Patent Publication Number: US-6341557-B1

Title: Non-ferrous/ferromatic laminated graphic arts impression dies and method of producing same

Description:
RELATED APPLICATION 
     This is a continuation-in-part of U.S. application Ser. No. 09/392,179 filed Sep. 9, 1999, and entitled NON-FERROUS/STEEL LAMINATED GRAPHIC ARTS DIES AND METHOD OF PRODUCING SAME. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of graphic arts and especially to graphic arts impression dies such as copper, magnesium, bronze or other non-ferrous metal/ferromagnetic laminated dies. It also relates to graphic arts impression die assemblies for use on various types of stamping or embossing apparatus, including sheet or web-fed graphic arts presses such as clamshell, vertical or horizontal presses, and to improved processes for preparing the graphic arts impression dies and to preparation of impression graphic arts die assemblies. As used herein, the term graphic arts “impression die(s)” means at least the categories of graphic arts dies including hot foil stamping/blocking dies, embossing dies, debossing dies, embossing/debossing dies, combination/fluted/one-shot/foil embossing dies, and any other graphic arts dies which combine any one or more of these general types of die functions on a single plate for smooth, lenticular, textured or grained surfaces, or any other similar graphic arts metal, polymeric or composite impression dies. 
     More particularly, the invention concerns a cladded metal graphic arts impression die plate having a non-magnetic layer of metal integrally joined with a ferromagnetic layer of metal. A relieved, design-defining surface is provided in the outer face of the non-magnetic layer of metal. The graphic arts impression die plate is mounted on a magnetic support member and held in position thereon at least in part by a series of permanent magnets embedded in the magnetic support member in disposition to magnetically attract and hold the ferromagnetic layer of the graphic arts impression die plate supported by the magnetic support member. 
     The magnetic support member with the cladded metal graphic arts impression die plate thereon is adapted to be affixed to the chase of a stamping or embossing machine such as a sheet or web-fed graphic arts press, in disposition with the design-defining surface of the graphic arts impression die plate in alignment with a predetermined design location. 
     The utilization of a cladded metal sheet having a non-magnetic layer integral with a ferromagnetic layer for the graphic arts impression die plate facilitates formation of a relieved design in the outer surface of the non-magnetic layer, either by way of a chemical etching process, mechanically using a pantograph milling machine, a computer numerically-controlled (CNC) laser or mechanical milling machine or an operator-controlled milling machine, or by hand-engraving. The cladded metal sheet having a photo-resist coating on the outer surface of the non-magnetic layer of the sheet may be affixed to a magnetic support member through the medium of a series of permanent magnets on the magnetic support member which attract the ferromagnetic layer of the sheet. The magnetic support member and the cladded metal sheet thereon may then be positioned in an etching machine for etching of the exposed areas of the non-magnetic layer of the cladded metal sheet which are not protected by the photo-resist coating. The magnetic support member having embedded permanent magnets therein may also be used to support the cladded metal sheet blank in a chemical etching machine, CNC, pantograph, or operator-controlled milling machines, or during hand engraving, resulting in a design-defining surface. The magnets embedded in the magnetic support member are especially important in stabilizing the central area of the relatively thin cladded metal sheet blank while it is being machined. 
     2. Description of Related Arts 
     Stamping dies have long been used in the graphic arts field to apply thin metal foil or thin layers of other transferable material to a substrate such as paper, cardboard, thin metal films or plastic in accordance with a design formed in the stamping surface of the die. Similarly, embossing dies have been provided to emboss or deboss a desired design in a suitable substrate, and to produce lenticular lines, texturing or graining impressions in the paper, plastic, thin metal film or cardboard. Combination dies which combine hot foil stamping or blocking, embossing or debossing, or formation of other surface feature designs are also well known in the art. 
     Graphic arts impression dies as described have long been prepared by etching or engraving a desired design in the outer surface of a metal plate, usually magnesium, copper or brass. These metal plates generally were of sufficient thickness, as for example about ¼ in., to cause the plate to be essentially self-sustaining. In the case of relatively long embossing or stamping runs involving as many as hundreds of thousands of impressions, it has been past practice to employ relatively long lived graphic arts impression die plates made of a metal such as copper or brass. For intermediate length runs, the plates were usually made out of magnesium which was less expensive and easier to engrave or etch a relieved design area than with copper or brass. 
     In those instances where the runs are shorter and any inherent wear of the die surface is acceptable from a final product quality standpoint, non-metal graphic arts impression dies have largely supplanted copper and brass, and even magnesium plates in more recent times by less costly and simpler non-metal dies. For example, steel-backed photo polymer graphic arts impression die plates have been developed in which a hardened photo polymeric composition representing the desired design is supported on a steel backing plate. These steel-backed photo polymer plates can be used with conventional foil stamping and embossing equipment. 
     Photo polymer graphic arts impression die plates are generally thinner than conventional magnesium, copper or brass graphic arts impression dies, and therefore a spacer plate has been required between the photo polymer graphic arts impression die plate and the chase of the stamping or embossing machine to avoid the necessity of modifying the embossing or stamping equipment. 
     Pat. No. 5,904,096 (“&#39;096”) of May 18, 1999, shows and illustrates one type of spacer plate that can be used to support a photo polymer graphic arts impression die plate on the chase of an embossing or stamping machine. The spacer plate of the &#39;096 patent is provided with a series of permanent magnets which are described as being capable of magnetically attracting and holding the steel plate portion of the graphic arts impression die plate and thereby the photo polymer die assembly on the spacer plate. Use of a spacer plate of an appropriate thickness serves to support the photo polymer die in the required spaced relationship from the surface of the chase. 
     There is a need though for a graphic arts impression die which substantially has the longevity of conventional copper or brass dies, yet is less costly and easier to manufacture than conventional metal dies made of copper or brass. There has also been a need for decreasing the make-ready time involved in mounting of a hot foil stamping or blocking, embossing or debossing die on stamping or embossing equipment, particularly from the standpoint of proper alignment of the die with respect to the image onto which the foil is to be applied, or the image to be embossed or debossed. A further important need in the graphic arts impression die field is to provide a die which may be changed out and replaced in the stamping or embossing equipment or apparatus in a significantly shorter period of time than is presently the case. 
     SUMMARY OF THE INVENTION 
     An improved metal graphic arts impression die is provided which is made up of a cladded metal die plate having a design-defining non-magnetic metal layer such as copper, magnesium, bronze, or other non-ferrous metal which may be cladded to a ferromagnetic support layer that for example may be a steel sheet. A relieved area in the non-ferrous layer defines the design to be foil stamped, embossed, debossed or impressed. In a preferred form, the laminated metal graphic arts die plate has a layer of copper clad to a sheet of carbon steel. 
     In view of the fact that the laminated die plate is thinner than conventional one-piece magnesium, copper or bronze stamping dies or embossing dies, a die plate support is preferably provided for holding the laminated die plate on the chase of a foil stamping or embossing machine. An improved magnetic support plate is provided for the steel-backed, graphic arts impression die assembly made up of a non-ferrous support member having a die mounting surface which substantially complementally receives the cladded steel or steel-backed graphic arts impression die. A plurality of specifically spaced magnetic elements are embedded in the support member substantially through the full extent thereof. The attractive force of the steel backing to the magnetic surface of the support plate is enhanced by positioning of the magnets embedded in the support member such that adjacent pairs of the magnets have their north and south poles oriented oppositely, and a ferro-magnetic component is positioned in bridging relationship to each pair of magnets against the faces thereof opposite the die support face of the plate to enhance the magnetic flux emanating from each of the pairs of magnets. 
     The provision of a magnetic plate for supporting a steel-backed impression die has a major benefit in the use of the assembly in that minute adjustments in the position of the die on the support plate after mounting of the assembly on the chase of the sheet or web-fed press may be accomplished with greater facility and more rapidly than in past mounting practices wherein repositioning of the die could be accomplished only by time-consuming manipulation of a number of fastening devices. 
     In a preferred embodiment of the invention, the magnets are of square shape, with each pair of magnets being in specifically spaced relationship from one another, and from adjacent pairs of magnets. The magnets of each pair are positioned such that their north and south pole axes extend through the major faces of each of the magnets, with the length and width dimensions of each of the magnets being substantially greater than the thickness of each magnet. The ferro-magnetic component is preferably in the nature of a steel plate that extends between and engages the major face of each of the magnets which is most remote from the die mounting surface of the support member. 
     The steel strip which extends between and engages the major face of each of the magnets most remote from the die mounting surface of the support member, enhances the holding power of the bridged magnets by directing and concentrating the magnetic field surrounding those ends of the magnets in closest proximity to the die assembly supporting surface of the support member. The ferro-magnetic component also functions to decrease the flux leakage from the magnets at the perimeter of the magnetic field created by respective pairs of magnets. 
     The individual magnets are embedded in the non-ferrous support member in positions causing the major faces thereof in closest proximity to the die mounting surface of the support member to be spaced inwardly from the plane of the outer die mounting surface. The magnets are not however spaced so far from the die mounting surface to significantly decrease the magnetic attractive flux of the magnets or the die assembly. In this way, the magnets are protected against wear or breakage during the frequent attachment to and detachment of the magnetic support plate graphic arts die assemblies from the magnetic support member. Furthermore, a smooth and consistent outer die support surface is presented that is not interrupted by the outer surface of the magnets to thus minimize any distortion of the design-defining layer. 
     The magnetic support member of this invention is also useful to support a cladded steel or steel-backed graphic arts impression die made up of a non-ferrous, design-defining layer backed by steel during removal of material from the surface of the non-ferrous layer by etching to form the design image in the outer surface thereof. Where the design-defining image is produced by a chemical etching process, a photo-resist composition is first applied to the outer surface of the non-ferrous layer of the cladded metal sheet. The photo-resist composition is configured to define the portion of the non-ferrous layer which is not to be removed by an etchant solution in the etchant bath equipment. 
     A magnetic die support plate for the graphic arts support die is preferably fabricated of plastic or other etchant-resistant material and is provided with a series of pairs of permanent magnets embedded therein as described and in disposition to magnetically attract the steel layer of the cladded metal sheet to at least partially hold the cladded metal die on the die plate support. The assembly of the cladded metal die plate with the photo-resist composition on the outer face of the non-ferrous layer of the die plate, and the support for the die plate may then be placed in an etchant machine to effect etching of a desired design in the outer surface of the non-ferrous layer. 
     Upon completion of the etching step and removal of the photo-resist composition from the surface of the non-ferrous layer of the cladded metal die plate, the die plate is ready for attachment to the spacer plate and then to the chase of the embossing or stamping machine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a graphic arts metal impression graphic arts impression die plate constructed in accordance with the preferred embodiment of this invention; 
     FIG. 2 is a fragmentary perspective view of one corner of the die illustrated in FIG. 1, to better illustrate the configuration of the die structure; 
     FIG. 3 is a fragmentary, enlarged, essentially schematic view of the corner of the die as depicted in FIG. 2, with the entire original outer surface of the graphic arts impression die plate having been removed by an etching process; 
     FIG. 4 is a fragmentary, enlarged, essentially schematic view of a larger segment of the die as shown in FIG.  1  and depicting areas of the die which have been removed by etching or milling, as well as areas which have not been removed by an etching process or by milling; 
     FIG. 5 is a plan view of one form of movable support structure for supporting the graphic arts impression die plate during etching thereof, and illustrating a series of individual, embedded permanent magnets for attaching the graphic arts impression die plate to the support structure; 
     FIG. 6 is a horizontal cross-sectional view taken substantially on the line  6 — 6  of FIG. 5; 
     FIG. 7 is a plan view of a another form of movable support structure for supporting the graphic arts impression die plate during etching or milling thereof, and illustrating a series of strip-shaped embedded permanent magnets for attaching the graphic arts impression die plate to the support structure; 
     FIG. 8 is a horizontal cross-sectional view taken substantially on the line  8 — 8  of FIG. 7; 
     FIG. 9 is a plan view of a third form of movable support structure for supporting the graphic arts impression die plate during etching or milling thereof, and illustrating shiftable clamps for attaching the graphic arts impression die plate to the support structure, along with a central permanent magnet for holding the central part of the die against the support structure; 
     FIG. 10 is an end elevational view of the support structure shown in FIG. 9; 
     FIG. 11 is a side elevational view, partly in vertical cross-section, of etching apparatus usable to etch the graphic arts impression die plate while it is carried by a support structure as shown in FIGS. 7,  8  or  9 ; 
     FIG. 12 is a fragmentary perspective view of a stamping die assembly which includes a graphic arts metal impression graphic arts impression die plate positioned on a magnetic support member and held in place thereon by a series of spaced pairs of magnetically-enhanced magnets embedded in the magnetic support member; 
     FIG. 13 is a fragmentary, essentially schematic vertical cross-sectional view through a portion of the assembly as shown in FIG. 12; 
     FIG. 14 is a plan view of a fourth form of movable support structure for supporting the graphic arts impression die plate during etching or milling thereof; 
     FIG. 15 is a fragmentary, cross-sectional view taken along line  15 — 15  of FIG. 14, looking in the direction of the arrows, and further illustrating a graphic arts impression die plate against one face of the support structure; and 
     FIG. 16 is a perspective view of one of permanent magnets embedded in the support structure of FIGS.  14  and  15 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Graphic Arts Impression Die Plate 
     A metal laminated graphic arts impression graphic arts impression die plate constructed in accordance with the preferred concepts of the present invention is broadly designated by the numeral  20  in FIGS. 1-4 of the drawings. The graphic arts impression die plate  20  may be of a type including a hot foil stamping or blocking die, an embossing die, a debossing die, a lenticular line die, a texturing die, a graining die, combinations of any of these die designs on a single graphic arts impression die plate, or other similar graphic arts metal impression dies (herein collectively referred to as “graphic arts impression dies”). 
     The blank for preparation of graphic arts impression die plate  20  is preferably a cladded metal plate made up of a steel sheet or layer  22 , and a non-ferrous sheet or layer  24  which is integral throughout the extent thereof with layer  22 . Utilization of a cladded metal plate for preparation of a graphic arts impression die having a ferromagnetic base layer while the layer of material that is cladded to the base layer is a non-ferrous metal, allows advantage to be taken of the ability of the cladded plate to be attracted to and held in place in a desired location by support structure which includes a plurality of permanent magnets. 
     Accordingly, a cladded graphic arts impression die plate blank which is useful in the present invention has a ferromagnetic base layer, although the non-ferrous metal layer cladded to the base layer may be of various materials, such as copper, bronze, magnesium and similar metals which are amenable to etching by a suitable etchant solution, or can be mechanically machined to produce the required design-defining image in the surface of the non-ferrous layer of the plate. Copper is a metal of choice for the non-ferrous layer of the cladded metal graphic arts impression die plate in that it can readily be etched in with a ferric chloride solution, and especially a ferric chloride solution containing an additive for controlling the degree and rate of the etching process. Magnesium is another non-ferrous material that may be cladded to the steel base layer, in that the magnesium may be etched in a conventional manner with a nitric acid solution of well known composition in the engraving die field. Bronze on the other hand, is a metal of choice for the non-ferrous layer of the cladded metal graphic arts impression die plate in instances where the design image in the outer surface of the non-ferrous layer is formed by a pantograph milling machine, a CNC laser or mechanical milling machine, or an operator-controlled milling machine, or by hand-engraving. 
     In the cladding process, which may be carried out in a manner that has long been conventional in the cladding industry, a strip of non-ferrous metal is brought into surface engagement with a strip of ferromagnetic material such as steel and the two layers in proximal relationship are fed between one or more compression rollers which apply extremely high surface pressures on opposite sides of the non-ferrous metal and steel sheets. In order to assure integration of the non-ferrous metal sheet with the steel sheet, as depicted schematically in FIGS. 2 and 3, the pressure applied to the interengaging non-ferrous metal and steel sheets should be sufficient to assure complete cladding of the non-ferrous metal to the steel layer. 
     A preferred cladded copper and steel die blank is manufactured by applying sufficient pressure to the interengaging copper and steel sheets which is sufficient to cross-sectionally deform the copper at least about 50% under cold welding conditions. If the copper and steel cladding process is carried out at an elevated temperature, as for example from about 800° C. to about 1100° C., then required integration of the interengaging surfaces of the copper and carbon steel may be accomplished at somewhat less pressure and in shorter time. The copper and carbon steel cladded product may be annealed at a temperature of about 480° C. if desired to increase the flexibility of the product. 
     In the case of a cladded metal graphic arts impression die plate  20  of steel and copper, the copper layer is desirably of from about 0.020 in. (0.508 mm) to about 0.090 in. (2.286 mm) in thickness, and the steel layer is from about 0.008 in. (0.0203 mm) to about 0.20 in. (5.080 mm) in thickness. The preferred copper/ferromagnetic cladded graphic arts impression die plate blank has a steel layer which is nominally 0.030 in. (1.076 mm) in thickness and a copper layer which is nominally 0.040 in. (1.016 mm) thick. A blank of that total thickness presents a relatively rigid structure, and is therefore useful in flat bed applications. However, if a somewhat flexible final die is preferred, allowing the die to be formed into a semi-circular configuration for mounting on the cylinder of a rotary press, a cladded metal blank having a steel layer of nominally about 0.008 in. (0.0203 mm) thick and a copper layer of nominally about 0.020 in. (0.508 mm) thick is preferred. 
     In those instances where the total thickness of the copper/ferromagnetic blank cladded plate is less than about 0.060 in. (1.524 mm) thick, it is desirable that the cladded metal die blank be annealed for approximately 1 hour at about 480° C. to about 650° C. and then air-cooled. Annealing serves to make the grain of the copper more uniform. At cladded blank thicknesses exceeding about 0.060 in. (1.524 mm), annealing is not usually required. 
     In the preferred embodiment, the steel layer of the copper/ferromagnetic cladded die blank is type 1008 carbon steel of conventional specifications, while the copper layer is desirably type C10700 copper sheet having a melting point of about 1083° C., a density of about 0.323 lbs/cu. in. at 20° C., a co-efficient of thermal expansion of from about 0.0000170 to about 0.0000177 per ° C. from 20° C. to 300° C., a modulus of elasticity of about 17,000 ksi, a modulus of rigidity of about 6400 ksi, and a thermal conductivity value of about 224 btu per ° F. from 68° F. to about 572° F. The copper should be substantially oxygen and lead-free, contain minimal zinc, and typically includes about 0.85% wt of silver. Other useful copper/ferromagnetic cladded metal die blanks may be employed where the layer of copper meets standard copper alloy specifications C10200-C11600 inclusive, and particularly copper alloy specification Nos. C10200, C10300, C10400, C10500, C10700, C10800, C11100, C11300, C11400, C11500, and C11600. The bronze layer of a bronze/ferromagnetic cladded metal die blank preferably meets standard copper alloy specification No. C22000, for commercial bronze 90%. 
     The representative, relatively rigid copper/ferromagnetic cladded graphic arts impression die plate  20  as for example shown in FIGS. 1-4 may be prepared from a cladded metal blank having a nominal total thickness of about 0.070 in. (1.778 mm). In this exemplary cladded graphic arts impression die plate, the carbon steel layer  22  has a nominal thickness of about 0.015 in. (0.0381 mm), while the copper layer  24  is about 0.055 in. (1.397 mm) throughout its extent prior to etching of the surface thereof. Part of the copper layer  24  is then removed by an etchant solution or mechanical milling to present a relieved design image  26 , as depicted in FIGS. 1 and 2. 
     In order to effect controlled etching of the copper layer  24  of copper/ferromagnetic graphic arts impression die plate  20  to produce the design image  26  for a graphic arts impression die, an image is placed on the outer surface  24   a  of copper layer  24  which is a negative of the desired design image  26 . The surface  24   a  is then spray-coated with an ultra-violet light-sensitive, positive or negative-working, photo-resist composition. The positive resist solution may consist of a photo-active compound including a mixture of diazonapthoquinone, phenolic resins, surfactants, plasticizers, and 1-methoxy-2-propanol. A negative photo-resist may be a mixture of a photosensitive polymer such as a methacrylate with an initiator, surfactant and/or plasticizers. The solid content of the resist is normally about 12%. A film mask is placed over the coated surface  24   a  of copper layer  24  and held closely by means of a vacuum system. The plate is exposed to UV light for a sufficient period of time to change the properties of the photo-resist, depending upon the photo-resist being used. Development of the coated plate by washing with a dilute alkaline solution such as sodium metasilicate removes the exposed area. 
     The coated plate with the photo-resist coating on surface  24   a  of layer  24  is then preferably etched with a ferric chloride solution having a ferric chloride concentration ranging from about 25 to about 40 Bé, and nominally about 30 Bé FeCl 3  solution. 
     A preferred etching machine is illustrated and described in U.S. Pat. No. 5,364,494 (“&#39;494 patent”), owned by the assignee hereof and which is specifically incorporated herein by reference thereto. The etchant solution in the etching machine is normally maintained at a temperature of about 21° C.-25° C. The cladded metal graphic arts impression die plate  20  with the developed photo-resist design image thereon is clamped to the rotatable turntable of the etching machine shown in the &#39;494 patent and the turntable is rotated at about 3-5 rpms. The flow of the etchant into the etching machine is maintained at about 45-57 1/min. The paddles in the etching machine of the &#39;494 patent, rotated at about 500-650 rpm, cause the etching solution to be splashed against the surface  24   a  of the cladded metal graphic arts impression die plate  20 . The depth of etch is a function of the etch rate of about 0.001 in./min (0.0254 mm/min). Therefore, a depth of 0.010 in. (0.254 mm) requires about 10 minutes of etching time. 
     The reaction of ferric chloride with copper metal (Cu°+2FeCl 3 →CuCl 2 +2FeCl 2 ) is an isotropic process and therefore occurs uniformly in all directions. Thus, as metal is removed and a relief is formed in the surface  24   a  of plate  24 , lateral etching can occur, usually termed “undercutting.” To minimize undercutting and form a beveled surface at a desired angle, protectant and stabilizing additives may be incorporated in the etching solution. 
     As ferric chloride reacts with the copper metal, cuprous ions react (chelate) with the additives to form a film on the surface of the copper metal. The extent of film forming is related to the concentration of the additives. Foramidine disulfide dihydrochloride and the ethylenethiourea are the key additives for maintaining a desired bevel angle. These additives are added to the etchant in varying amounts, depending on the reading from a given test target. The proper balance of the ferric chloride content, protective agent and elemental copper in the etching solution is adjusted based on the results of immersing a copper test target in the etch solution for 5 minutes. This test target contains a scale of a series of half-tone images maintained at certain percentages and also contains various other lines and images. After removal from the solution, an experienced operator visually interprets the test target to determine, based on experience, whether more additive or additional ferric chloride should be added to the solution, or whether the copper content has reached a level that dictates preparation and use of etchant solution. Those skilled in the art appreciate that this interpretation is subjective, depends on certain variables, and is carried out most effectively by an operator that has had requisite training and experience, and therefore has the necessary skill to use the test target results as a guide to determine the protective behavior of the etching solution. 
     Where the copper layer  24  of the graphic arts impression die is to have a nominal thickness of about 0.055 in. (1.397 mm), the die may for example be subjected to the etching operation for a period of time and under conditions to remove unprotected areas of the copper to a depth of about 0.030 in. (0.762 mm), leaving about 0.025 in. (0.635 mm) of the initial copper layer remaining. Thus, in FIG. 4, the height of the design image  26  in the representative example, is approximately 0.030 in. (0.762 mm), the remaining copper layer  24   b  defining the design image  26  is about 0.025 in. (0.635 mm), while the steel layer  22  is about 0.015 in. (0.381 mm). Upon removal of the photo-resist from the outer surface of the design image  26 , the graphic arts impression die plate  20  is ready for use in stamping, embossing or debossing operations. 
     Although cladded metal graphic arts impression die plate  20  is shown in FIGS. 1-4 as being of planar configuration, it is to be understood that the graphic arts impression die plate is sufficiently flexible that it can be bowed to an extent as required to complementally fit on the rotary cylinder of a stamping, embossing or debossing press. In this instance, therefore, the graphic arts impression die plate will in use be of semi-circular configuration. The design-defining relieved image in the copper surface  24  of the graphic arts impression die plate  20  may be configured to accommodate the intended bowing of the graphic arts impression die plate  20  for use, as may be necessary, and as is well understood in the graphic arts field. However, a preferred graphic arts impression die for rotary press use has a ferromagnetic layer  22  of steel about 0.008 in. (0.203 mm) and a total non-ferrous layer  24  of about 0.020 in. (0.508 mm). In this case, the non-ferrous layer is desirably etched to a depth of from about 0.002 in. (0.050 mm) to about 0.020 in. (0.508 mm). 
     Graphic Arts Impression Die Assembly 
     One specially useful application of a graphic arts impression die plate  20  as described above is in a clamshell-type hot foil stamping or blocking die press having a stationary heated chase and a movable pressure plate. This equipment is constructed for mounting of a conventional magnesium, copper or brass die on the heated chase, with metal foil being moved into position over the die, a paper sheet or other media onto which the foil is to be applied is interposed between the foil and the pressure plate, and then the plate is rotated through an arc to apply pressure against the paper and foil pressed against the die. The resulting pressure and heat from the die causes foil conforming to the configuration of the design in the die to be transferred to the surface of the paper or other substrate. Rigid magnesium, copper or brass dies designed for this type of application are conventionally about 0.25 in.  (6.35  mm) in thickness in the case of the “Americas” (North, Central and South America) and about 7 mm (0.276 in.) for the “rest of the world” (ROW). 
     In order to use the graphic arts impression cladded metal graphic arts impression die plate  20  in a conventional stamping machine such as a clamshell press, a backing member for the plate  20  may be necessary in view of the fact that the graphic arts impression die plate is of less thickness than conventional rigid magnesium, brass or copper graphic arts impression plates. The backing member though, must be capable of transferring adequate heat from the heated chase of the clamshell press to the design image-defining copper layer  24  of graphic arts impression die plate  20 . Steel is desirably used for the layer  22  of cladded graphic arts impression die plate  20  not only because of its high strength to weight ratio, but also because of its ferromagnetic properties. 
     A preferred magnetic support or backing member  28  for cladded metal graphic arts impression die plate  20  is illustrated in FIGS. 12 and 13 of the drawings. The backing or magnetic support member  28  preferably comprises a flat, relatively rigid, non-ferrous metal or plastic plate  30  of width and length dimensions greater than the graphic arts impression die plate  20  that is to be mounted thereon, so as to provide complete support for the graphic arts impression die plate  20  throughout the width and length thereof. The magnetic support member  28  is preferably fabricated of a plastic or etchant-resistant material such as PVC, an acrylic resin, nylon, a polycarbonate polymer, a glass fiber reinforced epoxy composition, a plastic composite reinforced with carbon fibers, tempered glass, titanium, or a ceramic material. Plate  30  should be of a thickness such that when a graphic arts impression die plate  20  is mounted thereon as illustrated in FIGS. 12 and 13, the combined thickness dimension of plate  30  and graphic arts impression die plate  20  is approximately equal to the thickness of a conventional magnesium, copper or brass hot foil stamping or blocking die, or about 0.25 in. (6.35 mm) for the Americas and about 7 mm (0.276 in.) for the ROW. 
     Alternately, the die assembly may comprise a layer of polymeric material presenting the design image which is applied to and firmly affixed to a ferro-magnetic sheet such as steel backing sheet  22 . The polymeric material is preferably a thermoset resin selected from the group consisting of allyl polymers, epoxy polymers, furan, melamine formaldahyde, melamine phenolic polymers, phenolic polymers, polybutyldiene polymers, thermoset polyester and alkyd polymers, thermoset polyimide polymers, thermoset polyurethane polymers, flexible thermoset silicone polymers, silicone epoxy polymers, and thermoset urea polymers, all of which have properties and characteristics permitting their utilization in a well known manner to prepare what is conventionally known in the graphic arts field as a polymeric die. 
     Magnetic support member  28  preferably comprises a relatively rigid, non-ferrous metal plate  30  (or of non-heat conductive materials such as plastic or wood for non-heat applications) of width and length dimensions greater than the die plate  20 , or a steel-backed polymeric die plate assembly that is to be mounted thereon, so as to provide complete support for the die plate assembly throughout the width and length thereof. The support plate  30  is preferably fabricated of materials such as bronze, brass, copper alloys, aluminum alloys, magnesium alloys, nickel, zinc, titanium, wood, thermoplastic and thermoset synthetic resin compounds, synthetic resin composites comprising tempered glass fiber, metal fiber, carbon fiber or graphite fiber reinforced thermoset resins such as epoxies or bakelite, with copper alloy being a preferred material. 
     Plate  30  should be of a thickness such that when a die plate  20 , or a steel-backed polymeric die assembly is mounted thereon, as illustrated in FIGS. 12 and 13, the combined thickness dimension of plate  30  and die plate  20  is approximately equal to the thickness of a conventional graphic arts impression die, i.e., about 0.250 in. (6.350 mm) for the Americas, and about 7 mm (0.276 in.) for ROW. Therefore, the thickness of the magnetic support member  28  should not exceed about 0.230 in. (5.842 mm) in the case of the Americas, and about 6.502 mm (0.256 in.) in the instance of ROW, taking into account the minimum thickness of a die plate of about 0.020 in. (0.508 mm). 
     In view of the fact that a cladded die plate such as die plate  20 , or a polymeric die carried by steel backing, are both of less thickness than conventional rigid magnesium, steel, brass or copper graphic arts impression dies, the magnetic support member  28  of this invention functions to not only carry the die assembly, but also serves as a shim between the die plate and the chase of the press. In the case of a hot foil stamping press, the backing member must be capable of efficiently transferring adequate heat from the heated chase of the web or sheet-fed graphic arts press to the design image-defining copper layer  18  of die plate  20 , or a polymeric die plate. Therefore, steel is desirably used for the layer  22  of die plate assembly  20 , as well as for a polymeric die assembly, not only because of its heat retention properties and its high strength to weight ratio, but also because the steel is magnetically attracted to and held by the die mounting surface  30   a  of magnetic support member  28 . 
     In the embodiment of the invention illustrated in FIG. 13, the plate  30  has a series of elongated, generally rectangular recesses or cavities  32  in the rear face thereof which may be formed for example by machining operations and that terminate in spaced relationship from the die plate mounting surface  30   a  of the plate. Each of the cavities  32  houses a pair of rectangular magnets  33  and  35  which are of a width and length substantially greater than the thickness thereof. The thickness of each of the magnetic elements is from at least about 0.040 in. (1.016 mm) to about 0.220 in. (5.588 mm) for the Americas, and about 0.246 in. (6.248 mm) for ROW. A preferred magnet may for example be of square configuration having dimensions of 0.5 in. (12.7 mm)×0.5 in. (12.7 mm) in width and length and 0.10 (2.54 mm) in. in thickness. In the preferred embodiments of the invention, the magnets  33  and  35  are spaced apart a distance of about 0.5 in. (12.7 mm). Magnets may be used that are from about 0.25 in. (6.35 mm)×0.25 in. (6.35 mm) to about 2 in. (50.8 mm)×2 in. (50.8 mm) with a spacing between adjacent magnets being about 0.10 in. (2.54 mm) for smaller magnets to about 3 in. (76.2 mm) for larger magnets within the specified magnets may be used. It is also to be understood in this respect that the cavities  32  should be spaced such that the distance between magnets in adjacent cavities are substantially within the ranges set forth for the magnets  33  and  35  in each cavity  32  and the spacing therebetween, depending upon the sizes of the magnets and the corresponding spacing between magnets  33  and  35  in each cavity  32 . As is most evident in the embodiment shown in FIG. 12, the cavities  32  are arranged in aligned rows extending transversely of the plate  30 . For example, as shown in FIG. 12, the cavities  32  of the row  37  thereof, are offset with respect to the cavities  32  of the next adjacent row  39 . The offset positions of the cavities  32  repeats from row to row with the cavities  32  of adjacent rows being offset from one another. Thus, with respect to FIG. 12 for example, the spacing between adjacent rows  37  and  39  is preferably about 0.5 in. (12.7 mm) in the instance where the magnets  33  and  35  are 0.5 in. (12.7 mm)×0.5 in. (12.7 mm) and the spacing between such magnets is 0.5 in. (12.7 mm). Similarly, the spacing between cavities  32  in each row  37  and  39  should be about 0.5 in. (12.7 mm) in the exemplary embodiment. 
     A ferro-magnetic component  36  in the form of a steel strip is located within each of the cavities  32  in bridging, engaging relationship to the outer surfaces  33   a  and  35   a  respectively of magnets  33  and  35  which are remote from the die mounting surface  30   a  of plate  30 . The ferro-magnetic component  36  may be steel, but vanadium-iron-nickel alloy (Permendor) is preferred because of its enhanced magnetic permeability, and is of a thickness of from about 0.010 in. (0.254 mm) to about 0.190 in. (4.826 mm) for the Americas and 0.216 in. (5.486 mm) for ROW. A preferred component has a thickness of about 0.060 in. (1.524 mm). The total thickness of each magnet  33  and  35  and the associated ferro-magnetic component  36  is at least about 0.050 in. (1.270 mm). A preferred thickness of magnetic support member  28  is about 0.180 in. (4.572 mm) for the Americas and 0.206 (5.232 mm) in ROW, with the distance between the die mounting surface  30   a  of member  30  and the adjacent upper surfaces of magnets  33  and  35  being about 0.020 in. (0.508 mm). An epoxy potting compound  38  serves to permanently affix the magnets  33  and  35  in respective cavities  32 . The recommended operating temperature during use of the magnetic support member  28  is usually within the range of about ambient to 500 F. 
     The magnets  33  and  35  within each cavity  32  are positioned such that the north pole of magnet  33  for example is in closest proximity to the mounting surface  30   a  of plate  30  while the south pole of the magnet  35  is in adjacent relationship to the strip  36 , as illustrated schematically in FIG.  2 . As shown schematically in that same figure, the south pole of the magnet  35  is in closest proximity to the die assembly mounting surface  30   a  of plate  30 , and the north pole of that magnet is adjacent strip  36 . Thus, magnets  33  and  35  are mounted in each of the cavities  32  with opposite polarity. 
     The strength of magnets  33  and  35  is a function of the amount of magnetic flux available from a unit volume of the magnet material and the shape of the magnet, and is generally expressed in units of MGOe (Mega gauss orsted). The preferred magnet material for the present invention is selected from the group of samarium-cobalt (SmCo) having an MGOe of 16-32 and neodymium-iron-boron (NdFeB) having an MGOe of 24-48. Aluminum-nickel-cobalt (Alnico) having an MGOe of 2-8 can be used in certain instances provided the material is adequately engineered to produce a stronger magnet assembly. SmCo magnet material is most preferred because of its low temperature of remanence (Br), making it well suited for strong holding magnet assemblies operating at higher temperatures, as is the case with hot foil stamping/blocking dies. 
     Magnetic support member  28  serves to removably and releasably hold a graphic arts impression die thereon as depicted in FIGS. 12 and 13, wherein the steel layer  22  of die  20  for example rests against and is magnetically attracted to the die mounting surface  30   a  of plate  30  by magnets  33  and  35 . 
     It is known that a magnetic circuit is the path which the magnetic flux from a magnet chooses to travel. Components in a magnetic circuit include the magnet, which acts as the source, along with air, other magnetic insulating material, and ferro-magnetic materials. All components other than the magnets act as impediments or reluctance to the flow of magnetic flux. The magnetic flux will choose to travel through the path that presents the least reluctance. Thus, reluctance in a magnetic circuit reduces the amount of magnetic flux from the magnet. 
     The magnetic attraction of a steel-backed die  20  to the magnetic support member  28  is significantly enhanced by the steel strips  36  bridging magnets  33  and  35  within each cavity  32  because of the significantly greater magnetic permeability of the steel as compared with air and the material from which plate  30  is fabricated. 
     Three dimensional boundary element method analyses have demonstrated that the magnetic holding force of two 32 MGOe 0.5×0.5×0.1 in SmCo magnets spaced 0.5 in. apart and in which the magnets  33  and  35  are bridged by a steel strip  36  confirms that the magnetic holding force is at least approximately three times greater than that of the holding force of a magnet arrangement wherein a steel strip bridging the two magnets is omitted. Furthermore, in the same test setup, the degree of leakage of magnetic flux from the arrangement in which a steel strip  36  between magnets  33  and  35  is provided is reduced by a factor of thirteen as compared with an arrangement in which the bridging steel strip  36  is not provided. 
     Mounted within each of the segments  32   a  of the openings  32  and adhesively held in place therein is a permanent magnet element  38 . Each of the magnetic elements  38  is of a size and located such that the upper surface  38   a  there of is generally parallel with face  34  of magnetic support member  28 , and located with the outer surface thereof slightly below the plane of face  34 . The number, relative spacing and directional orientation of the maximum magnetic field of each of the permanent magnets  38  are selected to assure that a graphic arts impression die plate  20  positioned thereon, as illustrated in FIG. 12, will hold the graphic arts impression die plate in the position where it is initially placed on the magnetic support member  28 , unless deliberately shifted from that initial location. An advantage of the use of a number of permanent magnets  38  is the fact that even though graphic arts impression die plate  20  is not a sufficient thickness to be as rigid as a conventional magnesium, copper or brass stamping die, the magnetic attraction of the steel layer  22  of graphic arts impression die plate  20  to the magnets  38 , causes the graphic arts impression die plate to lay in flat and uniform direct engagement against the face  34  of magnetic support member  28 , throughout the extent of the graphic arts impression die plate  20 . 
     Alternate magnetic support plate structure usable in this invention is illustrated and described in application Ser. No. 09/466,611 filed Dec. 17, 1999, entitled MAGNETIC SUPPORT PLATE FOR CLADDED STEEL AND STEEL-BACKED POLYMER STAMPING/BLOCK AND EMBOSSING GRAPHIC ARTS DIES, and assigned to the assignee herein, which is specifically incorporated herein by reference thereto. 
     Although not specifically illustrated in FIGS. 12 and 13, it is to be understood that if more secure attachment of graphic arts impression die plate  20  to magnetic support member  28  is desired than afforded by the multiplicity of the magnets  33  and  35  within respective cavities  32 , that fixation may be accomplished by providing a series of adjustable clamps carried by magnetic support member  28  at strategic locations to engage opposed edges of the graphic arts impression die plate  20 . 
     Alternatively, pin structure may be employed to prevent lateral movement of the graphic arts impression die plate  20 , especially in those instances where the relieved design-defining image to be formed in the layer  24  of the graphic arts impression die plate  20  using a milling machine, or hand-manipulated tools. A series of holes may be provided in the magnetic support member  28  for selective receipt of individual pins engageable with corresponding edges of the graphic arts impression die plate  20 . Desirably, the graphic arts impression die plate holding pins are positioned on all sides of the graphic arts impression die plate  20 , with two spaced holding pins on each side of the graphic arts impression die plate being provided. 
     The assembly of cladded metal graphic arts impression die plate  20  and backing or magnetic support member  28  as shown in FIGS. 12 and 13 may be mounted on the heated chase of a conventional clamshell hot foil stamping or blocking die press in the same manner as a conventional rigid magnesium, copper or brass die. As is well known to those skilled in this art, the heated chase of a conventional clamshell hot foil press is of so-called honeycombed design having a large number of openings for receipt of adjustable clamps for securing the die to the chase. In this manner, die may be located in a desired position relative to the overall extent of the chase. 
     Even though a chase is conventionally provided with a relatively large number of clamp-receiving openings, there are instances where it is desirable to further adjust the position of the die relative to the substrate to be impressed, and such desired die movement cannot always be accommodated because of the fixed relative positions of the clamp-mounting holes in the chase. 
     With the present assembly, however, magnetic affixation of the graphic arts impression die plate  20  to the backing or magnetic support member  28  allows the user to adjust the position of the graphic arts impression die plate on the magnetic support member in even minute amounts if desired, after the assembly of the support or backing member  28  and graphic arts impression die plate  20  have been attached to the chase. Make-ready time of a press can therefore be significantly reduced by virtue of the fact that it is not necessary to mount the die on the chase with the precision that has heretofore been required. Instead, the graphic arts impression die plate can readily be adjusted by simply relocating the graphic arts impression die plate  20  on the magnetic support member  28  within the overall dimensional limits of the latter after mounting of the magnetic support member  28 . 
     The present invention therefore provides the operator of the stamping, embossing or debossing machine to more quickly make a press ready for final operation because of the ease with which the graphic arts impression die plate assembly may be correctly aligned with an image onto which foil is to be applied, or the image embossed or debossed. This enhanced and more efficient make-ready is attributable to the press operator&#39;s ability to make precise and very small adjustments if necessary in the position of the graphic arts impression die plate assembly on the chase of the press, without the heretofore required necessity of manipulating each of the clamps attached to the chase to permit trial and error repositioning of the graphic arts impression die plate. 
     Another important advantage of the graphics art impression die assembly comprising graphic arts impression die plate  20  and magnetic support member  28  is the decreased time required to change over from one graphic arts impression die plate to another. In the past, this has required manual unclamping of all of the clamps holding the graphic arts impression die plate on the chase of the press, removal of the graphic arts impression die plate, placement of another graphic arts impression die plate on the chase, and fixation of that graphic arts impression die plate to the chase by further manual locking of all of the clamps around the perimeter of the graphic arts impression die plate to respective edges of the die. Significant time and effort was required to effect this manual change-out of a die, especially because of the necessity of aligning the die with the image area to be embossed or stamped, frequently requiring unclamping and clamping of the die as minute adjustments are made in its position on the chase. That clamping and unclamping is largely eliminated by use of the present graphics art die assembly in that the magnetic support plate  28  may be secured to the chase of the press with conventional clamps in what amounts to a macro position, with the necessary adjustments in the graphic arts impression die plate position for precise alignment purposes requiring only shifting of the position of the graphic arts impression die plate  20  on the magnetic support member  28  to whatever degree is required, including very minute adjustment distances. Final positioning of the graphic arts impression die plate  20  on the magnetic support member  28  can therefore be accomplished without repeated clamping and unclamping of the graphic arts impression die plate itself as has been required in the past. The time necessary for die change-out in this respect has been substantially decreased, even in those instances where foil must be displaced in stamping operations, or embossing carried out on sample substrate having a design image thereon, to verify that the die is correctly positioned, or if not, how much the die must be shifted on the chase in order to obtain the necessary alignment with the image. 
     Method of Preparing Cladded Metal Graphic Arts Impression Die 
     FIG. 11 of the drawings illustrates etching apparatus as shown and described in the &#39;494 patent, and which is useful for etching of graphic arts impression die plate  20  utilizing an etching composition and processing conditions previously described. 
     The etching apparatus  40  as depicted in FIG. 11 includes an etchant solution holding tank  42 , a containment basin  44  in which the tank  42  is located, and an open-topped basin  46  defined by four upright side walls and a bottom wall. A shallow pool  48  of the etchant solution is maintained in the bottom of the basin  46  through the use of a weir. Three paddle wheel assembly units  50  serve to direct etchant solution upwardly against the overlying graphic arts impression die plate to be etched. 
     The pivotal hood assembly  52  overlying basin  46  normally closes the opened upper end thereof, but may be swung upwardly and backwardly to gain access to the interior of the basin  46  of the etching apparatus  40 . 
     Hood assembly  52  has a depending frame assembly  54  which carries underlying, rotatable graphic arts impression die plate support structure broadly designated  56 . The support structure  56  is rotated about a vertical axis through the medium of a shaft  60  connected thereto which is operably connected to and driven by an electric motor  62 . 
     A preferred embodiment of support structure  56  comprises a plastic magnetic support member  64  of PVC shown in FIGS. 5 and 6. As is evident from these figures, magnetic support member  64  is of cross-shaped, planar configuration and has four legs  66 ,  68 ,  70 , and  72  integral with a central section  74 . The magnetic support member  64  has a plurality of openings  76  therein, each of which receives a respective magnet  78 . Preferably two permanent magnets such as magnets  33  and  35  and an associated steel plate  36  in bridging relationship thereto as illustrated in FIG.  13  and described above, are mounted within each opening  76  of rectangular configuration and adhesively held in place therein. However, only one magnet  78  within a respective opening  76  as depicted in FIG. 1 has been found to provide satisfactory holding power in most instances for releasably securing a cladded metal die plate  20  on support structure  56  for etching purposes, noting in this respect that the displacement forces exerted on the cladded metal die plate  20  during etching attributable to rotation of the support  64  are not nearly as great or severe as the forces imposed on the die plate during mechanical milling or hand engraving thereof, or when the die plate is secured to a hot foil stamping or embossing chase of a graphic arts press. At least two diagonally-positioned mounting apertures  80  are provided in magnetic support member  64  to facilitate attachment thereof to a rotatable frame assembly  82  forming a part of rotatable support structure  56 . 
     In use, a blank cladded metal graphic arts impression die plate  20  having a design-defining layer of photo-resist on the copper layer  24  is positioned on magnetic support member  64 . The graphic arts impression die plate  20  is located such that the steel layer  22  engages the face  70  of the magnetic support member  64  whereby the magnetic attraction of layer  22  by magnets  78  causes the blank graphic arts impression die plate to be firmly affixed to magnetic support member  64 . That magnetic support member normally will be pre-attached to the frame assembly  82  of the support structure  56  of the etching apparatus  40 . Blank graphic arts impression die plate  20  is oriented with the copper layer  24  thereof facing outwardly away from magnetic support member  64 . Thus, during operation of apparatus  40 , the etching solution will impinge against the exposed surface of copper layer  24  to remove the copper to form the required relieved design-defining image. 
     An alternate embodiment of structure for supporting the blank graphic arts impression die plate during etching thereof in apparatus  40  is shown in FIGS. 7 and 8. The support structure  164  as shown in these figures is of the same cross-shaped configuration and construction as magnetic support member  64  except that elongated, spaced strip-defining magnets  178  are substituted for the permanent magnets of magnetic support member  64 . At least two apertures  180  are provided in magnetic support member  164  for attachment of the magnetic support member to frame assembly  82  of the etching apparatus  40 . 
     Each of the magnets  178  is complementally received within a respective elongated, rectangular recess  179  in each of the legs  166 - 172 , and held in place therein by suitable adhesive. The magnets  178  are preferably spaced a distance less than the width of each magnet  178  and oriented such that they extend longitudinally of a respective leg  166 ,  168 ,  170 , and  172  of magnetic support member  164 . 
     The magnetic support member  164  is used in the same manner as described with respect to magnetic support member  64 , in that a blank graphic arts impression die plate placed thereon with the steel layer  22  in engagement with the face  184  of magnetic support member  164  is held in place by the magnetic attraction of steel layer  22  to strip magnets  178 . 
     A further alternate embodiment of structure for supporting the blank graphic arts impression die plate during etching of the plate in apparatus  40  is illustrated in FIGS. 9 and 10 designated by the numeral  264 . The magnetic support member  264  is of suitable etchant-resistant material and is also of cross-shaped configuration. The legs  266 ,  268 ,  270  and  272  of magnetic support member  264  are each provided with an elongated slot  286  therein extending longitudinally of a respective leg. Each of the legs  266 ,  268 ,  270  and  272  has an elongated groove  288  in the normally rearmost face  290  of each of the legs aligned with and of greater width than a corresponding slot  286 , as indicated by the dotted line representations of FIG.  9 . 
     A graphic arts impression die plate clamp  292  is shiftably mounted on each of the legs  266 ,  268 ,  270  and  272  for movement along the length of a respective slot  286 . Each clamp  292  includes a threaded fastener  294  provided with an enlarged rectangular head portion  294   a  slidable in a respective groove  288 , and an externally-threaded extension  294   b  which projects through a corresponding groove  288 . The rectangular plate  298  forming a part of each clamp  292  has an opening therein (not shown) which receives a respective extension  294   b . Each plate  298  normally extends transversely across a corresponding slot  286  and is provided with an edge groove  298   a  therein which is sized and configured to receive an edge of a blank cladded metal graphic arts impression die plate  20  carried by the magnetic support member  264 . At least one nut  300  is threaded over each of the extensions  294   b  and may be rotated on the respective extension until brought into engagement with the adjacent face of a respective plate  298 . Magnetic support member  264  has a relatively large circular recess  302  in the central part of the cross-shaped member in alignment with all four legs  266 - 272  of magnetic support member  264 . A permanent magnet  304  is located within recess  302  in disposition such that the outer face of the magnet  304  is flush with the normally outermost face of the magnetic support member  264 , as shown in FIG.  10 . Magnet  304  may be adhesively secured to magnetic support member  264  within recess  302 . Again, a pair of spaced magnets joined by a steel bridging element therebetween may be provided in lieu of the magnet  304  as depicted in FIG.  9 . The magnetic support member  264  also has at least two apertures  280  therein for attachment of the magnetic support member to frame assembly  82  of etching apparatus  40 . 
     A blank cladded metal graphic arts impression die plate  20  having design-defining photo-resist on the outer face of copper layer  24  is positioned on pre-attached magnetic support member  264  with the steel layer  22  engaging face  270  of the magnetic support member  264 . After loosening of each of the nuts  300 , respective clamps  292  are shifted along the length of corresponding slots  286  until the grooved portion  298   a  of each plate  298  receives a respective edge of graphic arts impression die plate  20 . The grooves  298   a  are configured such that the effective height thereof is slightly less than the thickness of the graphic arts impression die plate so that when a corresponding nut  30  is tightened down on a respective extension  294 , the plate  298  will engage and force the edge of the graphic arts impression die plate  20  tightly against the face  270  of magnetic support member  264 . 
     In view of the fact that the central part of the graphic arts impression die plate  20  resting on magnetic support member  264  is magnetically attracted to and engages the magnet  304  located in the center of the magnetic support member  264 , the graphic arts impression die plate  20  lays flat against the face  270  of the magnetic support member  264  throughout the extent of the graphic arts impression die plate  20 , even though it is held only at the edges thereof by respective clamps  292 . 
     Etching of the outer face of the copper layer of a blank graphic arts impression die plate  20  carried by magnetic support member  264  within etching apparatus  40  is accomplished in the same manner as with respect to magnetic support members  64  and  164 . 
     Another alternate embodiment of the support structure or member for graphic arts impression die plate  20  is shown in FIGS. 14-16 and comprises a disc  364  generally circular configuration which is also constructed of an etchant-resistant material of the type previously described. The disc-shaped magnetic support member  364  a series of semi-circular slots  386  in the perimeter thereof for receiving attachment devices for securing the disc member to the rotatable support structure  56  of an etching machine such as the apparatus  40  as shown in FIG. 11. A number of circular openings  388  are provided in the magnetic support member  364  which extend through the thickness of the disc-shaped member. 
     A number of circumferentially spaced, radially extending, relatively short, elongated slots  332  are provided in one face  366  of magnetic support member  364 . As is apparent from FIG. 16, the slots  332  do not extend through the full thickness of the disc-shaped member  364 , but terminate in spaced relationship from the face  368  of member  364 . It is also to be observed from FIG. 14 that each of the slots  332  is oriented with the longitudinal axis thereof extending through the axis of member  364 . 
     Each of the slots  364  receives a permanent magnet  338  that may be of rectangular configuration as depicted in FIG. 16, or alternatively, two spaced magnets such as magnets  33  and  35  bridged by a steel plate  36 . Each of the magnets  338  is located against the bottom surface  334  of a respective slot  332 . A filler  336  of epoxy or the like retains each of the magnets  338  in position against the surface  334  of corresponding slots  332 . The epoxy filler  336  may be introduced into slots  332  as a liquid and allowed to harden in place in filling relationship to a respective slot  332 . 
     The magnets  338  are constructed and they are oriented such that the maximum magnet field emanating therefrom is present at the normally uppermost face  370  thereof. It is to be recognized that by virtue of the relatively thin portion  372  of disc  364  which remains in overlying relationship to each of the slots  332 , such portion  372  does not detract to any significant degree from the magnetic properties exhibited by permanent magnets  338  embedded within slots  332 . 
     The advantage of embedding magnets  338  in slots  332  which extend virtually all of the way through the thickness of disc  364 , but in fact terminate in spaced relationship from the face  370  of the magnetic support member  364 , provides a completely flat surface defined by face  370  for receipt of a graphic arts impression die plate  20  thereon. Furthermore, the relatively thin portions  372  of magnetic support member  364 , which are integral with the main body of the support, fully protect the magnets from etchant solution splashed there against during etching of a graphic arts impression die plate blank removably positioned on magnetic support member  364 . 
     Magnetic support member  364  is used in the same manner to support a graphic arts impression die plate  20  as the magnetic support members  64 ,  164  and  264  previously described. 
     Although the preferred magnetic support member  364  is provided with a plurality of circumferentially spaced and radially extending slots  332  which receive respective permanent magnets  338 , in lieu of the plurality of magnets, a relatively thin, circular magnetic ferrite sheet may be adhesively or otherwise affixed to the face  370  of magnetic support member  364 . The magnetic ferrite sheet should have adequate magnetic attraction to firmly hold a graphic arts impression die plate  20  to the magnetic support member  364 , to substantially the same degree as is accomplished by the embodiment of magnetic support member  364  having a permanent magnet  338  within each of the slots  332 . The ferrite sheet should be provided with cutouts corresponding to at least slots  386 , and if desired, respective openings  388 .