A method and apparatus for the manufacture of pre-inkable rubber stamps

TECHNICAL FIELD The present invention belongs to the sphere of printing industry, more specifically to the manufacture of pre-inkable rubber stamps .

BACKGROUND OF THE ART In the field of manufacturing pre-inkable rubber stamps microporous thermo-plastic materials have been used, because their surface pores can be closed by heating and simultaneous compressing, in this way producing areas, which are impermeable to stamp ink, and can therefore serve as non-printing elements of a rubber stamp's printing surface. By selectively heating and simultaneously compressing the elements of the printing surface of a blank printing plate, which correspond to the non-printing elements of the required stamp pattern, a printing plate is produced which, after it has been filled with ink and mounted into a stamp mount forms a rubber stamp capable of producing a large number (tens of thousands) of imprints with no need for an ink-pad.

A method is known for manufacturing pre-inkable printing plates, which consists in selectively closing surface pores of the microporous thermo-plastic material by way of heating and compressing with a heated relief mould (GB patent No. 1112776, B 44 C 1/24, Bankers and Merchants, Inc., 1968, JP patent No. 6024114, b 41 K 1/50, Tokyo Ohka Kogyo Co., Ltd., 1994). According to this method, first of all a relief master mould is made, which has grooves corresponding to the printing elements of the printing plate being manufactured. The master mould is then heated to the temperature above the melting point of the microporous thermo-plastic material used, after which a sheet of the

microporous thermo-plastic material is pressed against the heated mould. At the location of the non-printing elements the sheet of the microporous thermo-plastic material and the heated mould come into contact and the surface of the sheet of the microporous thermo-plastic material melts and its pores collapse, producing a layer which is impermeable to stamp ink. At the grooves of the master mould the surface areas of the microporous thermo-plastic sheet do not melt and remain porous, forming the printing elements.

When using a relief master mould for the manufacture of printing forms from a microporous thermo-plastic material, not only is the surface of the sheet of the thermo-plastic material heated selectively, but it is also compressed selectively. In case of such selective compressing, the pressure exerted to the non-printing areas simultaneously with heating cannot be high, because it is difficult to produce such tiny relief elements which would be sufficiently high and strong. Therefore in the non-printing elements of the printing-plate, insufficiently compressed at the time of heating, there will remain microscopic pores that will soil the imprint.

A method has been devised for closing the surface pores of a microporous thermo-plastic material selectively, heating it by means of light radiation and simultaneously compressing against a smooth transparent body (EE Patent application P 9500001, AS Humal Elektroonika, 1995). This method presupposes use of a microporous thermo-plastic material with a light absorbing surface. A typesetting pattern is drawn on a transparent or semi-transparent means using a light-absorbing colour. The means is then placed between a transparent smooth-surfaced body and the microporous thermo- plastic material, whereas the typesetting pattern on it is separated from the light-absorbing surface of the microporous thermo-plastic material by at least a 0.02 mm thick

separating film. Through the smooth transparent body, the means with the typesetting pattern on it, and the separating film, the surface of the microporous thermo-plastic material is exposed to an impulse of light. At the areas of the surface of the microporous thermo-plastic material which correspond to the transparent (non-printing) elements of the typesetting pattern, the heat generated by the light impulse melts the surface and its pores will be closed as a result of the applied pressure, whereas at those areas which correspond to the printing elements the typesetting pattern absorbs the light, causing no heat to operate on the surface of the microporous thermo-plastic material and its pores to remain open. Pressing against a smooth- surfaced body simultaneously with selective heating makes it possible to exert enough pressure to close the pores completely.

Also, a method is known for manufacturing printing plates by heating and melting the surface of a thermo-plastic material by means of a thermal head (EP patent 0557013, B 41K 1/32, Brother Kogyo Kabushiki Kaisha, 1993). A thermal head having a large number of electrical heating elements on its surface that can be selectively switched on and off by means of a control device, is used for producing a typesetting pattern on the surface of a thermo-plastic material by selectively melting its surface.

As far as the manufacture of microporous thermo-plastic printing plates, especially according to the last two methods described above, is quite productive, it appears that in the manufacturing process of rubber stamps the most time- consuming stages are inking of the printing plates and mounting them into stamp mounts. Besides that, the mounting of pre-inked printing plates requires use of glues emitting noxious fumes. There is no good solution for re-inking of such rubber stamps either, because the rear surface of the

printing plate is glued to the stamp mount and is not quite suitable for re-inking.

One of the known solutions to the above problem is the use of specially designed stamp mounts as proposed by Japan Business Connections. Their stamp mount has a case for a printing plate, in the lower opening of which a microporous thermo- plastic printing plate can be mounted by melting along its perimeter, using for this purpose a special melting device. The rear surface of the printing plate is supported by a porous plate and the latter in its turn is supported by a framework filling the upper part of the printing plate's case, forming an ink reservoir. As the printing plate is fixed only along its perimeter and its rear surface is supported by a porous plate, this stamp can easily be refilled with ink through the opening in the upper part of the printing plate's case.

The use of the special mount described above will help solve the problem of refilling the stamp with ink, the mounting of the printing plate, however, which is time- consuming and needs possession of a special melting device, is left to the stamp manufacturer.

The technical solution closest to the invention being described herein was proposed by Yamahachi Chemical. The said company produces stamp mounts for manufacturing rubber stamps from microporous thermo-plastic materials by way of heating with the use of relief master moulds. Their stamp mounts have a printing plate case, into the lower opening of which a blank printing plate made of a microporous thermo- plastic material is mounted by melting along its perimeter. The rear surface of the blank printing plate is supported by a plate from a porous material and the latter in its turn is supported by a framework; the two together forming an

ink reservoir. The ink reservoir can be filled with ink through an opening in the upper part of the printing plate's case. This stamp mount is used for manufacturing pre-inkable rubber stamps in the following way.

First, a relief master mould is produced which corresponds to the typesetting pattern required, having grooves in place of the printing elements. The master mould is heated above the melting point of the microporous thermo-plastic material of the blank printing plate. The printing plate's case together with the blank printing plate mounted in it is placed into a special device which presses the surface of the blank printing plate for a short time against the relief mould. At the place of the non-printing elements the printing surface of the blank printing plate and the heated master mould come in contact with each other, the surface of the printing plate melts and its pores collapse under the pressure of the master mould, producing a layer impermeable to ink. At the place of the grooves in the mould the surface of the blank printing plate does not melt and remains porous, thus forming the printing elements.

The method described above will save the stamp manufacturer from having to mount the printing plate and might thus make stamp manufacturing very quick and economical. The difficulty, however, lies in the necessary pre-stage: the manufacture of the relief master mould. How to use the stamp mount described above without a relief mould, for example, selectively heating the surface of the blank printing plate by light radiation or by a thermal head is not known. The porous plate and framework supporting the printing plate in the stamp mount described above are rigid enough to withstand the relatively low pressure exerted selectively by the relief mould, but not to withstand the pressure necessary for totally closing the pores in the non-printing elements

when the whole printing surface is pressed against a smooth- surfaced body and selectively heated.

DESCRIPTION OF INVENTION It is the aim of the present invention to provide a method for fast and highly productive manufacture of re-inkable rubber stamps from a microporous thermo-plastic material, which would make it possible to produce high-quality imprints with clean non-print elements, and would be refillable by users without any special skills, with emanation of noxious fumes in the process of manufacture minimized or totally eliminated. The other aim of the present invention is to provide an apparatus for the manufacture of pre-inkable rubber stamps according to the proposed method.

These objects are achieved when using selective closing of pores in the printing surface of a blank printing plate from a microporous thermo-plastic material, glued of melted by its perimeter into the printing plate's case, without the use of a relief mould. The selective closing of the pores of the printing surface can be performed, for example, by selective heating by light radiation or a thermal head, while simultaneously with the selective heating both heated and non-heated elements of the printing surface of the blank printing plate are pressed against a supporting body. As the said supporting body a transparent smooth-surfaced body can be used, through which light radiation is selectively conducted to the printing surface of a blank printing plate, or a thermal head can be used for selective heating of the elements of the printing surface. The printing surface of the blank printing plate is pressed against the support body, the pressure being not less than 0.2 MPa. In order to press the printing surface of a blank printing plate against the said supporting body, prior to selective heating, some stamp ink is poured into the printing plate's case, so that it

covers the rear surface of the blank printing plate, and the printing plate's case is fixed so that the blank printing plate mounted in it is placed against the said supporting body. Now the blank printing plate is pressed with its printing surface against the said supporting body by way of creating inside the printing plate's case hydrostatis over¬ pressure, e.g. by connecting it to a source or compressed air. To avoid the outward deformation of the printing plate's case under the hydrostatic over-pressure, the printing plate's case is pressed against the said supporting body by means of a hollow mould supporting the printing plate's case from above and at the sides.

An apparatus for the manufacture of pre-inkable rubber stamps is proposed which consists of a supporting body, e.g. a glass plate, of a source of impulse light, of a means for fixing the printing plate's case with the printing surface of a blank printing plate against the said supporting body, and of a means for connecting the cavity in the printing plate's case to a source of compressed air.

BRIEF DESCRIPTION OF DRAWINGS

Fig.l - printing plate's case together with a blank printing plate, glued or melted by its perimeter into it Fig.2 - realization of the method Fig.3 - placement of the light absorbing pattern Fig.4 - forming of the printing plate

List of items

1 - shell of the printing plate's case

2 - blank printing plate 3 - supporting plate to the printing plate

4 - opening for filling the printing plate's case with ink

5 - printing surface of the printing plate

6 - glass plate

7 - transparent means with a light absorbing pattern on it

8 - light absorbing elements of the light absorbing pattern 9 - separating film

10 - mould for fixing the printing plate's case

11 - stamp ink

12 - source of impulse light

13 - light radiation 14 - areas with closed pores

15 - printing plate ready for use

BEST WAY FOR APPLICATION OF THE INVENTION The method is realized by means of an apparatus consisting of a supporting body, e.g. a glass plate, of a source of impulse light under the supporting body and of a means for fixing the printing plate's case so that the blank printing plate mounted in it is supported at its printing surface by the supporting body. According to Fig.l a printing plate's case is used which consists of a plastic shell 1, a blank printing plate 2 glued or melted to the latter by its perimeter, and a plate 3 that supports the blank printing plate and either has penetrating holes in it or is made of some porous material. For filling with ink, the printing plate's case has an opening 4, which can be closed by a plug or cover. The microporous material of the blank printing plate should be light absorbing by nature , if not, then the printing surface should be made light absorbing by application of a suitable dye or pigment.

The manufacturing process of the rubber stamp is performed as follows (Fig. 2, 3, and 4). Onto a glass plate 6 a transparent means 7 is placed which has on it a light absorbing pattern of typesetting 8 , covered by a transparent separating film 9. Upon the separating film the printing

plate's case of the design described above is placed, with the printing surface of the blank printing plate in contact with the separating film 9. The printing plate's case is fixed by a mould 10 to avoid its shifting from the glass plate 6 or outward deformation. Then some stamp ink 11 is poured into the porous plate 3 through the opening 4. The stamp ink 11 reaches the rear surface of the blank printing plate and covers the pores on its surface. Now the cavity of the printing plate's case is connected via the opening 4 to a source of compressed air. As the rear surface of the blank printing plate is covered with stamp ink 11, compressed air cannot penetrate into the microporous material, but presses the printing surface 5 of the blank printing plate 2 against the glass plate 6. After that the printing surface 5 of the blank printing plate 2 is exposed to a source of impulse light 12 through the glass plate 6, the transparent means 7 and the separating film 9. On the areas of the means 7, where there are no light absorbing elements 8, the light radiation 13 which is absorbed by the printing surface 5 of the blank printing plate 2 heats the material, the microporous thermo- plastic material melts and the pressure exerted by compressed air closes the pores on the surface, producing non-porous areas 14. On the areas with light absorbing elements 8 the light never reaches the printing surface 5 of the blank printing plate 2, instead, it is absorbed by the absorbing elements 8. As the printing surface 5 is separated from the light absorbing elements 8 by the separating film 9, only part of the generated heat reaches the surface 5, therefore in these areas the microporous material does not melt and its pores remain open. As a result the blank printing plate is converted into a printing plate 15 having both printing and non-printing elements (Fig.4) on it. After exposure the printing plate's case is disconnected from the source of compressed air and detached from the glass plate 6. After some time the stamp ink will sink into the printing plate 15, filling its pores and reaching the printing

surface 5. The elements of the printing surface that during the exposure were covered by the light absorbing elements 8 now form printing elements, whereas the elements without light absorbing elements 8 are now covered with a melted layer of the thermo-plastic material, impermeable to ink, thus forming non-printing elements. The printing plate's case together with the printed plate 15, glued or melted into it along its perimeter is mounted into a suitable stamp mount and the rubber stamp is ready for use.

INDUSTRIAL APPLICATIONS

For manufacturing rubber stamps according to the method described above, a printing plate's case shown in Fig. 1 was used. A blank printing plate 2 made of a microporous polyethylene, whose size was 4x26x30 mm, was melted into the lower opening of the printing plate's case, protruding by 1 mm above its walls. Plate 3 was performed as a construction from polyethylene, the lower side of which was a 2 mm thick framework with ca 20 3 by 3 mm holes in it. The upper part of this framework was supported by 30 small columns whose diameter was 2 mm.

The printing surface 5 of the blank printing plate 2 was darkened with graphite powder. Onto the glass plate 6 of 10 mm thickness according to Fig. 2 a semi-transparent means 6 was placed with a light-absorbing typeset pattern 8 on it. The means 6 was made of vellum paper on which the type-face was printed by laser printer. For better light transparency the vellum paper was impregnated with vegetable oil . The vellum paper was then covered with a 0.1 mm thick separating nitrocellulose film 9 . The printing plate's case, into which a blank printing plate 2 was melted, was placed onto the transparent means 7 from vellum paper and the separating film covering the glass plate 6 and was fixed by a mould 10 as shown on Fig.2. Into the printing plate's case was poured ca 2 ml of stamp ink. After that the cavity of the

printing plate's case was connected by means of a tube 16 with a source of compressed air at the pressure ca 0.8 MPa. The impulse light source under the glass plate 6 consisted of Xenon discharge tubes with diameters of 10 mm and the distance between the electrodes 130 mm. An array of parallel discharge tubes with 20 mm pitch was used. In each tube a discharge with electrical energy of 200 J was created. After that the printing plate's case was disconnected from the compressed air source and detached from the glass plate 6, the separating film 9 and the means 7. On the printing surface 5 of the printing plate 15 one could clearly distinguish between microporous areas corresponding to the pattern 8 and glossy areas with melted pores which were ca 0.2 mm deeper. In about 2 hours the ink had filled the printing plate 15 and the rubber stamp manufactured in that way left on paper a fine high quality imprint, exactly corresponding to the typeset pattern 8 printed by laser printer on vellum paper.