Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating
Reexamination Certificate
2002-05-10
2004-09-14
La Villa, Michael (Department: 1775)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
C264S001310, C264S001340, C029S017800, C072S199000, C072S365200, C072S379600
Reexamination Certificate
active
06790387
ABSTRACT:
TECHNICAL FIELD
This invention relates to the formation of optical diffraction gratings (surface relief patterns) on the surfaces of sheets, foils and finished articles made of metals. The invention also relates to the articles exhibiting diffraction patterns thereby produced and to methods of making tools for impressing diffraction gratings onto surfaces of the articles. More particularly, although not exclusively, the invention relates to the patterning of sheets, foils and finished articles made of aluminum or aluminum alloys, such as beverage cans, foils, sheets, and the like, by imparting optical diffraction effects.
BACKGROUND ART
Aluminum and aluminum alloys are used nowadays for the formation of an ever increasing number of finished articles, including containers, foil packages and the like, intended for sale to the public at large either as finished products in themselves, or as part of the packaging of other items, such as foods and beverages. Articles of this kind are normally required to have a decorative finish, e.g. an applied coating, a paint layer, a transfer, a patterned transfer, or the like, in order to make such articles more attractive, more noticeable and therefore more saleable. Decorative finishes are an important marketing tool in an increasingly competitive market place.
As an example, it is to be noted that beverage cans made of aluminum are manufactured in large numbers and are usually decorated by the application of a paper label or, more commonly, by direct painting, lacquering or printing, or by the application of a decal. While such decorating methods make it easy to apply a wide range of patterns, messages and logos, etc., to the outer surface of the can body, they have the disadvantage that they make the recycling of used beverage cans somewhat more difficult (since the paper, paints, inks, etc., have to be removed in some way to avoid contamination of the recycled metal or interference with the remelting process) and, more importantly, they have the potential for causing pollution because they require the use of solvents or other harmful materials. As a result, expensive solvent recycling and scrubbing systems are required, or new technology making use of water-based inks, paints or lacquers, or the use of very small amounts of conventional solvents, must be developed.
It has long been known that very appealing decorative effects may be obtained by creating a diffraction grating on a surface of an article. A diffraction grating is a surface relief pattern formed by a series of ridges and/or grooves formed on or in the article surface, the ridges and/or grooves having spacings (i.e. distances separating one from another) in the order or the wavelength of visible light (about 1 micron). When illuminated with diffuse white light, diffraction gratings create light interference effects and produce visible coloured patterns with hues and intensities that often change with viewing angle (e.g. rainbow-like patterns). Such patterns are extremely attractive and eye-catching, and they may be created without having to apply any foreign substance onto the article surface.
One way of providing the outer surface of an article with a diffraction grating is to emboss the pattern of ridges or grooves directly onto the article surface by means of an impressing tool such as a die, roller, or the like, the tool itself having a bearing surface provided with a mirror image of the desired diffraction grating (i.e. projecting ridges where there are intended to be recessed grooves, or vice versa). Embossing techniques are effective for applying diffraction or holographic patterns to relatively soft materials, such as plastics (e.g. for applying security features to credit cards and bank cards). The patterns are typically applied using a nickel shim master, supported on a roll. The resulting embossed diffraction gratings may then be metallized (aluminized) by known methods to produce bright diffractive patterns or holograms.
However, it is not as easy to emboss diffraction gratings directly onto articles having harder surfaces, e.g. metal surfaces, particularly when subsequent metal treating operations are required. As an example, the formation of aluminum beverage cans is considered. The cans are first formed by drawing a metal cup from sheet metal and then lengthening the sides of the cup by passing the cup through two or three successively smaller ironing rings. A lid is then applied to the can body to close the container. If a diffraction grating were to be embossed on the surface of the stock sheet metal, it would be eliminated by the surface smoothing action of the drawing and ironing stages. On the other hand, if the finished can were embossed with the diffraction pattern, this would require high embossing pressures because the drawing and ironing steps make the metal “hard” and it would be difficult as a result to avoid distorting the cans, even if they were well supported. A proposed method of carrying out such an embossing procedure on a can body is described in U.S. Pat. No. 5,881,444 which issued on Mar. 16, 1999 to Aluminum Company of America, in which the can is supported by a mandrel and pressed against a hardened embossed tool. It is noted that the process requires sufficient pressure to cause transfer of the embossed pattern to the can surface, while the pressure should not be so high that the wall of the can is distorted or deformed. In the experience of the inventors of the present invention, these two requirements cannot be satisfied satisfactorily at the same time, and an efficient pattern transfer is generally accompanied by an unacceptable distortion of the metal substrate. Similarly, if the pressure is reduced to avoid distortion, poor pattern transfer occurs. Moreover, the embossing step would have to be carried out very quickly in order not to delay the production rate of the cans (a typical line speed for can production is about 300 cans per minute). These factors make the embossing procedure unattractive for can bodies.
On the other hand, if the metal is soft enough to make the application of a diffraction pattern relatively easy (e.g. if the metal is in the form of a thin foil), the pattern may be distorted during the impressing step because the thickness reduction that will occur will be accompanied by an elongation or spread of the substrate metal.
U.S. Pat. No. 4,725,111 to Weitzen et. al., which issued on Feb. 16, 1988 to American Bank Note Holographics, (and corresponding divisional U.S. Pat. No. 4,773,718 which issued on Sep. 27, 1988) discloses a process of the above-mentioned type of applying diffraction patterns and holograms directly onto surfaces of metal materials, such as household aluminum foil and beverage cans. This is achieved by passing a metal foil or plate between a heated contoured embossing roller and a cooled counter roller, thus embossing a “negative” of the diffraction pattern provided on the embossing roller directly onto the metal foil or plate. When this procedure is applied to beverage cans, the embossing procedure is carried out as a final step on the formed can bodies. A wheel is provided with a number of cooled mandrels on which the can bodies are positioned and then, as the wheel rotates, it brings each can body in turn into contact with a heated embossing roller which compresses the sidewall of the can between the surface of the embossing roller and the mandrel. As will be appreciated, this procedure has the disadvantages mentioned above, i.e. that a potentially slow additional step must be carried out on a metal that has been hardened by drawing and ironing. The heating steps soften the metal to some extent, but slows down the overall processing speed. The process as taught relies on careful control of pressure such that it is sufficient to transfer an embossed pattern but not so high as to result in “calendering” of the material (calendering is a term normally associated with paper mills and is the polishing or smoothening of the surface which occurs as a result of sliding of the product against a roll).
Ball Melville Douglas
Fairlie Matthew James
Sang Harry
Alcan International Limited
La Villa Michael
LandOfFree
Process of producing diffraction gratings on the surface of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process of producing diffraction gratings on the surface of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process of producing diffraction gratings on the surface of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3231732