Abstract:
An automatic spray gun includes a nozzle ( 4 ) for ejecting the product that is to be sprayed, controlled by a needle valve ( 9 ), and means ( 14, 10 ) for controlling the movement of the needle ( 9 ). A tabular elastic sealing means ( 13 ) surrounds the needle valve ( 9 ) and is mounted trapped between the front ( 1   a ) and rear ( 1   b ) parts of the body. The interior surface of the tabular means ( 13 ) is tailored to the diameters of the needle valve ( 9 ) and of a control slide ( 14 ) mounted on the needle valve in order to move the needle thereof.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent. 
     The invention is especially useful for high-pressure and high-frequency atomization of paints that can be cross-linked by ultraviolet radiation. 
     This type of automatic spray gun comprises a pressurized propellant gas chamber supplied with pressurized propellant gas, generally compressed air, and a product chamber that is supplied with the product to be atomized under pressure. 
     A nozzle for atomization of the product to be atomized under pressure is connected to the product chamber. The product is supplied upstream from the spray gun and emerges through a calibrated opening of the nozzle. 
     Moreover, within the spray gun, there is a slide that passes through the gas and product chambers and that is provided on its front end with a needle valve that can block said nozzle. 
     The opening of the needle is triggered by the action of the propellant gas, generally compressed air. 
     On its back end, the slide is made integral with the actuating means, generally a pneumatic cylinder that can move in translation to block or clear said nozzle. The pneumatic cylinder comprises a piston returned to the closed position by a spring and moved into the open position by the propellant gas. 
     To prevent the propellant gas from travelling from the gas chamber to the product chamber along the slide and conversely to prevent the product to be atomized from travelling from the product chamber to the gas chamber, there are sealed oscillating means crossed by said slide. 
     During atomization of the product, the propellant gas, intended for atomization, facilitates atomization of the product through the central opening of the nozzle and forms a concentrically oriented jet formed by peripheral air vents. 
     In such a known automatic spray gun, said slide slides each time that it is necessary to open and close the nozzle. 
     In the known manner, there is an offset for opening the atomization air before discharge of the product and for ensuring the blocking of the nozzle before the closing of the atomization air. 
     On the nozzle side, the surface of the slide is in contact with the product that is to be atomized and that is often abrasive and/or corrosive, as is the case, for example, for enamel. 
     The sealing means can be static or deformable: the static means can be O-ring seals, scraper seals, or catches similar to braces, whereas the deformable means can comprise a bellows, a membrane or other flexible surface mounted between a fixed part and a movable part of the spray gun. 
     Document FR 2 863 512 thus describes an automatic atomizing spray gun comprising a flexible bellows on the side of the chamber of the product to be atomized. This flexible bellows is attached to the body of the spray gun on its back end and comprises a movable front end that forms a seal around the needle. 
     The needle comprises a shoulder that works with the seal formed by the front end of the bellows. 
     The spray gun of document FR 2 863 512, however, comprises a bellows with coils, of which the surface is difficult to rinse and clean, in particular in the depressions and corners between the coils. 
     Document EP1063018 likewise discloses an automatic spray gun with a membrane in which the needle valve is integral with the membrane and crosses it, in such a manner as to be connected to a pulsation control element. 
     However, due to the significant surface area of the membrane in contact with the product, the membrane must be protected by a coating, for example by a polytetrafluoroethylene layer. 
     SUMMARY OF THE INVENTION 
     A first objective of the invention is to improve the known prior art by providing a new automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent and which is easy to rinse and clean. 
     A second objective of the invention is to provide a new automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent, in which the sealed oscillating means have a small surface area in contact with the product to be atomized. 
     A third objective of the invention is to provide a new automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent, in which a protective coating of the sealed oscillating means is no longer necessary. 
     The object of the invention is an automatic spray gun for atomizing a product such as an abrasive paint, a lacquer, or an enamel, which is water-soluble or dissolved in a solvent, comprising a body having a front part that defines a chamber of the product to be atomized, and a back part for control and admission of a propellant gas, an atomization head and a nozzle that discharges the product controlled by a movable needle valve, as well as means for controlling the movement of the needle valve, an elastic sealing means surrounding the needle valve between the product to be atomized and the propellant gas while being mounted clamped between the front and back parts of the body, characterized by the fact that the elastic sealing means has an elongated tubular shape whose inner surface is adjusted to the diameters of the needle valve and a control slide mounted on the needle valve for moving it, so as to withstand the pressure of the product to be atomized. 
     According to other alternative characteristics of the invention: 
     The elastic sealing means is fixed on its front end so as to be locked against a shoulder of the needle valve and to prestress the elastic sealing means in compression. 
     The elastic means is mounted clamped against one part by screwing of a cylindrical jacket. 
     The cylindrical jacket for clamping the elastic sealing means defines the chamber of the product to be atomized. 
     The elastic sealing means has an external shape that is continuously concave in contact with the product to be atomized. 
     The travel of the needle valve is short, roughly several millimeters, and the variation of the radial dimensions of the elastic sealing means is small, roughly several tenths of a millimeter. 
     The elastic sealing means has a sleeve shape, with a front part bulging radially inwards and a back part bulging radially outwards. 
     The elastic sealing means is made of a perfluorinated elastomer material. 
     Control means work with a mechanical offset corresponding to a clearance and ensuring the opening of the propellant gas before the opening of the product to be atomized and the closing of the product to be atomized before the closing of the propellant gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by the following description given by way of a nonlimiting example with reference to the attached drawings in which: 
         FIG. 1  schematically shows a longitudinal cutaway view in the direction of line II-II of  FIG. 2  of an automatic spray gun according to the invention. 
         FIG. 2  schematically shows a top view in the direction of arrow I of  FIG. 1  of an automatic spray gun according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , an automatic spray gun according to the invention comprises a front part  1   a  and a back part  1   b , of which at least one,  1   a  or  1   b , is made of lightweight metal. 
     The front part  1   a  contains a cylindrical jacket  2  made of material that is resistant to abrasion, for example stainless steel, which accommodates an atomization head  3  by screwing it in. 
     The atomization head  3  comprises a central opening  3   a  and several peripheral openings  3   b  or  3   c . Based on the product discharge opening  3   a  and the propellant gas exit openings  3   b  or  3   c , atomization of the product is obtained by atomization of the product supplied to the spray gun under constant pressure, in combination with feed of atomization air that, besides atomization, makes it possible to control the shape and/or the dimensions of the jet of atomized product. 
     The atomizing spray gun according to the invention comprises atomization air feed means that discharge in the vicinity of the atomization nozzle  4  to improve atomization and to form the product jet. 
     The atomization air is supplied independently of the product exit using a mechanical offset that supplies the atomization air in an annular chamber formed in the atomization head and that communicates with the first set of openings  3   b  or air nozzles discharging quite near the exit  3   a  of the nozzle  4  and with a second set of openings  3   c  located in a ring located on the front end of the body of the spray gun. This arrangement allows the openings of this second set of openings  3   c  to point the atomization air jets at a much greater angle toward the product jet emerging from the nozzle  4 . 
     The nozzle  4  is shaped as a ball valve  5  comprising a module that can be interchanged by screwing it into the cylindrical jacket  2 . 
     The product to be atomized is supplied under constant pressure by a pipe to the feed chamber  7  of the product delivered to the front part of the spray gun, with an essentially cylindrical ring shape, discharging directly into a central channel  8  of the atomization head  3  that connects the feed chamber  7  of the product to an atomization nozzle  4 . 
     To control the feed of the product to the nozzle  4  and especially to cut off this feed when the spray gun is no longer actuated, the automatic spray gun according to the invention uses a needle valve  9  that blocks the exit of the product through the nozzle  4 . 
     The return of the needle valve  9  to clear this exit is controlled by pneumatic control means that act on the piston  10  of a cylinder  11  moving the needle valve  9  to the rear. When the pneumatic control means are no longer actuated, a return spring  12  acts on the piston  10  in the opposite direction to move the needle  9  forward to close the exit of the product. 
     When the piston  10  is pushed back by a control air pulsation to the rear, compressing the spring  12  up to its back position, the end of the needle valve  9  releases the ball  5  and clears access to the atomization nozzle  4  for the pressurized product that travels from the product input chamber  7  to the nozzle  4  through the annular space defined between the internal wall of the central channel  8  and the external surface of the needle valve  9 . 
     To improve atomization and to better shape the jet of atomized product that is emerging from the atomization nozzle  4 , the spray gun is likewise provided with atomization air feed means that discharge near the atomization nozzle  4 . These atomization air feed means comprise a compressed air pipe connected to an air duct discharging into an air input chamber  17  that is circular overall and that is arranged around a valve  16  in the back part  1   b  of the body of the spray gun. This air input chamber  17  is connected to an external circular chamber  19  arranged in the atomization head  3  via successive communicating annular passages  18  that extend into the back part  1   b  and front part  1   a  of the body. This external chamber  19  is in communication with the atmosphere through the openings  3   b  or  3   c  discharging on the front surface of the atomization head  4 . 
     The control piston  10  of the needle valve  9  is surrounded by a piston  10   a  that is coaxial to the piston  10  and is prestressed into the position closed by a spring  12   a.    
     The front end of the piston  10   a  narrows to form a poppet valve  16  applied to a seat, formed in or integral with the back part  1   b , by the pressure of a spring  12   a.    
     There is a predetermined play j of the displacement between the piston  10  and the piston  10   a  such that the movement of the piston  10  is controlled in both directions with an offset by movement of the piston  10   a , for both opening and closing. 
     The connection between the needle valve  9  and the piston  10  is ensured by a connecting slide  14  that traverses the front part  1   a  and back part  1   b  of the body of the spray gun. 
     The slide  14  is shaped as a pin threaded on its front end on the needle valve  9  and is made integral on its back end with the piston  10  and a support plate of the spring  12 . 
     The slide  14  is mounted on the needle valve  9  so as to clamp a front shape  13   a  of an elastic sealing element  13  against a shoulder  9   a  of the needle valve  9  and to prestress the sealing element  13  in compression. 
     The back end  13   b  of the elastic sealing element  13  is clamped between the front part  1   a  or its jacket  2  made of stainless steel and the back part  1   b  of the body of the spray gun to ensure interlocking and static sealing. 
     Due to the elasticity of the sealing element  13 , elongation and shortening of the element  13  can be done at increased actuating frequencies, around 2 Hz, while ensuring a service life of 10 million cycles at increased pressures of roughly 120 bar. 
     Advantageously, the elastic sealing element  13  has an elongated tubular shape, of which the inside surface is adjusted to the diameters of the needle valve and the slide, so as to withstand the pressure applied by the product to be atomized on the external surface of the element  13 . 
     Preferably, the elongated sealing element  13  is made in the form of a tubular sleeve with two bulging ends  13   a ,  13   b.    
     The front end  13   a  is bulging radially inwards, whereas the back end  13   b  is bulging radially outwards. 
     Due to the fact that the action of the needle  9  is to keep the ball  5  in contact with its seat or to allow its separation, the travel of the needle  9  is small, roughly two millimeters. 
     The variation of radial dimensions is again smaller, roughly a tenth of a millimeter; this allows use at increased actuating frequencies, roughly 2 Hz. 
     The continuously concave shape of the external surface of the sealing element  13  facilitates rinsing and cleaning of this external surface and eliminates the difficulties of rinsing and cleaning that are encountered with the bellows and membranes of the prior art. 
     The essentially cylindrical shape of the outside surface of the sleeve  13  likewise makes it possible to reduce the surface area in contact with the product to be atomized by comparison with the bellows and membranes of the prior art. 
     The material used for making the sleeve  13  can be a polyurethane in the case of a specification of resistance to water-soluble products. 
     Polyurethanes are economical and easy to shape, preferably by molding, or if necessary by machining. 
     In the case of resistance to all types of products to be atomized, water-soluble or dissolved in a solvent, the sleeve  13  can be made of a perfluorinated elastomer material with high chemical inertia. 
     A perfluorinated elastomer that is used is a perfluorinated elastomer specially formulated for a high-pressure environment and explosive decompressive conditions, which is satisfactory. 
     The invention is not limited to the described embodiment, but on the contrary covers any modification of shape and any variant embodiment within the framework and the spirit of the invention. 
     Thus, the material used for the sleeve  13  can be modified without departing from the scope of this invention and if necessary can be composed of a metallic or nonmetallic material.