EP1109954A1

EP1109954A1 - Electrochemical marking stencil, method and system

Info

Publication number: EP1109954A1
Application number: EP00900982A
Authority: EP
Inventors: Patrick F. King; Donald L. Roxby
Original assignee: Polaroid Corp
Current assignee: Siemens Energy and Automation Inc
Priority date: 1999-01-08
Filing date: 2000-01-03
Publication date: 2001-06-27
Also published as: WO2000040785A1

Abstract

A method and system for the electrochemical marking conductive metal parts is disclosed, the method and system being reliant upon a thermoplastic resin sheet material having expansive regions impervious to electrolyte solution and an imagewise pattern of perforations formed thermally in a predetermined pattern. The stencil, when placed between a firing electrode and a grounded metal part, enables communication of the firing electrode and grounded part via the electrolyte solution and etching of the conductive part in regions corresponding to the perforations. Identification marking stencils can be made on demand transmitting digital information corresponding to the desired identification pattern from a central processing unit to a thermal recorder (perforation) apparatus.

Description

Electrochemical Marking Stencil, Method and System

This invention relates to a method and system for the electrochemical marking of conductive metal surfaces. More particularly, it relates to a method and system for electrochemically etching the surfaces of conductive metal parts, such as precision-made parts, with markings that can be read or decoded for identification, inventory control and like purposes.

The electrochemical marking of metallic surfaces has found widespread application in the manufacture of precision-made and other metal parts. In general, the electrochemical marking process is accomplished by firing an electrode against a grounded conductive part between which are present an electrolyte and a stencil having open regions corresponding to the marking to be etched into the surface of the part. The etched design or pattern can then be read or decoded by a hand-held imager or other reading apparatus. In a manufacturing operation, it will oftentimes be desired that a serial number or other marking be applied to an individual part to provide the ability to relate the part to its manufacture and use history. A different stencil will need to be generated for each desired marking pattern. The possibility of mismarking an expensive precision-made part by the use of a stencil intended for use with another part is a decided disadvantage associated with the storage, handling and use of a number of different stencils.

Various methodologies have been employed for the production of stencils useful in the electrochemical marking process. One conventional method involves a mechanical process in which a paper fiber material coated with colored wax is struck with a heated metal die to press the wax coating onto a removable backing sheet, leaving an opening in the stencil material. This process, while suitable for the application of human-readable markings, does not provide the fidelity required to apply machine-readable symbols. Another method utilizes a mechanical cutter to remove regions of a sheet material according to the desired stencil pattern. The interconnection or contiguity of multiple open regions promotes floppiness of the stencil and hampers the efficient handling of the stencil, and particularly, accurate positioning and placement of the stencil.

Various chemical methods have been utilized for the manufacture of stencils, including the utilization of photoresist imaging methods involving the exposure (through an image of the desired pattern) of a photoresist composition on a suitable mesh or other porous web support, and the wet development of the desired stencil. This multi-step manual process is both slow and expensive. Other methods have involved the utilization of a mesh or other porous material (such as is used commonly in the well-known screen printing method) onto which are deposited regions of polymeric material, in areas where passage of electrolyte is to be prevented. Uncovered regions allow electrolyte to contact the conductive metal part and the electrode material of a marking head, in order to provide the desired etched marking upon firing of the electrode. Stencil-making methods that rely upon the imagewise deposition of polymeric material onto a mesh or other porous material, such as thermal wax transfer methods, ink-jet or hot-melt wax transfer methods, are oftentimes disadvantaged by the resolution limits of such methods. In addition, softness of the polymeric coating material may result in melting of the material in undesired regions, particularly when using higher electrochemical etch power settings. Moreover, large areas of the mesh or porous stencil material need to be covered in order to provide a stencil having a relatively small pattern defined by openings within an expansive area impervious to electrolyte. Discontinuities or voids in the desired coverage allow the passage of electrolyte therethrough, with corresponding and undesired marking of the metal part. It has been found that a stencil having especial application in an electrochemical marking method and which permits the efficient marking of finely resolved patterns on the surface of conductive metal parts can be provided by the thermal perforation (imagewise in the desired marking pattern) of an electrolyte- impervious polymeric sheet material. In a method aspect of the present invention, there is provided a method of electrochemically marking conductive metal surfaces. Thus, the surfaces of metal parts can be marked (etched) with an identifying marking or other pattern of indicia by a method, which includes the steps of providing a marking head having an electrode material connectable to a switchable power supply for applying an electrical potential across the electrode material and a conductive metal surface; placing a stencil between the electrode material of the marking head and the conductive metal surface, said stencil comprising a thermoplastic resin sheet material having expansive regions impervious to electrolyte solution and having an imagewise pattern of perforations for the electrical communication of the electrode material and the conductive metal surface through the perforations via the electrolyte solution, the imagewise pattern of perforations being formed thermally through said thermoplastic sheet material in a predetermined marking pattern; and applying a firing potential across the electrode material and the conductive material while they are in contact with one another via the electrolyte solution in regions of said perforations, thereby to electrochemically etch the surface of the conductive metal according to the predetermined marking pattern while etching is prevented in the expansive electrolyte-impervious regions of the stencil.

For a more full understanding of the invention, reference is had to the following description, taken in connection with the accompanying drawings in which: FIG. 1 is a cross-sectional edge view of a composite sheet material for the production of a stencil, the composite sheet material comprising a combination of a porous fabric or paper sheet and a thermoplastic sheet material.

FIG. 2 is an enlarged view of perforations formed thermally in a stencil sheet material such as is shown in FIG. 1.

FIG. 3 is an illustration of a woven fabric material for use as a porous sheet of the composite sheet material of FIG. 1.

FIG. 4 is a model diagram showing in exploded fashion a stencil comprising a porous sheet material of randomly arranged fibers and a thermoplastic sheet material having perforations formed therethrough for the passage of electrolyte solution.

FIG. 5 is a model diagram showing in exploded fashion a stencil comprising the woven fabric material of FIG. 3 shown along the line 3-3, and a thermoplastic sheet material having perforations formed therethrough for the passage of electrolyte solution.

FIG. 6 is an illustration of an electrochemical marking apparatus including a marking head for the marking of conductive metal parts.

FIG. 7 is an exploded view of the marking head of the marking apparatus of FIG. 6, the marking head including a stencil for the electrochemical marking (etching) of metal parts.

FIG. 8 is a block diagram of an apparatus for scanning the artwork of a desired identification code and processing the digital information thereof for production of a thermally perforated stencil sheet.

FIG. 9 is a depiction of a binary matrix code representative of information embodied thermally into a stencil and correspondingly marked electrochemically onto a conductive metal using the electrochemical marking method and system of the subject invention.

FIG. 10 is a schematic structural view of a thermal recording apparatus for the thermal perforation (imaging) of a stencil sheet material. Detailed Description of the Invention

As mentioned previously, the present invention is based upon an electrochemical etching method reliant upon the utilization of a stencil especially adapted to such a method and capable of providing finely resolved etching patterns in a simple and effective manner.

A stencil suited to utilization in the subject electrochemical method and system will typically comprise a sheet material bearing the desired pattern to be marked, such pattern being in the form of openings in the sheet material through which electrolyte solution can pass for completion of an electrical path between a marking-head electrode and the metallic surface to be marked or etched. Referring to FIG. 1 , there is shown a cross-sectional edge view of stencil material 10 which comprises a composite sheet or laminate including a paper or fabric sheet element 12 and a thermoplastic layer or sheet element 14.

Thermoplastic layer or sheet 14 comprises a thermoplastic resin material which can be melted in an imagewise manner, for example, by heated print- head elements or a laser, to provide a predetermined pattern of openings or perforations in the imaged thermoplastic resin material. Thermoplastic layer 14 of stencil material 10 comprises a resin material that is impervious to the electrolyte solutions that are typically used in well known and conventional electrochemical methods used for the marking of metal parts. Such impermeability is important to the production of patterns corresponding only to the perforations or openings created by the imagewise melting (patterning) of the stencil material and, accordingly, voids or discontinuities in layer 14 through which electrolyte can pass and cause unintended and undesirable corresponding markings on a metallic surface will be decidedly disadvantageous.

A variety of natural and synthetic polymeric resin materials can be employed as the material from which a suitable electrolyte-impervious resin layer 14 can be made or formed. Suitable resin layers 14 include those made or formed of any of a variety of polymers, inclusive of polyester, polyethylene, polypropylene, polycarbonate, polyvinylchloride, polyvinylidene chloride and polyvinylchlorine/polyvinlidene copolymer. A variety of waxes can also be used, such as carnuaba wax and stearin. Other resin materials can, however, be suitably employed. Preferred resin materials will be those which can be perforated thermally to provide openings or dots, the edges of which comprise film lumps or thickened regions formed by the added thickness of melted resin material and without merging of adjacent dots or perforations. An example of a preferred resin material for this purpose comprises a polyester resin, for example, a terephthalic acid/ethylene glycol polyester, suitable examples of which are well known and commercially available. Resin layer 14 of composite stencil materials 10 will preferably comprise a film layer which has been subjected to a stretching or orienting operation. Biaxially stretched polyester film materials are examples of such film materials. Preferably, when a polyester film is employed, it will have a melting point in the range of from 190° to 230°C and longitudinal and transverse stretching magnifications of about four.

Resin layer 14 of composite stencil material 10 can vary in thickness with desired handling requirements and the temperature and other operational parameters of the thermal perforation apparatus to be employed. Good results can be obtained using a film having a thickness in the range of from 0.5 to 20μm, and preferably in the range of from 0.6 to 1 Oμm.

Paper or fabric element of composite stencil material 10 comprises a porous material through which electrolyte solution can pass. Paper or fabric element 12 functions in part as a support material for thermoplastic film layer 14 and permits the production of a stencil material 10 that can be handled readily and that can be traversed through thermal printer and other apparatus with minimal propensity toward wrinkling of the stencil material.

Porous element 12 can be made of any of a variety of known materials, including papers, cloth and woven or non-woven fabrics of natural or synthetic fibers. Porous paper sheet materials that can be employed are exemplified by Japanese and other papers made of natural fibers, such as Manila hemp, pulp, Mitsumata (Edgeworthia papyrifera Sieb.) and Kozo (Broussonetia kazinoki Sieb.) Sheet materials made of synthetic woven or non-woven fibers can also be used, including those from such fibers as synthetic polymeric fibers of polyester, polyacetate, nylon or rayon and synthetic metallic or glass fibers.

Porous sheet element 12 owes its perviousness to electrolyte solution to the spaces or gaps between the fibers of the sheet. In general, the porous sheet element will be characterized by the presence of spaces or gaps smaller than the pixels to be formed in thermoplastic layer 14 of stencil material 10. Pixel size will typically be determined by the multiple of the respective primary and secondary scanning pitches. Gaps in porous sheet element 12 that are smaller than pixel size will generally occupy 60 to 100% of the total area of all the fiber gaps, and preferably, 80 to 100% of the total area of all the fiber gaps. Porous sheet element 12 can vary in thickness depending upon desired handling characteristics and the operational parameters of the thermal perforation apparatus used for stencil making.

In additions, the thickness of such material should be sufficient to provide electrical insulation at the voltages used for electrochemical marking. Thicknesses in the range, for example, of 5 to 1 OOμm can be employed. A preferred range of thickness is 10 to 60μm. If desired, stencil sheet material for use in the production of a stencil that can be used in the method and system of the subject invention can comprise only a thermoplastic film material made of such materials as mentioned hereinbefore in connection with stencil material 10 of FIG. 1.

Porous sheet element 12 of the stencil material 10 of FIG. 1 can be eliminated with attending cost savings. The particular resin film material of the resulting element and the thickness thereof can be varied to suit the particular conditions used for thermal perforation and to minimize problems with sheet handling, including wrinkling of the relatively thin thermoplastic film. In general, it will be preferred to utilize stencil material having a porous paper or fabric support element for realization of ease in handling and processing.

The thickness of porous element 12 may also vary with the nature of the element as a woven or non-woven sheet. Sheets of fibers woven into rectangular grids will typically be of somewhat greater thickness than conventional papers, cloth and non- woven fabrics of natural or synthetic fibers.

As mentioned, it will be preferred to use a stencil material having a thermoplastic film layer 14 affixed to a porous support element 12, such as is shown in FIG. 1. When the porous support element is affixed to the thermoplastic layer, transferring efficiency of the stencil sheet is improved. In FIG. 2 is shown an enlarged view of a stencil 20 having perforations 22 formed thermally in a stencil sheet material. Melted and solidified film portions 24 define the outer periphery of perforations 22 shown in FIG. 2. Shrinkage of a stencil sheet is controlled by the thermoplastic film layer 14 being affixed to porous element 12. Shrinking back of the film at the periphery of the perforations is restricted by the porous support element and the generation of wrinkles is thereby prevented. Melted (and solidified film) portions 24 about the periphery of perforations 22 form ridges or lumps and the presence of melted resin in the perforations is avoided. As a consequence, the stencil 20 is neatly perforated and substantially no melted resin is present in the perforations. While applicants do not wish to be bound by any particular theory or mechanism in respect of phenomena occurring as the result of thermal processing of stencil sheet material, it is believed that resin material is rendered flowable and caused by capillary action to flow in part to the periphery of the perforations (as shown in FIGS. 3, 4 and 5). Additionally, melted resin material is believed to be impregnated in part into porous element 12 (or porous element 42 or 34 of FIGS. 4 and 5, respectively).

Shown in FIG. 3 is an illustration of a woven fabric 30 comprising warp fibers 32 and filling (or woof) fiber 34. In general, a porous sheet material having a grid or woven pattern of openings 36 will be preferred as a porous element of a stencil material such as is shown in FIG. 1. Porous sheets having a grid structure are preferred from the standpoint of control of the melting of the affixed thermoplastic film, such melting being desirably confined generally within the openings defined by the fibers. Neat perforations and minimization of the shrinking of thermoplastic material back into the perforations are realized. In addition, grid or mesh structures permit a more uniform wetting of the stencil with electrolyte.

When a porous support element is affixed to the thermoplastic resin film, adhesion can be accomplished by thermal fusion or lamination or through a heat-sensitive or pressure-sensitive adhesive. Such a composite stencil forming material will be processed, for example, by heat-generating elements facing the thermoplastic resin layer.

If desired, porous element 12 can comprise a paper the fibers of which are substantially random in orientation. Such an element is shown as paper sheet 42 in the stencil 40 of FIG. 4, shown in exploded fashion. Electrolyte solution can pass through paper 42 and through the openings 47 in the thermoplastic film 44.

As shown in FIG. 4, the periphery of each perforated dot is provided with a continuous swelled portion 45 formed from the thermoplastic film resin material melted at the time of perforation and solidified upon cooling.

Shown in FIG 5., in exploded fashion, is a representation of a stencil 50 comprising a thermoplastic resin film 54 in combination with the woven fabric of

FIG. 3 viewed along line 3-3. As shown in FIGS. 3 and 5 fabric 30 contains warp fibers 32 and filler fiber 34. Perforations 57 are formed in thermoplastic resin film 54, the periphery thereof having swollen or lump portions of the melted and then solidified thermoplastic resin material. Suitable methods of making stencils useful in the subject electrochemical etching (marking) method and system are known in the art and are described for example in such United States patents as U. S. Pat. No. 5,483,883 (issued Jan. 16, 1996 to Noboru Hayama); U. S. Pat. No. 5,522,313 (issued Jun. 4, 1996 to Koichi Okusawa); U. S. Pat. No. 5,526,032 (issued Jun. 11, 1996 to Jun Nakamura); U. S. Pat. No. 5,559,074 (issued Sep. 24, 1996 to Hideo Watanabe); and U. S. Pat. No. 5, 617,787 (issued Apr. 18, 1976 to Nagon Takita). Other methods that assure the provision of perforations in an electrolyte-solution impervious thermoplastic resin film can, however, also be employed. Stencils of the character aforedescribed can be employed conveniently in known electrolytic marking (etching) methods and systems to advantage. A suitable apparatus adapted to the receipt of such a stencil is shown in FIG. 6. As therein shown, a suitable electrolyte marking (etching) apparatus includes power supply 60 having on/off switch 62 and supplying alternating or direct current selected via switch 64. Potentiometer 66 permits suitable voltage to be applied between the electrode (not shown) of marking head 70 and the metal surface or part (not shown) grounded via grounding plate 180. Applied amperage can be read on ampmeter 68.

In practice, a stencil, for example, stencil 80 of FIG. 7 having marking pattern 82 is placed over stencil holder 76 and held into position in housing

75 of marking head 70 by retainer 78. Insulating cover 72 is affixed to housing 75 via retainer clips 74.

For the application of the marking 82 of stencil 80 onto the surface of an electrically conductive metal part, the part is placed onto grounding plate 180 and electrolyte solution is applied to the stencil, for example, by contact of marking head

70 and stencil 80 with a sorbent pad (not shown) impregnated with the desired electrolyte solution. Upon wetting of the stencil with the electrolyte solution, the marking head is brought into contact with a metal part (not shown) grounded via grounding plate 180. A preselected voltage is applied across terminal 82 (via cable 63 and clip 65) and marking head 70 via cable 61. Completion of an electric circuit is through the electrolyte solution passable through perforation pattern 82 of the stencil 80.

If desired, markings can be applied to conductive metal parts by alternative methods. For example, grounding plate 180 of FIG. 6 can be eliminated and the metal part to be marked can itself comprise a grounding electrode, in which case, clip 65 need only be fastened onto any convenient portion of the part to be marked. In addition, stencil 80 need not be retained in marking head 70. In lieu thereof, stencil 80 can be placed directly onto the metal part to be marked. Upon wetting of the stencil with electrolyte and contact of the electrode of a marking head with the stencil positioned onto the part to be marked, and actuation of the electrode of the marking head, electrolytic etching (marking) of the part can be accomplished.

According to a preferred embodiment of the subject invention, there is provided a computer-controlled or otherwise automated system for on-demand production of a perforated stencil for use in an electrochemical marking (etching) method.

FIG. 8 is a block diagram depicting such a system. As shown therein, a desired marking pattern of bar code, matrix code, human-readable alphanumeric characters, optical character recognition (OCR) characters, graphical logos or the like can be embodied into code artwork 82 on a suitable substrate. The artwork can then be scanned using a conventional optical scanner 83 for acquisition or capture of the visual image information into a series of electronic impulses. Scanner 83 can be a light-sensitive electronic array, an optical CCD camera, a linear array scanner, a laser reader adapted for two-dimensional scanning, or the like. The electronic impulses produced by scanner 83 are transmitted to digitizer 84 which converts the electronic impulses into a series of computer- recognizable binary bits that correspond to the scanned image. Each visual cell is assigned a binary numeric value based upon the intensity of light sensed by optical scanner 83. Visual cells which are absolute black and absolute white are assigned the highest and lowest values, respectively. Shades in between are assigned incremental values forming an electronic digitized image of the scanned artwork 82a. The digitized image is then transmitted to a central processing unit (CPU) of a computer 85 ("CPU") which stores a bit-mapped image of artwork 82a and a part of its surrounding area as a reference within its memory. The desired image pattern stored in computer 85 can be viewed on monitor 86 before printing via printer 88. The stencil, as made conveniently and inexpensively, can be deployed directly for the marking of parts. Placing of the stencil into inventory and retrieval can be avoided. Stencils can be made and used and discarded and made again as needed; and mismarking can be thereby avoided.

Alternatively, an on-demand system of producing perforated stencils for use in electrochemical etching methods can be practiced without the requirement of preparing artwork 82a or using optical scanner 83 and digitizer 84. In such a case, commercially available software can be used in lieu thereof for the supply directly to computer 85 digitized representations of conventional bar code, matrix code and the like. Such digitized representations can be viewed on monitor 86 and can be transmitted to printer 88 for imagewise production of a stencil via thermal perforation.

Printer 88 (alternatively, a write engine or stencil sheet perforation device) can comprise any of a number of known printing devices that includes means for feeding a stencil sheet material past a thermal print head or laser for perforation according to the digital representation of the desired marking pattern. Suitable printer or thermal recorder devices can be employed for this purpose as known in the art. A thermal recording device 100 that can be employed for carrying out the thermal perforation stencil-forming method is shown in FIG. 10. In the illustrated thermal recording device, stencil sheet material 10 is held between a pair of conveyor rollers 102 and is traversed in the direction indicated by the arrow A (the secondary scanning direction) until the sheet material is placed between a platen roller 104 and a thermal head 106. Heat-emitting elements (one of which is shown as heat-emitting element 108) are contacted directly with sheet material 10 at the thermal recording surface 10a thereof. The desired marking pattern, in the form of perforations, is provided by selective heating of the heat-emitting elements 108 of thermal head 106 and traversal of thermal head 106 along the axis of platen roller 104, i.e., in a direction (the primary scanning direction) orthogonal to the direction of arrow A.

Thermal head 106 is provided with a plurality of rectangular heat- emitting elements 108 arranged in a row at a prescribed pitch in the primary scanning direction. Each of the heat-emitting elements is provided with an electrode

(not shown) at each end thereof along the secondary scanning direction such that electric power can be supplied individually to each of the heat-emitting elements 108.

If desired, a laser printer an be used to effect thermal perforation of the thermoplastic resin film layer 14 of sheet material 10. Such a printer will be equipped with a laser which can comprise a coherent beam directed toward the sheet material 10 for the imagewise perforation thereof or a plurality of laser-emitting elements, e.g., a plurality of semiconductor laser diodes, for direction of multiple beams toward the sheet material 10 to be imaged. Light absorbing compounds, for example, infrared-absorbing dye compounds, for absorbing irradiation in the range of the laser irradiation and for converting the irradiations into heat, can be employed to facilitate the perforation process.

Among known thermal recording devices that can be used for the production of thermally perforated stencils useful herein are those described in the following aforementioned United States patents: U. S. Pat. Nos. 5,522,313;

5,526,032; 5,592,209; and 5,617,787.

The thermal perforation method and system of the present invention can be used for the production of a variety of stencils and have particular usefulness in the electrochemical marking methods. Preferred marking patterns that can be embodied into such stencils include conventional bar codes and matrix symbols.

Stencils embodying matrix and stacked bar codes that are classified as "two- dimensional" symbols by the Automated Identification and Data Collection (AIDC) industry can be used to advantage. While stencils can be prepared for the marking of machine-readable, two-dimensional identification symbols of all types, a preferred symbol for embodiment into a stencil used for electrochemical marking is the matrix code.

Matrix codes were developed to overcome many of the deficiencies inherent to the first (linear bar codes) and second generation (stacked bar codes) machine-readable identification symbol formats. One of the primary changes is the use of squares as a carrier of data in lieu of the strips of variable widths used in linear and stacked bar codes. The use of a data element of known size and shape makes the matrix code more versatile. In the matrix code format, black squares (data cells) represent a binary "1 " and white data cells represent a binary "0". When these binary values are used together in specific sequences, they represent alphanumeric characters. Equal-sized data cells provide for an easier decoding logic decision process that for bar codes. By knowing the size and shape of a symbol and its individual data cells, decoding software can quickly reconstruct damaged portions of the code. Matrix symbols can be produced in both square and rectangular format and scaled in size to fit into an available marking area.

Matrix codes designed for application to and identification of any of a variety of articles and products are known and are described in detail, for example, in U. S. Pat. No. 4,939,354 (issued Jul. 3, 1990 to D. G. Priddy, et al.). A matrix code can store from one to 2335 alphanumeric characters in any language. An encoding scheme for use with such a symbol has a high degree of redundancy which permits most marking defects to be overcome. 16-bit cyclic redundancy check and data reconstruction capabilities are included in one version; and Reed-Solomon error correction is included in another. Up to 16 symbols can be concatenated. Error correction and checking (ECC) code 200 is preferred. In FIG. 9, there is depicted a binary code, generally indicated as a matrix 82b, that can be embodied into a thermally perforated stencil in the manner aforedescribed. Binary code matrix 82b has a perimeter 90 formed by intersecting sides 91 formed of solid lines and intersecting perimeter sides 92 formed of dark perimeter squares 93 and light perimeter squares 94 in an alternating pattern. Data, generally indicated as 96, is stored within perimeter 90 of matrix 82b.

Data 96 is stored within the perimeter of matrix 82b by converting each character to be stored into a visual binary code represented by dark and light squares corresponding to ones and zeros of binary information, as aforementioned.

Accordingly, a letter or number represented by the binary code 0001 may be represented by a string of data cells, each cell containing either a dark square or light square. Accordingly, the data representing 0001 would appear as a series of three light data cells and one dark cell. For example, the numbers 0 through 9 are stored within matrix 82b as a pattern of light cells 97 and dark cells 98.

It will be appreciated that an identification pattern in finely resolved perforations will oftentimes be readable visually only with some difficulty. Visualization can, however, be improved by using colorants in the stencil material for contrast with regions opened by perforation. Alternatively, registration marks, logos or other indicia can be applied to the stencil material by conventional printing methods. For example, a printing station can be included in thermal recorder 100 of FIG. 10 to print by ink or other means such indicia as registration marks, to facilitate location of the pattern within the general expanse of the stencil material and to assure electrochemical marking of parts in the precise location desired.

Example 1

A marking stencil for the electrochemical etching (marking) of a steel plate-like substrate was prepared in the following manner.

A composite sheet of tissue-like material (comprising a porous paper support of randomly distributed and partially bunched paper fibers overlaid with a layer of thermoplastic polyester, available as sheet material S-2284 from Riso

Kagaku Corporation, Tokyo, Japan) was fed into a thermal perforation (printer) apparatus comprising roller means for traversing the composite sheet over and past a thermal head equipped with multiple thermally addressable heating elements, the thermoplastic polyester side of the composite sheet material facing the thermal head. The printer apparatus was a thermal printer available commercially as Model 3440, from Intermec Technologies Corporation, 6001 36^th Avenue West, Everett, Washington. A bar code pattern comprising an arrangement of lines of varying width spaced apart by varying distance was generated by computer software and transmitted to the printer. The heat elements of the printer head were actuated imagewise in accordance with the pattern as the head was moved past the composite sheet material in a direction orthogonal to the feed direction. The composite sheet material was perforated in accordance with the pattern to provide a marking stencil. The resulting stencil was positioned onto a plate-like steel substrate and a pad impregnated with saline electrolyte solution (an aqueous solution of Corrosion Free Electrolyte (CFE), manufactured by Electro-Chem Etch Metal Marking, Inc. 450/B Apollo Street, Brea, California) was brought into contact with the stencil. A marking head having an electrode was superposed upon the stencil and a firing potential (about nine volts) was applied across the electrode and the ground state steel substrate. The desired pattern was etched into the surface of the steel substrate. Splotchiness was evident in the pattern attributable to the localized bunching of fibers in the porous paper support and consequent prevention of free passage of electrolyte therethrough and wetting of the substrate through perforations in the thermoplastic layer.

Example 2

A marking stencil having a data matrix (two-dimensional) pattern was made and used for the electrochemical marking on a stainless steel substrate, in the following manner. A composite stencil sheet material as described in Example 1 was imaged using a thermal printer having a resolution capability of 400 dpi (a

Model 3240 thermal printer, also available from Intermec Technologies Corporation). A central processing unit controlling a digital image information file generated by Strandware Label Matrix software was used to transmit the desired image pattern to the printer. The composite stencil sheet material was thermally processed (imaged) in less than three seconds, for production of the desired stencil. The resulting stencil was used in the method and apparatus described in Example 1 for the electrochemical etching (marking) of a stainless steel plate substrate. The etched pattern showed evidence of splotchiness attributable to bunching of paper fibers in the paper support of the composite stencil sheet material from which the stencil was prepared.

Example 3

A marking stencil was made and used for electrochemical marking in the manner described in Example 2, except that, in place of the thermal printer there described, there was used a thermal printer having a resolution specification of 300 dpi (a Model THT 360X thermal printer available from Brady Corporation, 6555 West Good Hope Road, Milwaukee, Wisconsin). Good results, comparable to those obtained in the case of Example 2, were obtained.

Example 4

Marking stencils for the electrochemical etching (marking) of a steel plate-like substrate were made and used in the following manner.

A composite sheet material comprising a woven mesh having a layer of thermoplastic polyester adhered thereto (available as sheet material 200P-45 from Riso Kagaku Corporation, Tokyo, Japan) was used for the making of stencils. In each instance, the composite sheet material was fed into a thermal perforation (printer) apparatus. The printer apparatus included roller means for traversing the composite sheet over and past a thermal head equipped with multiple thermally addressable (imaging) heating elements, and roller means for discharging the imaged sheet material. Samples of the composite sheet material were fed into the printer in all instances with the thermoplastic polyester side facing the thermal head. Commercially available printers were used for imaging (perforating) the composite sheet material (models 3240 and 3440, from Intermec Technologies Corporation, 6001 36th Avenue West, Everett, Washington). Conventional bar code and matrix code patterns were used for the making of stencils, in each case generated by computer software and transmitted to the printer. The heating elements of the printer head were actuated imagewise in accordance with the pattern as the head was moved past the composite perforatable sheet material in a direction orthogonal to the feed direction. In each case, the composite sheet material was perforated in accordance with the particular pattern to provide the marking stencil. In each case, the stencil was positioned onto a plate-like steel substrate and a pad impregnated with saline electrolyte solution (an aqueous solution of Corrosion Free Electrolyte (CFE), manufactured by Electro-Chem Etch Metal Marking, Inc. 450/B Apollo Street, Brea,

California) was brought into contact with the stencil. A marking head having an electrode was superposed upon the stencil and a firing potential (about nine volts) was applied across the electrode and the ground state steel substrate. In the aforedescribed manner, the bar code and matrix code patterns were etched into the surface of the steel substrate. The resulting patterns were readable (decoded) using a hand held imager apparatus.

Claims

1. A method of electrochemically marking a conductive metal surface, said method including the steps of: providing a marking head comprising an electrode material adapted to connection with a switchable power supply for applying an electrical potential across said electrode materials and said conductive metal surface; placing a stencil between said electrode material of said marking head and said conductive metal surface, said stencil comprising a thermoplastic resin sheet material having expansive regions impervious to electrolyte solution and having an imagewise pattern of perforations for the electrical communication of said electrode material and said conductive metal surface through said perforations via said electrolyte solution, said imagewise pattern of perforations being formed thermally through said thermoplastic sheet material in a predetermined marking pattern; applying a firing potential across said electrode material and said conductive material while they are in contact with one another via said electrolyte solution in regions of said perforations, thereby to electrochemically etch the surface of said conductive metal according to said predetermined marking pattern while etching is prevented in said expansive regions impervious to said electrolyte solution.

2. The method of Claim 1 wherein said stencil comprises said thermoplastic resin sheet material affixed to a porous support element comprising fibers which define spaces or gaps pervious to said electrolyte solution.

3. The method of Claim 2 wherein said thermoplastic resin sheet material has a thickness in the range of from 0.5 to 20μm.

4. The method of Claim 3 wherein said thickness is in the range of from 10 to 60μm.

5. The method of Claim 2 wherein said porous support element comprises paper fibers.

6. The method of Claim 2 wherein said fibers of said porous support element are in substantially random orientation.

7. The method of Claim 2 wherein said fibers of said porous support element define a grid or mesh structure.

8. The method of Claim 2 wherein said porous support element comprises a woven fabric.

9. The method of Claim 2 wherein melted and solidified ridge or lump portions of said thermoplastic resin sheet material define the periphery of said perforations.

10. The method of Claim 2 wherein said thermoplastic resin sheet material comprises a stretched or oriented film.

11. The method of Claim 10 wherein said film comprises biaxially stretched polyester.

12. The method of Claim 1 wherein said pattern of perforations comprises a binary code matrix.

13. An on-demand method of producing an electrochemical marking stencil and using said stencil for the electrochemical marking of a conductive metal surface, said method comprising the steps of: generating a series of electronic impulses representative of a desired marking pattern to be applied to a conductive metal surface; transmitting said electronic impulses to a thermal recorder for thermal perforation of a stencil material, said stencil material comprising a thermoplastic resin sheet impervious to electrolyte solution and perforatable by the imagewise application of heat in response to said electronic impulses; perforating said stencil material in response to said electronic impulses thereby to provide a stencil having a pattern of perforations pervious to said electrolyte solution; positioning said stencil between an electrode material of a marking head and a conductive metal surface to be marked; wetting said stencil with electrolyte solution; applying a firing potential across said electrode material and said conductive metal surface while they are in contact with another via said electrolyte solution in regions of said perforations, thereby to electrochemically etch said conductive metal surface according to said marking pattern.

14. The method of Claim 13 wherein said stencil comprises said thermoplastic resin sheet material affixed to a porous support element comprising fibers which define spaces or gaps pervious to said electrolyte solution.

15. The method of Claim 14 wherein said porous support element comprises a sheet of paper fibers in substantially random orientation.

16. The method of Claim 14 wherein said porous support element comprises a sheet and said fibers define a grid or mesh structure.

17. The method of Claim 16 wherein said support element comprises a sheet of woven fabric.

18. A system for electrochemically etching a desired marking pattern into the surface of a conductive metal comprising: means for providing a series of electronic impulses representative of a desired marking pattern; a thermal recorder for receiving said electronic impulses and recording said representation by the thermal perforation of a stencil material, said stencil material comprising a thermoplastic resin sheet material perforatable by the imagewise application of heat in response to said electronic impulses, said thermal recorder including means for conveying a sheet of said stencil material past a heat- emitting element for thermally perforating said stencil material and providing a marking stencil having said desired marking pattern in perforations pervious to the passage of electrolyte solution; a marking head for electrochemically etching said pattern into the surface of said conductive metal, said marking head comprising an electrode material adapted to connection with a switchable power supply for applying an electrical potential across said electrode material and said conductive metal surface, said marking head being adapted to receive and retain said marking stencil for positioning of said stencil between said electrode material and said conductive metal surface when said marking head is brought into contact with said conductive metal surface; and means for wetting said stencil with electrolyte solution for electrical communication of said electrode material and said conductive metal surface through said perforations upon application of said electrical potential.

19. The system of Claim 18 wherein said stencil material comprises said thermoplastic resin sheet material affixed to a paper or fabric support element having spaces or gaps pervious to said electrolyte solution.

20. The system of Claim 19 wherein said support element comprises a woven mesh fabric material.