Abstract:
A thermal transfer printer having a print head and a media support platform in the form of a removable tray 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 urging the media item against the edge of the mask with the print head avoiding contact with the retainer on the printing area, the printer having a mechanism to displace the retainer and sense whether a media item is properly placed in the cutout and retained by the retaining apparatus, and the printer having a print head unit that can be flipped to the side of the printer for replacement of the print ribbon or access to the print head and internal components for maintenance and servicing.

Description:
BACKGROUND OF THE INVENTION 
     This application is a continuation-in-part of our application Ser. No. 10/136,552 entitled “THERMAL PRINTER FOR COMPACT DISKS AND OTHER MEDIA”, filed Apr. 30, 2002, now U.S. Pat. No. 6,580,444. 
     This invention relates to a thermal printer of the type described in U.S. Pat. No. 6,312,174 B1, entitled “THERMAL PRINTER FOR COMPACT DISKS” issued Nov. 6, 2001. 
    
    
     This invention relates to a thermal transfer printer for printing on the surface of a compact disk and other media using a linear thermal transfer head. 
     The invention optimizes printing on irregularly shaped media and incorporates features to prevent damage to the thermal transfer print head and to facilitate ease of use and maintenance. 
     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 ink jet 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 ink jet printing. Additionally, use of inks that are water resistant is difficult and expensive requiring specialty print heads. 
     Thermal transfer printers can print with greater speed and print on disks 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 was 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 a marking medium 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 a 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 disk shaped media, the print head pressure to the media will change as the print head travels across the disk. 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 disk shaped 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 disk to compensate for the variation in width of printable surface, or 
     b) hold the disk in a manner that effectively presents an unchanging width of contact area for the print head as it moves across the disk. 
     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 disk 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 manually or robotically 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 disk shaped objects, where the print head&#39;s area of contact with the media varies as the print head moves relative to the disk. 
     The base layer (compressible surface) and the mask layer (surface with cutout area in the shape of the media) may have one or 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 disk 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 disk printing application is a non-compressible material such as polycarbonate. CD-ROM and CD-R disks are typically made from molded polycarbonate. 
     SUMMARY OF THE INVENTION 
     The thermal printer of this invention is designed to print on various configurations of 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 unique features of this invention include a print head chassis that flips open for easy replacement of the print transfer ribbon and maintenance and servicing of the print head and internal components. Additionally, the improved thermal printer includes a replaceable media transport carrier in the form of a removable tray. 
     The thermal transfer printer of this invention includes a rigid 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 configurations 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 certain applications, the media item may require a topographically tailored media support surface. The feature of the removable support tray permits a variety of trays with different tray templates to be provided including trays with custom, multi-level, complex support configurations to optimize print transfer. 
     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 centrally located retainer that is activated to hold the media item against the edge of the media mask. The retainer is part of a retainer mechanism that is incorporated into the removable tray and is connected to a latching mechanism in the printer for operation. 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. The retainer, in the form of a button when used for compact disks 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. In the improved printer the retainer is positioned at the leading edge of the disk which is printed as the tray is retracted into the printer. 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 button. This allows the disk to be printed with no chance of collision between the media holding retainer 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 improved actuatable retainer mechanism to hold the media item in place during printing. Other embodiments of a retainer include a shuttle bar for straight edge media such as truncated disks and rectangular recordable media popular for business cards, specialty jigs for tags and other printable items collectively defined as media items as described herein. 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 of this invention and a connected general purpose computer. 
     FIG. 2 is a perspective view of the thermal printer of FIG. 1 with a cover to the print head unit removed. 
     FIG. 3 is a top view of the thermal printer with the housing removed and the print head unit pivoted to one side. 
     FIG. 4 is a plan view of the underside of the typical print or media tray removed from the thermal printer. 
     FIG. 5 is a partial elevational view schematically illustrating the print head displacement mechanism and tray drive mechanism. 
     FIG. 6 is a plan view of one embodiment of the print or media tray for a typical compact disk. 
     FIG. 7 is a plan view of another embodiment of the print or media tray for a truncated disk, commonly used as a business card. 
     FIG. 8 is a plan view of another embodiment of the print or media tray for a rectangular business card. 
     FIG. 9 is a plan view of another embodiment of the print or media tray for a tag array carried on a jig. 
     FIG. 10 is a flow diagram of the printer operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 housing  16  with a control panel  18  for entry of user commands and display lights  19  for visual feedback of user entries and prompts generated by the printer  10 . Within the thermal printer  10  is housed a controller  11  that coordinates the electronic and mechanical operations involved in the automated printing of a media item. The most common media item is a recordable compact disk  20  shown in the extended media holding tray  22  for the embodiment of the print tray  22  shown in FIG.  1 . The thermal transfer printer of this invention is designed to print on non-rectangular shaped media and, as noted, is particularly adapted to print label information on compact disks. The printer 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. 
     The external housing  16  includes an easily removable cover  24  to provide access to the print head unit  26  shown in the perspective view of FIG. 2 with cover removed. The print head unit  26  has a print head chassis  28  that is mounted on top of and pivotally connected to a main print tray drive and controller chassis  30 . A pivot mechanism  29  that includes bracket tabs  32  which engage a pivot rod  34  is mounted on the side of the main chassis  30 . The pivotal connection of the print head unit  26  enables the print head unit  26  to be flipped to the side of the main chassis  30  for replacement of the print ribbon or access to the internal components of the printer for maintenance or servicing. A pin and slot stop mechanism  36  limits the pivot of the print head chassis for convenient access to the underside of the print head chassis and internal components of the main chassis. 
     As shown in FIG. 2, the printer  10  has an easily replaceable print ribbon supply roller  38  carried on a spindle  40  having one end that seats in a slot  42  and the other end in a bearing (not visible) at the other side of the chassis  28 . A take-up roller  44  is mounted on a spindle  46  on which is mounted a gear  48  that is driven by a motor gear  50  through an intermediate idler gear  52 . In the improved thermal printer  10  of this invention, the disk  20  seated in the print tray  22  is printed as the tray is retracted into the printer. In this manner, the take-up roller  44  draws the print ribbon  54  from the print ribbon supply roller  38  at the same speed of tray travel to prevent smearing as the print head presses against the transported media and a media mask  56 . The controller  11  (shown in FIG. 3) is located under the housing  16  in the main chassis and coordinates the printing operation. The controller  11  is electronically connected to the print head unit  26  through a ribbon cable  58 . Similarly, the user commands from the control panel  18  are electronically connected to the print head unit  26  and controller  11  through a ribbon cable  60 . Referring now to FIG. 3, a top view of the thermal printer  10  with the F housing and print tray removed is shown. The print head unit  26  is pivoted to a vertical, out-of-the-way position. The pin and slot stop mechanism  36  ordinarily limits the pivot to a position less than perpendicular, and the exaggerated pivot is shown for purposes of this description to prevent the print head unit  26  from obscuring the internal components of the main chassis  30 . With the housing  16  removed, the electronic controller board  62  for the controller  11  is mounted in the chassis next to a fan  64  and a power supply connector  66  with an on/off power switch  68 . Under the fan  64  is a tray sensor  69  to sense the home position of a retracted tray. 
     The main chassis  30  has ends  70  and side walls  72  for mounting the print tray support mechanism  73  and the print tray drive mechanism  74 . The side walls  72  provide shaft bearings for the components that support and guide the transport of the tray  22 . Spaced roller shafts  75  and  77  have spaced rubber rollers  76  which engage the underside of the removable tray  22  on each side of a rack unit  78  shown in FIG.  4 . Between the spaced roller shafts  75  and  77  is located a roller shaft  80  with spaced elongated rubber rollers  82  for support of the removable tray  22  under the contact location of the print head shown in FIG.  5 . 
     The top of the removable and replaceable tray  22  is retained by two spaced guide shafts  86  and  88 , shown in part in FIG.  3 . The guide shafts  86  and  88  include small plastic contact rollers  90  that engage the media mask  56  on the top of the tray  22 . 
     A print tray drive mechanism  74  for accurately moving the tray in and out of the printer  10  under the print head unit  26  cooperates with the rack unit  78  shown in FIG.  4 . The drive mechanism  74  includes a stepping motor  92  with a belt sprocket  94  that drives a driven sprocket  96  under a mounting plate  98 . The driven sprocket  96  has a shaft  100  that is common to a pinion gear  102 . The pinion gear  102  cooperates with two spaced guide rollers  104  to maintain the linear travel of the tray when in engagement with the rack unit  78 . Supported on the mounting plate  98  is a slide plate  106 . The slide plate  98  carries a nylon pressure roller  108  that is biased by a tension spring  110  connected at one end to the mounting plate  98  and at the other end to the slide plate  106 . The slide plate  106  has a slot  112  for the common shaft  100  to enable the slide plate  106  to displace under the pinion gear  102 . In this manner, the pressure roller  108  and guide rollers  104  cooperate to maintain the engagement of the drive mechanism  90  with the rack unit  78 . 
     Above the drive mechanism  74  is a support plate (not shown) spanning the two side walls  70  for positioning a reflective tab  114  that is shown in part in FIG.  3 . The reflective tab  114  seats in a notch  116  on the guide shaft  86  to avoid contact with the ribbon. The reflective tab  114  cooperates with a photo sensor (not shown) in the print head unit  26  to signal when the print ribbon  54  has run out, or that the print head chassis  28  has been pivoted up from the main chassis  30 . 
     The print head unit  26  has a side  118  with a handle  120  allowing the print head unit  26  to be easily flipped to the side when the cover  24  is removed. In order to ensure that the print head unit  26  is not inadvertently dislodged, the side plate  118  includes a notched tab  122  that engages the threaded shaft  124  of a chassis lock knob  126 . 
     When the print head unit  26  is lowered to its operating position a rod roller  128  engages and lifts the print ribbon  54  behind the print head  130 , a shown in greater detail in FIG.  5 . 
     In order to insure that the tray  22  when inserted is not inadvertently dislodged, a latching mechanism  132  is provided. The latching mechanism  132  includes an elongated square rod  134  with cylindrical ends  136  and a pair of central by-pass segments  138  to clear the rack unit  78 . The rod  134  carries a screw mounted latch plate  140 . The latch plate  140  has an end tab  142  connected to a tension spring  144  that is connected to a bracket tab  146  on the bottom  148  of the main chassis  30 . The end tab  142  includes a downwardly directed flag  150  that is pivoted to a photo-sensor  152  under the rod  134  when the latching mechanism  132  is in engagement. This typically occurs when the tray  22  is extended for receipt of or removal of a media item. 
     Referring to FIG. 4, the underside  154  of the media tray  22  is fabricated from a rigid, preferably metal, support plate  156 . The rack unit  78  includes a U-channel or track  158  mounted on the surface of the support plate  156 . The guide track  158  has two rails  160  and  162 . When the removable tray  22  is installed, rail  160  engages the guide rollers  104  and pressure roller  108 , displacing the slide plate  106  so that the rail  160  tracks between the spring biased pressure roller  108  and the pair of guide rollers  104 . Mounted along the inside of rail  160  is a nylon rack  164 . On installation, the rack  164  engages the pinion gear  102  of the drive mechanism  74 , which then displaces the tray  22  on rotation of the gear  102  by the stepping motor  92 . In this manner, movement of the tray  22  is accurately controlled by controlled operation of the stepping motor. 
     Under and adjacent the track  158  is the cooperating latch portion of the latch mechanism  132 . Seated in a recess  166  in the support plate  156  under the track  158  is a latch arm  168  mounted on a pivot pin  170 . The contour of the recess  166  limits the pivot of the latch arm  168 . At the distal end of the latch arm  168 , adjacent the rail, is a perpendicular latch hook  172 . A tension spring  174  that is anchored to the support plate  156  by screw  175  is connected to the hook  172 . The latch hook  172  is configured with an angled lead edge  176 , as shown in FIG. 5, to contact the latch plate  140  on inserting the removable tray  22  and rotate the square rod  134  allowing the latch hook  172  to clear the latch plate  140 . The tension spring  144  connected to the latch plate  140  biases the square rod  134  to position the latch plate  140  at an angle. As shown in FIG. 3, the latch plate  140  has a trailing edge  178  that overhangs the square rod  134 . As the media tray  22  is extended from the printer housing  16  the latch hook  172  snags the trailing edge  178  of the latch plate  140  and limits further displacement of the tray  22  from the main chassis, as shown in FIG.  5 . 
     The latch hook  172  can be released from engagement with the latch plate  140  by pressing a release button  180  on the extended end  136  of the square rod  134  which displaces the rod along its axis and positions the latch hook  172  at the adjacent, by-pass segment  138 . The latch hook  172 , biased by the tension spring  174 , clears the necked-down segment  138  of the rod  134 , allowing removal of the tray  22 . A compression spring  182  under the release button  180  returns the square rod to its normal axial position. 
     In conjunction with latching the tray to restrict displacement, the latch portion of the latch mechanism  132  actuates the retainer mechanism  184  for holding the media item on the tray  22 . The pivoting latch arm  168  is linked to an elongated tongue  186 , which is linearly displaceable in a guide channel  188  under the track  158  of the rack unit  78 . One end  190  of the elongated tongue  186  is raised over the latch arm  168  linked to the arm by a pivot pin  192 . The raised end  190  is displaceable in a slot  194  in the track  158 . The tongue  186  transfers the angular displacement of the spring biased latch arm to a linear displacement of a media retainer  196  on the topside  198  of the tray  22 , as shown in FIGS. 6-9. When the tray is extended and the latch hook  172  is engaged with the latch plate  140 , the limited additional travel of the tray against the bias of the latch arm spring  174  retracts the retainer  196  allowing placement or removal of the media item from the tray  22 . 
     Referring to the enlarged schematic view of FIG. 5, the print head displacement mechanism  199  for the printer  10  is shown with the print head unit  26  in its operating position and a thermal print head  200  displaced to its print position against the media mask  56  of the media tray  22 . The media tray  22  is shown in part extended to illustrate the engagement of the latching hook  172  with the pivoted latching plate  140  on the square pivot rod  134 . Normally, the tray  22  would be further withdrawn into the printer before the print head  200  is lowered against the tray  22  for printing. Operation of the print head is controlled by the programmed controller  11 . 
     The fragile thermal print head  200  is mounted on a carriage assembly  201  of the print head displacement mechanism  199 . The thermal print head  200  is fastened to an inner mounting plate  202  having end hinges  204  coupled the inner plate to an intermediate bracket  206 . The coupled mounting plate  202  and intermediate bracket  206  are connected to an outer bracket  208  by a transverse pin  210  to allow some limited side-to-side wobble to the print head  200  for distributing the force of the contact edge  212  of the print head uniformly across the top surface  214  of the tray  22  during printing. 
     The outer bracket  208  is fastened to a pivotal carrier sled  216  with side arms  218  coupled to a pivot rod  220  spanning the side walls  70  of the print head chassis  28 . A central tab  222  on the carrier sled  216  carries a roller  224  that engages the eccentric inside cam surface  226  of a cam wheel  228 . The cam wheel  228  is mounted on a shaft  230  that is carried on an assembly bracket  232  (shown in part) mounted to the print head chassis. The shaft  230  also carries a sensor flag unit  234  having a dark inner spacer wheel  236  and a disk plate  238  having two oppositely directed perpendicular flags  240  spaced one hundred eighty degrees apart. The flags  240  selectively align with one of a pair of photo sensors  242  located on the bracket  232 . A driven gear  244  is also mounted on the cam wheel shaft  230  which engages a drive gear  246  on the drive shaft  248  of a d.c. drive motor  250 . Operation of the drive motor rotates the drive gear  246  which drives the driven gear  244  to rotate the cam wheel  228  that raises or lowers the carrier sled  216  and mounted print head  200 . The flags  240  indicate the position of the cam wheel  228  when one of the oppositely positioned flags is in proximity to its photo sensor  242 , thereby selectively indicating the raised or lowered position of the print head  200  to the controller  11 . 
     An angled yoke  252  of the carrier sled  216  provides a mount for one end of one or more tension springs  254 . The tension springs  254  are connected at their opposite ends to an internal anchor mount  256  (partially shown) projecting from the print head chassis  28 . The springs  254  are selected to pull the carrier sled  216  downwardly, forcing the carrier sled roller  224  against the inside cam surface  226  of the cam wheel  228 . The springs  254  limit its downward force of the print head  200  against the tray surface  214 . If this force is exceeded, for example, when the print head engages a dislodged disk, the print head will rise against the force of the springs to avoid excessive damage to the print head. 
     The print ribbon  54  from the supply roll  38  is carried over a rod roller  258  rotatably mounted to the print head chassis  28  and under the contact edge  212  of the print head  200 . The print ribbon  54  is then lifted by the rod roller  28 , guided by the pivot rod  220  of the carrier sled and wound on the take-up roll  44 . The take-up roll  44  is gear driven by the d.c. drive motor  259 . Tension in the print ribbon  54  is limited by a spring clutch (not shown) in the spindle  40  of the supply roll  38 . Control of the d.c. drive motor  259  is coordinated with the positioning of the print head  200  by the programmed controller  11 . The controller generally includes basic program instructions generic to the various applications of the printer. Certain tasks such as graphics and print generation are preferably performed using the auxiliary computer  12 . 
     However, the printer  10  includes a memory buffer  261  on the controller board  62  to capture the last label graphic for continuation or restart of a printing operation. The controller  11  can have full personal computer capability with an auxiliary display or small integrated display, thereby dispensing with the external computer  12 . 
     This flexibility in controlling operations is beneficial for tailoring the printing operation to different media items. The replaceability of the printing tray enables the printer  10  to be adapted to thermal printing of a variety of media items as exemplified in FIGS. 6-9. 
     Referring to FIG. 6, the embodiment of the removable tray  22  is in the form of a standard compact disk tray  260  with a five inch compact disk  262  having approximately a 4¾ diameter outline shown in phantom. The tray  260  has a top surface  264  primarily comprising a media mask in the form of a stiff, non-compressible mask layer  266  fabricated of a plastic, such as polycarbonate, having structural characteristics similar to those of the compact disk. 
     A moderately compressible base layer  268  is exposed in the cut-out  270  of the mask layer  266 . The cut-out  270  is in a template configuration  272  for a compact disk. The CD template configuration  272  provides an oversized opening for seating a disk manually or robotically. To position the disk  262  for printing, a pair of small protuberances  274  in the cut-out  270  provide a two point contact for seating the disk  262  during printing. At the opposite side of the cut-out  270  is located the media retainer  196  in the form of a flat button  276  carried on the spring loaded tongue  186 . When actuated on extension of the tray, the button  276  retracts into a slot  278  in the mask layer  266  and base layer  268  allowing placement of the disk  262 . When the tray is retracted into the printer for the printing process the button  276  engages the edge of the disk and holds the disk firmly against the protuberances  274  with the selected force of the tension spring  174 . A hole  279  through the tray  22  is provided for compact disks of various sizes and configurations to facilitate removal. The underside of the tray  22  includes the rack unit  78  as described with reference to FIG. 4. A similar arrangement with a cut-out having a smaller template configuration can accommodate a 3″ CD using a button retainer as shown in FIG.  6 . 
     Referring to FIG. 7, the embodiment of the removable tray  22  is in the form of a compact disk business card tray  280  which receives a truncated 3″ compact disk  282  in a cut-out  284  for a 3″ compact disk. The tray  22  is modified with a media retainer  196  in the form of a flat shuttle  286  with a straight contact edge  288  that contacts one of the opposite straight edges  290  of the truncated disk  282 . 
     The flat retainer shuttle  286  has a tab  292  that retracts into a slot  294  and is connected to the actuator tongue  186  by a pin  296 . The template configuration  298  of the mask layer  266  of the media mask  56  includes protuberances  300  and is designed to accommodate a truncated disk  282 . The tray  280  has a hole  279  and underside with a rack unit  78  similar to that described with reference to FIG.  4 . 
     Referring to FIG. 8, the embodiment of the removable tray  22  is in the form of a rectangular card, compact disk tray  302 . The rectangular compact card disk  304  is a newly popular media item for business cards. The form is generally rectangular with rounded corner segments  306  which are thinner than a central, truncated disk portion  308  which includes an inner circular recording area  310 , shown in dotted line. 
     This stepped topography on the underside of the rectangular card disk  304  requires a media mask  56  with the mask layer  266  having a matching template configuration  312  with a complex topography. A complex topography is one having multiple levels. The template configuration  312  has a cut-out  314  and corner seats  316  provided by milled recesses in the mask layer  266 , such that the flat top surface of the compact card disk  304  is supported substantially flush with the top surface  318  of the mask layer  266 . A template configuration with a complex topography matching the topography of the media item allows a substantially even pressure to be applied by the print head across the media item during thermal printing. 
     The rectangular card disk tray  302  has a retainer  196  in the form of the flat shuttle  286 , as shown in FIG. 7, and includes a similar tray hole  279  as in the previous embodiments. 
     Referring now to FIG. 9, the embodiment of the removable media tray  22  is in the form of a general media item tray  320 . In the particular example of FIG. 9, the general media item tray  320  supports a preformed tag array  322  with a matrix of pre-cut tags  324  on snap-off tabs  326 . The preformed tag array  322  is seated on a metal printing jig  328  that seats in a mask cut-out  330  having a template configuration  332  conforming to the outline of the jig  328 . The jig  328  has a recess  334  with a milled complex topography matching the underside of the tag array to present a top surface  335  of the tags  324  flush with the mask layer  336 . The mask layer  336  is selected to have structural characteristics similar to the combined jig and media item. 
     The jig  328  is maintained in position by a retainer in the form of pins  338 , which are flush with the jig  328  to avoid damage to the print head. The tray hole  279  is omitted unless desirable to facilitate dislodgment of the jig and carried media item. Where the underside of the media item is flat, the jig may be omitted and the item retained by the mask area directly on the base layer. 
     These embodiments of the removable media tray are described as examples of the variety of different media items that are suitable for printing with this thermal printer and are not intended to limit the scope of the invention defined in the claims. 
     In operation, the thermal printer  10  of this invention has a unique method of insuring that malfunctions are minimized and damage to the print head is avoided. Although tasks performed by the computer  12  and controller  11  can be differently allocated as noted, the operating procedure for the preferred printer with a controller  11  having a buffer  261 , as described, is described with reference to the block diagram of FIG.  10 . 
     Starting with an initial condition with the thermal printer  10  powered off and the tray  22  removed, the computer  12  is connected to the printer via the cable  14 , generally the parallel port cable. The disk label software is loaded into the computer  12  by a variety of conventional means such as disk, local area network or wide area network. After initial set-up, preferably with the printer power off, the tray for the selected type of media item is installed into the printer without a media item as referenced in block  340  of FIG.  10 . 
     In block  342 , the printer  10  is turned on, for example, by manually activating the power switch  68 . The printer controller  11  fully retracts the tray  22  into the printer  10  to its home position which is sensed by sensor  69 . The printer  10  is ready and awaits a print command. In block  344  a print tray command is received by the controller  11 , for example, by manually pressing a print tray command key on the control panel  18 . As noted, in robotic systems the manual key commands are replaced by commands from a system controller or system computer. 
     In block  346  the printer responds by fully extending the tray  22 , which simultaneously retracts the retractable media retainer  196  and engages the latching mechanism  132  to set the flag  150  in the sensor  152 . The tray is ready for a media item with the retainer retracted maximizing the area in which to drop a media item. A media item is manually or robotically dropped in the mask cut-out and, using the computer  12 , a label is sent to the printer using the disk label software as represented in block  348 . Note, the last label file may be in the controller buffer and used instead of a new file. 
     In block  350 , a go print command is received by the controller and the tray  22  retracted by the stepping motor  92 . The steps of the stepping motor are counted and at a predefined number of steps, the sensor  152  is checked to determine if the latch flag  150  is present at block  352 . 
     Since the retainer mechanism  184  for the displaceable retainer  196  is linked to the pivoting latch arm  168  of the latching mechanism  132 , the latch arm  168  does not fully return in its recess  166  under bias of its spring  174  when the media item is properly seated in the tray and its edge engaged by the displaceable retainer  196 . 
     If at the predefined steps the flag  150  is detected, then the procedure flows to block  354 , where no media item is present or a media item is present, but is dislodged from the retainer  196 . In block  356  the controller  11  responds by extending the tray  22  and signalling the computer  12 , no media item is found. The controller  11  also alerts the user by a no item signal on the control panel  18 . 
     If at the predefined steps the flag  150  is not detected, meaning the retainer  196  is in proper engagement with the media item, then the flow proceeds to retained media item block  358 . 
     At block  360 , the controller begins the print operation by lowering the print head  200  and advancing the tray  22  into the printer in increments matching the print resolution of the formatted graphics in the label file. The print ribbon motor  259  (where thermal ribbon printing is being performed) is also activated to set the proper tension in the ribbon during printing. 
     In block  362 , on completion of the printing, the print head is raised, the tray fully extended with the retainer retracted for removal of the printed media item. The flow loops back to block  346 , where the tray  22  is ready for placement of another media item of the type previously printed, and the printer  10  is ready to receive a print command. 
     It is to be understood that this method of media item detection is for disk trays where the displaceable retainer mechanism is employed. In other media retainer systems an override command or an alternate conventional media item check is used. For example, a photo sensor directed at the media item to detect the presence of the item may be utilized. 
     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.