Patent Publication Number: US-2007102458-A1

Title: Spray gun for dispensing precise small liquid volumes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This patent application claims the benefit of U.S. Provisional Patent Application No. 60/735,497, filed Nov. 10, 2005. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to spray guns, and more particularly, to spray guns for dispensing relatively small, precisely controlled, quantities of liquid.  
     BACKGROUND OF THE INVENTION  
      Spray guns are known for dispensing substantially uniform quantities of liquid during each operating cycle. The quantity of liquid dispensed in each cycle, however, is not identical. Normal manufacturing tolerances and other imprecisions in the gun design can result in significant variations in the liquid quantities dispensed during each operating cycle. In a spray gun designed to dispense 1 ml. of liquid during each cycle of operation, for example, variances of 0.1 ml and more frequently occur.  
      When it is desired to dispense small droplet-sized quantities of liquid, such as 0.1 ml or less, more precise control is required. Heretofore, it has been difficult to reliably dispense such small liquid quantities without expensive precision equipment. Also, by virtue of the small volume of the liquid to be dispensed, it has been difficult to selectively vary or change the volume of dispensed liquid with precision.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide an improved spray gun that is adapted for dispensing precisely controlled, relatively small volumes of liquid during each operating cycle.  
      Another object is to provide a spray gun as characterized above which can be easily adapted to selectively change the precisely controlled volume of dispensed liquid.  
      A further object is to provide a spray gun of the above kind that is relatively simple in construction and which lends itself to economical manufacture and use.  
      Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a vertical section of an illustrated spray gun in accordance with the invention, showing the liquid dispensing piston thereof in a retracted liquid receiving position;  
       FIGS. 2   a  and  2   b  are fragmentary sections depicting downward movement of the liquid dispensing piston during a dispensing cycle;  
       FIG. 3  is an enlarged fragmentary section of the liquid dispensing chamber of the illustrated spray gun;  
       FIG. 4  is an enlarged fragmentary section depicting the end of the liquid dispensing piston as it enters the dispensing chamber; and  
       FIG. 4   b  is a fragmentary section, similar to  FIG. 4 , depicting the spray gun with an alternative liquid dispensing piston effective for dispensing smaller volumes of liquid during each operating cycle. 
    
    
      While the invention is susceptible of various modifications and alternative constructions, a certain illustrated embodiment thereof has been shown in the drawings and will be described below in detail. In should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring now more particularly to the drawings, there is shown an illustrative spray gun  10  in accordance with the invention which is adapted for dispensing relatively small, precisely controlled volumes of liquid during each operating cycle. The illustrated spray gun  10  includes a two part body  11  comprising upper and lower body members  11   a ,  11   b  coupled together by longitudinal bolts  12 . The nozzle body  10  has a liquid inlet port  14  coupled to an appropriate liquid supply line  15  which communicates through a relatively small diameter inlet  14   a  with a main axial liquid passageway or passage section  16  in the nozzle body member  11   b , which in turn communicates with a spray tip or discharge orifice member  18  mounted on a downstream end of the body  11 . To facilitate aligned assembly of the upper and lower body members  11   a ,  11   b , the upper body member  11   a  is formed with a depending annular collar  20  for positioning against an annular sealing gasket  20   a  within an annular recess  21  in the lower body member  11   b . The upper body member  11   a  in this instance also is formed with an upstanding annular nipple  22  to facilitate mounting of the spray gun on a mounting bracket  23  or the like for a particular installation.  
      The spray tip  18  in this case is mounted on an axial hub  19  of the body member  11   b , being retained within a counter-bore  26  of the hub  19 . The spray tip  18  has an annular retaining flange which is secured with the hub  19  by a threaded clamping ring  28 , with an annular sealing gasket  29  interposed between the flange and spray gun body.  
      The spray gun body liquid passageway  16  includes a relatively small diameter nozzling section  16   a  defined in a wall of the body member  11   b  that communicates with an enlarged passage section  16   b . The spray tip  18  has a liquid flow passageway  24  communicating with the body passageway  16  defined by a large diameter inlet section  24   a  and a smaller diameter nozzling passage  24   b  extending through a protruding nose  25  of the spray tip  18 .  
      In accordance with the invention, the spray gun has a cyclically operable liquid dispensing piston arrangement adapted for dispensing a precisely controlled predetermined small quantity of liquid during each cycle of operation of the spray gun. To this end, the illustrated spray gun  10  includes a liquid dispensing piston  30  supported for axial reciprocating movement within the nozzle body  11 . The liquid dispensing piston  30  in this instance has an enlarge diameter piston head  31  disposed within a cylindrical counter-bore  32  of the upper body member  11   a  and a smaller diameter dispensing stem  34  disposed within the liquid passageway  16  of the lower body member  11   b . For biasing the piston  30  in a downstream liquid dispensing direction, a coil spring  35  is disposed about the piston head  31  in interposed relation between an upper end of the counter-bore  32  and an outwardly extending radial flange  36  of the piston head  31 .  
      For retracting the piston  30  and liquid dispensing stem  34  thereof in an upstream direction against the force of the biasing spring  35 , the nozzle body member  11   b  is formed with an air inlet port  38  for coupling to a suitable pressurized air supply line  39 , which communicates through with a smaller diameter inlet  38   a  with a lower or downstream side of the piston head  31 . To seal the piston head  31  within the counter-bore  32 , an annular U-cup seal  40  is retained within a channel defined between axially spaced flanges  36 ,  41  of the piston head  31 . The U-cup seal  40  is mounted such that an air pressure receiving pocket defined by the seal is oriented on an underside thereof. A second U-cup seal  42  in this case is disposed about an upstream of the liquid dispensing stem  34  adjacent a smaller diameter wall opening  44  of the spray gun body through which the stem extends. Hence, pressurized air directed through the air inlet port  38  communicates with a lower side of the piston head  31  and cup seal  40  for moving the piston  30  in an upstream direction against the force of the biasing spring  35  (as shown in  FIG. 1 ).  
      In carrying out the invention, the spray gun body passage  16  defines a predetermined size liquid dispensing chamber  43  ( FIG. 3 ) downstream of the liquid inlet  14   a  and the liquid dispensing stem  34  of the piston  30  is sized and designed for causing precisely controlled predetermined small quantities of liquid to be dispersed during each reciprocating cycle of the piston  30 . In this case, a liquid dispensing chamber  43  has a precise predetermined volume defined between a downstream side  43   a  of the liquid inlet  14   a  and an end wall or stop surface  43   b  of the body member  11   b , and the liquid dispensing piston stem  34  is provided with a pair of annular sealing rings  46 ,  48  in predetermined spaced relation to a downstream end of the stem  34 , dependent upon the quantity of liquid to be dispensed during each operating cycle of the spray gun. The illustrated seals  46 ,  48  are quad seals of a commercially available type, which each have four annular sealing nibs  46   a - d ,  48   a - d , with two radially inward nibs  46   a, b  and  48   a, b  of each seal being disposed in axially spaced sealing contact with the liquid dispensing stem  34  and the two radially outward nibs  46   c, d ,  48   c, d  being disposed in axially spaced sealing contact with a wall of the liquid passageway  16 . The downstream outer nib  46   c  of the spray gun body quad seal  46  in this instance is disposed in predetermined spaced relation to the downstream lower end of the liquid dispensing stem  34  for controlling the quantity of liquid to be dispensed in an operating cycle, and the upstream quad seal  48  is disposed so as to be upstream of the liquid inlet  14   a  during both retraced and extended positions of the piston  30 .  
      For preventing liquid downstream of the nozzling liquid passage section  16   a  from being drawn back into the liquid passage  16  during retraction of the piston  30 , a rubber check valve  50  is provided on a downstream side of the nozzling passage section  16   a , which is biased into a closing position by a spring  51  interposed between the check valve  50  and a downstream end of the spray tip inlet passage section  24 . To facilitate proper positioning of the check valve  50  in its closed position, the spray gun body is formed with a tapered seating section  52  on a downstream side of the passage section  16   a.    
      In operation of the spray gun, introducing pressurized air to the air inlet port  38  will cause retracting movement of the piston head  31  and stem  34  upwardly against the biasing force of the spring  35 , relieving pressure within the downstream dispensing chamber  34  and enabling the check valve  50  to be biased into a closed position to prevent liquid downstream of the nozzling passage section  16   a  from being drawn back into the dispensing chamber  43 . As the lower quad sealing ring  46  moves upwardly past the liquid inlet  14   a , continued upward movement of the piston  30  will create a suction within the dispensing chamber  43  drawing liquid into the liquid passage  16  from the liquid inlet port  14 . Termination of pressurized air to the air inlet port  39  will relieve pressure on the underside of the piston head  31 , with the resultant downward movement of the piston head  31  and stem  34  under the biasing force of the spring  35 . As the lower quad ring  46  of the stem  34  moves toward the liquid inlet  14   a  excess liquid in the chamber will be forced back through the inlet port  14 . Upon the lower nib lobe  46   c  of the quad seal ring  46  moving downwardly past the downstream side  43   a  of the inlet  14   a , the dispensing chamber  43  is thereupon sealed with a predetermined quantity of liquid to be dispensed. With the piston biasing spring  35  being significantly stronger than the check valve biasing spring  51 , the piston biasing spring  35  will continue to force the piston stem  34  downwardly into the dispensing chamber  43  forcing the liquid to open the check valve  50  and dispense the liquid through the spray tip. When the piston stem  34  bottoms out against the end wall  43   b  of the dispensing chamber  43 , a relatively precise, predetermined quantity of liquid, measured by the volume of the dispensing chamber, less the displacement volume of the stem  34  downstream of the quad seal lower nib  46   c , will have been dispensed.  
      It will be appreciated by one skilled in the art that the dispensing gun  10  is relatively simple in construction and operation, while enabling precise control in the volume of the dispensed liquid during each operating cycle. In this regard, the volume of liquid dispensed is not dependent upon the length of retracting movement of the piston  30  in a upstream direction. The piston  30  need only be retracted sufficiently to fill the liquid passage  16  downstream of the lower quad sealing ring  46 . Any excess quantity of liquid drawn into the spray gun  10  during retraction of the piston  30  is simply forced back out the liquid inlet port  14  during downward dispensing movement of the piston  30  until such time as the quad seal lower nib  46   c  passes the inlet port  14   a . On the other hand, the volume of dispensed liquid can be precisely controlled by the size, and hence displacement volume of the stem  34  downstream of the lower quad ring nib  46   c . In other words, since the volume of the displacement chamber  43  downstream of the liquid inlet  14   a  is constant, precise design in the length “l” of the stem downstream of the lower quad seal nib  46   c  will enable precise control in the volume of liquid dispensed during a cycle of operation. Hence, it can be seen that the volume of dispensed liquid is precisely determinable by the volume of the liquid dispensing chamber  43  downstream of the liquid inlet  14   a , less the displacement volume of the stem section “l” downstream of the lower quad ring nib  46   c . Since the nib  46   c  is relatively small in diameter, in relation to the overall size of the quad sealing ring  46 , the displacement volume is precisely established. In practice, cyclic dispensing of liquid droplets as small as 0.05 mm can be accurately and cyclically dispensed.  
      In carrying out a further feature of the invention, the spray gun  10  can be easily adapted for changing the volume of the dispensed liquid by simply changing the dispensing piston  30 . To this end, upon unscrewing the fastening screws  12 , the body parts  11   a ,  11   b  may be separated, and the piston  30  removed from the body and replaced with a piston  30  having a stem of a different length “l” downstream of the quad seal lower nib  46   c . For example, replacement of the piston  30  with a piston having a longer stem length “l” downstream of the lower quad seal nib  46   c , and hence a larger liquid displacement volume, will cause a predetermined lesser quantity of liquid to be dispensed during each operating cycle of the spray gun. Likewise, replacing the piston  30  with a piston having a stem  34  with a shorter length “l”, and hence a smaller liquid displacement volume, will result in a predetermined larger quantity of liquid to be dispensed during each operating cycle.  
      From the foregoing, it can be seen that the spray gun of the present invention is adapted for dispensing precisely controlled relatively small volumes of liquid during each operating cycle. The spray gun also is relatively simple in construction, lends itself to economical manufacture and reliable operation, and can be easily altered to change the volume of dispensed liquid.