Patent Publication Number: US-2017348710-A1

Title: Spray gun

Description:
TECHNICAL FIELD 
     The invention relates to spray guns. 
     BACKGROUND ART 
     One of spray guns that have been known is a low-pressure atomizing air spray gun. The low-pressure atomizing air spray gun has a nozzle tip that is formed to have a front end with a truncated conical outer shape and include a circular cross-sectional internal hole to which a substantially V-shaped cross-sectional groove is formed to create a lip-like opening. Through this nozzle tip, the spray gun ejects paint at a pressure ranging from 1 to 6 kgf/cm 2 , and atomizes the sprayed paint by using a compressed air flow under a pressure ranging from 0.5 to 2 kgf/cm 2 . The spray gun has an air cap that creates an annular air ejection flow between the outer periphery of the front end of the nozzle tip and the central hole of the air cap. The front end face of the nozzle tip is located within an area where an annular air hole is formed between the front end face and the central hole of the air cap. (Patent Document 1) 
     In the Patent Document 1, spray air pressure is set low to solve a problem associated with high spray air pressure which is the most common cause of paint scattering during air spray. The spray gun disclosed in the Patent Document 1 achieves sufficient atomization even with the low spray air pressure, and uses a simple device to perform spray painting without using a device like a special high-pressure pump. 
     According to the description about the operation disclosed in the Patent Document 1, more specifically, the paint that is ejected from the nozzle at a pressure high enough to eject the paint is ejected from the lip-like opening through the groove in the front end of the nozzle, in the form of a flat flow in a folding-fan shape. The paint is atomized by colliding with an air flow ejected from the annular air opening that is formed by the conical outer periphery of the nozzle and the central hole of the air cap. Immediately after being atomized in the central hole, the paint is further atomized by colliding with the air flow that is ejected from auxiliary air holes opened on minor-diameter sides of the nozzle tip opening. 
     In the downstream thereof, a substantially rectangular spray flow is created by an air flow that is discharged from auxiliary air holes opened on major-diameter sides of the nozzle tip opening to collide with the paint. Subsequently, in the further downstream, air flows ejected from a pair of horn air holes formed on the major-diameter sides further atomize the spray flow that is not sufficiently atomized. Eventually, the whole paint is thoroughly atomized. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Examined Patent Application Publication No. H07-024796 
     SUMMARY OF INVENTION 
     Technical Problem 
     The method disclosed in the Patent Document 1 includes multiple steps of spray impingement of the air flow from a large number of auxiliary air holes against the paint for sufficient atomization, so that a large number of air holes need to be formed in the air cap. This complicates the structure of the air cap (gas cap). 
     The present invention has been made in light of the above-mentioned circumstances. An object of the invention is to provide a spray gun capable of setting a spray gas pressure at a relatively low middle-low pressure and performing good atomization without making auxiliary gas holes in a gas cap. 
     Solution to Problem 
     Spray gas pressure can be set at relatively low middle-low pressure and a good atomization is performed without providing a gas cap with auxiliary gas holes. 
     The spray gun of the invention includes a nozzle portion in which a substantially V-shaped groove is formed in a circular section of a truncated conical front end with a cone angle ranging from 20° to 90°, and an internal hole is opened as a liquid ejecting port by forming the substantially V-shaped groove; and a gas cap including a cap face which is provided with an atomized gas opening portion having an opening diameter larger than the circular section, the gas cap forming a circular slit-like gap between the gas cap and an outer periphery of the truncated conical front end, the gap being configured to eject gas for atomizing liquid. The circular section of the truncated conical front end has a diameter ranging from 0.8 mm to 2.8 mm. The atomized gas opening portion has the opening diameter that is equal to or larger than 1.0 mm and smaller than 3.0 mm. The gas is ejected from the circular slit-like gap at a flow rate ranging from 40 L/min to 160 L/min and an ejection velocity ranging from 100 m/sec to 2900 m/sec, so that the liquid can be atomized without providing the cap face with auxiliary gas ejecting holes for atomizing the liquid. 
     The invention will be understood through the following configuration. 
     (1) The invention provides a spray gun comprising a nozzle portion in which at least one substantially V-shaped groove is formed in a circular section of a truncated conical front end with a cone angle ranging from 20° to 90°, and an internal hole is opened as a liquid ejecting port by forming the substantially V-shaped groove; and a gas cap including a cap face which is provided with an atomized gas opening portion having an opening diameter larger than the circular section, the gas cap forming a circular slit-like gap, which ejects gas for atomizing liquid ejected from the liquid ejecting port, between the gas cap and an outer periphery of the truncated conical front end, in which the circular section of the truncated conical front end has a diameter ranging from 0.8 mm to 2.8 mm; in which the atomized gas opening portion has the opening diameter that is equal to or larger than 1.0 mm and smaller than 3.0 mm; and in which a flow rate of the gas ejected from the circular slit-like gap ranges from 40 L/min to 160 L/min; and an ejection velocity of the gas ejected from the circular slit-like gap ranges from 100 m/sec to 2900 m/sec, so that the liquid can be atomized without providing the cap face with auxiliary gas ejecting holes for atomizing the liquid.
 
(2) In the configuration according to (1), a supply pressure of the gas at a gas supply port ranges from 0.07 MPa to 0.25 MPa, and a supply pressure at a time when the gas is introduced into the circular slit-like gap ranges from 0.05 MPa to 0.2 MPa.
 
(3) In the configuration according to (1) or (2), the gas cap includes horn portions having a gas channel extending from an outer periphery of the cap face in an ejecting direction of the liquid, and the horn portion is provided with a pattern-adjusting gas ejecting port that opens in the gas channel, the pattern-adjusting gas ejecting port being configured to eject gas toward the liquid that is atomized and adjust a spray pattern shape with respect to a target to be applied with the atomized liquid.
 
(4) In the configuration according to any one of (1) to (3), the truncated conical front end of the nozzle portion is located between where the truncated conical front end is in plane with an end face of the atomized gas opening portion of the cap face, the end face being located on a downstream side of the liquid ejecting direction, and 0.6 mm inside the gas cap or 0.4 mm outside the end face of the atomized gas opening portion, which is located on the downstream side of the liquid ejecting direction.
 
(5) In the configuration according to any one of (1) to (4), the configuration includes a liquid nozzle disposed on a front end side of a spray gun body and including a nozzle tip position adjuster on the downstream side of the liquid ejecting direction; a nozzle tip provided as the nozzle portion that is disposed with a rear end-side portion inserted in the nozzle tip position adjuster; and a nozzle tip holder including an opening through which a front end-side portion of the nozzle tip is inserted and configured to fix the nozzle tip to the liquid nozzle. The nozzle tip has a tapered portion in which an external diameter of a rear-end outer peripheral surface is reduced toward a rear end. The liquid nozzle has such a shape that an internal diameter of the nozzle tip position adjuster is reduced toward a rear end of the liquid nozzle correspondingly to the tapered portion of the nozzle tip.
 
     A female thread structure that is threadedly engaged with the nozzle tip holder is formed in a front end-side inner peripheral surface of the liquid nozzle. A male thread structure that is threadedly engaged with the female thread structure of the liquid nozzle is formed in a rear end-side outer peripheral surface of the nozzle tip holder. The tapered portion of the nozzle tip is inserted in the nozzle tip position adjuster of the liquid nozzle. The nozzle tip holder is then attached to the liquid nozzle so that the front end of the nozzle tip passes through the opening of the nozzle tip holder. At the same time, the nozzle tip holder is threadedly engaged with the liquid nozzle to fix the nozzle tip to the liquid nozzle. The tapered portion of the nozzle tip is thus tightly sealed to the nozzle tip position adjuster, and the nozzle tip is positioned coaxially with the liquid nozzle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a spray gun according to an embodiment of the invention. 
         FIG. 2  is an exploded cross-sectional view of a liquid nozzle, a nozzle tip, and a nozzle tip holder according to an embodiment of the invention. 
         FIG. 3A  is a perspective view of the nozzle tip according to an embodiment of the invention, and  FIG. 3B  is a partially cut away view of the nozzle tip shown in  FIG. 3A . 
         FIG. 4A  is a perspective view of the liquid nozzle, the nozzle tip, and the nozzle tip holder according to an embodiment of the invention, which are assembled together, and  FIG. 4B  is an elevation view of  FIG. 4A . 
         FIG. 5  a partial elevation view of the spray gun according to an embodiment of the invention. 
         FIGS. 6A and 6B  shows a modification example of the nozzle tip according to an embodiment of the invention and corresponds to  FIGS. 4A and 4B . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for carrying out the invention (hereinafter, referred to as embodiments) will be explained below in detail with reference to the attached drawings. The same constituent elements will be provided with the same reference marks throughout the description of the embodiments. 
     In the following explanation, the terms “front end (side)” and “front” refer to a position or direction on the downstream side of a liquid flowing direction (liquid ejecting direction) in each part. The terms “rear end (side)” and “rear” refer to a position or direction on the upstream side of the liquid flowing direction (direction opposite to the liquid ejecting direction). 
       FIG. 1  is a cross-sectional view of a spray gun  10  according to an embodiment of the invention. 
     As shown in  FIG. 1 , the spray gun  10  includes a spray gun body  20 , a liquid nozzle  30  having a nozzle tip position adjuster  31  on the liquid ejecting direction side, which is located on a front end side of the spray gun body  20 , a nozzle tip  40  provided as a nozzle portion that is disposed with a rear end-side portion inserted in the nozzle tip position adjuster  31  of the liquid nozzle  30 , a nozzle tip holder  50  having an opening through which a front end-side portion of the nozzle tip  40  passes and configured to fix the nozzle tip  40  to the liquid nozzle  30 , and a gas cap  60  disposed on a front end-side portion of the nozzle tip  40 . 
     The spray gun body  20  has a gas supply port  21  through which gas is supplied. A gas supply pipe (not shown) for supplying the gas is connected to the gas supply port  21 . The gas supplied from the gas supply port  21  passes through a gas channel  22   a . After a flow rate of the gas is adjusted by a whole gas flow rate regulation valve  23 , the gas is supplied to a gas channel  22   b.    
     The present embodiment describes a state in which an attachment component  21   a  for detachably attaching the gas supply pipe (not shown) to the spray gun body  20  is disposed in the gas supply port  21 . However, a method for attaching the gas supply pipe (not shown) to the spray gun body  20  is not limited to the above-mentioned method which uses the attachment component  21   a . The attaching method may be properly changed. 
     There is no particular limitation in kinds of the gas that is supplied to the gas supply port  21 . The gas may be chosen from air, nitrogen, argon, etc., as appropriate. 
     The gas supplied to the gas channel  22   b  is subsequently supplied to an on-off valve  25   a  that is disposed in a needle  25  configured to move in a front-back direction by an operation of a trigger  24 . The on-off valve  25   a  is movable in the front-back direction with the needle  25 . 
     When the needle  25  is operated to move rearward by the operation of the trigger  24 , the on-off valve  25   a  also moves rearward, and the gas supplied to the on-off valve  25   a  is supplied to a gas channel  22   c.    
     The gas supplied to the gas channel  22   c  is further supplied through a gas channel  22   d  to a gas cap  60  side. 
     A front end  28   a  of a needle  28  of a pattern adjuster  27  is located on a front end side of the gas channel  22   d , so that a position of the front end  28   a  can be adjusted in the front-back direction by operating the pattern adjuster  27 . 
     An opening degree of an opening  29  therefore can be adjusted in a range between a fully closed position and a fully open position by adjusting the position of the front end  28   a  of the needle  28 . The gas is supplied through the opening  29  to a horn portion  62  side. The horn portion  62  extends from an outer periphery of a cap face  61  of the gas cap  60  in a liquid ejecting direction. 
     The gas supplied to the horn portion  62  is ejected from pattern-adjusting gas ejecting ports  64  so as to be blown toward atomized liquid which has been atomized through atomization of the liquid that is ejected from the front end of the nozzle tip  40  due to the gas ejected from an atomized gas opening portion  63  of the cap face  61 . 
     The gas supplied through the gas channel  22   d  to the opening  29  side diverges before the gas that is not supplied to the horn portion  62  side reaches the opening  29 , according to the opening degree of the opening  29  which is determined by the position of the front end  28   a  of the needle  28 . 
     The gas that has diverged is supplied through gas channels  32  of the liquid nozzle  30  to a space A around the nozzle tip  40 , which is located behind the cap face  61 . 
     The gas supplied to the space A is ejected outside through a circular slit-like gap that is formed by an outer periphery of the front end of the nozzle tip  40  and the atomized gas opening portion  63  of the cap face  61  and extends along the outer periphery of the front end of the nozzle tip  40 . 
     The spray gun body  20  has a liquid supply port  26  to which liquid is supplied. A liquid supply pipe (not shown) for supplying liquid is connected to the liquid supply port  26 . 
     The embodiment describes a state in which the attachment component  26   a  for detachably attaching the liquid supply pipe (not shown) to the spray gun body  20  is disposed in the liquid supply port  26 . However, a method for attaching the liquid supply pipe (not shown) to the spray gun body  20  is not limited to the above-mentioned method which uses the attachment component  26   a . The attaching method may be properly changed. 
     The liquid supplied to the liquid supply port  26  is supplied through a liquid channel  33  of the liquid nozzle  30  into a liquid channel  41  of the nozzle tip  40 . 
     A liquid ejecting port  42  for ejecting liquid is formed in the front end of the nozzle tip  40 . 
     A front end of the needle  25  is inserted in the liquid ejecting port  42  so as to be in contact with the liquid ejecting port  42  due to a biasing force of an elastic body  25   b  which is disposed on the rear end side of the needle  25  and made up of a coil spring. While the front end of the needle  25  is in contact with the liquid ejecting port  42 , the liquid ejecting port  42  is blocked with the front end of the needle  25 . 
     The liquid is not ejected from the liquid ejecting port  42  unless the trigger  24  is pulled. The liquid is ejected from the liquid ejecting port  42  when the trigger  24  is pulled. 
     As described above, the operation of the trigger  24  doubles as an opening/closing operation of the on-off valve  25   a  which controls the ejection of the gas. When the trigger  24  is pulled, therefore, the liquid is ejected from the liquid ejecting port  42  of the nozzle tip  40 , and simultaneously, the gas is ejected from the circular slit-like gap that is formed by the outer periphery of the front end of the nozzle tip  40  and the atomized gas opening portion  63  of the cap face  61 , and ejected also from the pattern-adjusting gas ejecting ports  64 . 
     The following explains in more detail about the liquid nozzle  30 , the nozzle tip  40 , the nozzle tip holder  50 , and the gas cap  60 . 
       FIG. 2  is an exploded cross-sectional view of the liquid nozzle  30 , the nozzle tip  40 , and the nozzle tip holder  50 . 
     (Liquid Nozzle) 
     As shown in  FIG. 1 , the liquid nozzle  30  is disposed on the front end side of the spray gun body  20  and thus makes up an attachment portion to which the nozzle tip  40  provided as the nozzle portion is attached. 
     As shown in  FIG. 2 , the liquid nozzle  30  has the nozzle tip position adjuster  31  for adjusting the position of the nozzle tip  40  provided as the nozzle portion as described above. 
     The nozzle tip position adjuster  31  has a shape in which an internal diameter is reduced toward the rear end side correspondingly to a tapered portion  43  having an external diameter that is reduced toward the rear end side and formed in a rear end outer peripheral surface of the nozzle tip  40 . 
     A female thread structure  34  that is threadedly engaged with the nozzle tip holder  50  is formed in an inner peripheral surface of the front end-side portion of the liquid nozzle  30 . In the outside of the nozzle tip position adjuster  31  and the female thread structure  34 , the plurality of gas channels  32  are formed around the nozzle tip position adjuster  31  and the female thread structure  34 . 
     (Nozzle Tip) 
     The nozzle tip  40  is a portion that makes up the nozzle portion configured to eject the liquid supplied from the liquid nozzle  30  side. As shown in  FIG. 1 , the nozzle tip  40  is disposed so that the tapered portion  43  on the rear end side is inserted in the nozzle tip position adjuster  31  of the liquid nozzle  30 . The nozzle tip  40  is thus fixed to the liquid nozzle  30  by the nozzle tip holder  50 . 
       FIG. 3A  is a perspective view of the nozzle tip  40 , and  FIG. 3B  is a perspective view of the nozzle tip  40  shown in  FIG. 3A , which is partially cut away to show the inside thereof. 
     As is apparent from  FIGS. 3A and 3B , a substantially V-shaped groove  44   a  is formed in a circular section  44  of a truncated conical front end of the nozzle tip  40 . An internal hole is opened by forming the substantially V-shaped groove  44   a  to make a liquid ejecting port  42  having an oval shape as viewed in a front view. 
     The truncated conical portion is formed to have a cone angle θ shown in  FIG. 2  ranges from 20° to 90°. 
     When the nozzle tip  40  is positioned as shown in  FIGS. 1 and 2 , the substantially V-shaped groove  44   a  extends in a vertical direction as viewed in the drawings. 
     Since the liquid ejecting port  42  is formed in the above-described manner, the liquid ejected from the liquid ejecting port  42  is ejected in a flat film-like shape due to the oval shape of an opening of the liquid ejecting port  42  as viewed in the front view and guidance by the substantially V-shaped groove  44   a.    
     As shown in  FIG. 3A , a stepped portion  45   a  with which the nozzle tip holder  50  is engaged is provided in a part of the outer periphery of the nozzle tip  40 . As shown by dotted circles, a pair of grasped faces  45  are formed in a circumferential lateral face of a front part of the stepped portion  45   a  to be arranged in parallel with each other. The nozzle tip  40  thus can rotate around a central axis thereof. Accordingly, if the pair of grasped faces  45  formed in the front end portion are held by, for example, a spanner or the like, and the nozzle tip  40  is rotated around the central axis, the substantially V-shaped groove  44   a  of the front end portion of the nozzle tip  40  is adjusted to be at a regular position relative to the pattern-adjusting gas ejecting ports  64  of the gas cap  60  shown in  FIG. 1 . 
     As described above, the rear end outer peripheral surface of the nozzle tip  40  is provided with the tapered portion  43  (see  FIG. 2 ) having the external diameter that is reduced toward the rear end. 
     Since the tapered portion  43  is provided as mentioned above, and the nozzle tip position adjuster  31  of the liquid nozzle  30  has the internal diameter that is reduced toward the rear end side correspondingly to the tapered portion  43 , the tapered portion  43  and the nozzle tip position adjuster  31  are tightly fitted together to accomplish sealing between the nozzle tip  40  and the liquid nozzle  30 , when the nozzle tip  40  is secured to the liquid nozzle  30  by the nozzle tip holder  50 , and the nozzle tip  40  is automatically positioned coaxially with the liquid nozzle  30 . 
     (Nozzle Tip Holder) 
     The nozzle tip holder  50  is configured to fix the nozzle tip  40  to the liquid nozzle  30 . As shown in  FIG. 2 , a male thread structure  52  that is threadedly engaged with the female thread structure  34  of the liquid nozzle  30  is formed in the rear end-side outer peripheral surface of the nozzle tip holder  50 . 
     The front end-side portion of the nozzle tip holder  50  is provided with an opening  51  through which the front end-side portion of the nozzle tip  40  passes. Formed in the opening  51  is a rib  53  to be engaged with the stepped portion  45   a  of the nozzle tip  40 . 
     After the tapered portion  43  of the nozzle tip  40  is inserted in the nozzle tip position adjuster  31  of the liquid nozzle  30 , the nozzle tip holder  50  is capped on the nozzle tip  40  so that the front end-side portion of the nozzle tip  40  passes through the opening  51 . Subsequently, the nozzle tip holder  50  is threadedly engaged with and fastened onto the liquid nozzle  30  in such a way that the nozzle tip  40  is pressed toward the liquid nozzle  30 . The nozzle tip  40  is thus fixed to the liquid nozzle  30  by the nozzle tip holder  50 . 
     At this time, as described above, the tapered portion  43  of the nozzle tip  40  is tightly fitted to the nozzle tip position adjuster  31  of the liquid nozzle  30  to accomplish sealing between the nozzle tip  40  and the liquid nozzle  30 . When the tight fit between the tapered portion  43  and the nozzle tip position adjuster  31  is accomplished, the nozzle tip  40  is automatically positioned coaxially with the liquid nozzle  30 . 
       FIG. 4A  is an elevation view showing a state in which the nozzle tip  40  is fixed to the liquid nozzle  30  by the nozzle tip holder  50 , and  FIG. 4B  is a perspective view of  FIG. 4A . 
     When the nozzle tip  40  shown in  FIGS. 1 and 2  is positioned as illustrated in  FIG. 4 , the substantially V-shaped groove  44   a  extends in the vertical direction. 
       FIG. 4  omit the spray gun body  20  and merely show the liquid nozzle  30 , the nozzle tip  40 , and the nozzle tip holder  50 . 
     As is clear from  FIGS. 4A and 4B , the gas channels  32  formed on an outward side of the liquid nozzle  30  are arranged around the nozzle tip  40  at substantially regular intervals. The gas is therefore supplied evenly into the space A around the nozzle tip  40 , which is behind the cap face  61 , as described with reference to  FIG. 1 . 
     (Gas Cap) 
       FIG. 5  is an elevation view of the gas cap  60  of the spray gun  10  and parts around the gas cap  60 .  FIG. 5  also shows an enlarged view of an area around the liquid ejecting port  42 . 
       FIG. 5  further shows an example of a case in which the substantially V-shaped groove  44   a  of the front end portion of the nozzle tip  40  is adjusted to the regular position relative to the pattern-adjusting gas ejecting ports  64  of the gas cap  60 .  FIG. 5  shows a case in which, for example, both major-diametrical end portions of the opening of the liquid ejecting port  42  in a bottom of the substantially V-shaped groove  44   a  of the nozzle tip  40  are positioned on lines leading to the pattern-adjusting gas ejecting ports  64  of the gas cap  60 , which are disposed with the nozzle tip  40  intervening therebetween. 
     As shown in  FIGS. 1 and 5 , the gas cap  60  has the cap face  61  with the atomized gas opening portion  63 , and the horn portions  62  with the gas channels  62   a  extending from the outer periphery of the cap face  61  in the liquid ejecting direction. 
     The following description of the gas cap  60  relates mainly to how the gas cap  60  is associated with liquid atomization and also refers to a peripheral configuration including the atomized gas opening portion  63  of the cap face  61 . The horn portion  62  will be described later. 
     As shown in the enlarged view of  FIG. 5 , the atomized gas opening portion  63  formed in the cap face  61  of the gas cap  60  has an opening diameter larger than a diameter of the circular section  44  of the truncated conical front end of the nozzle tip  40  which makes up the nozzle portion. 
     The outer periphery of the front end of the nozzle tip  40  and the atomized gas opening portion  63  of the cap face  61  form a circular slit-like gap (see a hatched part B of the enlarged view in  FIG. 5 ) around the front end of the nozzle tip  40 . 
     The liquid ejected from the front end of the nozzle tip  40  is atomized chiefly by the gas ejected from the circular slit-like gap. 
     Hereinafter, the hatched part B of the enlarged view in  FIG. 5  is referred to as a gas ejecting portion B. 
     More specifically, the liquid ejected from the liquid ejecting port  42  in a flat film-like shape is covered with the gas that is ejected from the gas ejecting portion B in an annular shape. 
     Since the front end of the nozzle tip  40  has the shape like a truncated cone, the gas ejected from the gas ejecting portion B flows along the outer periphery of the nozzle tip  40  and is ejected so as to converge conically. The gas is then caused to obliquely collide with the liquid. Since the liquid is ejected in a flat film-like shape, a contact area of the liquid with respect to the gas is large, which facilitates the atomization of the liquid. The liquid is thus sheared and comes into an atomized state. 
     If the gas is caused to collide with the ejected liquid at a right angle, a colliding force of the gas with respect to the liquid is increased. This increases a shear force which shears the liquid. 
     On the other hand, if the gas is caused to collide with the ejected liquid at a right angle, the gas collides with the liquid sideways with respect to the liquid ejecting direction, and the liquid is suppressed from flowing in the ejecting direction. This makes it impossible to effectively apply the atomized liquid to a target, and deteriorates a liquid application efficiency. 
     In this respect, if a collision angle at which the gas collided with the ejected liquid is reduced, the liquid is less likely to be suppressed from flowing in the ejecting direction, and the deterioration of the liquid application efficiency with respect to the target is prevented or reduced. In this case, however, the shear force which shears the liquid is decreased, resulting in inadequate atomization of the liquid and uneven liquid application to the target. 
     In order to achieve good atomization of the liquid and good application efficiency of the liquid with respect to the target, the front end of the nozzle tip  40  is formed in the truncated conical shape with the cone angle θ ranging from 20° to 90° as explained with reference to  FIG. 2 . 
     The inventors found that a flow rate of the gas ejected from the gas ejecting portion B and an ejection velocity of the gas were important in materializing a good atomized state, as described below. The inventors materialized a stable and good atomized state by obtaining proper flow rate and ejection velocity of the gas. 
     The following description will specifically explain relationship between the liquid atomization and the gas flow rate and ejection velocity, and will further explain a configuration of the present invention based on the relationship. 
     For example, if the flow rate of the gas ejected from the gas ejecting portion B is increased to make a large amount of gas collide with the liquid, the shear force is increased, and the liquid can be efficiently atomized. 
     However, in case that the atomization takes place using a large amount of gas, some of the liquid is overly atomized. The liquid that is overly atomized spatters before being applied to the target and fails to reach the target, which deteriorates the application efficiency. 
     If the gas flow rate is reduced, the shear force is decreased. In this case, some of the liquid is not sufficiently atomized, which contributes to uneven liquid application with respect to the target. 
     The conventional art described above provides auxiliary air holes. It seems that the auxiliary air hole supplies air flow for assisting the atomization of part of sprayed liquid, which is likely to be insufficiently atomized, and therefore enables a generally good atomization. 
     However, if atomization is carried out without using the auxiliary gas holes, some of the liquid is overly atomized when the flow rate of the gas ejected from the gas ejecting portion B is increased. If the gas flow rate is reduced, some of the liquid is insufficiently atomized. 
     In order to avoid the use of the auxiliary gas holes, it is required to find a new factor that is unrelated to a tradeoff relationship described above. 
     It is considered difficult to restore the overly atomized state to the state before the atomization. The inventors then conducted their study as to how to solve the insufficiency of the shear force when the gas flow rate is set to such a value that the over atomization does not occur without using the auxiliary air holes. 
     In other words, the inventors studied as to whether there is any factor that improves the shear force applied to the liquid without changing the gas flow rate while maintaining the gas flow rate that does not incur over atomization. 
     As a result, the inventors found that, even without changing the gas flow rate, the shear force applied to the liquid can be increased by increasing the ejection velocity of the gas ejected from the gas ejecting portion B. 
     More specifically, the ejection velocity V(m/sec) of the gas ejected from the gas ejecting portion B is expressed by V=Y/X, where X(m 2 ) is a cross-sectional area of the gas ejecting portion B, and Y(m 3 /sec) is the flow rate of the gas ejected from the gas ejecting portion B. 
     As can be seen from the above relationship, the ejection velocity V can be changed by changing the cross-sectional area X of the gas ejecting portion B while the gas flow rate Y is unchanged. 
     The increase of the ejection velocity V increases the colliding force of the gas with respect to the liquid. It is then possible to increase the shear force which shears the liquid. 
     Therefore, if the shear force which becomes deficient when the gas flow rate is set to a value that does not incur over atomization is compensated by the ejection velocity, the gas flow rate that does not incur over atomization can be maintained, and the good atomization of the liquid is achieved. 
     In order to increase the ejection velocity without increasing the flow rate of the gas ejected from the gas ejecting portion B, it is necessary to reduce the cross-sectional area of the gas ejecting portion B. 
     In other words, it is necessary to reduce a cross-sectional area of the circular slit-like gap formed around the front end of the nozzle tip  40  by the outer periphery of the front end of the nozzle tip  40  and the atomized gas opening portion  63  of the cap face  61 . 
     The cross-sectional area of the circular slit-like gap can be reduced by reducing width of the circular slit-like gap. However, the cross-sectional area increases in proportion to the square of a diameter ratio, so that if the front end of the nozzle tip  40  (diameter of the circular section  44  of the truncated conical front end) has a large outer peripheral diameter, the width of the circular slit-like gap needs to be extremely small. 
     The diameter of the circular section  44  of the truncated conical front end and the opening diameter of the atomized gas opening portion  63  of the cap face  61 , which form the circular slit-like gap, are then required to be made with high accuracy. 
     Therefore, the circular section  44  of the truncated conical front end is formed to have a small diameter ranging from 0.8 mm to 2.8 mm. 
     In addition, the inventors found that, when the flow rate of the gas ejected from the circular slit-like gap is set to fall in a range from 40 L/min to 160 L/min as a gas flow rate that does not incur over atomization, the good atomization can be accomplished by adjusting the ejection velocity into a range from 100 m/sec to 2900 m/sec. 
     The stable and good atomized state can be accomplished especially by setting the ejection velocity into a range from 300 m/sec to 700 m/sec. 
     The opening diameter of the atomized gas opening portion  63  is preferably equal to or larger than 1.0 mm and less than 3.0 mm so as not only to reduce the cross-sectional area of the gas ejecting portion B and achieve necessary gas ejection velocity but also to secure the gap (clearance) between the truncated conical front end and the atomized gas opening portion  63 , which is equal to or larger than approximately 0.1 mm, in consideration of a manufacturing process. 
     The truncated conical front end of the nozzle tip  40  is preferably positioned to point forward within a distance of 1.0 mm from an inner end face (end face which faces the space A in  FIG. 1 ) of the atomized gas opening portion  63  of the cap face  61  in the liquid ejecting direction. 
     For example, the truncated conical front end of the nozzle tip  40  is preferably located within 0.6 mm inside the atomized gas opening portion  63  from where the truncated conical front end is in plane with an outer end face (end face on an outlet side) of the atomized gas opening portion  63  of the cap face  61  or located within 0.4 mm outside the outer end face (end face on the outlet side) of the atomized gas opening portion  63  from where the truncated conical front end is in plane with the outer end face of the atomized gas opening portion  63  of the cap face  61 . 
     At the same time, the circular slit-like gap extending along the outer periphery of the front end of the nozzle tip  40  is surely formed by the outer periphery of the front end of the nozzle tip  40  and the atomized gas opening portion  63  of the cap face  61 , so that the ejecting state of the ejected gas is properly maintained. 
     For a more specific example, in a state where a supply pressure (pressure in the space A shown in  FIG. 1 ) of the gas introduced into the circular slit-like gap was 0.1 MPa by setting the diameter of the circular section  44  of the truncated conical front end at 1.9 mm, the opening diameter of the atomized gas opening portion  63  at 2.5 mm, and a supply pressure of the gas to the gas supply port  21  at 0.15 MPa, and if the flow rate of the gas was 70 L/min, and the ejection velocity of the gas ejected from the gas ejecting portion B was 563 m/sec, a very good atomized state was obtained, in which an average particle size was approximately 125 μm. 
     The horn portion  62  will be explained below. 
     As shown in  FIG. 1 , the horn portions  62  is formed to extend from the outer periphery of the cap face  61  in the liquid ejecting direction. A horn portion  62  includes a gas channel formed to extend in the direction where the horn portion  62  extends, that is, the gas channel  62   a  extending from the outer periphery of the cap face  61  in the liquid ejecting direction. 
     The pattern-adjusting gas ejecting ports  64  are disposed in a front end-side portion of the horn portion  62 . The pattern-adjusting gas ejecting ports  64  eject the gas toward the atomized liquid at a slant with respect to the liquid ejecting direction. The pattern-adjusting gas ejecting ports  64  open in the gas channel  62   a.    
     The present embodiment refers to a case in which two pattern-adjusting gas ejecting ports  64  are provided with respect to each horn portion  62 . The number of the pattern-adjusting gas ejecting ports  64  may be changed as necessary. 
     In the present embodiment, the liquid is ejected in the flat film-like shape when ejected from the front end of the nozzle tip  40 . At the same time as the atomization of the liquid by the gas ejected from the gas ejecting portion B, the atomized liquid forms a circular pattern along the ejection pattern of the gas. 
     The pattern of the atomized liquid can be changed into an oval pattern by causing the gas ejected from the pattern-adjusting gas ejecting ports  64  of the pair of horn portions  62  opposed to each other as shown in  FIG. 5 , to collide against the atomized liquid in the circular pattern. 
     The atomized liquid is thus widened in a longitudinal axis direction of the oval to form a pattern suitable to a case in which the liquid is applied to a wide area. 
     The gas ejected from the pattern-adjusting gas ejecting ports  64  also accelerates the atomization of the liquid and discourages unification of the atomized liquid, to thereby soften particles of the atomized liquid. 
     However, the acceleration of the atomization preferably takes place slowly to avoid over atomization. To that end, it is preferable that total cross-sectional area of openings of the pattern-adjusting gas ejecting ports  64  is relatively large. 
     Considering that the gas supplied from the gas supply port  21  shown in  FIG. 1  diverges to be supplied to the pattern-adjusting gas ejecting ports  64  and the atomized gas opening portion  63  as described above, it is preferable that, in a state where spray gas pressure, namely, a supply pressure (pressure in the space A shown in  FIG. 1 ) of the gas introduced into the circular slit-like gap is middle-low pressure ranging from 0.05 MPa to 0.2 MPa by setting a supply pressure of the gas supplied to the gas supply port  21  at a value in a range from 0.07 MPa to 0.25 MPa, the total cross-sectional area of the openings of the pattern-adjusting gas ejecting ports  64  is set equal to or larger than 8.5 mm 2  to slow the acceleration of the atomization, and the number of the pattern-adjusting gas ejecting ports  64  and the opening area of each of the pattern-adjusting gas ejecting ports  64  are set proper in order to obtain the gas ejecting state that allows the pattern adjustment as a primary function to be carried out. 
     Although the invention has been explained with reference to the specific embodiment, the invention is not limited to the embodiment. 
     For example, as shown in  FIGS. 6A and 6B , it is possible to form two substantially V-shaped grooves  44   a  in the circular section  44  of the truncated conical front end of the nozzle tip  40  so that the internal hole is opened by arranging the substantially V-shaped grooves  44   a  in a substantially cross shape in an elevation view to form the liquid ejecting port. 
     This further increases a surface area of the ejected liquid, which contacts the gas. It is therefore possible to facilitate the atomization and reduce the deficiency amount of the insufficient shear force. 
     The number of the substantially V-shaped groove  44   a  is not required to be limited to one. More than one substantially V-shaped groove  44   a  may be provided to achieve a good atomized state. 
     As described above, although the ejection pattern of the liquid ejected from the liquid ejecting port  42  is flat, the atomized liquid is formed into the circular pattern along the gas ejection pattern from the gas ejecting portion B at the same time as when atomized by the gas ejected from the gas ejecting portion B. Therefore, the substantially V-shaped groove  44   a  in the circular section  44  of the truncated conical front end of the nozzle tip  40  is not required to be formed along a perpendicular or horizontal direction as shown in  FIGS. 4 and 6 . The substantially V-shaped groove  44   a  may be obliquely formed. 
     The embodiment refers to a case in which the pair of horn portions  62  are vertically disposed. However, the horn portions  62  may be horizontally disposed, and more than one pair of horn portions  62  may be provided. The number and layout of the horn portions are not limited as long as the atomized liquid in the circular pattern can be properly changed into a predetermined oval pattern. 
     Needless to say, the invention can be applied to not only liquid such as paint but liquid which needs to be applied in an atomized state. 
     The invention is not limited to the above-described embodiment. The invention may be appropriately changed or modified. It is obvious from claims to one skilled in the art that such changes and modifications are also included in the technical scope of the invention. 
     The embodiment makes it possible to provide the spray gun in which the spray gas pressure can be set to relatively low middle-low pressure and which is capable of performing good atomization without making the auxiliary gas holes in the gas cap. 
     At least the following technical ideas can be understood from the above-described embodiment. 
     [1] A spray gun comprising: 
     a nozzle tip ( 40 ) in which at least one substantially V-shaped groove ( 44   a ) is formed in a circular section ( 44 ) of a truncated conical front end with a cone angle ranging from 20° to 90°, and an internal hole is opened by forming the substantially V-shaped groove to provide a liquid ejecting port ( 42 ); and 
     a gas cap ( 60 ) including a cap face ( 61 ) which is provided with an atomized gas opening portion ( 63 ) having an opening diameter larger than the circular section, and a circular slit-like gap (B) that ejects gas for atomizing liquid ejected from the liquid ejecting port is formed between the gas cap and an outer periphery of the truncated conical front end, 
     wherein the circular section ( 44 ) of the truncated conical front end has a diameter ranging from 0.8 mm to 2.8 mm; the atomized gas opening portion ( 63 ) has the opening diameter that is equal to or larger than 1.0 mm and smaller than 3.0 mm; a flow rate of the gas ejected from the circular slit-like gap (B) ranges from 40 L/min to 160 L/min; and an ejection velocity of the gas ejected from the circular slit-like gap (B) ranges from 100 m/sec to 2900 m/sec, so that the liquid can be atomized without making auxiliary gas ejecting holes for atomizing the liquid in the cap face ( 61 ). 
     [2] The spray gun described in [1], wherein a supply pressure of the gas at a gas supply port ( 21 ) ranges from 0.07 MPa to 0.25 MPa, and a supply pressure at a time when the gas is introduced into the circular slit-like gap (B) ranges from 0.05 MPa to 0.2 MPa.
 
[3] The spray gun described in either [1] or [2], wherein an ejection velocity of the gas ejected from the circular slit-like gap ranges from 300 m/sec to 700 m/sec.
 
[4] The spray gun described in any one of [1] to [3], wherein the gas cap ( 60 ) includes the horn portions ( 62 ) having a gas channel ( 62   a ) extending from an outer periphery of the cap face ( 61 ) in an ejecting direction of the liquid; the horn portion is provided with the pattern-adjusting gas ejecting ports ( 64 ) which ejects gas toward the liquid that is atomized and is open to the gas channel that adjusts a spray pattern with respect to a target to which the atomized liquid is applied.
 
[5] The spray gun described in [4], wherein the gas cap ( 60 ) includes one or more pairs of the horn portions ( 62 ) formed to extend in a vertical or horizontal direction of the spray gun.
 
[6] The spray gun described in [4] or [5],
 
     wherein the gas cap ( 60 ) includes one or more pairs of the horn portions ( 62 ); and 
     wherein both end portions of the at least one substantially V-shaped groove ( 44   a ) are adjusted to be located on a line connecting the pattern adjusting gas ejecting ports ( 64 ) in each of at least one pair of the horn portions. 
     [7] The spray gun described in any one of [4] to [6], 
     wherein a total cross-sectional area of openings of the pattern-adjusting gas ejecting ports ( 64 ) is equal to or larger than 8.5 mm 2 . 
     [8] The spray gun described in any one of [1] to [7], 
     wherein the at least one substantially V-shaped groove ( 44   a ) is formed along a vertical, horizontal or oblique direction of the spray gun. 
     [9] The spray gun described in any one of [1] to [7], 
     wherein the at least one substantially V-shaped groove ( 44   a ) comprises two substantially V-shaped grooves arranged in a substantially cross shape in an elevation view. 
     [10] The spray gun described in any one of [1] to [9], 
     wherein the truncated conical front end of the nozzle tip ( 40 ) is located between where the truncated conical front end is in plane with an end face of the atomized gas opening portion ( 63 ) of the cap face ( 61 ), the end face being located on a downstream side of the liquid ejecting direction, and 0.6 mm inside the gas cap ( 60 ) or 0.4 mm outside the end face of the atomized gas opening portion ( 63 ), which is located on the downstream side of the liquid ejecting direction. 
     [11] The spray gun described in any one of [1] to [10], comprising: 
     a liquid nozzle ( 30 ) disposed on a front end side of a spray gun body and including a nozzle tip position adjuster ( 31 ) on the downstream side of the liquid ejecting direction; 
     a nozzle tip ( 40 ) provided as the nozzle portion that is disposed with a rear end-side portion inserted in the nozzle tip position adjuster ( 31 ); and 
     a nozzle tip holder ( 50 ) having an opening ( 51 ) through which a front end-side portion of the nozzle tip ( 40 ) is inserted and configured to fasten the nozzle tip ( 40 ) to the liquid nozzle ( 30 ), 
     wherein the nozzle tip ( 40 ) has a tapered portion ( 43 ) in which an external diameter of a rear-end outer peripheral surface is reduced toward a rear end; 
     wherein the liquid nozzle ( 30 ) has such a shape that an internal diameter of the nozzle tip position adjuster ( 31 ) is reduced toward a rear end of the liquid nozzle ( 30 ) correspondingly to the tapered portion ( 43 ) of the nozzle tip ( 40 ); 
     wherein a female thread structure ( 34 ) that is threadedly engaged with the nozzle tip holder ( 50 ) is formed in a front end-side inner peripheral surface of the liquid nozzle ( 30 ); 
     wherein a male thread structure ( 52 ) that is threadedly engaged with the female thread structure ( 34 ) of the liquid nozzle ( 30 ) is formed in a rear end-side outer peripheral surface of the nozzle tip holder ( 50 ); 
     wherein the tapered portion ( 43 ) of the nozzle tip ( 40 ) is inserted in the nozzle tip position adjuster ( 31 ) of the liquid nozzle ( 30 ); the nozzle tip holder ( 50 ) is then attached to the liquid nozzle ( 30 ) so that the front end of the nozzle tip ( 40 ) passes through the opening ( 51 ) of the nozzle tip holder ( 50 ); and the nozzle tip holder ( 50 ) is threadedly engaged with the liquid nozzle ( 30 ) to fix the nozzle tip ( 40 ) to the liquid nozzle ( 30 ), so that the tapered portion ( 43 ) of the nozzle tip ( 40 ) is thus tightly fitted to be sealed to the nozzle tip position adjuster ( 31 ), and the nozzle tip ( 40 ) is positioned coaxially with the liquid nozzle ( 30 ). 
     [12] The spray gun described in any one of [1] to [11], 
     wherein the gas cap ( 60 ) includes one or more pairs of the horn portions ( 62 ) extending from an outer periphery of the cap face ( 61 ) in the liquid ejecting direction, and each of the horn portions is provided with the pattern-adjusting gas ejecting ports ( 64 ); 
     wherein the at least one substantially V-shaped groove ( 44   a ) is configured to eject liquid in a flat film-like shape from the liquid ejecting port ( 42 ); 
     wherein the circular slit-like gap (B) is configured to eject gas so as to atomize the flat film-like liquid and form the liquid into a circular pattern; and 
     wherein the pattern-adjusting gas ejecting ports ( 64 ) are configured to eject gas so as to shape the circular-pattern atomized liquid into an oval pattern and accelerate atomization. 
     The foregoing description merely explains several embodiments of the invention. One skilled in the art could easily understand that the embodiments described above may be changed or modified in various ways without substantially deviating from new teachings and advantages of the invention. Therefore, it is intended to include within the technological scope of the invention all aspects added with such changes or modifications. 
     The embodiments of the invention have been described with reference to several examples to facilitate the understanding of the invention, and the embodiment is not presented to limit the invention. Needless to say, the invention may be changed or modified without deviating from the gist of the invention, and includes equivalents thereof. The constituent elements described in the claims and the description may be arbitrarily combined or omitted within a scope where at least part of the above-mentioned problem can be solved or a scope where at least part of the advantages can be exerted. 
     The present patent application claims priority to Japanese Patent Application No. 2014-258282 filed on Dec. 22, 2014. The entire disclosure of Japanese Patent Application No. 2014-258282 filed on Dec. 22, 2014 including description, claims, drawings and abstract is incorporated herein by reference in its entirety. 
     The entire disclosure of Japanese Examined Patent Application Publication No. H07-024796 (Patent Document 1) including description, claims, drawings and abstract is incorporated herein by reference in its entirety. 
     REFERENCE SIGNS LIST 
     
         
           10  spray gun 
           20  spray gun body 
           21  gas supply port 
           21   a  attachment component 
           22   a  gas channel 
           22   b  gas channel 
           22   c  gas channel 
           22   d  gas channel 
           24  trigger 
           25  needle 
           25   a  on-off valve 
           25   b  elastic body 
           26  liquid supply port 
           26   a  attachment component 
           27  pattern adjuster 
           28  needle 
           28   a  front end 
           29  opening 
           30  liquid nozzle 
           31  nozzle tip position adjuster 
           32  gas channel 
           33  liquid channel 
           34  female thread structure 
           40  nozzle tip 
           41  liquid channel 
           42  liquid ejecting port 
           43  tapered portion 
           44  circular section 
           44   a  substantially V-shaped groove 
           45  grasped face 
           45   a  stepped portion 
           50  nozzle tip holder 
           51  opening 
           52  male thread structure 
           53  rib 
           60  gas cap 
           61  cap face 
           62  horn portion 
           62   a  gas channel 
           63  atomized gas opening portion 
           64  pattern-adjusting gas ejecting port 
         A space 
         B gas ejecting portion