The present invention relates to a frame for supporting the screen of a screen printing machine and in particular relates to reinforcement for such a frame to prestress and/or camber the frame.
Screen printing machines utilize a tensioned screen of fabric or mesh mounted on a supporting frame. A squeegee forces ink or other fluid media through the screen over an area whose shape is defined by a stencil. The squeegee is moved under pressure across the screen to deflect the screen downwardly toward the substrate to provide for transfer of the fluid medium onto the substrate.
The screen of a screen printing machine is typically secured to a rectangular frame having coupled members which retain the edges of the tensioned screen. Prior art frames include frames known as xe2x80x9cstretch and gluexe2x80x9d frames in which a screen is adhered to a supporting frame while in a tensioned condition. Also known in the art are frames known as xe2x80x9croller framesxe2x80x9d in which rollers are coupled together and rotatably supported by connecting members. Each of the rollers retains an edge of the screen for tensioning of the screen through rotation of the rollers. A locking mechanism secures the rollers to the connecting members to maintain the desired tension in a print screen. A typical roller of a roller frame includes a hollow cylindrical tube made of aluminum and having a longitudinally extending channel in which an edge of the screen is retained.
For larger frames, the coupled members of the frame can become long enough that the members become subject to undesirable transverse and torsional deflections. Such deflections may include inwardly directed deflections resulting from loads applied to the frame by the tensioned screen as well as sagging resulting from gravity forces acting on the frame. Undesirable vibration of the frame and the supported tensioned screen may result from forces applied to the screen by the squeegee. Vibration may also result from vertical reciprocation of the printing frame in certain printing machines. Such vibrations may require significant delay between print cycles to provide time for the vibrations to subside.
Tension forces applied to the frame by the screen may also cause downward and inward bending of the frame members resulting in an out of plane warping of the frame. As a result of the warping, the print area of the screen in reduced. Furthermore, such warping may also lead to undesirable inconsistencies during printing.
Reinforced structures for resisting deflections of frame members of screen printing machines are known in the art. U.S. Pat. Nos. 5,255,600 and 5,097,761 to Hamu show large and rugged truss-like structures for resisting transverse deflections of a roller frame. The disclosed structures are passive structures whose elements, apart from gravity effects, are not stressed in the absence of external loading. The truss-like structures of Hamu, when located inwardly of the rollers as shown in FIG. 1 of the ""600 patent, encroach upon the printing area. The structures of Hamu provide rugged support at the expense of added weight as well as adding to shipping, handling and fabrication costs.
U.S. Pat. No. 4,345,390 to Newman, which is incorporated herein by reference, shows a roller frame having screen retaining rollers coupled together by corner members and inwardly disposed roller support members adjacent the rollers. The disclosed support members resemble I-beam and box beam sections having a curved flange portion referred to a xe2x80x9climit stopxe2x80x9d and an opposite member which are connected by one or two webs, respectively. The curved flange portion provides a support surface which confronts the roller to resist inward deflections of the screen supporting roller. The disclosed support members, however, are passive structures whose elements, as described above, are not stressed in the absence of external forces. Therefore, in order to provide for increases in bending resistance utilizing the passive structures of the ""390 patent, a deeper section is required for a given material. Use of thinner walls and deeper sections allows for increasing stiffness while limiting increased weight. However, the use of thinner walls results in increased stresses imposed on the support member for a given loading. Additionally, increasing the depth of the section results in encroachment of the support member towards or into the print area of the screen.
U.S. Pat. No. 3,908,293 to Newman, which is also incorporated herein by reference, discloses a roller frame in which the rollers supporting the screen are coupled together by corner members. The roller frame includes tensioning members extending between the corner members to camber the rollers. The tensioning members are located at a distance from the rollers toward the print area of the frame and therefore can only provide for one direction of cambering. Also, the roller is unsupported between the ends and therefore, with the exception of torsional restraint applied at the ends, remains unrestrained against transverse and torsional deflections.
What is needed is a frame for supporting the screen of a screen printing machine having coupled frame members in which the stiffness of the frame members against transverse and torsional deflections are increased effectively without undesirable increases in weight or encroachment on the print area of a supported screen. The present invention provides such benefits through the incorporation of reinforcement in intimate proximity to the frame member for prestressing and/or cambering of the frame member.
According to the present invention, there is provided a frame for supporting the screen of a screen printing machine. The frame includes elongated frame members coupled together each providing support for an edge of the screen.
The frame further includes a reinforcing member supported by the frame to extend in intimate proximity along at least a portion of at least one of the frame members and provide a prestress and/or camber of the frame member.
According to one embodiment of the invention the reinforcing member includes a cable extending within the interior of a frame member and having a tensioner attached adjacent to one end of the cable for tensioning the cable between bearing surfaces of the frame. The bearing surfaces may be defined by corner members coupling the frame members or alternatively by the frame member itself.
According to another embodiment of the invention the reinforcing member includes an elongated secondary member affixed to the frame member. The secondary member applies prestress to the frame member resulting from affixation of the secondary member while the members while an applied condition imposed differential strains on confronting surfaces of the frame member and the secondary member. The differential strains create a prestress and/or camber following affixation and removal of the imposed conditions. Differential strains may be imposed through applied loading to create transverse deflection of the frame member and the secondary member. Differential strains may also be imposed by loading to create axial compression of the frame member.