Abstract:
An improved reversible airless spray tip inhibits dripping, spitting, and undesirable paint accumulation on the spray guard, while improving safety. A positioning detent on the spray tip carrier handle snaps positively into place when a nozzle carrier is rotated into spray position, indicating that the tip is properly positioned for spraying. A spray guard with airfoil-like cross-members protects the user from injury while they inhibit turbulence and prevent paint accumulation. An improved piston seal has a slot-like fluid passage, which is preferably substantially rectangular in cross section. A rearward end of the piston seal is sealed by a resilient ring compressed directly against the face of an attached spray gun. An improved tip retainer is expanded by swaging after insertion, which forces a lip into a mating slot. The tip retainer also has an expanded chamber which diffuses reverse fluid flow for safety.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to spray tip assemblies for airless, high pressure spraying, and more particularly to reversible spray tip assemblies provided with a tip guard for safety. 
     2. Description of the Related Art 
     Reversible spray tip assemblies are widely used for high pressure, airless spraying of paint and other fluids. In a typical reversible spray tip assembly, a small spray nozzle is carried in a cylindrical, rotatable nozzle carrier. The nozzle carrier can be rotated 180 degrees, thereby reversing the direction of paint flow through the nozzle for cleaning nozzle obstructions. Typically the nozzle carriers are interchangeable with other nozzle carriers carrying nozzles of various diameters and capacities. Prior reversible spray tip assemblies, although successful, continue to be plagued by several problems which affect their convenience, safety and utility. 
     Safety for the user is a primary concern. Airless high pressure sprayers eject a very high velocity, narrow jet, which disperses and slows as it atomizes. In the area near the nozzle (within approximately one inch), where the jet is most narrow and has highest velocity, there is a risk of injection injuries to a user. In recognition of this risk, prior sprayers have included various styles of spray guards to prevent the user&#39;s body from being hit by the spray jet near the spray nozzle orifice and to warn the user of the hazard. 
     While such spray guards reduce the risk of injection, prior spray guards have generally suffered from a tendency to accumulate paint during spraying. Accumulated paint can then drip from the guard, creating a mess and potentially staining clothing or surfaces in the work area. In addition, accumulated paint can be splattered from the tip guard by the aerodynamic forces of the spray, causing imperfections on the painted surface. When this occurs, the user may be tempted to remove the spray guard, thereby risking injury for the sake of convenience. 
     Some efforts have been made to reduce the tendency for the spray guard to accumulate paint. For example, U.S. Pat. No. 4,685,621 to Scherer et al. (1987) features a tip guard having two pairs of vanes extending forward and radially outward from a base, each pair of vanes joined by a crossbar. Scherer&#39;s tip guard allows air flow through the side of the spray guard, and is somewhat successful in reducing buildup of paint on the spray guard. Nevertheless, the accumulation of paint from overspray is not completely eliminated by Scherer&#39;s design, and users may still be tempted to remove the spray guard. 
     Another approach to the problem is taken by Eull in his U.S. Pat. No. 4,165,836 (1979). This patent describes a safety tip guard which is coupled to the sprayer in such a way that the spray tip will not operate if the tip guard is removed. This approach improves the safety of the spray guard, but paint can still accumulate and drip. In addition, the user may be forced to stop to wipe the spray guard occasionally; if the sprayer is actuated while the user has positioned fingers inside the guard for wiping, injection injury could result. 
     While prior attempts to improve the spray guard have improved the situation to some degree, none of the prior guards is considered convenient, safe and trouble free. 
     A related problem with existing reversible tip spray tips arises from their reversible tip feature. It is a major benefit of such devices that a user can easily rotate the spraying nozzle into a reverse flow position. This enables the user to quickly remove any particles that have plugged the very small orifice in the spray tip, by injecting paint through the spray tip in the reversed flow direction, dislodging the obstruction. However, with existing reversible tip devices it is possible to accidentally rotate the spray tip out of position if the tip handle gets bumped in the course of handling or moving the spray gun. It is also possible for a user to fail to rotate the spray tip completely into position before activating the sprayer. Either of these circumstances can yield a condition where the tip is not properly aligned when fluid pressure is applied, which can result in accidents ranging in severity from minor nuisance to serious injury or damage. 
     Prior reversible spray tips commonly include rotation stops, so that the tip cannot be overrotated inadvertently. For example, U.S. Pat. No. 4,165,836 to Eull (1979) includes a handle with a shoulder. The shoulder has a partially rounded shape to permit tip rotation and a flattened portion which contacts a flange to limit the range of rotation. While it does prevent overrotation, the flattened portion of the shoulder does not prevent improper positioning by underrotation of the tip. Other tips similarly limit the range of rotation but do not positively lock the tip into position. Thus, prior spray tips do not completely solve the problem of inadvertent tip misalignment. 
     In addition to misalignment problems, prior reversible spray tips are subject to “spitting” and dripping problems when the spray gun is being triggered on or off. These problems are related to the seal design. For sealing the rotatable metal cylinder, a floating cylinder seal is commonly provided with a forward sealing face that conforms with the outer cylindrical contour. High pressure tends to force the floating seal into sealing engagement with the cylinder during spraying, preventing leakage. 
     To prevent leakage during start up conditions, an initial compressive loading is typically applied to the seal. For example, in the U.S. Pat. No. 4,715,537 to Calder (1987), the floating seal is biased by a spring to provide initial sealing pressure during start up. The floating seal is sealed against leakage from its rearward face by an annular (O-ring) seal. 
     Existing seals exhibit, in varying degrees, a tendency to cause a “spit” or drip from the spray nozzle, particularly when pressure is suddenly removed. Moreover, these seals in many cases are difficult to assemble in proper alignment, as is necessary for an effective seal. Some existing tips have a further problem: when the rotatable metal cylinder is partially rotated out of alignment with the fluid supply port, seal leakage can occur due to the paint “bridging” the seal between the port and an outside surface. This troublesome “bridging” situation is illustrated by FIG.  1 . This figure shows the position of the nozzle carrier  1  when it has been turned partially so that the nozzle axis  2  does not align with the longitudinal axis  3  of the fluid passage  4 . If the dimension w o  is not sufficiently narrow to be fully covered by the concave face  5  of the piston seal  6  while in this intermediate position, the seal formed by the contact between the concave face  5  and the nozzle carrier  1  is bridged, and fluid (symbolized by flow line  7 ) is allowed to escape by flowing around the concave seal face  5 . Therefore, to prevent bridging the seal, the arc defined by the opening of the rear nozzle carrier orifice  8  must be smaller than the arc defined by the concave seal face  5 . This limitation is defined by a complex relationship, but for small concave faces (as used for practical sealing faces) and assuming that the fluid passage  4  is centered in the piston seal  6 , it is sufficient to prevent bridging if the width w o  is less than (d ps −w)/2, where w is the width of the fluid passage  4 , d ps  is the outside diameter of the piston seal  6 , and w o  is the width of the rear orifice in the spray nozzle carrier  1 . 
     Prior reversible spray tips have had problems related to the manner of retaining a spray nozzle  9  in the rotatable cylindrical spray nozzle carrier  1 . Typically, a small tungsten carbide spray nozzle is installed in a transverse bore of the nozzle carrier  1 , so that the axis of the nozzle is perpendicular to the axis of the nozzle carrier  1 . The transverse bore of the carrier  1  has a small step or bevel  10  which limits movement of the spray nozzle in the forward direction. A retainer  11  is installed behind the nozzle to secure its position in the bore. The nozzle must be mechanically retained in the carrier  1  such that fluid will not leak past the nozzle in either the forward or reverse flow direction. Furthermore, the nozzle must be mechanically retained in the carrier securely, to prevent it from being dislodged or ejected under very high fluid pressure (as high as 25,000 P.S.I in either the forward or reverse direction). It is also desirable that, in the reverse flow direction, some device is provided to diffuse the fluid stream to reduce the potential of injury from fluid injection while cleaning the spray tip by reverse flow. A transverse pin is often positioned across the fluid flow port for this purpose. 
     Previous reversible spray tips have generally retained the spray nozzles in the cylindrical carriers by either (a) threading the retainer into the carrier behind the nozzle, or (b) press fitting the retainer into the carrier behind the nozzle. The threaded retainer has high reverse load capacity but is costly and difficult to assemble. The difficulty arises because the spray pattern is not circularly symmetrical. The asymmetrical spray pattern must be oriented to the axis of the carrier (and therefore also to the spray tip assembly) to orient the maximum pattern width in the direction of spray gun movement. Since the threaded spray nozzle is rotating as it is screwed into the retainer, it is difficult to effect and maintain precise alignment of the nozzle in its seated position. 
     With a press fitted nozzle retainer, on the other hand, rotational alignment is not as great a problem. However, press fitting requires very tight tolerances and precise pressing technique to insure retention. In addition, the wall thickness of the retainer must be heavy enough to provide high compression pressure at the press fit interface. The wall thickness required causes the press fit hole to be so large that it will sometimes bridge the fluid seal in some positions and allow troublesome fluid leakage. 
     Some prior reversible spray tips have an additional problem related to the seal between the rearward end of the floating piston seal and the forward end of the spray gun. For example, Eull in his U.S. Pat. No. 4,165,836 discloses the use of a resilient sealing member interposed between the forward face of the spray gun and the rearward face of the piston seal, the sealing member having a forward end bevel which is received by a conical seat in the piston seal. This arrangement is disadvantageous in that the inside diameter of the sealing member is exposed to the fluid to be sprayed. The resilient sealing member is typically made from an organic elastomer, which can undergo chemical reactions with the fluid being sprayed, causing the elastomer to swell. The swelling of the elastomer then tends to constrict or choke off the flow of fluid through the tip, rendering the spray tip inoperable. In addition, the resilient sealing member contributes to “spitting” through the spray nozzle by reducing the rate at which fluid pressure rises and falls in response to the gun being triggered on and off. 
     Another problem with existing reversible tips is that they are not easily identified by the user by quick visual inspection. Although the handles of the interchangeable spray tip assemblies are frequently marked, for example with embossed part numbers, in a painting environment such markings are eventually obscured by buildup of paint or other contaminants. The paint buildup is not easily wiped from the handle, especially if it is partially dried, as is common after a long spraying session. This problem somewhat depreciates the value of the interchangeability feature of the spray tips. One cannot take full advantage of interchangeable tips if they cannot be conveniently distinguished in a workplace environment. 
     SUMMARY OF THE INVENTION 
     The invention is an improved reversible airless spray tip with several features which cooperate to inhibit dripping, spitting, and undesirable paint accumulation on the spray guard, while improving safety and convenience for the user. 
     An improved, aerodynamic spray guard having airfoil-like crossbars protects the user from accidental injection injury. The airfoil design of the crossbars inhibits turbulence and prevents paint accumulation on the spray guard, which would otherwise tempt the user to recklessly remove the spray guard. 
     A positioning detent on the spray tip carrier handle snaps positively into place when the tip carrier nozzle carrier is rotated into spray position, providing tactile feedback indicating to the user that the reversible tip is properly positioned for spraying. The positioning detent also resists accidental rotation of the nozzle carrier, which would otherwise cause accidents. 
     The invention also includes an improved floating seal with a slot-like fluid passage, which is preferably substantially rectangular in cross-section, with the longer dimension substantially perpendicular to the direction of rotation of the tip carrier . The fluid flow rate is improved by the increased cross-section presented by the rectangular fluid passage, as compared to conventional fluid passages with round cross-sections. This advantage is attained without concurrently increasing the likelihood of paint bridging the seal when the nozzle carrier is partially rotated (which would allow pressurized paint to escape). The rectangular cross section of the fluid passage also provides an asymmetry for a tool to engage for rotating the seal into the proper orientation, thereby facilitating proper installation and a proper initial seal. 
     A rearward end of the floating piston seal is sealed by a resilient, annular ring, preferably oval in cross-section. The ring is confined and compressed by a face of the spray gun on its rearward side, a housing on its outer circumference, and the floating seal on its inside circumference and its forward side. This configuration shortens the length of the floating seal as compared to existing spray tips, and enables placement of a spray gun needle valve closer to the spray tip&#39;s outlet nozzle, thereby reducing spitting and dripping problems. An additional benefit is that this configuration prevents the resilient seal from interfering with fluid flow by preventing inward expansion or distortion. 
     A nozzle assembly is retained in the rotatable nozzle carrier by a nozzle retainer inserted behind the nozzle. The nozzle retainer has a lip which is insertable into the transverse bore of the nozzle carrier, but which is expanded during assembly by applying pressure with a swage tool, which causes the lip to engage a corresponding groove in the nozzle carrier. The swaging process also creates and expansion chamber in the retainer, which acts to diffuse liquid flowing in a reverse direction through the nozzle assemble (as for cleaning). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a prior art reversible spray tip assembly; 
     FIG. 2 is an exploded perspective view of one embodiment of the invention; 
     FIG. 3 is a sectional view of the embodiment shown in FIG. 2; 
     FIG. 4 is a cross-section of the invention taken along section line  4 — 4  in FIG. 3; 
     FIG. 5 is a simplified sectional view schematically showing assumed streamlines of air flow around the spray guard of FIG. 2; 
     FIG. 6 is an elevation view of a handle used in the embodiment of FIG. 2; 
     FIG. 7 is a lower plan view of the handle of FIG. 5; 
     FIGS. 8 a ,  8   b ,  8   c  and  8   d  are a series of sectional views of the handle&#39;s cam portion, illustrating how it can be rotated through successive positions relative to a stationary surface; 
     FIG. 9 is a frontal view of a piston seal used in the embodiment of FIG. 2; 
     FIGS. 10 and 11 are sectional views of the piston seal, taken along section lines  10 — 10  and  11 — 11 , respectively, in FIG. 9; and 
     FIG. 12 a  is an exploded sectional view of the nozzle assembly and the spray nozzle carrier of the invention, in their pre-assembly form, together with a swaging tool applied during assembly; and 
     FIG. 12 b  is a sectional view of the nozzle assembly as it appears after it has been inserted into the nozzle carrier and swaged. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the exploded view of FIG. 2, an internally threaded retaining nut  12  with scalloped gripping surfaces  13  allows a user to mount the entire spray tip assembly  14  onto a conventional pressurized spray gun  15  (shown only partially for clarity) having complementary threads. A metal body  16  inserts axially through retaining nut  12  into a spray guard  17 . A cylindrical spray nozzle carrier  18 , which slidably and rotatably fits into a transverse bore  20  in the body  16 , can be rotated into spray position (shown) or a reversed cleaning position (180 degrees rotated from the position shown) by turning an attached handle  22 . 
     The body  16  has a longitudinal bore  24  substantially perpendicular to the transverse bore  20  which it intersects. This longitudinal bore  24  receives a substantially cylindrical piston seal  26  with a concave forward sealing surface  28  which mates with the cylindrical contour of the nozzle carrier  18 . An annular seal  30  seals the rearward end of the piston seal  26  as it is compressed between the rearward shoulder  34  of the piston seal  26  and the forward face  35  of the conventional pressurized paint spray gun  15 . 
     A fluid passage  36  with a preferably rectangular cross section extends longitudinally through the piston seal  26 . When the nozzle carrier  18  is rotated into spraying position or cleaning position, a nozzle assembly  38 , which is mounted diametrically in a bore through nozzle carrier  18 , aligns axially with the fluid passage  36 . As pressurized fluid is supplied from the attached spray gun (mounted behind annular sealing member  30 ), the fluid is allowed to flow forward through fluid passage  36 , then through nozzle assembly  38 , escaping externally in a fan shaped spray pattern along longitudinal axis  42 . 
     The nozzle carrier  18  slidably passes through an opening in the body portion of spray guard  17  to insert in the transverse bore  20  of the body  16 . As in prior reversible spray tip devices, the handle  22  is attached to the nozzle carrier  18 , suitably by pressing a splined shaft  44  of nozzle carrier  18  into a compatible bore  46  in handle  22  (as shown in FIG.  3 ). The nozzle carrier  18  is thus constrained to co-rotate with the handle  22 . The handle thus connected enables the user to rotate the nozzle carrier  18  between a spray position and a reversed flow, cleaning position. 
     The spray guard  17  includes a body section  48 , suitably four support arms  50  extending outward and forward, and typically two aerodynamic airfoils  52 , each supported by and spanning the forward ends of two support arms  50 . This spray guard  17  helps to prevent objects, especially a user&#39;s hand from intercepting the high velocity spray jet near the nozzle assembly  38  (where the jet velocity is highest and the stream most narrow). Although fingers can fit into the guard between the “wings”, the guard serves as a warning and establishes a safe distance reference boundary. 
     More detailed internal structure can be seen in FIGS. 3 and 4, which show the assembly mounted on a spray gun  15  and aligned with the nozzle carrier  18  in its spray position. Pressurized fluid flows forward through fluid passage  36  in the piston seal  26 , then continues forward through nozzle assembly  38  which is mounted in the diametric bore through nozzle carrier  18 . When the nozzle carrier  18  is rotated into the spray position as shown, pressurized fluid (typically paint) is forced forward through the nozzle assembly  38  and exits at high velocity along the central longitudinal axis  42 . A seal is created by the close contact between the nozzle carrier  18  and the semi-cylindrical face  28  of the piston seal  26 . The piston seal  26  is also sealed at its rearward end by the annular seal  30 , which is compressed between the spray gun face  35  on its rearward portion and a shoulder  34  of the piston seal  26  on its forward portion, the metal body  16  on its outside periphery and a neck portion  64  of the piston seal  26  on its inside diameter. Fluid pressure acting on annular seal  30  forces the piston seal  26  against the nozzle carrier  18 . The effective area of annular seal  30  is greater than that of fluid passage  36  which results in increased sealing force between piston seal  26  and nozzle carrier cylinder  18  in proportion to pressure applied. 
     It can be seen in FIG. 3 that the spray guard  17  has (preferably two) airfoils  52 . Each airfoil  52  has a characteristic aerodynamic design similar to a wing, with a curved outer surface  70  and a relatively flat inner surface  68  (analogous to the top and bottom, respectively, of an airplane wing). The airfoil cross-sections reduce air turbulence and create higher pressures near the inner surfaces  68  of the spray guard  17 . 
     FIG. 5 shows by streamlines the pattern of air flow generated in the region near the spray guard  17  when paint is sprayed in a fluid stream  76 . As the high velocity fluid stream  76  is sprayed forward, air is necessarily drawn into and along the fluid stream  76 , following the streamlines  78 . Each airfoil is situated with a rounded leading edge  80  disposed upstream (toward the fluid stream) and a substantially sharper trailing edge  82  disposed downstream. The air near the spray guard flows over the airfoil inner and outer surfaces  68  and  70  and merges easily into the atomized fluid stream, without turbulence. The air on the outer airfoil surfaces  70  of the guard will have lower pressure, while the air flowing across the inner airfoil surfaces  68  will have increased pressure due to the airfoil effects. 
     The angle α of the airfoil relative to the axis  42  of the spray jet  76  should preferably be small, in the neighborhood of 5 to 30 degrees. If the angle is too large, a stalling condition may result, causing turbulence and increasing paint accumulation on the spray guard. 
     Provided that stalling is avoided, the airflow design of the spray guard allows the air to flow easily without turbulence, which reduces the accumulation of paint overspray on the spray guard and the spray gun (as compared with prior spray guard designs). The reduced accumulation of paint enhances both the efficiency and the safety of the paint sprayer: efficiency because it allows the user to continue spraying for longer periods without interruption for wiping; safety because it reduces the motivation for the reckless user to remove the spray guard, which would cause increased risk of injection injury. 
     Efficiency and safety are also enhanced in one embodiment of the invention by an improved nozzle carrier handle  22  shown in FIG. 6 and 7 (detached from the nozzle carrier  18  for clarity). The handle  22  includes a cam  84  which is preferably integral with the handle, and is preferably made from an slightly elastically deformable material such as an organic polymer. The rim of the cam  84  has a substantially rounded portion  86  coaxial with said nozzle carrier  18 , and (preferably two) substantially flat rotation stops: a spray position stop  88  and a clean position stop  90 . Both stops  88  and  90  are substantially parallel to the axis of the nozzle carrier  18 . The spray position stop  88  and the clean position stop  90  are positioned to limit rotation of the handle  22  by contacting a stationary surface  92  (shown in FIG. 8 a ) of a counterstop (preferably a flange-like forward surface of the retaining nut  12 ) at the limit of rotation in either direction, giving the handle  22  and the nozzle carrier  18  the freedom to turn through approximately 180 degrees from stop to stop. These rotational limits position the tip in either the clean or spray positions, allowing either forward or backward fluid flow through the nozzle assembly  38 . 
     A position stop  88  is offset by a detent  96  which extends to a greater distance from the nozzle carrier axis than the adjacent surface of the rounded portion  86 . The detent  96  contacts the stationary surface  92  before the handle has rotated fully against the spray position stop  88 . The interference between the detent  96  and the stationary surface  92  causes elastic deformation of the detent  96  and the cam member  84  as it is forcibly rotated by a user into the spray position. As shown in FIG. 3, a portion  98  of the shaft  80  has a reduced diameter, thereby providing a slight space between the shaft portion  98  and the nozzle carrier  18 . This space permits the elastic deformation required for the cam member to rotate past the detent  96  and into the spray position. The same result could be reached by providing an enlarged portion of the bore  82 , which would also provide the necessary clearance. 
     FIGS. 8 a  through  8   d  illustrate a sequence of rotating the stop from the clean position of FIG. 8 a  into the spray position of FIG. 8 d . In FIG. 8 a  the handle is in the clean position, with clean position stop  90  engaged against the stationary surface  92 . In FIG. 8 b  the handle has been rotated so that the cam surface  86  is not in contact with the stationary surface  92 , allowing free rotation of the handle and attached nozzle carrier  18 . In FIG. 8 c  the handle has been rotated further so that the detent  96  contacts the stationary surface  92 . At this rotational position the interference between the detent  96  and the stationary surface  92  produces a torsional resistance to rotation which can be felt by the user, providing tactile feedback as to the position of the spray tip. The phantom outline  99  shows the position which the cam  84  would have taken but for the deformation caused by the pressure from the stationary surface  92 . Once the detent  96  is rotated beyond the center plane of the handle  22 , the elastic return of the deformed cam urges the detent against the stationary surface  92 , tending to aid rotation until the spray position stop  88  is in full contact with the stationary surface  92  as shown in FIG. 8 d . In passing from FIG. 8 c  to FIG. 8 d  the handle can be felt to snap into position. This indicates positively to the user that the spray tip is in spray position and ready to spray. 
     Because of the interference between the detent  96  and the stationary surface  92 , to rotate the handle  22  out of position a much higher force is required than that needed to overcome only the friction of the nozzle carrier  18  against the body  16  and the fluid seal  26 . This requirement of higher turning force (torque) serves to better hold the tip in alignment until the user rotates it deliberately. The result is improved safety and productivity. 
     Safety, cleanliness, efficiency, and versatility of the spray tip are all enhanced by an improved piston seal  26  with a non-cylindrical fluid passage  36  which is preferably rectangular in cross section. Unlike the piston seals of prior spray tips, which have fluid passages generally round in cross-section, the piston seal  26  of the invention features a fluid passage  36  with a slot-like, rectangular cross-section of length L and width w s  as shown in FIGS. 9 10  and  11 . The longer dimension L of the fluid passage should be oriented substantially parallel to the axis of the nozzle carrier  18  and the (coaxial) transverse bore  20  in body  16 . The width w s  of the fluid passage  36  should be sufficiently narrow to prevent bridging when the tip is reversed by rotating the cylindrical tip carrier  18 . 
     As in the prior art, the critical maximum width of w s  to prevent bridging depends on several factors, as illustrated in FIG.  1  and discussed in connection with the prior art. The maximum width permitted thus depends upon several dimensions, but there are practical constraints on each dimension. First, the diameter of the spray nozzle orifice depends upon the material to be sprayed and the flow rates desired. For high density materials such as roof coating, and high flow rates, an orifice in the range of 0.070 inches or larger is desirable. The contact width of the piston seal with the nozzle carrier cannot exceed the width of the spray nozzle carrier. The spray nozzle carrier size is in turn constrained because very large diameters become difficult for a user to turn due to friction caused by dried paint and/or seal pressure being increased and imposed on a greater radius. Nozzle carriers with cylinder diameters in the range of ¼ to ½ inch are desirable, and a diameter of approximately {fraction (7/16)} inch is common. In a typical embodiment, a fluid passage  36  with w s  of 0.080 inches and an L of approximately twice w s  are suitably used with a nozzle carrier  18  of approximately {fraction (7/16)} inch diameter and a piston seal with an outer diameter of {fraction (7/16)} inches. 
     The non-cylindrical fluid passage  36  of the invention is advantageous because it allows the cross-sectional area of the fluid passage  36  (cross-section taken normal to direction of fluid flow) to be made larger (for a given size piston seal) while having a desirably wide sealing land in the plane of tip rotation as compared to a conventional round fluid port with diameter w. To prevent bridging when the nozzle carrier  18  is being rotated, the useable maximum diameter of any round fluid port is limited (as discussed above). The maximum cross sectional area of a conventional round fluid port is thus limited to πw s   2 /4 (because r=w s /2 and area=πr 2 ). A rectangular port, in contrast, with dimension L greater than or equal to w, can achieve a significantly greater cross-sectional area (equal to 1×w s ). 
     The increased available cross-sectional area of the fluid passage presents less restriction of the fluid flow and permits the use of larger spray tip orifices. Alternatively, if the design goal is primarily to reduce leakage or reduce size, the rectangular passage is advantageous in allowing a reduced size for the concave face  28 , the nozzle carrier  18 , and the piston seal  26  for a given fluid passage cross-section and flow rate requirement. 
     Many shapes other than a rectangular cross section could be used for the fluid passage in the invention, provided that the chosen shape has a longer dimension in a direction substantially parallel to the axis of rotation of the nozzle carrier  18 . For example, oval or elliptical orifices could be used. Such variations are within the intended scope of the invention. 
     The rectangular fluid passage (or one of the aforementioned variations) is also useful in manipulating the piston seal  26  during installation into the body  16 . For example, a slotted port can accept a correspondingly shaped tool (in the manner of a mortise and tenon) for rotating the piston seal  26  during installation into the body  16 ; a round port cannot engage such a tool. 
     The method employed by the invention to seal the rear portion of the piston seal  26  shortens its length as compared to prior spray tips, and enables placement of a spray gun needle valve closer to the spray tip&#39;s outlet orifice  112 . To reduce spitting, it is highly desirable that the fluid passage  36  through the piston seal  26  be as short as possible. Commercial paint mixes commonly include entrapped air or other compressible components, making the liquid somewhat compressible. When pressure is suddenly removed, for example by closing a needle valve on the spray gun, a small volume of paint trapped in the fluid passage  36  does not cleanly stop flowing, but rather expands as the pressure drops, resulting in spitting of paint. This troublesome effect is mitigated by reducing the volume of the fluid passage  36 , thereby reducing the volume of pressurized paint trapped between the spray gun&#39;s needle valve and the outlet orifice  112 . This volume is best reduced by shortening the length of the channel rather than its cross-section, as a small cross-section tends to inhibit paint flow. Therefore, the reduced length of the fluid passage  36  within the piston seal  26  offered by the invention is very important in reducing spitting. 
     The rearward sealing arrangement of the invention reduces the length of the fluid passage  36  as compared to prior spray tips. The fluid seal of the present invention shown in FIG. 2 requires only one resilient annular seal  30 . The annular seal  30  encircles a neck portion  64  of the piston seal  26  and is surrounded on its outside perimeter by the longitudinal bore  24  in the body  16 . The seal  30  is compressed by the shoulder  34  of the piston seal  26  as it is forced toward the forward face of the spray gun  60 , when the entire tip assembly is mounted by screwing the mounting nut  10  onto the spray gun  60 . 
     The resilient annular sealing member  30  itself provides a bias for the floating piston seal  26 , eliminating the need for a spring and the additional length previously required to accommodate the spring. The present seal thus shortens the fluid channel  36  and thus the volume available to pressurized fluid downstream from the spray gun valve. This reduces the volume of entrapped pressurized paint, and thereby reduces the tendency of the spray tip to spit when the pressure is released. 
     The approach taken by the invention is also an improvement over the design disclosed by Eull in his U.S. Pat. No. 4,165,836 (discussed above). Significantly, in contrast with the sealing member used by Eull, in the present invention the inside diameter of the resilient annular sealing member  30  is not free to contract inward, constricting paint flow. The outer surface of the piston seal&#39;s neck  64  contacts the inside diameter of the resilient annular sealing ring  30  and prevents it from contracting under any conditions, so that paint flow cannot be restricted by sealing ring swelling. 
     The resilient sealing ring  30  should preferably be made of a somewhat resilient elastomeric, solvent resistant material such as a saturated ethylene-octene copolymer. The resilience of the material will provide pressure on the piston seal  26  so that the seal will not leak upon initial start up (application of paint pressure). The seal is preferably not round in cross-section, but rather elongated in one direction (for example, oval). This shape accommodates greater range of compression in the direction of elongation, and produces greater compressive force to properly bias the floating piston seal  26  while sealing between the floating piston seal and the forward face  35  of the pressurized spray gun  15 . 
     Details of a nozzle assembly  38  of the invention are shown in FIGS. 12 a  and  12   b . The assembly includes a spray nozzle  130  (with spray orifice  112 ), a compressible nozzle gasket  132  which is inserted behind spray nozzle  130  into the transverse bore  20  in the spray tip carrier  18 , and a spray tip retainer  134 , which is inserted into the transverse bore  20  behind the gasket  132  and retains the assembly in the bore  20 . 
     The retainer  134  is preferably a substantially cylindrical turned part with a small longitudinal inner fluid channel  135  and a radial lip  136  on the outside diameter. The cylindrical spray tip carrier  18  has a radial groove  138  in the transverse bore which is disposed to correspond with the radial lip  136  after assembly. Before assembly, the entrance  140  to the transverse bore  20  has a diameter which is larger than the radial lip  136  and smaller than the diameter of the groove  138 . On the forward side of the groove  138 , the diameter of the transverse bore closes to a diameter smaller than the radial lip  136 , providing a land  142  for the radial lip  136  to bear against for positioning during a swaging process. 
     To assemble the spray tip assembly  38 , the spray nozzle is first inserted into the transverse bore  20  in spray tip carrier  18  and positioned at the forward end of the bore  20 , where it is stopped by the forward shoulder  144  of the bore  20 . The orifice  112 , which is typically non-symmetrical, is manually aligned in relation to the axis of the spray tip carrier (by rotating it about the longitudinal axis of the bore  20 , thereby aligning the resulting paint spray pattern). The fluid sealing gasket  132  is then installed in the bore behind the spray nozzle  130 . The tip retainer  134  is inserted behind the gasket  132 , with the retainer&#39;s smaller-diameter end facing outward (rearward). A tapered swaging tool  145  is then pressed into the entry hole  135  of the retainer  134 , preferably to a predetermined depth. This pressing forces the retainer  134  into the land  142  which compresses the gasket  132  to a pre-determined thickness. Because of the pressure exerted by the swaging tool  145 , the outside features of the retainer  134  expand causing the radial lip  136  to expand into the groove  138 . The engagement of the retainer radial lip  136  with the groove  138  secures the retainer, and hence the spray tip assembly  38 , within the carrier  18 . The outer diameter of the retainer  134  expanded, by the same swaging action, into tight contact with the transverse bore, creating an almost seamless joint. The outside diameter of the tip carrier  18 , with the spray tip assembly  38  installed, is then preferably ground (by centerless grinding) to remove any portion of the retainer  134  which projects above the cylindrical surface of the carrier  18 , resulting in a smooth, cylindrical surface (which mates closely with the piston seal  26 , as previously described). 
     FIG. 12 b  shows the assembly seated in the carrier after swaging. A flared expansion chamber  148  is visible near the rear of the retainer  134 . This chamber  148 , which is formed by inserting the tapered swaging tool  145  under pressure, expanding the small inside bore, creates a venturi effect in the bore of retainer  134 . As a result of the expansion chamber  148 , fluid flowing in the reverse flow direction, as when the carrier is reversed for spray tip cleaning, becomes diffused as it exits the spray nozzle assembly  38 , rather than exiting in a narrow jet. This enhances safety of the device without distorting the spray pattern (as do some pin-type diffusers). 
     A final feature of the invention is an improved identifying mark or feature which allows a user to identify the size or type of a spray nozzle quickly and with certainty even in an environment which includes excess paint, as from overspray, mis-sprays, spills, or other problems which vex a painter. As in prior spray tips, various nozzle assemblies are available, and are easily interchanged by sliding out and replacing the entire nozzle carrier  18  with attached handle  22 . In a preferred embodiment of the invention, the handle  22  is perforated with an identifying perforation  150  (visible in FIGS.  1  and  2 ), which is a mark or symbol identifying the size and type of nozzle assembly  38  in the attached nozzle carrier  18 . For example, as illustrated by FIGS. 1 and 2 the alphanumeric identifier “ 515 ” is perforated through the handle to identify one particular spray nozzle. The user can easily inspect the perforation while the nozzle carrier  18  is fitted into or removed from the bore  20 , making spray nozzle identification quick and convenient. Paint does not tend to accumulate inside a perforation as readily as it does on, for example, embossed lettering; any paint which does accumulate is more easily cleaned from a perforation than from embossed lettering, for instance by passing a cleaning implement completely through the perforation. Thus the identifying perforations do not easily become unrecognizable due to paint accumulation, as do prior spray tip markings. 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.