Patent Publication Number: US-6987524-B2

Title: Thermal printer assembly

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/436,168, filed Dec. 23, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention relates to thermal printer assemblies and particularly to the curved path of print media through such assemblies. 
     BACKGROUND OF THE INVENTION 
     Platen rollers are commonly used in linear or serial printing to provide a firm foundation for various forms of print heads. In thermal printers, platen rollers are used for squeezing print media against a thermal print head to provide proper thermal conduction between the print head and media. Platen rollers also allow accurate movement of print media due to minimal friction characteristics. Certain types of thermal print heads are restricted to using a maximum radius platen roller, thus requiring a certain roller wrap and curvature to the print media. Unfortunately, the curvature introduced into the print media path by a platen roller can create design challenges for printer mechanisms which use multiple print heads, because this curvature is cumulative. The pulling of print media through such a distorted path presents even more design challenges for the apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a thermal printer assembly for use with pulled print media, including an elongated thermal print head oriented substantially orthogonally to a print media path, and a platen roller aligned with and opposed to the elongated thermal print head and adapted to pressure print media against the print head. The platen roller defines a curvature to the print media path, and a second roller is located adjacent and substantially parallel to the print head and adapted to correct at least a portion of the curvature of the print media path for providing a more linear overall print media path. 
     The thermal printer assembly may include a third roller located adjacent and substantially parallel to the print head and adapted to further correct the curvature of the print media path. The second and third rollers may be located adjacent to opposing elongated sides of the thermal print head and they may be fixedly mounted. 
     One of the print head and the platen roller may be fixedly mounted and the other may be moveably biased. Also, the second roller may be a second platen roller, and the assembly may further include a second print head mounted adjacent to the first platen roller and aligned with the second platen roller. The first and second print heads may be adapted to print on opposing sides of the print media and across the print media path. 
     The first and second print heads may form a first print head assembly, and the overall assembly may further include a second print head assembly oriented to print across the print media path. The first and second thermal print heads of each print head assembly may be located sequentially along the print media path, and the first and second print head assemblies may be adjacently located sequentially along the print media path. A last sequentially located platen roller of a first sequentially located print head assembly may be adapted to bend the print media path in one direction, while a first sequential platen roller of an adjacent next sequential print head assembly is adapted to bend the print media path opposite to the one direction. 
     This print head assembly provides a very efficient arrangement of print heads and their respective platen rollers and substantially eliminates overall curvature of the print media path. This arrangement can perform single or double sided printing across a print media path that may be less than, equal to or wider than the elongated direction of the individual print heads. The linear efficiency of this arrangement minimizes the unprintable space which occurs at the tops of individual print media pages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description of various preferred embodiments thereof taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a side view diagram of one embodiment of a thermal print head assembly according to the invention; and 
         FIG. 2  is a perspective diagram of one embodiment of a multiple print head assembly according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a side view block diagram of a thermal printer assembly  10  which generally includes a thermal print head  12  and a platen roller  14 . A section of print media  16  is physically biased by roller  14  against print head  12  as it travels along a print media path  17  (represented by an arrow). As mentioned, the necessity of biasing print media  16  with roller  14  necessarily causes a curvature in the print media path, which curvature appears in the downward direction for roller  14 . Also included is a second roller  18  which is located adjacent to print head  12  and substantially parallel to platen roller  14 . It should be understood with respect to  FIG. 1  that thermal print head  12  and rollers  14  and  18  are elongated in the direction perpendicular to the plane of the diagram. Print media path  17  likewise has a width which extends orthogonal into the plane of  FIG. 1 . In this manner, print head  12  and rollers  14  and  18  are oriented substantially across print media path  17 . The location of second roller  18  on the opposite side of the print media path from platen roller  14 , causes the path of print media  16  to curve in the opposite direction from the curvature caused by platen roller  14 . 
     Also shown is a roller  20 , which is also located adjacent to print head  14  and substantially orthogonal across the path of print media  16 . Because roller  20  is likewise located opposite and substantially parallel to platen roller  14 , roller  20  also induces curvature into the path of print media  16  which curvature further corrects the curvature introduced by platen roller  14 . 
     In  FIG. 1 , print head  12  is represented as being fixedly mounted and platen roller  14  is represented as being movably mounted and biased towards print head  12  by a spring mechanism  22 . In other applications, the types of mounting may be respectively reversed so long as one of the print head or platen roller is movably biased for pressuring print media  16  against the other. Any suitable form of mounting may also be used for rollers  18 ,  20  and their respective mountings may be similar or different. 
     In a further refinement of the assembly  10 , a second print head  24  is shown opposed to second roller  18 . Second print head  24  is shown to be movably mounted as discussed above. In this arrangement, second roller  18  is provided in the form of a second platen roller and second print head  24  is aligned therewith. Whereas non platen rollers such as roller  20  may be provided in any suitable form, platen rollers typically include an outer rubber surface to better pressure print media uniformly against their respective print heads. 
     Although the assembly  10  has been illustrated with print heads  12  and  24  on opposite sides of print media  16 , it will be understood that both print heads can be mounted on the same side of the print media. 
       FIG. 2  shows a perspective view of a multiplicity of print heads and platen rollers serially located along the path of a section of print media  30 . Print heads  32 ,  34  are shown to be sequentially located on the bottom side of print media  30 , and print heads  33 ,  35  are shown to be sequentially inter-digitated with the first print heads  32 ,  34 , except for their being located on the top side of the path of print media  30 . Each print head  32 – 35  is shown to have a respective platen roller  42 – 45  located for pressuring print media  30  against the respective print head. A multiplicity of lines  48 – 51  are shown crossing print media  30  and are intended to represent bends in the curvature of print media  30 . 
     Although the print heads  32 – 35  are illustrated as being staggered across the print media path, it will be understood that the print heads can be arranged in any positioning such as, for example, two or more of the print heads can be located in line with each other. 
     By the arrangement shown in  FIG. 2 , print heads  32  and  33  are located sequentially along the path of print media  30  and are adjacently located. Likewise print heads  34  and  35  are sequentially and adjacently located along the path of print media  30 . In this manner, print heads  32  and  33  and their respective platen rollers  42  and  43  represent a first print head assembly and print heads  34  and  35  and their respective platen and rollers  44  and  45  represent a second print head assembly. It is also appreciated that the last sequentially located platen roller  43  of the first sequentially located print head assembly is adapted to bend print media in one direction (generally downward as shown) and the first sequential platen roller  44  of the adjacent next sequential print head assembly is adapted to bend the print media  30  opposite to the one direction (generally upwardly as shown). Further, in the embodiment illustrated, the first and second print head assemblies are located across substantially different lateral portions of print media path  31 . 
     In the manner described, the print head assembly of the present invention provides a very efficient arrangement of print heads and their respective platen rollers and eliminates overall curvature of the print media path. This particular arrangement performs double sided printing across a print media path that is wider than the elongated direction of the individual print heads. The linear efficiency of this arrangement minimizes the unprintable spaces which occur at the tops and bottoms of individual print media pages. The unprintable area is determined by the distance  54  between the first sequential print head  32  and the last sequential print head  35 . Because print media path  31  is designed to eliminate the overall curvature of the print media path, print heads  32 – 35  are located in the closest possible proximity, thereby minimizing the unprintable area. 
     Although the invention has been described in detail with respect to various preferred embodiments thereof, it will be recognized by those skilled in the art that the invention is not limited thereto but rather that variations and modifications can be made therein which are within the spirit of the invention and the scope of the amended claims.