GB2034636A

GB2034636A - Intaglio printing members

Info

Publication number: GB2034636A
Application number: GB7931053A
Authority: GB
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1978-09-15
Filing date: 1979-09-07
Publication date: 1980-06-11
Also published as: GB2034636B

Abstract

The printing surface of an intaglio, especially gravure, printing member is formed of a rigid polymeric composition that can be engraved by an ion; electron or laser beam to give a sharp print without a rim around the engraved area. The polymeric composition e.g. a polyacetal, is chosen so that it volatilises in the image areas, but remains as a rigid solid in non-image areas. The member is preferably cylindrical and the polymeric composition provided as a cylindrical sheet, for example by butt-jointing a flat sheet, which is heat shrunk onto the cylinder and a seal is preferably provided to prevent ink penetrating between the cylinder and the sheet. The engraved surface may subsequently receive a metal plating.

Description

SPECIFICATION Improvements relating to printing members Printing members for intaglio printing, particularly gravure printing, must be made of a material which can be readily engraved with an image to be printed, but which must have a high wear-resistance to combat wear by the doctor blade and printing substrates; a high solvent resistance so that it is not chemically affected by the ink, or ink solvents; be dimensionally very stable because of high pressures generated durinc the printing process; and, finally be relatively cheap as the printing member is discarded at the end of a particular print run. To meet these apparently mutually incompatible requirements printing members have conventionally comprised a steel substrate with a printing surface formed of a continuous plated copper coating and the image pattern is engraved into this plated copper coating.

Alternatives to copper as the printing surface have been proposed. Since making the invention described below it has been proposed in British Patent Specification No. 1,544,748 to form the printing surface of a polymer having a particular tensile strength, to form ink cells in the surface by mechanical engraving means and to use a gravure printing doctor blade formed of a polymer having a particular Izod impact strength.

Polymers named for use as the print surface include polyethylene, polyvinyl chloride, polyamides, polyesters, and polycarbonates. It is said that the polymer can be coated onto a printing cylinder by extrusion moulding, spray coating, brush coating, powder coating or blade coating.

Normal intaglio print surfaces are formed of a continuous sheet of material which is engraved. Various methods of conducting the engraving are known. Attempts have been made to engrave a continuous metal print surface by use of a laser, for instance a pulsed laser beam, each pulse of energy being used to form a gravure cell the size or depth of which depended on the energy of the pulse. As explained in British Patent Specification No. 1,299,243 this method tended to result in the deposition of a rim of metal around each cell, thus impairing the printing properties of the printing surface.In an attempt at overcoming these difficulties il was described in that Specification and subsequently to form the printing surface of two materials, one material defining cells of the required cell pattern and the other material filling the cells and being more easily decomposed or evaporated than the first material. The laser beam was then used to evaporate or decompose the second material to leave cells defined by the first material. Various materials have been proposed for use as the second material. As explained in Specification No. 1,299,243 the second material could be softer than the first material that had to be hard to give wear resistance. Examples quoted as seconc material are polythene, glass and antimony. In Specification Nos. 1,465,364 and 1,498,811 the use of epoxy resins as the second material is proposed.

These methods all suffer from various disadvantages, including the fact that they involve the initial formation of cells of the first material.

It would therefore be desirable to be able to engrave a printing surface of a continuous sheet using a laser, electron or ion beam and to avoid these disadvantages. However the difficulty still remains of formulating the printing surface of a material that can be engraved by a laser, electron or ion beam to give a clean engraving and which will serve as a good printing surface. As mentioned the conventional metals are unsatisfactory and we have found that polymeric materials also tend to suffer from the same disadvantage as metals, namely that they result in the formation of a rim in the zone around the area being struck by the beam.

According to the invention an intaglio printing member comprises a print surface formed of a continuous sheet of rigid polymeric composition that, when struck by an ion, electron or laser beam in an area, is converted to volatile products and volatilises throughout the entire area while remaining as a rigid solid in the zone adjacent the area where it volatilises.

The printing member is initially formed with a smooth print surface but an engraved print surface, carryinc the desired image, is made by striking the areas that are to be engraved with a laser, electron or ion beam.

The polymeric composition absorbs the energy of the beam and is wholly or mainly vaporised in substantially only the areas struck by the beam and the vaporised material escapes into the atmosphere with substantially none of it being deposited on the printing surface and polymeric composition not volatilised remains as a rigid solid, i.e. it is sufficiently rigid that it does not flow into the engraved area and has properties suitable for intaglio printing. In particular it is necessary that the polymeric composition in the zone adjacent the area where volatilisation occurs shall remain substantially unaffected during engraving and shall not flow or melt at all, or shall not flow or melt sufficiently to impair the clarity of printing.Melting and flow can be minimised, and preferably avoided, by using a polymeric composition having high thermal conductivity, local heat thus being dissipated, or by using a polymeric composition having very low thermal conductivity, substantially no heat being transferred from the area struck by the beam to the surrounding zone. Rigidity can also be maintained by providing fibrous reinforcement in the polymeric composition the fibrous reinforcement thus preventing flow of the polymer and holding it in the substantially rigid state even though the polymeric component of the composition may be temporarily above its softening or melting point.

In general the melt flow index of polymer compositions for use in the invention should be from 1 to 12, most preferably 1 to 5, especially 1 to 3.

The polymeric composition preferably is a composition having a sharp melting point. The composition preferably consists of one or more polymers and optionally various fillers and reinforcements. The one or more polymers preferably change from a substantially rigid state to a molten state within a temperature range of 300C or less, preferably 1 00C or less, e.g. 0.2 to SOC. Preferably the melting point is below 250 C, preferably 130 to 180 C.

Throughout the area struck by the beam some or all of the polymeric composition volatalises to form volatile products. The products must remain in the gaseous state while they are close to the printing surface and preferably they are gaseous at 20 C. They will normally be low molecular weight products, for example having a molecular weight below 500 and preferably below 100. Volatilisation of the polymer preferably occurs at temperatures below 350 C, most preferably below 300 C.

Preferably the polymer material has a chain including oxymethylene orthiomethylene units. Suitable starting materials for such a polymer material include thioesters, mercaptans, thioaldehydes, ketones and thioketones. Preferred polymers are acetal polymers. An acetal homopolymer may be used but preferably the polymer is a copolymer with a comonomer introducing ethylenic or high alkylene groups into the polymer chain, for instance a copolymer with a cyclic ether containing an alkylene chain of at least 2 carbon atoms, for example ethylene oxide or 1.3-dioxolane.The copolymer has greater resistance to uncontrolled "unzipping" and thus is more resistant to degradation by chemicals or mechanical damage than the homopolymer, but still retains the ability to be volatilised into low molecular weight volatile constituents where it is struck by a beam and to remain substantially unaffected elsewhere.

The polyacetal should have a fairly high molecular weight, for instance above 10,000and often above 20,000, especially from the point of view of imparting adequate wear properties. For instance the molecular weight may be from 20,000 to 50,000 e.g. 40,000. A suitable material is sold under the trade name "Kematal" M25, which has a molecular weight of 40,000 and a melt flow index of about 2.5. When such a polymer is engraved with a laser, electron or ion beam, it dissociates and forms entirely gaseous products. This ensures that no upstanding rim or lip is formed surrounding the engraved pit or groove since the material is ablated entirely from the surface.The oxymethylene homopolymer dissociated and can release some formaldehyde when it is engraved using a laser, electron or ion beam but when the copolymer is engraved with a laser, electron or ion beam the copolymer dissociates to form carbon dioxide and water vapour. Formaldehyde is rather unpleasant in an industrial atmosphere in view of the lachrymose nature of formaldehyde. This is a further reason why the copolymer is preferred to the homopolymer.

Other polymers that are suitable for use in the invention, even though they are generally less satisfactory then the polyacetals, include epoxy resins but the beam of energy needs to be focused further beneath the surface than with polyacetals, and fibre reinforced, especially glass fibre reinforced, polyamides.

Non-reinforced polyamides and polymers such as polycarbonates polyethylene, polyvinyl chloride and polyesters generally form a rim around the area being engraved and/or do not give a sharp print.

It is naturally necessary to select the combination of radiation and polymeric composition such that the polymeric composition remains in the solid state where it is not struck by the radiation and vaporises where it is struck, with substantially no transition zone between the solid and vaporised areas. Additives may be included in the polymeric composition in order to increase the absorption of the composition so that a composition which would otherwise not absorb sufficient energy to be volatilised by a particular ion, electron or laser beam can be volatilised by that beam. For instance the polymeric composition may consist of the polymer or polymers and carbon black that will have the effect of making the composition absorb the intended radiation.For instance we have found that a polymeric composition comprising an acetal polymer can easily be engraved by a carbondioxide laser but good engraving with a YAG laser may require the incorporation of carbon black into the polymeric material in order to increase the absorption atthe wavelength of the YAG laser. Instead of using carbon black certain other organic and inorganic pigments may be used, for example based on titanium dioxide. The amount of carbon black or other pigment is generally 0.5 to 10% by weight of the polymer composition, preferably 1 to 5%. Apart from such filler, and glass or other reinforcement when present, the composition preferably consists substantially only of organic polymeric material and preferably at least 50%, usually 60% or 80% or more by weight of the composition is one of the materials discussed above, preferably polyacetal.

The continuous sheet of polymeric composition is generally carried on a substrate. The substrate, and therefore the printing member and the sheet, may be flat but preferably the substrate, sheet and printing member are cylindrical. The sheet is preferably preformed as a solid and then is secured onto the cylindrical substrate. The sheet may be preformed as a cylindrical sleeve or it may be formed as a flat sheet which is then converted into a sleeve, for instance by jointing the edges of the sheet. The jointing is preferably conducted so that the joint between the edges of the sheet of polymer material is welded so that it is 'indistinguishable from the remainder of the sleeve. In this way the entire outer surface of the cylindrical printing member can be used to receive an engraved image. The outer surface of the cylindrical printing member substrate is usually skimmed on a lathe to ensure that it has a perfectly true surface after the sleeve has been fitted to the cylindrical support. It is essential that the joint between the two edges of the sheet is complete and void free along its length and that there are no measurable defects throughout the joint. This ensures that no print defects will result in the completed printing member from the presence of, for example, an air bubble included in the joint which has been exposed as an unwanted pit after the skimming operation on the substrate. Further, any imperfections in the joint can lead to damage to the doctor blade used with the cylindrical printing member or damage to the printing member as a result of severe contact between the doctor blade and any irregularities in the joint.

Suitable apparatus for jointing comprises two opposed elongate platens at least one of which includes a strip heater extending along it for welding the plastics material together, with thermally insulating material extending along both sides of the heater or heaters, the apparatus also including clamp means to clamp the platens together to sandwich the plastics material between them with the joint extending along the platens between the strip heater or heaters. Reference should be made to our co-pending application 7931052 filed even date herewith by us.

The cylinder sleeve is preferably heat shrunk onto the cylinder substrate. Preferably this is achieved by the sleeve having been stretched in the peripheral direction (e.g. before butt jointing from a flat sheet) and cooled, being positioned around the cylinder while having a larger internal diameter than the external diameter of the cylinder, and then being heated to release the strain and shrink into a tight fit with the cylinder.

Despite the very tight fit between the cylinder and the sleeve that this can produce it is very desirable to provide a seal around the ends of the cylinder to prevent ink getting between the sheet material and the cylinder. There may be a groove in the end of the cylinder to receive sealing material, which may be an extension of the sleeve or a separate sealant. For instance there may be an annular ring of sealant, such as a silicone rubber, the groove then being the chamfered edge of the cylinder, or the groove may be inset from the edge and the sleeve may be wrapped around the end of the cylinder and pressed into it. Sealing can also be achieved using an extension of the sleeve, or silicone rubber, without a groove.

After machining or otherwise smoothing the print surface, the surface may be engraved in known manner by an electron, ion or, preferably laser beam. It may be conducted so as to form cells in conventional distribution over the surface or in such a manner as to form elongate grooves arranged helically around the surface. Preferably it is engraved for gravure printing.

The polymeric material naturally is preferably a material that has good resistance to moisture, printing inks and normal printing wear. If its properties are not entirely satisfactory the surface of the engraved polymeric composition may be metal plated. This has the additional advantage that local corrections can then be carried out more easily. The precise method of achieving the plating of metal will depend upon the polymer that is used. Generally the polymer is a polymer that can be chemically etched in which event the method may involve etching with acid and/or alkali prior to electrolysis plating of the plating metal. For instance acetal polymers can be etched by a strong oxidising mineral acid. Plating can be by conventional methods using, for instance, copper or nickel, but preferably chromium.

The invention is illustrated in the accompanying drawings in which Figure 1 is an exploded view of a cylindrical substrate and a sleeve ready for fitting over it, Figure 2 is a plan view of a gravure printing roller according to the invention and Figures 3 and 4 are enlarged cross-sections of a part of this roller.

In the drawings a cylinder 1 mounted on axle 2 is covered with a cylindrical sleeve 3. To seal between the cylindrical outer surface of the cylinder 1 and the sleeve 3 the material of the sleeve 3 may extend around the end 4 of the cylinder as shown in Figure 3 and may be pressed into an annular groove 5. An alternative sealing arrangement is shown in Figure 4 in which silicone rubber 6 seals between the outer surface of the cylinder 1 and the sleeve 3.

The following is an example of the production of a printing cylinder by the invention.

A base cylinder having a diameter of 32 cm is manufactured of finished copper in conventional manner to a form having a good surface finish and free from indentations. A sheet of acetal copolymer sold under the trade name "Kematal" M25 is wound around the cylinder and a weld is effected along the butted seam. The acetal copolymer used is supplied in sheet form, at a nominal thickness of 0.76 mm. The material may be supplied in a form such that it is uniaxially stressed by up to 15% dimensionally or it may in fact be so stressed on static equipment.

The stressed assembly is subjected to an air temperature of circa 80 C. This will result in stress-relieving of the sleeve and subsequent shrinkage onto the base cylinder. The ends of the sleeved assembly are sealed with a silicone-based composition or as shown in Figure 3. When a silicone based sealant is to be used, the surface of the plastics material should be roughened where the composition is to be applied, degreased and then rimed using a primer for silicone rubber sealants (a suitable primer is Dow Corning Primecoat 1200), and dried in warm air before application of the sealant.

The cylinder surface is then diamond turned to generate a surface having properties consistent with non-print characteristics. The cylinder is then laser engraved using a CO2 laser to the desired pattern, and is then proofed.

Correction and/or plating may be carried out on the cylinder as required. An example of a subsequent surface conditioning process comprises the following steps: 1. The engraved surface of the polymer material is degreased by wiping it with a solvent such as 1,1,1-trichloroethane.

2. A fine surface-etching is carried out on the surface of the polymer material to provide it with a slightly roughened surface to provide a key. This fine surface etching is accomplished by immersing the surface in a phosphoric acid/sulphuric acid solution to which a surfactant has been added for a period of five to ten minutes at a temperature of between 40 to 60 C.

A suitable etchant composition is: ortho-phosphoric acid (sg 1.75) 15% Sulphuric acid (Conc. GPR) 25% deionised water 59.5% surfactant 0.5% The surfactant may be a material such as X1 57 which is After etching, the surface of the polymer material is rinsed thoroughly in hot and cold water.

3. The etched surface is catalysed in a colloidal palladium based solution for 5 minutes at ambient temperature. A suitable solution is: Activator 2% HCL (conc) 28% Deionised water 70% The activator is a stannous chloride based palladium solution, for instance sold under the trade name Metex Activator 9070M.

After treatment the surface is rinsed with cold water.

4. The resultant product is then treated with sodium hydroxide of between 25 and 60% concentration, e.g. about 50%, for about 5 minutes at ambient temperatures and is then rinsed with water.

5. Electroless deposition of copper or nickel is then carried out on the engraved surface of the printing member. Once the copper or nickel plating has been achieved the surface can then be overplated with a chromium plating to provide the required very hard, durable and resistant surface to the printing member.

As another example, the copolymer sheet may contain 5% carbon black and the engraving may be by a YAG laser.

Claims

1. An intaglio printing member comprising a print surface formed of a continuous sheet of rigid polymeric composition that, when struck by an ion, electron or laser beam in an area is converted to volatile products and volatilises throughout the entire area while remaining as a rigid solid in the zone adjacent the area where it volatilises.

2. A printing member according to claim 1 in which the composition comprises a polymeric material having a chain including oxymethylene orthiomethylene units.

3. A printing member according to claim 1 which the composition comprises a polymer selected from epoxy resins, reinforced polyamides and polyacetals.

4. A printing member according to claim 1 in which the composition comprises a polyacetal.

5. A printing member according to claim 1 in which the composition comprises a polyacetal copolymer containing alkylene units of at least 2 carbon atoms.

6. A printing member according to any preceding claim in which the composition contains carbon black.

7. A printing member according to any preceding claim that has been engraved by an ion, electron or laser beam.

8. A printing member according to claim 7 in which the engraved surface is plated with a metal.

9. A printing member according to claim 7 or claim 8 in which the engraved surface has subsequently been plated with a metal.

9. A printing member according to any preceding claim comprising a cylindrical substrate carrying a cylindrical sleeve of the polymeric composition and that has been shrunk onto the cylindrical substrate.

10. A printing member according to claim 9 in which there is a seal of sealing material at each end to prevent ink penetrating between the substrate and the sheet.

11. A printing member according to claim 10 in which there is a groove in each end of the cylinder containing sealing material.

12. A printing member according to claim 11 in which the groove is the chamfered edge of the cylinder.

13. A printing member according to any of claims 10 to 12 in which the sealing material is silicone rubber.

14. A printing member according to claim 11 including an annular groove in each end of the cylinder and the sealing material is an extension of the sleeve which has been pressed into the groove.

15. A printing member according to any of claims 9 to 12 in which the sleeve was initially made with strain in its peripheral direction and has been heated while on the cylinder to release the strain and shrink the sleeve into a tight fit with the cylinder.

16. A method according to any of claims 9 to 13 in which the sleeve has been made by jointing a flat sheet of the polymer composition.

New claims or amendments to claims filed on 24 January 1980.

Superseded claims claim 8 New or amended claims:

8. A printing member according to claim 7 in which the engraved surface has subsequently been chemically etched.