EP0960731A1

EP0960731A1 - An apparatus for tensioning silk screens

Info

Publication number: EP0960731A1
Application number: EP98830328A
Authority: EP
Inventors: Sergio Tosi; Gianfranco Magnani
Original assignee: Serimec di Magnani Gianfranco e C SNC
Current assignee: Serimec di Magnani Gianfranco e C SNC
Priority date: 1998-05-27
Filing date: 1998-05-27
Publication date: 1999-12-01

Abstract

The tensioning apparatus is destined for use with silk screens essentially constituted by a flat frame (10) on which a screen material (2) or sieve is arranged. It comprises means for fixing the screen material (2) peripherally on the frame (10), and means for tensioning, associated to the frame (10), which operate between the frame (10) and the screen material (2) in order to tension the screen material (2).

Description

Specifically, though not exclusively, the invention is useful in the manufacturing of flat silk screens which are essentially constituted by a flat framework on which the sieve or screen is stretched.
A correct tensioning of this material is fundamental to all silk screen applications.
In the prior art, this tensioning is usually carried out in one of two ways:
a) with special fixed apparatus;
b) by means of mechanically-adjusted self-tensioning frames.
Fixed apparatus are rather unwieldy machines, having special gripping organs by means of which the material is gripped at edges thereof and brought to, and kept at, a predetermined tension.
Once the material is suitable tensioned, a frame is fixed to it, normally by use of glue.
The prior art as above contains serious limitations and drawbacks.
A first of these limitations is connected with the fact that the fixed tensioning apparatus are very laborious to use, being heavy, quite massive and slow to a significant degree.
A drawback which is a direct consequence of the tensioning operation consists in the differentiated deformation the frame is subject to when, after having been fixed to the stretched material, the material is detached from the gripping organs it was stretched in.
The problem is manifested because the sides of the frame, normally straight, are fixed to the already-stretched material when the frame is not subject to any forces, and it is only after the fixing procedure (when the silk screening material is detached from the gripping organs it was stretched by) that the frame becomes subject to stress forces, especially flexional forces, originating from the material itself, which tends to return elastically into the original undeformed configuration.
A further typical drawback of the prior art consists in the impossibility in practice of readjusting and/or keeping a silk screen at the predetermined tension.
Mechanically-adjustable self-tensioning frames need no such unwieldy external tensioning apparatus but are structured in such a way that the material can be fixed to the frame itself (to a sort of counterframe) and brought up to the desired tension through successive mechanical adjustments actuated by means of screw devices.
With these self-tensioners the stretching can be done one side at a time, but cannot be preadjusted and furthermore does not enable a uniform tension to be achieved except with great difficulty.
The main aim of the present invention is to create an apparatus which is not external, but rather stably associated to the frame of the screen, and which obviates the above-mentioned drawbacks and limitations inherent in the prior art.
An advantage of the invention consists in the possibility of regulating the tension of the screen material constantly and uniformly at least in the two main directions, longitudinal and transversal, independently of the deformability and the real deformations of the frame.
A further advantage of the invention consists in the simplicity and the basic ease with which the tension of the screen material can be adjusted.
A still further advantage of the invention is that the whole tensioning operation is made extremely rapid.
These aims and advantages and more besides are all attained by the object of the invention, as it is characterised in the appended claims.
Further characteristics and advantages of the present invention will better emerge from the detailed description that follows of some preferred but non-exclusive embodiments of the invention, illustrated purely by way of non-limiting examples in the accompanying figures of the drawings, in which:
figure 1 is a schematic plan view from above;
figure 2 is a schematic section, made according to line II-II of figure 1, wherein two different operative configurations are shown, relative to a first embodiment of the invention;
figure 3 is a schematic section, made along line III-III of figure 1, showing two different operative configurations, relative to a first embodiment of the invention;
figure 4 shows, in the same section of figure 2, a second embodiment of the invention:
figure 5 shows, in the same section of figure 2, a third embodiment of the invention;
figure 6 shows, in the same section of figure 2, a fourth embodiment of the invention;
figure 7 shows, in the same section of figure 2, a further embodiment of the invention.
With reference to the figures, 1 denotes in its entirety a flat silk screen constituted by a flat silk screen frame constituted by a flat frame 10 in a form somewhat like a picture frame, on which the screen material or sieve 2 is fixed.
The frame 10 is a structure, normally made of metal, constituted by sections 3 or 3' which are straight and connected at the ends thereof consecutively, with the interpositioning of connection portions.
The sections 3 and 3' are predisposed so that at least one expansion chamber 4 is located on at least a part of the external border of the frame 10.
In the first illustrated embodiment, represented in figure 2, the chamber 4 is formed by a tubular element 5 predisposed longitudinally on the external border of the frame 10 in a special longitudinal grooved seating 11.
The tubular element 5 is elastically deformable 5 and suitable for sealedly and internally receiving a pressurised fluid.
The fluid, which can be, according to needs, gassy (air) or liquid, has the task of causing the expansion of the tubular element 5 so that the external-facing mobile portions of the element surface (with respect to the frame) are caused to displace externalwise of the frame 10, away from the seatings 11 of the sections 3.
Means for fixing the screen material 2 are located at the lower side of the sections 3, opposite to the upper side where the material 2 is stretched. The means for fixing comprise a groove 12 into which the screen material is introduced and fixed by means of a small rod 13 insertable and press-jointable in the groove 12 itself. Once the screen material is sketched on the frame 10 and fixed by means of pressing the rod 13 into the groove 12, the screen can be tensioned simply by introducing pressurised fluid into the tubular elements 5. The externally-facing mobile surfaces of the tubular elements 5, are forced to displace ever more externalwise, pressing against the facing portions of screen material 2 and thus progressively tensioning the screen.
The above-described process can advantageously be operated on all the frame 10 sides, operating only by means of a fluid-actuated system.
In the very common case of square or rectangular screens it is easy to understand how a constant and uniform tension can be achieved in the two principal direction (longitudinal and transversal). The constancy of the tension is guaranteed by keeping constant the fluid pressure inside the element 5 or the tubular elements 5 provided on the frame 10. Uniformity is guaranteed by the fact that the screen material is stretched by an evenly-distributed load along the entire contact zone with the external surfaces of the tubular elements 5.
Furthermore, the considerable elastic deformability of said tubular elements 5 is able further to guarantee a uniform stretching action on the screen material 2 even where the material itself has not been sufficiently homogeneously and carefully fixed to the frame 10.
In all cases it is possible to carry out a very accurate stretching adjustment and obtain a perfect tensioning corresponding exactly to the desired degree.
In a second embodiment of the invention, the mobile surfaces, localised at the external edge of the frame 10, are constituted by outward-facing surfaces of at least one relatively-rigid mobile element 6 coupled on the single sections 3 forming the frame 10 and slidable in at least one direction which is parallel to the frame 10 plane. In this case the exapansion of the chamber 4, constituted in the example by the tubular element 5, pushes the mobile element 6 externalwise. Thus it its the mobile surfaces, i.e. those facing externally, which displace by pressing against the facing portions of screen material 2 so as progressively to tension the screen.
In a third embodiment, illustrated in figure 5, the configuration is similar to that of the second embodiment, but the mobile element 6 is substituted by a differently-conformed mobile element 6' constituted by a section exhibiting a C-shaped transversal section, coupled slidably on the parallel and opposite surfaces of the section 3.
A variant of the embodiment of figure 5 is shown in figure 6, where instead of the mobile element 6' there is a mobile element 14 constituted by a closed section slidably internally housing the section 3. In this case the screen material 2 must be fixed on the mobile element 14 which for the purpose is specially provided with a longitudinal cavity 15 for anchoring the material 2 similarly to the previously-described embodiments.
In a further embodiment, illustrated in figure 7, the mobile surfaces, localised at the external edge of the frame 10 and predisposed to interact with the corresponding facing portions of material 2, are constituted by the external surfaces of at least one rigid mobile element 9 which is hinged longitudinally to a section 3' - constituting the structure of the frame 10 - by means of a pin 16. In this case, during expansion, the chamber 4, once more constituted by a tubular element 5, produces a rotation of the mobile element 9 about the pin 16 which, by distancing the mobile element 9 from the section 3', tensions the screen material 2.
All the illustrated embodiments of the invention enable a rapid, efficient and accurate adjustment of the material. The tensioning is also constant and uniform and dispenses with the need for laborious and unwieldy external machinery.
The method for controlling the screen tension is extremely simple, as it is merely necessary to control the fluid pressure inside the expansion chambers.

Claims

An apparatus for tensioning silk screens, said screens being essentially constituted by a flat frame (10) on which a screen material (2) or sieve is stretched, characterised in that it comprises, associated to said frame (10) means for fixing the screen material (2) peripherally to the frame (10), and means for tensioning, operating between the frame (1) and the screen material (2) in order to stretch a part of the screen material (2) comprised internally of a perimeter of the frame (10).
The apparatus of claim 1, characterised in that said means for tensioning comprise mobile surfaces located in an external edge of said frame (10), which mobile surfaces are in contact with corresponding portions of the screen material (2); said mobile surfaces acting on command on corresponding portions of the screen material (2) in order to produce a displacement thereof, a direction of the displacement being outwards from a central zone of said frame (10).
The apparatus of claim 2, characterised in that the frame (10) is constituted by a section (3, 3') predisposed in such a way that at least one expansion chamber (4) is situated on at least a part of the external edge of the frame, an expansion of said expansion chamber (4) being able to produce an outwards displacement of said mobile surfaces.
The apparatus of claim 3, characterised in that the expansion of the at least one expansion chamber (4) is produced by means of introducing a pressurised fluid into said at least one expansion chamber (4).
The apparatus of claim 4, characterised in that the at least one expansion chamber (4) is formed by a tubular element (5) predisposed longitudinally on the external edge of said frame (10).
The apparatus of claim 5, characterised in that said tubular element (5) constituting the at least one chamber (4) is made of a material characterised by having a high degree of elastic deformability.
The apparatus of claim 6, characterised in that said mobile surfaces located at the external edge of said frame (10) coincide with the portions of external surface of said tubular element (5) which face externalwise of said frame (10) and which are in contact with corresponding portions of the screen material (2) predisposed on the frame (10).
The apparatus of any one of claims 4, 5 or 6, characterised in that said mobile surfaces located at the external edge of said frame (10) are constituted by external surfaces of at least one rigid mobile element (6, 6') coupled on the sections (3) forming said frame (10) and being slidable in at least a parallel direction to the frame (10) plane; said at least one expansion chamber (4) being arranged so as to operate between said section (3) and the mobile element (6, 6').
The apparatus of claim 8, characterised in that said mobile surfaces located at the external edge of said frame (10) are constituted by external surfaces of at least one rigid mobile element (6), which is constituted by a section housed slidably in a groove (8) longitudinally arranged in the external edge of the section (3) forming the frame (10); said at least one expansion chamber (4) being arranged in such a way as to operated between said section (3) and said mobile element (6).
The apparatus of claim 8, characterised in that said mobile element (6') has a C-shaped section; arms of said C-shaped section embracing and being coupled slidably on parallel and opposite surfaces of the section (3) constituting said frame (10).
The apparatus of any one of claims 4, 5 or 6, characterised in that said mobile surfaces located at the external edge of said frame (10) are constituted by external surfaces of at least one rigid mobile element (9) which is hinged longitudinally to the section (3') constituting the frame (10); said at least one expansion chamber (4) being arranged in such a way as to operate between said section (3') and said mobile element (9).