Abstract:
Method for marking hot glass article having a surface uses a flexible carrier ribbon bearing a laser ablatable, high temperature, diffusely reflective coating, preferably white in color. A pattern is imaged in said coating on carrier ribbon by laser ablation. The patterned carrier ribbon is pressed against the surface only for a time adequate for transferring the patterned coating to the surface. The carrier ribbon then is released from pressing against the surface. The transferred image thickness may be limited by solid particles within the coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to product marking and identification and more specifically to the marking of hot glass, as typified by picture tube components. 
     There is a need to piece identify hot glass articles. Picture tube components, for example panels and funnels, start life by being solidified in one of several, say 10, molds. Each piece contains mold related dimensional defects and is uniquely stressed as it is handled, cooled, and then annealed. Of the initially molded pieces, typically more than 30% never have the dimensional accuracy and strength to make it out of the plant. 
     This 30% loss is tolerable only because the broken (cull) glass can be recycled (one or more times) and, in fact, contributes to a better breed of glass. However, the scrap loss becomes very costly if much processing is done prior to scrapping. 
     Piece tracking will permit the plant operator to test and update the database for each piece and, thereby, determine if its history supports being scrapped rather than processed. 
     Suppose that the plant operator knew that mold #7 (and its associated shell) currently was producing dimensionally defective pieces and that they should be scrapped at the lehr exit, where they are known to be “dead on arrival”. The downstream costs of processing these parts, through to the first gauging point, could be saved. This, of course, is a simplistic example, because the reason for known defects commonly may involve the interaction of two (or more) machines prior to annealing. The only way such interactions can be discovered quickly is through individual piece tracking. 
     In the mold #7 hypothetical, the average production rate is assumed to be 5 pieces/minute and that the costs associated with unnecessary post lehr processing is $4.00/piece (this figures includes labor, equipment amortization, consumables (e.g., grinding and polishing materials), maintenance, power, technical support, gauging costs, etc.). If the plant operator can scrap the 10% of production (those pieces formed by mold #7 or another currently defective mold) prior to downstream processing, the plant operator will save over $2.00/minute (approximately $500,000/year). If the post lehr processing equipment throughput is in fact limiting on plant production (especially when a machine is down), the savings can be significantly higher, because a “good” shippable piece can replace every predictably “dead” piece. An additional good piece, of course, is worth far more than $4.00. The beneficial results of piece tracking include more production throughput and a savings when the operator eliminates unnecessary processing of bad pieces. 
     A variety of techniques for marking hot glass (picture tube panels and funnels) as they exit the forming mold at between about 400° and 650 ° C. can be envisioned. These techniques are listed below along with the problems associated with each: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Direct laser marking 
                 Poor contrast 
               
               
                 See for example U.S. PAT. NO. 
                 Possible shard/crack generation 
               
               
                 6,227,394 (Shinoda) 
               
               
                 Also see C. Buerhop and R. 
               
               
                 Weismann, “Temperature development 
               
               
                 of glass during CO 2  laser irradiation- 
               
               
                 Part 1”, Glass Technology, Vol. 37 No. 
               
               
                 2 (April 1996) 
               
               
                 Glass tag (frit bonded) 
                 Fragile edges of tag 
               
               
                   
                 Frit melt/temperature/cure 
               
               
                   
                 match is delicate 
               
               
                 Spray background (then laser cut 
                 Overspray 
               
               
                 away) 
                 Delicate balance between 
               
               
                 See for example U.S. PAT. NO. 
                 cut/shard 
               
               
                 4,323,755 (Nierenberg) 
                 Possible shock to glass 
               
               
                   
                 (nebulizing air) 
               
               
                   
                 Spray reliability 
               
               
                 Spray background (then laser blacken) 
                 Overspray 
               
               
                   
                 More liquid material &amp; thermal 
               
               
                   
                 shock 
               
               
                   
                 Spray reliability 
               
               
                 Pad apply laser darkenable patch 
                 Pad transfer buildup 
               
               
                   
                 Multiple stamps-requires 
               
               
                   
                 significant time 
               
               
                 Tape apply laser darkenable patch 
                 May require 2 stations 
               
               
                   
                 (cure time) 
               
               
                   
                 Difficult to formulate 
               
               
                   
                 adhesion &amp; 
               
               
                   
                 (clean/strong/black) 
               
               
                   
                 markability together in 
               
               
                   
                 one tape coating 
               
               
                   
               
             
          
         
       
     
     Thus, all of the tabulated approaches lead to complicated, difficult to maintain and/or messy equipment. A new approach to labeling hot glass for identification, therefore, is needed. 
     BRIEF SUMMARY OF THE INVENTION 
     Method for marking hot glass article having a surface uses a flexible carrier ribbon bearing a laser ablatable, high temperature, diffusely reflective coating, preferably white in color. A pattern is imaged in said coating on carrier ribbon by laser ablation. The patterned carrier ribbon is pressed against the surface only for a time adequate for transferring the patterned coating to the surface. The carrier ribbon then is released from pressing against the surface. A “pattern” for present purposes includes alphanumeric characters, numbers, graphics, and bar codes (e.g., laser scanable and vision system readable bar codes). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and advantages of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which: 
     FIG. 1 depicts a cross-section of the carrier bearing the laser ablatable, high temperature, diffusely reflective coating, which has been partially laser ablated; 
     FIG. 2 depicts details on how the image is created on the carrier of FIG. 1; 
     FIG. 3 is a plan view of a system designed to label hot glass picture tube panels with the carrier of FIG. 1; 
     FIG. 4 is an enlarged view of the carrier of FIG. 1 being pressed against a hot glass panel; and 
     FIG. 5 is an enlarged view like FIG. 4, except that large particles have been added to the laser ablatable, high temperature, diffusely reflective coating carried by the carrier. 
    
    
     The drawings will be described in detail below. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Marking of surfaces using a selectively ablated coating has application beyond the marking of hot glass articles. The coating might be liquid or “tacky” (especially if solvent based) paint and reside on the product when ablatively laser imaged. These same coatings also might be applied in two layers. For example, the first underlying layer might be (unablatively imaged) black and the top layer might be ablatively imaged white. This would create indicia, which would have good black/white contrast independent of the underlying product color, such as taught in U.S. Pat. No. 6,007,929. 
     The invention here proposes a tape coating, which is imaged prior to being pressed upon a warm or hot surface. The remaining patch colorant will be left as an imprint on the product, as if by a (programmable) stamp pad. 
     The invention, however, will be illustrated by specific reference to the marking of hot glass where high temperatures and short contact times are necessary. Such description is by way of illustration, however, and not by way of limitation of the present invention, as other substrates are appropriate as are variations of the coating carried by the carrier. 
     Referring initially to FIG. 1, shown is an end of a tape that includes a carrier assembly,  10 , which can be a single layer (e.g., aluminum foil) or multiple layers. Shown in FIG. 1 is the multiple layer configuration, which includes, for example, an optional supporting structure or backing (e.g., 0.005″ thick paper),  12 ; a heat resistant layer (e.g., 0.001″ thick aluminum foil),  14 ; and an optional (very thin) release layer,  16 . Atop carrier assembly  10  is a patch coating,  18 . An ablated zone,  22 , is shown for illustration in patch coating  18 . Patch coating  18  has the following desirable properties: 
     a. Patch coating  18  contains no significant solvent content (including water), so that a bonding inhibiting barrier (e.g., steam barrier) is not created when the top surface,  20 , of coating patch  18  is pressed against hot glass. A low solvent content also will ensure that the tape will not thermally (heat of vaporization) shock or craze the hot glass when pressed against it. 
     b. Surface  20  is not “sticky” to the outer surface,  24 , of backing  12  (i.e., the outer surface of carrier assembly  10 ) at ambient temperature so that the laminated carrier can be wound into a coil and subsequently freely unwound for use. 
     c. Surface  20  needs to become tacky or melt when pressed against hot glass in the temperature range of between about 400° C. and 650° C. and the softened coating material  18  needs to wet the hot glass surface upon which it is pressed. 
     d. Patch coating  18  needs to preferentially go with the hot glass and release from carrier assembly  10  when stripped. Optional release layer  16  can help facilitate this release. 
     e. The pigments in coating patch  18  generally are white in color and produce a generally white coating patch on the cooled glass, which coating patch on the cooled glass is diffusely reflective of incident (bar code scanner) light. 
     f. The coating patch on the cooled glass must remain firmly attached to the glass article and not significantly powder or release from the glass as it experiences several subsequent reheat (lehr) cycles. 
     g. The pigments in coating patch  18  and the composition of the resin in coating patch  18  together produce a patch, which may be cleanly ablated while at or near ambient temperature and while on carrier assembly  10 . 
     In a preferred embodiment, carrier assembly  10  consists of strong paper backing  12  (e.g., 2 to 10 mil inch thick, paper) with aluminum foil layer  14  (e.g., 0.5 to 2 mils thick). Release layer  16  is an acrylic/vinyl film (e.g., 0.00001 to 0.0005 inch thick). Patch coating layer  18  preferably is between about 0.5 and 2 mils thick. The following coating formulation for patch coating  18  has been developed to meet the needs outlined above. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 INGREDIENT 
                 % BY WEIGHT 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Mono ammonium phosphate (25 wt-% in 
                 69.75 
               
               
                   
                 water) 
               
               
                   
                 TiO 2  (opacifying agent) 
                 15.0 
               
               
                   
                 Ceramic beads (White Zeeospheres 3M 
                 15.0 
               
               
                   
                 Company, St. Paul, MN)) 
               
               
                   
                 Darvan C (ammonium polymethacrylate 
                 0.25 
               
               
                   
                 dispersing aid, R.T. Vanderbilt Co., 
               
               
                   
                 Norwalk, CT) 
               
               
                   
                   
               
             
          
         
       
     
     After formulation, this slurry is applied (e.g., doctor blade, roller, air assisted atomization, etc.) onto carrier  10  and is conductive heat or hot air dried to a state whereby coating layer  18  is dried (is no longer moist) and the tape can be rolled without offsetting or sticking onto carrier back surface  24 . 
     Referring now to FIG. 2, a tape,  26 , having a pair of edges,  28  and  30 , is depicted. Tape  26  bears patch coating  18 , which is separated into frames by edge bands,  28  and  30 , and interlabel strips,  32 , and  34 , and  36 . During laser ablation, edge bands  28  and  30  can be laser ablated to avoid build up on the application pressure roller or pad, while interlabel strips  32 , and  34 , and  36 , can be laser ablated to provide clean edges on the transferred label at the leading and trailing edges. 
     A pair of frames,  38  and  40 , are depicted in FIG.  2  and are representative of a series of frames formed in tape  26 . Patch coating  18  is ablatively removed (where shown in black) to produce areas where the coating is absent and will not be transferred onto the hot glass or other object being marked. The ablated zones (e.g., zone  22  in FIG.  1 ), thereby, appears “black” to a scanning laser when scanning the indicia,  42  and  44 , marked on tape  26 , because the scanning beam either passes through the article (e.g., glass) or the article (e.g., leaded glass) appears black when compared to the transferred, diffusively reflecting coating white forming images  42  and  44 . It should be noted that the images  42  and  44  depicted in FIG. 2 are as seen from the backside of carrier  10  (i.e., as viewed from side  24 ). The laser markings, images  42  and  44 , must be mirror images of the desired ultimate markings on the glass article subject to marking. 
     FIG. 3 illustrates the use of coated ribbon  26  to coat a hot glass picture tube panel,  46 . Panel  46  (shown seal edge down) is momentarily stopped (e.g., for 1 second) against an indexing stop,  48 , while progressing generally in the direction of arrow  50 . The scheme set forth in FIG. 3 is designed to mark a lip,  52 , of glass panel  46 . 
     Wound tape or ribbon  26  is supplied as a free wheeling supply roll,  54 . A drive roller  56 , pressured against an idler roller,  58 , advances ribbon  26  one frame at a time from roll  54 . A laser marking unit,  60 , selectively and ablatively removes selected coating material at the area designated by numeral  62  such that the remaining coating region defines, for example, the (mirror image) white of the ultimate label to be applied at lip  52  of glass panel  46 . Alternatively, the ablative coating removal could proceed using a one-axis galvanometer, while drive roller  56  is stepped in the manner as taught in U.S. Pat. No. 5,855,969. 
     The laser marking described above is repeated whenever a sensor,  64 , determines that a supply loop,  66 , needs more tape or label material. The information or data printed at zone  62  will be applied to a glass panel or funnel several units of production behind glass panel  46  shown in FIG.  3 . Of course, the plant operator must ensure registry and correspondence between the label and the glass panel marked therewith. 
     When a new panel appears at stop  48 , e.g., panel  46 , a second drive roll,  68 , working against a second idler,  70 , advances tape  26  such that a new selectively marked label will be pressed against lip  52  when a roller,  72 , is brought forward to the position identified by numeral  72 ′ by an actuator,  74  (details not shown in FIG. 3, but are provided in conventional fashion). After actuator  74  is engaged, a second actuator,  76  (again details not shown in FIG. 3, but are provided in conventional fashion), draws application roller  72 ′ across lip  52 , thereby impressing the remaining label coating onto lip  52  in a manner that produces a “nip”. To accomplish this nip, a constant (CW) torque is applied by a drive roller,  78 , against an idler,  80 . Alternatively, a relatively flat foam pad formed from a temperature resistant material, such a silicone rubber, can replace roller  72  and be used to simply “tamp” the image onto lip  52  in one very brief stroke. Upon advancement of tape  26 , a label length of scrap (the carrier segment from a previous label) is fed into a scrap barrel,  82 . 
     Since glass panel  46  is hot (e.g., in the range of from about 400° C. to 650° C.), shield plates,  84  and  86 , limit the exposure of tape  26  (and the coating pattern it carries) from this heat. Shield plates  84  and  86  can be fabricated, for example, from reflective, low emissivity aluminum, or other suitable heat-resistant metal, ceramic, or like material. 
     While the foregoing procedure describes a general technique for producing imaged labels for application to hot glass, work on the present invention has revealed that the application of the imaged label to a hot glass article is sensitive to a variety of variables: (1) pressing time, (2) pressure applied, (3) temperature dependent cure/flow rate of the coating, and (4) the mechanical limits on the contact/pressure pad or roller. Controlling all four of these variables in a production machine presents the operator with a very difficult task. 
     FIG. 4 illustrates the problem the operator faces: attempting to identify a hot product,  88 , using a coating,  90 , of nominal thickness, T 0 , which has been laser ablatively patterned, as at  92 . A carrier, constructed from a foil,  94 , and substantial substrate,  96  (e.g., paper), carries coating  90  to product  88 . A pressure pad or roller,  98 , and pressure, P, in the direction of arrow  100 , are utilized to imprint patterned coating  90  onto hot product  88 . Unfortunately, at the hot glass temperatures encountered (e.g., in the range of from about 400° C. to 650° C.), coating  90  is rapidly heated and flows freely. Even when the pressure, P, is small, the free flowing coating, unless inhibited, tends to continuously thin and, thereby, flows into ablated areas openings (e.g., area  92 ) in coating  90 , thus, closing them or filling them in. When these ablated areas become filled in, the pattern is lost or distorted, and cannot be properly read by laser scanners/readers. 
     Also, it is impractical to mechanically “flat” limit the compression of (nearly liquid) coating  90  over the relatively large indicated label area (e.g., 1 sq. in.) upon a variably dimensioned product. 
     The solution to this conundrum is illustrated in FIG.  5 . Coating  90  is seen to contain thinning limiting particles,  102 . Limiting particles  102  are sized to be nominally smaller in diameter than nominal coating thickness T 0 . The nominal size of limiting particles  102  is D 0 , wherein T 0 &gt;D 0 . Under the influence of pressure  100  (and the high temperature of hot glass article  88 ), coating  90  flows in all directions until the thickness of softened coating  90  reaches a nominal thickness of D 0 . 
     The use of limiting particles  102 , wherein T 0 &gt;D 0 , will not prevent a partial closure of ablated area  92 . Because the liquefied coating is incompressible, a large area of coating  90  might flow parallel to hot article  88  to fill any available voids, e.g., area  92 , while thinning from T 0  to D 0 . Therefore, it is important to limit the residual flow, parallel to the surface of hot article  88  to limit the closings of laser cuts, such as cut  92 . 
     Techniques to further limit such undesirable flow include: 
     1. reduce the contact (pressing) time to minimize the time during which such (viscosity limited) flow is forced. 
     2. provide a “highly volatile” thin release layer between foil  94  and coating  90 , such as, for example, nitro cellulose. This release layer helps to “loft” the approaching coating  90  from carrier foil  94  onto hot article  88  and, thereby, minimize the necessary contact time. There also is the possibility of utilizing such lofting to transfer coating  90  even if limiting particles  102  are larger in size (diameter) than T 0  (thereby preventing any significant parallel flow). 
     While the invention has been described with reference to a preferred embodiment, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts and percentages are by weight, unless otherwise expressly indicated. Also, all citations referred herein are expressly incorporated herein by reference.