US3541957A - Pneumatic tensioning of stencil screens - Google Patents

Description

United States Patent [72] inventor Henry J. Bubley Deerlleld, Illinois [21] Appi. No. 674,995

[22] Filed Oct. 12, 1967 [45] Patented Nov. 24, 1970 [73] Assignee American Screen Process Equipment Company Chicago, Illinois [54] PNEUMATIC TENSIONING 0F STENCIL SCREENS 15 Claims, 14 Drawing Figs.

[52] U.S.Cl. l01/127.l, 10l/4l5.l; 160/378 [51] int. Cl B411 13/02; B41f27/04 [50] Field ofSearch 10i/127.1,

Primary Examiner-William B. Penn A ttorney- Neal .1. Mosel ABSTRACT: In the preparation of screen process stencils, a stencil frame is enclosed tightly by a pneumatically operated frame. The stencil screen is secured on all sides by the movable portions of the surrounding pneumatic tensioning frame. The pneumatic tensioning frame is then inflated uniformly with compressed air and extended to cause the screen to be tensioned uniforrniy in all directions. The screen is then tacked or stapled or otherwise fastened to the stencil frame and has the desired degree of stretch for stencil printing. The pneumatic frame comprises a plurality of pneumatically operated frame members having a fixed member engageable with the stencil frame and a movable member on which the stencil screen is secured for movement under pneumatic pressure. The pneumatic frame members are provided with con nection means for assembling the individual frame members end to end or at right angle to provide a rectangular frame enclosure of any desired size in which the movable frame member is adapted to move under pneumatic pressure to tension a screen stencil in all directions uniformly and continuously around the periphery of the stencil frame.

PNEUMATIC TENSIONING OF STENCIL SCREENS SUMMARY OF THE INVENTION This invention comprises a new and improved pneumatic tensioning or stretching frame for screen fabrics. More particularly, the invention comprises a pneumatic frame having a plurality of frame members arranged and secured together end-to-end or end-to-end in the formof a rectangular frame having movable portions to which the screen fabric is secured and which are operated pneumaticallyto tension or stretch the fabric.

The individual pneumatic frame components comprise extruded members which telescope in assembly and which are arranged for actuation by a pneumatic tube. The fixed member of the pneumatic frame is provided with a tongue member extending across one end and grooves extending across the other end and along one side for tongue-in-groove connectionend-to-end or end-to-side. The movable member of the frame is provided with an elongated slot into which the fabric is secured by a cam lock bar. Upon inflation of the pneumatic tube, the movable member is moved outward. The arrangement of the frame members compactly around the printing frame on which the fabric is to be stretched makes possible the uniform stretching of the fabric in all directions and facilitates the attachment of the stretched fabric to the printing frame, all with a minimum waste of screen fabric.

BACKGROUND OF THE lNVENTlON ln screen process printing, the printing frame, usually of wood or metal, has a screen stretched tightly across it and secured around the entire periphery of the frame. The screen may be secured by tacks or staples or by a'continuous cord or rope which secures the edge of the screen into a retaining groove on the frame and also assists in the tensioning of the screen. A suitable design is formed on the screen by conventional techniques. A suitable printing ink, usually a paste or thixotropic ink, is placed on the stencil and is forced through the stencil onto the printing stock bya rubber squeegee. When multicolor designs are used, the designs are applied using a different stencil for'each separate color, with intermediate drying of the colors prior to application of the next color.

In the preparation of the screen process stencils, the tensioning ofthe screen at the time of application to the printing frame is of utmost importance. The screen fabric must be stretched properly on the printing frame in order to produce quality work. Badly stretched fabric causes blurred prints. As the squeegee moves across a poorly stretched screen, the fabric bunches up in front of the squeegee, thereby causing blurred prints. Also, the normal expansion and contraction of fabrics due to atmospheric conditions causes the registration of the screen design to vary. Poorstretching and slack screens allow even more variation in register. Slackness in fabrics on screen process stencils lead to early stencil breakdown because the stencil pattern is flexed excessively during the printing stages, thereby hastening cracking, drying, etc. Finally, it should be noted that most types of stencils, including adhering film and direct contact stencils'and carbon tissue stencils, adhere more successfully when the fabric is very taut.

DISCUSSION OF THE PRIOR ART The earliest methods of tensioning screen fabrics on printing frames used a strictly manual technique. The fabric would be tacked or stapled along one edge of the frame and stretched by hand to the other edge and tacked or stapled in place. This method had the disadvantage that the tensioning of the fabric was not always uniform and did not permit a ten-' sioning of the fabric in both directions on the frame. Another technique which has been used to a very great extent involves the use of stencil frames having a peripheral retaining groove into which the edge of the fabric is secured by a cord or rope. The fabric is secured along one edge of the frame by rope secured in the peripheral groove. The fabric is then stretched manually and the rope hammered into the groove forcing the fabric to stretch ash is being secured in the groove. This technique has been very time consuming and does not provide the-uniformity'of tensioning in all directions which is needed. Furthermore, this technique does not permit the tensioning of screens for a number of frames simultaneously.

There are a number of mechanical screen fabric stretching devices available which have attempted to solve some of the problems of stretching fabric uniformly in all directions. One

commercial screen fabric stretcher uses two sets of parallel spacer bars extending at right angles to each other. The stretcher bars are mounted at their ends on worm gears which have handles atone end for rotation. Rotation of one handle will cause two of the parallel spacer bars to move apart or advance toward each other. Rotation of the other handle will cause the other pair of stretcher bars to be moved together or apart. When the screen fabric is fastened to four stretcher bars, which have been set to a size slightly larger than the printing frame, and the gear rotating handles are turned, the stretcher bars move outward in each direction to stretch the fabric. This apparatus is moderately effective butis very expensive and not easily available to small or even moderate size printing shops.

Another screen fabric stretcher which has been quite successful is the American-Omega screen fabric stretcher. This stretcher has a frame to which the screen fabric is secured. The printing frame is placed under the fabric on adjustable corner supports which are arranged for movement by a rotary screw. Upon rotation of the screw at each corner of'the stretcher frame, the'printing frame is slowly advanced upward and presses againstthe screen fabric. As the printingframe is moved upward, the fabric is tightened in all directions. This screen fabric stretcher is quite effective and has been successful commercially but there is a slight tendency to tear the fabric when theframe is pressed upward. It is, therefore, necessary to use printing frames which have specially rounded corners and rounded edges to prevent damage to the screen fabric. Another disadvantage of this type of fabric stretcher is that a substantial'excess of the screen fabric isrequired which .is wasteful.

The Robustina screen'fabric stretcher, manufactured by the ltalian firm, S. Robustelli, has four parallel stretcher bars, two in each direction, which are arranged to be moved by a gear mechanism operated by rotary handles. The stretcher bars are segmented and provide for connection to the fabric at 7 separate points so that adjustments may be made in the amount of stretch at various points along the stretcher bar. For very large printing frames, the ltalian firm, S. Robustelli, manufactures a large electrically operated fabric stretcher known as the Robusta. This fabric stretcher has parallel stretcher bars which handle frame sizes ranging from 34 inches to 118 inches and up to 33 feet in length. The parallel stretcher bars which stretch the screen fabric in opposite directions are operated by a gear system driven by electric motors and controlled from a control panel at the end of the apparatus. This fabric stretcher apparatus, of course, is extremely expensive and is used only by very large screen process printers who print carpets and rugs or other extremely long pieces of fabric.

Recently, several attempts have been made to solve the problem of uniform stretchingof screen fabrics by a pneumatic apparatus. The devices which have been marketed commercially for this purpose, however, have been quite expensive and have been inflexible in their operation or have in some way produced a nonuniform stretching of the fabric.

- One pneumatic fabric stretcher is manufactured and sold by the French firm, Tripett and Renaud. This apparatus has a plurality of pneumatic units, usually 30 or 40 or more, which are arranged in fixed positions on a supporting table and which are interconnected pneumatically for simultaneous operation.

This apparatus is quite expensive and does not fit closely around the frame on which the fabric is being stretched. Also, in this apparatus, there is a discontinuity in the points of application of stretching force with the result that some portions of the fabric may be stretched more than others. Still another approach to the pneumatic stretching of screen fabrics is found in the pneumatic fabric tensioner manufactured by M and M Research Engineering Company. This fabric tensioner consists of an elongated clamp for securing the edge of the screen which is mounted for movement by an air cylinder. The fabric stretcher comprises a plurality of separate units each arranged to secure a portion of the edge of the fabric being stretched and, upon operation of the air cylinders simultaneously, to stretch the fabric uniformly in all directions. This apparatus has the disadvantage of being very expensive and of not being flexible in terms of fitting closely around the frames being equipped with fabric, particularly when there is a substantial variation in frame size. Another disadvantage, as in the case of the French pneumatic tensioning device, is that there is a discontinuity in the application of tension at points on the fabric between adjacent clamps.

STATEMENT OF OB JECTS AND FEATURES OF THE INVENTION It is, therefore, one object ofthis invention to provide a new and improved apparatus for tensioning screen fabrics uniformly in all directions for application to a printing frame.

Another object of this invention is to provide a new and improved apparatus for pneumatic tensioning of screen fabrics uniformly around the entire periphery of the fabric.

Another object of this invention is to provide a new and improved pneumatic tensioning frame.

One of the features of this invention is the provision of an improved pneumatically operated tensioning frame which applies a tensioning force uniformly and continuously around a screen fabric and tensions the fabric uniformly in all direction.

Another feature of this invention is the provision of an improved pneumatically operated screen fabric tensioning frame comprising a plurality of frame members which are adjustably secured in the form of a rectangular frame adjustable to any desired size and operable to apply a uniform tensioning force to a screenfabric secured in the frame.

Still another feature of this invention is the provision of an improved pneumatic frame component comprising a pair of frame members telescoped together and having an elongated pneumatic tube for operation, one of the telescoping members being provided with means for securing the edge of fabric to be stretched and movable by inflationof the pneumatic tube to stretch a fabric, said frame members being arranged for tongue and groove assembly end-to-end or end-to-side.

Other objects and features of this invention will become apparent from time to time throughout the specification and claims as hereinafter related.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, to be taken as a part of this specification, there is clearly and fully illustrated a preferred embodiment ofthe invention, in which drawings:

FIG. 1 is a plan view of a pneumatically operated screen fabric stretching frame representing a preferred embodiment of this invention;

FIG. 2 is a sectional view taken on the line 2-2 of FIG. 1 and showing the connection of the fabric to the tensioning frame and its relation to the printing frame when in an unstretched position;

FIG. 3 is a sectional view of the apparatus as shown in FIG. 2 with the tensioning members of the frame moved to an actuated position wherein the fabric is stretched for application to the printing frame;

FIG. 4 is a plan view of one of the pneumatic frame members, slightly enlarged in relation to FIG. 1;

FIG. 5 is a view in front elevation of the frame member shown in FIG. 4;

FIG. 6 is a view in rear elevation of the frame member shown in FIG. 4;

FIG. 7 a view in left elevation of the frame member shown in FIG. 4;

FIG. 3 is a view in right elevation of the frame member shown in FIG. 4;

FIG. 9 is a sectional view taken on the line 9-9 of FIG. 4; FIG. 10 is a sectional view taken on the line l0-10 of FIG.

FIG. 11 is a detail view showing the point of connection of the screen fabric to the movable portion ofthe pneumatic tensioning frame;

FIG, 12 is a detail view of a cam lock bar used to secure the fabric in the frame and shown in the end view in FIG. 11;

FIG. 13 is an isometric view of the pneumatic tube used in the screen tensioning frame; and

FIG. 14 is a detail view of an alternate clamp arrangement for the pneumatic frame.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings by numerals of reference. more particularly to FIG. I, there is shown a pneumatically operated tensioning frame I which comprises a plurality of pneumatic frame members 2, 3. 4, and 5 arranged in square or rectangular form surrounding the printing frame 6. The pneumatic frame 1 is preferably arranged with the frame members 2, 3, 4, and 5 abutting the entire periphery of printing frame 6. It should be noted, however, that in large printing frames of this type which handle the stretching of fabric for a number of stencils the frame members may not abut all of the printing frames. The pneumatic frame members 2, 3, 4, and 5 are provided with valve stems 7,8, 9, and 10, respectively, which are connected by suitable conduits or hoses, illustrated diagrammatically as dotted lines in FIG. 1, to a source of compressed air.

In FIG. 2, the use of the frame in stretching or tensioning a screen fabric over a printing frame is seen in more detail. A screen fabric 11 which may be silk, nylon, polyester, Teflon,

- stainless steel, etc., is secured in a movable portion of the frame members 2, 3, 4, and 5 when the printing frame (herein shown as a wooden frame) is positioned adjacent the fabric for ease of attachment thereto. When compressed air is supplied to the printing frame, i.e. to each of the pneumatic tubes in the several printing frame members, the movable portion of the printing frame members 2, 3, 4, and 5 moves outward as shown in FIG. 3 to tension or stretch screen fabric 11. In this stretched position, the fabric may be attached to frame 6 as by staples 12, other suitable fastening means, such as tacks, cement or glue, or rope or cord securing the screen in a peripheral groove on the printing frame.

The pneumatic frame members 2, 3, 4, and 5 are substantially identical and their construction and assembly and operation will be understood more fully by reference to FIGS. 4 to 13 of the drawings. Since the frame members are essentially identical, only one of them, frame member 2, will be described.

Frame member 2 comprises two elongated extruded members 13 and 14 which are of approximately U-shaped cross section (as seen in FIGS. 7 to 9). Extruded member 13 has a lower flange 15 and upper flange 16 extending from end wall 17. Wall 17 is provided with grooves 18 and 19 and upwardly extending flange portion 20. Extruded member 14 comprises lower flange portion 21 and upper flange portion 22 extending from end wall 23. The upper, horizontally extending, flange portion 22 is provided with upwardly extending flange 24 which abuts flange portion 20 on end wall 17 of extruded member 13. Flange portion 24 is provided with a longitudinally extending groove 25 having a narrow entrance or neck 26.

A spacer block 27, preferably of wood, is positioned between flanges 16 and 21 to secure extruded members 13 and 14 against relative vertical movement. Spacer block 27 extends for substantially the entire length of frame member 2, as is seen in FIG. 10. In the space between the end wall 17 and spacer block 27, which runs longitudinally for the entire length of frame member 2, there is positioned a rubber innertube which is seen in more detail in FIG. 13. Innertube 28 is provided with an elongated valve stem 7 which extends through passageway 30 in spacer block 27.

At opposite ends of frame member 2, there are provided end walls in the form of interlock members 31 and 32, respectively. Member 31 is provided with groove 33 which is substantially identical in size, shape, and vertical spacing to groove 18. On the inside of member 31, there is provided a tongue member 34 which serves no function in this position. Member 31 is secured in place by pins 35 and 36 which may be screws or bolts if desired, or may be secured by welding or cementing. Member 32 is provided with tongue member 37 which is of the size and shape to mate with groove 33 or groove 18 on the end or side of an adjacent frame member to which frame member 2 is to be connected. On the inside of member 32, there is provided groove 38 which has no function in this position. Member 32 is held in position by pins 39 and 40 which may be screws or bolts if desired or may be secured by welding or cementing. It is seen that interlock members 32 and 31 are identical in construction and, together with groove 18, provide for tongue-in-groove assembly of the frame members either end-to-end or end-to-side.

In FIG. 11, there is shown a detail view, on a slightly enlarged scale in relation to the other FIGS., of the retaining grobve for holding the screen fabric in position for stretching or tensioning. Fabric 11 is positioned in groove 25 and an elongated cam lock bar is positioned in the groove to hold the fabric tightly positioned therein. Cam lock bar 41 is a round metal bar having an elongated flat 42 machined along its entire length. The dimension'of bar 41 across a diameter which intersects the midpoint of flat 42 is slightly less than the narrow opening or neck 26 in groove 25. Cam lock bar 41 is turned on edge for insertion through the neck opening 26 and then is rotated into the position shown in FIG. 11 to secure fabric 11 tightly in place. Cam lock bar 41 is of a size to fit tightly in groove 26 when the fabric 11 is positioned in the groove. Any other suitable means may be used for attaching fabric 11 to movable member 14.

In FIG. 13, there is shown a detail isometric view of pneumatic tube 28. Pneumatic tube 28 may beof any suitable con-" struction but is preferably formed from upper and lower rubber sheets 43 and 44 which are heat sealed together around the periphery as indicated'at 45. Valve stem 7 is sealed in upper wall 43 of pneumatic tube 28.

The assembly and operation of frame member 2 and of tensioning frame 1 should be apparent from the description of the various components. The assembly and operation, however, will be repeated in slightly more detail for a more thorough understanding of the invention.

OPERATION In assembling frame member 2, end wall members 31 and 32 are secured between flanges and 16 of extruded member 13 and secured in place by pins 35, 36, 39, and 40. Members 31 and 32 are secured with tongue portion 38 exposed at one side and groove portion 33 exposed at the other side, as shown in FIG. 10, to permit assembly of the frame members end-to-end or end-to-side (as shown in FIG. 1).

Next, spacer block 27 is positioned on flange 21 of extruded member 14 with passageway 30 alined with a corresponding opening 46 in end wall 23. Pneumatic tube 28 is positioned with valve stem 7 extending through passageway 30 and opening 46. The members 13 and 14 with spacer block 27 and pneumatic tube 28 in place are then telescoped together into the position shown in FIG. 9. In this position, application of air pressure through valve stem 29 will cause pneumatic tube 28 to inflate and press against wall 17 and the end of spacer block 27. This will cause member 14 to move to the right as viewed in FIG. 9.

When the pneumatic frame members 2, 3, 4, and 5 are assembled as shown in FIG. 9, they may be formed into square or rectangular stretchingor tensioning frames as illustrated in FIG. 1. When frames of relatively small size are to be assembled, four of the pneumatic frame members are assembled with tongue 37 secured in groove 18. This results in an assembly as shown in FIG. 1 in which the size of the square or rectangular opening may be varied by sliding the members relative to each other with the tongue-in-groove construction holding the frame together. If a longer or larger frame is required, two or more of the frame members are assembled end-to-end with tongue 37 of one member secured in groove 33 of the adjacent member. The rectangular or square frame is then further assembled by having the tongue 37 of the end pneumatic frame member fitted into groove 18 on the pneumatic frame member to which it is assembled.

In operation, the stretching or tensioning frame is formed with frame members 2, 3, 4, and 5 abutting tightly against printing frame 6 on which stencil screen ll is to be stretched and secured. Stencil screen 11 is secured in grooves 25 by cam lock bars 41 on all four sides of frame 6. It is to be noted that the close proximity of grooves 25 to the edge 'of printing frame 6 requires only a very slight excess of screen fabric and thus is not wasteful of fabric in the stretching operation. When pneumatic pressure is applied to pneumatic tubes 28 in each of the pneumatic frame members 2, 3, 4, and 5, uniform pneumatic force is applied in each of the frame members causing movable member 14 on each of the pneumatic frames to move outward as viewed in FIG. 1. This applies a uniform and continuous outward stretch in all directions to fabric 11 which is stretched over printing frame 6. The application of tensioning or stretching force is uniform and continuous in all directions. There is no break in the application of tensioning force between adjacent members and there is no tendency to have an irregular or diagonal pull as sometimes occurs with prior art stretching or tensioning devices. When the fabric is tensioned or stretched to the desired degree, determined by the amount of air pressure applied to pneumatic tubes 28, the

fabric 11 is then secured on printing frame 6 by tacks or stapics 12 or by any other suitable securing means, such as cord or rope securing the stencil screen in the peripheral grooves on the printing frame.

AN ALTERNATE EMBODIMENT pivoted at 54 on abutment 55 on frame member 14. Lever 56 is pivotally connected to arm 53 through toggle arm 57. When lever 56 is moved from the vertical, arm 53 raises clamp bar 51. When lever 56 is in the position shown in FIG. 14, clamp bar 51 clamps the fabric against insert 50 in groove 25.

While this invention has been described fully and completely with special emphasis upon a preferred embodiment, it should be understood that within the scope of the appended claims, the invention may lie-practiced otherwise than as specifically described herein.

Iclaim: p

1. A pneumatic stretcher apparatus comprising at least four frame member assemblies arranged as a rectangular frame, means on each frame member assembly for securing the same to an-adjacent frame member assembly to form a frame adjustable in size, each frame member assembly comprising a first elongated channel-shaped member and a second elongated member assembled on and guided by said channelshaped member for relative movement therebetween, said first and second elongated members being arranged for relative movement in each frame member assembly with one of said members being a fixed member and the other being a movable stretcher member, means on said movable stretcher member to secure a screen fabric thereto for stretching in preparation for application to a fabric supporting frame, and

pneumatically operated means in each channel-shaped member for moving each of the movable stretcher members laterally of said frame member assemblies for stretching screen fabric when secured thereon.

2. An apparatus as defined in claim 1 in which said frame member assembly securing means comprises a retaining groove along the end wall of each fixed member and a tongue member on an adjacent fixed member securing said frame member assemblies together by a tongue-in-groove assembly.

3. An apparatus as defined in claim 2 in which each frame member assembly has a tongue member at one end of said fixed member adapted to be assembled in the groove in the fixed member of an adjacent frame member assembly for assembling said apparatusas a rectangular frame.

' 4. An apparatus as defined in claim 3 in which each fixed member has a retaining groove at the end opposite said tongue member for assembly of a plurality of said frame member assemblies end-to-end 5. An apparatus as defined in claim 1 in which said pneumatic means comprises an inflatable pneumatic tube positioned in said channel-shaped member and operable upon inflation to move said movable stretcher member outward.

6. An apparatus as defined in claim 5 in which said fabric securing means on said movable member comprises an elongated groove therein adapted to receive screen fabric and an elongated cam lock rod adapted to be fitted in said elongated groove to secure screen fabric in place therein.

7. An apparatus as defined in claim 6 in which said screen fabric retaining groove has a necked opening and said cam lock rod is of a shapepassing through said necked opening in one position and retained therein to secure a screen fabric in place when rotated to another position.

8. An apparatus as defined in claim 5 in which said frame member assembly securing means comprises a retaining groove along the end wall of said fixed member and a tongue member on an adjacent fixed member securing said frame member assemblies together by a tongue-in-groove assembly.

9. An apparatus as defined in claim 5 in which each fixed member has a tongue member secured at one end adapted to be assembled in the groove in an adjacent fixed member for assembling said apparatus as a rectangular frame.

10. An apparatus as defined in claim 9 in which each fixed member has a retaining groove at the end opposite said tongue member for assembly of a plurality of said frame member assemblies end-to-end.

11. A pneumatic frame member comprising a first elongated hollow member and a second elongated member guided in said first member forrelative movement laterally thereof, said members being adapted to be positioned with one member fixed as part ofa frame and the other member movable laterally of the fixed member. and an inflatable pneumatic tube positioned in said hollow member and operable upon inflation to move said movable member laterally outward of said fixed member.

12. A pneumatic frame member as defined in claim 11 in which said movable member includes means to secure a screen fabric thereon for stretching.

13. A pneumatic frame member as defined in claim 12 in which the fabric securing means in said movable member comprises an elongated groove adapted to receive screen fabric and an elongated cam lock rod adapted to fit in said groove to secure screen fabric in place therein.

14. A pneumatic framemembcr as defined in claim 13in which said fabric retaining groove has a necked opening and said cam lock rod is of a shape passing through said necked opening in one position and secured tightly in said groove upon rotation to another position.

15. A pneumatic frame member as defined in claim 12 in which the securing means on said movable member comprises mechanical clamping'means for securing fabric thereon.

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