Patent Publication Number: US-8120631-B2

Title: Thermal printer with reduced donor adhesion

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a Divisional of application Ser. No. 11/747,821 filed May 11, 2007 now U.S. Pat. No. 7,868,906, now allowed, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to methods of printing and printers and in particular to methods of borderless printing and printers for providing borderless prints. 
     BACKGROUND OF THE INVENTION 
     Many photographers use digital cameras to capture images. Unlike conventional wet processing of silver halide film and papers, digital images can be printed directly onto sheets of paper. Color images can be printed using ink jet printers, multicolor transferable toner printers, heat sensitive coated paper printers, or thermal dye transfer printers. Many mass-market retail establishments have user-friendly kiosks where shoppers can make color prints. A large number of these kiosks use thermal dye transfer printers. Because the kiosks use large amounts of paper, such kiosks often use a continuous web of paper for printing such images. The images are later separated from each other and from the web by a suitable cutter or knife. In such kiosk printers, the cutting process is typically performed in a manner that ensures that the printed images provided to the user have dye images extend to the edges of the print in both the latitudinal and longitudinal directions. These prints are known as borderless prints and are the most popular prints. 
     Thermal dye transfer printers generate very high quality images. As such, a number of photographers want their own thermal dye transfer printer. However, it is impractical and not cost effective to supply continuous web paper for use in home printing. It is also expensive to supply built-in paper cutters and knives to provide borderless prints. To attempt to meet the demand for borderless prints, there are known methods of extending the latitudinal edges so that there is no border on the tops and bottoms of prints. See, for example, U.S. Pat. Nos. 5,441,353; 5,196,863; and 5,499,880. However, those techniques cannot provide prints that extend to the longitudinal borders. 
     One approach to providing borderless prints in a sheet fed thermal printer is modeled after the technique used by home printers. In this approach receiver sheets are provided that are pre-perforated at a distance from each longitudinal edge.  FIG. 1  shows a prior art thermal dye transfer printer  2  that is intended to provide monotone, multi-tone or full color borderless printing using a perforated sheet  4  is shown in  FIG. 1 . Printer  2  records images on a sheet  4  that is driven along a print path by a pair of pinch rollers  6   a  and  6   b  connected to a motor  7 . A print head  8  is located opposite a free spinning platen  10  through which sheet  4  is passed during printing. Donor take-up roller  12  and donor supply roller  14  support a donor web  16  of thermal dye donor material and are positioned on opposite sides of print head  8  so that donor web  16  passes between print head  8  and platen  10 . A bias spring  18  presses print head  8  against donor web  16  to urge donor web  16  against platen  10 . 
     Prior to printing, a leading edge  4 . 1  of sheet  4  is fed between rollers  6   a  and  6   b . Rollers  6   a  and  6   b  pull sheet  4  and donor web  16  between thermal print head  8  and platen  10  where thermal print head  8  causes donor material to be transferred to sheet  4 . 
     Sheet  4  is perforated to provide a separable perforated leading portion  4 . 1  and a trailing portion  4 . 3  bordering a central portion  4 . 2 . Donor material, such as a dye, is transferred to sheet  4  such that an image is formed that causes the entire central portion  4 . 2  and that optionally extends to leading portion  4 . 1  and trailing portion  4 . 3  slightly beyond the perforations. When leading portion  4 . 1  and trailing portion  4 . 3  are removed at the perforations, central portion  4 . 2  bears a printed image that extends from edge to edge, and the print appears to be borderless. 
     A key drawback of this solution is the requirement for special paper with perforations on the leading and trailing edges. Such paper is expensive to manufacture and has little or no other market outside of printing digital images. In addition, customers can be dissatisfied with the requirement of tearing off the perforated edges of the printed images. 
     Another approach to providing borderless sheet fed prints is to provide a thermal printer with systems that precisely detect the leading edge of a sheet  4  and that precisely positions a leading edge of a sheet at a print head. However, it will be understood that print lines in thermal printers can be arranged on the order of 300 or more lines per longitudinal inch of a sheet. It also will be understood that even a minor border on the order one print line will not be acceptable as a borderless print. This requires that the sensing and positioning equipment in the thermal printer be very precise, thus raising the cost of the printer. 
     Further, it will be understood that even minor variations in a length of sheet  4 , in the sheet transport systems used to position sheet  4  for printing, or in the equipment used to sense the position of sheet  4  can allow for a portion of sheet  4  to be advanced past print head  8  such that a border will appear. Such variations can also cause the leading longitudinal edge to be positioned before the print head such that the printer will begin printing before the longitudinal edge is positioned to receive the print. When this occurs, the leading edge longitudinal edge will appear to be borderless, however, because the printer begins printing without the leading edge at the print head and because the printer will print a predefined length, this problem creates a risk that the printer will stop printing before the trailing edge of the sheet has reached the print head thus creating a border at a trailing edge. 
     What is needed therefore is a printer and a method for printing that can achieve borderless printing without requiring the use of tear off receiver medium and/or slitting without precision sensing and positioning, and without a border. Such a system should be inexpensive and highly reliable. 
     SUMMARY OF THE INVENTION 
     Thermal printers and methods for operating a thermal printer having a thermal print head with an array of thermal elements position opposite a platen at a printing nip and with a donor web having thermal donor material positioned between the print head and the platen. The thermal elements are adapted to heat the donor web in accordance with received control signals so that donor material can be transferred from the donor web to the receiver medium. In accordance with the method a leading edge of the receiver medium is moved proximate to the printing nip. A sequence of thermal print head control signals is generated that is adapted to cause the array of thermal elements that causes the donor material to transfer from the donor web in a manner that is modulated in accordance with the image data and attenuated in accordance with an attenuation pattern. The leading edge of the receiver medium is urged through the printing nip while the thermal printhead control signals are transmitted to the thermal print head to cause the donor material to transfer from the donor web in an image modulated pattern having a longitudinal length that is larger than a longitudinal length of the receiver medium. The attenuation pattern provides a relatively high level of attenuation at a portion of the printing wherein there is a greater risk that the receiver medium will not be within the printing nip than at a portion of the printing wherein there is a lesser risk that the receiver medium will not be within the printing nip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a mechanical schematic view of a prior art printer; 
         FIG. 2  is a mechanical schematic view of one embodiment of a printer; 
         FIGS. 3A and 3B  are, respectively, schematic views of the apparatus with the receiver medium aligned to a sensor and an enlarged partial view of the receiver medium and the sensor; 
         FIGS. 4A and 4B  are, respectively, views of the next step where the receiver medium is driven to an initial position proximate the heat/print line or the print head including an enlarged partial view of the initial position; 
         FIGS. 5A and 5B  are views of the next step where the print head has moved vertically to clamp the receiver medium between the donor web and the platen with  FIG. 4B  showing a detail of that clamping operation; 
         FIG. 6  shows a step of the process where the receiver medium is ejected from the apparatus; 
         FIGS. 7A and 7B  illustrate a leading web and a trailing web formed by donor adhesion; 
         FIGS. 8A and 8B  illustrate one method for operating borderless thermal printer to achieve borderless printing on a receiver medium with reduced possibility of donor adhesion problems; 
         FIG. 9  shows one embodiment of an attenuation pattern; 
         FIG. 10  shows another embodiment of an attenuation pattern; 
         FIG. 11  shows another embodiment of an attenuation pattern; and 
         FIG. 12  shows another embodiment of a method for operating a thermal printer to achieve borderless printing with reduced possibility of donor adhesion problems. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2-6  illustrate one embodiment of a borderless thermal printer  30 ,  FIGS. 7A and 7B  illustrate a leading web and a trailing web formed by donor adhesion, and  FIGS. 8A and 8B  illustrate one method for operating borderless thermal printer  30  to achieve borderless printing on a receiver medium  20  with reduced possibility of donor adhesion problems. In the embodiment illustrated, borderless thermal printer  30  has a thermal print head  32  with an array of thermal elements  34  positioned opposite a platen  36  at a printing nip  38 . Platen  36  is coupled to a platen stepper motor  40  by a suitable transmission  41  such as a belt. Those skilled in the art understand that  FIGS. 2-6  are schematic in nature and other suitable structure are possible for connecting platen stepper motor  40  to platen  36  in order to turn platen  36 . Such other structures include and are not limited to gear trains, and transmissions. A control circuit  42  sends suitable signals to control the operation of platen stepper motor  40  in a manner that will be described in greater detail below. 
     The array of thermal elements  34  is adapted to heat a donor web  44  in accordance with control signals received from control circuit  42  and to heat in response thereto so that thermally transferable donor material can be transferred from donor web  44  to receiver medium  20 . 
     Donor web  44  is supplied by a donor supply roller  46  on one side of the thermal print head  32  and collected by a donor take-up roller  48  on an opposite side of thermal print head  32 . As will be discussed in greater detail below, during printing, donor web  44  travels across the array of thermal elements  34  and is wound on a donor take-up roller  48 . Donor web  44  carries thermally transferable donor material such as dyes, colorants, protective coating and/or metallic or other materials. Donor web  44  can comprise a single color of thermally transferable donor material for monotone printing, but it preferably comprises at least three sequential sections of differently colored thermally transferable donor material in order to provide full-color print and a clear section for applying a protective cover on the print. In a conventional fashion, thermal printer  30  has a bias spring  53  that urges thermal print head  32  toward platen  36 . 
     As is shown in  FIG. 2 , borderless thermal printer  30  has a receiver medium advance system  50  that is adapted to move a leading edge  22  of receiver medium  20  between a loading position, for example, in receiver supply  23  and a print start position. In the embodiment of  FIGS. 2-6 , receiver medium advance system  50  comprises a pick roller  52 , surface guides  54 ,  56 , entrance urge rollers  58 ,  60 , urge stepper motor  62 , transmission  64 , and print head guide  65 . 
     Entrance urge rollers  58 ,  60  are disposed at one end of surface guides  54 ,  56 . Entrance urge rollers  58 ,  60  are biased together by a suitable spring or other biasing structure, not shown, so that rotary motion imported to one roller is transmitted to the other. Entrance urge roller  58  is a pinch roller and entrance urge roller  60  is driven by an urge stepper motor  62  and a transmission  64 . Transmission  64  is shown as a belt but may be any suitable transmission known in the art that can be used to connect the rotary motion of stepper motor  62  to entrance urge roller  60  including, but not limited to, a gear train or transmission. In operation, receiver medium  20  is advanced by urge rollers  58  and  60  through a nip between urge rollers  58 ,  60  and is then supported by print head guide  65 . Print head guide  65  directs leading edge  22  to printing nip  38  as urge rollers  58  and  60  of receiver medium advance system  50  continue to advance receiver medium  20 . Entrance urge rollers  58 ,  60  can be permanently engaged or can be selectively engaged. To selectively engage entrance urge rollers  58 ,  60 , an entrance urge roller  58  can be spring biased away from urge roller  60  and an actuator (not shown) controlled by control circuit  42  is operable to move urge roller  58  or entrance urge roller  60  into or out of engagement with each other. 
     As is shown in  FIGS. 3A and 3B , an edge sensor  70  is positioned relative to print head guide  65 . Edge sensor  70  can be, without limitation, an optical, mechanical, electromechanical, electromagnetic or a combination of optical, mechanical, electromechanical or electromagnetic device that at least senses receiver medium  20 . Edge sensor  70  is coupled to control circuit  42  and generates a sensor signal from which control circuit  42  can determine when leading edge  22  and, optionally, trailing edge  24  of receiver medium  20  are advanced through an area sensed by edge sensor  70 . Any conventional edge sensor  70  can be used including a wide variety of sensors that are well-known in the printing and photocopying art. In addition, edge sensor  70  may be combined with a suitable gate (not shown) or other signal modifying, amplifying or attenuating circuits so the sensor signal is provided to control circuit  42  in a manner that is readily usable thereby. Those skilled in the art understand that in this embodiment receiver medium  20  has its lateral sides aligned and deskewed so that leading edge  22  of receiver medium  20  is transverse to a path  43  of travel and is substantially aligned parallel to the linear array  34 . 
     As is shown in  FIGS. 4A and 4B , when edge sensor  70  generates a signal indicating that leading edge  22  of receiver medium  20  has been detected, control circuit  42  drives urge stepper motor  62  a predetermined number of steps. This causes urge rollers  58 ,  60  to drive receiver medium  20  toward array of thermal elements  34  and to stop receiver medium  20  with its leading edge  22  at an initial position that is a distance D from the array of thermal elements  34 . The initial position is close enough to array of thermal elements  34  so that leading edge  22  of receiver medium  20  will be captured between thermal print head  32  and platen  36 . In  FIG. 4B , leading edge  22  of receiver medium  20  is shown at location D that is approximately 0.020 inches (0.0368 cm) from the array of thermal elements  34 , however, the separation can be greater or lesser as desired. 
     After receiver medium  20  is in the initial position, control circuit  42  causes a motor (not shown) to drive thermal print head  32  downward in the direction of arrow  53  in order to clamp receiver medium  20  and donor web  44 , between thermal print head  32  and platen  36 , as is shown in  FIGS. 5A and 5B . With receiver medium  20  in the initial position, control circuit  42  actuates platen stepper motor  40  such that receiver medium  20  and donor web  44  driven past array of thermal elements  34 , while array of thermal elements  34  are energized in an imagewise fashion so as to transfer thermally transferable donor material from donor web  44  to receiver medium  20  to record an image on receiver medium  20 . 
     As shown in  FIG. 6 , after receiver medium  20  passes array  34 , a peel plate  72  sharply alters the direction of donor web  44  relative to receiver medium  20  to separate donor web  44  from receiver medium  20  as receiver medium  20  continues to travel in generally the same direction or until it is redirected in another direction. In one embodiment, receiver medium  20  can continue such travel along an exit path that causes receiver medium  20  to pass out of printer  30  as a completed printed image. 
     In the embodiment illustrated, receiver medium  20  passes into a receiver medium return system  80  after donor web  44  is separated from receiver medium  20 . Receiver medium return system  80  comprises an arrangement of guides, gates, rollers or other structures that is responsive to control circuit  42  and that can direct receiver medium  20  along a return path  82  that returns receiver medium  20  to a position that allows receiver medium advance system  50  to reload receiver medium  20  so that additional images can be printed thereon or to an exit path  84 . 
     As shown in the embodiment of  FIGS. 2-6 , receiver medium return system  80  comprises exit guides  86 ,  88  which lead receiver medium  20  from printing nip  38  and into the nip of exit urge rollers  90  and  92 . Exit urge rollers  90  and  92  are likewise driven to rotate by an actuator (not shown) that is under control of control circuit  42 . Exit urge rollers  90  and  92  are operable to rewind and feed receiver medium  20  back toward urge rollers  58 ,  60 . This can be done to rewind receiver medium  20  so that it is reloaded for a second printing, for example, to enable multicolor printing and/or laminating. In other embodiments, a return system  80  can provide a recirculating type return path (not shown) that guides leading edge  22  from exit urge rollers  90 ,  92  to entrance urge rollers  58 ,  60  without passing receiver medium  20  through printing nip  38 . A wide variety of recirculating return paths of this type are known in the printing, scanning, and photocopying arts. Examples of such recirculating return paths are illustrated in, for example, co-pending U.S. application Ser. No. 11/176,147 entitled “Printer with Multi-Pass Media Transport” filed by Cloutier et al. on Jul. 7, 2005; or U.S. Pat. No. 5,838,357 to Maslanka et al., issued Nov. 17, 1998, entitled “Thermal Printer Which Uses Platen to Transport Dye Donor Web Between Successive Printing Passes”; U.S. Pat. No. 5,798,783 to Maslanka et al., issued Aug. 25, 1998, entitled “Thermal Printer with Sensor for Leading Edge of Receiver Sheet”; U.S. Pat. No. 5,841,460 to Maslanka et al., issued Nov. 24, 1998, entitled “Thermal Printer which Recirculates Receiver Sheet Between Successive Printing Passes”; and U.S. Pat. No. 5,850,246 to Maslanka et al., issued Dec. 15, 1998, entitled “Thermal Printer with Improved Print Head Assembly”. After printing is completed, control circuit  42  sends signals to an actuator (not shown) that cause exit urge rollers  90 ,  92  discharge receiver medium  20  along an exit path  84  into a discharge bin  94 , as shown in  FIG. 2 . 
     It will be appreciated that receiver medium advance system  50  provides relatively precise location of leading edge  22  of receiver medium  20  in that edge sensor  70  can sense leading edge  22  of receiver medium  20  with a great deal of accuracy and in that stepper motors can be used to precisely drive sets of exit urge rollers  90 ,  92  and platen  36  to move receiver medium  20  by an initial amount from the detected position. In this way, thermal-dye transfer material may be transferred from leading edge  22  of receiver medium  20  to trailing edge  24  of receiver medium  20 . However, it will be understood that in other embodiments, any other conventional system known for positioning a receiver medium  20  proximate to an array of thermal elements  34  can be used. 
     It will also be appreciated, however, that even with the most precise placement of leading edge  22 , minor variations in the length of receiver medium  20 , the receiver medium advance system  50  used to position receiver medium  20  for printing, platen  36 , transmission  41  or platen stepper motor  40 , or in the equipment used to sense the position of the receiver medium can cause receiver medium  20  to be positioned at the start of printing such that multiple lines of thermal donor material will be printed before leading edge  22  of receiver medium  20  enters printing nip  38  between thermal elements  34  of thermal print head  32  and platen  36 , thus creating a risk of thermal adhesion of the thermally transferable donor material to platen  36 . As is conceptually illustrated in  FIG. 7A , this can cause a section of thermally transferable donor material to be transferred to platen  36  at the start of printing creating a leading edge web  138  of molten, semi-molten or form of donor transfer material between platen  36  and donor web  44  which either interferes with the movement of receiver medium  20  as it enters printing nip  38  by blocking movement of receiver medium  20  through printing nip  38  or creates unintended image artifacts in an image printed using receiver medium  20 . Similarly, the same factors can cause printing to continue after all of receive medium  20  has passed through printing nip  38 , forming a trailing web  148  between donor web  44  and platen  36 , which can create similar interference or image artifacts. 
     To avoid these outcomes a thermal printer  30  and method for operating a thermal printer  30  are provided that can reduce the possibility of thermal donor adhesion, such as might cause a leading edge web  138  to form while still providing borderless prints. 
       FIG. 8  illustrates one embodiment of a method for operating the borderless thermal printer  30  of  FIGS. 2-6  to reduce the possibility of thermal donor adhesion. As is illustrated in  FIG. 8 , the printing process begins when thermal printer  30  receives a print order from an interface  100  (step  122 ). The print order provides instructions sufficient for control circuit  42  to begin a print sequence and can include an instruction to print an image, the image data for the image to be printed, print quantity information or information identifying a selected receiver medium  20  upon which the image is to be printed. The print order can also contain other information including but not limited to as delivery date, delivery destination information, consumer information, and point of sale information. 
     Interface  100  can incorporate any known circuits or systems that are capable of receiving a print order or data forming part of the print order. By way of example, and not limitation, in the embodiment illustrated in  FIGS. 2-6 , interface  100  has a user input system  102 , a communication system  104 , and a memory reader  106  that can be used for obtaining information forming all or at least a part of a print order. 
     User input system  102  can comprise any form of transducer or other device capable of receiving an input from a user and converting this input into a form that can be used by control circuit  42 . For example, user input system  102  can comprise a touch screen input, a touch pad input, a 4-way switch, a 6-way switch, an 8-way switch, a stylus system, a trackball system, a joystick system, a voice recognition system, a gesture recognition system or other such systems. In the embodiment illustrated in  FIG. 2 , user input system  102  includes a keypad  108  and mouse  110  for receiving input from a user. A display  112  is connected to control circuit  42  and provides information and feedback to the user to facilitate user input actions and for other purposes. 
     Communication system  104  is adapted to enable communications between thermal printer  30  and an external devices, networks and systems. Examples of such external devices include but are not limited to local, regional and international data and telecommunication networks, computers, databases, printers, cameras, cellular phones, personal digital assistants, internet appliances, the internet and any associated devices, televisions, assistive technology devices and any other communication, data or other devices that can be used to generate, process, edit, distribute or otherwise send or receive data that can be related to a print order or other function that is performed by thermal printer  30 . 
     Communication system  104  can be for example, an optical, radio frequency or transducer circuit or other system that converts image and other data into a form that can be conveyed to such external devices such as a remote memory system by way of an optical signal, radio frequency signal or other form of signal. Communication system  104  provides control circuit  42  with information and instructions from signals received thereby. 
     In the embodiment of  FIGS. 2-6 , memory interface  118  comprises a memory card slot  114  that holds a removable memory  116  such as a removable flash card or other form of memory card or memory device and has a removable memory interface  118  for communicating with removable memory  116 . Data including but not limited to control programs, digital images and metadata can also be stored in a remote memory system. In other embodiments, removable memory  116  can take other forms such as, a removable optical, magnetic or other disk memory (not shown). 
     Each print order generally provides information from which control circuit  42  can determine what images are to be printed, how the images are to be printed and the quantity of each of the images that is to be printed. A print order can be associated with digital image data representing the image to be printed and instructions for printing such an image. However, other print orders can be associated with digital image data by providing reference information instead of digital image data with the reference information being useable by control circuit  42  to obtain digital image data from an external source such as by using communication system  104  or memory interface  118 . In some cases, the printing instructions can be provided in the form of digital print order format (DPOF) data that allows a user of a digital camera or other type of display device to define which of a set of stored images are to be printed, and can also provide information that identifies number of copies or other image information that can be used in fulfilling a print order. 
     Control circuit  42  processes the print order data to determine what images to print and in what manner. In this regard, control circuit  42  processes non-image data in the print order to determine factors such as quantity information, print type information, enlargement or reduction factors, collation information and the like (step  124 ). 
     Control circuit  42  then obtains digital image data for each image in the print order (step  126 ). As is noted above, such image data is typically transmitted with the print order data and can be obtained therefrom. In certain circumstances, the print order data provides information indicating how the digital image data can be obtained, such as by providing address and, optionally, access information, allowing such data to be downloaded from a removable memory  116  or from a source connected to borderless thermal printer  30  by way of communication system  104 . 
     Control circuit  42  then determines control signals that are adapted to cause thermal printer  30  to print borderless images on receiver medium  20 . To do this control circuit  42  generates control signals for array of thermal elements  34  that cause the array of thermal elements  34  to radiate heat as necessary to cause donor material to transfer from donor web  44  to receiver medium  20  to form a printed image on the receiver medium  20 . Typically, such control signals are based upon the image data for the image to be printed and any print order data indicating a print size, shape or orientation, and include control signals enabling the printing of an image having a longitudinal length that is longer than a longitudinal length of the receiver medium. For the purposes of the examples herein it will be assumed that the print order calls for a borderless printing of a single image. 
     As is noted above, to ensure that such borderless printing occurs, receiver medium  20  is positioned with a leading edge  22  separated from printing nip  38  by a distance D, thus allowing printing to begin just before leading edge  22  reaches nip, so that it is certain that leading edge  22  will receive donor material deposited in an imagewise fashion. 
     However, as is illustrated in  FIG. 7A , this creates a risk that donor material will be applied to platen  36  in a fashion that allows such donor material to adhere to platen  36  and create leading edge web  138  of molten and/or semi-molten donor material between platen  36  and donor web  44 . Where such a leading edge web  138  is formed from a substantial amount of donor material, web  138  can impede or block receiver medium  20  from passing through printing nip  38  or can create unwanted image artifacts. Accordingly, in the embodiments described a step of forming attenuated control signals (step  128 ) comprises forming such signals in accordance with an attenuation pattern that reduces the amount of or density of thermal donor material to minimize the risk of forming a type of leading edge web  138  that can impede or block the transit of receiver medium  20  through printing nip  38 . 
     In the embodiment of  FIG. 8A , control circuit  42  performs step  128  by first converting the digital image data into control signals that can be used by thermal print head  32  (step  130 ) and then attenuating the control signals according to an attenuation pattern (step  132 ). 
     In this embodiment, this step of converting the digital image data into control signals (step  130 ) involves converting the digital image data into printer code values or other data types that represent specific colors to be printed on receiver medium  20  to form an image. This is typically done in accordance with so-called calibration information that provides a logical association between the colors called for in the image data and printer code values that are assumed to cause such colors to be printed by thermal print head  32 . Such calibration information can also include information that printer  30  can use in determining printing actions to be taken in response to particular code values. 
     The calibration information can be predetermined using calibration data that is established during an initial set up phase at a manufacturer&#39;s facility or elsewhere. However, because many aspects of printing, particularly color printing, are influenced by environmental conditions, printing process variations, and donor and receiver material variations, it is understood that, from time to time, it may be useful to recalibrate the initial printer settings to ensure that the colors that are printed correspond to colors called for in the print data. Accordingly, the process of converting digital image data into code values can adapt to such conditions by recalibrating printer settings according to feedback from using sensors that monitor such conditions, feedback from sensors that monitor the colors printed in response to particular code values or using manual feedback systems. 
     During printing, the code values are converted into control signals which govern the extent to which thermal elements in array  34  are energized. The thermal elements of array  34  radiate heat in proportion to the extent to which they are energized and the thermal donor material transfers from donor web  44  in proportion to the amount of heat applied thereto. Accordingly, in a conventional thermal printer, an image is formed using the thermal donor materials by supplying control signals to the thermal print head  32  as the donor web  44  and receiver medium  20  are moved by the platen to advance the receiver sheet and the donor web past array of thermal elements  34 . 
     In the embodiment illustrated in  FIG. 8A , the control signals are generated on a line by line basis and, during printing, control circuit  42  transmits a first line of control signals, then control circuit  42  transmits signals to platen stepper motor  40  causing platen stepper motor  40  to rotate in a manner that advances platen  36  by a distance that is intended to position receiver medium  20  so that the array of thermal elements  34  can print a second print line. Control circuit  42  then sends a second line of control signals to array of thermal elements  34 . Printing continues in this manner until all print lines for the image have been printed. 
     However, in the embodiment of  FIG. 8A , before the image based control signals are used for printing, control circuit  42  attenuates the control signals to lower the density of the printing according to an attenuation pattern. 
       FIG. 9  illustrates one embodiment of this attenuation pattern  140 . In the attenuation pattern of  FIG. 9 , the extent of attenuation indicated by attenuation pattern  140  is relatively high during a first few lines of printing and can be as high as 90% or more in during these lines, however, the extent of attenuation is decreased as additional steps are taken. This reflects, in a general way, the probability that leading edge  22  of receiver medium  20  will be positioned in printing nip. More specifically, it will be appreciated that, a borderless thermal printer  30  using receiver medium advance system  50  that anticipates positioning receiver medium  20  with a leading edge  22  separated from printing nip  38 , it is unlikely that leading edge  22  will be positioned at printing nip  38  when the first line of the image is printed. Thus, the attenuation is greatest at the first line of printing of the image and is reduced as printing continues. 
     However, with each step that platen stepper motor  40  causes platen  36  to take, it becomes more likely that leading edge  22  of receiver medium  20  will enter printing nip  38 . Accordingly, attenuation pattern  140  provides a density decreasing the extent of attenuation with the extent of urging supplied by platen  36  and platen stepper motor  40  until the extent of the attenuation reaches to a minimum level of attenuation, shown in  FIG. 9  as a 0% level of attenuation. It will be appreciated that using such an approach ensures that even where some donor material is transferred onto platen  36  during the first line(s) of printing, the amount of donor material transferred will be minimized so that, if necessary, leading edge  22  can be thrust through leading edge web  138  by the action of platen  36  to avoid blockage and with minimal risk of unwanted image artifacts. 
     The attenuated control signals are then used for printing as generally described above (step  134 ). 
     In the embodiment of  FIG. 8B , control circuit  42  performs step  128  by attenuating the digital image data for the image to be printed according to an attenuation pattern (step  136 ) and then converts the attenuated digital image data into attenuated control signals that can be used by thermal print head  32  (step  138 ). In this embodiment, the step of attenuating the digital image data comprises determining which portions of the digital image data represent portions of the image that will be printed during the first few lines of printing and adjusting the density of the colors called for in those lines according to the attenuation pattern. The attenuated image data is converted to create attenuated control signals in the same fashion that is used to convert any form of digital image data into control signals, such as the fashion described above with respect to step  130 . 
     It will also be appreciated that, in order to ensure that a digital image is printed at a trailing edge  24  of receiver medium  20 , it is useful to continue printing the image data such that it can be anticipated that printing of the image will continue so that all portions of receiver medium  20 , of any length or that is positioned within any range of positions relative to the initial position, will be printed. Thus, there exists a possibility that a trailing web  148  of donor material can be formed between platen  36  and donor web  44  under certain circumstances. This can interfere with the movement of subsequent receiver mediums through printing nip  38 . Accordingly, attenuation pattern  140  can provide for a trailing edge density ramp down, such as is illustrated in  FIG. 10 , during which the control signals are attenuated in an increasing fashion to reach a relatively high level of attenuation as the printing of the image nears an end. This density ramp down can be initiated when control circuit  42  receives signals from edge sensor  70  indicating that trailing edge  24  has passed edge sensor  70  or at some predetermined number of steps of movement of receiver medium  20  after trailing edge  24  has passed edge sensor  70 . 
     Here too, attenuation pattern  140  is generally based upon the probability that a particular line will be printed with no receiver medium  20  positioned at printing nip  38 . As this probability increases, the extent of the density ramp down is increased. Printing is then performed using the attenuated control signals (step  132 ). 
     The attenuation patterns, illustrated in  FIGS. 9 and 10 , are exemplary only and a wide variety of other attenuation patterns may be used. A variety of factors can be used to determine an attenuation pattern for a material. 
     For example, and without limitation, it will be appreciated that different types of donor material may exhibit different tendencies to form a leading edge web  138  or a trailing web  148 . Donor material that has a tendency to deposit more material per level of heat applied may have a propensity to form a leading edge web  138  or a trailing web  148  at lower temperatures than a donor material that has a tendency to deposit less material per level of heat applied. For example, in color printing where donor web  44  carries a plurality of different types of differently colored donor material, a darker color may comprise donor material that is denser or that applies more donor material than a lighter colored material. Similarly, donor material that has metals in it may be denser than donor material that does not have metals in it. In such cases, an attenuation pattern  140   a , such as the one illustrated in  FIG. 11 , can be used for the donor material that deposits more material per level of heat applied, while a different attenuation, pattern  140   b , is used for donor materials that deposit less donor material per unit area. As can be seen, attenuation pattern  140   a  provides a greater extent of attenuation as well as a more gradual rate of attenuation ramp down. 
     It will be appreciated that other factors related to donor material type can also have a similar influence, such as a known viscosity or cooling rate of the donor material, inherent adhesive properties if any of the donor material, the type of material used to support donor material on donor web  44 , or any other known materials or properties of donor web  44  or the donor material thereon. Properties of platen  36 , such as cooling rate, adhesion characteristics, the shape and size of platen  36 , and other factors, may also influence the attenuation pattern. 
     Further, it will be appreciated that during printing different environmental conditions can increase probability of forming a leading edge web  138  or trailing web  148  that impede or block the transit of receiver medium  20 . For example, when platen  36  is colder, donor material can cool faster and is more likely to adhere to platen  36  in a manner that creates problems. In contrast, when platen  36  is warmer there is less likelihood of such adhesion, accordingly as shown in  FIG. 12 , where it is determined that platen  36  is or should be warm such as where a temperature sensor (not shown) indicates that platen  36  is warm or when there have been a significant number of printed images within a period of time to warm platen  36 , attenuation patterns  140   a  and  140   b  can be adjusted, to allow for lesser or greater attenuation based upon such factors. 
       FIG. 12  shows the embodiment of  FIG. 8A , with the generating step  128  further comprising optional additional steps of determining whether an attenuation condition is met, suggesting that an attenuation pattern should be used. In one example, it is determined that the attenuation condition is met when leading (or trailing) edge of the image to be recorded on receiver medium  20  has image elements that require high density printing (step  137 ). Typically, this determination_can be made by analyzing the density of the colors identified in the digital image data for the leading edge or trailing edge of an image to be printed, by analyzing code values derived from the digital image data, or by analyzing control signals generated for such areas. As noted above, during the printing in these areas there is a higher risk that at least some printing will occur with no receiver medium  20  present between the array of thermal elements  34  of thermal print head  32 . 
     The purpose of this analysis is to identify when the printing of an image may occasion the delivery of sufficient amounts of thermal donor material proximate to a leading or trailing edge of the image to create a risk of the formation of a leading edge web  138  or trailing web  148 . This analysis can be performed by totaling the digital image data, code values, or control signals, by calculating an average, for regions of the image proximate to a longitudinal edge. Where this is done a threshold is set as a total above which it is determined that attenuation is to be performed and below which no attenuation is performed. Similarly, statistical means, or other statistical analysis of the digital image data, code values, or control signals can be performed with the outcome thereof compared to a threshold. 
     Alternatively, this can be done by comparing a pattern of threshold values to a pattern of digital image data, code values or control signals representing areas proximate to an edge of receiver medium  20 , such as areas that are printed at a beginning portion of the image or as the printing of the image nears an end. It will be appreciated that any other know statistical, mathematical, neural or other form of analysis can be used in order to analyze the digital image data, code values or control signals for portions of the image that are proximate to an edge of the image to be printed and to characterize the amount of donor material that will be conveyed to print such portions such that a determination can be made as to whether an attenuation condition is met. Such a determination can be made for each donor patch, or for the printed images as a whole. Such a determination can be made based upon image data, code values or control signals for portions of the image to be printed that are proximate to a leading edge of the image, proximate to a trailing edge of the image to be printed or both. 
     As is noted above, printing conditions may influence the potential for a leading edge web  138  or a trailing web  148  to form. Such printing conditions can be used to adjust a threshold used in the determining step (step  137 ), or to adjust the analysis performed during the determining step (step  137 ). For example, for some printers and donor material, where the temperature of the printer is higher then the threshold can be lowered in that there is a lesser chance of donor material adhering to platen  36  at such higher temperatures. 
     Where it is determined that no such high density printing condition is called for (step  137 ) the attenuation pattern is not applied to the control signals and the control signals (step  139 ) are used for printing (step  134 ). Where it is determined that such a high density printing condition is called for the steps of generating attenuated control signals (step  128 ) and printing using the attenuated control signals (step  134 ) can be performed. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                   
                  4 
                 sheet 
               
               
                   
                  4.1 
                 leader portion or leading edge 
               
               
                   
                  4.2 
                 central portion 
               
               
                   
                  4.3 
                 trailing portion 
               
               
                   
                  6a 
                 pinch roller 
               
               
                   
                  6b 
                 pinch roller 
               
               
                   
                  7 
                 motor 
               
               
                   
                  8 
                 print head 
               
               
                   
                  10 
                 platen 
               
               
                   
                  12  
                 donor roller 
               
               
                   
                  14  
                 supply roller 
               
               
                   
                  16  
                 donor web 
               
               
                   
                  18 
                 bias spring 
               
               
                   
                  20 
                 receiver medium 
               
               
                   
                  22 
                 leading edge of receiver 
               
               
                   
                  23 
                 receiver supply 
               
               
                   
                  24  
                 trailing edge of receiver 
               
               
                   
                  30 
                 thermal printer 
               
               
                   
                  32 
                 thermal print head 
               
               
                   
                  34 
                 linear array of thermal elements 
               
               
                   
                  36 
                 platen 
               
               
                   
                  38 
                 printing nip 
               
               
                   
                  40 
                 stepper motor 
               
               
                   
                  41 
                 transmission 
               
               
                   
                  42 
                 control circuit 
               
               
                   
                  43 
                 path of travel 
               
               
                   
                  44 
                 donor web 
               
               
                   
                  46 
                 donor supply roller 
               
               
                   
                  48 
                 donor take-up roller 
               
               
                   
                  50 
                 receiver medium advance system 
               
               
                   
                  52 
                 pick roller 
               
               
                   
                  53 
                 bias spring 
               
               
                   
                  54 
                 surface guide 
               
               
                   
                  56 
                 surface guide 
               
               
                   
                  58 
                 entrance urge roller 
               
               
                   
                  60 
                 entrance urge roller 
               
               
                   
                  62 
                 urge stepper motor 
               
               
                   
                  64 
                 transmission 
               
               
                   
                  65 
                 print head guide 
               
               
                   
                  70 
                 edge sensor 
               
               
                   
                  72 
                 peel plate 
               
               
                   
                  80 
                 receiver medium return system 
               
               
                   
                  82 
                 return path 
               
               
                   
                  84 
                 exit path 
               
               
                   
                  86 
                 exit guides 
               
               
                   
                  88 
                 exit guides 
               
               
                   
                  90 
                 exit urge roller 
               
               
                   
                  92 
                 exit urge roller 
               
               
                   
                  94 
                 discharge bin 
               
               
                   
                 100 
                 interface 
               
               
                   
                 102 
                 user input system 
               
               
                   
                 104 
                 communication system 
               
               
                   
                 106 
                 memory reader 
               
               
                   
                 108 
                 keypad 
               
               
                   
                 110 
                 mouse 
               
               
                   
                 112 
                 display 
               
               
                   
                 114 
                 memory card slot 
               
               
                   
                 116 
                 memory 
               
               
                   
                 118 
                 memory interface 
               
               
                   
                 122 
                 receive print order step 
               
               
                   
                 124 
                 process print order step 
               
               
                   
                 126 
                 obtain image data step 
               
               
                   
                 128 
                 form attenuated control signals step 
               
               
                   
                 130  
                 convert image data into control signals step 
               
               
                   
                 132  
                 attenuate control signals step 
               
               
                   
                 134  
                 print using control signals step 
               
               
                   
                 135  
                 convert attenuate digital data into control signals step 
               
               
                   
                 136  
                 attenuate digital image data step 
               
               
                   
                 137  
                 determine high density printing step 
               
               
                   
                 138 
                 leading edge web 
               
               
                   
                 139 
                 print using control signals step 
               
               
                   
                 140  
                 attenuation pattern 
               
               
                   
                 140a 
                 attenuation pattern 
               
               
                   
                 140b 
                 attenuation pattern 
               
               
                   
                 142  
                 attenuation pattern 
               
               
                   
                 148  
                 trailing web