GB2412335A

GB2412335A - Base film for use in the manufacture and mounting of base plates

Info

Publication number: GB2412335A
Application number: GB0406679A
Authority: GB
Inventors: Matthew Barker
Original assignee: PRO SHIM SUPPLIES U K Ltd
Current assignee: PRO SHIM SUPPLIES U K Ltd
Priority date: 2004-03-25
Filing date: 2004-03-25
Publication date: 2005-09-28
Anticipated expiration: 2024-03-25
Also published as: GB2412335B; GB2412335B8; GB2412335A8; GB0406679D0

Abstract

The present invention provide an improved base film 1, for use in the production and mounting of a flexographic printing plate 7. The base film 2, which provides a base structure on to which a cured photopolymer relief 6 image is bound, is usually mounted on to a printing press carrier means using double-sided adhesive tape. The improved base film 1 of the present invention eliminates the stage of applying the adhesive tape by providing a base film with a pre-production applied solvent acrylic adhesive 4 and thus eliminates the need for the removal of trapped air bubbles. In order to protect the adhesive layer during the rigours of the flexographic printing plate 7 production process, a release liner 3 is also provided. The adhesive may be applied to the base film sheet in strips.

Description

24 1 2335 An Improved Base Film for use in the Production and Mounting of

Flexographic Printing Plates

Field of the Invention

The present invention relates to the general field of printing plate manufacture and more specifically to the manufacture of flexographic printing plates.

Background of the Invention

Within the printing industry there are a number of accepted mechanisms for producing printing templates for use in medium and large scale printing runs. One such mechanism is the production of flexographic printing plates.

The process of manufacturing flexographic printing plates (sometimes known as liquid photopolymer printing plates) involves providing a layer of a particular photopolymer resin on a polyester base film or backing and then covering the photopolymer resin with a negative image template before exposing the photopolymer to an ultraviolet(UV) light source. The nature of the photopolymer resin is such that those areas that are not covered by the negative template are cured and as a result harden, where as the protected areas remain soft. By this process the pattern of the negative image template is transferred to the photopolymer which is in turn bonded to the polyester base film.

The next stage of the process is to remove the excess, non-hardened photopolymer resin and clean up the printing plate. This is initially done by the use of scrapers to manually remove the excess. Following the manual treatment the plate is subjecting to a machine wash, which removes the remainder of the excess photopolymer resin.

Once the excess photopolymer resin has been removed, the plate is subjected to a final curing stage. Following the curing stage the flexographic printing plate is ready to be mounted on to a printing press carrier. The carrier may take the form of a drum-like structure, a cylindrical sleeve, or a continuous belt for example.

At present, this stage of the printing process involves a skilled printing technician firstly applying a double-sided adhesive sticky tape to the back of the printing plate, i.e. the polyester base film. This procedure can be difficult and time consuming due to the tendency for air bubbles to get trapped between the sticky tape and the back of the printing plate. The presence of trapped air bubbles is extremely undesirable due to the levels of accuracy required to obtain efficient printing results.

Although a skilled printing technician can greatly reduce the number of air bubbles trapped during the above process, the air removal process is time consuming and can still result in the technician having to pop air bubbles to release the trapped air.

Once the flexographic printing plate is successfully mounted on the double-sided adhesive tape the plate needs to mounted onto the printing press carrier. In this second application of the double-sided adhesive, the same problems as discussed above are again encountered, viz. trapped air bubbles. At this stage of the process the technician must be even more careful when handling the printing plate because the removal of trapped air bubbles can be more difficult once the plate is mounted upon the printing press carrier.

In view of the time consuming nature of the final stages of the preparation of a liquid photopolymer printing plate and the potential for errors (i.e. trapped air bubbles), there is a need for an more efficient and reliable method for producing and mounting flexographic printing plates.

Summary of the Invention

In a first embodiment of the present invention there is provided an adhesive base film for the use in the production of flexographic printing plates, comprising a base sheet on which the flexographic plate is formed, a release liner sheet, and a solvent acrylic adhesive that laminates the sheets together, wherein the adhesive adheres to the base sheet more powerfully than to the release liner sheet, thus permitting the release liner sheet to be removed without removing the adhesive from the base sheet.

Preferably the adhesive does not cover the entire surface of the base sheet. Advantageously, the adhesive can be applied in such a way that air channels are formed between the regions where adhesive is present. A preferred application of the adhesive would provide the adhesive in strips, thereby providing intermediate strips where there is no adhesive that act as air channels.

Preferably, the adhesive is applied at a coat weight of 25-35 grams per square inch.

Preferably, the release liner sheet is made of a material that permits UV light to pass therethrough. More preferably, such material is polyester.

This is advantageous because it stops the release liner sheet from interfering with the curing of the photopolymer resin.

Advantageously the release liner sheet has a thickness of between 30 and 70,um, although a thickness of 50,um is most preferable.

Usually the adhesive base film of the present invention would be stored in a rolled up form, and preferably with the release liner sheet on the inside of the roll.

In a second embodiment of the present invention there is provided a method of producing and mounting a flexographic printing plate comprising the steps of: a) placing an image negative template and then a protective cover layer onto the work surface of a UV developing bed, said bed having a UV light source above the work surface and a UV light source beneath the work surface; b) providing a layer of the adhesive base film of the present invention over the protective cover layer; c) providing a layer of a photopolymer resin over the adhesive base film; d) subjecting the work surface to a UV light from above thereby partially curing the entire surface of the photopolymer resin to create a floor of cured photopolymer on the adhesive base film; e) subjecting the work surface to a UV light from below, thereby curing only the areas of the photopolymer resin that are not protected from the UV light by the image negative template to transfer a corresponding image onto the photopolymer; f) cleaning the flexographic plate, which comprises the substantially cured photopolymer resin and the adhesive base flim, to remove the excess, uncured photopolymer resin; 9) subjecting the flexographic plate to at least one further hardening process; h) removing the release liner from the adhesive base film and mounting the flexographic plate onto a printing press carrier.

The flexographic plate may be cleaned manually by scraping off the excess, uncured photopolymer. The flexographic may be cleaned using a machine wash. Preferably, a machine wash uses water jets and is run at a high temperature, typically between 50 and 70 C. Advantageously, the cleaning step of the present invention comprises both a manual cleaning step and a machine washing step.

Preferably, the layer of photopolymer resin provided in step c is of a thickness of between 1 and 6.5,um.

Preferably, the UV lighting used in the present invention is provided by a 60 Watt UV light.

Advantageously, the UV light from above is used for a period of about 1.5mins. Preferably, the UV light from below is used for a period of 1 5mins.

Brief Description of the Drawings

The present invention will be herein described with reference to the drawings, wherein: Figure 1 shows a cross-sectional view of the adhesive base film of the present invention; Figure 2 shows a perspective view of a preferred form of the adhesive base film of the present invention; and Figure 3 shows a cross-sectional view of the flexographic printing plate formed using the adhesive base film of the present invention with the release layer partially removed.

Detailed Description of the Invention

The present invention provides an adhesive base film for use in the production and mounting of flexographic printing plates. At present, flexographic printing plates are produced using an established method, which is well known in the art and has already been discussed in the Background of the Invention section.

In this established methodology, a standard polyester base film is used as a base structure for the formation of a flexographic printing plate from a photopolymer resin, an example of which is a resin supplied under the name of Chemence_.

The main problems with the method used in the state of the art arise when it comes to mounting the flexographic printing plate onto a printing press carrier, an example of which being a drum-like structure. In order for a high quality printing finish the flexographic printing plate needs to be accurately positioned on the carrier. This means that any imperfections created during the mounting stage must be greatly reduced, if not completely eliminated.

Trapped air bubbles between the various layers of the system are a major cause of imperfections and as a result must be removed by the printing technician where ever possible.

Air bubbles generally become trapped during the mounting stage of the process. This stage firstly involves the application of a double-sided adhesive tape to the exposed base film portion of the flexographic printing plate (i.e. the side of the base sheet that is not covered by the cured photopolymer), followed by the application of flexographic printing plate to mounting plate, that can then be attached to the printing press carrier, or directly on to the carrier.

The process of ensuring that all of the trapped air bubbles are removed is time consuming and is usually carried out by a skilled printing technician.

In its broadest form the adhesive base film 1 of the present invention (as shown in Fig.1) removes the need for the first part of the mounting stage by providing a base sheet 2 that already possesses an adhesive layer 4. It will be appreciated that the base sheet of the present invention may be a standard base film sheet as used in the current methods of production. The base sheet of the present invention may preferably be made of polyester having a thickness of 125,um.

The provision of this adhesive base film 1 reduces the time that the printing technician needs to spend ensuring that there are no air bubbles are trapped between the base film and the adhesive. During the production of the flexographic printing plate a release liner 3 protects the adhesive layer 4.

As can be seen from Fig.2, in a preferred embodiment of the adhesive base film 1 the adhesive layer 4 is applied in strips, thus creating intermediate strips 5 on the base film 2 without any adhesive. By providing an adhesive base film 1 of the preferred embodiment, it is possible to virtually eliminate the time spent by the printing technician removing trapped air bubbles.

Whilst the strips of adhesive 4 provide the required sticking power to retain the flexographic printing plate 6 on the printing press carrier, the strip regions 5 without adhesive serve as air channels 5 and provide a means of egress for any air trapped between the base sheet 2 and a mounting surface(i.e. the carrier mounting or the carrier directly).

Although the presence of air channels 5 does not necessary prevent the formation of trapped air bubbles, it does greatly reduce the number of air bubbles formed. Furthermore, any air bubbles that do get trapped can easily be worked towards the nearest air channel 5 by the printing technician so that they can be released.

Due to the harsh environmental treatment that a standard base film must endure during the various stages of the production of a flexographic printing plate, the adhesive and the release liner have optimum characteristics that contribute to producing the most consistent performing adhesive base film.

The main stages of the established production process that can effect the adhesive are the cleaning stage, the hardening stage and the drying stage. More specifically for example, during the cleaning stage the post-UV treatment photopolymer covered base film is subjected to a machine wash that involves the use of hot water jets to flush out the remaining excess uncured photopolymer resin. The temperatures in the washing machine, about 60 C, causes the formation of steam, which can get between the base sheet 2 and the release liner sheet 3 and greatly reduce the adhesives effectiveness.

Following numerous tests it was found that a solvent acrylic adhesive, which is specially designed to reject water and moisture, provided the best adhesion levels following the flexographic printing plate production process.

Furthermore, it was discovered that the amount or coat weight of the solvent acrylic adhesive required to provide the necessary sticking power was between 25 and 35gsi (grams per square inch). Although the most effective amount of the solvent acrylic adhesive is about 3Ogsi.

In the preferred form of the adhesive base film where the adhesive is applied in strips, the strips of adhesive advantageously up at least 50% of the base film surface. One example of a preferred distribution are adhesive strips having a width of between 3mm and 15mm, but preferably 5mm, and air channels having a width of between 2mm and 4mm, but preferably 3mm.

Although the present invention discloses strips of adhesive as a preferred distribution pattern, it is appreciated that alternative distribution patterns(e.g. diamond shaped adhesive regions) may provide an equally effective means for allowing trapped air to escape.

As with the adhesive layer used in the present invention, the flexographic printing plate production process caused certain problems to the release liner used with the present invention.

More specifically for example, during the hardening stage wherein the post-cleaning flexographic printing plate is usually submerged in water bath whilst it is subjected to germicidal lights to harden off the photopolymer, it was found that water was starting to soak into the release liner sheet causing it to come away from the base sheet.

Following further testing it was found that thicker release liner sheets were more resilient to the water bath treatment. Preferred release liner thickness range from between 30 and 70,um, although a thickness of 50,um was found to be the most effective.

An additional advantage of the present invention, beyond the time saving aspect(which can save up to 20 minutes per job), is the production of a more sturdy nature of the flexographic printing plate during production. It was found that during the production process, the presence of the release liner sheet made the intermediate flexographic plate more sturdy and thus easier to handle.

Another key feature of the present invention is the fact that the adhesive layer 4 adheres more firmly to the base sheet 2 than it does to the release liner sheet 3. This is possible due to the fact the adhesive sticks more strongly to the glossy finish side of a polyester sheet than it does to the matt finish side of a polyester sheet.

In this way the easy removal of the release liner sheet 3 before mounting is facilitated (see Fig.3) It will be appreciated by the skilled man that nature of the adhesive base film of the present invention is such that it can simply be used in the place of a standard base film in the flexographic printing plate production processes known in the art with out the need for any alteration of the standard process. The only alteration to the production process comes when the printing technician comes to mount the finished printing plate, at which stage the time needed for the mounting process is greatly reduced.

Claims

1. An adhesive base film for the use in the production of flexographic printing plates, comprising a base sheet on which the flexographic plate is formed, a release liner sheet, and a solvent acrylic adhesive that laminates the sheets together, wherein the adhesive adheres to the base sheet more powerfully than to the release liner sheet, thus permitting the release liner sheet to be removed without removing the adhesive from the base sheet.

2. The adhesive base film of claim 1, wherein the adhesive does not cover the entire release liner facing surface of the base film sheet.

3. The adhesive base film of claim 2, wherein the adhesive is applied to the base film sheet in such a way that air channels are formed between the regions where adhesive is present.

4. The adhesive base film of claim 3, wherein the adhesive is applied to the base film sheet in strips, thereby providing intermediate strips where there is no adhesive that act as air channels.

5. The adhesive base film of any of the preceding claims, wherein the release liner sheet is made of a material that permits UV light to pass therethrough.

6. The adhesive base film of any of the preceding claims, wherein the release liner sheet has a thickness of between 30 and 70,um.

7. The adhesive base film of any of the preceding claims, wherein the adhesive base film is stored in a rolled up form.

8. A method of producing and mounting a flexographic printing plate comprising the steps of: a) placing an image negative template and then a protective cover layer onto the work surface of a UV developing bed, said bed having a UV light source above the work surface and a UV light source beneath the work surface; b) providing a layer of the adhesive base film according to any of claims 1 to 7 over the protective cover layer; c) providing a layer of a photopolymer resin over the adhesive base film; d) subjecting the work surface to a UV light from above, thereby partially curing the entire surface of the photopolymer resin to create a floor of cured photopolymer on the adhesive base film; e) subjecting the work surface to a UV light from below, thereby curing only the areas of the photopolymer resin that are not protected from the UV light by the image negative template to transfer a corresponding image onto the photopolymer; f) cleaning the flexographic plate, which comprises the substantially cured photopolymer resin and the adhesive base film, to remove the excess, uncured photopolymer resin; g) subjecting the flexographic plate to at least one further finishing process; h) removing the release liner from the adhesive base film and mounting the flexographic plate onto a printing press carrier.

9. The method of claim 8, wherein the flexographic printing plate is cleaned manually by scraping off the excess, uncured photopolymer.

10. The method of claim 8 or 9, wherein the flexographic printing plate is cleaned using a machine wash.

11. The method of claim 10, wherein the machine wash uses water jets and is run at a high temperature, typically between 50 and 70 C.

12. An adhesive base film for the use in the production of flexographic printing plates, substantially as described, with reference to the drawings, herein.