GB2243618A - Electroforming mandrel; making continuously electroformed thickness modulated or perforated metal foil - Google Patents

Abstract

An electroforming mandrel for making metal foil with holes therein comprises areas capable of behaving as an efficient cathode together with a predetermined pattern of more resistive or dielectric areas where plating efficiency is impaired or is zero. The mandrel can be prepared by local chemical conversion of the surface of the mandrel e.g. by ion implantation, ion assisted plasma vapour deposition or anodisation to form the resistive areas. Metal foil is also formed by printing an electroformed foil with a pattern of plating resist and thereafter plating the foil to give a compound foil thickness modulated or having apertures or lacunae or cavities. Applications mentioned are the manufacture of printing screens, and security labels.

Description

IMPROVEMENTS IN THE PRODUCTION PROCESS FOR MAKING CONTINUOUSLY ELECTROFORMED THICKNESS MODULATED OR PERFORATED METAL FOIL Continuously electroformed metal foil is useful in a variety of applications wherein subsequent processing is used to open arrays of slots or apertures for applications such as printing screens, battery electrode manufacture, particle sieves or metal components in markers for retail security applications.

Foil for such applications is frequently electroformed onto a cylindrical cathode rotating partly submerged in a plating electrolyte, the foil being peeled continuously from the cylinder as it leaves the electrolyte.

Attempts have been made in the past continuously to electroform metal foil, especially nickel, with the holes already formed, so that no subsequent operations are required. This has been tried using pits or holes in the mandrel filed with a dielectric such as an epoxy resin, the structure being polished flat before use.

This type of mandrel construction has resulted in the possibility of an opening or fissure gradually propagating between the dielectric and metal portions of the mandrel. After only a few cycles of use the perforated foil tends to stick on these'boundaries so that the foil tears and the continuous process is interrupted.

In a recent example of an improved process for mandrel manufacture, designed to overcome this difficulty, the mandrel is coated with a slurry made from a fusible inorganic dielectric paste. (EPO 030774A, Stork Screens B.V.). Local melting of the paste is then achieved by laser or electron beam pulse treatment to form dielectric zones fused onto and into the conducting mandrel. The surplus dielectric slurry powder is then removed and the mandrel polished flat so that there is presented to the electrolyte a conducting surface containing embedded dielectric regions. The disadvantages of this method are that (1) the heating pulse, be it laser or electron beam, has a gaussian spatial distribution, and (2) the layer of dielectric powder, slurry or reaction mixture is difficult to apply uniformly and contains a distribution of particle size.These factors together lead to irregular dielectric islands attained after the washing and polishing steps, and consequent defects in the electroformed product.

According to one aspect of the present invention, there is provided an electroforming mandrel whose surface comprises areas capable of behaving as an efficient cathode and a predetermined pattern of more resistive or dielectric areas where plating efficiency is impaired or is zero.

This invention describes new processes for the preparation of such a mandrel using ion implantation, ion assisted Plasma Vapour deposition, or anodisation (plasma or electrolytic) to achieve a very durable mandrel having excellent release properties. A novel methdd for the continuous manufacture of foil having thickness modulation is also described. A simple subsequent process whereby this may be converted to a form with holes according to a predetermined format is also disclosed.

Preferably, the bulk resistivity of the high cathodic efficiency surface regions lies in the range 0 ohm cm to 100 ohm cm and the bulk resistivity of the low efficiency regions falls in the range 1014 ohm cm down to 10 ohm cm. The thickness of the or each high resistivity (i.e. low plating efficiency) region is advantageously in the range l0nm to imam. These regions are conveniently made by local chemical conversion (according to a predetermined pattern by for instance masking) of the high cathodic efficiency material forming the bulk of the mandrel, or by local or otherwise chemical conversion of a thin layer applied to the surface of the mandrel. The undesired portions of a low resistivity skin on the mandrel may be removed by a physical or chemical process, e.g. cutting, sand blasting or etching.The high resistivity regions on the mandrel may be formed by the process of ion implantation using for instance ions of elements from Groups III to VI of the periodic table especially boron, nitrogen, phosphorus, oxygen, sulphur and selenium either singly or on combination.

Alternatively, the high resistivity regions may be formed by the deposition, locally or otherwise, of high resistivity regions formed by a physical or chemical thin film deposition method as sputtering, chemical vapour deposition, evaporation, atomic layer deposition or plasma spraying.

A raised resistivity skin on the surface of the mandrel formed by applying a thin solid film of an element or mixture of elements chosen from Groups III to V of the periodic table, singly or in combination to the mandrel surface either though a deposition mask or over the whole surface followed by diffusion of this layer into the bulk of the mandrel by whatever process, having removed if necessary any unrequired portions of the thin film. An example is the deposition of a layer of boron through a mask onto a titanium mandrel followed by heating of the mandrel to say 6500C for 2 hours to allow the boron to diffuse into the titanium mandrel.

The low cathodic efficiency regions may be formed by anodisation of the high conductivity skin of the mandrel itself, or anodisation of a skin material applied to the mandrel. A mask having openings formed according to a predetermined pattern may be used to define the portions of the mandrel surface to be anodised. The anodisation process may be conducted in an ion plasma containing ions from Groups III to VI of the periodic table. Electrolyte anodisation is an acceptable technique.

According to another aspect of the present invention, there is provided a process which comprises printing an electroformed foil with a pattern of plating resist and thereafter further plating the foil to give a compound foil having apertures or lacunae or cavities. The metal used to plate onto the patterned electroformed foil may be the same as, or different from, that of which the foil itself is made.

In a further aspect, the invention provides a cylindrical mandrel formed from a dielectric ceramic with conducting areas provided by the local introduction of a conductor, e.g. titanium nitride. A specific example is where the ceramic is Syalon from Vesuvius Ltd., such that the local introduction of the polytype titanium nitride causes a local grade conversion from for instance Syalon 201 to Syalon 501 the former having good insulating properties, and the latter being a good conductor.

The invention will be described further, by way of example, in the following Examples.

EXAMPLE 1 In an example of the invention a polished mandrel, made for instance from titanium, and prepared for the continuous electroforming of, for instance nickel, is tightly wrapped with a single layer of resist foil (e.g. nickel) already perforated with the required aperture pattern. Only minimal overlap of the foil at the join is used, so as to give the smallest possible interruption in the otherwise continuous pattern.

The prepared mandrel is then placed on a spindle at the job end of an ion implanting machine, and the mandrel is rotated whilst being implanted with oxygen ions accelerated to 150kV, or an implant depth of around 0.15pm. The dose is arranged so that the implanted layer is substantially TiO2.

After removing the patterned resist foil the implanted mandrel is unaffected under the solid part of the resist foil, but has the surface properties of TiO2 in the regions which were exposed to the oxygen ion implantation through the resist foil apertures.

EXAMPLE 2 In an alternative example of the process the polished mandrel, of for instance titanium or stainless steel is rotated within 9 vacuum system, glow discharge cleaned and coated with a layer of a second material, for instance boron, silicon, titanium or aluminium, by, for instance, evaporation of the second material from a source or plurality of sources within the same vacuum chamber. The drum, now coated with say 10 to 1000nm of the second metal is removed from the vacuum chamber and treated by further processing according to the first example. After removal from the implant system the unimplanted material from the vacuum deposition step may be removed by for instance chemical etching.In the case of a 150nm layer of aluminium on a titanium mandrel implanted with oxygen, the implanted parts are substantially alumina, Al203, the remainder of the coating may be dissolved in iM NaOH solution without effect on the titanium substrate.

EXAMPLE 3 In another example of the principle of the invention a prepared cylindrical mandrel of stainless steel or titanium is wrapped with a perforated mask as in Example 1 and rotated within an implant system equipped with a deposition source, so that for instance aluminium. boron or silicon may be deposited to a thickness of 10 to 1000nm followed by implantation of say oxygen or nitrogen at up to 200kV. A layer of a dielectric such as boron nitride or alumina is thus produced. The implantation stage may be followed by further cycles of evaporation and implantation till the required dielectric thickness has been attained on the exposed portions of the mandrel surface.

EXAMPLE 4 In this example the prepared cleaned mandrel is uniformly dipped, sprayed or roller coated with a uniform layer of an organometallic solution designed to give pin hole free dielectric films of for instance silica or titanium dioxide. Such solutions are sold as proprietary mixtures by Emulsitone Corp. as for instance Silicafilm, and may contain other ingredients such as phosphorus or boron to alter the properties of the dielectric to for instance diffusion of ions. The coated mandrel is carefully dried and the layer partly densified by heating the mandrel in clean air to 3000C.

Further local densification of the dielectric layer is achieved by ion implantation through apertures in a superimposed mask as has been shown in the examples above.

The local implantation causes the implanted zones to be far less reactive than the partly densified shielded areas, so that immersion of the drum in aqueous 4% ammonium bifluoride completely removes the shielded Silicafilm, leaving the implanted zones substantially unaltered.

EXAMPLE 5 The electroforming mandrel is coated with a photo laquer and the photolaquer is patterned through a mask to actinic radiation so as to yield the inverse of the required metal foil pattern in the photolaquer. After appropriate development and postbake the mandrel is plated with a resist metal. The mandrel may then be processed omitting the masking step by the routes shown in the examples above. Alternatively an ion stopping or attenuating resist of appropriate thickness may be applied to the mandrel directly, so avoiding the masking steps described in claims 2 to 4.

EXAMPLE 6 This example describes a method for the continuous electroforming of metal sheet with apertures opened part way through the thickness of the sheet. This form is particularly useful for the production of screen mesh. The basis for the plant is a standard foil electroforming plant, depositing for example continuous copper or nickel, complete with'rinse and dry stages.

Following this is a printer which applies to the foil as it leaves the electroformer a pattern of spots or disks of a dielectric ink such as an acrylic or nitrocellulose based composition suitable as a one shot plating resist. The foil then passes via a second electroforming drum or a further stage or compartment of the same vat, through a second electroforming stage which builds up the thickness of the first metal layer or applies a different metal, as required. This process allows a foil with thickness modulations to be cheaply formed, with the advantages that the ink mask is used once only and is replaced without effect on the electroforming mandrels or process. Thickness modulated nickel is particularly useful as a deactivator material for retail security applications.

The subsequently deposited material may also be a different metal. For instance the first bath may contain an acid copper electrolyte, and deposit 8pm of copper, and the second might apply a further 15pm of nickel. The plating is not effective on the resist coated parts of the foil, and these remain at the original thickness. The metal deposited by the fist stage bath may then be etched away, if required, using a selective etchant, or the foil may be useful in this form as it stands. Similarly the ink pattern may be left on, or not.

EXAMPLE 7 In a variation of the process outlined in Example 6 we describe a method especially useful in the production of anti-pilferage labels. A roll of magnetically inert substrate material such as Mylar foil is coated with a conducting material (for instance copper or nickel, henceforth metal 1) in a reel to reel vacuum process to give a substantially continuous thin conducting skin. On a separate plant the coated roll is then further processed as follows. Firstly the conducting coating is partly masked by printing on a pattern of insulating dots using for instance an ink jet printer. After for instance W, evaporative or thermal curing of the resist pattern the foil passes under an electrical contact comb made from for instance carbon fibres. This contact provides the means to apply a thin bright layer of Permalloy alloy (80-20 Nickel Iron, henceforth metal 2) by electroforming. The foil leaves the Nickel iron bath, and after a rubber wipe and spray rinse meets a nip roller formed from a rubber roller opposed to a cathodic titanium roller with deposition shield at the surface of a second electroforming 'solution. Fifteen microns of columnar nickel (henceforth metal 3) is applied to the metal 2 on metal 1 layer. By this means a magnetic foil with holes having no preferred magnetic direction within the plane of the foil is formed on a continuous non magnetic substrate suitable for the direct production of anti pilferage labels.

Claims (22)

CLAIMS:

1. An electroforming mandrel whose surface comprises areas capable of behaving as an efficient cathode and a predetermined pattern of more resistive or dielectric areas where plating efficiency is impaired or is zero.

2. An electroforming mandrel according to claim 1 where the bulk resistivity of the high cathodic efficiency surface regions lies in the range 0 ohm cm to 100 ohm cm and the bulk resistivity of the low efficiency regions falls in the range 1014 ohm cm down to 10 ohm cm.

3. A mandrel accqrding to claim 2, wherein the thickness of the low plating efficiency region is in the range lOnm to lmm.

4. A mandrel as claimed in claim 2 or 3, wherein the dielectric or high resistivity regions are made by local chemical conversion (according to a predetermined pattern by for instance masking) of the high cathodic efficiency material forming the bulk of the mandrel, or by local or otherwise chemical conversion of a thin layer applied to the surface of the mandrel.

5. A mandrel according to claim 4, wherein after causing the conversion of a thin film or skin on the mandrel to a more sensitive form the undesired portion of the skin is removed by an appropriate physical or chemical step such as cutting, sand blasting or selective etching.

6. A mandrel according to claim 4, wherein the chemical conversion is effected by the process of ion implantation using for instance ions of elements from Groups III to VI of the periodic table especially boron, nitrogen, phosphorus, oxygen, sulphur and selenium either singly or on combination.

7. A mandrel as claimed in claim 1, 2 or 3, wherein the dielectric or high resistivity regions are formed by the deposition, locally or otherwise, of high resistivity regions formed by a physical or chemical thin film deposition method as sputtering, chemical vapour deposition, evaporation, atomic layer deposition or plasma spraying.

8. A mandrel as claimed in claim 1, 2 or 3, wherein a raised resistivity skin on the surface of the mandrel formed by applying a thin solid film of an element or mixture of elements chosen from Groups III to V of the periodic table, singly or in combination to the mandrel surface either though a deposition mask or over the whole surface followed by diffusion of this layer into the bulk of the mandrel by whatever process, having removed if necessary any unrequired portions of the thin film.

9. A mandrel according to claim 1, 2 or 3, wherein the low cathodic efficiency regions are formed by anodisation of the high conductivity skin of the mandrel itself, or anodisation of a skin material applied to the mandrel.

10. A mandrel according to claim 9, wherein a mask having openings formed according to a predetermined pattern is used to define the portions of the mandrel surface to be anodised.

11. A mandrel according to claim 9 or 10, wherein the anodisation process is conducted in an ion plasma containing ions from Groups III to VI of the periodic table.

12. A mandrel according to claims 9 and 10 wherein the anodisation process is carried out electrolytically.

13. A process which comprises printing an electroformed foil with a pattern of plating resist and thereafter further plating the foil to give a compound foil having apertures or lacunae or cavities.

14. A process according to claim 13, wherein the printed foil comprises a metal different to that applied subsequently.

15. A process according to claim 13, wherein the printed foil is of similar composition to the metal subsequently deposited.

16. A process according to claim 13 or 14, wherein the continuous (first applied) layer of foil is removed by a physical or chemical method capable of doing so without affecting the perforated foil.

17. A process according to claims 13, 14 and 16 where the aperture containing foil so produced is further plated on one or both sides with either further material of the same sort or a dissimilar material.

18. A process substantially as described in Example 7, wherein the Mylar substrate is replaced by any magnetically inert film capable of surviving the subsequent processing.

19. A process substantially as described in Example 7, wherein an electrically conductive carrier film is used and no first metal deposition step is performed.

20. A process according to claim 19, wherein the film is a polyunsaturated hydrocarbon plastic doped to render it conductive, for example a polymer based on a halogen-doped alkyne.

21. A process substantially as described in Example 7, wherein the second metal layer is electrically conductive, and the third metal layer is formed of the same material or another metal.

22. A cylindrical mandrel formed from a dielectric ceramic with conducting areas provided by the local introduction of a conductor, e.g. titanium nitride.