Patent Publication Number: US-7591955-B2

Title: Method for forming an etched soft edge metal foil and the product thereof

Description:
This patent application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/699,927, filed Jul. 14, 2005, entitled “METHOD FOR FORMING AN ETCHED SOFT EDGE TECHNOLOGY AND THE PRODUCT THEREOF” the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The invention relates generally to metal processing and more particularly, to the processing of thin metal foils or metal strips. 
     BACKGROUND OF THE INVENTION 
     Many industries require thin metal parts with precise dimensions. Such parts include optical components, encoder discs, switch contacts, spacers and gaskets, meshes, thin screens, flat springs, and foils. There are currently numerous methods known to produce thin metal parts. The most popular methods are electroforming, etching, punch and die stamping, and cutting, either by laser or water jet. Electroforming is a process by which a metal part is produced on a base form. The base form is negatively charged and placed in a solution containing a salt of the metal desired for the part. The metal is deposited onto the base form and the part is formed. In contrast, the other methods begin with a thin metal sheet and remove material until the desired part is produced. Etching uses an acid to remove the base metal by chemical or electrochemical corrosion. An etch-resistant protective coating is adhered to the object in areas where the metal should remain. During the etch, only the unprotected parts are removed by corrosion. 
     For precision applications electroforming, etching, and punch and die stamping have a clear advantage over cutting because heat or mechanical energy generated during the cutting process deforms the edges of the cut. Electroforming and etching have a further advantage over stamping because the stamping process cannot readily create grooves or notches of a depth approximating half of the material thickness in the metal part. Although electroforming and etching can both produce seemingly very similar parts, the parts have geometries at a microscopic level that are quite different. 
     One application where thin metal parts are used is electric foil-head shavers. As shown in  FIG. 1 , a foil-head shaver  1  has a cutting element  5  which oscillates under a shaver foil  10 . The shaver foil  10  serves as a guard to protect the user&#39;s face. The shaver foil  10 , as shown in  FIG. 2 , has a skin side  20 , a cutter side  25  and a plurality of whisker holes  30  which allow the user&#39;s hair to pass through and be cut by the cutting element  5 . 
     Conventionally, shaver foils are produced by electroforming of nickel or another metal. A cross-sectional view of a shaver foil made by electroforming is shown in FIG.  3   a . The cross section shown passes through the center of the whisker hole and includes the thinnest part of the shaver foil between adjacent whisker holes. An advantage of the electroformed foil is that the edges  56  of the whisker holes  30  on the face side  20  of the shaver foil  10  are rounded, providing user comfort. On the other hand, the edges  51  of the whisker holes  30  on the cutter side  25  are blunt resulting in poor cuts. This results in a shaver which is comfortable to use but does not perform optimally. 
     An alternative method of producing shaver foils is by etching the whisker holes in a preexisting metal foil.  FIG. 3   b  shows a cross section corresponding to  3   a  of a shaver foil made by etching The etching produces sharp edges  57  and  52  on both the face side  20  and the cutter side  25 , respectively, of the shaver foil  10 . The sharp edges  52  on the cutter side  25  improve the cutting ability of the shaver, but the sharp edges  57  on the face side  20  are uncomfortable for the user. The etching process also allows the metal foil to be produced from various wrought metal alloys including stainless steels which have more desirable properties than can be produced by the electroforming process. 
     SUMMARY OF THE INVENTION 
     The present invention is for a thin-metal processing method which removes material from a thin metal foil or a metal strip. The processing method involves etching to create removed areas in the metal foil or strip. Novel surface preparation steps allow edges surrounding the removed areas to be either sharp or rounded. The processing method further allows both the sharp and rounded edges to be made in a single etching pass. 
     The present invention also relates to an improved shaver foil for a foil-head shaver. The shaver foil provides both face comfort and superior cutting ability over current foils. 
     The shaver foil includes a skin side, a cutter side and a plurality of whisker holes. The whisker holes allow a user&#39;s hair to pass through the shaver foil and be cut by a cutting element while the shaver foil protects the user&#39;s skin. The whisker holes have edges on both the face side and the skin side of the shaver foil. The skin side edge is a rounded “soft-edge” which provides comfort for the user&#39;s face. The cutter side edge is sharp to improve cutting ability. 
     The aforementioned thin-metal processing method is used to produce the improved shaver foil and includes a plurality of steps. The plurality of steps include (i) application of photo-resist or any other substance known in the art that resists corrosion during etching to an unclean surface of a thin metal foil or metal strip where a soft edge is desired, wherein the photo-resist or other corrosion resistant substance will improperly adhere to the surface due to the presence of elements such as metal particles, oils, small organic materials and/or oxidation on the unclean surface and the adhesion will fail during an etching step, allowing a slight corrosion of the base material to occur at areas where adhesion has failed; (ii) application of photo-resist or any other substance known in the art that resists corrosion during etching to a clean surface of a base material where a sharp edge is desired, wherein the photo-resist or other corrosion resistant substance will optimally adhere to the surface and prevent corrosion of the base material under that substance; and (iii) etching of the base material to produce a foil with a number of openings, each opening having a sharp edge on one side and a soft edge on another side. 
     Another metal processing method of the present invention includes the steps of (i) preparing a metal surface to produce marginal adhesion between the metal surface and a layer of etch-resistant material; (ii) applying the etch resistant material to the metal surface; and (iii) etching the metal. During the etching step the adhesive bond is progressively degraded with cumulative residence time in the etching acid, causing the resist bond to fail and delaminate only at or near the end of the etching step. Only limited edge rounding takes place locally on the immediate surfaces adjacent to the perimeter of the etched part, rather than generally across the entire surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention in which: 
         FIG. 1  is a perspective view of a prior art foil-head shaver; 
         FIG. 2  is a perspective view of a prior art shaver foil; 
         FIG. 3   a  is an enlarged cross-sectional view of a single whisker hole within a prior art shaver foil produced by electroforming; 
         FIG. 3   b  is an enlarged cross-sectional view of a single whisker hole within a prior art shaver foil produced by etching; 
         FIG. 4  is an enlarged cross-sectional view of a single whisker hole within a shaver foil produced in accordance with the present invention; 
         FIG. 5  is a bottom view of a shaver foil in accordance with the present invention; 
         FIG. 6   a  is an enlarged cross-sectional view of a thin metal foil coated with etch resistant material and a protective coating on one side in accordance with the present invention; 
         FIG. 6   b  is an enlarged cross-sectional view of a thin metal foil coated with etch resistant material and a protective coating on both sides in accordance with the present invention; 
         FIG. 7   a  is an enlarged cross-sectional view of a thin metal foil coated with partially exposed etch resistant material in accordance with the present invention; 
         FIG. 7   b  is an enlarged cross-sectional view of a thin metal foil with an attached pattern of etch resistant material in accordance with the present invention; and 
         FIG. 8  is an enlarged cross-sectional view of a thin metal foil and an attached pattern of etch resistant material after it has been etched in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , a conventional foil-head shaver  1  includes a shaver foil  10  and a cutter element  5 . The shaver foil  10  shown in detail in  FIG. 2  includes a skin side  20 , a cutter side  25  and a plurality of openings or holes  30 . The openings, or whisker holes  30 , allow a user&#39;s hair to pass through the shaver foil  10  and be cut by the cutter element  5 .  FIG. 4  shows a cross-section of one whisker hole  30  in accordance with an embodiment of the present invention, wherein the whisker hole is circular. The cross section shown in  FIG. 4  passes through the center of the circular whisker hole  30  and shows the thinnest part of the shaver foil  10  between adjacent whisker holes  30 . An edge profile  40  defines the perimeter of the whisker hole  30  and extends from the skin side  20  of the shaver foil  10  to the cutter side  25  of the shaver foil  10 . A skin-side edge  50  is formed where the edge profile  40  meets the skin side  20 , and a cutter-side edge  55  is formed where the edge profile  40  meets the cutter side  25 . The edge profile  40  should be angled inward from the skin side  20  to the cutter side  25 , such that the skin-side circumference of the hole  30  is larger than the cutter-side circumference of the hole  30 . In other words, an angle  45  between the cutter side  25  of the shaver foil  10  and the edge profile  40  is acute. Thus, the cutter-side edge  55  is sharp. In contrast, an angle  47  between the skin side  20  and the edge profile  40  is obtuse. Additionally, the skin-side edge  50  is rounded. 
     The geometries of the skin-side edge  50  and the cutter-side edge  55  provide an improved shaver foil  10  in contrast to conventional shaver foils. The rounded skin-side edge is comfortable to the user&#39;s skin. There are no sharp edges which rub across the user&#39;s face. In addition, the sharp cutter-side edge  55  provides superior cutting ability over conventional shaver foils. When the sharp cutter-side edge  55  and the cutter element  5  come into close contact, any hairs positioned therebetween have a higher probability of being cut than if the cutter-side edge is blunt. 
     In a preferred embodiment, as shown in  FIG. 5 , a relief  60  is formed over the majority of the cutter side  25  of the shaver foil  10 . Only a margin  65  around the whisker holes  30  protrudes. The relief  60  helps reduce the amount of material which comes into close contact with the cutter. This reduces friction and the heat resulting thereof. 
     The present invention also relates to a method for producing the shaver foils. The preferred production method is described below in detail. However, the steps presented are not required to be carried out in a particular order, nor are the specific materials discussed the only materials which can be used to perform the method of the present invention. 
     Surface preparation is an important aspect of the method used in the present invention. Stock metal, as received from metal distributors, is never completely clean. Although the metal may not appear “dirty,” the surface is likely to contain at least one contaminant. These contaminants may include organic materials, oils, small particles of metal resulting from the stock metal processing and slight oxidation of the surface. In order to produce an optimal adhesion between the etch-resistant coating and the metal surface, all contaminants must be removed. If the contaminants are not removed, the adhesion will fail during etching and the etch-resistant coating will not protect the metal surface. 
     The present invention uses controlled adhesion failure between the metal surface and the etch-resistant coating to create rounded edges at the boundaries of the etched part. In contrast, conventional etch preparation with optimal adhesion between the metal surface and the etch-resistant coating produces sharp edges around the etched perimeter. An embodiment of the preparation and etching method in accordance with the present invention is described in detail below. 
     A metal foil as received from a metal distributor is moderately cleaned. The cleaning process used should remove only a fraction of the contaminants on the metal surface. One embodiment of the moderate cleaning process in accordance with the present invention uses a microetch to remove a fraction of the contaminants. The microetch includes sulfuric and ferric chloride acids. In an alternative embodiment of the moderate cleaning process, an acid wash is used to moderately clean the metal foil. Regardless of the cleaning process used, some contaminants remain on the surface of the metal foil. Alternatively, the surface of the stock metal could be deliberately contaminated as a pre-treatment step to assure desired response to an optional cleaning process. 
     As shown in  FIG. 6 , the moderately clean metal foil  100  has a first side  110  and a second side  115  and is coated with a first layer of etch-resistant material (photo-resist)  120  on the first side  110 . An imperfect adhesion between the first layer of photo-resist  120  and the first side  110  is made because of the remaining contaminants on the surface  110  of the metal foil  100 . The first layer of photo-resist  120  is then covered with a protective coating  128 ; polyester film is used for the protective coating  128  in a preferred embodiment. The second side  115  is then properly cleaned. No harm is done to the first layer of photo-resist  120  during the cleaning because of the protective coating  128 . As shown in  FIG. 6   b , a second layer of photo-resist  122  plus an additional protective coating  128  is then applied to the second side  115  of the metal foil. An optimal adhesion is formed between the second side  115  and the second layer of photo-resist  122 . Due to the differences in adhesion, there are different bond strengths between the layers of photoresist and the metal foil. 
     Before etching the foil  100 , sections of the photo-resist layers  120 / 122  are removed to create a pattern of photo-resist  124  on the metal surfaces  110 / 115 . This will allow only some sections of the foil  100  to be removed during the etch. The sections of the metal foil  100  that are protected by the photo-resist pattern  124  will not be removed. The removed sections of photo-resist in the preferred embodiment include larger circles on the first side  110  and smaller circles on the second side  115 . This will produce etched holes that angle inward from the first side  110  to the second side  115 . 
     Various methods may be used to create the pattern of photo-resist  124  on the foil  100 . The methods depend on the type of photo-resist used. Either positive or negative photo-resist may be used. Positive photo-resist becomes more soluble when exposed to ultraviolet light. Any exposed sections of positive photo-resist are easily removed from the applied surface in a solution. The unexposed sections remain on the surface. In contrast, negative photo-resist polymerizes when exposed to ultraviolet light causing the polymer to become more stable and prevents dissolution in developer chemicals. The exposed sections of negative photo-resist remain on the surface while the unexposed sections of negative photo-resist are removed in a solution. In both methods, sections of the photo-resist layers  120 / 122  are exposed to ultraviolet light in order to create the photo-resist pattern  124 . There are various methods that may be used to control which sections of photo-resist are exposed to the ultraviolet light. One method uses a filter at the source of the ultraviolet light, to project a pattern onto the photo-resist layers  120 / 122 . An alternative method uses an opaque mask covering the photo-resist layers  120 / 122  to block some of the ultraviolet light. The preferred embodiment uses negative photo-resist and ultraviolet light that is filtered at the source.  FIGS. 7   a ,  7   b  and  8  illustrate the preferred embodiment of the present invention. 
     As shown in  FIG. 7   a , both layers of photo-resist  120 / 122  are exposed to a pattern of ultraviolet rays  140 . The ultraviolet rays  140  produce a pattern of exposed photo-resist  124  and a remainder of unexposed photo-resist  126 . During the exposure, the photo-resist polymerizes. The unexposed photo-resist  126  is then removed from the foil  100  in a solution, and only the pattern of exposed photo-resist  124  remains. The metal foil  100  with the attached pattern of exposed photo-resist  124  is shown in  FIG. 7   b . The boundaries of the pattern of exposed photo-resist  124  define first edges  152  where the exposed photo-resist  124  and metal foil  100  meet on the first side  110  and second edges  153  where the exposed photo-resist  124  and metal foil  100  meet on the second side  115 . 
     The metal foil  100  is then subjected to an etching acid and holes  30  are formed. In the preferred embodiment, high specific gravity ferric chloride etching acid is used. The resulting part is shown in  FIG. 8 . During the etch, areas of the metal foil  100  that were not protected by photo-resist are removed completely forming a plurality of holes  30  in the metal foil  100 . The optimal adhesion between the pattern of photo-resist  124  on the second side  115  and the metal foil  100  keeps the photo-resist on the second side  115  intact during the etch. This produces a sharp edge  155  where the side  140  of the hole  30  meets the second side  115  of the metal foil  100 . In contrast, the adhesion between the pattern of exposed photo-resist  124  on the first side  110  and the metal foil  100  fails during the etch. The failure begins at the first edges  152  of the photo-resist pattern  124  on the first side  110 . As the adhesion fails the areas under which failure occurs come into contact with the etching acid. As a result, the etching acid corrodes the metal in those areas and creates rounded edges  150 . 
     In an alternative embodiment (not shown) the adhesion between the photo-resist pattern  124  and the first side  110  of the metal foil  100  fails almost entirely during the etch. The adhesion does not fail upon immediate contact with the etching acid, rather, the photo-resist pattern is slowly peeled from the edges  152  until it is nearly entirely removed. As a result, the areas of the metal foil  100  under the edges  152  of the photo-resist pattern  124  erode more than the areas under the majority of the photo-resist pattern  124  and a gradual curve is formed from the edge of the hole  30  to the first side  110  of the metal foil  100 . 
     After etching, the photo-resist polymer patterns are removed and the resulting part is the shaver foil. The first side  110  is the skin side  20 , the second side  115  is the cutter side  25  and the holes  30  are the whisker holes  30  of the shaver foil  10 . 
     In an alternative embodiment of the present invention the metal foil may be first properly cleaned to remove all contaminants from the metal surface and photo-resist may be applied to those areas where optimal adhesion is desired to produce sharp edges. An adhesion inhibiting material may then be applied to the metal foil to inhibit the adhesion of a second layer of photo-resist applied to the metal foil. This will result in a failure of the adhesion and rounded “soft” edges will result. Alternatively, the adhesion inhibiting material may be applied to the second layer of photo-resist before application and both layers of photo-resist may be applied simultaneously. 
     Although the preferred form of the invention has been shown and described, many features may be varied, as will readily be apparent to those skilled in this art. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.