Peter
Martin
Peter
Martin
| 1. | A method of preparing a patterned screen printing plate for use in proofing comprising using a laser to 5 ablate discrete areas of material from a continuous substrate to form a pattern of discrete perforations corresponding to the pattern to be formed on the final printing screen, the impingement of the laser with the substrate being controlled firstly by pattern information ° corresponding to the pattern to be formed, and secondly by the laser only being allowed to impinge on the substrate at areas corresponding to those at which perforations are present in the final printing screen. |
| 2. | A method according to claim 1, in which the 5 continuous substrate is formed by axially oriented polyethylene teraphthalate film. |
| 3. | A metthod according to claim 1 or 2, in which the laser beam is only allowed to impinge on the substrate at areas corresponding to those of perforations in the final 0 printing screen by covering the continuous substrate with all or part of a typical preperforated rotary printing screen having all of its perforations open. |
| 4. | A method according to claim 1 or 2, in which the laser beam is modulated by both the pattern information 5 and information corresponding to the pattern of perforations on a printing screen. |
| 5. | A method according to claim 4, in which the information, corresponding to the pattern of perforations on a printing screen is obtained by directly scanning a 0 preperforated printing screen using a conventional scanning head. |
| 6. | A method according to claim 5, in which the printing screen which is scanned by the scanning head is the same printing screen that is to be used to print the final 5 pattern. |
| 7. | A method according to claim 4, in which a pattern corresponding to that of the perforations in a printing screen is generated entirely electronically. |
In the production of printed textiles, for example, by a screen printing operation the inks are applied to the textile through a perforated nickel screen printing screen. The perforations are usually arranged in a generally hexagonal pattern and the screen is coated with a lacquer which fills all of the perforations in the screen. The perforations are then selectively opened to create the pattern to be printed by the screen printing member. Traditionally the lacquer is photosensitive and is exposed to light via a high contrast film having the pattern to be printed in the form of opaque and transparent regions on the high contrast film. The photosensitive lacquer is then developed and this enables the lacquer to be selectively removed from the perforations through which it is required to supply ink. More recently the lacquer has been removed by ablating it with a laser to open the perforations. When a laser is used to prepare a printing screen in this way it is not necessary to use a photosensitive lacquer.
GB-A-2050104 describes the production of a screen for screen printing textiles by photographic or direct laser engraving techniques in which the apertures through the printing screen are tailored individually to suit the required ink flow characteristics to suit the pattern to be printed by the printing screen. The apparatus for preparing the printing screen combines the image data produced by scanning an original image with previously stored signals relating to the desired aperture patterns and only enables the laser to engrave the printing screens when both signals are co-incident. US-A-3981237 describes the construction of a plastics printing screen to replace the conventional nickel printing screens.
When a new textile design is created it is desirable to be able to produce a small quantity of cloth as a proof to check the design and to show to customers when soliciting their orders. It is uneconomic to produce printing screens, particularly printing screens for each different colour ink that is required and to set up an entire multi-colour printing operation merely to print a few metres of material as a sample. Accordingly, it is usual to produce a sample of the new textile design using a flat bed silk screen printing press which prints a single pattern repeat or some small sample from the pattern. Naturally a number of single pattern repeats are printed one after the other along the sample length of fabric. The flat bed silk screen printing press uses a fine square mesh of silk or plastics material which is again coated with a lacquer to fill the interstices between adjacent threads of the screen. The lacquer is photosensitive and the pattern through which the ink is forced in the silk screen printing press is produced by a photographic process in a similar way to that used on the final rotary printing screen.
A sample produced on a flat bed silk screen printing press gives some idea of the new design which has been created but since the substrate forming the printing screen is of a totally different nature from the rotary nickel screen it does not have the same pitch or have the apertures through which ink passes at the same location as those of the final printing screen. Accordingly, the result that is obtained from the flat bed silk screen printing press is different from the final result that is obtained in production. It is also impossible to check the sample for various printing defects such as to check for the absence of Moire effects and accurately to check for any colour corrections, compensation and colour balance assessments.
According to this invention a method of preparing a patterned screen printing plate for use in proofing comprises using a laser to ablate discrete areas of material from a continuous substrate to form a pattern of discrete perforations corresponding to the pattern to be formed on the final printing screen, the impingement of the laser with the substrate being controlled firstly by pattern information corresponding to the pattern to be formed, and secondly by the laser only being allowed to impinge on the substrate at areas corresponding to those at which perforations are present in the final printing screen.
By starting with a continuous substrate and then removing discrete areas to form perforations in the substrate only at areas where both it is required that a pattern is formed and there is a perforation at that point in the final printing screen which is to be used for printing large quantities of material, it is possible to manufacture an exact replica of the final printing screen. Accordingly by using such a printing screen on flat bed printing press it is possible to get a substantially exact reproduction of what would eventually be produced when the same pattern information is used to produce a rotary, pre-perforated printing screen and then the resulting rotary pre-perforated printing screen used to print a textile. This can be done using the information in the same digitised form as required for preparing t)e printing screen and makes it unnecessary to expose a film just to produce a sample of the design. Preferably the continuous substrate is formed by axially oriented polyethylene teraphthalate film sold under the trade name of MYLAR. By using such a substrate as the material for a printing screen this can easily be ablated using a laser of reasonable power. The substrate material is cheap and the resulting screen can be
produced readily and quickly, certainly very much more quickly than the production of a set of rotary pre-perforated printing screens. Further, the resulting printing screen can be used in a conventional flat bed silk screen printing press to print initial trial samples of materials easily and readily.
The laser beam may only be allowed to impinge on the substrate at areas corresponding to those of perforations in the final printing screen by covering a continuous substrate with all or part of a typical rotary pre-perforated printing screen having all of its perforations open. In this way, by only allowing the laser beam to impinge on the substrate through a typical rotary perforated printing screen the laser beam only ablates the substrate material in areas immediately beneath the perforations in the rotary printing screen. This arrangement, whilst simple, enables the pattern to be formed in the continuous substrate very quickly. The laser beam is merely modulated in accordance with the pattern information as the laser beam is scanned over the surface of the continuous substrate and printing screen. Where the laser beam is turned ON over the solid areas of the printing screen it is reflected or absorbed by printing .screen but where the laser beam is turned ON over the perforations of the printing screen it passes through the printing screen and ablates material from the substrate but only beneath the perforations in the printing screen.
Alternatively the laser beam may be modulated by both' the pattern information and information corresponding to the pattern of perforations on a printing screen. This information may be information obtained by directly scanning a printing screen and if absolutely exact correspondence is required by scanning the perforations of the printing screen that is
subsequently going to be used to print the pattern, using a conventional scanning head. Alternatively, a pattern corresponding to that of perforations in a printing screen may be generated entirely electronically. In either event the information corresponding to the pattern of the perforations on the printing screen is used as an enabling signal only to allow the laser to be fired when it is focussed at points corresponding to the perforations of the printing screen. Naturally whether or not the laser is actually fired in these regions depends upon the pattern information that is also used to modulate the output of the laser.
The continuous substrate may be engraved whilst it is held on a cylindrical surface in an apparatus such as that sold by us under the trade name ZEDCO SCREEN, or for example, as described in detail in GB-A-2050104, using the data corresponding to the pattern of the perforations in the printing screen as the signals relating to the required apertures. Alternatively the flat printing screens may be prepared on a flat bed by using means to scan the laser beam preferably in a raster pattern over the continuous substrate as it is held on a flat bed.