GB2286257A - Three-dimensional engraving using photoresists - Google Patents

Abstract

A continuous tone image A is used to expose photoresist C through support B and developed to produce a resist of different depths. Support B is stripped off and resist C is bonded by adhesive E to the substrate F prior to shotblasting. As shotblasting proceeds, firstly the least resistant areas and then gradually the relatively more resistant areas of the resist are progressively worn away, exposing the different areas of the substrate to the shotblasting process for different amounts of time and thus producing in the substrate an image comprising differential depths of erosion as predetermined by the various levels of resistance built into the image in the resist. <IMAGE>

Description

IMPROVED METHOD OF ENGRAVING This invention relates to a novel resist and the use thereof in shotblast engraving.

Resists are widely used in shotblast engraving. When ciosely bonded on to a suitable substrate, the resist allows a chosen image to be engraved by protecting certain areas of the substrate and leaving others exposed. The longer the substrate is exposed to the shotblasting process, the deeper the engraving will tend to be.

One of the most widely used types of engraving resists is the screen printed paper resist. These are produced by a multiple stage process which first involves making a printing screen of the desired image. Using this prepared screen, plastisol ink is printed onto paper and then the printed image is heat cured to produce the finished resist. In use, the resist is bonded onto the substrate and, whilst the cured ink protects any areas that it covers, the shotblasting process soon disposes of any exposed areas of paper and thus exposes those areas of the substrate to its action.

A second method of producing engraving resists is to cut them out from selfadhesive vinyl or rubber sheet. This may be done by hand or, more usually, by means of a computer controlled cutter and may be done whilst the resist material is still in sheet form or it may be done after it has been adhered to the substrate. In either case, the resist having been adhered to the substrate and any desired areas picked out, any substrate that remains thus exposed may then be engraved.

A third category of resist may be made by exposing a photopolymer film to actinic light through a transparency of the desired image. This results in hardening of the exposed areas, whilst the unexposed areas, shaded by the desired image on the transparency, remain soluble. The soluble areas are washed away and the resultant resist is then adhered to the substrate by one of a number of (mostly proprietary) methods and shotblast engraved as above.

A primitive three dimensional effect may be produced from any of these types of resist by shotblasting the unprotected areas. Then removing selected areas of the remaining protecting resist and shotblasting the whole area again. Thus the parts that are freshly exposed are shotblasted once and the parts that were originally exposed are shotblasted twice, resulting in two different depths of engraving according to the two different amounts of time for which the substrate was exposed to the shotblasting. This may be undertaken a number of times to produce a variety of different depths.

This method of producing three dimensional engraving has two substantial disadvantages. Firstly, it can be extremely time consuming for operatives to pick out the various next areas of the design to produce the required varying depths. Each piece having to be shotblasted, then the resist picked out, then shotblasted and then picked out as many times as the design requires depths. Secondly, it is virtually impossible to avoid noticeable steps being produced between the various depths and a smoothly graduated image cannot be produced.

Alternative methods of producing three dimensional images are by means of using a relatively small shotblasting nozzle compared to the scale of the workpiece and using it to selectively carving away areas of the substrate. Or by using the traditional methods of copper wheel engraving or the similar and more recently developed diamond wheel engraving.

Any of these processes require considerable skill and time in their operation. It takes a number of years for a copper wheel engraver to serve an apprenticeship. It would then take this highly skilled craftsperson a substantial amount of time to engrave a design.

We have now produced an effective engraving resist which is utilised by a simple process and which substantially mitigates the disadvantages of prior art shotblast resists and of other methods of engraving.

Thus according to the invention we provide an engraving resist which has a given image comprising differential thicknesses and thus having differential resistances to the shotblasting process.

Thus according to a further aspect of the invention we provide an engraving resist which has a given image comprising differential resiliences and thus having differential resistances to the shotblasting process.

Thus according to a further aspect of the invention there is provided the use of the resist of the first and/or second aspects of the invention which comprises a) bonding the resist to the substrate b) shotblasting the resist so that, by means of the differential thicknesses or resiliences of the image and thus its differential resistances to the shotblasting process, it is progressively worn away and thus exposes the substrate to the shotblasting process for differential amounts of time and thus produces a corresponding image in that substrate having predetermined differential depths of engraving.

The resist of the invention may be made by a number of methods using commercially available materials such as photosensitive direct stencil emulsions as used in the screen printing industry, filmic photo resist materials and liquid photoresists as used in the electronics industry, capillary photostencil films, or by preparing and coating a custom-made formulation comprising one or more ethylenically-unsaturated monomers and oligomers, one or more photo initiators and synergistic reagents, a water-soluble, aqueous-developable or solvent-soluble binder and, optionally, a dispersed or dissolved photopolymer.

By way of background explanation, if these types of resist film are exposed to a relatively small quantity of light through the film base, only a thin layer of the film, immediately adjacent to that base, will harden sufficiently to withstand the processing or washing out. The rest may then be washed away and thus only a relatively thin resist will be produced. If, however, the resist film is exposed to a relatively large quantity of light through its film base, the whole thickness of the film will harden sufficiently for none of it to be processed away. Thus producing a correspondingly much thicker resist. This progressive relationship between exposure and thickness provides the mechanism for producing variable thickness resists.

Further, several of these types of resist film show a variation in toughness and resilience to shotblasting which is not dependent on the relative thickness of the exposed resist layer, but is dependent upon the method and rate of exposure, the type of actinic electromagnetic radiation source used, the temperature during exposure and the nature of the photohardening chemicals and system used in the resist film. Resist films incorporating hardening systems based on a photopolymerisation process often suffer from oxygen inhibition of the photopolymerisation as oxygen diffuses into the resist layer during exposure. This means that different rates and conditions of actinic exposure result in different polymerised and crosslinked molecular structures, which have different resistances to the shotblasting process. This difference in resilience of areas of resist film exposed to similar overall doses of actinic radiation but under different exposure conditions, provides the mechanism for producing variable toughness resists.

In order to engrave a particular three dimensional image, the production of a resist may utilise, but is not limited to the following process: - a master image is produced in such a manner that the areas required to be engraved to the deepest level are represented by a specific dark level of tone and the areas to be engraved to the shallowest level are represented by a specific and relatively much lighter level of tone, with the intermediate levels of engraving being represented by correspondingly intermediate levels of tone and areas not to be engraved at all being left clear of tone; - this image, usually produced on a computer to give the required precise levels of tonal control, is made or output as a continuous tone transparency; - the photopolymer resist film is then contact exposed through its film base and through this continuous tone transparency of the image to an actinic source of electromagnetic radiation for a specific quantity of exposure and the varying levels of radiation transmission through the various tones of the transparency result in a controlled differential exposure of the resist film; - this correspondingly results in progressive hardening and toughening through the resist film from its film base according to the various levels of actinic radiation to which it is exposed. The unhardened areas are then washed out and a resist of variable thickness and resilience is thus produced for use in the process of the invention.

Suitable photopolymer films which may be mentioned for use in the process of the invention include but are not limited to the following: - photoresistfilms used in the electronics industry (examples are Du Pont "Riston", Dynachem "Laminar" and similar products from Hoechst, Fuji and Hitachi); - by using liquid resists or emulsions, such as KPR Resist (Kodak), AHR Resist (Asahi Chemical), Autosol Plus (Autotype International Ltd) and Dirasol 902 (Sericol Group Ltd), coating this liquid resist onto a transparent film support (usually, but not limited to, polyethylene terephthallate) and drying to produce a photosensitive resist film; - pre-coated capillary photostencil films as used for stencil production in the screen printing industry, such as Ulano CDF4 (Ulano Corporation, USA) and One-Pot Film (Murakami Screen, Japan); - specialist resist films for producing shotblast engraving masks, such as SuperMask Fine and SuperMask Heavy Duty (SuperMask Ltd, UK).

In a preferred embodiment of this invention, the photopolymer films are SuperMask Fine photoprocess shotblast resist film and SuperMask Heavy Duty photoprocess shotblast resist film, as sold in the UK by Collins Clayton & Co, Devon and distributed by SuperMask Ltd, Oxfordshire.

Suitable methods of transferring this film from its film base onto the substrate which may be mentioned for use in the process of this invention include but are not limited to the following: -signwnterstransfertape, such as Dorotape (Dorotape Ltd, UK); - a liquid applied material which dries to a flexible and conformable film which performs the function of an intermediate carrier film for the resist layer. such liquid formulations include but are not limited to viscous solutions of natural or synthetic organic polymers or admixtures of these polymers with plasticisers and other additives. Examples are organic solutions of shellac, nitro-cellulose, polymethylmethacryl ate, polyb utylmethacrylate and copolymers of acrylic acid, methyl methacrylate and butyl methacrylate, dissolved in organic hydrocarbon solvents. An example of such a liquid applied material is SuperMask Carrier Film (SuperMask Ltd, UK).

In a preferred embodiment of this invention, the transfer method used is SuperMask Carrier Film, as sold in the UK by Collins Clayton & Co, Devon and distributed by SuperMask Ltd, Oxfordshire.

Suitable adhesives which may be mentioned for use in the process of the invention include but are not limited to the following: - Pressure sensitive spray adhesives such as Mistic Spray Adhesive (Mander Graphic Supplies Ltd, UK); - Water based dextrine adhesives such as D1058 (Enterprise adhesives & Chemicals Ltd, UK) and SuperMask Water Based Adhesive (SuperMask Ltd, UK).

In a preferred embodiment of this invention, the adhesive used is SuperMask Water Based Adhesive, sold in the UK by Collins Clayton & Co, Devon and distributed by SuperMask Ltd, Oxfordshire.

The resists according to the invention may find applications in a variety of techniques. For example, for shotblast engraving into brittle substrates such as glass, stone, hard plastics or ceramics.

A specific embodiment of the invention will now be described with reference to the following diagrams, all drawn in perspective except for figure 1, in which: Figure 1 shows a sample of original artwork (here shown as a halftone on paper but, in the process of the invention, this would be a continuous tone image on transparent film); Figure 2 represents a continuous tone transparency of part of the the original artwork; Figure 3 shows a photopolymer film being contact exposed through the transparency; Figure 4 shows the photopolymer film being washed out to produce the resist; Figure 5 shows a liquid carrier film being applied to the resist; Figure 6 shows the film base being removed from the combination of dried carrier film and resist; Figure 7 shows the combination of carrier film and resist being squeegeed onto the substrate; Figure 8 shows the carrier film being removed from the resist; Figure 9 shows the resist adhered to the substrate, prior to shotblasting; Figure 10 shows an early part of the shotblasting process; Figure 11 shows a later part of the shotblasting process; Figure 12 shows the remains of the resist and adhesive being removed, leaving behind the engraved substrate.

Referring to the drawings, the original artwork transparency A has various levels of tone which correspond to the desired image to be engraved in the substrate, see figures 1 & 2. The darker the shade of tone on the transparency. the lesser the amount of light that will be allowed to pass through it during exposure.

Correspondingly, the lighter the shade of tone, the more the amount of light that will be allowed to pass through it. In areas where there is no tone at ail, virtually all of the light will be allowed to pass through.

The photopolymer film C is contact exposed to actinic light through the transparency A and through its own film base B, see figure 3, for a specific amount of time. This amount of time is sufficient to allow the photopolymer film to cure right through to its desired thickness in the fully exposed areas, but it is short enough so that the areas differentially shaded by the varying tones on the transparency will only receive the desired partial exposures and thus produce varying thicknesses of hardening and levels of resilience within the film.

After exposure, the areas of the photopolymer film that have received insufficient light to become insoluble are then washed away, see figure 4, leaving behind the insoluble areas of the photopolymer film (hereinafter referred to as the resist), still on the original film base. This resist is then dried.

To assist in supporting the resist whilst transferring it from its original film base B onto the desired substrate, a liquid carrier film is applied, see figure 5. This carrier film is then dried.

The film base B is then removed, see figure 6.

The resultant laminate of carrier film and resist is floated on a film of adhesive E onto the substrate F. As much as possible of the adhesive is then squeezed out from behind the resist using a squeegee G, see figure 7. The assembly is then allowed to dry.

When the adhesive E has dried, bonding the resist C to the substrate F, the carrier film is peeled off, see figure 8.

This leaves just the resist C and a thin layer of adhesive E, closely bonded to the substrate, see figure 9.

The resist is then shotblasted. The thinnest areas of the resist are worn away first, thus exposing the substrate underneath to the action of the shot, see figure 10.

As the shotblasting process proceeds, so the thicker areas of the resist are progressively worn away, exposing more and more of the substrate to the action of the shot and thus automatically producing a controlled and graduated three dimensional cut into the substrate, see figure 11.

Finally, when all of the desired image area of the resist has been worn away and the substrate underneath has been exposed to the required amount of shotblasting, the remains of the resist C and the bonding adhesive E are removed, see figure 12, leaving the desired three dimensional image engraved into the substrate F.

Claims (9)

1 A differentially resistant shotblast resist which includes a given image comprising given relatively different resistances to shotblasting in given different areas of that image and which can be worn through by the shotblasting process in those areas relatively more or less quickly in accordance with their differential resistances and thus expose the corresponding areas of the substrate to the shotblasting for a greater or lesser proportion of the time taken for the shotblasting to proceed and thus result in differential depths being shotblasted into the substrate corresponding to and as predetermined by those differential resistances.

2 A differentially resistant shotblast resist as claimed in Claim 1 wherein the different resistances of the resist are brought about by differences in the thickness of the material of the resist.

3 A differentially resistant shotblast resist as claimed in Claim 1 or Claim 2 wherein the different resistances of the resist are brought about by differences in the resilience or toughness of the material of the resist.

4 A differentially resistant shotblast resist as claimed in any preceding Claim wherein a photosensitive resist film is given differential resistances by exposing given areas of it to different amounts of actinic electromagnetic radiation.

5 A differentially resistant shotblast resist as claimed in any preceding Claim wherein a photosensitive resist film is given differential resistances by exposing given areas of it to different amounts of actinic electromagnetic radiation by means of filtering the radiation through a continuous tone film transparency of the image with differential properties of transmission or reflection of that radiation.

6 A differentially resistant shotblast resist as claimed in any preceding Claim wherein a photosensitive resist film is differentially exposed and then unhardened areas and layers are washed or developed away to give a resist film of differential thicknesses.

7 A differentially resistant shotblast resist as claimed in any preceding Claim wherein the image is digitally mastered.

8 A differentially resistant shotblast resist as claimed in any preceding Claim wherein the image is photographically mastered.

9 A differentially resistant shotblast resist substantially as described herein with reference to Figures 1 - 12 of the accompanying drawings.