Abstract:
A thermal transfer printer having a print head and a media support platform that is displaced relative to the print head, the media support platform having a rigid support surface for a media item having a non-rectangular configuration such as a compact disk, the support platform having a mask with a cutout substantially in the shape of the non-rectangular disk, the mask and media item combining to form a contact surface for the print head to uniformly distribute a constant force of the print head in a uniform pressure across the mask and media item during printing, the mask providing, in addition, a holding apparatus for the media item which is contacted by a displaceable retainer pin urging the media item against the edge of the mask with the print head avoiding contact with the retainer pins on the printing area, the printer having a mechanism to displace the retainer pin and sense whether a media item is properly placed in the cutout and retained by the retaining apparatus.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a thermal transfer printer for printing on the surface of a compact disk, particularly a recordable compact disk known as a CD-R disk. 
     The invention optimizes printing on irregularly shaped media and incorporates features to prevent damage to the thermal transfer print head. 
     Compact disks are an inexpensive medium for storing digital information that may relate to audio, video and/or any type of information or data that is conveniently stored in digital form. When compact-disks are manufactured in large quantities, the side opposite the recording side of the disk is customarily printed in a mass printing process such as silk screening. The label information applied to the disks is generally identical for each disk and related to the pre-recorded content of the disks. 
     With the development of the CD-R disk, disks can be sold in blank with the informational content later recorded by a CD-R recorder. In order to appropriately label such disks with regard to the content that is recorded on the disk, programmable disk printers, such as ink jet printers and thermal transfer printers have been devised. These printers print the surface of the disk with graphics and other information that can be customized to correspond to the information recorded on the disk by the CD-R recorder. One drawback in using an inkjet printer is the extended time required to print an individual disk. Another drawback is the additional expense of disk blanks which require a precoated surface for inkjet printing. 
     Thermal transfer printers can print with greater speed and print on disk blanks prepared with an inexpensive lacquer coating. Thermal transfer printers include a print head that applies a contact pressure to the media to be printed. 
     One type of thermal transfer printer will typically consist of a mechanism that has a stationary print head, a ribbon, and assembly that moves the media under the print head. The print head contains an array of heating elements. The ribbon is a plastic film with a wax or resin compound deposited on one side. The print head is in contact with the ribbon during printing, and the ribbon is in contact with the media. 
     By heating the areas of the ribbon, the wax or resin compound is deposited on the media. Printing occurs by moving ribbon and the media at the same rate across the print head, while firing the heating elements in a desired pattern. The print head must exert some pressure on the media for successful transfer of the wax or resin to the media. 
     A second type of thermal printer is a direct transfer printer, which uses thermally sensitive media that changes color when heated, therefore a ribbon is not required. With thermally sensitive media, the print head marks the media by generating a pattern of heated and non-heated areas on the surface of the media, as it moves under the print head. The invention described is applicable to-both types of thermal printers. 
     Thermal transfer printers require the print head to contact the printable surface at a uniform pressure for optimum transfer of wax or resin from a ribbon to the media (or heat in the case of direct thermal transfer printer). Variations in print head pressure to the media result in improper printing on media such as non-printed areas or uneven print density. 
     Printing on rectangular objects, such as a piece of paper is relatively straight forward, since the print head pressure remains constant during the entire printing process. The pressure remains constant because the area of contact between the print head and the media does not change. For example, in printing a 5″ wide piece of paper the print head is always in contact with 5″ of media. In contrast, printing on an 5″ diameter disk, the area of contact would initially be very small as the print head is at the edge of the disk, but then increases to 5″ as the print head crosses the center of the disk. After crossing the center of the disk, the area of contact decreases as the print head travels the far edge of the disk. 
     When the force of the print head applied to the media is constant and the print head travels across a rectangular shaped media, the pressure per unit area is constant. If the print head travels across a disc shaped media, the print head pressure to the media will change as the print head travels across the disc. When the force of the print head applied to the media is constant and the print head travels across a disk shaped media the pressure per unit area changes as the contact area increases and decreases. 
     To successfully, print on disc shape media, the printer must be constructed to either: 
     a) vary the force of the print head applied to the media as it travels across the disc to compensate for the variation in width of printable surface, or 
     b) hold the disc in a manner that effectively presents an unchanging width of contact area for the print head as it moves across the disc. 
     The process described in point a) can be achieved by using a complicated system of cams, gears, and sensors. 
     The process described in point b) can be achieved by using a simple system based on the invention that incorporates a media holding tray that puts the print head in contact with the media and a supplemental surface. The combination of the surfaces which are in contact with the print head present a surface of uniform width (width that does not change as the disc is printed). This supplemental surface comprises a mask, that has a thickness and structural characteristics that are substantially the same as the media. 
     The invention described below consists of a thermal printer that utilizes a tray type of media holder with materials arranged in such a manner as to maintain a uniform print head pressure to media as the media moves relative to the print head. 
     The media to be printed is placed in the media tray which consists of a base layer of compressible material (mounted on either a platform or platen) and a second mask layer of material similar to the thickness and composition of the media. The mask layer has a cutout in which the media is positioned. This arrangement allows the printable surface of the media to be at the same level as the unmasked areas of the compressible surface. 
     The key feature of this arrangement is that as the print head passes over the media, the area of contact between the print head and the sum of the areas of the media and the surface of the media holder remains constant. This results in uniform (unchanging) print head pressure on the media during the entire printing process. 
     By careful selection of the materials of the media holder, the proper print head to media pressure can be maintained without the use of complex print head pressure control systems. In addition, proper print head pressure can be maintained when printing odd shaped, non-rectangular media, such as disc shaped objects, where the print head&#39;s area of contact with the media varies as the print head moves relative to the disc. 
     The base layer (compressible surface) and the mask layer (surface with cutout area in the shape of the media) may have one of more layers of material, so long as the surface of the mask layer has similar mechanical characteristics to the item being printed. 
     A typical composition of the base layer would consist of a material that compresses to the appropriate degree needed to maintain proper print head pressure distribution on the media. The preferred embodiment for the disc printing application would require a base layer material that has a compression value of 40-70 durometer which could include materials such as neoprene and other rubber-like substances. 
     A typical configuration of the mask layer would consist of a material that does not compress or has the same compression characteristics as the media. The preferred material for the mask layer of the disc printing application is a non-compressible material such as polycarbonate. CD-ROM and CD-R discs are typically made from molded polycarbonate. 
     SUMMARY OF THE INVENTION 
     The thermal transfer printer of this invention is designed to print on non-rectangular media, and in particular, on disk-shaped media, such as a compact disk. The invented printer resolves the problem of printing with a uniform pressure across irregular-shaped media. 
     The thermal transfer printer of this invention includes a carrier having a flat media support surface with a resilient base layer and a top mask layer. The top mask layer has a media mask with a cutout having a configuration that matches the configuration of the media item to be printed. The media mask is fabricated from a material having physical and structural characteristics that are substantially the same as the media item being printed. Additionally, the media mask has a thickness that matches the thickness of the media item. 
     In this manner, the thermal contact element in the print head of the thermal transfer printer distributes its contact force across both the media item and the mask. The resulting pressure per unit area applied to the media item thereby remains constant during each advance of the carrier relative to the contact edge of the print head. 
     Additionally, the thermal transfer printer of this invention includes an improved retaining mechanism to retain a media item in position during the printing process. The retaining mechanism is designed to avoid damage to the fragile thermal resistors forming the linear array of pixel generating elements in the contact edge of the print head. 
     The retaining mechanism includes a retainer pin that is activated to hold the media item against the edge of the media mask. In the case of a compact disk having a circular perimeter, the mask includes two small edge protuberances that project into the complimentary circular-shaped cutout area of the mask layer opposite the retainer pin. The pin is activated against the edge of the disk to urge the disk against the protuberances, thereby positioning the disk on the centerline between the protuberances. 
     This arrangement avoids the use of multiple contact pins that may damage the fragile pixel generating elements in the contact edge of the thermal print head. Furthermore with this system, the printer is able to place the contact edge of the print head at the leading edge of the disk just behind the single disk holding pin. This allows the disk to be printed with no chance of collision between the media holding pin and the print head. 
     The invented transfer printer also includes a mechanism to detect the carrier position and detect whether a media item is properly positioned on the carrier before contact by the print head. The detection mechanism is incorporated into the preferred actuatable retainer mechanism to hold the media item in place during printing. Other embodiments of a retaining means include non-conductive pins and pins with curved or chamfered profiles which are utilized to avoid damage during inadvertent contact with the thermal contact edge of the print head. These and other features are described in greater detail in the detailed description of the preferred embodiments that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the thermal printer and a personal computer, the printer having an extended tray with a compact disk in the tray. 
     FIG. 2 is a perspective view of the thermal printer with the casing removed and the tray extended. 
     FIG. 3 is a side sectional view of the thermal printer showing the internal components of the printer with the tray retracted. 
     FIG. 4 is a partial end view showing the support mechanism for the tray. 
     FIG. 5 is a multilevel plan view showing an actuator mechanism for a disk retainer pin. 
     FIG. 6 is a partial side sectional view detailing a portion of the printer shown in FIG.  3 . 
     FIGS. 7A-7C are a series of schematic views of the pin actuator mechanism and a sensor for detecting disk loading conditions. 
     FIGS. 8A-8E are a series of schematic view of different embodiments of retainer pins. 
     FIG. 9 is a schematic view of a print head contact edge. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The thermal transfer printer of this invention is shown in one preferred embodiment in FIG. 1, and is designated generally by the reference numeral  10 . The thermal transfer printer  10 , hereafter, thermal printer, is shown coupled to a general purpose computer  12  by a cable  14 . The general purpose computer  12  conveniently carries an application program to create and manage graphic images and text that are to be transferred to the media by the thermal printer  10 . An ordinary personal computer is typically adequate for creating labels for compact disks, the primary use for which this printer was invented. 
     The thermal printer  10  has an external casing  16  with a control panel  18  for entry of user commands and a display  19  for display of user entries and prompts generated by the printer  10 . Within the thermal printer  10  is housed a controller  11 , shown in FIG. 3, that coordinates the electronic and mechanical operations involved in the automated printing of a media item. The preferred media item is a recordable compact disk  20  shown in the extended media holding tray  22  of the preferred embodiment. The thermal transfer printer of this invention is designed to print on non-rectangular-shaped media and is particularly adapted to print label information on compact disks. The embodiment described utilizes a ribbon having a thermally sensitive transfer coating that is transferred from the ribbon to the media when heated by a print head. 
     Referring to the perspective view of FIG. 2 with the external housing  16  removed, the media tray  22  is shown fully extended from a housing frame  24 . The media tray  22  has a carriage  26  with side rails  28  that engage rollers  30  mounted on internal journal brackets  32  fixed to the side walls  34  of the frame  24  as shown in FIG.  4 . The portion of the media tray that is extended external to the housing frame  24  comprises a media support platform  35  having a flat, rigid top surface. 
     Mounted on the top surface  36  of the tray  22  is a rectangular resilient pad  38  with a center cutout  40 . On top of the resilient pad  38  is a mask  42  having a rectangular perimeter with a substantially circular cutout  44  that approximately conforms to the circular perimeter of the recordable compact disk  20  of FIG.  1 . The mask  42  has a thickness equal to the compact disk  20  and has similar physical and structural characteristics, preferably being fabricated of poly-carbonate, the same material as the disk. The circular cutout  44  has two edge protuberances  46  (exaggerated in FIG. 2) spaced about 45° apart to center a disk when deposited in the circular cutout  44  of the mask  42 . The cutout  40  of the resilient pad  38  allows the disk  20  to seat flat on the pad  38  by accommodating an annular ridge around the centerhole of a typical compact disk. When seated in the cutout of the mask, the top surface  43  of the disk  20  is at the same level as the top surface  45  of the mask  42 . 
     The circular cutout  44  of the mask  42 , the pad  38  and the end of the tray  22  have a slot  48  to accommodate a displaceable retainer pin  50 . The pin  50  engages the edge  51  of a disk that is deposited manually or mechanically in the circular cutout  44  and holds the disk against the protuberances  46  during printing. 
     The flat tray  22  forms a rigid support surface, the resilient pad of uniform thickness forms a base layer and the mask forms a mask layer, which layer includes the combination of the mask and a flat media item when printing. The base layer is constructed of a rubber-like material such as neoprene sheet of 40-70 durometer hardness. The base layer is approximately one-eighth inch in thickness and allows for uniform distribution of forces across the printing edge of a print head. The material forming the mask  42  in the mask layer is relatively non-compressible and has physical and structural characteristics that match the non-rectangular contour of the media item. In the case of a recordable compact disk that is relatively non-compressible and stiff, with only a limited degree of flexibility, the mask in the mask layer must have similar characteristics, since the local pressure of the print head is distributed over an expanded area of the base layer by the relatively rigid components forming the mask layer. The mask  42  has the same thickness and preferably the same composition as the media item. 
     Printing is accomplished by a thermal print head  52  having an elongated contact edge  54 . Along the length of the contact edge are pixel generating elements  55 , shown schematically in FIG.  9 . The elongated contact edge  54  has a length and a narrow width that distributes the force applied to the print head across the surface contacted. The pixel generating elements  55  include a line of thermal resistors (not shown) that are selectively activated to cause the transfer of discrete spots of wax on resin ink from a coated ribbon  78  to the media. The discrete spots form the print pixels of the bit-mapped label image. The composite of the label image is formed in lines as a sequence of linear print segments. The media is advanced relative to the print head in steps that are the width of the pixel elements to form the next linear segment of print. Contact of the print head with the media is direct in the case of a media item having a thermally activateable coating, or indirect in the case where a ribbon coated with a thermal transfer coating is interposed between the print head and the media. 
     The print head of the preferred embodiment includes over 1500 pixel generating elements in a linear array approximately five inches in length. The print head  52  is pressed firmly against a media item with the ribbon  78  interposed between the contact edge  54  and the media item. When the force of the print head  52  against the media item is constant, and the number of pixel generating elements in contact with the media item is the same for each line of printing, the printing is uniform. 
     However, if the number of pixel generating elements in contact with the media item changes from one line to the next, for example, when printing a non-rectangular media item such as a compact disk, then the printing, which is pressure dependent, will become uneven. Printing at the edge of a disk distributes the force of the head over a lesser number of pixel elements than printing at the center of the disk where a greater number of pixel elements are in contact with the disk. 
     To maintain a constant print pressure per unit area over a non-rectangular shaped media item, the media compatible mask  42  is used. The mask plus the media item presents a combined contact surface to the contact edge of the print head that is designed to be substantially uniform with a constant width for each linear segment of printing. 
     Although the entire length of the contact edge  54  of the print head need not contact the media item and mask, the portion of the contact edge in contact with the combined surfaces of the media item and mask must remain substantially constant for uniform printing. As shown in FIG. 1, the top surface  43  of the disk  20  plus the top surface  45  of the mask  42  form a combined surface that is rectangular. The sum of the mask surface  45  and the media surface  43  in contact with the contact edge of the print head is thereby constant for each linear segment of printing as the tray is displaced during the printing operation. 
     Referring to FIG. 3, the media holding tray  22  is shown retracted by action of a lead screw  58 , which is connected to the carriage  26  and holding tray  22  by a lead screw nut  60 . The lead screw  58  is mounted in end walls  62  and  64  in bearings  66  and  68 . A stepping motor  70  connected to a timing belt assembly  71  rotates the lead screw  58  in counted pulses and precisely controls the linear displacement and location of the carriage  26  and the tray  22 . A photosensor  72  on a bracket  74  mounted to the frame  24  detects a flag  76  depending from the underside of the carriage  26  to mark a “home position.” This reference is used by a control program in the controller  11  for determining the precise tray position to sequence the various print operations including raising and lowering of the print head  52 . 
     The print head  52  presses firmly against the media layer with a force ranging from 5-12 pounds. Even when fully distributed across the entire contact edge  54 , there is an applied force of over a pound per linear inch along the edge  54 . Contact with a disk holding pin can easily damage the fragile thermal resistors of the pixel generating elements  55 . The retaining pin  50  is preferably located outside the area of printing. Use of the single pin  50  in conjunction with the protuberances  46  on the mask  42  avoids contact with the pin, since the print head  52  is lowered with precision onto the disk  20  and mask  42  adjacent the pin and prints with movement away from the pin. 
     The thermal transfer medium is coated on the ribbon  78  carried on a supply roll  80 . The ribbon  78  is passed around perimeter guide rods  82  and under the print head  52  to a takeup roll  84 . A photosensor  85  detects the presence of the ribbon  78  and signals the controller when the end of the ribbon is reached. A drive motor  86  has a drive gear  88  that engages a pair of gears  90  and  92  in a gear train, with gear  92  engaging a spindle gear  94  for the takeup roll  84 , as shown in FIG.  2 . The drive motor  86  takes up the ribbon during printing as the print head  52  presses the ribbon against the media and mask on the moving media holder tray  22 . 
     The print head  52  is raised and lowered by the print head control mechanism  56 . Referring to FIG. 6, the print head  52  is mounted on a print head carrier  96  with a carrier spindle  98  engaging a vertical slot  100  in each side wall  34  allowing limited vertical displacement. The carrier  96  is suspended in a bracket  102  spanning the spindle  98  and has an adjustment screw  104  to adjust the angle of the print head  52  relative to the tray  22 . The bracket  102  has a central tab  106  that has a pivot  108  connecting the bracket  102  to a lever arm  110  that is connected to a fixed pivot pin  112 . The central tab pivot  108  allows some lateral roll along the printing edge  54  of the print head  52  to distribute the force applied to the print head  52  uniformly along the contact edge  54  when applied against the media to be printed. A tension spring  114  maintains the carrier spindle  98  forward in the slot  100  to eliminate play on print head positioning. 
     The lever arm  110  has a distal end  116  with a pivot  118  connecting the lever arm  110  to a bracket  120  on which is mounted a motor  122  with a displaceable screw  124 . The displaceable screw  124  connects the motor bracket  120  to an end bracket  126  with a pivot  128  and a spring trap  130 . The spring trap  130  includes a floating bracket  132  and a compression spring  134 . The motor bracket  120  and end bracket  126  form a variable length link using the displaceable screw  124  as the adjustment mechanism. When contracted, the print head is lowered to the media, here the recordable compact disk  20  and mask  42 . Upon further contraction, the bracket  132  that traps the compress ion spring  134  is drawn toward the motor bracket  120  compressing the spring. The force of compression is magnified by the moment arm to pivot  112  and is translated to the contact edge  54  of the print head  52  as a controlled force against the disk  20  and mask  42 . 
     When the variable length link is expanded, by reversing the motor and extending the displaceable screw  124 , the spring trap  130  is driven to the end plate  136  where the lock nut  138  seats and the distal end  116  of the lever arm  110  is forced to rise to the position “A” as indicated in FIG. 6, thereby lifting the print head  52 . A tab  140  on the lever arm  110  is detected by a photo sensor  142  to stop the motor and signal the controller that the print head  52  is in its raised position. 
     In FIG. 6, the print head  52  is shown pressing the ribbon  78  against the compact disk  20  at the beginning of the print cycle. The contact edge  54  is positioned proximate the retainer pin  50  without contacting the pin. The pin  50  is located at the end of an actuator rod  144  slidable in a pair of bearings  146  and  148  in a hollowed out portion of the underside of the tray  22 . The pin  50  is urged against the edge of the disk  20 , which is pressed against the protuberances  46  at the edge of the mask  42  by a compression spring  150  compressed between one of the bearings  148  and a clip  152  on the rod  144 . At the end of the rod  144  is a cylindrical cam  154  that contacts one end of a lever  156  having a center pivot pin  158 , shown in FIG.  5 . The other end of the lever  156  is pivotally connected to a push rod  160  having a depending flag  162  that actuates the push rod  160  and hence the actuator rod  144  for the pin  50 . 
     When the flag  162  contacts the end wall  62  on extension of the tray  22 , the push rod is urged back and the pin actuator rod  144  is urged forward by action of the lever  156 . 
     The schematic views of FIGS. 7A-7C show the movement of the actuator rod  144  as the tray  22  is extended from the end wall  62  of the printer. The flag  162 , linked to the actuator rod  144 , is used in conjunction with a second flag  164  with a slot  166  to detect whether a disk  20  is loaded and engaged by the pin  50 . The second flag  164  is mounted to the underside of the tray. As the tray  22  extends, flag  162  is off-set from slotted flag  164  as shown in FIG.  7 A. When the tray  22  reaches maximum extension as shown in FIG. 7B, the flag  162  contacts the wall  62  and is urged back along-side flag  162 . In this position, slot  166  is blocked and pin  50  is advanced by lever  156  of FIG. 5 allowing a disk  20  to be deposited on the tray  22 . 
     As the tray  22  begins to be retracted, the pin  50  returns to the position of FIG. 7A, unless the pin  50  contacts the edge of the disk  20 . If a disk is present and properly engaged by the pin  50 , as shown in FIG. 7C, the full return of the actuator rod  144  is halted and the slot  166  of the slotted flag  164  remains blocked by the flag  162 . This blockage is detected by a photosensor  170  which is activated at a predefined tray position and a signal is sent to the controller to indicate a disk is loaded and properly seated permitting the printing cycle to commence. If a disk is not loaded or if improperly seated, with pin  50  returning to the position of FIG. 7A, then the photosensor  170  detects the uncovered slot  166  and the printing cycle will not be triggered. When the tray  22  returns to the position of FIG. 7B in the return cycle, the pin  50  is again displaced releasing the disk  20  for manual or automatic removal from the tray  22  and replacement with the next disk to be printed. 
     Referring again to FIG. 6, the disk  20  has a thickness matched by the mask  42  forming the mask layer. The force of the print head  52  at the start of the printing operation is largely distributed across the mask  42  on each side of the edge of the disk. The pad  38  forms the base layer and is adhered to the rigid top surface  36  of the tray  22 . As the tray  22  is extended a distance equivalent to the pixel width formed by a pixel generating element  55  in the linear array, discrete thermal resistors are activated causing the ink transfer to the media in a bit-mapped pattern created by the labeling program and transferred to the printer controller as an instruction set to activate the thermal resistors. 
     Referring to FIGS. 8A-E, certain details relating to the holding pin are illustrated. In FIG. 8A the pin  180  is shown engaging a media item  182  adjacent a mask layer  184  displaced from the top surface  186  of the media item. In this manner the contact edge of a print head will avoid contact with the pin  180 . The degree of displacement in part depends on the compressibility of an underlying base layer, and should be approximately ¼-⅓ of the media thickness. 
     In FIG. 8B, a retaining pin  188  is shown seated on a compressible plug  190  in the base material  192 . Contact with a print head will depress the pin and avoid damage to the print head. 
     In FIG. 8C, a retaining pin  194  is in the form of a cap  196  on the end of an actuator rod  198  with a compression spring  200  allowing displacement of the pin  194  on contact with the print head. 
     Alternately, a pin  202  as shown in FIG. 8D has a rounded edge  204  opposite the edge  206  that contacts the media item  208 . Or, as shown in FIG. 8E the pin  210  has a chamfered top  212 , sloping downward and away from the contact edge with the media item  210 . In each case, damage on contact by the printing head will be minimized or avoided. 
     Preferably, a non-conducting material is used for the pins to avoid short circuiting of the resistor elements if contacted. 
     While, in the foregoing, embodiments of the present invention have been set forth in considerable detail for the purposes of making a complete disclosure of the invention, it may be apparent to those of skill in the art that numerous changes may be made in such detail without departing from the spirit and principles of the invention.