Patent Publication Number: US-11040548-B1

Title: Thermal transfer printers for deposition of thin ink layers including a carrier belt and rigid blade

Description:
TECHNICAL FIELD 
     This application relates to systems and techniques for thermal transfer printing. 
     BACKGROUND 
     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. In a continuous band thermal printing apparatus, the band is recirculated within the system and re-inked with each revolution of the band in the system. The substrate to be printed is advanced continuously past the printhead during each printing operation. The printhead includes a plurality of selectively energizable printing elements that enable a pixel of ink to be transferred to the substrate. The energization of the printing elements is controlled to transfer ink to the substrate in a desired pattern. The printhead contacts an ink-free side of the inked band, and presses the opposite, inked, side of the band against the substrate to transfer pixels of ink from the ribbon to the substrate by heat. The length of time that a pixel of ink is exposed to a heated printing element prior to the pixel being transferred from the band to the substrate affects print quality; there is an optimum heating period to achieve a satisfactory transfer, with patchy and inconsistent prints if the ink is not heated for long enough or a blurred or smeared print if heated for too long. Various methods of manufacturing the inked band are also possible. 
     SUMMARY 
     This disclosure is based, in part, on the discovery that using a metal rigid coating blade levels the ink when used with a compliant opposing surface and produces a uniform coating height of the ink. A deformable carrier belt transports the printing band around the printer and can be used in conjunction with the rigid coating blade. The carrier belt can have a minimum Shore Hardness A of in the range of 50 to 100 (inclusive) to prevent excessive deformation and to prevent an overly thick ink coating. In some implementations, the carrier belt has a Shore Hardness A in the range of 60 to 90, or 70 to 80, 70 to 90, or 80 to 100 (all inclusive). 
     In some embodiments, a printing apparatus comprises 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 to thermally transfer a portion of hot melt ink from the band to the substrate to print on the substrate, an ink feed device configured to add hot melt ink to the band and to heat the hot melt ink on the band, and a rigid blade configured and arranged to cause hot melt ink to pass between a blade edge of the rigid blade and the band, wherein the rigid blade is shaped to minimize a contact area between the rigid blade and the band while controlling ink thickness of the hot melt ink on the band. 
     In some implementations, the rigid blade has a blade edge with a radius of curvature between 0.15 and 0.3 mm. The rigid blade has a front surface that forms an angle between 30 degrees and 90 degrees with respect to the band. The rigid blade is configured to restrict ink from passing beyond lateral edges of the rigid blade. The rigid blade has a lateral curvature that funnels ink towards a midline of the band. The rigid blade has side shields at the lateral edges of the rigid blade. The rigid blade has a rear surface that defines an ink/air interface and has an angle of above 30 degrees and below 90 degrees with respect to the band. The ink feed device comprises a slot die in communication with a slot within a body of the rigid blade that delivers hot melt ink to the band. 
     In some embodiments, a printing apparatus comprises an ink band capable of holding hot melt ink thereon, an ink feed device configured to deposit hot melt ink on the ink band, ink band rollers configured and arranged to hold and transport the ink band with respect to a substrate, a printhead configured to thermally transfer a portion of hot melt ink from the ink band to the substrate to print on the substrate, a rigid blade configured to control a thickness of the hot melt ink deposited on the ink band, a carrier belt in contact with the ink band, and carrier rollers configured and arranged to hold and transport the carrier belt with respect to the ink feed device, wherein the carrier belt is formed of at least a first material component and a second material component, the first material component providing the carrier belt with compliance that controls the thickness of the hot melt ink deposited on the ink band in conjunction with the rigid blade, and the second material component prevents or reduces elongation of the carrier belt. 
     In some implementations, the carrier belt is formed from a single material that comprises both the first material component and the second material component. The first and second material components are respective first and second layers. The first layer comprises a material with a Shore A Hardness between 50 and 100. The first layer is silicone rubber. The first material component layer comprises a material with a Shore A Hardness between 60 and 100. The second material component comprises a material that has a friction coefficient of greater than 0.1 when in contact with the carrier rollers. The second layer is Kevlar. A steering mechanism maintains a position of the ink band with respect to the carrier belt. The steering mechanism includes a rotatable shaft attached to one of the carrier rollers that is configured to adjust a position of the carrier roller in a direction perpendicular to a direction of travel of the carrier belt. 
     In some embodiments, a printing apparatus comprises an ink band capable of holding hot melt ink thereon, an ink feed device configured to deposit hot melt ink on the ink band, ink band rollers configured and arranged to hold and transport the ink band with respect to a substrate, a printhead configured to thermally transfer a portion of hot melt ink from the ink band to the substrate to print on the substrate, a carrier belt in contact with the ink band, the carrier belt being formed of at least a first material component and a second material component, the first material component providing the carrier belt with compliance that controls the thickness of the hot melt ink deposited on the ink band in conjunction with the rigid blade, and the second material component prevents or reduces elongation of the carrier belt carrier rollers configured and arranged to hold and transport the carrier belt with respect to the ink feed device, and a rigid blade configured and arranged to cause hot melt ink to pass between a blade edge of the rigid blade and the ink band, wherein the rigid blade is shaped to minimize a contact area between the rigid blade and the ink band while controlling ink thickness of the hot melt ink on the ink band. 
     In some implementations, the compliant layer provides a compliance amount that is matched to a pressure exerted by the rigid blade, which produces a desired ink coating thickness. 
     The systems described herein advantageously allow the deposition of a thin, uniform layer of ink onto the ink band. Thin ink layers and consistent ink layers improve print quality. The system advantageously reduces wear on the ink band, potentially increasing the life of the ink band while maintaining high speeds. Other advantages include that the system can work in any orientation and requires no extra process to remove ink from the band to create a fresh ink coating. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of a thermal transfer printer with a rigid blade. 
         FIG. 2  shows an example of a rigid blade that can be used in the thermal transfer printers of  FIGS. 1, 6, and 8 . 
         FIG. 3A-B  show cross-sectional views of an example of a rigid blade used in the thermal transfer printers of  FIGS. 1, 6, and 8 . 
         FIG. 3C-D  show results of computer modelling visualization of the flow stream at the rigid blade. 
         FIG. 4  shows an additional example of a rigid blade tip that can be used in the thermal transfer printers of  FIGS. 1, 6, and 8 . 
         FIGS. 5A-B  show an additional example of a rigid blade tip that can be used in the thermal transfer printer of  FIGS. 1, 6, and 8 . 
         FIG. 6  shows an example of a thermal transfer printer with a compliant carrier. 
         FIG. 7  shows an example of a thermal transfer printer with a rigid blade and a compliant carrier. 
         FIG. 8  shows an example of a thermal transfer printer with a rigid blade and details of a compliant carrier. 
         FIG. 9  shows an example of an ink monitoring control subsystem, which can be used in the thermal transfer printers of the present application. 
         FIG. 10  shows a front view of a portion of the thermal transfer printer of  FIG. 8  including a slot die ink delivery. 
         FIGS. 11A and 11B  show an example of a rigid blade used with the slot die of  FIG. 10  and includes an ink feed pocket and slot. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a thermal transfer printer  100 . The thermal transfer printer  100  includes an ink band  105  that is held and transported using guides and/or rollers, which can include routing rollers  110 , and a drive roller  112 . The drive roller  112  holds the ink band  105  and is responsible for the motion that transports the ink band  105  through the thermal transfer printing apparatus  100 . The drive roller  112  is advantageously located to pull the ink band  105  locally relative to a re-inking station (rather than pushing it locally through the re-inking station or pulling through a longer length of the band), however the drive roller  112  can be positioned at other locations, or more than one drive roller is possible. The band can be made of various materials, as described in detail below. Selection of an appropriate thickness for a given type of band material can result in good heat transfer characteristics through the ink band  105 , allowing high quality prints at high speed, while also maintaining the durability of the ink band  105 . A print roller  115  can be used to transport a substrate  120  (e.g., paper or plastic) proximate to the ink band  105 . A thermal transfer printhead  125  is adjacent to the substrate  120  and is used to transfer hot melt ink from the ink band  105  to the substrate  120 . In some implementations, the printer  100  can be reconfigured to position the substrate  120  adjacent the printhead  125  on a printing platen that can replace the roller  115 . 
     In some implementations, an inking platen  130  contacts a back side (i.e., non-ink side) of the ink band  105  and holds the ink band  105  in position relative to a re-inking station while the ink band  105  can slide over the surface of the platen  130  that holds it in position relative to the re-inking station. Alternatively, a roller (e.g., a rotatable platform) can be used in place of the platen  130 , and the contacting surface of the roller moves with the ink band  105 . The features described below with respect to the platen  130  can be implemented with a roller instead, in various implementations. 
     In some implementations, the platen  130  has a fixed position. In other implementations, the platen  130  (or roller) is moveable, such as in response to a control signal during printing or for purposes of installing or replacing the ink band  105  in the printer  100 . The platen  130  presents a firm surface to the back side of the ink band  105 . For example, the platen  130  can be made of metal and be generally unyielding when pressure is applied. In other implementations, the platen  130  is compliant (e.g., includes a compliant exterior layer). 
     The thermal transfer printer  100  includes an ink feed device  135  to add additional hot melt ink to the ink band  105  (as needed) and a blade support  180 , which holds a rigid (e.g., metal) blade  184  that is pressed against the ink band  105 . There are various ways to implement the blade  184 , including those described below as blades  200 ,  250 ,  260 , and  370 . Methods of inking the ink band  105  are described in WO 2018/065959, the contents of which is incorporated herein by reference. In various implementations, the rigid blade  184  is made of metal, such as aluminum, stainless steel, titanium, or a combination of these. In addition, in some implementations, the rigid metal blade  184  is coated with an additional material to prevent or reduce wear and abrasion. For example, in some implementations, the rigid metal blade  184  is coated with an amorphous fluoroplastic, such as one or more types of Teflon® PTFE (Polytetrafluoroethylene) coating materials, available from E. I. Du Pont de Nemours and Company (also known as DuPont) of Wilmington Del. 
     The blade  184  can be held orthogonal or at another angle to the direction of travel of the ink band  105 . During printing operations, the blade  184  is pressed against the platen  130 , trapping the ink band  105  against the platen  130 . 
     In some implementations, the platen  130  is heated to ensure the hot melt ink on the ink band  105  is in a molten state as it approaches the blade  184 . Additionally or alternatively, a heater  142  can be included to heat the ink so that it is fully melted before it reaches the blade  184 . The heater  142  can be an infrared lamp or other radiant heater. An example of a radiative heat source is described in WO 2018/065959. In general, one or more heating devices are included. For example, in addition to using a heated platen  130 , a heater  142 , or both, the ink feed device  135  can be a heated ink feed device. In any case, at least one heating device should be close enough to the blade  184  to ensure that the hot melt ink is maintained in a molten state at the location of the blade  184 . Moreover, the specific sequence of components leading up to the blade  184  can be changed, e.g., a heated ink feed device  135  can be placed before or after the heater  142  in the direction of travel of the ink band  105 . 
     One or more controllers  160  are also provided, each or all of which can be included in the thermal transfer printer  100  or be separate from the printer  100  but still be included in a larger printing apparatus or system. In some implementations, controller(s)  160  operates the various components of the printer  100 , including the printhead  125 , the heated ink feed device  135 , the heater  142 , the blade support  180 , and potentially a heated platen or roller  130 . The controller(s)  160  can be implemented using special purpose logic circuitry or appropriately programmed processor electronics. For example, the controller(s)  160  can include a hardware processor and software to control the printer  100 , including controlling the speed of the ink 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 ink band  105  and substrate  120  continue to move at the same speed (e.g., 400 mm/s). 
     The controller(s)  160  for the printer  100  can provide control signals to a blade support  180  to position the blade  184  relative to the speed of the ink band  105 , (e.g., during set-up or after replacement of an ink band  105 ) or to prevent wear during periods of non-printing. The controller(s)  160  can include (or be coupled with) one or more sensors to assist in carrying out its functions. For example, a speed sensor can be associated with the ink band  105  to monitor the speed of the ink band  105 . Alternatively, the speed of the band can be known by the controller(s)  160 , without the use of a sensor, as when the controller(s)  160  controls the speed of the ink band  105 . In addition, a thickness sensor can be associated with the ink band  105  to monitor a thickness of the hot melt ink on the ink band  105  after the blade  184 . A temperature sensor  146  can be located near the ink band  105  to determine the temperature of the ink being melted onto the band. A temperature sensor  140  can also be part of the ink feed device  135  and register a temperature of the heated ink. The one or more sensors can include a deformation sensor to maintain the uniform coating height. In some implementations, the deformation sensor can be a spring-steel lever connected to a strain gage, such as P/N MMF307449 from Micro-Measurements (Raleigh, N.C.). Note that the controller(s)  160  can be divided into various subcomponents, which can operate in cooperation with each other or separately control the components of the printer  100 , and further details regarding examples of control subsystems are described below in connection with  FIG. 9 . 
       FIG. 2  shows an embodiment of rigid blade  200  used in the thermal transfer printer of  FIG. 1 . The rigid blade  200  has a blade edge  210  with a specified radius of curvature  205  at the tip and angles leading thereto. As shown in  FIG. 2 , the blade edge  210  of the rigid blade  200  also has a lateral curvature  212 . The lateral curvature  212  is shaped so as to push ink toward a center line of the rigid blade  200 . This pushing of the ink can help prevent overflow of the ink beyond the lateral sides of the rigid blade  200 . The lateral curvature can also prevent bulging in the ink band  105 , which affects the ink deposition onto the ink band  105 . 
     The width of the blade can be between 25 mm to 130 mm. The width of the blade  200  depends on the width of the ink band  105  and the printhead  125 . In many implementations, the blade  200  will be wider than the printhead  125  of the thermal transfer printer, and the width of the ink band  105  will also be wider than the printhead and may be wider than the blade  200 . In various implementations, the printhead is from 32 mm to 128 mm (e.g., 53 mm) wide. 
     Referring to  FIGS. 3A-B , the blade  200  is positioned with respect to the ink band  105 , where the ink band  105  which can overlie a compliant material on a platen or roller, or overlie a compliant material carrier belt as part of the ink band  105  at the surface in contact with the blade edge  210 . To reduce pressure on the ink band  105 , the blade edge  210  advantageously presents a small surface to the ink band  105 . For example, a tooling radius of 0.2 mm can be used to produce a small radius. In some embodiments, the radius of the blade edge  210  can be less than 0.5 mm, less than 0.4 mm, less than 0.3 mm, less than 0.2 mm, or less than 0.1 mm. As the radius is increased, the contact surface  215  or ink coating zone between the blade edge  210  and the ink band  105  increases, and the resulting ink height increases, potentially more than is desirable. 
       FIG. 3B  shows ink  214  travelling in the direction  216  with respect to the ink band  105 . The position of the blade  200  is represented, although the blade itself is not shown. The ink coating zone  215  is shown in greater detail; if minimized as much as possible the pressure on the ink band  105  (and a carrier band  170  discussed in detail below) will decrease and lead to lowered stress imposed on the ink band  105 . The length is limited to approximately 0.3 mm (the minimum possible with current tooling limitations). 
     The lead-in slope  225  is the angle at which the blade edge  210  creates a funnel through which the ink  214  must pass. The lead-in slope  225  has an angle above 30 degrees and below 90 degrees, the angle being used in any given implementation based on the ink used. For example the angle can be 45 degrees. 
     Functionally, the ink  214  entering into the angle area of the blade along the lead-in slope  225  develops a vortex  244  (shown in  FIGS. 3C and 3D ) within the ink  214  caused by the relative movement of the band  105  to the blade  200 . The slope of the angle determines how far under the blade  200  the vortex is wedged and how much upward pressure is exerted on the blade  200  and how much downward pressure is exerted on the band  105 . 
       FIGS. 3C and 3D  are graphs showing a result of a computational model of the velocity field that develops in the angled area of the blade lead-in, the ink vortex  244  at the point represented by the blade edge  210  of  FIG. 3B . The graphs show the vortex  244  in a vertical orientation (e.g., rotated with respect to  FIG. 3B ) with the blade  200  on the left portion of the figure and the area to the right of the vortex  244  being the band  105  (and compliant carrier band  170 ).  FIG. 3D  is an enlargement of the top portion of  FIG. 3C . The size of the vortex is viscosity dependent. The ink  214  used in some implementations is non-Newtonian (e.g., shear thinning) and thus the vortex  244  can have a thinning effect on the ink. 
     Modelling was carried out for visualization of the flow stream at the blade. In this computer testing, ink viscosity μ is a function of temperature T and shear rate γ, ink pressure P is a function of μ, ink velocity v (which is equivalent to band speed v) and the contact area of the blade is A. The displacement of the rubber carrier belt u is a function of P and rubber shore hardness S. The ink height h is a function of u. 
     The desired ink thickness, h, can be 4 microns. To make h as small as possible, it is desirable to minimize the deflection of the carrier belt material. However, a solid surface with no compliance increases the pressure P and risks tearing the thin band. To allow a higher durometer carrier belt without risking tearing the ink band, P is reduced. This is carried out by an increase in T to above a critical level, e.g., 120° C. for the ink used in testing (with ink materials of ethylene vinyl acetate (EVA), wax, resin) with μ around 10 Pa·s. This can be achieved by increasing heat supplied to the coating mechanism. The band speed v can be reduced (undesirably) as can the blade area A (making the blade edge as small as possible). The area can be reduced by removing the edge radius so that the blade goes directly from a 45 degree entry angle to the blade length  215  (as shown in  FIG. 3B ). An upper maximum of 1200 mm/s coating speed has been demonstrated to date. Shore hardness S can be increased by choosing a harder rubber to above a level of 75 Shore A Hardness and it was ensured that the surface irregularities were as small as possible. Results of the modelling visualization of the flow stream at the blade is shown in  FIG. 3C  with a zoomed in view in  FIG. 3D . 
     The rigid blade  200  can be curved in the Z dimension as seen in  FIG. 2 . As best seen in  FIG. 2 , the blade  200  is curved to keep ink from rolling off the band at the edges of the blade and can funnel the ink  214  in more than one dimension. 
     The blade exit slope  228  is the angle at which the blade edge  210  creates a funnel through which the ink exits. The blade exit slope was determined to have an angle of above 30 degrees and below 90 degrees. On the same edge, the beginning of the ink/air interface  232  is where the ink creates an interface surface with external air. 
       FIG. 4  shows an additional example of a rigid blade  250 . The rigid blade  250  has a blade edge  265  configured to cause the ink to funnel between the body of the rigid blade  250  and the ink band  105 . The rigid blade  250  includes side shields  255  at either side of the blade  250 . These side shields prevent ink overflow beyond the lateral edges of the blade  250 . 
     Control of the ink includes control of the ink thickness. Generally, the position of the blade support  180  relative to the platen  130  controls the pressure exerted by the blade  200  or  250  and the ink thickness on the ink band  105 , i.e., the height of the ink as it leaves the blade at the ink/air interface  232 . The controller(s)  160  can provide control signals to the blade support  180  to reposition the blade  200 ,  250  in accordance with a viscosity of the hot melt ink and the speed of the ink band  105 . However, the viscosity of the ink decreases as the ink is heated. Rather than repositioning the blade due to the viscosity of the hot melt ink, the desired ink height (e.g., exiting from the ink/air interface  232  in  FIG. 3B ) can be achieved by modulating the viscosity of the hot melt ink by adding more heat to the ink. High relative speeds between the blade  200 ,  250  and the ink band  105  can be achieved by adding more heat to the ink, thereby reducing the force exerted by the ink. Thus the blade  200 ,  250  is fixed in place and only the variation of heat energy from the heater  142  is used to regulate the ink height. 
     In addition, in some implementations, the controller(s)  160  provides control signals to adjust a position of the blade  200 ,  250  to compensate for wear of the blade material, which alters the mechanical properties of the blade  200  over the course of time. In some implementations, the controller(s)  160  also receives an input from a sensor monitoring the coating thickness. Thus, the controller(s)  160  can implement a closed loop control system controlling the ink thickness based on the sensor signal by varying the ink viscosity. These adjustment mechanisms are described in further detail below in connection with  FIG. 9 . 
       FIGS. 5A and 5B  show an additional example of a rigid blade  260  that can be used with the system  100  and the systems described below. The blade is connected to an apparatus containing a narrow channel configured to deliver ink to the blade. The channel can be heated. The apparatus can also contain a reservoir of ink. The ink can be moved from the reservoir through the narrow channel to the blade by pressurizing the ink supply. Ink can be metered according to the usage of ink while printing. 
       FIG. 6  shows another example of a thermal transfer printer  300 . The thermal transfer printer  300  has many of the same features as the thermal transfer printer  100  of  FIG. 1 . In addition to an ink band  105 , the thermal transfer printer  300  includes a carrier belt  170 . The carrier belt  170  is held and transported using carrier rollers  175 ,  177 . The carrier roller  177  can be a driver roller that pulls the carrier belt  170 , and thus the ink band  105  from bottom to top in this figure. The carrier belt  170  supports the ink band  105  and is at least partially responsible for the motion that transports the ink band  105  through the thermal transfer printing apparatus  300 . As the carrier belt  170  and the ink band  105  are separate bands, a steering mechanism maintains the relative position between the two bands. The steering mechanism keeps the ink band  105  centered under the print head  125  and a rigid blade  186  and steers the ink band  105  relative to the carrier belt  170 . The blade  186  can be a traditional rigid blade. The steering mechanism can include a rotating shaft  114 . This rotating shaft  114  is a steering mechanism that causes tension on one edge of the band and slack on the other edge of the band, causing the band to track toward the tensioned side. A non-contact edge sensor (e.g., an infrared LED transmitter and photo diode receiver or an ultrasonic sensor) is used to sense if the band is off track. The rotating shaft  114  thus acts to keep the ink band  105  centered on the carrier belt  170 . The rotating shaft arm  114  attaches to the carrier roller  175 , and causes the carrier roller  175  to move slightly to either side along a direction perpendicular to the direction of travel of the band  105  (e.g., into and out of the plane of the page of the figure). In some implementations, the rotating shaft  114  works in conjunction with a band position sensor that detects a position of the ink band  105  relative to the carrier belt  170 . The rotating shaft  114  adjusts the centerline of the carrier belt  170  along the axis perpendicular to the direction of travel to compensate for any drift of the bands relative to each other. This action keeps the centerline of the ink band  105  aligned with the centerline of the carrier belt  170 . Additionally, a flange on one or both of the carrier rollers  175 ,  177  can hold the carrier belt  170  in place, e.g., along the centerline of the rollers. Roller  110  can be configured to be a dancer arm to take up slack in the ink band  105 . 
     Referring to  FIG. 7 , the carrier belt  170  is designed as a seamless carrier loop that transports the ink band  105  around the printer  300 . The carrier belt  170  is made from two layers, a top compliant layer  230  and a bottom substrate layer  235 . The top compliant layer  230  acts to control the thickness of the ink layer when pressed against the rigid blade  186 , while the bottom substrate layer  235  acts as a carrier belt for transporting the compliant layer  230  and to prevent or reduce elongation of the carrier belt. 
     The bottom substrate layer  235  of the carrier belt  170  is in contact with the rollers  175  that move and guide the carrier belt  170  around the printer  600  and can be in contact with the platen  130 . The bottom layer  235  is made of a firm material. For example, the bottom layer is made of Kevlar. The bottom layer  235  is a material with a high friction coefficient; such a high friction coefficient ensures that the carrier belt  170  remains in contact with the rollers  175 ,  177  and platen  130  without slipping. For example, the friction coefficient between the surface of the bottom substrate layer  235  and the rollers  175 ,  177  or platen  130  can be between 0.1 and 1. In some instances, the rollers  175 ,  177  and platen  130  can be coated with a silicone rubber, although any suitable material that can achieve a friction coefficient with Kevlar in the above range can be used. The platen  130  can be heated to provide consistent heat to the carrier belt to improve the coating process. 
     The bottom substrate layer  235  provides a firm backing for the ink band  105 , enabling the ink band  105  to be transported around the printer  300  and also provides a firm support to the top compliant layer  230  of the carrier belt  170 . The firmness of the bottom substrate layer  235  enables the blade  186  to exert pressure on the top compliant layer  230  without affecting the speed of travel of the ink band  105 . For example, the bottom substrate layer  235  is made of a material with a Shore A Hardness of at least 60. For example, with a Shore Hardness A in the range of 60 to 90, or 70 to 80, 70 to 90, or 80 to 100 (all inclusive). 
     The top compliant layer  230  is made of a more compliant material than the bottom substrate layer  235 . The compliant material of the top layer  230  provides a deformable material that enables the ink band  105  in contact with the top layer  230  to be coated with ink using the rigid blade  186 . However, if the material is too soft, then the deformation of the rubber is too large and the coating process produces an ink layer that is too thick. For example, the top compliant layer  230  is made of a material with a Shore A Hardness of minimum  50  and maximum of 100. For example, with a Shore Hardness A in the range of 60 to 90, or 70 to 80, or 70 to 90. The top compliant layer  230  can be uniformly smooth to facilitate an even coating, e.g., have a surface roughness smaller than 5 μm, such as less than 4 μm, less than 3 μm, less than 2 μm. 
     In some instances, the material of the top compliant layer  230  can be rubber. In some implementations, hyperelastic polymers are used. Examples of materials that can be used include silicone, VITON® or EPDM or KALREZ. VITON® is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings. EPDM rubber (ethylene propylene diene monomer (M-class) rubber) is a type of synthetic rubber, which is an elastomer characterized by a wide range of applications. KALREZ® by DuPont is a perfluoroelastomer. The elastomer can be combined with chemicals or fillers to improve heat conduction, reduce friction, reduce compression set and control hardness, etc. In other implementations, other materials can be used for the compliant layer. In general, the compliant layer should be matched to the desired ink coating thickness and chemical compatibility. 
     If the material of the top compliant layer  230  is too solid (e.g., a linear elastic material), the pressure from the rigid blade  186  risks destroying the ink band  105  supported by the carrier belt  170 , e.g., deforming or tearing the ink band  105 . Proper rubber characteristics permit a controlled gap between the rigid blade  186  and the ink band  105  and allow only a small amount of ink to pass there between. In this manner, the rigid blade  186  and compliant carrier ink band  105  can create an ink coating that is less than 4 μm thick. 
     Furthermore, the two layers of the carrier belt  170  do not expand significantly under thermal stress. Such a thermal property ensures that the gap between the coating rigid blade  186  and the inked ink band  105  will be constant for a fixed speed. For example, the materials of both the top compliant layer  230  and the bottom substrate layer  235  tend to have a coefficient of thermal expansion in the range of 1e −6  to 3e −4  [1/K]. The materials also have an operating temperature up to 600 [K]. 
     In some instances, the bottom substrate layer  235  is woven Kevlar and the top layer is 230 silicone rubber that is cast directly onto the bottom substrate layer  235 . In such an arrangement no adhesive is used. In other instances, the top compliant layer  230  is adhered to the bottom substrate layer  235 . The hyper elasticity of the rubber ensures that there is no permanent deformation as long as the rubber is not stretched beyond the deformation limit. 
     The ink band  105  can be operated at variable speeds while also being coated with ink to the correct thickness. By taking advantage of the shear-thinning properties of the ink, whereby the viscosity of the ink drops as the temperature increases, the thermal transfer printer  100  can produce a thin coating thickness (e.g., 4-10 μm) at a low cost. 
       FIG. 8  shows another example of a thermal transfer printer  350 . The thermal transfer printer  350  has many of the same features as the thermal transfer printer  100  of  FIG. 1  as well as the features of the thermal transfer printer  300  of  FIG. 6 . The thermal transfer printer  350  includes a carrier belt  170  that supports the ink band  105  and is at least partially responsible for the motion that transports the ink band  105  to be inked, using the rigid blades described herein. The rigid blade  184  can be any of the rigid blades described, including rigid blades  200 ,  250 , or  260  described above, or blade  370  described below. 
       FIG. 9  shows a portion  600  of a thermal transfer printer, including an example of an ink monitoring control subsystem  460 , which can be used with the thermal transfer printers of the present application. For example, the portion  600  can be used with implementations that employ the carrier belt  170  and use blade  186 , such as shown in  FIG. 6 . 
     The thermal transfer printer includes a band  410 , a roller  415 , and returning hot melt ink  420  on the band  410 . In addition, a blade  440  conditions the ink on the band  410  and can be repositioned by translation, rotation, or both. The roller  415  can be a platen. A heater system  465  can add heat to the ink on the band  410  or the ink being applied to the band  410 . 
     A speed sensor  430  can be used to monitor the actual speed of the band  410 . The speed sensor  430  can be a roller attached to a rotary encoder, or any other appropriate device to measure speed. Moreover, in some implementations, the control system controls the speed of the band  410  and thus already knows the speed of the band without using a speed sensor. Nonetheless, it can be beneficial to include a speed sensor  430  to confirm the speed information. In any case, the speed can be monitored by the control system, which can apply a transfer function (Kb)  445  to the speed signal to determine the angle of the blade. In some implementations, the transfer function Kb is a linear function, e.g., the change in angle is directly proportional to the change in speed. In other implementations, the transfer function Kb is a non-linear function. The exact form of the function can be determined by the temperature and resulting viscosity of the ink on the band  410 . In some implementations, the transfer function uses the shear and temperature dependent viscosity to extract the optimal blade angle based on the pressure generated by the coating speed. 
     For various implementations, to determine precise values to use for ink viscosity and coating speed, various computational modelling programs can be used, such as Computational Fluid Dynamics (CFD) software and/or Finite Element Analysis (FEA) software. For example, for a given ink, CFD software and FEA software can be used to generate a rheological characterization of the ink that shows the shear thinning of the ink and simulation results of the pressure change the ink undergoes when being applied to the band. Various methods can be used to measure the material&#39;s response to changing temperature, time and stress/strain, such as (1) a strain sweep method (the ink&#39;s response to increasing oscillating shear stress is measured at various predefined temperatures while holding frequency constant), (2) a thermal sweep method (the frequency and strain are held constant while the temperature is ramped between two values, e.g., from 70° C. to 140° C. at a rate of 5° C./minute), (3) a frequency sweep method (the time dependence of the ink&#39;s flow properties are measured while the strain and the temperature are held constant), and/or (4) a flow method (the dependence of viscosity on shear rate is measured at various predefined temperatures over a shear rate range, e.g., a shear rate range of 0.1 sec −1  to 1000 sec −1 ). Using such methods and known computer simulation programs, the ink(s) to be used can be analyzed to determine rheological characterizations corresponding to ink properties, such as ink viscosity shear and temperature dependence, which then informs the design of the thermal transfer printer system, as described herein. 
     In addition, an ink thickness sensor  435  observes the levelled ink  425  on the band  410  and provides a data signal to indicate whether the desired thickness is being achieved. The ink thickness sensor  435  can be a laser or ultrasonic sensing device, or any other appropriate device that can achieve the necessary resolution, e.g., a resolution that is at least ten times higher than the desired ink thickness. The desired ink thickness (T) can be received as an input, or be predefined for a given thermal transfer printer, and is used to control the heat added to the heater  440 . The ink monitoring control subsystem  460  implements a closed loop control algorithm using the thickness value feedback from the ink thickness sensor  435 , fed through a filter  450  implementing a transfer function (K t ) and a filter  455  implementing a forward transfer function (K f ). The exact value of the transfer functions K t  and K f  is determine by the mechanical layout of the final printer system and can be adjusted using standard control techniques, which are well understood in the field. The control algorithm can be implemented using electronic circuits or more typically a software algorithm within a control system microcontroller. 
     In some implementations, rather than using an ink feed device that is separate from a rigid blade, the ink feed device  135  and the rigid blade (and potentially the heater as well) can be combined into a single component, such as a slot die.  FIGS. 10, 11A, and 11B  show an example of a system  380  that uses a slot die system  365  with a blade  370 . In such instances, the ink delivery includes a heatable slot die where ink is transferred between an ink reservoir and the slot die via a pump (such as a piston pump). The ink is delivered to a channel  355  in the rigid blade  370  (shown in  FIGS. 11A and 11B ) that terminates in an opening of a slot  378  positioned near to the ink band  105 . The blade  370  has an edge portion  372  and a body portion  374  that when joined together forms a rigid blade that includes a pocket  376  fluidically connected to the channel  355  and to the slot  378  through which ink is delivered to the band  105 . 
       FIG. 11B  shows a view of the edge portion  372  of the blade  370  removed from the body portion  374 . The surface  376 A of the pocket  376  belonging to the edge portion  372  and the surface  378 A of the slot  378  belonging to the edge portion  372  are both visible. 
     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, pressure applied to the band can be used in addition to elevated temperature to sinter the band, such a pressure chamber or physical force on the band with particles. Moreover, the actions recited in the claims can be performed in a different order and still achieve desirable results. Accordingly, other embodiments are within the scope of the following claims.