Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1998-10-19
2001-05-01
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C101S048000, C101S112000, C101S128400
Reexamination Certificate
active
06225026
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a printing stencil prepared by utilizing radiation curing.
BACKGROUND OF THE INVENTION
Printing processes include stencil duplicating (mimeography) and screen-printing. These are processes which utilize stencils and in which ink is allowed to pass through ink permeable areas on the stencil (image areas) to the paper, thus printing an Image. Duplicating, usually a rotary process, is used for printing finer details than screen printing which is a flat bed process.
Various stencil making processes have been developed and used over the years, both for mimeography and screen-printing. The process of stencil making involves cutting out image areas in the stencil. This was originally done by hand cutting of the stencil material, Other methods used various mechanical means to cut the image areas in impregnated tissue or in free standing films. Electro-optical methods were also used for stencil cutting by IR absorption or Electro-erosion. Since the 1980's, thermal digital duplicators have been developed. In these machines, the stencil is cut using a thermal head to make holes in a thin polyester layer.
Examples of existing printing stencils and of their preparation are illustrated in
FIGS. 1 and 2
.
FIG. 1
illustrates a thermal stencil and describes the method for its preparation. The thermal stencil
10
consists of a thin polyester film
12
laminated on a fine tissue base
14
. Holes
16
are made in the polyester film
12
using a thermal head
18
. During printing, ink
15
becomes accessible to paper
13
through holes
16
. This technology gives reasonable quality, short preparation time and is suitable for a digital work flow. It is used in all digital duplicators made at present.
FIG. 2
illustrates a laser thermal stencil and describes the method for its preparation. The laser thermal stencil
20
consists of an IR sensitive layer
22
and a thin polyester film
24
laminated on a paper tissue base
26
. IL patent U.S. application No. 09/402,205, assigned to the common assignees and co-inventors of the present invention, discloses the use of a laser beam for cutting a stencil similar in its structure to the thermal stencil. Instead of using an electric thermal element, a laser beam
28
is used to generate the heat in a radiation-absorbing layer, such as the IR sensitive layer
22
, cutting holes
21
in the IR sensitive layer
22
and polyester layer
24
. During printing, ink
25
becomes accessible to paper
25
through holes
21
. A high-resolution stencil may be quickly produced in this method. So far there is no commercial implementation of this method.
An example of an existing UV stencil for screen printing and of its preparation is illustrated in FIG.
3
.
Stencil
30
is made of a UV curable material (photo polymer)
32
and a fine mesh
38
. A photographic film
34
, containing image
33
, is laid over the photo polymer
32
, and acts as a mask during the exposure of stencil
30
, in a contact exposure process. The UV curable material hardens as it is exposed to UV light
36
through transparent areas
31
in the photographic film
34
. The film
34
is removed and the stencil is washed out, in the developing process, leaving the mesh
38
and the cured material
39
. The stencil is then used for printing ink
37
on paper
35
. Ink
37
is inaccessible to paper
35
in the areas defined by the cured material
39
, and will contact paper
35
only in the non cured areas
33
′ around the cured material
39
. The Screen-printing stencils thus produced have relatively high quality. Numerous versions of this principle exist, with direct (on-mesh processing) and indirect (off-mesh processing) emulsions and photo polymer films.
FIG. 4
illustrates a digital flexographic printing plate and describes the method for its preparation, such as described in U.S. Pat. Nos. 5,262,275 and 5,607,814. The flexographic printing process is a relief printing process which does not utilize stencils. Flexographic plate
40
consists of a UV sensitive photo polymer
48
coated by a dense black IR sensitive layer
42
. Layer
42
is etched out by a powerful IR laser
44
, forming an integral mask having holes
43
through which the plate
48
is exposed to UV light
46
. The UV exposure from light
46
forms, by curing, a relief image
41
on the plate. Additional exposure to UV lamp
49
forms a cured plate base
41
′. After processing, relief image
41
can be used for printing ink
47
on paper
45
. The process requires powerful exposures for both IR and UV light. It is as lengthy as for normal UV plates but it gives high quality since contact exposure with a film is no longer required.
UV curable material was utilized for screen printing, as described in FIG.
3
and for flexographic plate printing, as described in
FIG. 4
, but has never been utilized in the preparation of printing duplicating stencils and has never been used in an on-press process
The existing methods for producing printing stencils for duplicating (such as described in the above patents and in
FIGS. 1 and 2
) are limited in the quality of stencils which can be produced due to various effects of the thermal interaction of the material either with the thermal head or with the laser. Present stencil duplicating is limited in its application to simple jobs with very coarse halftone images.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a printing stencil and a method for its preparation which overcome the limitations of the known printing stencils.
There is thus provided, in accordance with a preferred embodiment of the present invention, a printing stencil especially useful for stencil duplicating processes. The printing stencil comprises a porous stencil carrier layer, preferably of a non-woven tissue, which is impregnated with a solid or liquid radiation curable material. The printing stencil may further comprise a mask laid upon or integral to the stencil layer for defining areas transparent and areas non transparent to curing irradiation. The mask may be, for example, an IR sensitive layer or an ink jet printed layer in which the areas impermeable to curing radiation are areas not ablated by IR irradiation or areas containing ink.
There is thus also provided, in accordance with a preferred embodiment of the present invention, a method for the preparation of a printing stencil, said printing stencil comprising a stencil carrier layer and a stencil layer, which is impregnated with a radiation curable material. The method comprises the step of selectively irradiating the stencil layer with curing radiation, thereby forming ink permeable and ink impermeable areas, said ink permeable areas consisting of uncured material and said ink impermeable areas consisting of cured material.
In most cases, the uncured material is removed from the stencil, prior to printing, to form the ink permeable image areas.
The method may further comprise the step of creating a mask on the stencil layer prior to irradiating it with curing radiation, for selectively blocking curing radiation by defining, on the printing stencil, areas exposed and areas unexposed to curing radiation.
The mask may be removed prior to removing the uncured material from the stencil.
It will be appreciated that “area”, in the present invention, refers to a surface of a layer, e.g. the mask, or to the full thickness of the layer under laying the surface, e.g. the impregnated stencil.
The printing stencil of the present invention may be used in stencil duplicating processes for printing fine details, graphic elements and color printing at a higher quality than existing stencils, approaching offset lithographic standards. Thus, the invention utilizes a technically simple process for achieving a quality presently achievable only in complicated printing processes.
REFERENCES:
patent: 4262084 (1981-04-01), Kinney
patent: 4268576 (1981-05-01), Montmarquet, Jr.
patent: 5262275 (1993-11-01), Fan
patent: 5607814 (1997-03-01), Fan
patent: 5888700 (1999-03-01), West et
Figov Murray
Lifshitz Lior
Ron Hannoch
Afrion Digital Ltd.
Eitan Pearl Latzer & Cohen-Zedek
Huff Mark F.
Mohamedulla Saleha R.
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