AU2006316485B2

AU2006316485B2 - Flexible fine screen mat

Info

Publication number: AU2006316485B2
Application number: AU2006316485A
Authority: AU
Inventors: Franz Anibas; Hermann Fritz; Franz Renner
Original assignee: Binder and Co AG
Current assignee: Binder and Co AG
Priority date: 2005-11-24
Filing date: 2006-11-21
Publication date: 2011-05-12
Anticipated expiration: 2026-11-21
Also published as: AU2006316485C1; PL1957210T3; AT8742U1; EP1957210B1; RU2008119321A; EP1957210A1; ES2553460T3; WO2007060155A1; RU2428264C2; AU2006316485A1

Description

FLEXIBLE FINE SCREEN MAT AREA OF THE INVENTION The present invention relates to a flip-flop screening machine comprising a first moved system having a first cross-member and a second moved system having a second cross member, first and second moved systems being situated movably in relation to one another, and at least one screen mat being provided, which is fastened to both an assigned first cross-member and also an assigned second cross-member to achieve alternating compression and stretching of the screen mat, according to the preamble of Claim 1, and to a screen mat unit for a for a flip-flop screening machine having a screen mat and a clamping element provided for fastening to a cross-member of a moved system of the flip-flop screening machine and situated in the edge area, according to the preamble of Claim 12. Flip-flop screening machines are already known. They are used in both the coarse grain and also the fine grain ranges and are distinguished by compression and stretching of the screen mat, which has the result that the stuck grain formation is significantly less than in screen machines having unmoved screen mat. PRIOR ART Typical screen mats are manufactured in most cases from polyurethane, the screen openings being produced by punching. These screen mats produced in a typical manner are distinguished by robustness and long-term resilience, both criteria which are decisive for use in flip-flop screening machines, because alternating strains arise due to the alternating stretching and compression of the screen mats, which negatively influence the long-term resilience of the materials used. However, the punching of the screen openings is a time-consuming and also costly procedure which requires a high production time for a screen mat. In addition the 2 diameter of the screen openings may not be reduced arbitrarily during punching for technological reasons or in other words a limiting value exists for the dimensions of the screen openings, from which it is no longer possible to punch them, both in regard to the technology per se and also in regard to cost-effectiveness. In practice, this limiting value is a mesh width of approximately 1 mm for square screen openings and approximately 0.4 mm x 4 mm for slotted screen openings. However, practice has shown that sufficient demand exists for screening machines for extremely fine-grained screened product, no adequate solution having been found until now to respond to this need appropriately in the field of flip-flop screening machines because of the cited technological difficulties. For example, a flip-flop screening machine having two moved cross-member systems is known from US 4,430,211, a first and a second moved system being situated so they are movable in relation to one another, plastic screen mats being provided which are each fastened to an assigned first cross-member and also to an assigned second cross-member to achieve alternating compression and stretching of the screen mats. The screen mats are fixed on the cross-members in this case using conventional clamping bars. Providing screen mats made of wire fabric is known from other fields of screening, but not in the field of flip-flop screening machines. Such a screen mat is known, for example, from DE 38 11 641 Al. These screen mats to allow the production of especially small screen openings, but their use in a flip-flop screening machine is not possible because of the material, however, because the alternating compression and stretching of the screen mats occurring in a flip-flop screening machine is not possible with screen mats made of wire fabric and/or results in breakage in a short time. Using fine screen mats made of plastic fabric, such as polyamide fabric, is known from other fields of screening for especially fine-grained screened product. These fine screen mats do offer the possibility of especially fine-meshed implementation of the screen mats because of their manufacturing as a fabric, in contrast to the punched polyurethane 3 solution, which is also accompanied by a significantly larger open screen area, which, inter alia, also results in a small overall size of screening machines equipped with such fine screen mats. However, the fine screen mats manufactured from plastic fabric have the problem of insufficient ultimate elongation for use in flip-flop screening machines, so that until now such plastic fabric has not been used in flip-flop screening machines. As already noted, the screen mats are alternately compressed and stretched in flip-flop screening machines to avoid stuck grains, the screen mats being so overstretched during each stretching procedure following a compression procedure that the occurring whipping effect almost entirely prevents stuck grain formation and clears open clogged screen openings. In typical screen mats made of polyurethane used in flip-flop screening machines, this occurring whipping effect is not problematic because polyurethane has an ultimate elongation of approximately 400%. The term ultimate elongation is a term used for elastomers which is referred to as elongation at fracture for other materials and specifies how many percent a material may be stretched before it breaks. Because of the high ultimate elongation, polyurethane screen mats have proven to be insensitive to the whipping effect occurring upon use in flip-flop screening machines. The fabric made of plastic used in other screening machine fields as screen mats, only has an ultimate elongation of approximately 30% to 40%, however, because of which such screen mats would tear if such screen mats were used in flip-flop screening machines and the screen mats were overstretched in this manner and/or if the whipping effect occurred. Dispensing with the use of the whipping effect would drastically reduce the screening performance because of increased occurrence of stuck grains and would require shutdown times, during which the stuck grains have to be removed manually from the screen. DESCRIPTION OF THE INVENTION 4 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge 5 in the field relevant to the present invention as it existed before the priority date of each claim of this application. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or 10 step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. It is therefor the object of the present invention to prevent this disadvantage and to improve known flip-flop screening machines in such a manner that they are also usable 15 for extremely fine-grained screened product. It is a further object of the invention to adapt flip-flop screening machines in such a manner that the use of fine screen mats made of plastic is possible without destroying the screen mats and/or without reducing their long-term resilience to values 20 unacceptable in practice. According to one aspect of the present invention there is provided a flip-flop screening machine comprising a first moved system having a first cross-member and a second moved system having a second cross-member, first and second moved systems being 25 situated so they are movable in relation to one another and at least one screen mat manufactured from plastic fabric being provided, which is fastened to both an assigned first cross-member and also an assigned second cross-member to achieve alternating compression and stretching of the screen mats wherein the screen mat is implemented as a fine screen mat and is connected via at least one attachment element to the 30 assigned first cross-member and/or the assigned second cross-member, which has a greater ultimate elongation than the fine screen mat manufactured from plastic fabric. According to a further aspect of the present invention there is provided a screen mat unit for a flip-flop screening machine having a screen mat manufactured from plastic 35 fabric and a clamping element, provided for fastening to a cross-member of a moved system of the flip-flop screening machine and situated in the edge area wherein the 4A screen surface is implemented as a fine screen mat manufactured from plastic fabric, and a clamping element comprises an attachment element, which has a greater ultimate extension that the fine screen mat. 5 In flip-flop screening machines known perse, comprising a first moved system having a first cross-member and a second moved system having a second cross-member, first and second moved systems being situated so they are movable in relation to one another and at least one screen mat manufactured from plastic fabric being provided, which is fastened to both as assigned first cross-member and also an assigned second 10 cross-member, to achieve alternating compression and stretching of the screen mats, the screen mat is implemented as a fine screen mat and is connected via at least one attachment element in each case to the assigned first cross-member and/or the assigned second cross-member, which has a greater ultimate elongation in comparison to the fine screen mat manufactured from plastic fabric. 15 Because of the already noted low ultimate elongation of the plastic fabric in comparison to the punched polyurethane screen mats, by positioning one or more elastic attachment elements between fine screen mat and first and/or second moved system and/or between fine screen mat and assigned first cross-member and/or assigned 20 second cross-member, the whip stroke effect is damped, without preventing its effect of cleaning openings, however. The exact tuning of the material of the attachment element and/or of the ultimate elongation of the material (for example, via the modulus of elasticity) of the attachment element is important. This must be selected in such a manner that the 25 5 suddenly occurring strain during alternating compression and stretching of the fine screen mats is not too strongly damped, so that the whip stroke effect is prevented, because of which stuck grains would not be cleaned out. On the other hand, the material may also not be selected so that too strong oscillation of the fine screen mats occurs. According to a preferred embodiment variant of the invention as specified in Claim 2, at least one attachment element is a spring element, a typical coiled spring being able to be used here, for example, which fastens the fine screen mat to the first moved system, for example, to a screen box and/or to the second moved system, for example, to an oscillating frame, via the particular cross-member. According to a further preferred embodiment of the invention as described in Claim 3, the attachment element is a clamping element made of plastic, which is fastenable to the assigned first cross-member and/or the assigned second cross-member and in which the fine screen mat is embedded. This embodiment variant is distinguished by robustness in particular. A further preferred embodiment of the invention according to Claim 4 provides a plastic bed as the attachment element, which is embedded in a receptacle of a clamping element fastenable to the assigned first cross-member and/or the assigned second cross-member. The, fine screen mat is in turn embedded in this plastic bed. The clamping element may certainly be manufactured from a harder material in this case and not have the ultimate elongation and/or elasticity required for the present application in a flip-flop screening machine, because the element damping the whip effect in this embodiment variant is the plastic bed implemented in the clamping element, in which the fine screen mat is embedded. The fastening of the fine screen mat in the plastic bed may be performed by casting the fine screen mat in plastic or by vulcanizing or gluing on, for example. According to the characterizing features of Claim 5, the receptacles are implemented as slotted, by which particularly simple fastening of the fine screen mats in the receptacle and/or in the plastic bed located therein is possible.

6 According to a further preferred embodiment variant as described in Claim 6, the attachment element and the fine screen mat form a replaceable screen mat unit. It is thus possible to already prefinish such a screen mat unit and fasten it, clamp it, for example, to the flip-flop screening machine on location. In particular if a clamping element is provided, its implementation may already have a shape corresponding to the fastening capabilities, in particular the cross-member, on the flip-flop screening machine, so that rapid and nonetheless secure fastening of the screen mat unit to the flip-flop screening machine is possible. According to another preferred embodiment variant of the invention as provided in Claim 7, the attachment element is a preferably separate elastic element fastened to the assigned first cross-member and/or the assigned second cross-member. In this case, the fine screen mat made of plastic fabric may be fastened directly to the assigned first cross-member and/or the assigned second cross-member. In such a case, according to Claim 8, fastening means are provided which connect the elastic element on one hand to the fine screen mat and on the other hand to the assigned first cross-member and/or the assigned second cross-member. According to a further preferred embodiment of the invention as described in Claim 9, and at least one of the two thread groups comprising warp and weft of the plastic fabric forming the fine screen mat is situated running diagonally to the cross-members, i.e., neither parallel nor perpendicular thereto. In this manner, the plastic fabric has a higher ultimate elongation and/or elasticity in the compression or stretching direction of the flip flop screening machine than would be possible without an inclined position. The attachment element may thus be implemented as somewhat stiffer without the danger that the fine screen mat made of plastic fabric will tear. In a special embodiment according to Claim 10, the fine screen mat made of plastic fabric itself functions as the attachment element and the required ultimate elongation for use of 7 the fine screen mat in flip-flop screening machines is produced by corresponding inclined position of warp and weft in relation to the cross-member. In this case, an additional attachment element for fastening the fine screen mat to the cross-members is no longer required and the fine screen mats made of plastic fabric may be fastened to the cross members in a typical manner. According to the characterizing features of Claim 11, the mesh width of the fine screen mats made of plastic fabric is less than 2 mm x 2 mm for square screen openings and/or 2 mm x X millimeters for oblong hole openings (X = greater than 2 mm). Claims 12 through 15 relate to a screen mat unit for a flip-flop screening machine having a screen mat and a clamping element situated in the edge area provided for fastening to a cross-member of a moved system of the flip-flop screening machine, according to the present invention, the screening surface is implemented as a fine screen mat manufactured from plastic fabric and the clamping element comprises an attachment element which has a greater ultimate elongation than the fine screen mat. The positioning of the attachment element used as a damping element for the whipping effect directly on the screen mat made of plastic fabric allows the use of fine screen mats made of plastic fabric having correspondingly smaller mesh width on flip-flop screening machines, attachment element and fine screen mat forming a replaceable unit, which makes replacement of the entire screen mat unit on the flip-flop screening machine easier. According to a preferred embodiment variant of the invention as described in Claim 13, the clamping element itself, which produces the connection between fine screen mat and the particular cross-members, is implemented as an attachment element having greater ultimate elongation. According to another preferred embodiment variant as described in Claim 14, the clamping element has a preferably slotted receptacle having a plastic bed which is used as an attachment element for the fine screen mat made of plastic fabric and to which the fine screen mat is connected, for example, by embedding.

8 BRIEF DESCRIPTION OF THE FIGURES The invention is described in greater detail hereafter on the basis of an exemplary embodiment. In the figures: Figure 1 shows an axonometric view of a flip-flop screening machine Figure 2 shows a schematic side view of a screen mat unit, comprising screen mat and attachment element, having spring element Figure 3 shows a schematic side view of a screen mat unit, comprising screen mat and attachment element, having clamping element and plastic bed Figure 4 shows a schematic side view of a screen mat unit, comprising screen mat and attachment element, having clamping element without plastic bed Figure 5 shows a schematic side view of a screen mat unit in a state fastened to cross-members Figure 6 shows a top view of a screen mat unit Figure 7 shows an alternative embodiment variant of a clamping element Figure 8 shows a schematic side view of a screen mat unit in a state fastened to cross-members Figure 9 shows a schematic side view of a screen mat in a state fastened to a cross member having separate attachment element Figure 10 shows a top view of a screen mat unit having inclined warp and/or weft 9 Figure 11 shows a top view of a screen mat unit having inclined warp and weft WAYS OF IMPLEMENTING THE INVENTION Figure 1 shows, as a preferred construction, a flip-flop screening machine I having a screen box 3 having screen jaws 3a, 3b as the first moved system, which is supported on springs 15 in relation to a fixed foundation and a drive 16 which sets the screen box 3 into oscillations, which may be implemented as a single or double unbalanced drive. The screen jaws 3a, 3b are connected to one another via multiple screen box cross-members 2 situated parallel to one another, forming first cross-members. A second moved system is implemented on the screen box 3, in the present case as an oscillating frame, which comprises the screen jaws 3a, 3b and/or pushrods 17 mounted on strips 18 situated thereon. The pushrods 17 are mounted via elastic elements, such as rubber blocks or spring elements (neither shown), so that the fundamental oscillation of the screen box 3 excites the oscillating frame into a further, amplitude-excited oscillation. The pushrods 17 are connected to one another via oscillating frame cross-members 4, which are situated parallel to one another and parallel to the screen box cross-members 2 and form second cross-members. The screen surface is constructed by individual screen mats 5, which are fastened alternately to one oscillating frame cross-member 4 and one screen box cross-member 2. The mode of operation of the flip-flop screening machine is as follows: the oscillating frame is also set into oscillations by the screen box 3, which is set into oscillation using the drive 16 implemented as an imbalanced drive. Due to the elastic mounting of the pushrods 17 of the oscillating frame on the screen box 3, an oscillation of the oscillating frame which is offset to the fundamental oscillation of the screen box 3 arises. Due to the alternating fastening of the screen mats 5 to the screen box 3 and to the oscillating frame 10 or more precisely to the screen box cross-members 2 and to the oscillating frame cross members 4, the screen mats 5 are alternately compressed and stretched or overstretched, which not'only causes the screening of the screened product, but rather also prevents the formation of stuck grains. This in turn causes an optimum screening action. The present exemplary embodiment shows a screening device having one screen level. It is obvious that it is also possible to situate multiple screen levels one above another and this is also typical in flip-flop screening machines. Either multiple pushrods 17 are elastically mounted on the screen box 3 or multiple oscillating frame cross-members 4 running in different levels are mounted on one pushrod 17. The present oscillating system may also be implemented in a banana implementation having continuously decreasing screen deck inclination. To also be able to use flip-flop screening machines in the fine grain range, according to the invention, at least one of the screen mats 5 is implemented as a fine screen mat made of plastic fabric which is connected via at least one attachment element 6, 7, 8 to an assigned oscillating frame cross-member 4 and/or an assigned screen box cross-member 2, this attachment element 6, 7, 8 having greater ultimate elongation than the fine screen mat 5 manufactured from plastic fabric. In general, one may differentiate between two different embodiment principles, namely on one hand the principle in which the fine screen mat forms a replaceable screen mat unit with the attachment element. In this embodiment principle, the attachment element and the fine screen mat are connected to one another, preferably inseparably. The second embodiment principle provides that the attachment element damping the whipping effect is an element situated on the corresponding screen box cross-member 2 or oscillating frame cross-member 4, whose ultimate elongation is greater according to the invention than that of the fine screen mat.

11 Figure 2, Figure 3, and Figure 4 each show exemplary embodiments which operate according to the first embodiment principle. Figure 2 shows the schematic illustration of a fine screen mat 5 which is connected via an attachment element implemented as a spring element 6 to a clamping element 12 known per se, on which no requirements are placed other than that it must be fastenable to a cross-member 2, 4 of one of the two moved systems. Figure 3 shows a screen mat unit having a fine screen mat 5 and a clamping element 10 which has a slotted receptacle 11. An attachment element implemented as a plastic bed 7 is located in the slotted receptacle 11, which is embedded in the receptacle or fastened therein in another way. The ends of the plastic screen fabric are embedded in the plastic bed 7. The clamping element 10 may also be manufactured in this case from a material having lower ultimate elongation, because the plastic bed 7 is provided as a damping elastic element having appropriate ultimate elongation, which contributes to the occurring whipping effect not resulting in destruction of the plastic fabric of the fine screen mat. Figure 4 shows a further embodiment variant of the first embodiment principle. An attachment element implemented as a clamping element 8 is cast directly with the fine screen mat 5 and forms the attachment element. In this embodiment variant, it is therefore necessary for the clamping element to have the required ultimate elongation, which must be higher according to the invention than the ultimate elongation of the plastic fabric forming the fine screen mat 5. In Figures 2 through 4, the clamping elements 8, 10, 12 each have a special design, which allows the secure and rapid clamping thereof on the screen box cross-members 2 and/or oscillating frame cross-members 4, as shown in Figure 5. The clamping elements 8, 10, 12 have a corresponding recess 19 for this purpose, using which they may be plugged onto the screen box cross-members 2 and/or- oscillating frame cross-members 4. For example, one or more pins 25 or a strip 25 is provided for spreading the clamping elements 8, 10, 12~on the screen box cross-members 2 and/or oscillating frame cross- 12 members 4, so that the clamping elements 8, 10, 12 are fastened unshakably on the screen box cross-members 2 and/or oscillating frame cross-members 4. The clamping elements as shown in Figures 2 through 4 are one possible embodiment variant. Of course, other embodiment variants are also conceivable, which allows secure fastening of the screen mat units to the screen box cross-members 2 and/or oscillating frame cross-members 4. For example, Figure 7 shows such an alternative embodiment variant of a clamping element. The screen mat unit comprises fine screen mat 5 and clamping element 20, the clamping element 20 having a different shape than shown in Figures 3 and 4, but having an identical construction, i.e., either having a receptacle having a plastic bed 7, in which the attachment element is embedded, or forming the attachment element itself and correspondingly being manufactured as elastic. The clamping element 20 is implemented flatly in contrast to the clamping elements 8, 10 of Figures 2, 3, however. In this case, a fastener 14, such as a screw, as shown in Figure 8, is used for fixing on the screen box cross-members 2 and/or oscillating frame cross-members 4. Figure 6 shows a fine screen mat 5 according to the invention made of plastic fabric as shown in Figures 2 through 5 and Figures 7 and 8 in a top view. The clamping element 8, 10, using which the fastening to the screen box cross-members 2 and/or oscillating frame cross-members 4 is performed, extends in the longitudinal direction of the fine screen mat along its edge area. Figure 9 shows an embodiment variant according to the second embodiment principle. The attachment element 13 is fixed as a preferably separate element to a screen box cross-member 2 and/or oscillating frame cross-member 4 via a fastener 14, such as a screw. The fastener 14 is also simultaneously used for direct fixing of the fine screen mat 5, which does not have to have a clamping element in this case, to the screen box cross member 2 and/or oscillating frame cross-member 4 using clamping between a binding element 21 and the attachment element 13. In this manner, a very elastic attachment of 13 the fine screen surface made of plastic fabric to the screen box cross-member 2 and/or oscillating frame cross-member 4 is also achieved. Figures 10 and 1 show a top view of a fine screen mat 5 made of plastic fabric according to a further preferred embodiment of the invention. The screen box cross-member 2 and/or oscillating frame cross-member 4 are schematically indicated for orientation. The thread group of the fabric forming the warp 23 is situated running diagonally to the screen direction 24 instead of parallel, as shown in Figure 10. In a further embodiment variant shown in Figure 11, in addition, the thread group forming the weft 22 is also situated running diagonally to the screen box cross-members 2 and/or oscillating frame cross-members 4. A higher elasticity of the plastic fabric is achieved in the screen direction 24 by the diagonally running configuration and the ultimate elongation is thus increased. The clamping elements 8, 10 shown in Figures 10 and 11 are no longer necessary with sufficiently inclined position of the thread groups, so that in this special case the plastic fabric itself forms the attachment element. In such a case, an additional elastic attachment element for attaching the fine screen mat made of plastic fabric to the screen box cross-member 2 and/or oscillating frame cross number 4 is no longer necessary. However, such fine screen mats having diagonal weft 22 and/or warp 23 may also be combined with clamping elements 8, 10 as shown in Figures 3 and 4 and/or with spring elements 6 as shown in Figure 2.

Editorial Note Case #2006316485 Due to amendments to the specification, page 14 has been deleted. The claims that follow this description begin on page 15.

Claims

1. A flip-flop screening machine comprising a first moved system having a first cross-member and a second moved system having a second cross-member, first and 5 second moved systems being situated so they are movable in relation to one another and at least one screen mat manufactured from plastic fabric being provided, which is fastened to both an assigned first cross-member and also an assigned second cross member to achieve alternating compression and stretching of the screen mats wherein the screen mat is implemented as a fine screen mat and is connected via at least one 10 attachment element to the assigned first cross-member and/or the assigned second cross-member, which has a greater ultimate elongation than the fine screen mat manufactured from plastic fabric.

2. The flip-flop screening machine according to Claim 1, wherein the attachment 15 element is a spring element.

3. The flip-flop screening machine according to Claim 1, wherein the attachment element is a clamping element made of plastic, which is fastenable to the assigned second cross-member and/or the assigned first cross-member and in which the fine 20 screen mat is embedded.

4. The flip-flop screening machine according to Claim 1, wherein the attachment element is a plastic bed embedded in a receptacle of a clamping element fastenable to the assigned first cross-member and/or the assigned second cross-member, in which the 25 fine screen mat is embedded.

5. The flip-flop screening machine according to Claim 4, wherein the receptacle is implemented as slotted. 30

6. The flip-flop screening machine according any one of Claims I to 5, wherein the attachment element and fine screen mat form a replaceable screen mat unit.

7. The flip-flop screening machine according to Claim 1, wherein the attachment element is a preferably separate elastic element situated on the assigned second cross 35 member and/or first cross-member. 16

8. The flip-flop screening machine according to Claim 1, wherein fasteners are provided, which connect the elastic element on one hand to the fine screen mat and on the other hand to the assigned first cross-member and/or the assigned second cross member. 5

9. The flip-flop screening machine according any one of Claims I to 8, wherein at least one of the two thread groups, comprising warp and weft, of the plastic fabric forming the screen mat is situated running diagonally to the cross-members and/or to the conveyance direction. 10

10. The flip-flop screening machine according to Claim 1, wherein the attachment element is the screen mat itself and at least one of the two thread groups, comprising warp and weft, of the plastic fabric forming the screen mat is situated running diagonally to the cross-members. 15

11. The flip-flop screening machine according any one of Claims I to 10, wherein the mesh width of the fine screen mats is less than 2 mm.

12. A screen mat unit for a flip-flop screening machine having a screen mat 20 manufactured from plastic fabric and a clamping element, provided for fastening to a cross-member of a moved system of the flip-flop screening machine and situated in the edge area of the screen mat wherein the screen surface is implemented as a fine screen mat manufactured from plastic fabric, and the clamping element comprises an attachment element, which has a greater ultimate extension than the fine screen mat. 25

13. The screen mat unit according to Claim 12, wherein the clamping element itself is implemented as the attachment element.

14. The screen mat unit according to Claim 12, wherein the clamping element has a 30 preferably slotted receptacle having a plastic bed as the attachment element, to which the fine screen mat is connected, preferably embedded.

15. The screen mat unit according to one of Claim 1 1 through 14, wherein the mesh width of the fine screen mats is less than 2 mm. 35 17

16. A flip-flop screening machine as hereinbefore described with reference to the accompanying drawings.

17. A screen mat unit for a flip-flop screening machine having a screen mat 5 manufactured from plastic fabric and a clamping element as hereinbefore described with reference to the accompanying drawings.