Method and apparatus for creating a graphic image on a reflective metal surface

A method for creating a graphic image on a reflective metal surface. A digital representation of the graphic image in a negative form is created on a computer and stored in memory. The negative image is then sent to an inkjet printer, where ultraviolet light curable ink representative of the negative image is printed onto the reflective metal surface using the inkjet printer. The portions of the reflective metal surface that are not covered with the ink remain exposed. The reflective metal in the exposed areas is mechanically scuffed to render it less reflective and visually distinct from the covered portions. The ultraviolet light curable ink is then removed with a high pressure water spray to reveal the graphic image made by the contrast between the original reflective metal and the scuffed metal surfaces.

FIELD OF THE INVENTION

The present invention relates generally to forming graphic images, and more particularly to a method of forming a graphic image on a reflective metal sheet.

BACKGROUND

Graphic images and designs have been formed on metal surfaces for a variety of uses for many years. One common practice employed in the electronics industry is to create printed circuit boards using a technique known as “print and etch”. This method uses a polymeric photoresist that is laminated onto a copper clad dielectric medium. The photoresist is photopolymerized with an ultraviolet light in selected areas, and the unpolymerized resist is washed off with chemicals, exposing the copper in certain areas. The exposed copper is then etched with strong acids to dissolve and chemically remove the copper. The remaining polymerized resist is then removed with additional harsh and environmentally damaging chemicals, to yield the patterned circuit. Numerous variations of this technique are used today thoughout various industries to provide patterned metal surfaces for a wide variety of uses. The problem with all of these techniques is that they require expensive equipment, they use large quantities of toxic chemicals that are harmful to the environment, and thus are subject to strict government regulation.

Alternate techniques seek to provide visual designs on metal surfaces by mechanically carving into the metal surface, removing some measurable amount of material by cutting, burning, or otherwise vaporizing the metal using cutting tools or lasers. Still other techniques emboss or stamp the metal surface to create a design.

Each of these techniques removes metal or distends the metal in one way or another, creating a three dimensional surface with measurable texture. They are limited in that they can only be used on metal substrates of sufficient thickness to allow for metal removal. And they are costly and slow. There is a continuing need to improve the quality, reduce the cost, and to simplify the manufacture of graphic images on a two dimensional metal surface that does not harm the environment.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method and apparatus components related to forming a graphic image on a reflective metal surface, which comprises a two-dimensional metal substrate, a substantially planar metal substrate, or other metal substrate. Accordingly, the apparatus components and methods have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

A method for creating a graphic image on a reflective metal surface in accordance with various embodiments will now be described. A digital representation of the graphic image is created on a computer and stored in memory. The digital file is then sent to an inkjet printer, where ultraviolet light curable ink representative of the image, or optionally of a negative version of the image, is printed onto the reflective metal surface using the inkjet printer. The portions of the reflective metal surface that are not covered with the ink remain exposed. The reflective metal in the exposed areas is mechanically scuffed to render it less reflective and visually distinct from the covered portions. The ultraviolet light curable ink is then removed with a high pressure water spray to reveal the graphic image made by the contrast between the original reflective metal surface and the scuffed metal surface.

Referring now toFIG. 1, a flowchart depicting one embodiment of the invention, a graphic image is created on a reflective metal surface.

The terms “reflective”, “polished”, and “shiny”, as interchangeably used herein refer to a surface that is capable of specular reflection. Referring now toFIG. 1, the law of reflection says that for specular reflection the angle10incident to the surface12equals the angle14reflected. Specular reflection forms an image. The most well known form of specular reflection is a mirror. Although all surfaces reflect light to a greater or lesser extent, diffuse reflection (FIG. 2) does not produce an image. When incident light20rays strike a rough or granular surface22, it bounces off in all directions24due to the microscopic irregularities of the interface. Thus, an image is not formed. This is called diffuse reflection. I find that metal that is polished to have a smooth, shiny surface provides adequate specular reflectance for use in forming a graphic image in accordance with my invention, in contrast to metal that diffuses light creating a dull or matte appearance. Brushed metal, for example, does not provide specular reflectance, but diffuses the light. Stainless steel is particularly suitable, although other metal sheets or sheets coated or clad with copper, aluminum, steel, nickel, and their alloys may also be used alone or in combination.

Referring now toFIG. 3, a flowchart depicting one embodiment of the invention, a graphic image formed on a two dimensional reflective surface begins30with a design stored in a computer memory in digital representation. Although the image might be created in any number of conventional ways, for example drawn on paper by a human hand using conventional tools such as pencils or markers, it ultimately is converted to a digital representation, by scanning, for example. More modern tools such as graphic arts design software store the image directly in digital format. Referring now toFIG. 4, the desired visual design40is created such that there are positive areas42, representing the image that the human eye perceives, and negative areas44, representing the “space” between the positive areas. Degrees of shading can be simulated by using halftone techniques in conventional manner. Where continuous tone imagery contains an infinite range of greys, the halftone process reduces visual reproductions to a binary image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion, that these tiny halftone dots are blended into smooth tones by the human eye.FIG. 5is a magnified example of a halftone. Although circular dots56are shown, other shapes are considered to be within the scope of the invention. Referring back toFIG. 3, the visual design stored in computer memory is then ported32to an inkjet printer, where a negative image of the visual design can be printed on the reflective metal surface. By negative image, I mean an image (FIG. 4)46that is the “opposite” of the desired visual design, where the positive portion and negative portions are reversed. The negative image can be created or stored in either the computer or in the inkjet printer. The terms “ink jetted image”, “ink jettable”, and “ink jet printed” all refer to an image created with an ink jet printing process employing a radiation curable ink composition. The image may be text, graphics, coding (e.g., bar coding), etc., being comprised of a single color, typically black. It has been found that inks cured by ultraviolet (UV) light, used in the printing industry for printing long lasting images such as outdoor signs, when polymerized or cured on the reflective substrate, can function as an effective “resist” for my invention. The metal substrate is placed on the printing bed of the inkjet printer after which the inkjet printer receives information from the computer to determine the precise position of the substrate on the printing bed. During the printing process34, a printer head of the inkjet printer moves along the substrate leaving droplets of the UV-light curable ink on demand. As the droplets of UV-curable ink are applied to the substrate, an ultraviolet light source located proximal the printer head of the inkjet printer exposes the UV-curable ink to ultraviolet light rays to polymerize or cure the UV-curable ink and bond it to the substrate. In regards to the above-mentioned UV inkjet printer, it is noted that UV inkjet printing hardware is commercially available from a number of sources including 3M Company of Saint Paul, Minn., Mimaki Engineering Co., Ltd of Tokyo, Japan, and Oce N. V. of The Netherlands. I have found that the Oce model 250 is particularly useful. These UV-curable inks are formulated by the various ink suppliers specifically for signage articles intended for outdoor usage. In the case of signage for traffic control, the articles are able to withstand at least one year and more preferably at least three years of weathering, temperature extremes, exposure to moisture ranging from dew to rainstorms, and colorfast stability under sunlight.

After ink jetting the negative image onto the reflective substrate, areas of the reflective metal surface that represent the “white space” or negative portions of the visual design are covered with cured ink, and those areas that will correspond to positive portions remain exposed or not covered. The exposed portions of the reflective surface are then mechanically roughened36to render the spectrally reflective surface diffusely reflective. Some exemplary means of mechanical roughening are scuffing, grazing, brushing, scratching, and scrubbing, among others. This operation can be performed by sanding, bead blasting, or abrading with a synthetic pad such as a nylon web impregnated with aluminum oxide abrasive. I find that the well known synthetic Scotch Brite pads sold by the 3M Company are useful to scuff exposed portions of the surface sufficient to alter the reflectance so as to cause a distinct difference between the original reflective polished surface and the scuffed surface. The pads are manually worked across the surface in a unidirectional motion, but can also be moved in two or more directions, or can be circular or random to create a variety of effects. Although the intent is to scuff the surface of the exposed polished metal, one does not need to be selective and only touch the metal. The UV cured ink, having been formulated to withstand harsh climates, is robust enough to withstand the scuffing and not detach from the metal surface or otherwise become degraded. Some roughening or scratching of the ink may occur, but this is inconsequential, and will not alter the appearance of the end product. Mechanical roughening can be performed manually or manually with assist by a portable power tool, or even completely automatic with a dedicated machine. The roughening is typically performed dry, although some type of lubricant may be used if desired. It is important to note that this operation is not intended to remove macro amounts of material from the surface, but merely to render it diffuse, i.e. microscopically rough. It is not necessary to etch or abrade into the surface.

After mechanically roughening the reflective surface, the UV-cured ink needs to be removed37. Since the ink has been cured and strongly bonds to the metal surface, and since it has been formulated to withstand water and mechanical abuse for years, conventional wisdom dictates that it would require harsh chemicals to remove. Indeed, in the prior art, ink resists have been commonly removed using a plethora of exotic chemicals such as chlorinated solvents, oxygenated solvents, acids, bases, etc. I have found that a novel method to remove the ink jetted UV-cured ink is to impart a very high pressure spray of water37onto the ink. A high pressure in excess of approximately 1000 pounds per square inch (PSI), and more preferably in excess of 1500 PSI, imparted by a pressure washer, is sufficient to remove this tenacious ink. The nozzle of the pressure washer is typically aimed at the cured ink at a shallow angle to remove the ink. It is not necessary to heat the water, and no chemicals are needed, with standard tap water from a metropolitan water supply at a neutral pH. As an example, the high pressure spray may be water having a neutral pH and less than 500 ppm total dissolved solids applied at a pressure greater than 1000 pounds per square inch. Once the ink has been pressure-washed off, and the metal article has been dried, the finished graphic design image is revealed38. The difference in reflection between those portions44of the original specular reflective surface and the scuffed portions42that are now diffused, creates the visual design, apparent to the human eye, where the diffuse areas form a positive representation of the visual design.

In one alternate embodiment, instead of printing inkjet ink in a negative pattern, the ink is printed in a positive pattern, and the reflective areas form a positive representation of the visual design on a diffuse background. The choice of whether to print a positive pattern of ink or a negative pattern of ink is a design choice left to the practioner.

In another alternate embodiment of the invention, the process of ink jetting, scuffing, and removing the ink with high pressure water spray can optionally39be repeated one or more times. These second and subsequent repetitions may use a different version of the visual design that exposes only some of those portions that were previously exposed and abraded. The second mechanical roughening process creates areas that are even more diffuse than those created during the first process, giving those areas greater intensity. Optionally, subsequent processes can also abrade the surface in a direction that is different than the first abrasion direction, for example, orthogonal or random. The surface can be mechanically roughened using a different type of scuffing media. A variety of effects can be achieved by the practioner to create visual designs or graphic images on a reflective metal surface.

In yet other alternate embodiments of the invention shown inFIG. 6, the process described herein can be used create articles of manufacture, such as metal doors62, metal drawers64, metal cabinets, etc.

In summary, a method for creating a graphic image on a reflective metal surface utilizes a digital representation of the graphic image that is printed by an inkjet printer onto a reflective metal surface. The exposed portions of the reflective metal surface that are not covered with the ink are mechanically scuffed to render them less reflective and visually distinct from the covered portions. The ink is durable enough to withstand the brief scuffing imparted by the Scotch Brite pads or other scuffing implements. The ultraviolet light curable ink is then removed with a high pressure water spray to reveal a graphic image formed by the contrast between the original reflective metal surface and the scuffed metal surface.