Patent Publication Number: US-2018037032-A1

Title: Apparatus and Method for Thermal Transfer Printing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims priority under 35 USC §120 to U.S. patent application Ser. No. 15/468,986, filed on Mar. 24, 2017, which is a continuation of U.S. patent application Ser. No. 15/078,906, filed Mar. 23, 2016, and issued as U.S. Pat. No. 9,604,468 on Mar. 28, 2017, which is a continuation of U.S. patent application Ser. No. 14/839,496, filed Aug. 28, 2015, and issued as U.S. Pat. No. 9,296,200 on Mar. 29, 2016, which is a continuation application of International Application PCT/US2014/059293, filed Oct. 6, 2014, which is a continuation of U.S. patent application Ser. No. 14/050,054, filed on Oct. 9, 2013 and issued as U.S. Pat. No. 8,922,611 on Dec. 30, 2014, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This specification relates to systems and techniques for thermal transfer printing. 
     Thermal transfer printing involves the use of a ribbon to carry a material (e.g., ink) to the location of a printhead, where heat is then used to transfer the material from the ribbon to a substrate (e.g., paper or plastic). Many different variations of this general process have been developed over the last sixty years, and various improvements have also been made in the configurations and control systems employed for thermal transfer printers. For example, U.S. Patent Pub. No. 2013/0039685 describes a motor control system, a method of operating a motor control system, a tape drive including a motor control system, a method of operating such a tape drive, and a printing apparatus including such a tape drive, as can be used with thermal transfer printing. 
     In spool-to-spool printers, ink is supplied in ribbon form rolled onto cores, which are mounted or pressed onto spools (a supply spool and a take-up spool) in the printer. The movement of the spools can be precisely controlled by an electric motor for each spool. During a standard print operation, the motors are controlled to move the ribbon in front of the printhead at the same speed as the substrate where ink is removed from the ribbon. In order not to waste ribbon, each print should land on the ribbon directly adjacent to the previous print. This typically requires backing up the ribbon between each print in order to allow enough space on the ribbon to accelerate the ribbon to match the substrate speed before printing. For each print, both motors are used to accelerate the ribbon to the substrate speed, move the ribbon forward at the print speed, decelerate to zero velocity, accelerate in the reverse direction, stop and then decelerate again in the reverse direction, stop and then start the entire process over again for the next print. All of this is often complicated by the fact that the diameters of both spools are changing as the supply side is used up and the take-up side grows. Similar limitations apply to traditional shuttled printers, where the pack rate is limited by the operations of the shuttle, which goes back and forth for each print, and the length of the print may be limited by the travel distance of the shuttle. 
     SUMMARY 
     This specification describes technologies relating to systems and techniques for thermal transfer printing. 
     In general, one or more aspects of the subject matter described in this specification can be embodied in one or more methods that include: transporting a band holding hot melt ink thereon in proximity to both a heating device and a thermal transfer printhead, where the thermal transfer printhead is adjacent a substrate; actuating heaters in the thermal transfer printhead to transfer a portion of the ink from the band to the substrate to create a print on the substrate; and operating the heating device to heat the band to cause ink on the band to re-melt, flow and replace at least some of the portion of the ink transferred to the substrate previously before arriving at the printhead again for a next print. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products. 
     Operating the heating device can include: using a heater to maintain a temperature of a solid heat conducting material of an ink roller, where the solid heat conducting material includes a textured outer surface; applying a first side of the solid heat conducting material of the ink roller to the band to re-melt ink on the band; and supplying new ink to a second side of the solid heat conducting material of the ink roller, such that the new ink is retained by the textured outer surface. The textured outer surface of the ink roller can have a surface roughness greater than or equal to 3.2 microns, and the method can include using a blade to control an amount of ink retained by the textured outer surface of the ink roller, such that a uniform coating of ink, between 3 and 7 microns thick, is applied to the band. 
     The supplying can include periodically putting solid ink in contact with the textured outer surface of the ink roller. The transporting can include continuously moving the band at a same speed as the substrate, in coordination with the actuating, to achieve a pack rate above 650 packs per minute. The method can include: moving the thermal transfer printhead from a non-printing position into a printing position against the band to press the band against the substrate before the actuating; and moving the thermal transfer printhead back into the non-printing position after the actuating. Moreover, the band can include a polyimide film, an engineering plastic, or a metal ribbon. 
     One or more aspects of the subject matter described in this specification can be embodied in one or more printing apparatus including: a band capable of holding hot melt ink thereon; rollers configured and arranged to hold and transport the band with respect to a substrate; a printhead configured and arranged to thermally transfer a portion of the ink from the band to the substrate to print on the substrate; and a heating device configured and arranged to heat the band to cause ink on the band to re-melt, flow and replace at least some of the portion of the ink transferred to the substrate previously before arriving at the printhead again for a next print. 
     The heating device can include an ink roller including a solid heat conducting material having an outer surface that is textured, where the textured outer surface of the ink roller can be configured and arranged to contact the band and to receive new ink on the textured outer surface, and the textured outer surface of the ink roller can have a surface roughness greater than or equal to 3.2 microns. The ink roller can have a heater, and the printing apparatus can include: a blade configured and arranged to control an amount of ink retained by the textured outer surface of the ink roller; and a reservoir configured and arranged to hold any excess ink proximate to the ink roller. 
     The ink roller can be configured and arranged to apply a uniform coating of ink, between 3 and 7 microns thick, to the band. The printing apparatus can include a device to periodically put solid ink in contact with the textured outer surface of the ink roller to cause ink to be melted into the textured outer surface of the ink roller. One of the rollers can be a drive roller, and another of the rollers can be a spring loaded tension roller. The printing apparatus can also include a control system configured to control the band to match a speed of the substrate and to print at a pack rate above 650 packs per minute. 
     The band can include a polyimide film, such as a Kapton® material. The band can include an engineering plastic, such as an engineering plastic having a heat transfer rate greater than 0.120 Watts/meter-Kelvin and a thickness less than 25 microns. The band can include a metal ribbon, such as a stainless steel ribbon. Other band materials are also possible. 
     Particular embodiments of the subject matter described in this specification can be implemented to realize one or more of the following advantages. High speed and high pack rate thermal transfer printing can be realized while also minimizing use of consumables, such as used thermal transfer ribbon spools. High speed, high pack rate, and high quality coding can be performed on flexible films, as may be used in the flow-wrapper market. A thermal transfer printer can include an inkable band that is re-inked within the printer, where the band can be transported at the rate of the substrate to be printed to achieve very high pack rates. However, even when lower printing rates are used, the advantage of waste reduction still remains, which can result in reduced costs. The ribbon waste (ribbon substrate material, unused ink left on the ribbon (note that typical prints use about 30% of the ink in the area of the print), and used cores) of traditional spool-to-spool type thermal transfer printers can be substantially eliminated. 
     Printer down time can also be reduced since ink supplies can be replenished without stopping the line, and the band can be durable enough to require infrequent replacement (e.g., substantially less often than replacement of an ink ribbon roll). Moreover, since the band length does not change, tension in the band can be readily maintained using a spring loaded roller or dancer arm. A feedback loop to the controller need not be included to monitor the band tension or length. Only one motor need be used to move the mass of the band in one direction, rather than two motors traditionally used to drive two spools, forward and backward, where those two motors should accelerate and decelerate the mass of a full ribbon roll without losing position. The durability of the band, the replacement of only the ink used, and the lack of a ribbon core have the added advantage of reduced costs for the customer. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the invention will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a thermal transfer printing system. 
         FIG. 2A  shows an example of a thermal transfer printing apparatus. 
         FIG. 2B  shows an example of components of the thermal transfer printing apparatus from  FIG. 2A . 
         FIG. 2C  shows further details of the example of components from  FIG. 2B . 
         FIG. 2D  shows an exploded view of components from  FIG. 2C . 
         FIG. 3  shows an example of a process for operating a thermal transfer printer. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a thermal transfer printing system  100 . The system  100  includes a band  105  entrained around rollers  110 . The band can be made of various materials, such as polyimide film, engineering plastic, or metal. Selection of an appropriate thickness for a given type of band material can result in good heat transfer characteristics through the band  105 , allowing high quality prints at high speed, while also maintaining the durability of the band  105 . A print roller  115  can be used to transport a substrate  120  (e.g., paper or plastic) proximate to the band  105 . A thermal transfer printhead  125  is adjacent to the substrate  120  and is used to transfer hot melt ink from the band  105  to the substrate  120 . In some implementations, the system  100  can be reconfigured to position the substrate  120  adjacent the printhead  125  on a platen, rather than a roller  115 . 
     A heating device  130  is positioned adjacent to the band  105  so as to heat and re-ink the band  105 . For example, the heating device  130  can include an ink roller  135  that resides at least partially within a reservoir that holds ink for the thermal transfer printing system  100 . In addition, the system can include a device  140  that periodically adds new ink. For example, the device  140  can periodically put solid ink  145  in contact with the ink roller  135  to cause ink to be melted onto the outer surface of the ink roller  135 , with any excess being retained in the reservoir. Note that the roller  135  can be heated such that contact by the solid ink  145  will readily melt new ink for the system  100 , similar to what would happen when touching a hot skillet with a crayon. In other implementations, the reservoir can be filled with molten or semi-solid ink that is then in contact with one portion of the roller  135 , or a foam or sponge roller can be impregnated with hot melt ink and put in contact with the heated ink roller  135  (e.g., with the pressure of the foam or sponge roller against the heated roller maintaining the proper amount of ink in pockets of the heated roller). In some implementations, the ink is a mixture of pigment, wax and resin for a total pigment concentration of 20%, although many wax and resin type hot melt inks can be used in various implementations. 
     A controller  150  can also be provided to operate the various components of the system  100 , including the printhead  125 , the heating device  130 , and the ink supply device  140 . The controller  150  can be implemented using special purpose logic circuitry or appropriately programmed processor electronics. For example the controller  150  can include a hardware processor and software to control the system  100 , including controlling the speed of the band  105  to match the speed of the substrate  120 , and the delivery of data to the printhead  125 . The data can be delivered digitally, and the data can be changed with each print while the band and substrate continue to move at the same speed (e.g., 3 m/s). 
     The controller  150  can include (or be coupled with) one or more sensors to assist in carrying out its functions. Moreover, the controller  150  can be divided into various subcomponents, which can be then be integrated together to operate in cooperation with each other, or separately control the components of the system  100 . In some implementations, the controller  150  can control the band speed to enable the printer to operate at the high end speeds used by HFFS (Horizontal Form Fill and Seal) machinery. For example, the target substrate speed can be three meters per second, and the target pack rate can be 600 packs per minute (ppm) or greater. Note that a relatively simple motor driver system can be used to operate the band  105  at the same speed as the print roller  115  during printing. For example, a rotary encoder can be put in contact with the print roller  115 , and a stepper motor can be used to drive the band  105 . A belt and pulley from the motor can be used to drive the ink roller  135 . In some implementations, a gear or belt arrangement from the print roller  115  can be used to drive the band  105  at the same speed as the print roller  115  without using a motor. 
       FIG. 2A  shows an example of a thermal transfer printing apparatus  200 . The thermal transfer printing apparatus  200  includes a band  205 , which can include materials such as described above in connection with  FIG. 1 . For example, the band  205  can be a polyimide film with a thickness of 7.5 microns. In some implementations, the polyimide film is a Kapton® material, available from E. I. du Pont de Nemours and Company of Wilmington Del. In some implementations, the band  205  can be an engineering plastic that has a heat transfer rate greater than 0.120 Watts/meter-Kelvin and a thickness less than 25 microns (e.g., 4.5 microns). In some implementations, the band  205  can be a metal such as stainless steel ribbon with a thickness of 10 microns or less, such as 5 microns. 
     The band  205  is held and transported using rollers, which include a drive roller  210 , routing rollers  215 , and a spring loaded tension roller  220 . These rollers carry the band  205  to a thermal printhead  225  and an ink delivery device  230 . The ink delivery device  230  includes a reservoir  235  to hold any excess ink proximate to an ink roller  240 . The ink delivery device  230  also includes a blade  245  to control an amount of ink retained by the ink roller  240 . The ink is applied to the band  205  as the band  205  contacts the roller  240 . In some implementations, the ink coating applied to the band  205  is a uniform coating between three and seven microns thick. In some implementations, the ink delivery device  230  has a removable top to give access to the reservoir  235 , which includes a slot for ink that is put in contact with the roller  240  within the reservoir  235 . 
     In some implementations, a DC motor can be used to revolve the heated roller  240  to match the band speed to the substrate speed. In some implementations, the heated roller  240  is connected to a motor that is computer controlled to match the band speed to the substrate speed. In some implementations, the motor is connected with pulleys and belts to the drive roller  210  and the heated roller  240 . In addition, the band  205  can be kept at approximately 6 Newtons of tension, such as by looping the band around the spring loaded tension roller  220 , which is attached to a linear slide, as shown. 
     The ink delivery device  230  can also be viewed as a heating device. In some implementations, the ink delivery device  230  can include a heater within the reservoir  235 . In some implementations, the ink delivery device  230  can include a heater within the heated roller  240 , which is part of the ink delivery device  230 .  FIG. 2B  shows an example of components of the thermal transfer printing apparatus from  FIG. 2A .  FIG. 2C  shows further details of the example of components from  FIG. 2B .  FIG. 2D  shows an exploded view of components from  FIG. 2C . An ink roller  240  is partially contained by the reservoir  235 . The ink roller  240  can be a solid heat conducting material having an outer surface that is textured  255 . For example the texture  255  can be formed by bead blasting (e.g., using ceramic beads) to create a pocketed surface on the roller  240 . In some implementations, the roller  240  can be a knurled roller or an anilox roll or gravure cylinder with a specific design for coating. In any case, the textured outer surface  255  of the roller  240  can be designed to receive new ink from the reservoir or from direct contact with solid ink, such as described above. For example, the textured outer surface  255  of the ink roller  240  can have a surface roughness greater than or equal to 3.2 microns (e.g., approximately 3.2, 6.3, or 12.5 micrometer surface finish). In some implementations, the roller  240  can be a wire wound roller, such as a K-bar as provided by RK Printhcoat Instruments of Litlington, Royston, UK. 
     Two blades  245  can be positioned on either side of the roller  240  to control an amount of ink retained by the textured outer surface  255  of the roller  240 . The blades  245  can be made from silicone. Stainless steel plates can support the silicone blades. One of the blades  245  can be used to doctor the ink, and the other blade  245  can be used to keep debris from rolling back into the ink in the reservoir. 
     The roller  240  can be heated and positioned to contact the band, such that ink on the band is re-melted as the band passes the roller  240 . The roller  240  can include a heater  250  within a center portion of the roller  240 , which can be operated to keep the roller  240  at an appropriate temperature to re-melt the ink on the band as it passes the roller  240 . For example, the ink can be a wax based ink with twenty percent carbon concentration, and the roller  240  can be kept at a temperature of about 80° C. to keep the ink at a tacky consistency able to coat the roller without becoming so liquid that it flows off the roller. The heater  250  inside the roller  240  can be powered using wires connected through a slip ring (rotating electrical connector) so the heater can rotate with the roller. For example, a rotary electrical connector, such as a 4 connector Mercotac Model 430, can be used for connecting to the heater and to a sensitive thermocouple for feedback signals to provide power to the heater. Other heating systems can also be used, such as heating the roller  240  from the outside using radiant heat (e.g., a heater placed within the reservoir proximate to the roller). 
       FIG. 3  shows an example of a process for operating a thermal transfer printer. At  400 , a band holding hot melt ink thereon is transported in proximity to both a heating device and a thermal transfer printhead adjacent a substrate. For the printhead side of the band, in some implementations, the thermal transfer printhead can be moved at  405  from a non-printing position into a printing position against the band to press the band against the substrate. This can be done using a pneumatic cylinder, a motor and a cam, or by another mechanism. As described above, the band can include a polyimide film, an engineering plastic, or a metal ribbon. 
     At  410 , heaters in the thermal transfer printhead are actuated to transfer a portion of the ink from the band to the substrate to create a print on the substrate. Ink is melted off the band and onto the substrate in accordance with instructions from a control system. At  415 , the thermal transfer printhead can be moved back into the non-printing position after the actuating. 
     For the heating device side of the band, the heating device is operated to heat the band to cause ink on the band to re-melt, flow and replace at least some of the portion of the ink transferred to the substrate previously before arriving at the printhead again for a next print. In some implementations, a heater is used at  420  to maintain a temperature of a solid heat conducting material of an ink roller, where the solid heat conducting material includes a textured outer surface. The maintained temperature can be between 70° and 90° C., or another temperature range, or a temperature value, dependent upon the printing material being used in a specific implementation. At  425 , a first side of the solid heat conducting material of the ink roller is applied to the band to re-melt ink on the band. As each portion of the band moves past the inked heated roller, the ink on the band is re-melted. 
     In addition, new ink can be supplied at  430  to a second side of the solid heat conducting material of the ink roller, such that the new ink is retained by the textured outer surface. For example, this can involve periodically putting solid ink in contact with the textured outer surface of the ink roller, as described above. The textured outer surface of the ink roller can have a surface roughness greater than or equal to 3.2 microns. Further, a doctor blade can be used at  435  to control an amount of ink retained by the textured outer surface of the ink roller, e.g., ink contained by pockets on the roller, such that a uniform coating of ink, between 3 and 7 microns thick, is applied to the band. Areas on the band that have had ink removed in the printing process are thus recoated with melted ink through contact with the roller. Ink is supplied to the roller both by re-melting the ink already on the band in contact with the first side of the roller, and by the supply of ink provided on the second side (e.g., the roller rolling through a reservoir area). 
     The operations of this process are depicted in the drawing in a particular order for simplicity, but some of the operations shown are in fact performed in parallel with each other. Sequential ordering of operations is not required, and not all of the illustrated operations need be performed to achieve desirable results. The transporting at  400  can involve continuously moving the band at a same speed as the substrate, in coordination with the actuating, to achieve a pack rate above 650 packs per minute (ppm), although some implementations can be operated at pack rates of 650 ppm or less. 
     For a traditional spool-to-spool type thermal transfer printer, the rate of acceleration for the direction changes of the spools and ribbon is dictated by the fact that the motors should not lose position while accelerating the mass of the ribbon rolls, which thus limits the pack rate. The supply and take-up spools are accelerated until the linear speed of the ribbon matches the speed of the substrate, the printhead is actuated, the printhead prints, the printhead is retracted, and the spools of ribbon are decelerated, stopped, accelerated in reverse, decelerated and stopped in the start position in preparation for the next print. The mass of the ribbon spools limits the acceleration and deceleration of the ribbon spool motors. This adds considerable time between prints for the printer to prepare for the next print which is what limits the pack rate. For example, the pack rate for printing a 20 mm print at 1 m/s with a traditional spool-to-spool type thermal transfer printer is about 172 ppm. 
     In contrast, with the re-inked band described herein, there need only be one motor that always drives the band in one direction. The pack rate is thus limited to how quickly the printhead can be actuated. With high abrasion resistant printheads, or with a low friction treatment (such as with a Teflon® material) to the printhead side of the re-inked band, there is a possibility that the printhead does not need to be lifted between prints. In this case the pack rate is only limited by the data transfer rate to the printhead. 
     Note that the print speed is the rate at which the head can print once the head is contacting the ribbon and substrate. The print speed is limited by the ability for the resistors in the printhead to heat and cool. Pack rate is related to how quickly the printer can prepare for the next print. For a traditional shuttled printer (where the shuttle has lower inertia than the mass of a roll of ribbon), for each print, the shuttle is accelerated to the speed of the substrate, the printhead is actuated, the printhead prints, the printhead is retracted, the shuttle is reversed to the start position, and the cycle starts again. Additionally, the length of travel of the shuttle also limits the length of the print. Current shuttle-type thermal transfer printers can achieve a pack rate of about 474 ppm. 
     With the re-inked band, the band can be run constantly in one direction and be controlled to match the speed of the substrate. The pack rate may thus be limited only by the actuation time of the printhead. Once the printhead is retracted, there need be no other mechanism that must be returned to a start position. The length of the print doesn&#39;t have to be limited by the travel distance of a shuttle. In some implementations, a pack rate of 845 ppm can be readily achieved. Moreover, in some implementations, where the printhead is down at all times, thus allowing essentially back-to-back printing, the pack rate can approach 4000 ppm. 
     Embodiments of the subject matter and the functional operations described in this specification can be implemented using digital electronic circuitry, computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented using one or more modules of computer program instructions encoded on a computer-readable medium (e.g., a machine-readable storage device, a machine-readable storage substrate, a memory device, or a combination of one or more of them) for execution by, or to control the operation of, data processing apparatus. The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     While this specification contains many implementation details, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Thus, particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, a system can employ a print platform to transport the substrate rather than a print roller. A system can employ a foam or sponge roller impregnated with hot melt ink and put in contact with the heated ink roller to supply ink. A system could reduce the number of guide rollers or guide the re-inked band by another mechanism, such as a rotating drum. A system could use a nip roller in conjunction with the drive roller to move the re-inked band. A system could use the force between the ribbon, pressed by the printhead, against the moving substrate to move the re-inked band in conjunction with or without the drive motor. Moreover, the actions recited in the claims can be performed in a different order and still achieve desirable results.