GB2160989A - Flexographic printing methods and materials - Google Patents

Abstract

A flexographic print member for use in flexographic printing of case blanks and other sheet materials comprises a cylinder 16 to which is secured a printing plate comprising a substrate 7 carrying image areas 10 and non-image areas 9 of photocured resin. The plate is releasably clamped to the cylinder by means 11 and 12 that are secured to the substrate and that engage with corresponding means 17 and 14, 15 and 18 on the printing cylinder. <IMAGE>

Description

SPECIFICATION Flexographic printing methods and materials One standard method of printing sheet material is by flexography, that is to say by contact with a flexographic print member comprising a plate that comprises a flexographic relief surface bonded to a substrate sheet and that is carried by a printing cylinder. Ink on the raised portion of the relief surface is transferred image-wise to the sheet material.

The relief surface is formed of flexible material. The sheet material that is to be printed may be a continuous web or may be separate pieces. Often it is card that is, or is to be used to make, carton or other case blanks.

if the length of the pattern that is to be printed, and thus the length of the print member and the circumference of the printing cylinder, is relatively short, for instance up to 30 cm, standard practice is to stick the substrate sheet to the printing cylinder by some adhesive system, for instance double-sided tape. An unfortunate consequence of this is that removal of the plate from the cylinder may result in some distortion of the plate but, with small plates and careful handling, this distortion may be sufficiently small that it is acceptable.

When the print members are to be much larger than this, for instance when a single member is to print corrugated cardboard with the pattern required on all the sides of a carton blank (typically above 50 and often above 70 cm in length) it is common to releasably clamp the plate to the cylinder, the clamping generally being by clamping means recessed within the surface of the cylinder along the length of the cylinder.

Originally the relief surface was made by engraving but now the normal method of making the plates is by image-wise photoexposing and photocuring photocurable resin supported on a flexible support sheet followed by removal of uncured material. Thus the flexible relief surface is cast onto the support sheet during the curing operation.

When the print members are relatively small, as discussed above, the support sheet may also serve as the substrate sheet of the printing plate and so the sheet may be stuck to the printing cylinder by, for instance, adhesive tape. Alternatively however the support sheet may itself be adhered to a separate substrate sheet by, for instance, double-sided tape.

This system is not suitable when the print members are larger, for instance because of the risk of damaging the plates, and at present print members that are releasably clamped to the cylinder are always made by adhering the support sheet of several smaller plates to a sheet substrate that is then clamped to the cylinder. The adhesion to the sheet substrate of each of the support sheets onto which the relief surface is cast is normally effected by the use of double-sided tape.

Large print members for printing case blanks will generally have several separate image areas, each corresponding with one face of the eventual case, separated by areas that are either totally non-image areas or that have only small image components, for instance a pattern of lines extending around the carton. At present therefore each of these large print members is made by forming an individual plate corresponding to an image area and then adhering each of these plates onto the sheet substrate in the appropriate position. Thus a typical 6-sided case blank may be printed using a printing plate made by adhering six separate printing plates onto a substrate sheet using double-sided tape.

The individual printing plates that are stuck to the substrate sheet at present to form, after application to the cylinder, the print member are made by photoexposure. Each can be photoexposed separately from the others or, if larger photoexposure apparatus is available, several different images may be used for simultaneous image-wise photoexposure to produce a product having a single backing sheet carrying a flexible relief surface that represents several images, and this product is then cut up into the individual plates which are then adhered to the final substrate.

It is necessary for each of the small plates to be positioned very accurately in order that satisfactory print quality is achieved. This is a particular problem when the print member is to be used for multi-colour printing as it is then essential that the individual small plates are in exact registration with one another. In practice registration often has to be finally determined on the printing machine, after a proof run, and so final adjustments of registration involve the print machine being out of operation. This clearly is very expensive and undesirable.

Good registration and print quality is achievable if the operator is sufficiently skilled and has sufficient time to achieve it. In practice however time pressures, and the availability of skilled personnel, are often such that print quality may suffer.

The present state of the art therefore is that flexographic printing plates made by photoexposure of photocurable resin in contact with a support sheet are bonded by the support sheet onto the printing cylinder if they are small or, if they are large, they are manually located on, and adhered to, a substrate sheet that is then releasably clamped to the cylinder. Bonding the support sheet of large print members onto the printing cylinder is technically unsatisfactory because of the risk of damage of the plates. Adhering small plates onto a substrate sheet that is then releasably clamped to the cylinder gives poor results unless sufficient skill and time is applied to achieve accurate registration, and in practice it is difficult to achieve this.

It has been our object to devise a flexographic system that is easier to operate and that avoids the disadvantages of existing methods and that is particularly useful when printing case or other carton blanks.

In the invention we make a flexographic print member that comprises a flexographic print plate that is secured to a printing cylinder and that comprises a sheet substrate carrying a flexographic relief surface formed of a cured photopolymer, characterised in that the plate is made by applying photocurable resin against the substrate, imagewise photoexposing and photocuring the resin and removing uncured resin, whereby the flexible relief surface of the plate is cast direct to the sheet substrate, and the plate is secured to the printing cylinder by releasably clamping the substrate to the cylinder. It is particularly preferred that the photocurable resin that is applied against the substrate is a liquid photocurable resin.

The invention includes a flexographic printing method in which a sheet material is printed flexographically using a flexographic printing member that comprises a flexographic printing plate that is secured to a printing cylinder and that comprises a substrate carrying a flexographic relief surface formed of a cured photopolymer, and in this method the flexographic relief surface is cast direct to the sheet substrate and the plate is secured to the cylinder by releasably clamping the substrate to the cylinder.

In the prior art the flexible relief surface was cast direct to the sheet substrate only when that sheet substrate was to be adhered to the printing cylinder and when the plate was small. In all other instances the flexible relief surface was cast direct onto a support sheet which was then adhered, for instance by adhesive tape, to a sheet substrate that was clamped to the printing cylinder. In the invention this intermediate step is eliminated and the flexible relief surface is cast direct to the sheet substrate, i.e. without the intervening step of casting it onto a support sheet and then adhering this support sheet onto the sheet substrate.

The invention is of particular value for printing carton blanks and other articles in which a substantial proportion is to be non-image area. Usually a majority (more than 50%) and often more than 75% of the surface is to be non-image area and so remains unexposed during the imagewise photoexposure and so remains uncured. This necessitates the removal of large amounts of resin after the photocuring stage but a particular advantage of forming the plate from liquid photocurable resin is that it is easily possible to recover this large proportion of uncured resin, and reuse it in the manufacture of further flexographic printing plates. If conventional solid photocurable resin is used for forming the relief surface there may be substantial wastage of material unless difficult recovery procedures are used.

The liquid photocurable resin may be any of the materials that are available for this purpose and will be chosen having regard to the flexibility and other properties required in the final product. Often the entire plate, apart from the substrate sheet, is formed of a single composition but it can be formed of two or more different compositions, for instance as described in British Patent 2,012,977. Suitable liquid photocurable resins are described in that patent.

The photoexposure is conducted while the liquid resin is in contact with the substrate sheet. The exposure is generally conducted through a negative transparency, usually protected by a cover film, with the liquid resin between the transparency and the substrate sheet. Preferably the transparency is laid on a glass or other transparent support surface, and covered by a protective film, and the liquid polymer is then spread over this as a uniform layer, for instance by a doctor blade.

The substrate sheet is then laid onto the upper surface of the liquid resin and the resin is then photoexposed upwardly through the negative. Resin in contact with the substrate must be photocured in order to form the plate. This may be achieved solely by the imagewise photoexposure but preferably the substrate sheet is transparent and the resin is generally also given an overall exposure through the substrate sheet, this overall exposure being sufficient to photocure only a minor proportion (for instance 5 to 30%) of the depth of the resin layer.

The photocured resin layer, with the substrate sheet cast to it, is then stripped from the transparency and its protective cover film and uncured resin removed in conventional manner. For instance in large unexposed areas the majority of the uncured material may be removed simply by drainage, possibly assisted by squeegee or air blowing.

The substrate sheet is normally of a substantially non-stretchable plastic film, that is preferably transparent. The preferred material is polyester. Other possible substrates that may be used include cellulose or other foam substrate sheets and reinforced paper. The thickness is preferably from 0.05 to 0.5 mm.

The depth of the photocured resin is generally from 0.75 to 10 mm, most preferably 1 to 7 mm. In the recessed areas the base film may be exposed or may be covered with photocured material, for instance as a result of the overall photoexposure mentioned above. This overall layer may be from 0.2 to 3 mm thick. The depth of the relief is generally from 0.5 to 6 mm.

The negative transparency or other image through which photoexposure is made will be the exact image-wise reproduction of the pattern that is to be printed on the sheet material. For instance art work may be prepared showing the image that is to be printed on a carton blank, before folding it to provide the six sides of a case, and this art work may then be photographed to form the desired negative transparency.

This transparency, and the size of the resultant plate, is generally rather large, typically above 50 cm and often above 70 cm in each direction, and it is impossible in practice to position the backing sheet, that is to serve as the substrate, accurately on the liquid resin layer with respect to the negative transparency. Previously this did not matter, since the exposed product was always cut and then positioned manually on a different substrate.

In the invention it is preferred that the photocured relief image, and the transparency through which the imagewise photoexposure was made, should include positioning marks, so that the photoexposed product includes positioning marks in relief. Plates having these marks in the relief image are novel.

These positioning marks can be utilised to position a resultant plate accurately on the cylinder and so any misalignment of the substrate with respect to the negative transparency does not result in misalignment of the printing plate on the cylinder. The marks may consist of two or more marks that are to define one edge of the plate that is to extend along the length of the cylinder. These marks may be registration marks that are to be brought into register with corresponding marks along the length of the cylinder or, preferably, trimming marks to which the plate is trimmed and the trimmed edge may then be utilised to position the plate accurately on the cylinder. Some or all of the positioning marks may be folding marks that will print folding marks in to the sheet material, to indicate the position at which it is to be folded.

There may be positioning marks along one edge only of the plate or upon opposite edges or upon all four edges.

The plate can be releasably clamped to the cylinder by any convenient clamping means to form the print member. Generally these clamping means comprise recesses within the surface of the cylinder along the length of the cylinder so that the clamping means do not form print areas. Clamping means similar to those conventionally used for clamping plates for printing carton blanks may be used. Thus the clamping means may comprise cooperating engagement means for engaging one edge of the plate with respect to the cylinder and tensioning means operating on the opposite edge of the plate for tensioning the plate around the cylinder. The engagement means may comprise a ridge secured to the plate, this ridge generally having been bonded to the plate by adhesive or double-sided tape.

The location of this ridge will determine the positioning of the plate on the cylinder and so must be accurately determined. This ridge may therefore be bonded to the described positioning marks or to the described trimmed edge of the plate. This ridge may engage in a channel, for instance in the surface of the cylinder. Eyelets may be provided in the opposite edge of the plate, for instance in a strengthening strip bonded to the surface of that edge, and this edge may be secured to the cylinder by tension straps hooked between the eyelets and appropriate fasteners on the cylinder.

If the case or other carton requires a total image area greater than can conveniently be provided by a single exposure, two or more plates can be made, each having its own backing sheet and clamping means, and these can then individually be clamped on the cylinder, generally in abutting relationship. Any print marks caused by the abuttment of the edges of the plates are preferably located in fold areas of the case.

The invention has many advantages. The image areas do not have to be preformed on a support sheet and then manually located on and adhered to a substrate. Accurate registration can be achieved much quicker and generally does not require prolonged proofing and adjustment on the printing machine.

It is no longer necessary to have both a support sheet and a substrate and it is no longer necessary to use double-sided tape or other means for bonding separate pieces to the substrate. Accordingly there is a large saving in materials. Location of the image areas on the plate is determined by the art work and photoexposure conditions and so high print quality can be achieved without the need for the use of very skilled manipulation of the small print plates on the substrate sheet and without having to perform this manipulation on the printing machine.

The invention makes it possible for the first time for printers, who do not have the equipment or the resources available to print other than single colour work by conventional methods, to produce large two- and threecolour sets with excellent colour registration.

The invention is illustrated in the accompanying drawings. In these Figures 1 is a cross-section through the plate during photoexposure and Figure 2 is a cross-section through the resultant flexographic printing plate. Figure 3 is a transverse section through a printing member carrying the plate.

Referring to Figure 1, a negative 1 is laid on a glass plate 2 and protected by a cover film 3. The negative has areas 4 where the resin is to remain unexposed and areas 5 where the resin is to be exposed. These areas 5 include positioning mark images 5a.

Liquid photocurable resin 6 is spread over the cover sheet and a transparent film 7 of polyester is rolled onto the top of the resin layer by roller 8.

The assembly is then given an image-wise exposure through the negative from beneath the glass plate 2 and a short overall exposure from above the polyester film 7. This results in the polyester film 7 becoming bonded to the layer 6 and resin in the layer 6 being exposed image-wise. The assembly of layer 6 and film 7 is removed from the cover film 3 and is squeegeed and then washed to remove uncured resin, resulting in the plate shown in Figure 2. This plate consists of the polyester substrate sheet 7 cast onto an overall layer 9 of cured photopolymer and raised image areas 10 and 1 ova. The image areas 10 correspond to the areas that are to be printed onto cardboard or other sheet material whilst the image areas 1 Oa are the positioning marks that are present to facilitate accurate positioning of the plate on the printing cylinder.

The plate is then trimmed along the inside edge of each positioning mark 1 Oa.

If these positioning marks were along the edges of the plate that are to be the axial ends of the printing member then the trimmed edges can be used for accurate axial location of the printing plate on the printing cylinder. If the positioning marks are at the lateral ends of the plate then the trimmed edges can be used for accurate radial positioning of the printing plate on the printing cylinder. For instance as shown in Figure 3 a strip of rigid material 11 is bonded along one trimmed end and a strip 12 of non-tearable material is bonded along the opposite trimmed end. There are a series of apertures 13 through this strip 12. Hooks 14 that extend from tensioning straps 15 are located through these apertures.The resultant plate can then be wrapped around and clamped to a cylinder 16 to form the print member, by engagement of the ridge 11 with the tensioning straps 15 or, more usually, by engagement of the ridge 11 with a groove 17 in the cylinder and by engagement of the ends of the tensioning straps 15 with appropriate securing means 18 in the cylinder.

The print member can then be inked in conventional manner and used for printing the chosen sheet material. The invention is of particular value when the sheet material is corrugated cardboard which is cut to form case blanks and subsequently folded into cases. However it can be a continuous web of material other than corrugated cardboard, for instance a web of card to make carton blanks.

When used for printing cases or other cartons the plate will generally contain up to six main image areas, one corresponding to each face of the case, although there may be some imaging interconnecting these image areas.

As an example of the invention, a plate is made by the general method illustrated in Figure 1. The negative includes not only the desired print image 4 and 5 but also positioning mark images 5a. The cover film 3 is formed of transparent polypropylene and is 30 microns thick. The liquid resin 6 is the product known by the trade name APR Flexo Resin from Asahi Chemical Industry (UK) Limited and is chemically based primarily on polyurethane. The thickness of the layer 6 is 6.35 mm. The substrate 7 is transparent polyester 180 microns thick.

The negative transparency carries an image consisting of the entire case and has dimensions of about 70 cm x 70 cm and has areas 5 concentrated primarily in four regions corresponding to the four side faces of a case.

More than 75% of the negative has areas 4, so that more than 75% of the area of the resin 6 remains unexposed during photoexposure through the transparency.

The assembly is given an overall photoexposure of 26 seconds at an intensity of 2 mW/cm2 (20W/m2) through the substrate 7 and then an imagewise photoexposure of 250 seconds of 2mW/cm2 (20W/m2) intensity through the plate 2. As a result the depth of the layer 9 is about 1.6 mm. The substrate 7, carrying the layer 9 and the image areas 10 and 1 ova, is stripped from the sheet 3, squeegeed or air blown to remove uncured resin.

The uncured resin is collected and re-used for manufacture of a subsequent sheet. After trimming and application of strips 11 and 12 the plate is releasably clamped on to the cylinder, as shown in Figure 3.

As a comparison, four small plates, each corresponding to one of the sides of the case, were formed by four separate exposures using a method generally as illustrated in Figure 1 and as described above except that the substrate 7 was replaced by a transparent support sheet formed of polyester 100 microns thick.

These four separate plates were then individually positioned in the approximate desired positions on a substrate sheet and were secured in those positions by double-sided adhesive tape. The substrate sheet was then releasably clamped on to the cylinder, by the general method shown in Figure 3, and a proof print run was conducted. This in practice always shows that the positioning of the individual sheets is insufficiently accurate and so minor adjustments have to be made, on the printing cylinder, in the location of one or more of the individual plates.

Claims (12)

1. A method of making a flexographic print member comprising a flexographic printing plate that is secured to a printing cylinder and that comprises a sheet substrate carrying a flexographic relief surface formed of a cured photopolymer characterised in that the plate is made by applying photocurable resin against the substrate, imagewise photoexposing and photocuring the resin and removing uncured resin whereby the flexible relief surface of the plate is cast direct to the sheet substrate, and the plate is secured to the printing cylinder by releasably clamping the substrate to the cylinder.

2. A method according to claim 1 in which the resin that is applied is a liquid photocurable resin and the majority of the area of the flexible relief surface remains unexposed during the imagewise photoexposure and the unexposed liquid photocurable resin is removed after photoexposure and reused.

3. A method according to claim 2 in which the plate is made by a method comprising applying the liquid resin over a transparency of the desired image, applying the substrate over this, photoexposing the resin through the transparency and photocuring the resin in contact with the substrate, stripping the resultant plate comprising the substrate and adherent photocured resin from the transparency and removing uncured resin from the plate.

4. A method according to any preceding claim in which the substrate is transparent and the resin is given the imagewise photoexposure and is also given an overall photoexposure through the substrate for a duration sufficient to photocure a minor proportion of the depth of the resin adjacent to and throughout the entire area of the substrate.

5. A method according to any preceding claim in which the photoexposure is conducted through a transparency of the desired image and in which the image includes positioning marks, and the resultant positioning marks in the relief surface are utilised to position the plate accurately on the cylinder.

6. A method according to claim 5 in which the plate is trimmed to the positioning marks and the trimmed edges are utilised to position the plate accurately on the cylinder.

7. A method according to any preceding claim in which the releasable clamping is by clamping means recessed within the surface of the cylinder along the axial length of the cylinder.

8. A method according to claim 7 in which the clamping means comprise co-operating engagement means for engaging one end of the plate with respect to the cylinder along the axial length of the cylinder and tensioning means operating on the opposite end of the plate for tensioning the plate around the cylinder.

9. A flexographic printing method in which a sheet material is printed using a flexographic printing member made by a method according to any of claims 1 to 8.

10. A flexographic printing method in whicha sheet material is printed flexographically using a flexographic printing member that comprises a flexographic printing plate that is secured to a printing cylinder and that comprises a sheet substrate carrying a flexographic relief surface formed of a cured photopolymer characterised in that the flexible relief surface is cast direct to the sheet substrate and the plate is secured to the printing cylinder by releasably clamping the substrate to the cylinder.

11. A flexographic printing method according to claim 9 or claim 10 in which the sheet material comprises case blanks.

12. A flexographic printing plate comprising a sheet substrate that carries a flexographic relief surface formed of a photocurable photopolymer and that includes means for releasably clamping the plate to a printing cylinder to form a printing member, characterised in that the resin is cast direct to the substrate and the relief surface includes, in relief, positioning marks for use in accurate positioning of the plate on the cylinder.