Screen printing stencil production

Details Screen printing stencil production Screen printing stencil production

2001-07-03

2003-10-21

Yan, Ren (Department: 2854)

Printing

Stenciling

Stencils

C101S129000, C430S308000

Reexamination Certificate

active

06634289

ABSTRACT:

BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention relates to the production of stencils for screen printing.
2. Related Background Art
The production of screen printing stencils is generally well known to those skilled in the art.
One method, referred to as the “direct method” of producing screen printing stencils involves the coating of a liquid light-sensitive emulsion directly onto the screen mesh. After drying, the entire screen is exposed to actinic light through a film positive held in contact with the screen in a vacuum frame. The black portions of the positive do not allow light to penetrate to the emulsion which remains soft in those areas. In the areas which are exposed to light, the emulsion hardens and becomes insoluble, so that, after washing out with a suitable solvent, the unexposed areas allow ink to pass through onto a substrate during a subsequent printing process.
Another method, referred to as the “direct/indirect method”, involves contacting a film, consisting of a pre-coated unsensitised emulsion on a support base, with the screen mesh by placing the screen on top of the flat film. A sensitised emulsion is then forced across the mesh from the opposite side, thus laminating the film to the screen and at the same time sensitising its emulsion. After drying, the base support is peeled off and the screen is then processed in the same way as in the direct method.
In the “indirect method” a film base is pre-coated with a pre-sensitised emulsion. The film is exposed to actinic light through a positive held in contact with the coated film. After chemical hardening of the exposed emulsion, the unexposed emulsion is washed away. The stencil produced is then mounted on the screen mesh and used for printing as described above for the direct method.
In the “capillary direct method” a pre-coated and pre-sensitised film base is adhered to one surface of the mesh by the capillary action of water applied to the opposite surface of the mesh. After drying, the film is peeled off and the screen then processed and used as described for the direct method.
In addition to the above methods, hand-cut stencils can be used. These are produced by cutting the required stencil design into an emulsion coating on a film base support. The cut areas are removed from the base before the film is applied to the mesh. The emulsion is then softened to cause it to adhere to the mesh. After drying, the base is peeled off. The screen is then ready for printing. This method is suitable only for simple work.
One problem generally associated with the prior art methods is that many steps are necessary to produce the screen, thus making screen production time-consuming and labour-intensive.
Another problem is that normal lighting cannot be used throughout the screen production process in any of the methods except hand cutting. This is because the stencil materials are light-sensitive. In addition, it is necessary to provide a source of actinic (usually UV) light for exposing the stencil. This usually incurs a penalty of initial cost, space utilisation and ongoing maintenance costs.
Other methods of preparing printing screens are available. CA-A-2088400 (Gerber Scientific Products. Inc.) describes a method and apparatus in which a blocking composition is ejected directly onto the screen mesh surface in a pre-programmed manner in accordance with data representative of the desired image. The blocking composition directly occludes sections of the screen mesh to define the desired stencil pattern.
EP-A-0492351 (Gerber Scientific Products, Inc.) describes a method wherein an unexposed light-sensitive emulsion layer is applied to a screen mesh surface and a graphic is directly ink-jet printed on the emulsion layer by means of a printing mechanism to provide a mask through which the emulsion is exposed before the screen is further processed.
Both the above methods require the use of very specialised equipment which incurs a certain cost as well as imposing restrictions arising from the limitations of the equipment, in particular in terms of the size of screen and its resolution. The second method also requires sensitised films and emulsions, requiring exposure units and vacuum frames.
Ink-jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can be used to produce a wide variety of printed materials, including text, graphics and images on a wide range of substrates. In many ink jet printing systems, ink is printed directly onto the surface of the final receiving substrate. An ink jet printing system where an image is printed on an intermediate image transfer surface and subsequently transferred to the final receiving substrate is disclosed in U.S. Pat. No. 4,538,156 (AT&T Teletype Corp.). Furthermore, U.S. Pat. No. 5,380,769 (Tektronix Inc.) describes reactive ink compositions containing at least two reactive components, a base ink component and a curing component, that are applied to the receiving substrate separately. The base ink component is preferably applied to the receiving surface using ink jet printing techniques and, upon exposure of the base ink component to the curing component, a durable, crosslinked ink is produced.
EP-A-0108509 (Pilot Man-Nen-Hitsu KK) describes a process in which a coating on a porous support is selectively chemically solubilised and then washed away.
EP-A-0770552 (Riso Kagaku Corporation) describes a machine in which a porous substrate forms the cylindrical surface of a drum. The substrate has a coating layer which is selectively solubilised prior to ink being passed outwardly through the substrate to be transferred to a substrate.
GB-A-180778 (Carter) describes a stencil paper which is coated with varnish prior to application of an ink. The ink and the varnish below is then washed away to form a stencil for a rotary duplicating machine.
Our co-pending Application PCT/GB97/01881 (WO99/02344, the content of which is incorporated herein by reference, describes a method of producing a screen-printing stencil which requires the image applied to a receptor element to be a negative image (this known as “negative working”). In the present application, a method is disclosed which uses a positive image (“positive working”). Each method has its advantages compared with the other, depending upon the circumstances of use.
One object of the present invention is to make screen-printing stencil production less time-consuming and labour-intensive.
Another object is to allow normal lighting to be used throughout the stencil production process and to avoid both the problems of prior art stencil materials which are light-sensitive and also the need to provide a source of actinic (usually UV) light for exposing the stencil.
SUMMARY OF THE INVENTION
The present invention provides a method of producing a screen-printing stencil having open areas and blocked areas for, respectively, passage and blocking of a printing medium, the method comprising:
providing a receptor element comprising an optional support base and an image-receiving layer comprising a first substance,
applying imagewise to the image-receiving layer a second substance in areas corresponding to the open areas of the stencil to be produced,
bringing the image-receiving layer into contact with a third substance applied in a layer to a screen-printing screen,
causing or allowing chemical reaction to take place to form on the screen a stencil-forming layer having areas of relatively higher and relatively lower solubility corresponding to the open and the blocked stencil areas respectively, and
washing away the stencil-forming layer in the higher solubility areas, thereby to produce the screen-printing stencil,
the first, second and third substances being such that the said chemical reaction takes place as stated.
Within this overall scope, four particular methods can be identified as preferred aspects of the invention, although the invention is not limited to these four prefe

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