GB2294905A

GB2294905A - Stencil holder

Info

Publication number: GB2294905A
Application number: GB9522706A
Authority: GB
Inventors: Alfred Fromm
Original assignee: Bengel & Bros Bebro
Current assignee: Bengel & Bros Bebro
Priority date: 1994-11-05
Filing date: 1995-11-06
Publication date: 1996-05-15
Also published as: ATE177365T1; EP0790892A1; GB9522706D0; WO1996014210A1; EP0790892B1

Abstract

A stencil holder, for a metal foil stencil (20) used for screen printing solder or adhesive onto p.c.b.'s, comprises a frame (10), and at least two stencil retaining webs (54 - 60) arranged on the flame to detachably hold the stencil (20). The stencil has retaining strips (44 - 50) adjoining a stencil area and extending at maximum over the respective length of an edge of the stencil area in the respective longitudinal direction thereof. Each retaining strip may be detachably fixed to one retaining web, and the stencil may be clamped by means of a stencil clamping device acting on the retaining web in a first clamping direction and in a second clamping direction running transversely to the first. The clamping may be effected by a tensionable cable (122) and associated pulleys, or by pressure actuators, and may act on all four strips or on only two. <IMAGE>

Description

Stencil Holder The invention relates to a stencil holder for metal foil stencils, masks or templates, preferably for screen printing, in particular for attaching adhesive to glue electronic components to printed circuit boards or solder paste for soldering electronic components onto printed circuit boards using the reflow method, comprising a frame, at least two stencil receptacles arranged on the frame to detachably hold the stencils and a stencil clamping device with which the stencil can be clamped in at least a first clamping direction.

Such stencil holders are known from the handbook "DEK MICRO MOUNT Advanced Stencil Clamping System" of DEK Precision Screen Division, 11 Albany Road, Granby Industrial Estate, Weymouth, Dorset DT4 9TH, England.

Here, the stencil is fixed, for example, by means of the two opposing stencil receptacles and is clamped in the first clamping direction extending between the stencil receptacles.

However, the stencil cannot be clamped uniformly in these known clamping devices.

Therefore the present invention seeks to improve a stencil holder of the type known in the art so that the stencil can be clamped as uniformly as possible.

This object is achieved according to the invention with a stencil holder of the aforementioned type in that the stencil has retaining strips adjoining a stencil area, that these retaining strips extend at maximum over the respective length of an edge of the stencil area in the respective longitudinal direction thereof, that each retaining strip may be detachably fixed to one respective stencil receptacle, and that the stencil may be clamped by means of the stencil clamping device acting on the stencil receptacles in a first clamping direction and in a second clamping direction running transversely to the first.

The advantage of the solution according to the invention is that the clamping forces can have an optimum effect on the stencil area here and are not changed, in particular reduced, by retaining strips which extend beyond the stencil area and are connected to one another in these areas extending beyond it, and as a result act unevenly on the stencil, thus causing warping in extreme cases.

A further considerable advantage in the solution according to the invention with respect to uniform clamping of the stencil is that this may be clamped in two clamping directions running transversely to one another and therefore in particular the stencil area is clamped in two clamping directions running perpendicular to one another, in contrast to the solution known from the prior art, in which clamping only occurs in one clamping direction.

In principle, it would be possible to construct the stencil area and the retaining strips using different material and, for example, connect the retaining strips to the stencil area in a suitable manner.

However, it is much more advantageous if the stencil area and the retaining strips are made of a single piece of metal foil.

In the solution according to the invention, the stencil is expediently constructed so that it has a rectangular stencil area and has a total of four retaining strips arranged in pairs opposite one another, and each pair of retaining strips can be clamped by retaining strips in one clamping direction.

It would be possible in principle to fix each retaining strip to several stencil receptacles. However, it is particularly advantageous if each retaining strip can be detachably fixed, in particular in its entirety, to a single stencil receptacle so that each stencil receptacle as a whole unit applies the clamping force to the entire retaining strip.

To detachably fix the retaining strips to the stencil receptacles, it is preferably provided that each retaining strip may be detachably fixed to the respective stencil receptacle by positive locking. Compared to a frictional engagement, a positive locking arrangement provides a much simpler solution which is more suitable for absorption of the clamping force than a frictional engagement.

In the simplest case, it is provided that each retaining strip may be detachably fixed by a row of positive-locking elements on the respective stencil receptacle, in which case the positive-locking elements are preferably arranged in a row one behind the other with essentially equal distances between them.

No further details have be given thus far with respect to the fixture of the stencil in the respective stencil receptacle. Therefore, a non-positive clamping arrangement would be conceivable, for example. However, it has proved particularly expedient to fix the stencil in the stencil receptacle using a positive locking arrangement, since a positive locking joint allows the clamping forces to be transferred much more favourably to the stencil.

Such a positive locking joint can be achieved by all types of positive-locking elements which are effective between the stencil receptacle and the holding web. In the simplest case, such positive-locking elements comprise openings in the retaining strips and holding elements engaging through the openings.

In the case of replaceable stencils, it has proved particularly advantageous if the stencil receptacle has a positioning device acting transversely to the clamping direction. This positioning device enables the stencil to be positioned exactly transversely to the clamping direction and thus automatically reach its exact position transversely to the clamping direction when a replacement stencil is inserted.

This is particularly expedient in a clamping device according to the invention, in which the stencil may be clamped in two clamping directions running perpendicular to one another. since such a positioning device then automatically allows the location of the stencil to be exactly fixed in the plane formed by the stencil.

Such a positioning device can be constructed in a wide variety of ways. Hence, such a positioning device could be formed by optical marks. However, the stencil is positioned particularly simply and securely by means of a positioning device if this comprises a fit.

In this case, the fit can be constructed in a wide variety of ways. It can be formed by two spaced fitting surfaces, between which a section of the stencil may be inserted.

However, it is particularly expedient if the fit is formed by a row of holes into which corresponding retention fingers engage.

A particularly advantageous embodiment provides that the openings and the holding elements engaging through them are constructed at the same time as fits for the exact positioning of the stencil in the stencil receptacle.

No further details have been given in conjunction with the previous explanation with respect to the structure of the stencil receptacles. Hence, an advantageous embodiment provides that two holding webs mounted opposite one another on the frame form the stencil receptacles in each clamping direction, and a retaining strip may be fixed to each holding web.

Each holding web is preferably constructed in this case so that it has holding elements engaging in openings in the retaining strip, in which case the openings are preferably arranged on the retaining strip in the form of a row of holes.

In order to fix the respective retaining strip on the respective holding web, so that the holding elements remain engaged in the openings in the respective retaining strip, it is preferably provided that in a position fixed on the respective holding web, each retaining strip is held with a holding-down means so that it lies on a fastening support surface of the respective holding web.

This ensures that the retaining strip is not lifted from the fastening support surface of the respective holding web and thus also that the holding elements do not become disengaged from the openings.

It is particularly expedient here if, in the fastening position, the holder engages with fingers over the respective retaining strips in the areas between the holding elements in the form of locating lugs to thus also prevent the metal foil of the retaining strips from warping in the areas between the locating lugs during clamping, since such warping would likewise cause folds to form in the stencil.

No further details have been given thus far with respect to positioning the stencil perpendicular to the plane in which it extends. Hence, it would be conceIvable, for example, to carry out this positioning via the position of the holding webs in this direction.

It has proved particularly advantageous, especially in order to provide a stable position of the stencil suitable for screen printing, if the stencil is supported between a stencil area and a toe on the holding web by a supporting web.

It would be possible here to act against the stencil with the supporting web. However, it is particularly advantageous if the holding web, which is displaceable in the clamping direction, abuts against the supporting web.

during clamping.

This may be achieved, for example, by the holding web pulling the stencil against the supporting web during clamping.

This may be achieved in a particularly advantageous manner if the stencil receptacles are arranged and may be moved on a side of a plane defined by the supporting web opposite the stencil. This also has the further advantage that the stencil receptacles do not disturb the movement of a doctor blade working on the stencil.

No further details have thus far been given with respect to the arrangement of the supporting web on the stencil holder in conjunction with the previous explanation of the individual embodiments.

Hence, a particularly advantageous embodiment provides that the stencil holder has an outer frame formed from frame supports in which a stencil field is arranged, and that the stencil field is defined by four supporting webs connected to the frame by means of struts. Therefore, in this solution the outer frame in a machine operating with the stencil enables the stencil holder to be guided and positioned exactly on its outer periphery, and moreover to likewise position the stencil exactly in relation to the outer frame by means of the supporting webs forming the stencil field which are firmly connected to the outer frame.

Positioning of the stencil by means of the support struts can be achieved particularly simply if each of the supporting webs has a supporting surface onto which the retaining strips of the stencil can be laid. This means that in this embodiment the stencil lies with the retaining strips on the supporting surfaces of the supporting webs so that the stencil area in particular lies inside the stencil field defined by the supporting webs, which is of particular advantage for fold-free and preferably uniform clamping of the stencil.

So that the retaining strips can be easily connected to the holding webs, it is preferably provided that each retaining strip extends beyond the respective supporting web as far as the respective holding web, thus allowing the holding web to engage with the retaining strips outside the stencil field defined by the supporting webs.

No further details have been given with respect to the mobility of the holding webs in the respective clamping directions in conjunction with the previous description of the solution according to the invention. Hence, an advantageous embodiment provides that at least one of the two holding webs effective in one clamping direction may be moved in the direction of the respective clamping direction by means of the stencil clamping device. The other holding web can be held immovably on the frame, for example, in the clamping direction.

No further details have thus far been given with respect to the arrangement of the holding webs which are movable in the respective clamping direction. Hence, it would be expedient, for example, if the holding webs are guided by any type of sliding guide.

It has proved particularly advantageous if the holding webs are mounted on a parallel guide effective in the respective clamping direction. This parallel guide could, for example, be in the form of a lever guide. However, it is particularly advantageous if the parallel guide comprises two parallel longitudinal guides.

The longitudinal guides are expediently constructed as guide rods onto which guide bushes slide.

As an alternative to the longitudinal guides in the form of guide rods with guide bushes which, for example, guide the holding webs inside their longitudinal extension, a solution to this provides that the holding webs are guided at their ends on longitudinal guides.

Longitudinal guides are preferably provided between the two ends of each holding web and the struts connected to the frame, said longitudinal guides in particular enabling the respective holding web to be guided exactly in the direction of its longitudinal extension and thus transversely to the respective clamping direction.

Each such longitudinal guide is expediently formed by a roll body running in a guide groove, in which case the guide groove are preferably arranged in the respective struts extending at the ends of the web and the roll bodies are held, for example, on the holding web.

All the sliding guides can be constructed so as to allow movement in the respective clamping direction with clearance transversely thereto. However, should the movement of the respective holding web be limited solely to the clamping direction, then the respective sliding guide is preferably provided with as low a clearance as possible.

No further details have been given with respect to the configuration of the stencil clamping device in conjunction with the previous description.

Hence, in an advantageous variation of the invention, the stencil clamping device is formed by hydraulic or pneumatic clamping elements effective in the respective clamping direction which are coupled to one another so that the same forces are applied simultaneously to the holding webs.

In the simplest case, the clamping device in this solution comprises at least one clamping cylinder working between the respective holding web and the frame to enable the respective holding web to move to clamp the stencils.

To ensure that all the clamping cylinders act equally and clamp simultaneously with the same clamping force in each clamping direction in the manner according to the invention, it is preferably provided that the clamping cylinders are switched in parallel and connected to a pressure line so that the same pressure builds up in each clamping cylinder, and therefore, with identical clamping cylinders, each clamping cylinder generates the same clamping force.

In principle, one clamping cylinder for each holding web would be sufficient. However, to generate as uniform an application of force on each holding web as possible, two clamping cylinders spaced apart from one another and acting in parallel are advantageously provided between the respective holding web and the frame.

As an alternative to the above-mentioned possible arrangements, an advantageous embodiment of a clamping device according to the invention provides a clamping winch which acts simultaneously on the two respective holding webs located opposite one another and movable in the same clamping direction, and may be clamped by a clamping element. The clamping winch is expediently a nonflexible winch.

The clamping winch preferably has a loop effective between the frame and the respective holding web, the size of said loop being variable in accordance with the pulling effect on the clamping winch. In this case, the clamping winch is in particular deflected via rollers to form the loop in order to prevent any jamming and thus prevent uneven forces from occurring on the holding webs.

A particularly advantageous embodiment, in which it is possible to clamp the stencil in two clamping directions, provides that the clamping winch serves to clamp all four holding webs.

In this case, the clamping winch is guided by means of deflection rollers in corner regions of the stencil holder.

The advantage of such a clamping winch in principle is that it can be clamped via a mechanical clamping element, e.g. a lever element or a clamping sleeve, with a clamping bolt by traction, and this tensile force remains constant for a long period so that such a clamping winch has advantages over hydraulic or pneumatic solutions because of its simplicity, particularly in the case where the stencil has to be clamped for a long period at the same clamping forces.

Moreover, such a clamping winch has the additional advantage that it can be guided in a simple manner in any desired direction by means of the deflection rollers.

No special details have been given with respect to the application of force on the clamping device. Hence, it would be possible, for example, to act on the holding web at two spaced locations by using a clamping winch.

However, it is particularly advantageous, especially to ensure easy mobility of the holding web and clamping thereof, if the stencil clamping device acts on the centre of the holding webs and the holding webs are preferably guided on both sides of the central application of force.

A wide variety of solutions are possible regarding the manner in which the stencil is clamped in the two clamping directions running transversely to one another. Hence, one advantageous solution provides that the stencil receptacles have two opposing holding webs which are displaceably guided on the frame solely in the direction of the first clamping direction; that the stencil clamping device has a clamping force source acting on both holding webs simultaneously with the same clamping force; that the stencil receptacles have two opposing holding webs which are displaceably guided on the frame solely in the direction of the second clamping direction; and that the stencil clamping device has a clamping force source acting on the two holding webs simultaneously with the same clamping force.

The advantage in this solution is that as a result of the exact guidance of the two holding webs solely in the first clamping direction and in the second clamping direction a defined position of the stencils fixed in the stencil receptacles transversely to the respective clamping direction is provided during clamping; and that, on the other hand, as a result of simultaneous application of the same clamping force on each of the holding webs, the stencil is thus clamped uniformly in the first direction so that the two holding webs move away from one another under the effect of the same clamping force in each case.

In this case, the stencil is preferably fixed in its position in relation to the frame because of the two clamping directions running transversely to one another and the guidance of the holding webs solely in these clamping directions. Hence, the stencil is simultaneously fixed in the first clamping direction as a result of the second clamping direction and the stencil is simultaneously fixed in the second clamping direction as a result of the first clamping direction. This means that it must be possible to position the stencil fixed to the stencil receptacle exactly in the two clamping directions with such a configuration of the solution according to the invention.

A particularly advantageous embodiment of this solution provides that a single clamping force source acts on all the holding webs.

As an alternative to this solution, another embodiment of the solution according to the invention provides that one of the two holding webs effective in the first clamping direction is firmly connected to the frame, and the holding web located opposite the fixed holding web is movable relative to the frame in the first clamping direction; and that the holding webs effective in the second clamping direction are movable relative to the frame in the first clamping direction and at least one of the holding webs effective in the second clamping direction is movable relative to the frame in the second clamping direction.This solution works from the feature that the stencil may be adequately positioned by the holding web fixed to the frame, and in this case it is necessary for the holding web effective in the second direction to be movable in the first direction in order to prevent creasing, this mobility being achieved by a guide clearance.

It is particularly advantageous in this embodiment if the holding web acting and movable in the first clamping direction is also movable relative to the frame in the second clamping direction, so that any creasing may also be compensated for here, the mobility in the second clamping direction likewise being achieved by a guide clearance.

A further embodiment of a solution according to the invention provides that at least one of the holding webs effective in the first clamping direction is mounted to move relative to the frame in the first clamping direction; that both holding webs effective in the first clamping direction are mounted to slide in the second clamping direction; and that the holding webs effective in the second clamping direction are movable relative to the frame in the first clamping direction and at least one of the holding webs effective in the second clamping direction is movable relative to the frame in the second clamping direction.

In both of the embodiments described above it is possible to arrange one of the holding webs effective in the second clamping direction so that it cannot slide in the second clamping direction and only the other holding web is movable. It is particularly favourable if both of the holding webs effective in the second clamping direction are mounted to move relative to the frame in this direction so as to achieve symmetrical clamping in the second direction thereby.

In conjunction with the solution that at least one of the holding webs effective in the first clamping direction is movable relative to the frame in the first clamping direction, an advantageous solution provides that one of the holding webs is mounted so as not to slide on the frame in the first clamping direction to thus predetermine a fixed position for the stencil in the first clamping direction.

However, it is also possible that both of the holding webs effective in the first clamping direction are mounted to move relative to the frame, and in this case the position of the stencil may preferably be determined by a marking on the stencil.

The invention also relates to a stencil made of metal foil, preferably for screening printing.

If the stencil is to be clamped in two clamping directions running essentially perpendicular to one another, it has proved advantageous according to the invention if the stencil has retaining strips adjoining a stencil area, these retaining strips extending at maximum over the respective length of the stencil area so that the clamping forces can have an optimum effect on the stencil area and not be reduced by retaining strips, which extend beyond the stencil area and are connected to one another in this area extending beyond it, thus acting unevenly on the stencil and causing warping in extreme cases.

Expediently, the stencil is constructed so that it has a rectangular stencil area and a total of four retaining strips arranged in pairs opposite one another.

It would also be possible, in principle, to construct the stencil area and the retaining strips from different material and for example, connect the retaining strips to the stencil area in any suitable manner.

However, it is much more advantageous if the stencil area and the retaining strips are made of a single piece of foil.

In principle, the foil may also be made from different materials.

However, because of the stress applied to the foil, it has proved advantageous if the foil is a metal foil.

No further details for the stencil have been given thus far with respect to the fixture of the retaining strips to the holding webs. Hence, it has proved particularly advantageous if each retaining strip has a row of holes for fixing it to the respective holding web so that the locating lugs arranged on the holding webs can be brought into engagement with the individual holes in the row.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a top view onto a first embodiment of a stencil holder according to the invention; Figure 2 is an enlarged partial view of a longitudinal side of the stencil holder; Figure 3 shows a section along line 3-3 in Figure 2; Figure 4 is an enlarged sectional view along line 4-4 in Figure 2 with the stencil receptacle firmly holding the stencil; Figure 5 is a view similar to Figure 4 with the stencil receptacle open; Figure 6 is a top view onto a stencil for the stencil holder; Figure 7 shows an enlarged section through a clamping element of the stencil holder; Figure 8 is a top view similar to Figure 1 onto a second embodiment of a stencil holder according to the invention;; Figure 9 is an enlarged sectional view similar to Figure 2 of the second embodiment; Figure 10 shows a section along line 10-10 in Figure 9; Figure 11 shows a section along line 11-11 in Figure 9; Figure 12 is a top view similar to Figure 8 onto a third embodiment of a stencil holder according to the invention, and Figure 13 is a top view similar to Figure 8 onto a fourth embodiment of a stencil holder according to the invention.

A first embodiment of a stencil holder according to the invention, shown in Figure 1, comprises a frame, given the overall reference 10, which is formed by four frame supports 12, 14, 16 and 18 forming the longitudinal sides of the frame. The frame 10 may be received and exactly positioned in the area of the frame supports 12 to 18, for example, in a screening printing plant, so as to exactly position a stencil, given the overall reference 20, for screen printing.

The stencil 20 is located in a stencil field 22 of the frame 10 defined by four supporting webs 24, 26, 28 and 30. The supporting webs 24 to 30 are in turn connected via struts 32, 34, 36, 38 forming an extension of said supporting webs and extending as far as the frame supports 12 to 18 to said frame supports (Figures 1 and 2).

As shown by way of example in the case of the supporting web 30 in Figures 2 and 3, each of the supporting webs 24 to 30 comprises a supporting surface 40 on which lateral retaining strips 44, 46, 48 and 50 of the stencil 20 may be laid to position the stencil 20 on a defined plane 52, the plane 52 being determined here by the supporting surfaces 40 of the supporting webs 24, 26, 28 and 30.

Additionally, each of the retaining strips 44 to 50 (Figure 6) extends beyond the respective supporting web 24 to 30 as far as a respective holding web 54, 56, 58, 60 on which the retaining strip 44 may be fixed.

For this, the respective retaining strip 44 to 50 has a row of holes 64 extending along an outer side edge 62 (Figure 6), through which locating lugs 66 of a row of locating lugs 68 arranged on the respective holding web 54 to 60 engage (Figures 3, 4, 5). In this case, to the side of the rows of holes 64 the respective retaining strip 44 to 50 lies on a locating support surface 70 of the respective holding web 54 to 60 and is held by a holdingdown means 72 to lie on the locating support surface 70, in which case the locating lugs 66 protrude from the locating support surface 70 and engage through the respective row of holes 64 in the retaining strip 44 to 50 lying on this locating support surface 70.

The holding-down means 72 preferably has a supporting strip 74 which extends parallel to the locating support surface 70 and, as shown in Figure 2, engages over the respective retaining strips 44 to 60 in its areas between the positioning peg 66 by means of fingers 76. The holding-down means 72 also comprises a guide arm 78 which abuts against one side 80 of the respective holding web 54 to 60 facing away from the respective supporting web 24 to 30, and may be moved transversely, preferably perpendicular, to the locating support surface 70 relative to the respective holding web 54 to 60 so that the supporting strip 74 may be moved towards the locating support surface 70 or away from it (Figures 4, 5). The holding-down means 72 is preferably subjected to pressure by a pressure spring 82 acting on the supporting strip 74 in the direction of a position of the supporting strip 74 releasing the respective retaining strips 44 to 50 (Figure 5), and may be moved against the force o the pressure spring 82 into a position (Figure 4) in which the respective retaining strip 44 to 50 abuts against the locating support surface 70, and may be fixed in this position by means of clamping screws 84, in which case the clamping screws 84 engage through elongated holes 86 in the guide arm 78 and thus, together with the elongated holes 86, act as guide for the guide arms 78 on the respective holding web 54 to 60.

According to the invention, both the holding webs 54 to 60 with the respective holding-down means 72 and the locating lugs 66 are arranged so that they lie in the fixing position for the retaining strips 44 to 50 beneath a plane 92 defined by an upper side 90 of the stencil 20 in the area of the stencil field 22 so that a wiper on doctor blade running over the upper side 90 can move beyond both the holding-down means 72 and the locating lug 66 without colliding.

Each of the holding webs 54 to 60 is movably guided in only one clamping direction 98 or 100 by a parallel guide, formed by two guide rods 94 and 96 parallel to the respective clamping direction 98, 100, said clamping directions 98 and 100 running parallel to the plane 92 and, moreover, standing perpendicular to one another. In this case, holding webs 54 and 58 are movable solely in clamping direction 98 and holding webs 56 and 60 are movable solely in clamping direction 100.

In this case, guide rods 94 and 96 are held, for example, on supporting webs 24 to 30. However, either additionally or alternatively thereto, they may also be held on the frame supports 12 to 18.

To allow the holding webs 54 to 60 to slide easily on guide rods 94 and 96, the holding webs are guided via guide bushes 102, preferably in the form c= ball-type bushes, to slide exactly and easily on the guide rods 94 and 96 in the form of round rods. As a result of the exactly defined sliding mobility of opposing holding webs 54 and 58 in clamping direction 98 and opposing holding webs 56 and 60 in clamping direction 100, the stencil 20 is exactly positioned in the plane 92 in the case of a row of holes 64 in the form of a fit and locating lugs 68, so that when the stencil 20 with the rows of holes 64 is inserted into the respective rows of locating lugs 68, perforations 110 provided therein are exactly positioned in the plane 92.

To clamp the stencil 20, the respective opposing holding webs 54 and 58 as well as 56 and 60 may each be moved in the corresponding clamping direction 98 and 100 respectively by means of a common stencil clamping device which comprises a support block 112 arranged on each of the holding webs 54 to 60 and a clamping roller 114 rotatably arranged on the respective support block 112, as well as deflection blocks 116 arranged opposite the support blocks 112 on the frame supports 12, 14, 16 and 18 having two deflection rollers 118 and 120, between which a clamping winch 122, formed in the simplest case by a steel cable, protrudes in the form of a loop 124 and loops around the clamping roller 114.The clamping winch 122 extends from a fixed end 126, still guided by corner deflection rollers 128, mounted in corner deflection blocks 130, from one deflection block 116 to the next, in which case the loop 124 leading to the respective holding web 54 to 60 being formed in the area of each deflection block 116. At the end 132 of the clamping pinch 122 opposite end 126, the clamping winch is clamped by means of a clamping element 134 shown in Figure 7 which has a clamping bolt 136 which is firmly attached to end 132 and has an external thread 138 which, in turn, engages into an internal thread 140 of a clamping sleeve 142, itself rotatably mounted on the frame 10, e.g. on frame support 14 (Figure 7).

Moreover, the clamping bolt 136 is mounted on the frame 10 so as to be non-rotatable, but able to slide in the direction of its longitudinal axis 144, preferably in a perforation 146 through the strut 34, so that it may slide in the direction of its longitudinal axis 144 by rotating the clamping sleeve 42 of the clamping bolt 136 in order to clamp the clamping winch 122 and thus generate the clamping force acting in the respective clamping direction 98 or 100 via each loop 124, and at the same time clamp the stencil 20 simultaneously in clamping directions 98 and 100, so that the retaining strips 44 to 50 come into abutment against the supporting surfaces 40 of the supporting webs 24 to 30, and so that the stencil 20 lies exactly on the plane 52 defined by the supporting surfaces 40 of the supporting webs 24 to 30, so that in addition to the exact positioning of the stencil 20 in clamping directions 98 and 100, an exact position may be achieved perpendicular to these clamping directions 98 and 100.

The stencil is preferably constructed so that it has a central stencil area 150, in which the perforations 110 necessary for screen printing are arranged, and adjoining them on side edges 154, 156, 158 and 160 of the stencil area 150 forming an edge of the stencil area 150, the retaining strips 44, 46, 48 and 50 extending at maximum over the length of the corresponding side edges 154, 156, 158, 160, so that the stencil 20 has corner sections, and as a result, the clamping effect in one clamping direction 98 and the clamping effect in the other respective clamping direction 100 are not reduced by the retaining strips 44 to 50, but are able to develop to the full over the respective extension of the stencil area 150 in the respective clamping direction 98, 100 (Figure 6).

In a second embodiment of a stencil holder according to the invention, the parts which are identical to those in the first embodiment are provided with the same references and therefore reference is made to the full explanations of the first embodiment for their description.

In a second embodiment of the stencil holder according to the invention, shown in Figures 8 and 9, clamping cylinders 172a,b, 174a,b, 176a,b and 178a,b arranged in pairs on the respective frame support 12, 14, 16, 18 with their cylinder housing 170 operate between the frame supports 12, 14, 16 and 18 and the holding webs 54, 56, 58, 60, and a piston rod 180 of cylinders 172, 174, 176,.

178 is connected to the respective holding web 54, 56, 58, 60.

A pressure connection 182 from each of the cylinders 172, 174, 176, 178 is connected to a pressure line 184 directed on the frame 10. The pressure line 184 can be connected centrally via a pressure connector 186 to a pressure source 188 which constantly supplies constant pressure, which results in each of the clamping cylinders 172, 174, 176, 178 acting with the same force on the respective holding web 54, 56, 58, 60, since the pressure line 184 builds the same pressure in the parallel connected clamping cylinders 172, 174, 176, 178.

In contrast to the first embodiment, each of the holding webs 54, 56, 58, 60, as shown in Figure 9 and Figure 11, is provided at both its ends 190, 192 with a pair of rollers 194, 196 of two idlers 198, 200 spaced apart from one another, which are guided in a groove 202, 204 in the webs 336, 32 allocated to the ends 190, 192, as shown by the example of the holding web 60 in Figures 9 and 11. The grooves 202, 204 are in this case dimensioned so that they guide the idlers 198, 200 exactly parallel to the respective clamping direction, clamping direction 100 in the case of Figures 9 and 11, so that the respective holding web 60 is also guided parallel to clamping direction 100.

The respective holding web 60 is guided transversely to clamping direction 100 in the embodiment shown in Figure 9 as a result of its ends 190, 192 on the respectively facing struts 36, 32 sliding on their upper surface 206, 208 facing the holding web 60 with its end surface 210, 212. An even more precise guidance of the respective holding web 58 transversely to the clamping direction 98 is also possible, in that a roller body guide is effective between the ends 190, 192 of the respective holding web E0 and the struts 36, 32 of the frame 10, by means of which guide the ends 190, 192 of the respective holding web 60 are then supported on the respective end struts 32, 36 arranged on the ends.

The connection between the piston rod 180 of each of the clamping cylinders 172, 174, 176, 178 is achieved by a hole 220 arranged in the respective holding web 58, through which an end section 222 of the piston rod 180 with a reduced diameter passes. This piston rod is provided at its end with a radially projecting disc 226 engaging over a ring flange 224 in the respective holding web, said disc allowing the tensile force of the respective clamping cylinder 176 to act on the respective holding web 58 via the ring flange 224.

In order to achieve a particularly exact positioning of the holding webs 54, 56, 58, 60 when the stencil 20 is clamped, an end stop in the form of two end stop elements 230 is preferably allocated to one of the two holding webs 54, 58 or 56, 60 arranged in pairs opposIte one another.

These end stop elements enable an end pcsition of the respective holding web 54, 56 to be fixed exactly when the clamping cylinders 172, 174, 176, 178 apply tensile force onto all the holding webs 54, 56, 58, 60. This means that the respective opposing holding web 58, 60 clamps the stencil starting from these end positions because of the clamping cylinder 176, 178 acting on this holding web, whereas the clamping cylinders 172, 174 hold the respective holding web 54, 56 in abutment against the stop pins 230. This enables the stencil 20 to be positioned by the stop elements 230 with even more precision than was possible with the longitudinal guides of the holding webs 54, 56, 58, 60.

In a variation of the second embodiment, however, it is also possible to guide the holding webs 54, 56, 58, 60 not only parallel to the respective clamping direction 98, 100 in which it is effective, but also to provide that each pair of holding webs 54, 58 or 56, 60 effective in one of the clamping directions 98 or 100 may be moved in the other respective clamping direction 100 or 98 at least to a restricted degree.For this purpose, as is shown in the example of the holding web 60 in Figure 9, the respective holding web 60 is guided as a result of its end 190, 192 being guided with clearance between the respective facing struts 36, 32 so that the end surfaces 210, 212 of the holding web 60 come into abutment against the respective upper surfaces 206, 208 of the struts 36, 32, while the respective holding web 60 is effective in its clamping direction 100 and is also guided in this direction by the idlers 198, 200 located in the grooves 202, 204 so as to move relative to the frame 10.

Although exact positioning of the stencil 10 is no longer possible during clamping in this variation of the second embodiment, it is possible to position the clamping frame exactly for screen printing by means of a marking on the stencil by using devices for moving the clamping frame so that the perforations 110 lie in the desired position.

In a third embodiment of a stencil holder according to the invention, shown in Figure 12, one of the holding webs 54, 56, 58, 60, e.g. holding web 54, is fixed so as not to slide on the frame 10 both in its effective clamping direction 98 and also transversely thereto, i.e. parallel to clamping direction 100.

The other holding webs 56, 58, 60 are mounted to move both in their effective clamping direction, i.e. in the first clamping direction 98 in the case of holding web 58 and in clamping direction 100 in the case of holding webs 56 and 60, while also preferably being able to move transversely to their effective clamping direction, i.e. in clamping direction 100 in the case of holding web 58 and in clamping direction 98 in the case of holding webs 56 and 60. In this case, holding webs 56, 58, 60 are preferably guided with clearance in the latter direction, as has been described in conjunction with the variation of the second embodiment.

In addition, holding webs 56, 58, 60 may still be subjected to pressure from clamping cylinders 174, 176 and 178 in the described manner in order to clamp the stencil 20.

In the third embodiment according to Figure 12, although holding web 54 is fixed to webs 38 and 3w, it is now possible to uniformly clamp the stencil 20 in both clamping directions 98, 100, on application of pressure from the clamping cylinders 174, 176 and 178, because holding webs 56, 60 are able to move transversely to their effective clamping direction 100, i.e. in clamping direction 98, with clearance between webs 36 and 32 and thus compensate for a slight deformation of the stencil 20 clamped from the direction of the fixed holding web 54.

Additionally, holding web 58 is also advantageously able to move transversely to its effective clamping direction 98, i.e. in clamping direction 100, with clearance between webs 38 and 34 in order to compensate any transverse movements resulting from the holding webs 56, 60 being clamped with clamping cylinders 174 and 178, i.e.

movements towards clamping direction 100.

Reference is made herewith to the full contents of the explanations regarding the first and second embodiments as regards description of the remaining features of the third embodiment.

In a fourth embodiment shown in Figure 13, holding web 54 and holding web 56 are no longer subjected to pressure from a clamping cylinder. Instead, these are fixed in their effective clamping direction 98 or 100 between end stops 230 and 232 or 234 and 236.

Holding webs 54 and 56 are mounted transversely to their effective clamping direction in each case, i.e. in clamping direction 100 in the case of holding web 54 and in clamping direction 98 in the case of holding web 58, with clearing between the respective struts 38 and 34 or 32 and 36, in which case holding webs 54 and 56 are preferably fixed in the same way as described in conjunction with the second embodiment, i.e. via grooves 202 and 204 in the corresponding webs 38 and 34 or 32 and 36 as well as via idler pairs 194, 196 arranged at the ends of holding webs 54 and 56.

The other two holding webs 58 and 60 are mounted in the manner already described and may be subjected to pressure from clamping cylinders 176 or 178 in the effective clamping direction 98 or 100 respectively.

In addition, holding webs 58 and 60 are also arranged to move transversely to their effective clampIng direction, i.e. in clamping direction 100 in the case of holding web 58 and in clamping direction 98 in the case of holding web 60, with clearance between the respective webs 34 and 38 or 36 and 32.

On the application of pressure from clamping cylinders 176 and 178, the corresponding holding webs 58 or 60 now act in the respective clamping direction 98 or 100, and at the same time, all the holding webs 54, 56, 58 and 60 are able to slide between the corresponding webs as a result of their mobility transversely to the effective clamping direction such that the stencil 20 may be uniformly clamped in both clamping directions 98, 100.

Reference is made herewith to the full contents of the explanations regarding the first and second embodiments as regards description of the remaining features of the fourth embodiment.

Claims

1. A stencil holder for metal foil stencils, comprising a frame, at least two stencil receptacles arranged on the frame to detachably hold the stencils and a stencil clamping device, wherein the stencil has retaining strips adjoining a stencil area, and these retaining strips extend at maximum over the respective length of an edge of the stencil area in the respective longitudinal direction thereof, wherein each retaining strip can be detachably fixed to one respective stencil receptacle, and wherein the stencil can be clamped by means of the stencil clamping device acting on the stencil receptacles in a first clamping direction and in a second clamping direction running transversely to the first.

2. A stencil holder according to Claim 1, wherein the stencil area and the retaining strips are made from a single piece of metal foil.

3. A stencil holder according to Claim 1 or 2, wherein it has a rectangular stencil area and has a total of four retaining strips arranged in pairs opposite one another.

4. A stencil holder according to any one of the preceding claims, wherein each retaining strip can be detachably fixed to a single stencil receptacle.

5. A stencil holder according to any one of the preceding claims, wherein each retaining strip can be detachably fixed by positive locking to the respective stencil receptacle.

6. A stencil holder according to any one of the preceding claims, wherein the stencil is fixed in the stencil receptacle through openings in the retaining strips and holding elements engaging through the openings.

7. A stencil holder according to any one of the preceding claims, wherein each stencil receptacle has a positioning device acting transversely to the clamping direction.

8. A stencil holder according to Claim 7, wherein the positioning device comprises a fit.

9. A stencil holder according to Claim 8, wherein the fit is formed by a row of holes into which corresponding retention fingers engage.

10. A stencil holder according to any one of the preceding claims, wherein two holding webs mounted opposite one another on the frame form the stencil receptacles in each clamping direction.

11. A stencil holder according to Claim 10, wherein each holding web has holding elements engaging in openings in the retaining strip.

12. A stencil holder according to Claim 10 or 11, wherein in a position fixed on the respective holding web each retaining strip is held with a holding-down means so that it lies on a locating support surface of the respective holding web.

13. A stencil holder according to Claim 12, wherein in the fastening position, the holding-down means engages with fingers over the respective retaIning strips in the areas between the holding elements in the form of locating lugs.

14. A stencil holder according to any one of the preceding claims, wherein the stencil is supported between a stencil area and a toe on the holding web by a supporting web.

15. A stencil holder according to Claim 14, wherein the holding web pulls the stencil against the supporting webs during clamping.

16. A stencil holder according to Claim 14 or 15, wherein the stencil receptacles are arranged on a side of a plane defined by the supporting webs opposite the stencil.

17. A stencil holder according to any one of Claims 14 to 16, wherein it has an outer frame formed from frame supports in which a stencil field is arranged, and the stencil field is defined by four supporting webs connected to the frame by means of struts.

18. A stencil holder according to any one of Claims 14 to 17, wherein each of the supporting webs has a supporting surface onto which the retaining strips of the stencil can be laid.

19. A stencil holder according to any one of Claims 14 to 18, wherein each retaining strip extends beyond the respective supporting web as far as the respective holding web.

20. A stencil holder according to any one of the preceding claims, wherein at least one of two holding webs effective in one clamping direction may be moved towards the clamping direction by means of the stencil clamping device.

21. A stencil holder according to any one of the preceding claims, wherein the movable holding webs are guided by a sliding guide.

22. A stencil holder according to Claim 21, wherein in the holding webs are mounted on a parallel guide effective in the respective clamping direction.

23. A stencil holder according to any one of the preceding claims, wherein the stencil clamping device has clamping elements effective in the respective clamping direction which are coupled to one another so that the same forces are applied simultaneously to the holding webs.

24. A stencil holder according to any one of the preceding claims, wherein the stencil clamping device comprises at least one clamping cylinder working between the respective holding web and the frame.

25. A stencil holder according to Claim 24, wherein the clamping cylinders are switched in parallel and connected to a pressure line.

26. A stencil holder according to Claim 25, wherein two clamping cylinders acting in parallel are provided between the respective holding web and the frame.

27. A stencil holder according to Claim 25, wherein when a stencil is clamped, the clamping cylinders are constantly subjected to pressure from a medium under pressure.

25. A stencil holder according to any one of Claims 1 to 23, wherein the clamping device comprIses a clamping winch and a clamping element.

29. A stencil holder according to Claim 28, wherein the clamping winch forms a loop effective in each case between the frame and the respective holding web.

30. A stencil holder according to any one of the preceding claims, wherein the stencil receptacles have two opposing holding webs which are displaceably guided on the frame solely in the direction of the first clamping direction, the stencil clamping device has a clamping force source acting on both holding webs simultaneously with the same clamping force, the stencil receptacles have two opposing holding webs which are displaceably guided on the frame solely in the direction of the second clamping direction, and the stencil clamping device has a clamping force source acting on the two holding webs simultaneously with the same clamping force.

31. A stencil holder according to Claim 30, wherein a single clamping force source acts on all holding webs.

A2. A stencil holder according to any one of Claims 1 to 29, wherein one holding web of the two holding webs effective in the first clamping direction is firmly connected to the frame and the holding web located opposite the fixed holding web is movable relative to the frame in the first clamping direction, and the holding webs effective in the second clamping direction are movable relative to the frame in the first clamping direction and at least one of the holding webs effective in the second clamping direction is movable relative to the frame in the second clamping direction.

33. A stencil holder according to Claim 32, wherein the holding web effective in the first clamping direction and movable therein is also movable relative to the frame in the second clamping direction.

34. A stencil holder according to any one of Claims 1 to 29, wherein at least one holding web of two holding webs effective in the first clamping direction is mounted to move relative to the frame in the first clamping direction, both holding webs effective in the first clamping direction are mounted to slide in the second clamping direction, and the holding webs effective in the second clamping direction are movable relative to the frame in the first clamping direction and at least one of the holding webs effective in tbe second clamping direction is movable relative to the frame in the second clamping direction.

35. A stencil holder according to any one of Claims 32 to 34, wherein both of the holding webs effective in the second clamping direction are mounted to move relative to the frame.

36. A stencil holder according to Claim 34 or 35, wherein one of the holding webs is mounted so as not to slide on the frame in the first clamping direction.

37. A stencil holder according to Claim 34 or 35, wherein both of the holding webs effective in the first clamping direction are mounted to move relative to the frame in this direction.

38. A stencil holder according to any one of the preceding claims, wherein the stencils are for screen printing.

39. A stencil holder substantially as herein described with reference to each of the accompanying drawings.

40. A stencil for a stencil holder, wherein the stencil has retaining strips adjoining a stencil area and that these retaining strips extend at maximum over the respective length of an edge of the stencil area in the respective longitudinal direction thereof.

41. A stencil according to Claim 39, wherein it has a rectangular stencil area and a total of four retaining strips arranged in pairs opposite one another.

42. A stencil according to Claim 40 or 41, wherein the stencil area and the retaining strips are made of a single piece of foil.

43. A stencil according to Claim 40, wherein the foil is a metal foil.

44. A stencil according to any one of Claims 40 to 43, wherein each retaining strip has a row of holes for fixing it to the respective holding web.

45. A stencil substantially as herein described with reference to each of the accompanying drawings.