Patent Publication Number: US-2004050739-A1

Title: One-sided covered marking on sheet metal surfaces

Description:
[0001] The invention relates to a method for hiding marks on thin metal sheets or similar surfaces, especially in two part beverage cans, as well as such marked metal sheets.  
       [0002] It often happens that distinguishing marks or other codes are embossed (or impressed in some other way) on the thin sheets used for making boxes and lids, especially for drinking liquids, The preferred method is stamping, or scratch impressions. One disadvantage of this method is that an image of the mark, sign, or code appears on the reverse side of the thin metal. Although a mirror image, it can be deciphered easily. This is often not desirable.  
       [0003] The object of the invention is to suggest a method by which covered marking is provided on thin sheets wherein formation of a mark impressed on one side, preferably embossed or otherwise, of a thin metal sheet. The image appearing on the other side of the metal sheet is not to appear according to the invention.  
       [0004] This is achieved by the method according to claim 1 or 14. It is provided that the reverse (other) side of the thin metal showing the image is subsequently subjected to a second embossing process in such a way that the visibility and legibility of the embossed mark on one side is unimpaired, while at the reverse side of the sheet the image is covered by the subsequent embossing and cannot be read or deciphered any more.  
       [0005] The subsequent embossing is carried out by means of a specially surface structured stamp which provides a textural, patterned or hatched impression (embossing) on the reverse side of the metal sheet which then reliably covers the image.  
       [0006] The original impression can, for example, be applied to the inside of a thin metal lid with its inside surface turned to the interior of the closed can. The second stamping process is then applied to the outside of the lid that is part of the visible surface of the closed can.  
       [0007] It often happens that a tear-out tab end can be opened with a shaped tab of thin sheet metal. An advantage of this is that the embossment or impression (which penetrates the surface) could be placed on the bottom side of the removable tab, thus being invisible (claim 17). The image of the mark appearing on the upper surface of the tab can then be covered by the second impression on the upper surface. The stamp used for the second procedure can be textured, patterned or hatched (claim 11).  
       [0008] The invention can also be applied to a lid made of thin sheet metal (claim 10) or a section of such a surface (claim 16) which has to be retained according to claim 1 or claim 14, or is actually retained after the embossing process. The steps of the procedure immediately indicate how the section of the surface would appear after being impressed.  
       [0009] The embossing processes both of which alter the structure of the surface differ in depth of penetration and in regard to lateral extent (claim 6). The second impression changes the properties, smoothness, or quality of the surface to hide the image of the first embossment. The first deformation is locally more pronounced and penetrates deeper into the surface material and is still visible after the second impression. The quality of the surface of the image can be compared with adjoining sections of the surface not affected by the first embossment.  
       [0010] Both the surface extension and the smaller depth enable the second impression to fulfill its task of hiding the first embossment without changing it (claim 8).  
       [0011] The “thinness” of the metal sheets means either that we are dealing with material in sheet form, or that the concept thin sheet metal (German “Blech”) is sufficient to indicate that it actually is a thin metal sheet. Although thicker sheet metal is made, the thickness of the kind used in boxes for juices is not greater than 0.3 mm (claim 15). The thickness of the metal can also be seen with relation to the depth of penetration of the first impression (claim 5). Only when an image appears on the reverse side, the metal is regarded as “thin” and the second procedure is required to hide it.  
       [0012] A surface change by means of a covering layer like paint is not really an altered surface. The outer layer can be regarded as being joined to the surface, but it can be removed. However, surface deformations produced by pressure, constitute a direct intrusion into the surface that cannot be removed from the metal sheet or from some other material like PET or plastic.  
       [0013] The direction of the deformation comprises a further advantage of impression by means of vertical pressure which alters the surface, since it can easily be implemented when boxes or lids are finished. Transverse motions cannot be applied directly, but such movements are not required.  
       [0014] The invention results in a clearly legible mark hidden in the finished product. The advantage of, for example, embossing the mark, is that it cannot be removed as could be done in the case a printed mark. The impressed mark invariably leaves a mirror image on the visible side of the metal sheet, but this can be scrambled, guaranteeing that the mark is completely hidden. A rough surface is impressed over the image, completely covering it and making it practically unreadable.  
       [0015] One way of making a “scrambling” stamp is by using spark erosion. This would extend over the reverse side of the first embossment. It is also possible to apply spark erosion directly to the reverse image (claim 14) as a second surface change opposing the original impression. This process can be compared with sandpaper which penetrates into the surface thus corresponding to deformation by pressure, or spark erosion which removes surface particles; in any case, it constitutes a surface penetrating change.  
       [0016] Experimental examples have shown that penetration depths of between {fraction (1/100)} and {fraction (2/100)} mm can be attained with the current metal thicknesses of lids and tab loops. The depth of penetration is appreciably greater on the side impressed first, whether embossing is done by pressing, scratching, carving or cutting. This depth lies between {fraction (4/100)} and {fraction (5/100)} mm.  
       [0017] Compared to the reflectivity of the smooth, high quality surface, light reflection is destroyed on the roughened section of the surface which is responsible for the visibility of the mark image even if it is not raised.  
     
    
    
     [0018] The invention is further explained by means of schematic drawings of several examples.  
     [0019] The drawings show:  
     [0020]FIG. 1 a partial perspective view of an upper section of a can, for beverages for example, which has a tear-out tab with a loop handle.  
     [0021]FIG. 2 is a partial view from above of the grip end of a loop handle which can be used for a tear-out tab.  
     [0022]FIG. 3 shows the cutout according to FIG. 2 of the reverse side.  
     [0023]FIG. 4 is a large-scale view of FIG. 3 after covering the image of the mark by means of a second impression according to the procedure.  
     [0024]FIG. 4 a,    
     [0025]FIG. 4 b  are cross sections of FIG. 4 illustrating the change of the surface after the first impression and the second impression.  
     [0026]FIG. 5 is a schematic representation of various further possible means of impression in the second process, combined in one image as a negative contour on the reverse side of a schematic stamp  20 .  
     [0027]FIG. 6,  
     [0028]FIG. 7,  
     [0029]FIG. 8 indicate sections of actual metal parts.  
    
    
     [0030] A container as shown in FIG. 1 consists of thin sheet metal and it is closed with a lid  3  which also consists of thin metal sheeting. The latter exhibits a seam in the area of the lid tab  3   a,  along which the tab can be torn out, at least partially, to result in an opening. To start and proceed with the tearing open, a loop  5  is provided which is fixed near one end to the lid tab  3   a  by means of a rivet  4 . A grip section  6  at the other end can be lifted, torn loose, and pressed into the opening.  
     [0031] An enlarged section of one end of the grip  6  of the grip loop  5  (see FIG. 1) is shown in FIG. 2 as seen from below. Thus, one sees surface  6   a , the end of the grip loop from below. An embossed mark has been impressed in the surface near the grip end. In this case a numerical code has been carved or scratched into the surface comprising three separate systems  7   a,    7   b,  and  7   c  as arranged in area  3   a′.    
     [0032] Because the sheet metal used is quite thin, it is inevitable that a tab image  8  of the impression of code  7  (see FIG. 2) is formed on the reverse side. In this case the FIGS. 8 a,    8   b  and  8   c  appear on the upper side  6   b  according to FIG. 3 as can be seen in section  3   a ′. Although the codes  7  embossed on the grip loop are hidden, being on the underside of grip section  6 , they can be deciphered directly from the outside as shown in FIG. 3.  
     [0033]FIG. 4 show that this image has been overlain by a textural, patterned, or crosshatched second impression. The visible outer side  6   b  of the grip end  6  of the grip loop is shown again. Corresponding to the position on the underside of the grip section where code  7  has been embossed, the upper side is textured by a second impression  10  as indicated by  10 ′ in FIG. 4. In this case the texturing is a simple (narrow) hatching.  
     [0034] The subsequent impression is made with a specially structured stamp  10 ′ exhibiting a corresponding pattern in a surface of a textured field.  
     [0035] The texture changes the surface without applying a covering layer to the surface. No paint or any other covering is laid over the image  8 , but the texture of the surface itself is changed (structure, hatching, or any other penetrating shaping process) and this cannot be removed by scraping or otherwise. This texture change is limited to the area where the images  8  of the previous impressions  7  are found, for example the regular delineated area  10 ′ shown in FIG. 4. This area lies within a strengthened edge section  6   c  (for example the reshaped edges of the sheet metal loop) which comprises the grip end (section  6 ).  
     [0036] Refer to the cross-section in FIG. 4 a  of section  3   a ′ for an explanation of the surface change. The right half of the image shows where the first impression  7  has been applied which formed a definite image  8   a ′,  8   a ″ on the reverse side, i.e. the outer surface  6   b.  The left half-image of FIG. 4 a  shows the surface change on the outside ( 6   b ) corresponding to those of FIG. 3. Here we have a texture in the shape of structure  11  (which will be explained further in terms of FIG. 5). The depth of penetration t 11  into this surface is shown.  
     [0037] The depth of penetration t 7  of the first impression  7   a ′,  7   a ″ is obvious. The cross-sectional structure of the sheet metal is changed and an image is pressed through on the other side. This image could either be raised or it is visible through an otherwise smooth surface because of the altered internal structure. For covering or obscuring the visible image  8   a ′,  8   a ″ corresponding to  8   a  in FIG. 3, a second surface change is required on the side of the image. This is firmly set into the surface and it alters its integrity, smoothness, or quality in the sense that disorder is caused in the area concerned by means of directly induced surface changes. The depth of penetration t 11  which has previously been described as obscuring the image is essentially less than t 7 .  
     [0038] The ratio between the two depths of penetration is that the stronger first impression  11  on side  6   b  is still visible on side  6   a.  However, the image  8   a ′,  8   a ″ formed previously cannot be recognized any more, having been hidden by texture  11 .  
     [0039] The second surface change is not only less deep, but it is also more surface oriented than the first one. The first impression is localized more strongly to make the mark recognizable. The second alteration is restricted to the surface to hide the mark. However, both affect the surface immediately and directly with the result that they are fixed and joined permanently with the area involved. Seen from one side ( 6   a ), the mark is obvious, but from the other side ( 6   b ) it is invisible. We are mainly concerned with sheet metal parts, but it can also be applied to other materials like plastic or any other substances whose surface structure can be reshaped. The concept of impression can be adapted to any suitable process by keeping the operation of the first and second penetrating intrusions in mind.  
     [0040] The texturing (structuring or hatching) can also be formed in other ways than those shown in FIG. 4. Several examples of impressing surfaces of a stamp  20  are shown in FIG. 5. At  11  a serrated texture is embossed. In section  12  a texture consisting of fine small tightly packed cross impressions is shown. At  13  a cross hatching is shown which appears negative after the second impression. All textures of FIG. 5 are depicted in combined form, and, when applied, they would cover the entire area  10 ″ to change the surface structure of side  6   b.  The extent of boundary  10 ″ is calculated to include the three images  8   a,    8   b,  and  8   c  and to form a regular boundary which has already approached a quadrilateral with boundary  10 ′ as explained in FIG. 4.  
     [0041] Instead of positioning the code and the second impression on an area  3   a ′ of the grip loop as shown in FIGS.  2  to  4 , the lid tab  3   a  can also carry a corresponding embossed code, but in this case also the appropriate area of the outer side of the lid must be textured or hatched by means of a second impression.  
     [0042] This process is applicable in any area where it is intended to obscure a code or mark impressed on a thin surface section.  
     [0043]FIG. 6 shows an example of a code  7 ,  7 ′,  7 ″ from the visible side. The image  8 ,  8 ′,  8 ″ appears in FIG. 7. We have an example of a round texture impression  11  (similar to the pattern of FIG. 5) in FIG. 8 still without the underlying code numbers  8  being impressed.  
     [0044] The “enciphered” impression follows from a combination of FIG. 7 with FIG. 8. Texture  11  is provided in a limited external area  10 ′″ which extends over the maximum width of codes  7 ,  7 ′,  7 ″. The boundary is regular and differs from the shape of the impressed lines of sevens arranged in a circle.