Abstract:
A printer including a supply roll of receiver media and a plurality of rollers to feed the receiver media through the printer. Means for maintaining a preselected tension of the receiver media between the supply roll and a pair of the plurality of rollers includes a motor with a torque limiter. The motor can be attached to the supply roll or to an optional pair of rollers adjacent the supply roll. A capstan roller having a high traction, non-marking surface is used in conjunction with the tension control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. 13/032,926, entitled “Thermal Printing,” by Paoletti et al. filed concurrently herewith, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention is directed to thermal printing. In particular, a printer apparatus and method is disclosed for printing on receiver media without damaging the media. 
     BACKGROUND OF THE INVENTION 
     Currently, most thermal printers achieve acceptable color to color image registration by the use of a capstan roller having sharp peaks, as shown in  FIGS. 3A-B , which penetrate the receiver for optimum traction to avoid slippage during receiver transport. While this presents no grave problems for a simplex print, since the marks produced are on the back side of the print, for duplex printing this unimproved method leaves noticeable impression marks on thermal receiver media constructed with a dual-sided voided layer. The impression marks appear in the image area of the print. These impression marks, or depressions, do not allow dye from the dye donor web to reach the receiver media which leaves behind discolored areas on the print, such as white dots. It is noted that the present invention is not limited in any way only to duplex printing methods because the non perforating design of the presently disclosed methods and apparatuses can be implemented in non-duplex printing systems. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention incorporate the use of a less aggressive capstan roller design along with a softer pinch roller to eliminate impression marks in the thermal receiver. To compensate for the less aggressive grip on the receiver, a tension differential across the capstan is controllably decreased. By increasing tension in the receiver on the roll side of the capstan during printing, an acceptable color to color image registration is produced. The capstan uses a straight (longitudinal) knurl pattern with ridges running along the length of the roller parallel to its axis of rotation, as shown in  FIGS. 4A-B . The ridges are disposed at a frequency of 10 to 30 ridges per inch. The depth of these ridges being at least 10 microns. Other methods of achieving high traction, non-marking surfaces include the use of plasma coatings, thin elastomeric coatings, and increasing the wrap angle of the receiver around the capstan. In the case of a duplex printer, this invention is applicable whether the printer incorporates a single web path with print heads on both sides of the receiver or separate web paths for individually imaging each side of the sheet. 
     The pinch roller of a preferred embodiment of the present inventions is composed of a steel shaft covered with an elastomeric material having a shore-A durometer ranging from 20 to 60, with a 50 micron Teflon sleeve covering the elastomer. The tension of the receiver between the receiver roll and the capstan, region  108 , produced during a printing phase should be maintained at more than 50% of the tension existing between the capstan and the thermal print head, region  107 . This amounts to less than 50% tension differential across the capstan roller. These preferred embodiments of the invention do not require slowing down the print speed. 
     A preferred embodiment of the present invention includes a printer comprising a thermal print head for applying thermal media onto a receiver, a capstan roller disposed between the thermal print head and a supply of receiver media for controlling a feed of the receiver to the print head, and means for maintaining a minimum tension of the receiver between the supply of receiver and the capstan roller. The capstan roller is uniquely knurled with a longitudinal knurl pattern having a depth of at least about 10 microns and does not contain sharp points that may penetrate the receiver. Rather, it comprises a high traction, non-marking surface. A pinch roller adjacent the capstan roller forms a nip for the receiver. The pinch roller comprises an elastomeric material thereon having a shore-A durometer ranging from about 20 to about 60, preferably closer to about 40. The means for maintaining tension comprises a motor and a torque limiter that drives the roll holding the supply of receiver. Alternately, the motor and torque limiter can be applied to a second pair of rollers between the supply of receiver and the capstan roller. In either embodiment, the receiver is maintained in a taut state in a region of the receiver adjacent the capstan roller. This controlled tension helps to maintain a printing registration within a preselected tolerance. 
     Another preferred embodiment of the present invention comprises a drive system with a first pair of rollers forming a nip for feeding a receiver medium toward a print head. A receiver supply roll has a supply of the receiver wound thereon and supplies the receiver to the first pair of rollers. A means for maintaining a minimum tension acts upon the receiver in a region between the receiver supply roll and the first pair of rollers. The means may include a control means for controllably rotating the receiver supply roll for maintaining the minimum tension, or it can include a second pair of rollers between the receiver supply roll and the first pair of rollers. The second pair of rollers can control a tension of the receiver between the second pair of rollers and the first pair of rollers in response to a movement of the receiver through the first pair of rollers. 
     Another preferred embodiment of the present invention comprises a printer including a supply roll of receiver media for printing thereon. A plurality of rollers in the printer feed the receiver media through the printer. A means for maintaining a preselected tension of the receiver media between the supply roll and a pair of the plurality of rollers can include a motor with a torque limiter. The motor can be attached to the supply roll for driving the supply roll. Alternately, it can be attached to an optional pair of rollers adjacent the supply roll. The pair of the plurality of rollers (not referring to the optional pair) comprises a capstan roller having a high traction, non-marking surface, which can comprise a longitudinal knurl pattern. 
     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. It is also noted that other approaches to this problem could include eliminating the capstan and letting the platen roller be the main drive roller or in the case of a duplex printer, filling in or coating over the holes left by the capstan before printing over them, 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 
         FIG. 1  illustrates a receiver feed mechanism. 
         FIG. 2  illustrates a receiver feed mechanism with tension control rollers. 
         FIGS. 3A-B  illustrate views of a sharp point capstan roller. 
         FIGS. 4A-B  illustrate views of a longitudinal knurled roller. 
         FIG. 5  illustrates in-track data points using different modifications. 
         FIG. 6  illustrates cross-track data points using different modifications. 
         FIG. 7  illustrates impression mark data points using different modifications. 
         FIGS. 8A-B  illustrate in-track and cross-track registration performance using different receiver tensions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1  there is illustrated a portion of a thermal printer&#39;s drive system. A roll  106  of receiver  105  is fed through a thermal printer  100  as shown by the receiver advancing past thermal print head  101 , as fed by thermal roller  102 , pinch roller  104  and capstan roller  103 . Dye donor web  109  (partially illustrated) is applied onto the receiver in predetermined patterns, as is well known in the art. The receiver is iteratively reversed and printed during several color applications of the dye donor web in the predetermined patterns. Tension in approximate region  107  relative to approximate region  108  affect an ability of the capstan and pinch rollers to effectively control movement of the receiver therethrough. 
     A preferred embodiment of the present invention comprises a less aggressive capstan roller  103  design, as is illustrated in  FIGS. 4A-B  wherein a knurled pattern provides a spike free configuration that does not perforate a surface of receiver  105  as would the spiked configuration of the capstan roller shown in  FIG. 3A-B . Together with a softer pinch roller  104 , impression marks are not formed in the thermal receiver as it passes between capstan and pinch rollers  103 ,  104 . To compensate for the less aggressive grip on the receiver, a tension differential across the capstan in approximate regions  107  and  108  is decreased. By increasing tension in the receiver on the roll side of the capstan  108  during printing, an acceptable color to color image registration is produced. This increase in the tension in approximate area  108  reduces the tension differential across capstan roller  103 . 
     Referring to  FIG. 2 , control of the tension in approximate region  208  of the receiver can be achieved by providing a properly sized clutch (torque limiter) on the output of the drive motor for receiver roll  206  (not shown). The clutch control can be used to adjust tension in the receiver in approximate region  208 . An alternative method for controlling the tension in approximate region  208  of the receiver includes adding rollers  210  which would likewise be driven by a motor with a properly sized clutch on its output. This would reduce the length of controlled tension approximate region  208  to that approximate portion indicated by the dashed line bracket  208   a . In a preferred embodiment, roll  206  or rollers  210  would feed receiver  205  faster than the capstan, thus causing the clutch to slip and maintain a constant torque, during a forward feed printing phase of printer  100  and reverse feed the receiver slower than the capstan, again causing the clutch to slip and maintain a constant torque during its rewind phase. Both of these adjustments, one each for forward feed and for reverse feed, increase tension in the receiver in approximate region  208 . 
     The capstan  203  uses a straight knurl pattern with ridges running along the length of the roller parallel to its axis of rotation as shown in  FIGS. 4A-B . The ridges are disposed at a frequency of 10 to 30 ridges/cm at a depth of at least 10 microns. 
     The pinch roller is composed of a steel shaft covered with an elastomeric material with a shore-A durometer ranging from 20 to 60, with a 50 micron Teflon sleeve covering the elastomer. This preferred embodiment is a softer and thinner version of conventional elastomer roller covers. A softer pinch roller aids in eliminating marks in the receiver but often results in more slippage of the receiver due to lower traction. Controlling tension in the receiver on both sides of the capstan roller can reduce or eliminate slippage. The tension of the receiver between the receiver roll and the capstan, approximate region  108 , produced during printing should be more than about 50% of the tension existing between the capstan and the thermal print head, approximate region  107 . This percentage is higher than the unregulated tension commonly existing in thermal printers. 
     The clutched motor, either used for roll  206  or for rollers  210 , or both, is designed to provide a predesigned load, which controls an amount of tension applied to the receiver at approximate region  108 . Manual trial and error clutch adjustment can be fine tuned by monitoring performance of the printer, then manually leaving the clutch set at the desired adjustment point. This procedure can be undertaken during the design phase to establish a factory setting. Depending on the design of the printer, characteristics such as thermal head drag and capstan traction might require more or less tension between the receiver roll and the capstan to achieve proper image registration. The receiver roll diameter ranges from about 7 inches diameter when full to about 3.5 inches when depleted for the spool diameter, which should be compensated by controlling motor speed and torque during depletion of the receiver media. In an eight inch printer width, a full roll weighs approximately 5-6 pounds. If the clutch is driving the paper roll, the RPM of the motor output must be determined based on the smallest possible roll diameter during the printing cycle and on the largest possible diameter during the rewind cycle to insure that the clutch slips and maintains tension properly. If the clutch is driving a second pair of rollers, for example, the alternate rollers  210 , the roll diameter is not a concern. 
     The clutch operates by attaching part of it to the shaft and another concentric part attached to a drive component such as a gear or pulley. These two parts of the clutch are coupled to each other only by friction which produces a limited amount of torque when slippage of one half relative to the other occurs. Typically, this friction coupling is adjustable for controlling an amount of mechanically transmitted torque. 
     To determine a value of the torque that the clutch must transmit to the receiver to achieve accurate registration, the torque can be varied in a stepwise fashion until the color to color registration is within specification. Some possible ways to vary the torque to determine an acceptable value are to use an adjustable clutch, a series of fixed-value clutches or a pulley and weight system attached to the paper roll. This same technique can be used whether the clutch is driving the paper roll or a second pair of rollers. The precision of the tension control will depend on the gripping capability of the capstan roller. The less the gripping capability, the more tension control is required. 
     Other more precise methods of controlling tension include (1) the use of a three-roll cluster, the middle roller being a “dancer” roller which has a wrap angle of approximately 180° and exerts a constant force on the web (receiver); and (2) using a closed-loop system in which a tension sensor feeds back a signal to a DC motor which drives either the receiver roll  206  or the second pair of rollers  210 . 
     With reference to  FIG. 5 , experimental testing measured in-track registration, i.e. same direction as receiver movement through the printer, with resulting data points as shown in this figure. Testing procedures used straight knurl capstan roller  502 , as described above, varying pinch roller hardness modifications  503 , different pinch roller pressure modifications as applied with pinch roller springs  504 , and different print head load pressure modifications  505 , also applied via springs. There is a data point for each of these different print head load pressure modifications  505  shown in the graph, which tests were repeated using the different pinch roller modifications and pinch roller pressure modifications as shown. Horizontal baseline  501  line indicates a preferred minimum in-track performance of about −6 thousandths of an inch. To illustrate the scale of the graph shown relative to this −6 performance, the data point at head load spring  505  value 3.2, pinch roller spring  504  value 3.8, and pinch roller  503  value 40 shore A durometer, shows an in-track performance of approximately −18 thousandths of an inch. 
     With reference to  FIG. 6 , experimental testing measured cross-track registration, i.e. perpendicular to in-track registration, with resulting data points as shown in this figure. Testing procedures used straight knurl capstan roller  602 , as described above, varying pinch roller hardness modifications  603 , different pinch roller pressure modifications as applied with pinch roller springs  604 , and different print head load pressure modifications  605 , also applied via springs. There is a data point for each of these different print head load pressure modifications  605  shown in the graph, most of which tests were repeated using the different pinch roller modifications and pinch roller pressure modifications as shown. Horizontal baselines  601 ,  606  indicate a preferred performance window between +6 thousandths of an inch  601  and −6 thousandths of an inch  606 , with zero cross-track error indicated by dotted line  607 . The two performances closest to zero cross-track error indicated in this figure was achieved with pinch roller hardness of 40 shore A durometer, pinch roller spring tension (measured in kgf) of 4.9, and head load spring magnitude (also measured in kgf) 2.8 and 3.2. 
     With reference to  FIG. 7 , experimental testing measured impression marks in the receiver caused by the capstan  702 , with resulting data points as shown in this figure. Testing procedures used straight knurl capstan roller  702 , as described above, varying pinch roller hardness modifications  703 , different pinch roller pressure modifications as applied with pinch roller springs  704 , and different print head load pressure modifications  705 , also applied via springs. There is a data point for each of these different print head load pressure modifications  705  shown in the graph, most of which tests were repeated using the different pinch roller modifications and pinch roller pressure modifications as shown. Horizontal baselines  701  indicate resulting performance. The lowest line indicates that the impression is invisible to the naked eye and requires a loop to be seen; the second lowest horizontal line indicates an impression mark that can be seen by the naked eye but is not obvious. The remaining three horizontal lines indicate, in an upward progression, increasingly noticeable impression marks. Performance having less noticeable impression marks is preferred. 
     With reference to  FIGS. 8A and 8B , experimental testing measured in-track and cross-track registration, respectively, with varying tension applied to the receiver in region  108 , with resulting data points as shown in this figure. Testing procedures were undertaken by measurably controlling the torque applied to roll  106 . Horizontal baselines  801 ,  802  indicate a preferred minimum in-track and cross-track performance of about −6 thousandths of an inch. As is illustrated in  FIG. 8A , in-track registration with zero error is achieved using approximately 7 newtons of added tension. Cross-track registration, shown in  8 B, begins to deviate below the baseline with added tension of this magnitude. 
     PARTS LIST 
     
         
           100  Printing System 
           101  Print Head 
           102  Roller 
           103  Roller 
           104  Roller 
           105  Receiver 
           106  Supply Roll 
           107  Receiver region 
           108  Receiver region\ 
           109  Donor 
           200  Printing System 
           201  Print Head 
           202  Roller 
           203  Roller 
           204  Roller 
           205  Receiver 
           206  Supply Roll 
           207  Receiver region 
           208  Receiver region 
           208   a  Receiver region 
           210  Rollers 
           220  Direction 
           221  Direction 
           501  Horizontal line 
           502  Field 
           503  Field 
           504  Field 
           505  Field 
           601  Upper axis 
           602  Field 
           603  Field 
           604  Field 
           605  Field 
           606  Lower axis 
           607  Zero axis 
           701  Horizontal axes 
           702  Field 
           703  Field 
           704  Field 
           705  Field 
           801  Lower axis 
           802  Lower axis