Abstract:
A thermal printer that includes a thermally conductive sacrificial member disposed between the thermal print head and the web. The sacrificial member is held generally under tension and prevents the web from directly engaging the thermal print head. The sacrificial member may take one of many different forms, including a fixed strip or belt, a rotatable, continuous belt which slowly recirculates during printing to equalize wear across its surface, or a Moebius loop which further provides that wear is equalized across both sides of the belt. By providing a sacrificial member between the print head and web, wear and exposure of the print head is reduced, thereby prolonging the life of the print head.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of the following U.S. Provisional Application Serial No. 60/264,858, filed Jan. 29, 2001 and Serial No. 60/266,496, filed Feb. 5, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to thermal printers, and more specifically relates to a thermal printer which uses a sacrificial member between a print head and the web on which the printer prints in order to reduce wear on the print head during the printing process. As used throughout this application, the term “thermal printer” shall mean thermal transfer printer as well as direct thermal printer. 
     BACKGROUND OF THE INVENTION 
     Direct thermal printers are well known in the prior art. In such printers, a web of paper or film having a thermally sensitive coating is interposed between a driven platen roller and a thermal print head having a line of selectively energized heating elements. To print onto the web, an electrical pulse is applied to a selected set of the heating elements, and a localized chemical reaction occurs at corresponding points in the thermally sensitive coating on the web which results in the formation of visible dots on the web. 
     After a line of dots is printed, the web is advanced to locate an adjacent location of the web over the print head heating elements, and the selecting and heating process is repeated to print an adjacent line of dots on the web. This process is repeated in order to print complete lines of text or graphics on the web. 
     The heating elements of the print head which are selectively energized during the printing process are typically covered with a protective ceramic overcoat. Webs which are used to print images thereon also typically have thermally sensitive coatings. During printing, the web moves across the print head; hence, the coatings on the web and print head rub against each other. The rubbing of the web on the print head during printing causes abrasion of the overcoat on the print head, and this is a common mode of failure and a limitation on print head life. Additionally, because the web contacts the print head, it has been required to use expensive print media, specifically print media with expensive coatings in order to limit the wear on the print head. 
     In some printing applications, reactive components or metallic ions are used to produce an image on the thermally sensitive coating on the web. This causes the print head to be exposed to the reactive components or metallic ions. Unfortunately, the reactive components which are used in some printing applications can be corrosive to the print head. Additionally, the print head can become contaminated as a result of being exposed to metallic ions. 
     Because the print head of a thermal printer is subject to so much wear and exposure during the printing process, the print head is often considered to be an expendable maintenance item, despite the fact that the print head is relatively costly. 
     Thermal transfer printers are also well known in the prior art. In these printers, a nonsensitized web is customarily used and a transfer ribbon is interposed between the print head and the web having a coating of wax or resin which is selectively melted and thereby transferred to or chemically reacted with the web. This allows nonsensitized webs to be imaged and provides for a wide range of materials that can be used to form the image. The transfer ribbon can similarly expose the print head to reactive components or metallic ions, resulting in reduced print head life unless expensive back coatings are applied to the transfer ribbon to reduce the wear. 
     While the preferred embodiment is disclosed in terms of a direct thermal printer, the subject invention is equally adapted to thermal transfer printers, in which case, the sacrificial member is disposed between the print head and the transfer ribbon. 
     OBJECTS AND SUMMARY 
     A general object of an embodiment of the present invention is to provide a thermal printer that positions a sacrificial member between a print head and the web in order to reduce wear on the print head. 
     Another object of an embodiment of the present invention is to sacrifice a sacrificial member, such as a belt or web of thermally conductive material, in lieu of or in addition to the print head overcoat by interposing the sacrificial member between the web and the print head. 
     Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a thermal printer that includes a sacrificial member disposed between the thermal print head and the web. The thermal printer holds the sacrificial member generally under tension, and the sacrificial member contacts the thermal print head and generally prevents the web from directly engaging the thermal print head. 
     Preferably, the thermal printer includes a platen roller, and the sacrificial member and web are disposed between the thermal print head and the platen roller, however, it should be noted that thermal and thermal transfer printing can be applied to webs that are held against the print head by web tension alone, that the claimed sacrificial member can be used with such structures, and that such structures are intended to lie within the scope of the appended claims. It should also be noted that the preferred embodiment is disclosed in terms of belt drives from a common motor, but that equivalent structures having gear drives or independent motors and drives for the web and the sacrificial member are intended to lie within the scope of the appended claims. 
     The sacrificial member may take one of many different forms. Regardless of the form the sacrificial member takes, by providing a sacrificial member between the print head and web, wear and exposure of the print head is reduced, thereby prolonging the life of the print head. 
     The thermal printer may take the form of a thermal transfer printer, in which case a thermal transfer ribbon is disposed between the sacrificial member and the web. 
     Another embodiment of the present invention provides a method of thermal printing wherein the steps include interposing a sacrificial member between a thermal print head and a web, and energizing the thermal print head to heat the web through the sacrificial member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein like reference numerals identify like elements in which: 
     FIG. 1 is a simplified schematic view of a thermal printer which is in accordance with the present invention, where the thermal printer includes a printer mechanism which is in accordance with any one of FIGS. 2-6; 
     FIG. 2 is a side orthogonal view of a printer mechanism which includes a continuous, recirculating belt disposed between a thermal print head and a web; 
     FIG. 3 is a side orthogonal view of a printer mechanism which includes a continuous, recirculating belt that includes a Moebius loop; 
     FIG. 4 is a side orthogonal view of a printer mechanism which includes a fixed belt or strip that is disposed between a thermal print head and a web; 
     FIG. 5 is a side orthogonal view of a printer mechanism which includes a sacrificial member which is fed from a supply roll to a take-up roll; and 
     FIG. 6 is a side orthogonal view of a printer mechanism for use in a thermal transfer printer, wherein the printer mechanism includes a thermal transfer ribbon disposed between a sacrificial member and a web. 
    
    
     DESCRIPTION 
     While the invention may be susceptible to embodiment in different forms, there are shown in the drawings, and herein will be described in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein. 
     FIG. 1 provides a simplified schematic of a thermal printer  6  which is in accordance with the present invention. The thermal printer includes a printer mechanism  8  which corresponds to any one of FIGS. 2-6. In other words, FIGS. 2-6 show five different printer mechanisms which may be employed with the thermal printer shown in FIG.  1 . 
     Each of the printer mechanisms shown in FIGS. 2-6 provide that a sacrificial member is employed between a thermal print head and a web in a thermal printer in order to reduce wear of the print head during printing. Since the cost of the sacrificial member and the labor to replace it are far less than the cost of replacing the print head, the result is a significant reduction in printing cost. Additionally, because the print head does not directly engage the web, less expensive print media can be used without causing excessive wear to the print head. 
     The printer mechanism shown in FIG. 2 will be described first, and then the printer mechanisms illustrated in the other FIGURES will be described emphasizing the differences. Because the different printer mechanisms have many similarities, like reference numerals are used to identify like parts. 
     FIG. 2 shows a printer mechanism which is intended for printing on a web  10  of thermally sensitized material. The printer includes a stepper motor  12  which drives a drive belt  14 . The drive belt  14  is engaged with pulleys  16  and  18 . Pulley  16  is connected to a platen roller  20  such that the stepper motor  12  uses the drive belt  14  and pulley  16  to drive the platen roller  20  in order to advance the web  10  during printing. 
     Pulley  18 , with which the drive belt  14  is also engaged, is connected a drive roller  22 . The drive roller  22  is engaged with a sacrificial member  24 . As shown, the sacrificial member  24  may be a continuous, relatively smooth belt  26  which is held in tension by the driver roller  22  as well as idle rollers  28  and  30  and a print head  32 . As shown, idle roller  28  is preferably engaged by a spring  34 , and the print head  32  is also engaged by a spring  36 . 
     The print head  32  includes a nip  38  at which printing occurs, and the nip  38  of the print head  32  is engaged with the belt  26 . The print head  32  preferably is a thermal print head and includes heating elements which can be selectively energized during the printing process in order to print onto the web  10 . The print head  32  also may include a protective overcoating, such as a ceramic overcoating. 
     The belt  26 , i.e. sacrificial member  24 , is preferably a continuous, smooth, thermally conductive material such as polyester, polyamide, or polyimide, e.g. Kapton™. Alternatively, the belt  26  may consist of a less costly material such as polyethylene terephthalate (PTE) if suitably thin. The belt  26  may consist of an unpigmented (i.e. uninked) ribbon with a heavy backcoat, in which case it is possible to use print media which does not include coatings that limit wear on the print head. In other words, less expensive print media can used due to the fact that the web  10  no longer directly contacts the print head  32  during printing. 
     As shown in FIG. 2, the belt  26  is held under tension by the nip  38  of the print head  32  (acting under pressure of spring  36 ), drive roller  22 , idler roller  28  (acting under pressure of spring  34 ), and idler roller  30 . The belt  26  is rotatable about the rollers  22 ,  28 ,  30  and print head  32  such that, during printing, the belt  26  recirculates, thereby wearing generally evenly about the entire surface of the belt  26 . Preferably, the diameters of the drive roller  22  and pulley  18  effectively work as a velocity differentiator and provide that the belt  26  moves much slower than the web  10  during printing, such as one tenth as fast. This prolongs the life of the belt  26 . 
     As an alternative to that which is shown in FIG. 2, a slack sacrificial belt can be used as the sacrificial member  24 . In which case, a pinch roller would be added proximate the drive roller  22 , or a pinch roller and brake would be added proximate idler roller  30 . 
     Still further, the printer can be configured such that the user must periodically, manually advance the belt  26 , as opposed to the belt  26  automatically advancing as the web  10  advances during printing. Alternatively, control of recirculation of the belt  26  may be foregone at the expense of belt life, in which case the belt  26  need not be actively driven (in other words, the belt  26  need not engage a drive roller, and instead may be engaged with only idler rollers and the print head). 
     In use, the stepper motor  12  dives drive belt  14  which is engaged with pulley  16 . This, in turn, drives platen roller  20  which works to advance the web  10 . When the drive belt  14  is driven, pulley  18  rotates, causing drive roller  22  to be driven. This causes the sacrificial belt  26  to circulate. The thermal print head  32  is selectively energized to heat the web  10  through the sacrificial belt  26 , thereby causing printing on the web  10 . 
     As discussed above, the printer mechanism shown in FIG. 2 provides that the belt  26  recirculates during printing, thereby causing the belt  26  to wear generally evenly about the entire surface of the belt  26 . To further distribute the wear on the belt, the belt may be provided with a Moebius loop  40  as shown in FIG.  3 . The Moebius loop  40  is provided by a twisted section  42  of the belt  26 , and the Moebius loop  40  provides that the belt  26  wears evenly on both sides  44 ,  46 . As shown in FIG. 3, three pinch rollers  50 ,  52 ,  54  can be provided to engage the twisted section  42  of the belt  26 . The twisted section  42  is constrained between a first nip  56  formed between drive roller  22  and pinch roller  50 , and a second nip  58  formed between second pinch roller  52  and third pinch roller  54 . 
     FIG. 4 shows an alternative embodiment wherein instead of the sacrificial member  24  comprising a recirculating belt, the sacrificial member  24  consists of a generally non-circulating belt or strip  60  (i.e. a fixed web) which is removably retained by a first spring clip  62  and a second spring clip  64 . The spring clips  62 ,  64  serve to keep the belt or strip  60  from moving substantially with the web  10  in either direction during printing, yet allow the belt or strip  60  to be replaced or repositioned. Because the belt  60  does not generally move during the printing process, the ability of the belt  60  to resist thermal deformation is important. Therefore, preferably, the belt or strip  60  consists of a material which is highly resistant to thermal deformation, such as Kapton™ polyimide. 
     FIG. 5 shows still another embodiment wherein the sacrificial member  24  does not circulate endlessly, nor is fixed, but rather comprises a ribbon  70  which is fed from a supply roll  72  to a take-up roll  74 . Preferably, the ribbon  70  is unpigmented (i.e. uninked) with a heavy backcoat, in which case it is possible to use print media which does not include coatings that limit wear on the print head. As shown, the ribbon  70  contacts, and is generally held in tension by, the print head  32  and roller  22  (and pinch roller  76 ). 
     The difference in diameters between the roller  22  and pulley  18  provides that the ribbon  70  moves slower than does the web  10  during printing. As such, the roller  22  and pulley  18  together effectively act as a velocity differentiator with respect to the ribbon  70  and web  10 . Providing that the ribbon  70  moves slower than the web  10  provides that the ribbon  70  need not be replaced as often. 
     FIG. 6 shows yet another printer mechanism, and is configured to be employed when the thermal printer (see FIG. 1) is a thermal transfer printer. As shown in FIG. 6, such case provides that a thermal transfer ribbon  80  is disposed between the sacrificial member  24  and the web  10 . While FIG. 6 is otherwise identical to FIG.  2  and shows that the sacrificial member consist of a circulating belt  26 , the sacrificial member  24  used in a thermal transfer printer may take any of the other forms identified herein (i.e. may include a Moebius loop  40  as shown in FIG. 3, may be a fixed belt or strip  60  as shown in FIG. 4, or may be fed from a supply roll to a take-up roll as shown in FIG.  5 ). 
     With regard to the material chosen to comprise the sacrificial member, preferably in the embodiment shown in FIG. 4, the sacrificial member consists of Kapton™ polyimide, as Kapton™ polyimide is highly resistant to thermal deformation, and resistance to thermal deformation is important in the case where the sacrificial member is fixed. In the other embodiments (i.e. shown in FIGS. 2,  3 ,  5  and  6 ), the sacrificial member moves during the printing process, hence thermal deformation is not as much of an issue. Hence, less costly materials can be used for the sacrificial member. 
     Regardless of the specific embodiment employed, using a sacrificial member between a thermal print head and a web in a thermal printer reduces wear of the print head during printing, reduces overall printing cost, and provides that less expensive print media can be used without causing excessive wear to the print head. 
     While embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.