Patent Application
20100055221
This invention relates to methods, apparatus, and assemblies for roller press actuated sheet material embossing. More particularly, this invention relates to specially configured magnet embedded rolls which are adapted for holding flexible magnetic embossing dies.
Magnet embedded rolls or rollers are commonly utilized for supporting flexible ferromagnetic or paramagnetic embossing plates. In such usage, a thin and flexible ferromagnetic embossing plate is wrapped circumferentially about the roll, allowing the roll's typically exposed and embedded magnets to securely magnetically hold the plate as a radial extension of the roll. However, such magnetic roll and ferromagnetic plate assembly often undesirably produces a “ghosting” effect (discussed below) upon the sheet material which is embossed by the assembly.
Where a magnetic roll and ferromagnetic embossing plate assembly, such as is described above, is utilized for heated embossing of very thin holographic film, an undesirably embossed pattern matching the pattern of the roll's magnets and channels may be impressed into the holographic film along with the intended embossed patterns and images. Such undesirable additionally embossed magnet pattern is commonly referred to as “ghosting”, such embossing irregularities typically ruining the intended holographic film work product.
The “ghosting” effects described above are understood to be attributable to difficulties associated with precision grinding of the typical outer or finished circumferential surface of a magnetic roll. Upon attempting to grind a magnetic roll to a precise consistent radial dimension, the grinding wheel variably contacts the roll's substrate or core material (which commonly comprises mild steel) and the exposed radially outer surfaces of the embedded magnets. Such two materials elastically respond differently at the locus of grinding, resulting in microscopic differences between the two ground surfaces. Such differences in the material characters of the surfaces to be ground are understood to contribute to or cause small localized variations in the roll's radius of curvature and radial dimension. Additionally, in heated embossing processes such as those typically performed in holographic film embossing, differences between the thermal characteristic of a magnetic roll's substrate material and that of the embedded magnets commonly result in thermal variations along the surface of the roll. Such thermal variations are understood to additionally contribute to or cause an undesirable “ghost” pattern upon the embossed work product.
The instant inventive assembly and method for magnetic embossing roll surfacing solves or ameliorates the problems discussed above by spraying a stream of high velocity molten metal droplets over the outer circumferential roll surface to create a metal deposition or a build-up layer, and by subsequently precision grinding such metal sprayed outer layer to a consistent radial dimension.
The base or supporting structural component of the instant inventive magnetic roll for sheet material embossing comprises a cylindrical core having an axial length and having a first or base circumferential surface. The cylindrical core's circumferential surface necessarily has a base radial dimension measured from a central axially extending axis of rotation within the cylindrical core. The cylindrical core functions as a substrate or base which supports overlying structures such as embedded magnets, embedded magnetic pole pieces, metal sprayed circumferential surface covering layers, and a magnetically attached embossing, as will be further discussed below.
In a preferred embodiment of the instant invention, the cylindrical core substrate comprises mild steel, such material being durable and being economically obtained. Other cylindrical core substrate materials such as brass, stainless steel or aluminum may be suitably substituted. Where the roll is to be utilized in a heated application such as is typically performed in holographic film embossing, the cylindrical core substrate is preferably specially adapted for heating via heated oil pumped through internal channels provided within the cylindrical core, or via internally mounted electrical resistance “cal rod” heaters.
Magnet receiving channels are preferably milled radially inwardly from the circumferential outer surface of the cylindrical core. Thereafter, magnets are preferably fixedly mounted within such channels. Preferably, the magnets are durably held within the channels through the use of an epoxy adhesive, and the magnets are specially arranged in a “NN, SS, NN, SS . . . ” magnetic pole configuration along the channels, such pole configuration enhancing lines of magnetic flux emanating from the surface of the roll and enhancing the plate fixing strength of the roll. For additional plate fixing magnetic strength, mild steel pole pieces are preferably adhesively mounted between each of the magnets. In many applications, high strength ceramic magnets may be mounted upon the roll. However, where the roll is intended for use in heated applications, temperature tolerant magnets such as a samarium cobalt magnets are preferably used. Upon installation of the magnets within such channels, the assembled cylindrical core, magnets, and pole pieces combination is preferably ground to a consistent base radial dimension, such dimension preferably being between 0.007″ and 0.008″ less than a desired finished radial dimension of the roll.
In a preferred embodiment of the instant invention, at least a first armature effect resisting metal band is preferably adhered to the cylindrical core's first or base circumferential surface, such band comprising a deposition or build-up of sprayed molten metal droplets. Such deposition is preferably accomplished via utilization of a commonly known metal arc spraying, metal flame spraying, plasma metal spraying, or HVOF (high velocity oxy-fuel) metal spraying process. In order to prevent the sprayed deposition metal band from magnetically armaturing across the roll's north and south magnetic poles, the metal chosen for use in the metal spraying application preferably is non-paramagnetic or non-ferromagnetic in character, such metal preferably comprising stainless steel, a stainless steel alloy, aluminum, an aluminum alloy, copper, a copper alloy, zinc, a zinc alloy, tin, or a tin alloy. In a preferred embodiment of the instant invention, metal which is sprayed in molten droplet form over the cylindrical core and over the magnets comprises stainless steel. In order to enhance the initial adhesion of the metal sprayed layer at the exposed outer surfaces of the embedded magnets, a thin bond coating comprising an alloy such as an aluminum and nickel alloy may be metal sprayed prior to the application of a thicker stainless steel metal spray coating.
In a preferred performance of the method, the cylindrical core is rotated swiftly at approximately 84 RPM contemporaneously with application of the metal sprayed coatings. Such rotation during metal spraying desirably helically extends an adhered metal band consisting of a deposition or build-up of initially molten metal droplets over the circumferential surface of the cylindrical core. Such metal band is preferably further extended in a partially overlapping configuration in several axial spraying passes until such metal deposition band makes up an axially and circumferentially continuous outer surface layer. Such outer surface layer preferably comprises a temporary outer circumferential surface having a temporary radial dimension, such temporary outer surface preferably having a radial dimension between 0.01″ and 0.02″ greater than the intended finished radial dimension of the roll.
Upon complete application of the metal sprayed layer, the roll is preferably precision ground to a consistent radial dimension matching the intended finished radial dimension of the magnetic roll.
When the finished roll is utilized, for example, for supporting a holographic film embossing die for embossing holographic film, no “ghost” pattern is embossed upon the holographic film work product.
Accordingly, objects of the instant invention include the provision of and assembly of components, and the performance of method steps such as are described above. Other and further objects will become known to those skilled in the art upon review of the Detailed Description which follows, and upon review of the appended drawings.
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In a preferred embodiment of the instant invention, the cylindrical core 4, along with its exposed magnets 14 and magnetic pole pieces 16 are initially ground to a radial dimension that is between 0.007″ and 0.008″ less than the intended finished radial dimension of the roll. Upon the metal spraying application of the metal deposition band 2A, such radial dimension is preferably temporarily increased to 0.01″ to 0.02″ greater than the roll's intended finished radial dimension. Thereafter, the roll 1 may be re-ground to the intended finished radial dimension. At such dimension, the metal sprayed layer 2 preferably has a radial thickness between 0.007″ and 0.008″, the thickness of the layer 2 indicated in
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While the principles of the invention have been made clear in the above illustrative embodiment, those skilled in the art may make modifications in the structure, arrangement, portions, components, and method steps of the invention without departing from those principles. Accordingly, it is intended that the description and drawings be interpreted as illustrative and not in the limiting sense, and that the invention be given a scope commensurate with the appended claims.
