" or shaping-compressed air driven from the same compressed air which supplies the nozzle (180, 178) air is supplied through an additional pressure reducer valve (52) to reduce its pressure below that of the nozzle (180, 178) air. The fan air reducer (52) employs a drilled orifice (208) of a calculated diameter. The atomizing air reducer (190) is of a needle (192) type that utilizes a controlling outside diameter.

Description: This invention relates an atomizing device, particularly to so-called high atomizing air volume-low atomizing air pressure, or HVLP, atomizing devices, such as hand-held spray guns. HVLP atomizing devices are defined as those with exit air pressures at or below 68947 N/m 2 (10 psig). Such an atomizing device is known from DE-U-8911932. The desirability of HVLP atomizing devices is explained in greater detail in U.S.S.N. 07/622,853 filed December 6, 1990, titled "Paint Spray Gun" and assigned to the same assignee as this application. Reference is here made to that application for its disclosure and particularly for that explanation. Prior to U.S.S.N. 07/622,853, HVLP atomizing devices were known. There are, for example, the atomizers described in U.S. Patents: 4,761,299 and the prior art mentioned therein, particularly, 4,232,824; 4,341,347; and, 3,093,309. The reader's attention is also directed to, for example, U.S. Patent 4,765,539, which is for the spraying of agricultural chemicals or the like, and to the Ransburg REA® III atomizing device available from Ransburg Corporation, P.O. Box 88511, Indianapolis, Indiana, 46208-0511. In order for an HVLP device to have an exit pressure at or below 68947 N/m 2 (10 psig), two methods of supplying air to the device are used. The first method employs a large and expensive turbine-type compressor that delivers a high volume of air at a low pressure through a large, typically approximately 2,54 cm (one inch), diameter hose. The second method employs conventional compressed "shop air" supplied through a small diameter hose. As most shops involved in finishing already utilize compressed shop air, it is far more convenient for the customer, and therefore in the interest of the atomizing equipment supplier, to offer HVLP atomizers that utilize shop air. The obvious problem with employing shop air, however, is that in order to supply a high volume of air to the atomizing device through a small diameter hose, the supply pressure must be high, i.e., between 413680 N/m 2 (60 psig) and 689467 N/m 2 (100 psig). After this high volume, high pressure air enters the atomizing device, this pressure must be reduced to 68947 N/m 2 (10 psig) or below. Two devices currently being used to perform this function are needle/venturi pressure reducers and fixed orifice pressure reducers. Needle/venturi pressure reducers are an approach that is both difficult and expensive, and is not tolerant of large fluctuations in supply pressure. The second type of reducer, the fixed orifice, is a very simple approach. A hole of a specific diameter is drilled in the gun body at a point at which air is supplied to the air and fluid nozzles. Both needle/venturi pressure reducers and fixed orifice pressure reducers satisfy the goal of reducing the incoming high air pressure down to a maximum of 68947 N/m 2 (10 psig), but in the event that some part of the system is changed, such as changing from an air nozzle with passageways of given sizes to an air nozzle with larger or smaller passageways, both needle/venturi and fixed orifice pressure reducers can permit the pressure to exceed 68947 N/m 2 (10 psig). Needle/venturi pressure reducers may require installation of a new needle/venturi. Fixed orifice pressure reducers may require a new gun body. Both of these alternatives are rather expensive. It has been established experimentally that air pressure has a greater influence on atomization quality than does air volume. Because of this, it is desirable to keep the atomizing air pressure as close as possible to the 68947 N/m 2 (10 psig) figure of merit for HVLP atomization. Adjustment of this fan air pressure valve increases or decreases the width of the spray pattern. In the prior art devices, the 68947 N/m 2 (10 psig) figure of merit for HVLP atomizing air pressure must be set with no fan air flow. Otherwise the atomizing air pressure can exceed 68947 N/m 2 (10 psig) under certain operating conditions of the gun, for example, with no fan air flow. Because both the fan and atomizing air are supplied by the same source, a drop in the atomizing air pressure inevitably occurs when fan air flows. All prior art HVLP spray guns of which applicants are aware suffer from this shortcoming. Prior art spray guns can experience as much as a 41368 N/m 2 (6 psig) drop in the atomizing air pressure when fan air flows. The HVLP spray gun of the present invention incorporates a specially designed fan air valve that reduces the magnitude of this pressure drop to the extent possible. Document JP-A-60110360 discloses a control valve provided with a bypass passageway for passing a necessary minimum amount of atomizing air. In contrast to that, the present invention discloses a control valve provided with a bypass passageway for passing a minimum amount of fan air in order to reduce the magnitude of said pressure drop to about 6895 - 13789 N/m 2 (1-2 psig) The fan air valve is designed so that when it is closed a 27579 N/m 2 (4 psig) bleed (with the spray gun trigger in spraying position) is experienced. 27579 N/m 2 (4 psig) was chosen as the bleed pressure since pressures over 27579 N/m 2 (4 psig) begin to affect spray pattern size. This 27579 N/m 2 (4 psig) bleed permits a higher volume of air to be supplied to the spray gun, while maintaining a predetermined level at the gun inlet. This causes less of a pressure drop to occur in the atomizing air pressure when the fan air valve is opened. Since the atomizing air pressure remains closer to the 68947 N/m 2 (10 psig) maximum allowable than prior art designs, the atomization quality is greatly enhanced, and the finish quality is higher than that which can be obtained with prior art designs. Disclosure of the Invention According to the invention, a means for coupling the second and third passageways to a source of the second fluid includes a first valve member and a first valve seat. The relative positions of the first valve member and first valve seat are adjustable selectively to increase and decrease flow of the second fluid through the third passageway. Means providing a bypass passageway around the first valve seat provides a flow of the second fluid through the third passageway regardless of the relative positions of the first valve member and the first valve seat. Brief Description of the Drawings The invention may best be understood by referring to the following detailed description and accompanying drawings which illustrate the invention. In the drawings: Fig. 1 illustrates a side elevational view of a HVLP hand spray gun constructed according to the invention; Fig. 2 illustrates a fragmentary longitudinal sectional view of the HVLP hand spray gun illustrated in Fig. 1; Fig. 3 illustrates an enlarged sectional view of the forward portion of the HVLP hand spray gun illustrated in Figs. 1-2, taken generally along section lines 3-3 of Fig. 2; Fig. 4 illustrates an enlarged fragmentary sectional view of the fan air valve assembly of the HVLP hand spray gun illustrated in Figs. 1-2, taken generally along section lines 4-4 of Fig. 2; Fig. 5 illustrates a fragmentary sectional view of the HVLP hand spray gun illustrated in Figs. 1-2, taken generally along section lines 5-5 of Fig. 1, illustrating the front end of the gun barrel with the air and fluid nozzles removed; Fig. 6 illustrates a fragmentary view of the HVLP hand spray gun illustrated in Figs. 1-2, taken generally along section lines 6-6 of Fig. 1; and, Fig. 7 illustrates an enlarged fragmentary sectional view through the handle of the HVLP hand spray gun illustrated in Figs. 1-2, taken generally along section lines 7-7 of Fig. 1. Modes for Carrying Out the Invention An HVLP hand spray gun 20 according to the present invention includes a handle portion 22 and a barrel portion 24. The handle portion 22 is provided with a hand grip 26 and a hanger hook 28. A trigger 30 is pivotally 32 mounted from the upper region of handle portion 22 and extends downwardly in front of hand grip 26 for easy manipulation by an operator when the operator is gripping hand grip 26. Handle portion 22 is provided with an internal compressed air passageway 36 and a high voltage connector passageway 38, both of which terminate at the lower end of hand grip 26 at connectors 40, 42, respectively. Air flow through air passageway 36 is controlled by a trigger 30-controlled air valve 44. A controlled air passageway 46 extends upwardly from air valve 44 and intersects a forwardly extending atomizing air passageway 48 and a forwardly extending fan air passageway 50 at the top of handle portion 22. A constant bleed fan control valve assembly 52 at the rear end 54 of fan air passageway 50 controls the supply of fan air forward from valve 52 in a manner which will be described later. Low magnitude voltage provided through a suitable low voltage cable 56 is supplied through connections (not shown) to a transformer housed in a transformer housing 60 integral with hanger hook 28. The transformer transforms low-magnitude voltage (≦ ± 24V) supplied through cable 56 up to about 5KV. Barrel portion 24 is mounted to handle portion 22 by a barrel 24-retaining ring 62 and cooperating barrel-retaining nut 64. A gasket 66 positioned between handle portion 22 and barrel portion 24 seals all of the passageways and connections between these two. Barrel portion 24 includes a high voltage cascade 68 and electrical connections (not shown) by which it is coupled electrically to the output terminals (not shown) of the high voltage transformer in housing 60. Cascade 68 rectifies and multiplies the approximately 5KV output voltage variations across the transformer's output terminals up to about -65KV for application to a liquid coating material charging needle 74 positioned at the front end of gun 20. Barrel portion 24 also includes an atomizing air passageway 76 which mates with passageway 48 at gasket 66, and a fan air passageway 78 which mates with passageway 50 at gasket 66. Passageways 76, 78 extend forward from gasket 66 to the front end 80 of barrel portion 24 and terminate at threaded openings 82, 84, respectively. A liquid coating material is introduced into gun 20 through a connector 86 provided on the underside of barrel portion 24 about halfway along the length thereof. Liquid coating material flows upward through connector 86 and then forward through a coating material passageway 88 which extends longitudinally along barrel portion 24 and terminates at the front end 80 of barrel portion 24. An air valve 44 pushrod 106 extends rearwardly from behind and adjacent trigger 30 into the body of air valve 44. A valve head 108 on the end of pushrod 106 and a compression spring 110 of valve 44 are captured in the valve 44 bore by a threaded air valve cap 112. Seepage of air from the front of the bore 114 accommodating pushrod 106 is minimized by a TEFLON® O-ring 118 held in place by a retaining nut 120 threaded into the forward end of bore 114. Barrel portion 24 includes a central valve needle passageway 130. Passageway 130 includes a reduced diameter extreme forward portion 132 and a larger diameter rearward portion 134. The rearward end 136 of portion 134 is threaded. A needle shaft assembly 138 includes a shaft 140 which extends forward, and a shaft 141 which extends rearward, from a trigger crossbar 142. A collar 144 adjacent the rearward end of shaft 141 provides a stop for a compression spring 146 captured between collar 144 and a valve spring cap 150 threaded into the rear end of a return spring housing 152 which houses spring 146 and the rearward end of shaft 141 including collar 144. A trigger adjusting nut 154 and a trigger adjusting lock nut 156 control the relative timing of the initiation and termination of the air and liquid coating material flows, as will be appreciated. Shaft 140 extends forward from trigger 30 into passageway 130 through a packing nut 160, a Belville spring washer 162, a seal and spring spacer 164, a rear needle seal retainer 166, a packing adjustment tube 168, a female chevron seal adapter 170, a chevron packing 172, and a male chevron seal adapter 174 to the front end 80 of barrel portion 24. An internally threaded, central, enlarged opening 176 is provided around the opening of passageway 130 in the front end 80 of barrel portion 24. A coating liquid nozzle 178 is threaded into opening 176. The charging needle 74 and liquid coating material flow control valve needle 180 respectively project from, and seat against, the coating material discharge orifice 182 in nozzle 178. An air nozzle 184 is placed over nozzle 178 and the remaining exposed portions of the front end 80 of barrel portion 24 to define an atomizing air chamber 186 and an atomizing air exit opening 188. Air nozzle 184 is captured on the front end 80 of barrel portion 24 by a retainer ring 187 which threads onto the front end 80 of barrel portion 24. In accordance with the invention, a pressure reducer 190 having a rearwardly extending needle 192 and a threaded 194 and slotted 196 head is threaded into opening 82, and a flow reducer 200 having a central flow control orifice 202, a threaded exterior surface 204 and slots 206, is threaded into threaded opening 84 before liquid nozzle 178 is threaded into opening 176. It is known that changes in the configuration of the air nozzle 184 can change the fan- and atomizing air flow dynamics, and therefore, the operating nozzle pressure of guns such as gun 20. The configuration of the illustrated embodiment permits pressure and flow reducers 190, 200 precisely matched to the flow characteristics of a particular air nozzle 184 to be packaged with that air nozzle 184 and supplied with it to the operator. When the operator changes an air nozzle 184, the pressure and flow reducers 190, 192 are changed at the same time, quickly and conveniently, with no more tools than a small screwdriver, to maintain the operating air pressure at nozzle 184 at the 68947 N/m 2 (10 psig) HVLP maximum. In order to reduce the sensitivity of the atomizing air pressure at the output side of pressure reducer 190 even further, a bleed pathway 208 is provided through fan air control valve assembly 52. The fan air adjusting knob 210 of assembly 52 is adjustable by screwing it in or out to reduce or increase, respectively, the fan air flow from the fan air openings 212 in air nozzle 184. It is known that the flow which results from fan air openings 212 when the fan airs pressure in chamber 214 provided in air nozzle 184 is 27579 N/m 2 (4 psig) generally does not affect the atomized coating material pattern. Only when the fan air pressure in chamber 214 exceeds 27579 N/m 2 (4 psig) does the atomized coating material pattern begin to "fan", or be affected in terms of its shape, by the fan air flow. Consequently, bleed ports 208 are provided in the fan control valve 52 which permit the pressure in chamber 214 to be 27579 N/m 2 (4 psig), just below the fan air shaping threshold, and air to flow from openings 212 at all times, even when fan air adjusting knob 210 is adjusted all the way in to "close" fan control valve 52. Because this 27579 N/m 2 (4 psig) bleed exists at all times and the pressure and flow reducers 190, 200 and air nozzle 184 are otherwise designed to account for it, the air pressure on the downstream side of pressure reducer 190 is "buffered" against atomizing air flow rate/pressure variations caused by increasing or decreasing the amount of fan air supplied to chamber 214 by turning fan air adjusting knob 210.

Claims: A device for atomizing a first fluid, the device comprising a first passageway (88) for supplying the first fluid, a second passageway (76) for supplying a second fluid for promoting atomization of the first fluid, and a third passageway (78) for supplying the second fluid to promote shaping of a cloud of the atomized first fluid, means (36,44,48,50) for coupling the second and third passageways (76,78) to a source of the second fluid, wherein said coupling means (36,44,48,50) include a control valve assembly (52) having a first valve member and a first valve seat, characterized in that the relative positions of the first valve member and first valve seat being adjustable selectively to increase and decrease flow of the second fluid through the third passageway (78), and include means providing a bypass passageway (208) around the first valve seat to provide a flow of the second fluid through the third passageway (78) regardless of the relative positions of the first valve member and the first valve seat. Device according to claim 1, wherein the means providing a bypass passageway (208) around the first valve seat comprises means providing said bypass passageway through the first valve body. Device of anyone of the claims 1 or 2, wherein the second passageway (76) having a first cross-sectional area at an atomizing end (80) of the atomizing device, the third passageway (78) having a second cross-sectional area at the atomizing end (80) of the atomizing device, a nozzle (184) is provided for attachment to the atomizing end (80) of the atomizing device, the nozzle (184) providing adjacent first (182), second (188) and third openings (212) and a first path for the first fluid therethrough, a first chamber (186) is provided adjacent the first opening (182) of the first path from the nozzle (184), the second passageway (76) providing atomizing fluid flow to the first chamber (186), the first chamber (186) is coupled to the second opening (188) to promote atomization of the first fluid as the first fluid exits from the first opening (182), a second chamber (214) is provided adjacent the first opening (182), the third passageway (78) providing shaping fluid flow to the second chamber (214), the second chamber (214) is coupled to the third opening (212) to promote shaping of the cloud of the atomized first fluid. Device according to claim 3, wherein a first pressure reducer (190) is inserted into the second passageway (76) or the first chamber (186), the first pressure reducer (190) providing a first orifice having a cross-sectional area smaller than the first cross-sectional area to reduce the pressure of the atomizing fluid at the second opening (188), whereby the first pressure reducer (190) is replacable in order to match to the flow characteristics of the nozzle (184). Device according to claim 4, wherein the first pressure reducer (190) is threadable into one of the second passageway (76) or the first chamber (186). Device of anyone of the claims 4 or 5, wherein the first pressure reducer (190) comprises a first valve needle (192), the second passageway (76) providing a second valve seat, the first orifice is defined between the first valve needle and the second valve seat. Device of anyone of claims 3 to 6, wherein a second pressure reducer (200) is inserted into the third passageway (78) or the second chamber (214), the second pressure reducer (200) providing a second orifice having a cross-sectional area smaller than the second cross-sectional area to reduce the pressure of the atomizing fluid at the third opening (212), whereby the second pressure reducer (200) is replacable in order to match to the flow characteristics of the nozzle (184). Device according to claim 7, wherein the second pressure reducer (200) is threadable into one of the third passageway (78) or the second chamber (214). Device of anyone of claims 7 or 8, wherein the second pressure reducer (200) comprises a flow reducer sleeve for threading into the atomizing end (80) of the third passageway (78).

Inventors: RODGERS MICHAEL C, RODGERS, MICHAEL C.
Applicants: RANSBURG CORP, RANSBURG CORPORATION
Application year: 1992
Main classification: B05B7
IPC codes: B05B7, B05B5