Abstract:
A printing system comprising two thermal printheads and an apparatus for smoothing a side of thermal media which may be compromised by an aggressive drive roller design. The smoothing apparatus in one embodiment is a heater which optionally comprises a heated roller for contacting and smoothing the receiver media.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. application Ser. No. 13/422,089, filed Mar. 16, 2012, which is hereby incorporated by reference in its entirety. This application is related to U.S. patent application Ser. No. 13/422,045, entitled “Printing Method of Reducing Printer Artifacts” which contains subject matter related, in certain respect, to the subject matter of the present patent application and which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to thermal printing, in particular, to heat treatment of a dye receiver layer surface exposed to a capstan roller to reduce print density differential. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is a well known practice within dye diffusion thermal transfer printers that, in order to controllably drive the paper and maintain traction for precise image registration between color passes, an aggressively textured drive roller, and a companion pinch roller that applies a load between the paper and drive roller, is commonly used. This type of drive system does not result in any image artifacts on the printed paper when printing only on one side, or simplex printing, because the aggressively textured drive roller is not contacting the printed side of the paper. This method does present a problem when printing a two-sided, or duplex print because the aggressively textured drive roller must contact both sides of the printed sheet. For two-sided or duplex printing, the paper surface that is in contact with the aggressively textured surface of the drive roller may become compromised by the aggressively textured surface. This compromised paper surface may not receive dye transfer as readily, resulting in a visible density difference between the area of the paper that saw contact with the drive roller&#39;s aggressive texture and the area that did not contact the aggressive texture. 
         [0004]    It is also common practice within the dye diffusion thermal transfer printer firmware to incorporate compensation algorithms that correct for across the page density variations, and/or down the page density variations. There may be limitations within the printer hardware or printer firmware such that compensation algorithms cannot completely compensate for printing artifacts generated by the drive roller. Due to these limitations, it becomes important to minimize the deviations in a print medium surface caused by the textured drive roller contacting the medium. 
         [0005]    With respect to  FIG. 1 , for two-sided or duplex dye diffusion thermal transfer printing, a common method is to use two thermal print heads  102 ,  109 , by first driving the rolled print medium  106  via drive roller (or capstan roller)  105  and pinch roller  104 , in cooperation with a motor drive (not shown) on roll  106 , to between platen roller  112  and one thermal print head  102  (print medium path of travel is shown in solid line), and printing on one side, Side A, of the print medium using dye donor  101 . A length of print medium received from print medium roll  106  driven through the drive roller  105  and pinch roller  104  exposes Side B to come into contact with the drive roller&#39;s surface texture, compromising the Side B surface for subsequent printing. The Side B surface is compromised via the textured drive roller  105  perforating, forming depressions, pitting, or indenting the outermost layer, or more layers, of the Side B surface. The print media is then re-positioned by reversing drive roller  105  and pinch roller  104  in cooperation with the motor drive on roll  106 , so that the lead edge of the paper is retracted toward the supply roll  106  and then diverted to the path represented by the dashed line. The rolled print medium  106  is driven via drive roller (or capstan roller)  105 , pinch roller  107 , in cooperation with the motor drive on roll  106 , to between platen roller  111  and the second thermal print head  109 . The non-printed surface, Side B, of the print medium is then printed using dye donor  110 . 
       SUMMARY OF THE INVENTION 
       [0006]    A preferred embodiment of the present patent application comprises a printer with two thermal printheads, a roll of duplex receiver media, and an apparatus for smoothing one side of the media prior to printing on it. The smoothing apparatus in one embodiment is a heater, which can optionally comprise a heated surface for contacting the receiver media. Optional embodiments could include radiant, air convection heaters, or a heated roller. A roller embodiment typically involves a second roller, optionally heated, for forming a nip with the heated roller and compressing the receiver media in the nip. Current passing through the roller, or other heated surface, comprises the heat source. 
         [0007]    Another preferred embodiment of the present invention comprises a printer with a roll of duplex printing media, a first thermal printhead for printing on media, a drive roller for drawing the media to the first thermal printhead, a smoothing device for smoothing another print side of the duplex printing media after the first side is printed, and a second thermal printhead for printing on the second side of the duplex printing media. The smoothing device comprises a heater for heating the duplex printing media, which may or may not require that the printing media be physically contacted by the heater such as by surface-to-surface contact. Such would be the case if the heater comprises a heated roller for pressing against the printing media. Typically, heat is generated in such a roller using an electric current. 
         [0008]    Another preferred embodiment of the present invention comprises an apparatus with a thermal printhead for printing on a receiver media, a drive roller, a pinch roller for forming a nip with the drive roller and for pulling the receiver media through the nip toward the thermal printhead. The drive roller and the nip are configured such that a surface of the receiver media is compromised as to its ability to evenly receive donor dye applied by another thermal print head on one side of the receiver media. Thus, a smoothing device is used to correct the compromised surface of the media before printing on it using the thermal printhead and donor dye. The smoothing device comprises a heater for heating the surface of the receiver media, which heater can be a heated roller. A circuit passes electric current through the heated roller for generating heat. 
         [0009]    These, and other, aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention and numerous specific details thereof, is given by way of illustration and not of limitation. For example, the summary descriptions above are not meant to describe individual separate embodiments whose elements are not interchangeable. In fact, many of the elements described as related to a particular embodiment can be used together with, and possibly interchanged with, elements of other described embodiments. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. The figures below are intended to be drawn neither to any precise scale with respect to relative size, angular relationship, or relative position nor to any combinational relationship with respect to interchangeability, substitution, or representation of an actual implementation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Preferred embodiments of the present invention will be more readily understood from the detailed description of exemplary embodiments presented below considered in conjunction with the attached drawings, of which: 
           [0011]      FIG. 1  illustrates duplex printing in a thermal printer apparatus. 
           [0012]      FIG. 2  illustrates a print medium positioned in a modified thermal printer apparatus. 
           [0013]      FIG. 3A  illustrates a print medium positioned in one embodiment of a modified thermal printer apparatus. 
           [0014]      FIG. 3B  illustrates a print medium positioned in one embodiment of a modified thermal printer apparatus. 
           [0015]      FIG. 4  illustrates experimental conditions used in a test fuser and the corresponding observations of a dye receiving layer&#39;s surface quality. 
           [0016]      FIG. 5  illustrates test results of dye receiving layer under heat treatment according to embodiments of the present invention. 
           [0017]      FIG. 6  illustrates a duplex receiver layer structure with thicknesses. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A preferred method and apparatus for printing is described in detail herein, and is illustrated in  FIG. 2  wherein components common to  FIG. 1  are operable as described above. After completion of printing on print medium Side A as described above, as the print media is being retracted toward the supply roll  106 , the pressure roller  103   a  will press the print medium against heated roller  108  and the combination of heat and pressure will reduce a size of the depressed points of the Side B surface left by the drive roller  105 . The printer is also operable in a reverse sequence as described herein wherein Side B is printed first and Side A depressions formed by drive roller  105  are reduced by compression between pressure roller  103   b  and heated roller  108 .  FIG. 2  shows pressure roller  103   a  forming a nip with heated roller  108 , however, both pressure rollers  103   a  and  103   b  are moveable to and from a position against heated roller  108  to form a nip therewith as needed. The heating roller is similar to a fuser roller used in electrophotographic printers which uses an electric current passing through the roller for heating or, in an alternative preferred embodiment ( FIGS. 3A and 3B ), the heating devices comprise two additional thermal print heads  208  and platen rollers  203  whose sole purpose is to smooth the printing medium surface. The heating devices are operable to heal the printing medium according to the present invention if disposed as illustrated in  FIG. 3A  or as illustrated in  FIG. 3B . An electric circuit for passing current through a heating element, such as a heated roller or thermal printhead, is well known and is not shown in the Figures. 
         [0019]    With reference to  FIG. 3B , after completion of printing on print medium Side A as described above, the print media is retracted toward the supply roll, as before, and then diverted along the pathway represented by the dashed line. The capstan and pinch roller drive the print medium between the printhead and platen roller such that a length of the print medium extends beyond the printhead. This is because the print medium is pulled from left to right, as shown in the Figures, during the thermal printing step. At this point, while the print medium is extended beyond the printhead and platen roller and is being pulled toward the printhead for printing, the pressure roller  303   b  will press the print medium against heated roller  308   b  and the combination of heat and pressure will reduce a size of the depressed points of the Side B surface left by the drive roller. The printer is also operable in a reverse sequence wherein Side B is printed first and Side A depressions formed by the drive roller are reduced by compression between pressure roller  303   a  and heated roller  308   a  prior to printing Side A. 
         [0020]    It is known from experimentation that, during the printing operation described above, holes, depressions, perforations, or indentations are created by the capstan roller on the side opposite to the side being printed. These holes are crescent shaped indentations in the outer dye receiver layer (DRL). Depending on the type of media, these holes might penetrate the DRL resulting in perforations in the DRL. Because the DRL is a flexible layer, it may be indented or perforated by the drive roller. Whether the DRL is indented or perforated by the drive roller, the heating step improves the DRL surface for receiving the dye donor and results in improved print quality. It should be noted that some duplex thermal printer designs are envisioned without a textured drive roller. Rather, a smooth drive roller is used with increased pressure against the pinch roller to compensate for lost traction due to lack of an aggressive texture on the drive roller. This increased pressure can also cause depressions or indentations, i.e. “tracks”, in the duplex receiver resulting in across the page density variations. The embodiments of the invention disclosed herein also serve to correct for these variations. 
         [0021]    With reference to  FIG. 6 , there is illustrated the thicknesses of various layers in the duplex receiver structure  601  contemplated by a preferred embodiment of the present invention. Other duplex receiver materials may be similarly improved with use of the embodiments of the present invention. The textured drive roller typically comprises protrusions extending from its cylindrical surface at a distance of about 25 microns. 
         [0022]    When this compromised DRL surface is printed the print density at areas corresponding to the capstan roller is lower than the print density found in the rest of the print. It was hypothesized (and observed by microscopy) that the holes do not get filled up with dye as intended by a thermal printing step and hence the half tone effect results in visibly lower print density. A two sided thermal receiver (medium) comprising a voided biaxially oriented polypropylene laminate was run through once for testing purposes. The one time run through means that the receiver was pulled or driven through the capstan rollers one time without printing. Experiments evaluated the effect of heat treatment on the unprinted DRL surface exposed to the capstan roller. The heat treatment was applied using an electrophotographic fuser breadboard. This breadboard allows temperature and line speed to be changed at a constant pressure between the nip formed by the heated roller  108  and pressure roller  103   a  or  103   b,  which is an elastomer nip. The measured nip width using a pressure sensitive medium was 5 mm This width is measured lengthwise along the print medium and is formed by the pressure of the heated roller against the compliant pressure roller with the print medium therebetween. Increased pressure increases the nip width as would a larger diameter heated roller, a larger diameter compliant pressure roller, or if either roller was made to be more compliant. Increased nip width increases an amount of heat transferred to the print medium. Typical pressure rollers are steel core with a thick silicone rubber layer, and a thin Teflon coating as an outer layer. The heated roller is similar in design to a fuser roller used in most electro-photographic printers. 
         [0023]    Ten feet of each variation was created to enable testing the heated capstan roller exposed DRL side in the printer. Observations were recorded as illustrated in  FIG. 4 . For a given temperature and line speed condition (e.g., 150 C, 70 mm/sec) the print medium (receiver) was run twice and thrice through the nip. We consider running the medium twice through a 5 mm nip as equivalent to exposing the medium to a 10 mm nip width (though in a discontinuous manner, because the receiver cools in between the heating steps) and running thrice as equivalent to exposure to a 15 mm of nip width (though in a discontinuous manner, as above).  FIG. 5  highlights difference in Delta L* (ΔL*) between a capstan roller compromised area of the print medium and a capstan-untouched portion of the print medium. L* is an arbitrary relative measure of lightness and the changes in L* shown in the graph of  FIG. 5  should be interpreted relative to the other measured magnitudes. The magnitudes are measured using a densitometer. It is observed that samples with heat treatment (150° C., 70 mm/s) shows lower ΔL*, i.e. there is less visible difference between untouched medium and a capstan compromised medium. 
         [0024]    Heat treatment shows promise in healing the capstan roller marks and minimizing ΔL*. Improvements in this procedure could include the ability to change pressure in the nip to enable a healing process, or to use a thermal head to heal the holes ( FIG. 3 ). Alternative heating methods include a heating zone located between the capstan roller and the supply roll. The heating zone could comprise a heated band which does not stick to DRL. The heating zone could also contain a non-contact heating source. 
         [0025]    The thermal dye receiving medium can be manufactured by various well known techniques and materials for duplex thermal receivers. A preferred method and materials are described in US Patent Application Publication 2011/0091667 A1, which is incorporated herein by reference in its entirety but for descriptions of a non-imaging reverse side of the print medium. 
         [0026]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
       PARTS LIST 
       [0000]    
       
           101  Donor 
           102  Thermal Print Head 
           103   a  Compliant Pressure Roller 
           103   b  Compliant Pressure Roller 
           104  Pinch Roller 
           105  Capstan Roller 
           106  Paper (Medium) Roll 
           107  Pinch Roller 
           108  Heated Roller 
           109  Thermal Print Head 
           110  Donor 
           111  Platen Roller 
           112  Platen Roller 
           203  Platen Rollers 
           208  Thermal Heads