This invention relates to- apparatus for use in stretching sheet materials and it has particular, though not exclusive, application in stretching fabric or woven screens, sometimes known as silk screens, for use in printing. It will be appreciated that such screens may be fabricated or woven from a variety of materials, including stainless steel and plastics materials.

The preferred embodiment of the invention to be described enables a screen to be stretched exceptionally uniformly and to bo maintained in the stretched condition in a comparatively simple way. Furthermore, with this embodiment, it is possibl-3 to position a screen and a stencil relative to one another comparatively easily, the positioning stop being carried out at ono location, and the screen with the stencil thereon being transferred to and arranged at a second location at which the printing step is carried out.

Arrangements for stretching silk screens have previously been described. For example, in U.K. Patent Specification No. 71 > _* ?-&9 • there is described an arrange¬ ment in which a screen is stretched by pushing or pulling the screen at or near to its edges into a channel or groove. However, th ^■ arrangements described in that specification do not provide any simple means for ensuring that the screen is stretched along its length by an amount different from that by which it is stretched across its width.

In th' embodiments of the present invent on to be described, moans are provided which enable more uniform stretching of a person to bo obtained in a comparatively simple way.

An embodiment of tlαe invention will now be described, by way of example, with, reference to the accompanying drawings in whichϊ-

Fig. 1 is a perspective view of the main parts of a frame assembly separated from one another;

Fig. 2 is a plan view from beneath of the parts shown in Fig. 1 assembled together;

Fig. 3 is a side view of the assembly as shown in

Fig. 1| Fig. h is an end view of the assembly with, the parts assembled;

Fig. is a section on the line A—A of Fig. 2 in the direction of the associated arrows;

Fig. 6 is a section on the line B-B of F±g. 2 in the direction of the associated arrows;

Fig. 7 is a section on the line C-C of Fig. 2 in the direction of the associated arrows; and

Fig. S is a section through a part ofanother arranganait. Referring now to the drawings, there is shown a rectangular main frame 1 having longitudinally extending side members 2" and 3 and side members h- and 5 defining the width of the frame. The members 2 and 3 are longer than the members k and 5 and at each of their ends there is an adjustable leg 6 having a foot 7« The legs 6 are arranged to space the assembly from a bed or platform on which it is placed by a given predetermined amount.

The outer edges of the side members 2, 3 _» ^ ^a nd 5 have flange portions extending therefrom as best seen in Figs. 5-7 where flanges 8 and 9 extend from members 2 and 5 respectively. The widths of the lower faces 11 and 12 of the side members 2 and 3 ^a x*e of both the same dimensions. The widths of the lower faces 13 and 14 of the side members and 5 are both of the same dimensions. The widths of the faces 11 and 12 are greater than the widths of the

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faces 13 and 1 , for reasons which will be explained below.

Clamping members 15-18 having a U-shaped cross section are held in position on the flange portions extending from the side members 2, 3, ^ a d 5» by means of butterfly headed screws 1 and knurled headed screws 20. There is no significance in the use of the different types of screws 19 and 20. The particular embodiment shown is a prototype which employs and illustrates the basic features of the invention. There are a number of different features of design that would be incorporated in a final embodiment.

A mesh screen 22 is shown in Fig. 1 extending over the main frame 1 and around the flanges projecting from the outer edges of the sides 2-5, as may best be seen from Figs. 5-7- The screen 22 is held in position by the clamping members 15-18 which are clamped on to the flange portions, e.g., 8 and 9 _» extending from the outer edges of the side members 2-5 by the screws 19 and 20. At this stage the screen 22 is held substantially in a plane against the faces 11-1 (Fig. ), s indicated by the solid lines in Figs. and h .

The major portion of the screen 22 is next displaced from the plane indicated by the solid lines in Figs. 3 and k to a second plane indicated by chain dotted lines in Figs. 3 and h , by means of a stretching frame 3.

The length of the stretching frame 3 is defined by longitudinally-extending side members Zk and 25 and its width is defined by side members 26 and 27. The side members 2^-27 have downwardly extending portions 28-31

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whose outer faces fit snugly against the inner faces of the side members 2-5 of the main frame 1 , and flange portions 3^-3 which overlap the upper faces of the s±de members 2-5. Countersunk headed screws 36 pass through holes 37-^0 in the flange portions 32 and 33 and engage threaded holes k l-kk in the sides 2 and 3 of the main frame .

The lower outer corner edges of the downwardly- extending portions 28 and 29 are bevelled to a greater degree towards their ends, as indicated at kβ and k on portion 29 and in Figs. 1 and 3 _» than they are at their centres.

The lower outer corner edges of the downwardly extending portions 30 and 31 are bevelled uniformly along their length, as indicated at k% in Figs. 1 and 3.

The variation in the bevel on the lower outer corner edges 29 of the portion of the member 25 may be clearly seen in the differences between the cross- sectional views shown in Figs. 5 and 6, where the screen 22 passes around the edges.

In use, the screen 22 is first attached to the frame 3 by locating it around the flange, for example flange 8, projecting from one of the longitudinally extending members and clamping it in position by means of the U-shaped member 15 and t o screws 19. The screen is then stretched by hand, as uniformly as possible, and located around the flange extending from the other longitudinally extending member 3 _> where it is clamped in position by the member 16 and the screws 19. A similar operation is then carried out to stretch the

screen 22 along its length from the position in which it is located around the flange portion 9 on member 5 and held in position by the clamping member 18 and the screws 20 to- the position in which it is located by the clamping member 17 around the corresponding flange extending from the member k . At this stage the screen 22 is held in a plane against the faces 11-14,as shown by the solid line in Fig. 3.

The stretching frame 2 is then lowered into position so that the lower edges of the downwardly extending portions 26-29 of the members 2 -27 rest on the screen 22.

The frame 3 is then drawn down in an even manner to press upon the screen 22 by screwing the screws 36 into the holes k , —hh in the sides 2 and 3 _of the main frame 1.

By this means the stretching frame 23 is drawn down into the main frame 1 until the flange portions 32-35 rest on the side members 2-5 and the major portion of the screen 22 is displaced to the plane indicated by the chain dotted lines in Fig. 3.

It will be noted that the plane defined by the chain-dotted lines 22 is arranged such that a small gap exists between it and a base 22a upon which the feet 7 rest.

A stencil (not shown) to be used in the printing process is placed on the base 22a upon which the feet 7 rest and the frame assembly 1 , 3 can easily be positioned accurately withrespect to the stencil so that the screen 22 is a little above the stencil. When the stencil and screen 22 are in tho correct registration, the screen 22 may be pressed down on to the stencil. The screen will pick-up the stencil and the assembly

complete with the stencil can be moved to a further location at which the printing step may be carried out. At the further location, the distance between the screen 22 and the material to be printed may be suitably adjusted, for example by adjusting the lengths of the legs 6.

In order to provide a more uniformly stretched screen 22, the dimensions of the main frame 1 and the stretching frame 23 are carefully determined. It will be noted that the screen 22 is rectangular in shape and, unless a different degree of stretching is applied to the width of the screen from that applied to its length, the stretching of the screen is not uniform. In some printing applications, the lack of such evenness of stretching may not be of great importance, however in others it is of great importance in order to enable accurate registration to be obtained.

In the particular embodiment described, it will be noted that the downwardly extending portions 28-31 of the members 24-27 are of equal depth and that they displace the screen 22 from the first to the second parallel planes, as indicated by the solid and chain-dotted lines 22 in Fig. 3 in a uniform manner.

The difference between the amount by which the screen 22 is stretched across its width and that by which it is stretched along its length is determined by the difference in the widths of the side members 2 and 3 and the side members 4 and 5 of the main frame 1. This difference is clearly visible in Fig. 2. This difference may also be appreciated by comparing Figs. 5 and 7 and noting the difference between the widths of the faces 11 and 14. Considering Figs. 5 and 7, a portion of the

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screen 22 forms in each case the hypoteneuse of a right angled triangle, and, in Fig. 7, the face 14. The other side of the triangle is in each case provided by the respective downwardly extending members 29 and 1 « The length of the side of the triangle provided by the respective members 2 and 3 is in each case the same since the lower edges of the members 29 and 31 of the stretching frame 27 have been moved downwardly from the level of the faces 11 and 14 defining the first planar position of the screen 22 to the position shown, by the same amount. The width of the screen 22 initially included the portions which were against the face 11 and the corresponding face 12 on the member 3» T e insertion of the stretching frame 2 has caused the portions which were against the faces 11 and 12 to be moved so that they form the hypoteneuse of the above-mentioned triangle and the portion between the sides of the frame 23 to be displaced directly downwards. It may thus be considered that the width of the screen 22 has been stretched by twice the difference between the width of the face 11 and the length of the above-mentioned hypoteneuse of the triangle shown in Fig. 5« Similar considerations apply to the length of the screen 22. The length of the screen 22 may be considered to have been stretched by twice the difference between the width of the face 14 and the length of the hypoteneuse of the above mentioned triangle shown in Fig. 7.

The use of the chamfered portions of the edges shown best at 46 and 7 in Fi . 3 have been found to be helpful in evening-out tensions that might overwise occur in the corner regions of the screen 22. Chamfering or rounding of the edges, as shown at 48 is helpful in reducing stresses on the screen 22.

It will be appreciated that the arrangement of the rounding or chamfering may be different from that shown and that variable degrees of chamfering may be given to the lower edges of the downwardly extending portions 30 and 1 of the members 6 and 27 in addition to or instead of that on the portions 28 and 29.

The method of clamping of the screen by means of the members 15—18 and the screws 19 and 20 may be replaced in a production model by the use of bars having a surface that grips the screen material and that are held in position by toggle-action clamps.

It is also possible for one of the side members for example member 2, to be of the same depth as the downwardly extending portions 29 of the stretching frame 23 so that the action of stretching occurs as a result of the dis¬ placement of the screen from its position against the lower face 12 of the side member 3 only.

In determining the difference between the amount of stretch given to the screen in one direction compared to the amount of stretch given in the other direction, it may be necessary to take account of differences in tension between the longitudinally-extending strands and the transversely-extending strands of the woven fabric screen material. Thus, even with a square frame it may be necessary to employ a degree of differential stretching in order to even but the tension in the screen material.

On the inside lower face of each of the clamping members 15—18, there may be provided a strip of resilient material having ridges and grooves along the length of its surface that mate with grooves and ridges in a similar strip of material on the underside of the flange portions,

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e.g. 8 and extending from the outer edges of the side members 2-5. In this arrangement, when the screws 19 and 20 are tightened, the action of the clamping members 15-18 is not only to clamp the screen, but also to apply further tension to the screen as the screen material is forced into the mating grooves by the ridges of the strips of resilient material.

In yet another embodiment, there is, attached to the inner face of the vertical wall of each of the clamping members 15-18, a strip of resilient material having on its surface upwardly-directed serrations or teeth that grip the screen material and draw the screen tighter across the frame as the screws 19 and 20 are tightened and the clamping members 15-1 are moved upwards relative to the side members - «

In one particular application of the embodiment described, it may be considered that three stages of operation are employed in setting-up the apparatus.

Firstly, there is the stage during which a screen 22 is stretched evenly and comparatively lightly, with the screen material held between the clamping members 15-18 and the sides 2-4 of the main rame 1.

In the second stage, the stretching frame 2 is introduced into the main frame 1 and the screws } ^' & are tightened to bring the stretching frame down into the main frame 1 until the screen 22 is displaced into the second plane indicated by the chain-dotted lines in Figs. J and 4. In this position the frame and the screen 22 are on the bed or base 22a upon which the feet 7 rest. Assuming that the bed or base 22a is flat, it is possible by this means easily to bring the stretching frame 2 and the screen 22 down on to the bed

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or base 22a,and to arrange that the screen is in even contact with the bed or base and therefore- also flat.

In the particular application being described, in a third stage, the length of each of the legs 6 is then extended by an equal amount. In the particular prototype, the legs 6 ax'e replaced by longer legs which, in effect, are gauges or spacers.

These longer legs 6 space the stretched flat screen 22, in the plane indicated by the chain dotted lines- in Figs. 3 and 4, from the base on which the feet 7 rest by a predetermined amount. The prede ermined amount is sufficient to enable a stencil film or coating to be arranged beneath, and in a given position relative to, the screen 22. Pressure on top of the screen 22, for example by a roller, results in the screen 22 ^' being brought down on to the stencil which is picked-up by the screen i a well-known way . It is alternatively possible to arrange that the screen 22 is moved down evenly on to the stencil by means of further downwards movement of the stretching frame 23, brought about by further tightening of the screws 6, in order to assist in picking-up the stencil and then to release the screws 36 sufficiently to bring the screen 22 back up into or towards the plane indicated by the chain-dotted lines in Figs. 3 and 4.

The longer legs 6 employed in this third stage are such that the distance between the screen and the base which is beneath the screen during printing is not sufficient to allow the screen to be stretched beyond a predetermined limit during printing.

The step of releasing the screws 6 after the stencil has been picked-up has the effect of releasing

the tension in the screen 22 slightly and thus the screen shrinks slightly a d grips the stencil better in its interstices .

It will be appreciated that other means than adjustable, hinged or replaceable logs 6 may be used to achieve the required spacings. For example, fixed studs of different predetermined lengths may be arranged upon the frame in such a way that, i any one stage of the process, only one set of the studs is operative. It is,

10 of course, possible to arrange for the spacers or studs to be on the base or bed on which the frame 1 rests and not upon the f m«--» Itself, or for separate and different spacer members of predetermined depth to be brought into position from an adjacent posi ion at the appropriate

15 stages in the process.

It will also e understood that, after a screen 22 has been stretched to a required even tension, a separate frame may be placed upon and glued to the screen 22. When the separate frame has become firmly attached to the

;0 screen 22, the screen may be cut-away around the separate frame and a separate frame having a screen with a particularly even tension is thereby obtained in a simple manner. In this use of the invention in the production of separate frames, the actual arrangement shown in the

_5 accompanying drawings is not itself used in a printing process .

It has been found that by adjusting independently individual ones of the screws 19 2nd 20 in the embodiment described, extremely accurate control of the tension in 0 local areε.s o the screen can be obtained.

In attaching a separate frame to a screen that has already been stretched, it is necessary to place the separate frame on to the screen or cloth and, in order to ensure a good contact with the screen or cloth suitable for gluing, it may be necessary to press the frame on to the screen while the glue between them sets or cures. This pressing or pushing will change the tensions in the screen or cloth that is in a stretched condition. If however, the separate or manufactured frame is placed between the stretching frame and the surface of the screen or cloth, and provided that the correct dimensions are maintained, the screen can be stressed accurately to the particular separate frame in one operation. After the frame has been glued to the screen, the manufactured or sub-frame can be cut away from the stretching frame assembly by cutting the screen around the separate frame in the way described above,

A cross—section through a part of an arrangement employing this method will now be described with' reference to Fig. 8 of the accompanying drawings.

In Fig. 8 there is shown one side 55 of a male tensioning frame with one box section side 56 of a previously manufactured separate frame arranged between the side 55 of the tensioning frame and a woven screen 57• A side 58 of the main (female) frame of "the assembly incorporates a spacer portion 59 and a flange 6θ having a longitudinally serrated lower surface 61. The spacer portion 59 locates the side 56 of the separate frame and similar spacer members, each aranged on another side of the separate frame, maintain the differential stretching relationship previously described. A tension¬ ing screw 62 is shown coupling the side 55 of the tensioning frame to the spacer portion 59 _• The

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serrated lower surface 6l of the flange 6θ extends along the length of the flange by an amount corresponding at least to the length of the separate frame 56. Co¬ operating with the serrated surface 61 is a similarly serrated surface 63 of a member 6k forming with the flange member 60 a pair of jaws. The jaw members 60 and 6k are held together by a channel section member 65 which slides over them.

In use the screen material 57 is arranged to extend between the jaw members 6θ and 6k and it passes around one of the members. In Fig. 8 the material 7 is shown at 66 passing around the member 6k to extend at 67. The material 57 s first pulled as evenly as possible manually and then the jaws 60 and 6k are brought together and held in an interlocked position by the channel member 65 _* The jaw members grip the material 57 firmly between tho serrated portions and provide an additional tensioning. The channel member 65 holds the jaw members together _. and grips the material 57 firmly. Further tensioning is carried out by means of the screws 62 as described previously.

Tho lower jaw 6k may bo hinged to the upper jaw 6θ on one side only, making sure that the serrations fully engage when the hinge is closed.

Alternatively the upper jaws may be attached loosely to their respective lover jaws so that the cloth or fabric may freely slide between them and so that any one side can be clamped independently of any or all of the other three.

To clamp any .side, the cloth is gently pulled to remove excess folds, wrinkles or creases and the jaws 6θ and 6 are pressed together. The channel section 65 is then slipped or pushed over the two closed

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lock them .

Vhen a screen or cloth is placed between the jaws and they are pressed together, the cloth is drawn into the jaws. If one end of the cloth offers resistance as a result of it having been clamped, the cloth continues to be drawn into the jaws until the frictional forces exerted by the jaws equalise the tensional forces on the cloth. To some extent, the forces required to deflect the cloth will play a part. Should there be a wrinkle, fold or crease in the cloth, no tensional forces will occur until the wrinkle, fold or crease is removed. Further advantages of this particular arrangement are that the cloth or fabric lays flat and is unencumbered by any protuberances. The only protuberances may be hinges on the jaws.

The spacer portions 59 need not extend along the whole length of a side, they may be placed at points where it is necessary to transfer the strain to the manufactured frame.

It will be appreciated that this development concerns a different approach from the usual to the stretching of sheet materials and cloths. Woven cloths used in screen printing are usually plain or twill woven and are so made as to give an interlocking action to each thread making it impossible to pull a thread out of the main body of the cloth. Each thread acts upon others in its vicinity. Stresses in other sheet materials act in a similar ma ner. The apparatus of the invention can stretch other sheet materials for example paper and sheet metals, and it can be used to prestress reinforcing materials.

By increasing the number of tension screws in the arrangement described and by making the female main frame flexible, the tensions in any given part of the screen surface can be varied comparatively easily.

It will be appreciated that, although the invention has been described, by way of example, with reference to particular embodiments, variations, modifications and combinations of the arrangements shown may be made.

For example, the frame may be other than rectangular in shape and the ridges and grooves on the members 15-18 and on the jaw member 6θ and 6k may be rounded at the p..ak≤ and in t e troughs,

Ic will also be understood that the terms cloth, fabric and screen are interchangeable in the context of this speci ication an that they may be made from any suitable material, including filament materials of plastics and metal, or that they may bo sheet material which may be perforated or not.

It will also be appreciated that material may be stretcl'.ed in shapes other than rectangular, for example a frame may be shaped such that material is stretched over a circular or an oval area.

It will also bo appreciated that differences in tensioning may be obtained by varying the depths of the downwardly-extending side portions 28, and 30,31 of the frame 23 along their lengths and/or of varying the spacings between the corresponding side portions 28,29 and 30,31 of the frame 23 and the outer edges of the corresponding side members 2, 3 ^a nd _f of the frame 1 along their lengths.

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