GB2157012A - Suction platen - Google Patents

Abstract

A suction platen comprises a thin sheet 1 of glass defining, together with a gas-impervious membrane 3, a suction enclosure to receive a thermal stencil 7 and an original document 6 which are to be together exposed to radiation for imaging the stencil. A resilient backing sheet 5 of open cell foamed material allows suction applied at a hose 8 to be communicated to all parts of the suction enclosure with a uniform contact pressure everywhere over the stencil and sheet. <IMAGE>

Description

SPECIFICATION Suction platen for document exposure The present invention relates to a suction platen for exposing documents to radiation, for example optical light. This platen is particularly useful in photographic work.

It is known to use a suction to hold a document flat for photographic exposure, and to do so by holding the document in firm contact with a transparent support plate, for example a sheet of glass.

In such case the document may be held by the application of suction to a space bounded on one side by the underside of the transparent support plate, and on the other side by a flexible membrane, such that the document held between the membrane and the glass wil be pressed against the glass by the action of the suction.

A problem in such prior art arrangements is to provide for uniform application of suction across the whole surface area of the glass or other transparent support sheet, and in the past this has been achieved by providing the contiguous surface of the flexible membrane with a dimpled profile which provides for minature air passages to distribute the suction uniformly over the surface of the plate. However, such dimpled backing membranes have the disadvantage of providing localized areas of higher contact pressure (in register with the dimples) and other areas of lower pressure, It is an object of the present invention to provide a suction platen in which a uniform contact pressure results from the uniform application of suction across the platen.

Accordingly, the present invention provides a suction platen for document exposure, comprising a transparent sheet, a gas-impervious membrane in association with the sheet so as to define therewith a suction enclosure to receive a document to be held and exposed through the transparent sheet, and a resilient backing sheet of open cell foamed construction in the suction enclosure between the membrane and the transparent sheet, whereby application of suction to one area of said suction enclosure can be evenly communicated over the whole of the extend of that suction enclosure.

Preferably, the membrane is a plain sheet of gasimpermeable material held in contact with the perimeter of the transparent support plate by means of a resilient peripheral strip on which the membrane is laid or held.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to one embodiment of a suction platen in accordance with the present invention.

In the sole Figure the transparent support plate 1 is a thin sheet of suitable glass which provides maximum transmissivity to the irradiating light coming from a lamp 2 schematically illustrated in the drawing.

In order to complete a suction enclosure of which one side if formed by the glass 1, an air-impervious membrane 3 is placed underneath the glass sheet and rests on a peripheral strip 4 of resilient material, for example a strip of foamed plastics material. In practice the glass plate 1 is rectangular and has the strip 4 extending along all four sides thereof.

Resting on the gas-impervious membrane 3 everywhere apart from its perimeter is an open cell foamed plastic sheet 5 on which rests an original 6 to be used for image-wise conversation of stencil material 7 to provide ink-pervious areas of the stencil in registration with dark areas of the original 6 and ink-impervious areas in registration with lighter non-image areas of the original 6.

Although for any contact printing operation it is important for the original and the copy medium to be in intimate contact, this is particularly important when imaging thermal stencils if only small amounts of radiant energy are to be applied to the original 6 because the absorption of the radiant energy by those black image areas will generate only a small amount of heat which is intended to be transferred by conduction directly to the thermally sensitive stencil medium 7. Thus, intimate contact is important for optimum imaging. If the sheets are not in contact, then the intensity of the radiant energy must be increased through partial conversation of the non-image areas.

In order to apply suction to the enclosure formed by the membrane 3 and the glass sheet 1, the membrane includes an integral suction hose 8 through which air is extracted, as shown by the arrow, by means not shown.

The purpose of the open cell foamed material 5 between the membrane 3 and the glass sheet 1 is to communicate the suction applied by the hose 8 at the righthand side of the drawing along the whole of the central area of the suction enclosure.

It is clear that the suction can communicate with all peripheral areas of the suction enclosure, but for adequate contact pressure between the stencil 7 and the original 6 near the centre of these sheets it is important for that peripheral suction to be applied also centrally. The open cell construction of the sheet 5 allows the extraction of air from even the central regions.

As a result of this ability to communicate suction uniformly over the whole of the area of the original sheet 6 without the need for dimples in the membrane 3 as has been tried previously, and by virtue also of the resilience of the sheet 5 which is compressed by the membrane 3 against the underside of the original 6 and of the glass sheet 1, a very uniform contact pressure is obtained between the original and the stencil 7.

It will of course be appreciated that the drawing shows the configuration before the application of suction.

The illustrated construction of suction platen has various advantages affecting both ease of manufacture and efficiency of operation.

Firstly, avoidance of dimples on the top face of the membrane 3, coupled with the use of a resilient backing sheet 5 to the original 6, gives a "dim ple-free' contact pressure of the original 6 against the glass sheet 1.

Secondly, whereas normally the conventional backing membrane would be integrally formed with a seal bead to engage the peripheral area of the transparent glass sheet such as sheet 1 in the drawing, the superposition of a plain membrane sheet 3 on an external rim strip 4 in this case provides a much simpler way of manufacturing the platen and allows for replacement of the rim strip 4 if, after prolonged use, the platen is found to lack sealing efficiency through permanent compressive set of the peripheral sealing strip 4.

One obvious advantage of the structure shown in the drawing is that a very thin sheet of glass can be used even where contact pressures of up to 1000 g/cm2 are required, because there will be pressure equilbration between the upper side of the glass sheet 1 and the underside of the membrane 3 when a high vacuum is pulled in the suction enclosure between the glass sheet and the membrane.

Claims (5)

1. A suction platen for document exposure, comprising a transparent sheet, a gas-impervious membrane in association with the sheet so as to define therewith a suction enclosure to receive a document to be held and exposed through the transparent sheet, and a resilient backing sheet of open cell foamed construction in the suction enclosure between the membrane and the transparent sheet, whereby application of suction to one area of said suction enclosure can be evenly communicated over the whole of the extend of that suction enclosure.

2. A suction platen according to claim 1, and including a suction hose integrally moulded with the membrane.

3. A suction platen according to either of the preceding claims, and further including a resilient peripheral strip extending around the periphery of the gas-impervious membrane outside the suction enclosure for pressing the periphery of the membrane into sealing contact with the transparent sheet when the platen is closed.

4. A suction platen according to any one of the preceding claims, wherein the transparent sheet is formed of glass.

5. A suction platen constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.