Screen printing is an important industrial printing technique. In conventional wet screen printing a porous screen is placed in contact with the substrate on to which the image is to be printed. The screen may be made of, for example, a mesh formed from metal or synthetic fibre, or from perforated metal foil. The screen supports a stencil or mask which obscures pores of the screen in the"non-image" areas. During printing, the print medium (e. g. ink) is applied to the screen surface and mask. Pressure is applied to the print medium by, for example, a squeegee. The print medium is forced through the open pores of the mesh screen (but not those pores blocked by the mask) to print the desired image onto the substrate. Wet screen printing normally requires a gap between screen and substrate. As the squeegee passes over the print area, it presses the screen momentarily into contact with the print receiving surface, to effect the ink transfer from the open parts of the screen. To provide the mechanical strength and flexibility required for this off-contact printing, woven screens are usually preferred.

Dry powder printing processes are also known where dry powder such as toner is applied to the screen and mask. In such so called"dry screen printing", the image defining mask is preferably in intimate contact with the print receiving substrate to avoid powder scatter. It may be desirable to print multiple images one over the other to obtain a more complicated design such as in colour printing. To optimize registration of images in multiple or superimposed printing, flexibility of the mask is undesirable. Perforated films and foils have been found to be eminently suitable for dry screen

printing. They are much cheaper than woven screens and give better print results.

In the simplest form of screen printing, the mask is cut from a laminated film material and stuck to the screen. In general, in industrial screen printing the mask may be formed photographically, as follows: 1) A mask film coating of light sensitive resin or photopolymer is applied to the screen surface to obscure the whole screen surface. To ensure complete coverage and smoothness of the mesh screen a number of successive coatings of light-sensitive resin or photoemulsion may be applied.

Prior to exposure (to light, for example UV light) the stencil film coating is soluble in aqueous solution.

Exposure hardens the mask film coating and renders it insoluble in aqueous solution.

2) The mask film coating of light-sensitive resin or photopolymer is photographically exposed to the image. If exposure is made through a photographic positive image, the coating will be hardened in the non-image areas of the photomaster.

3) After exposure, the complete screen surface is washed.

The washing removes non-hardened areas of mask film coating- the image areas of the photomaster-leaving the screen mesh open in these areas. The resultant screen with image may be termed a direct stencil, and may be used for screen printing in the normal way. Alternatively, rather than being directly applied to the screen mesh, the mask film coating of photopolymer may be disposed on a transparent carrier film.

The coated carrier film may be photographically exposed and washed in the usual way to leave the stencil on the carrier film. The stencil may then be laminated onto the screen mesh,

with subsequent removal of the carrier, to leave a masked screen mesh for printing. This method is known as indirect stencilling.

The shape of the mask is determined by a photomaster, which itself masks certain areas of the mask film coating from exposure. The mask film coating may be exposed to radiation either in contact with, or by projection through, the photomaster. After exposure it is the masked area of the mask film coating which is removed by aqueous development to provide the final stencil.

More recent methods have exposed the mask film coating directly by laser projection interfaced with digital data from a computer, that is by laser printing the mask film coating to provide a photostencil. The laser light projects a negative image into the mask film coating, and the area of the coating which was not exposed to laser light is then removed by washing. This process removes the need for a photomaster.

Another recent filmless computer-to-screen system of making photostencils is to jet print a hot-melt photo-opaque coloured wax image onto the mask film coating; photomaster is thus substituted by the jet printed wax image. After exposure to U. V. light, the non-exposed parts of the coating are removed by aqueous development to provide the mask. Like the digitally input laser projection, this process removes the need for a photomaster. However, it is still necessary to a) coat the screen with a mask film coating and b) remove the non-exposed area of the mask film coating with an aqueous development step.

According to the present invention there is provided a method of screen printing an image comprising the steps of:

[a] providing a printing screen for printing an image onto a printing substrate; [b] applying a film forming hot melt material to the printing screen to provide a stencil of the image to be printed; and [c] printing the image through the printing screen onto the substrate.

The film forming hot melt material may be applied to the printing screen directly. Alternatively, the film forming hot melt material may be applied to the printing screen via a carrier. Preferably, the carrier is a smooth surface in the form of a roller or a planar sheet.

The hot melt material may be a thermoplastic material.

The film forming hot melt material forms a mask in the shape of the negative of the image to be printed.

The present invention removes the need for a) coating the screen with a photoactive, mask film coating; b) exposure of the film coating; and c) subsequent aqueous development step.

The printing screen may be of synthetic fibre, wire cloth or perforate foil.

After printing, there may be some residual attraction between the screen and the substrate. In this case, the electrostatic attraction may be removed by applying an AC current to the screen (for example, using a corona). In electrically conducting screens the discharge may be achieved by earthing (grounding) the conductive screen or by applying an AC discharge through the screen. This discharge serves to aid separation of the printing screen and the printed substrate.

Preferably, the printing screen is a perforate polymer film.

Polymer films may be made electrically conductive to assist in separation from the substrate after printing.

Preferably the final printing step (step c) prints the image by a dry screen printing process. Dry screen printing processes are known (see US-A-5 355 794). These processes use dry printing powders or toners. The printing powder to be used may be of the kind used in conventional electrophotographic printing. The printing powder is preferably a mono-component toner, but may also be used as a two-component toner used with a carrier particle or bead.

If a two-component toner is used, the particle size of the carrier must be larger than the aperture of the screen to prevent the carrier passing through the printing screen.

Typical toner resins that may be used are polystyrene-n- butylmethacrylate resin, polystyrene-n-butylacrylate resin, polystyrene-butadiene resin, polyethylene, polyethylene vinylacetate resin, polyester resin or expoxy resin.

If the screen printing is to be by a dry powder printing step, then the printing screen may preferably be a perforate polymer film, for example a perforated polyester film.

In one alternative embodiment, the screen may comprise a fine, plain weave, stainless steel screen or may comprise a perforated metal foil. The screen preferably has a mesh opening within the range about 20um to about 50um. More preferably the screen has mesh opening size of about 36um.

It is also possible to use a finely perforated metal foil, preferably, the mesh opening size is within the range of about 20um to about 50um.

The printing substrate may be, for example, card, paper, glass, wood, metal or plastic.

Preferably the film forming hot melt material is a wax, for example in the form of a puck of solid wax. The wax may be, for example, coumarone-indene resin, rosin or derivative of rosin, a mineral, vegetable or petroleum wax, an alkyd, a terpene resin, a heat stable formaldehyde resin, or combination of one or more of these waxes. The wax is heated to a temperature usually in the range of 80-150°C, typically 130°C, to melt it, then pumped up to, for example, the piezo head of a conventional ink-jet printer, for stencilling.

Preferably, the film forming hot melt material is formed on the screen by a jet printer. This may be a conventional ink- jet printer adapted for use with hot melt film forming materials such as waxes. The jet printer may be controlled by computer (by, for example, conventional graphics and printer control software) to accurately print the film forming hot melt material onto the screen to form a mask as the negative of the image to be printed on to the substrate.

The definition or resolution of the jet printed mask on the screen is affected by the drop size of the film forming hot melt material and the size of aperture of the screen. To achieve a high definition mask, which is desirable for high screen print resolution, the drop size of the film forming hot melt material may be. reduced to below the size of the pores or apertures of the screen.

For a relatively coarse stencil image, for example using a print head giving a resolution 600-1000 dpi (dots per inch), the mask can be jet printed directly onto the screen. There may be some drop penetration of the mesh aperture, which may

result in extremely small holes in the mask (referred to as pin holes), but this can be overcome by repeated superimposed jet printing of the mask onto the screen.

For very high resolution printing, for example at resolutions between 1000-2600 dpi, it may be necessary to fill the apertures of the screen with a temporary filler prior to jet printing the mask onto the screen. The temporary filler prevents the droplets of film forming hot melt material penetrating the aperture of the screen. The temporary filler is removed prior to printing.

As stated, a solid powder printing method such as electrostatic toner deposition (as disclosed in US Patent No.

5,355,794) is preferred for the final printing step when the hot melt material is wax. If the film forming hot melt material used is wax, the wax image is fragile: solvents used in final wet ink printing steps would quickly remove the wax image. When used in conventional prior art screen printing processes to mask the mask film coating (as described above), the hot melt material is used merely as a mask for UV exposure, and is therefore not subject to significant mechanical or chemical treatment before removal to expose the unmasked area of the screen. Because the wax image can be fragile, it may be undesirable to subject it to the mechanical and chemical treatment inevitable when screen printing with a wet ink. The use of dry powders allows the final image to be printed without subjecting the wax to significant mechanical and chemical attack.

Use of a solid powder method is also advantageous in that it is particularly suited to automated operation. The invention provides that preparation and printing become quick, simple and fully controllable directly through computer input; the

present invention provides fully automated screen preparation.

Instead of a wax, a fusible metal alloy such as Wood's metal (bismuth 50%, lead 25%, tin 12.5%, cadmium 12.5%), which melts at about 70°C can be used as the film-forming hot melt material. Fusible metal alloys of this kind can be chosen so as to be resistant to water, solvents, and abrasion; allowing conventional wet printing methods to be readily used.

In a further aspect, the present invention also provides a method of applying a mask to a screen to form a stencil for screen printing, comprising applying a film forming hot melt material directly onto the screen.

Preferably, the film forming hot melt material is applied by jet-printing. Rather than applying ink, the printing head or nozzle of an ink-jet printer is adapted to print a film forming hot melt material such as a wax.

It will be appreciated that ink jets are already capable of a resolution in excess of 2000 dpi; the present invention can thus provide a high resolution printed image.

According to the present invention in a second aspect there is provided a screen for screen printing comprising a perforate polymer film.

Preferably the polymer film is a polyester film.

The perforate polyester screen may be formed from a sheet of non perforate polyester film, for example by rolling using a spiked roller.

The perforate polymer film may be used in methods of screen printing according to the present invention. The perforate

polymer film may also be used in known screen printing methods. For example, the perforate film may be coated with a light sensitive layer. After imaging the stencil, either through a hot melt wax stencil or a digitally produced photomask, or by computer controlled laser printing, the photostencil may be developed in warm water.

According to the present invention in a third aspect there is provided a method of screen printing an image comprising steps of: a providing a printing screen for printing an image onto a printing substrate; b cutting perforations in the printing screen with a laser to provide a stencil of the image to be printed ; and [c] printing the image through the printing screen onto the substrate.

It will be appreciated that by"cutting perforations"it is meant any way of causing a perforation in the screen using a laser, for example cutting away or ablating a region of the screen to form a perforation.

The printing screen may be a film such as a polymer film.

The printing screen may be a perforate or non-perforate film.

Preferably the printing step is effected by dry screen printing. The printing step may be effected by wet screen printing.

The printing screen may be cut from a non-perforate film.

Laser ablation apparatus is commercially available, and has been used in the paper, textile and electronics industries.

For example a film can be cut with a 1200Watt carbon

dioxide laser, radio frequency powered and polarized to minimise reflection from the smooth surface of the film.

Embodiments of the invention will now be illustrated with reference to the attached drawings, in which: FIGURE 1 shows a copy of a screen print (Example 1) made using a method embodying the invention; and FIGURE 2 shows a copy of a screen print (Example 2) made using a second method embodying the invention.

EXAMPLE 1 (First aspect of the invention).

A stainless steel screen printing screen (150/cm mesh count) is masked directly with a stencil in the form of a wax mask.

The mask is applied using a proprietary ink jet printer which has been modified to apply a hot melt wax instead of ink.

The printer is computer controlled by standard printing software so that the mask (which is a negative of the image to be printed through the screen) is printed as a thin layer of wax on the screen. The stencil or mask is printed in line scanning mode.

Once the stencil or mask has set, standard dry screen printing steps are carried out. These are fully described in US Patent No. US-A-5 355 794. The substrate to which the image is to be applied, in this case a paper sheet, is charged with a positive corona and toner (5-8hum particle size) applied using a brush. The image is fused using thermal radiation. Fig 1 shows the high definition image provided using this method. The directly jet masked screen, preferably used in conjunction with dry screen printing methods, can also provide one or more of the following advantages:

1. It is not necessary to coat the screen with a layer of photo-active mask film coating. The mask/stencil of the present invention can be printed directly from a jet printer.

Subsequent processing (UV exposure, washing) is not necessary.

2. The hot melt mask can easily be removed after use, for example by hot air blast into an absorbent material placed behind the screen or by a combination of heat and suction, thus permitting the recycling of the hot-melt mask film coating material.

3. The jet-printer may be incorporated in the screen printing machine, so that the steps of masking the screen, printing the image and removing the mask are done in situ.

Any or all of these steps may be fully computer controlled.

EXAMPLE 2 (second, third aspects of the invention) The screen used is a plastics sheet. In the case of Example 2, the screen was a ready perforated plastics sheet to replace the more expensive and less dimensionally stable woven screen. The perforate foil may then be masked with a hot melt stencil; the hot melt masks the screen, thus causing a blank area where the ink is not applied to the substrate.

The standard dry screen printing steps are carried out, as fully described in US Patent No. US-A-5 355 794. The substrate to which the image is to be applied, in this case a paper sheet, is charged with a positive corona and toner (5-8pu particle size) applied using a brush. The image is fused using thermal radiation.

It will be appreciated that similar results can be obtained using a non perforate sheet which is perforated in the shape

of the image to be printed using a laser (cut with a 1200 Watt carbon dioxide laser, radio frequency powered and polarized to minimise reflection from the smooth surface of the film), prior to the printing step. The laser can be incorporated in the screen printing machine, so that the steps of making the screen and printing the image may be done in situ.

The screen printing machine may incorporate tension rollers on two sides of the printing area. A full roll of non perforate plastics sheet is mounted on one of the rollers "the supply roller". A section of this sheet is unrolled across the printing area and fixed to the second roller"the receiving roller". The section of the sheet that is across the printing area forms the screen. This may be perforated (for example, by a laser) and used to print one or more images by standard screen printing methods. Once the screen is worn, or the print run finished, the used screen can be wound on to the receiving roller. In doing this a fresh screen is thus rolled over the printing area from the supply roller, forming a new screen.