Abstract:
A holder for the frame of a printing screen is connected to an adjustment plate which, in turn, is mounted on a support arm assembly. The support arm assembly swings the holder up and down, toward and away from a printing platen. The adjustment plate has elongated slots for receiving eccentric lugs of manually rotatable cam members. The adjustment plate is normally fixed to the screen holder by a locking mechanism, but the locking mechanism is releasable so that the cam members can be rotated to effect a desired adjustment of the plate and the associated holder to achieve a desired registration of the printing screen relative to the printing platen. The support arm assembly also is adjustable to change the angle of the screen relative to the platen when the screen is lowered.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a mechanism incorporated in silk screen printing equipment for making precise adjustments of the screen position to assure proper registration over an article being printed. 
     BACKGROUND OF THE INVENTION 
     Multistation silk screen printing systems have two or more printing platens spaced around a stand. A turntable is mounted on the stand and carries two or more screen holders for swinging from one platen to the next. Separate screens are clamped in the holders. Each screen can be used for printing a specific color at each platen. It is important that the screens register with the printing platens precisely so that the finished print is well defined, rather than having the colors overlap or otherwise not be properly delimited. In conventional equipment, obtaining such precise registration requires adjustment of the positions of the screens in their holders. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fine adjustment mechanism for screen printing machines in which a holder for the screen frame is movably mounted on a support arm assembly which swings the holder up and down, toward and away from a printing platen. An adjustment plate is interposed between the arm assembly and the holder. The adjustment plate has elongated slots for receiving eccentric lugs of manually rotatable cam members. The plate is normally fixed to the holder and the locking mechanism normally secures the adjustment plate in a selected position relative to the support arm assembly. The locking mechanism is releasable so that the cam members can be rotated to effect a desired adjustment of the plate and the associated holder for a desired registration of the screen relative to the support arm assembly and the printing platen. The support arm assembly translates the holder and its screen rearward away from a worker at a printing station as the arm assembly is swung up. The support arm assembly also is adjustable to change the angle of the screen relative to the printing station platen when the screen is lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a somewhat diagrammatic top plan of a screen printing machine having fine adjustment mechanism in accordance with the present invention; 
     FIG. 2 is a fragmentary side elevation of the machine of FIG. 1; and FIG. 3 is a corresponding fragmentary side elevation of such machine with parts in different positions; 
     FIG. 4 is a fragmentary top perspective of a portion of the machine of FIG. 1, namely, the screen clamping assembly and fine adjustment mechanism; 
     FIG. 5 is a somewhat diagrammatic vertical section taken along line 5--5 of FIG. 4; 
     FIG. 6 is a top perspective of the holder and fine adjustment mechanism in accordance with the present invention with parts in exploded relationship; and 
     FIG. 7 is a somewhat diagrammatic section along line 7--7 of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a screen printing machine 1 in accordance with the present invention has one or more holders 2 for the structural frames 3 of printing screens 4. Each holder is carried at the end of a swingable support arm assembly 5. In the illustrated embodiment, four such support arm assemblies and holders are rotatably mounted on a stand 6 by a turntable 7 above four printing stations which include platens 8 for articles on which the designs of the screens 4 are to be printed. Adjacent holders, screens and platens are spaced 90° apart. For multiple color designs, ink is passed through the openings of one screen onto an article at one station, such as by a roller or squeegee, whereupon the screen frame is raised and an adjacent screen is moved into position for application of ink of a different color. In a multicolor printing operation, it is important that the screens be positioned precisely for a crisp appearance of the final printed design with each color properly delimited. 
     With reference to FIGS. 2 and 3, the stand 6 can include a central column 10 rotatably supporting a lower turntable 11 for the printing platens 8, as well as the upper turntable 7 for the support arm assemblies 5. Suitable indexing mechanism is provided to achieve alignment of the support arm assemblies with the platens 8 during printing. Such indexing mechanism and the support structure for the platens are conventional. 
     With respect to the support arm assemblies 5 mounted on the upper turntable 7, such support arm assemblies carry the holders 2 for the screen frames 3 for moving the screens between the raised position illustrated in solid lines in FIG. 2 and the lowered printing position illustrated in broken lines in FIG. 2 and in solid lines in FIG. 3. Preferably, the screen holder normally is fixed to a short horizontal support member 12 cantilevered outward from an upright link 13. Such link connects the outer ends of swinging arms 14 and 15 which have their inner ends connected to the turntable at radially spaced locations by pivot pins 16. A torsion spring 17 can be connected between the turntable and one of the swingable arms to at least partially counterbalance the arm and its screen holder so as to retain them raised or lowered. 
     The radially swinging arm 14 is shorter than the radially inner swinging arm 15, although preferably longer than one-half the length of the inner arm. The pins 16 connecting the swinging arms to the turntable are aligned horizontally, but preferably the pins 18 connecting the outer ends of the swinging arms to the upright link 13 are spaced apart both vertically and radially. The pivot for the lower and radially outer swinging arm 14 is below and outward of the pivot for the upper and inner arm 15. The upper arm 15 is formed in two parts, one of which is telescoped inside the other. Such two parts are normally secured together by clamping bolts but such bolts can be loosened by manipulation of a locking lever 20 and knob 21 allowing the effective length of the upper arm 15 to be adjusted. 
     The primary purpose for adjusting the effective length of the upper arm 15 is to swing the screen holder and screen about an axis perpendicular to the length of the support arm assembly without changing the position of the support arm assembly so as to position to the screen frame 3 precisely parallel to the printing platen 8 when the screen is in its lowered position illustrated in FIG. 3. Shortening the upper arm raises the unclamped end of the screen relative to the clamped end, and lengthening the arm has the opposite effect. With the screen in the desired lowered position extending parallel to the platen 8, ink is forced through the screen onto an article A supported on the platen, such as by use of a squeege S. Then the screen is raised to the position illustrated in FIG. 2. 
     The preferred construction of the support arm assembly 5, including the swinging arms 14 and 15 and connecting link 13, results in the screen being partially translated rearward as it is raised for convenient access to the article A. In addition, the frame is tilted at a small acute angle rearward and downward which helps to prevent ink from dripping onto the article or the floor. In conventional systems, the screen is typically fixed to a single arm such that if the screen is raised to a large degree for convenient access to the article being printed, it will be tilted severely and the ink may run off or drip from the inner portion of the screen, whereas if the screen is raised only slightly to a moderately tilted position, access to the article being printed can be difficult. 
     The details of the screen holder 2 are shown in FIG. 4, FIG. 5 and FIG. 6. The mouth of a channel member 23 opens outward for receiving the structural frame 3 of the printing screen. Brackets or ears 24 project upward from the channel member 23 for mounting conventional toggle clamps 25. The reciprocating shafts 26 of such clamps extend downward through the top plate 27 of the channel member 23 and are connected to a long clamp plate 28 by conventional swivel joints 29. By manipulation of the actuating lever handles 30 of the toggle clamps, the clamp plate 28 can be locked in a downward projected position for clamping the screen frame 3 against the bottom plate 31 of the channel member 23. 
     In accordance with the present invention, an adjustment plate 32 is interposed between the holder channel member 23 and the cantilever support member 12 of the swinging support arm assembly. A connecting flange 33 projects downward from the adjustment plate 32 for bolting to the rear side of the upright web of the channel member 23. Preferably, the connecting flange 33 has a single circular bolt hole 34 at its center (FIG. 6) with an additional slot 35 concentrically arcuate relative to the bolt hole 34 for an additional connecting bolt. Thus, with the bolts loosened, channel member 23 can be tilted relative to flange 33 and adjustment plate 32 about an axis generally parallel to the length of the swinging support arm assembly to achieve a desired orientation of the screen holder 2 relative to the associated printing platen. Thereafter, the connecting bolts can be tightened to secure the adjustment plate 32 to the screen holder. 
     The upper portion of the adjustment plate 32 is elongated transversely of the length of the support member 12, in line with an upper, transversely extending connecting member 36 secured to the outer end of the support member 12. As seen in FIG. 6, such upper portion of the adjustment plate has a central slot 37 elongated transversely of the length of the plate. Slot 37 is flanked by slots 38 and 39, each of which is elongated lengthwise of the plate. Additional apertures 40 are provided at the outer end portions of the adjustment plate. 
     The connecting member 36 above the adjustment plate 32 has a through hole or aperture 37&#39;, 38&#39;, 39&#39;, or 40&#39; in alignment with each of the slots and apertures 37, 38, 39, and 40, respectively. The outer apertures 40&#39; of member 36 receive the shanks of clamping bolts 41 which have top lever arms or handles 42 and which are tightened with bottom nuts 43 for locking the connecting member 36 to the adjustment plate 32. With such locking levers loosened, the adjustment plate can be moved relative to the connecting member for adjusting the position of the screen holder relative to the swinging support arm assembly, including members 12 and 36 of the support arm assembly. 
     To achieve a quick and precise change in the position of adjustment plate 32 relative to connecting member 36, three cams 44, 45, and 46 have downward projecting lugs 47, 48, and 49, respectively, received in the slots 37, 38, and 39, respectively, of the adjustment plate. Such lugs are eccentric with respect to upward-extending shafts or shanks 50 which pass through the connecting member 36 and have large knobs 51 at their upper ends for manually rotating the cams. The adjustment action achieved by manually turning the knobs is best illustrated in FIG. 7. In the central position of the adjustment plate 32, lugs 48 and 49 are offset outward from the shafts of their cams such that small rotations of the associated knobs move the adjustment plate in and out relative to the support member 12 in generally the direction indicated by the arrows 52 and 53. In the central position of the adjustment plate, lug 47 of the center cam member is offset radially from its shaft such that small rotating motion of the associated knob moves the adjustment plate transversely in the direction of the arrow 54. Thus, with the locking bolts loosened the upper knobs 51 can be turned manually to achieve a desired precise adjustment of the screen holder relative to the support members 12 and 36, whereupon the outer locking levers 42 can be tightened to secure the holder in a desired registration relative to its swinging arm assembly. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.