Patent Publication Number: US-7589752-B2

Title: Two-sided thermal printing

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   Benefit of priority is claimed based on U.S. Provisional Application No. 60/644,772 of John L. Janning filed Jan. 15, 2005. 

   BACKGROUND 
   Direct thermal printing is a recognized means of printing quietly without toners or inks. It is a relatively mature technology that has been around for over forty years. Its use by retailers for printing of cash register receipts, mailing labels, etc. is now commonplace. 
   An example of early one-sided direct thermal printing is the thermal half-select printing as taught in U.S. Pat. Nos. 3,466,423 and 3,518,406 to John L. Janning. Such thermal half-select printing was accomplished by energization of electrically resistive thermal printing elements on both sides of thermal printing paper at the same time. The dual-sided coincident electrical current energization energy is additive to produce one-sided printing. The applied energy levels were such that, if applied on one side only, they were not sufficient enough to cause printing. By applying sufficient heat on both sides of the media simultaneously, the applied energies added and one-sided printing could occur. 
   Duplex or dual-sided direct thermal printing of transaction documents or receipts is described in U.S. Pat. Nos. 6,784,906 and 6,759,366. The printers were configured to allow printing on both sides of thermal media moving along a feed path through the printer. In such printers a direct thermal print head was disposed on each side of the media feed path. A print head faced an opposing platen across the feed path from the print head. 
   In direct thermal printing, a print head selectively applies heat to paper or other sheet media comprising a substrate with a thermally sensitive coating. The coating changes color when heat is applied, by which “printing” is provided on the coated substrate. For dual-sided direct thermal printing, the sheet media substrate may be coated on both sides. 
   Duplex or dual-sided direct thermal printing has been described for providing variable information on both sides of a paper receipt, e.g., to save materials and to provide flexibility in providing information to customers. The printing could be driven electronically or by computer using a computer application program which directs dual-sided printing. 
   Duplex or dual-sided direct thermal printing as described in U.S. Pat. Nos. 6,784,906 and 6,759,366 involves direct thermal print heads offset from one another while disposed on opposite sides of the media feed path for single-pass, two-sided printing. Unless there is a print head offset, uneven print density can potentially occur. This is because heat energy can be additive if it is applied simultaneously to both sides of the thermal printing paper when the print heads are directly across from one another. 
   SUMMARY 
   Dual-sided direct thermal printing of a thermal imaging element having thermally sensitive coatings on opposite sides of a substrate is described, where the thermal imaging element is provided along a feed path of a thermal printer having print heads disposed on opposite sides of the feed path. Printing on both sides of the thermal imaging element is achieved by applying variable energy heat pulses from the opposed print heads. Different energy levels of heat pulses are applied on opposite sides of the thermal imaging element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  schematically shows opposed print heads for dual-sided direct thermal printing in accordance with one exemplary variation of the invention. 
       FIG. 1   b  shows schematic detail of the print heads shown in  FIG. 1   a.    
       FIG. 2  shows exemplary energy level timing diagrams for heat pulses applied to the front and back of a thermal imaging element for two-sided “half-select” printing. 
       FIG. 3  shows exemplary energy level timing diagrams for heat pulses applied to the front and back of a thermal imaging element for two-sided “partial-select” printing. 
   

   DESCRIPTION 
   By way of example, various embodiments of the invention are described in the material to follow with reference to the included drawings. Variations may be adopted. 
     FIG. 1   a  of the drawings shows two thermal print heads  101   a  and  101   b  facing each other separated by thermal imaging element  104 , e.g., printing paper, provided along a feed path  105 .  FIG. 1   b  is an exploded partial view of  FIG. 1   a . Resistive printing elements  103  connect to electrical conductors  102 . Printing energies of variable energy heat pulses supplied by thermal print-heads  101   a  and  101   b  can add to implement direct thermal printing on one or both sides of the thermal imaging element  104  in a printer. 
   Two-sided direct thermal printing of front and back sides of thermal imaging element  104  is accomplished by simultaneous use of the adjacent two print heads  101   a  and  101   b  disposed on opposite sides of the feed path  105 , e.g., using thermal half-select printing as taught in U.S. Pat. Nos. 3,466,423 and 3,518,406. Thermal print heads  101   a  and  101   b  are energized to provide two available energy levels of heat pulses, and printing of one side of the thermal imaging element  104  is accomplished by use of the higher energy level heat pulses from one of print heads  101   a  and  101   b . Printing on both sides of thermal imaging element  104  is done by coincident use of lower energy level additive heat pulses from opposed print heads  101   a  and  101   b.    
   The charts in  FIG. 2  show two-level energies used for direct thermal printing from print heads  101   a  and  101   b  on both sides of thermal printing paper  104 . The lower level “half-select” energies are used for “same time-both sides” printing. Printing energy of heat pulses from each of print heads  101   a  and  101   b  is reduced to “half-select” levels when printing is to occur on both sides of the paper  104  at the same time. Otherwise, print density could cause an optical distraction in the area of print were higher energy levels used for simultaneous print on both sides of, e.g., paper  104 . The higher heat pulse energy levels shown in  FIG. 2  are used for printing on one side only of paper  104 . 
   In printing sequence—from print number 1 to print number 18 shown in  FIG. 2 , three prints (1-3) are made on the backside; followed by a single print (4) on the front; followed by a print (5) on both sides; followed by no print (6) on either side; followed by a print (7) on the backside; followed by a print (8) on both sides; followed by a print (9) on the front; followed by two prints (10-11) on the backside; followed by two prints (12-13) on the front; followed by a print (14) on both sides; followed by no printing on either side for two time periods (15-16); followed by a print (17) on the backside; and then followed by a print (18) on both sides of the dual-sided thermal imaging element, e.g., paper,  104 . 
   Thermal partial-select printing is accomplished in a similar manner except in the case where printing is to occur on one side only of thermal printing paper  104  having a thermal coating on both sides. In this case, coincident energies are applied by the print heads  101   a  and  101   b  in unequal or uneven energy levels with most of the printing energy supplied to the print head on the desired print side of the paper  104  while a lesser amount of energy is supplied by the element on the opposite side of the paper  104 . The two energies add and printing occurs on the side of the paper  104  with the greatest energy level applied.  FIG. 3  shows exemplary heat pulse energies for partial-select thermal printing. 
   In the embodiment shown in  FIG. 3 , three energy levels of heat pulses are supplied from both front and backside print heads  101   a  and  101   b . Printing cannot occur on either side of the paper  104  without help from both print heads  101   a  and  101   b  simultaneously, based on the selected energy levels chosen. For printing to occur on the front side only of the thermal imaging element  104 , a small energy level “partial” heat pulse is generated by the backside print head element while a large energy level “partial” heat pulse is generated by the front print head element. For printing to occur on the backside only, a small energy level “partial” heat pulse is generated by the front side print head while a large energy level “partial” heat pulse is generated by the backside print head. To print on both front and back of the thermal print paper  104 , a moderate energy level “partial” heat pulse is generated by both front and backside print heads  101   a  and  101   b.    
   In operation, heat pulses are generated by both front and backside printing heads  101   a  and  101   b . However, in the embodiment of  FIG. 3 , none of the heat pulses generated by the print heads  101   a  and  101   b  on the front or backside of the thermal paper  104  is chosen to be adequate enough to print a mark on either side of the paper by itself. 
   In printing sequence—from print number 1 to print number 18 in  FIG. 3 , three prints (1-3) are made on the backside of thermal imaging element  104 ; followed by a single print (4) on the front; followed by a print (5) on both sides; followed by no print (6) on either side; followed by a print (7) on the backside; followed by a print (8) on both sides; followed by a print on the front (9); followed by two prints (10-11) on the backside; followed by two prints (12-13) on the front; followed by a print (14) on both sides; followed by no printing on either side for two time periods (15-16); followed by a print (17) on the backside; and then followed by a print (18) on both sides of thermal imaging element  104 . 
   Thermal imaging element  104  may be constructed in a variety of ways, in a known manner, generally including thermally sensitive coatings on opposite sides of a substrate. Thermal imaging element  104  is provided along a feed path  105  of a thermal printer having print heads  101   a  and  101   b  disposed on opposite sides of the feed path  105 . Printing on both sides of the thermal imaging element  104  is accomplished by applying variable energy heat pulses from each of the print heads  101   a  and  101   b . The energy level of a heat pulse from one of the print heads  101   a  and  101   b  can be varied by varying the magnitude of a voltage that produces the heat pulse from the print head. Both sides of the thermal imaging element  104  are printed by coincident application of additive heat pulses from each of the print heads  101   a  and  101   b  as depicted in  FIGS. 2 and 3 . Printing on opposite sides of thermal imaging element  104  is controlled by the energy level of the heat pulses. 
   Heat pulses from each of print heads  101   a  and  101   b  can have at least two available energy levels where printing of one side of the thermal imaging element  104  is accomplished by use of higher energy level heat pulses from one of the print heads. Printing of both sides of the thermal imaging element  104  is accomplished by coincident use of lower energy level additive heat pulses from opposed print heads  101   a  and  101   b.    
   Where heat pulses from each of print heads  101   a  and  101   b  have at least three available energy levels, printing of one side of the thermal imaging element can be accomplished using the highest energy level heat pulses from one of the print heads and coincident use of the lowest energy level heat pulses from an opposed print head. Printing on one side only of thermal imaging element  104  can be accomplished by coincident use of intermediate energy level heat pulses from opposed print heads  101   a  and  101   b . Preferably, none of the three available energy levels would be selected to be adequate by itself to print a mark on either side of the imaging element  104 . The direct thermal printing on opposite sides of the thermal imaging element  104  is controlled by the timing of heat pulses from print heads  101   a  and  101   b  in this example of dual-sided direct thermal printing. 
   As taught in U.S. Pat. Nos. 3,466,423 and 3,518,406 to John L. Janning, a print head  101   a  or  101   b  may comprise a first group of parallel resistive heating elements disposed on one side of the feed path  105  and an opposed print head  101   a  or  101   b  may comprise a second group of parallel resistive heating elements disposed on the opposite side of feed path  105 , where heating elements of the first heating element group are disposed orthogonally to heating elements of the second heating element group. A dual-sided direct thermal printer is thus constructed in which the opposed print heads  101   a  and  101   b  each comprise electrically resistive thermal printing elements in the form of orthogonal row and column conductors disposed on opposite sides of feed path  105 . In such a dual-sided direct thermal printer, the printing occurs where coincidentally energized orthogonal row and column conductors overlap. Alternative dual-sided direct thermal printer constructions may be used, e.g., as illustrated in  FIGS. 1   a  and  1   b , where discrete electrically resistive printing elements  103  in print heads  101   a  and  101   b  may be adjacent one another and disposed on opposite sides of the feed path  105 . Dual-sided direct thermal printing on opposite sides of the imaging element  104  is accomplished by coincident current energization of the electrically resistive printing elements  103 . 
   The foregoing description above presents a number of specific embodiments or examples of a broader invention. The invention is also carried out in a wide variety of other alternative ways which have not been described here. Many other embodiments or variations of the invention may also be carried out within the scope of the following claims.