Patent Publication Number: US-6714232-B2

Title: Image producing process and apparatus with magnetic load roller

Description:
FIELD OF THE INVENTION 
     The invention relates to an image processing apparatus and a process for exposing an intended image on an imaging drum or the like, and, more particularly, to an image processing apparatus incorporating a magnetic load roller, and a process for loading media in an image processing apparatus. 
     BACKGROUND OF THE INVENTION 
     Pre-press color-proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high volume, printing press to produce an example of an intended image. An image may require several corrections and be reproduced several times to satisfy or meet the customers requirements resulting in a large loss of profits and ultimately higher costs to the customer. 
     One such commercially available image processing apparatus is arranged to form an intended image on a sheet of thermal print media. Dye is transferred from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor sheet material to form the intended image. This image processing apparatus generally includes a material supply assembly or carousel, and a lathe bed scanning subsystem or write engine, which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, load roller, and imaging drum, and thermal print media and dye donor sheet material exit transports. 
     Operation of the image processing apparatus includes metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then cut into sheet form of the required length and transported to the imaging drum. It is then registered, wrapped around, and secured onto the imaging drum. The load roller, which is also known as a squeegee roller, removes entrained air between the drum and the thermal print media. Next, a length of dye donor material (in roll form) is metered out of the material supply assembly or carousel, and cut into sheet form of the required length. It is then transported to the imaging drum and wrapped around it. A load roller is used to remove any air trapped between the imaging drum and the dye donor material. The dye donor material is superposed in the desired registration with respect to the thermal print media, which has already been secured to the imaging drum. 
     After the dye donor sheet material is secured to the periphery of the imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the dye donor sheet material on the spinning imaging drum while it is rotated past the printhead to form an intended image on the thermal print media. The translation drive then traverses the printhead and translation stage member axially along the axis of the imaging drum in coordinated motion with the rotating imaging drum. These movements combine to produce the intended image on the thermal print media. 
     After the intended image has been formed on the thermal print media, the dye donor sheet material is removed from the imaging drum without disturbing the thermal print media beneath it. The dye donor sheet material is then transported out of the image processing apparatus. Additional dye donor sheet materials are sequentially superimposed with the thermal print media on the imaging drum, further producing an intended image. The completed image on the thermal print media is then unloaded from the imaging drum and transported to an external holding tray on the image processing apparatus. 
     Although the presently known and utilized image processing apparatus is satisfactory, a need exists to improve the load roller in regard to its interaction with the imaging drum, as well as mechanical adjustment and loading, and efficient removal of any air entrained beneath the print media. 
     SUMMARY OF THE INVENTION 
     Briefly summarized, according to one aspect of the present invention, the invention resides in an image processing apparatus comprising an imaging drum for holding a sheet of dye donor material and a sheet of thermal print media, and a magnetic load roller, which improves alignment, provides uniform loading to the imaging drum, and removes entrained air beneath the media. The image processing apparatus receives the thermal print media and the dye donor materials for processing an intended image onto the thermal print media. The magnetic load roller could in fact be utilized in any mechanical apparatus that requires a load roller. 
     According to a preferred embodiment of the present invention, an image processing apparatus for writing images to a thermal print media, comprises: a) a rotatable, magnet-attracting imaging drum mounted for rotation about an axis, the imaging drum being arranged to mount a receiver sheet and a donor sheet in superposed relationship thereon; b) a linear drive motor for rotating the imaging drum; c) a sheet transport assembly for transporting the thermal print media and donor sheets to the imaging drum; d) a printhead; e) a lead screw for moving the printhead in a first direction, the printhead being mounted on the lead screw; f) a linear translation subsystem on which the printhead, imaging drum, and lead screw are mounted; and g) a magnetic load roller. 
     Also included herein is an image producing process for loading thermal print media or donor material on an imaging drum, comprising the steps of: 
     a) rotating an imaging drum in a first direction of rotation; 
     b) actuating a sheet transport assembly and driving a sheet of thermal print media to the imaging drum until a leading edge of the thermal print media sheet engages the imaging drum, and then stopping the sheet transport assembly; 
     c) rotating the imaging drum in a second direction of rotation and stopping the imaging drum at a first media load position, 
     d) moving a magnetic load roller into engagement with the leading edge of the thermal print media sheet; 
     e) rotating the imaging drum in a second direction of rotation until a trailing edge of the thermal print media sheet is under the magnetic load roller, and then stopping rotation of the imaging drum; and 
     f) moving the magnetic load roller away from the imaging drum. 
     The present invention provides: a self-aligning, more reliable magnetic load roller that does not require a crown, and a magnet-attracting imaging drum. The magnetic load roller efficiently removes entrained air, therefore eliminating the need for a second pass with the load roller over the media. Also, the imaging drum preferably has at least one magnet embedded in its surface to aid in disengagement of the magnetic load roller from the imaging drum. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention and its advantages will be apparent from the detailed description taken in conjunction with the accompanying drawings, wherein examples of the invention are shown, and wherein: 
     FIG. 1 is a side view in vertical cross-section of an image processing apparatus according to the present invention; 
     FIG. 2 is a perspective view of an image processing apparatus according to the present invention; 
     FIG. 3 is a top view in horizontal cross section, partially in phantom, of a lead screw according to the present invention; 
     FIG. 4 is an exploded, perspective view of a vacuum imaging drum according to the present invention; 
     FIG. 5 is a plan view of a vacuum imaging drum surface according to the present invention; 
     FIGS. 6A-6C are plan views of a vacuum imaging drum according to the present invention, showing a sequence of placement for thermal print media and dye donor sheet material; 
     FIG. 7 is a schematic side elevational view of a proofing printer according to the present invention; 
     FIG. 8 is a front perspective view of a material supply carousel of a proofing printer according to the present invention; 
     FIG. 9 is a partial schematic end view of an imaging drum and a magnetic load roller according to the present invention, shown in an unloaded position; 
     FIG. 10 is a partial schematic end view of an imaging drum and a magnetic load roller according to the present invention, shown in a loaded position; 
     FIG. 11 is a cutaway partial schematic end view of an imaging drum and an external magnetic load roller according to the present invention, showing a magnet embedded in the imaging drum; 
     FIG. 12 is a schematic end view of an imaging drum and an internal magnetic load roller according to the present invention, showing a magnet embedded in the imaging drum; 
     FIGS. 13 a-h  are partial schematic illustrations of a material supply handling system according to the present invention, showing the loading and unloading of material; 
     FIGS. 14 a-c  are charts showing imaging drum operating conditions at each of the steps shown in FIGS. 13 a-h ; and 
     FIG. 15 is a schematic end view of an imaging drum and magnetic load roller according to the present invention showing the various operating positions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, like reference characters designate like or corresponding parts throughout the several views. Also, in the following description, it is to be understood that such terms as “front,” “rear,” “lower,” “upper,” and the like are words of convenience and are not to be construed as limiting terms. Referring in more detail to the drawings, the invention will now be described. 
     Turning first to FIG. 1, an image processing apparatus according to the present invention, which is generally referred to as  10 , includes an image processor housing  12 , which provides a protective cover for the apparatus. The apparatus  10  also includes a hinged image processor door  14 , which is attached to the front portion of the image processor housing  12  and permits access to the two sheet material trays. A lower sheet thermal print material tray  50   a  and upper sheet input image material tray  50   b  are positioned in the interior portion of the image processor housing  12  for supporting thermal print media  32 , or an input image, thereon. Only one of the sheet material trays  50  will dispense the thermal print media  32  out of the sheet material tray  50  to create an intended image thereon. The alternate sheet material tray either holds an alternative type of thermal print media  32 , or an input image, or functions as a back up sheet material tray. In this regard, lower sheet material tray  50   a  includes a lower media lift cam  52   a , which is used to lift the lower sheet material tray  50   a  and, ultimately, the thermal print media  32  upwardly toward lower media roller  54   a  and upper media roller  54   b . When the media rollers  54   a, b  are both rotated, the thermal print media  32  is pulled upwardly towards a media guide  56 . The upper sheet input image material tray  50   b  includes an upper media lift cam  52   b  for lifting the upper sheet thermal print material tray  50   b  and, ultimately, the thermal print media  32  towards the upper media roller  54   b , which directs it toward the media guide  56 . 
     Continuing with FIG. 1, the movable media guide  56  directs the thermal print media  32  under a pair of media guide rollers  58 . This engages the thermal print media  32  for assisting the upper media roller  54   b  in directing it onto the media staging tray  60 . The media guide  56  is attached and hinged to the lathe bed scanning frame  202  at one end, and is uninhibited at its other end for permitting multiple positioning of the media guide  56 . The media guide  56  then rotates the uninhibited end downwardly, as illustrated. The direction of rotation of the upper media roller  54   b  is reversed for moving the thermal print medium receiver sheet material  32 , which is resting on the media staging tray  60 , under the pair of media guide rollers  58  upwardly through an entrance passageway  204  and up to the imaging drum  300 . 
     A roll  30  of dye donor material  34  is connected to the media carousel  100  in a lower portion of the image processor housing  12 , as shown in FIG.  1 . Four rolls  30  are ordinarily used, but, for clarity, only one is shown in FIG.  1 . Each roll  30  includes a dye donor material  34  of a different color, typically black, yellow, magenta and cyan. These dye donor materials  34  are ultimately cut into dye donor sheet materials  36  and passed to the imaging drum  300  for forming the medium from which dyes embedded therein are passed to the thermal print media  32  resting thereon. In this regard, a media drive mechanism  110  is attached to each roll  30  of dye donor material  34 , and includes three media drive rollers  112  through which the dye donor material  34  of interest is metered upwardly into a media knife assembly  120 . After the dye donor material  34  reaches a predetermined position, the media drive rollers  112  cease driving the dye donor material  34 . Two media knife blades  122  positioned at the bottom portion of the media knife assembly  120  cut the dye donor material  34  into dye donor sheet materials  36 . The lower media roller  54   a  and the upper media roller  54   b  along with the media guide  56  then pass the dye donor sheet material  36  onto the media staging tray  60  and ultimately to the imaging drum  300 . 
     FIG. 1 shows an imaging drum  300  and a magnetic load roller  350 . Once the thermal print medium receiver sheet material  32  is moved into position, the magnetic load roller  350  is moved into contact with the thermal print medium receiver sheet material  32  against the imaging drum  300 . The imaging drum  300  has a ferrous coating that attracts the magnetic load roller  350  to it, with the magnetic load roller aligning itself to the imaging drum  300 . 
     As shown in FIG. 1, a laser assembly  400  includes a quantity of laser diodes  402  in its interior. The lasers are connected via fiber optic cables  404  to a distribution block  406  and ultimately to a printhead  500 . The printhead  500  directs thermal energy received from the laser diodes  402 . This causes the dye donor sheet material  36  to pass the desired color across the gap to the thermal print media  32 . The printhead  500  attaches to a lead screw  250  (see FIG.  2 ). A lead screw drive nut  254  and drive coupling (not shown) permit axial movement along the longitudinal axis of the imaging drum  300  for transferring the data to create the intended image onto the thermal print media  32 . 
     For writing, the imaging drum  300  rotates at a constant velocity. The printhead  500  begins at one end of the thermal print media  32  and traverses the entire length of the thermal print media  32  for completing the transfer process for the particular dye donor sheet material  36  resting on the thermal print media  32 . After the printhead  500  completes the transfer process for the particular dye donor sheet material  36  resting on the thermal print media  32 , the dye donor sheet material  36  is removed from the imaging drum  300  and transferred out of the image processor housing  12  via a skive or ejection chute  16 . The dye donor sheet material  36  eventually comes to rest in a waste bin  18  for removal by the user. The above-described process is then repeated for the other three rolls  30  of dye donor materials  34 . 
     Continuing with FIG. 1, after the color from all four sheets of the dye donor sheet materials  36  has been transferred, the dye donor sheet material  36  is removed from the imaging drum  300 . The thermal print media  32  with the intended image thereon is then removed from the imaging drum  300  and transported via a transport mechanism  80  out of the image processor housing  12  and comes to rest against a media stop  20 . 
     Operation of the image processing apparatus  10  includes metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media  32  is then measured and cut into sheet form of the required length and transported to the imaging drum  300 . It is then registered, wrapped around, and secured onto the drum  300 . Next, a length of dye donor material (in roll form)  34  is metered out of the material supply assembly or carousel, measured, and cut into sheet form of the required length. It is then transported to the imaging drum  300  and wrapped around the imaging drum using the load roller  350 , so that it is superposed in the desired registration with respect to the thermal print media, which has already been secured to the imaging drum. 
     After the dye donor sheet material  36  is secured to the periphery of the imaging drum  300 , the lathe bed scanning subsystem  200  or write engine provides the scanning function. This is accomplished by retaining the thermal print media  32  and the dye donor sheet material  36  on the spinning imaging drum  300  while it is rotated past the printhead  500  that will expose the thermal print media  32 . The translator drive  258  then traverses the printhead  500  and translation stage member  220  axially along the axis of the imaging drum in coordinated motion with the rotating imaging drum  300 . These movements combine to produce the intended image on the thermal print media  32 . 
     Continuing with a description of the operation of the apparatus, the media carousel  100  is rotated about its axis into the desired position, so that the thermal print media  32  or dye donor material (in roll form)  34  can be withdrawn, measured, and cut into sheet form of the required length, and then transported to the imaging drum. To accomplish this, the media carousel  100  has a vertical circular plate, preferably with, though not limited to, six material support spindles. The support spindles are arranged to carry one roll of thermal print media, and four rolls of dye donor material. They provide the four primary colors, which are preferably used in the writing process to form the intended image. One roll is used as a spare or for a specialty color dye donor material, if so desired. Each spindle has a feeder assembly to withdraw the thermal print media or dye donor material from the spindles. 
     Turning to FIG. 2, the image processing apparatus  10  includes the imaging drum  300 , printhead  500 , and lead screw  250 , which are assembled in the lathe bed scanning frame  202 . The imaging drum  300  is mounted for rotation about an axis X in the lathe bed scanning frame  202 . The printhead  500  is movable with respect to the imaging drum  300 , and is arranged to direct a beam of light to the dye donor sheet material  36 . The beam of light from the printhead  500  for each laser diode  402  (shown in FIG. 1) is modulated individually by modulated electronic signals from the image processing apparatus  10 . These are representative of the shape and color of the original image. The color on the dye donor sheet material  36  is heated to cause volatilization only in those areas in which its presence is required on the thermal print media  32  to reconstruct the shape and color of the original image. 
     Continuing with FIG. 2, the printhead  500  is mounted on a movable translation stage member  220 , which is supported for low friction movement on translation bearing rods  206 ,  208 . The linear translation subsystem  210  includes the translation stage member  220 , the translation bearing rods  206 ,  208 , and the translator drive  258 . The translation bearing rods  206 ,  208  are sufficiently rigid so as not sag or distort between mounting points and are arranged as parallel as possible with the axis X of the imaging drum  300 , with the axis of the printhead  500  perpendicular to the axis X of the imaging drum  300  axis. The front translation bearing rod  208  locates the translation stage member  220  in the vertical and the horizontal directions with respect to axis X of the imaging drum  300 . The rear translation bearing rod  206  locates the translation stage member  220  only with respect to rotation of the translation stage member  220  about the front translation bearing rod  208 . This is done so that there is no over-constraint of the translation stage member  220 , which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the printhead  500  during the generation of an intended image. The translator drive  258  traverses the translation stage member and printhead axially along the imaging drum. 
     Referring to FIGS. 2 and 3, the lead screw  250  includes an elongated, threaded shaft  252 , which is attached to the translator linear drive motor  258  on its drive end and to the lathe bed scanning frame  202  by means of a radial bearing  272 . A lead screw drive nut  254  includes grooves in its hollowed-out center portion  270  for mating with the threads of the threaded shaft  252 . This allows the lead screw drive nut  254  axial movement along the threaded shaft  252  as the threaded shaft  252  is rotated by the linear drive motor  258 . The lead screw drive nut  254  is integrally attached to the to the printhead  500  through the lead screw coupling (not shown) and the translation stage member  220  at its periphery, so that the threaded shaft  252  is rotated by the linear drive motor  258 . This moves the lead screw drive nut  254  axially along the threaded shaft  252 , which in turn moves the translation stage member  220 , and ultimately the printhead  500  axially along the imaging drum  300 . 
     As best illustrated in FIG. 3, an annular-shaped axial load magnet  260   a  is integrally attached to the driven end of the threaded shaft  252 , and is in a spaced-apart relationship with another annular-shaped axial load magnet  260   b  attached to the lathe bed scanning frame  202 . The axial load magnets  260   a  and  260   b  are preferably made of rare-earth materials such as neodymium-iron-boron. A generally circular-shaped boss  262  part of the threaded shaft  252  rests in the hollowed-out portion of the annular-shaped axial load magnet  260   a , and includes a generally V-shaped surface at the end for receiving a ball bearing  264 . A circular-shaped insert  266  is placed in the hollowed-out portion of the other annular-shaped axial load magnet  260   b . It has an arcuate-shaped surface at one end for receiving ball bearing  264 , and a flat surface at its other end for receiving an end cap  268  placed over the annular-shaped axial load magnet  260   b , which is attached to the lathe bed-scanning frame  202  for protectively covering the annular-shaped axial load magnet  260   b . This provides an axial stop for the lead screw  250 . 
     Continuing with FIG. 3, the linear drive motor  258  is energized and imparts rotation to the lead screw  250 , as indicated by the arrows. This causes the lead screw drive nut  254  to move axially along the threaded shaft  252 . The annular-shaped axial load magnets  260   a ,  260   b  are magnetically attracted to each other, which prevents axial movement of the lead screw  250 . The ball bearing  264 , however, permits rotation of the lead screw  250  while maintaining the positional relationship of the annular-shaped axial load magnets  260 , i.e., slightly spaced apart. Mechanical friction between them is thus prevented, yet the threaded shaft  252  can continue to rotate. 
     The printhead  500  travels in a path along the imaging drum  300 , moving at a speed synchronous with the imaging drum  300  rotation and proportional to the width of the writing swath. The pattern transferred by the printhead  500  to the thermal print media  32  along the imaging drum  300  is a helix. 
     In operation, the scanning subsystem  200  or write engine contains the mechanisms that provide the mechanical actuations for the imaging drum positioning and motion control to facilitate placement of loading onto, and removal of the thermal print media  32  and the dye donor sheet material  36  from the imaging drum  300 . The scanning subsystem  200  or write engine provides the scanning function by retaining the thermal print media  32  and dye donor sheet material  36  on the rotating imaging drum  300 . This generates a once per revolution timing signal to the data path electronics as a clock signal, while the translator drive  258  traverses the translation stage member  220  and printhead  500  axially along the imaging drum  300  in a coordinated motion with the imaging drum rotating past the printhead. Positional accuracy is maintained in order to control the placement of each pixel, so that the intended image produced on the thermal print media is precise. 
     During operation, the lathe bed scanning frame  202  supports the imaging drum and its rotational drive. The translation stage member  220  and write head are supported by the two translation bearing rods  206 ,  208  that are positioned parallel to the imaging drum and lead screw. They are parallel to each other and form a plane therein, along with the imaging drum and lead screw. The translation bearing rods are, in turn, supported by the outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned therebetween. 
     The translation drive  258  is for permitting relative movement of the printhead  500  by means of a DC servomotor and encoder, which rotates the lead screw  250  parallel with the axis of the imaging drum  300 . The printhead  500  is placed on the translation stage member  220  in the “V” shaped grooves. The “V” shaped grooves are in precise relationship to the bearings for the front translation stage member  220  supported by the front and rear translation bearing rods  206 ,  208 . The translation bearing rods are positioned parallel to the imaging drum  300 . The printhead is selectively locatable with respect to the translation stage member; thus it is positioned with respect to the imaging drum surface. The printhead has a means of adjusting the distance between the printhead and the imaging drum surface, and the angular position of the printhead about its axis using adjustment screws. An extension spring provides a load against these two adjustment means. The translation stage member  220  and printhead  500  are attached to the rotational lead screw  250 , which has a threaded shaft, by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only forces parallel to the linear lead screw and rotational forces are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame  202 , where it is supported by deep groove radial bearings. At the drive end, the lead screw  250  continues through the deep groove radial bearing through a pair of spring retainers. The spring retainers are separated and loaded by a compression spring, and to a DC servomotor and encoder. The DC servomotor induces rotation to the lead screw  250 , which moves the translation stage member  220  and printhead  500  along the threaded shaft as the lead screw  250  is rotated. Lateral movement of the printhead  500  is controlled by switching the direction of rotation of the DC servomotor and thus the lead screw  250 . 
     The printhead  500  includes a number of laser diodes  402 , which are tied to the printhead and can be individually modulated to supply energy to selected areas of the thermal print media  32  in accordance with an information signal. The printhead  500  of the image processing apparatus  10  includes a plurality of optical fibers, which are coupled to the laser diodes  402  at one end and at the opposite end to a fiber optic array within the printhead. The printhead  500  is movable relative to the longitudinal axis of the imaging drum  300 . The dye is transferred to the thermal print media  32  as radiation is transferred from the laser diodes by the optical fibers to the printhead, and thus to the dye donor sheet material  36 , and is converted to thermal energy in the dye donor sheet material. 
     Referring to FIG. 4, the imaging drum  300  has a cylindrical-shaped vacuum drum housing  302 . The imaging drum is, by definition, hollow, and includes a hollowed-out interior portion  304 . The imaging drum  300  further includes a number of vacuum grooves  332  and vacuum holes  306  extending through the vacuum drum housing  302 . Vacuum is applied from the hollow interior portion  304  of the imaging drum  300  through these vacuum grooves and holes. The vacuum supports and maintains the position of the thermal print media  32  and the dye donor sheet material  36 , even as the imaging drum  300  rotates. 
     Continuing with FIG. 4, the ends of the imaging drum  300  are closed by a vacuum end plate  308 , and a drive end plate  310 . The drive end plate  310  is provided with a centrally disposed drive spindle  312 , which extends outwardly therefrom through a support bearing. The vacuum end plate  308  is provided with a centrally disposed vacuum spindle  318 , which extends outwardly therefrom through another support bearing. 
     The drive spindle  312  extends through the support bearing and is stepped down to receive a DC drive motor armature (not shown), which is held on by a drive nut. A DC motor stator (not shown) is stationarily held by the late bed scanning frame member  202  (see FIGS.  1  and  2 ), encircling the DC drive motor armature to form a reversible, variable DC drive motor for the imaging drum  300 . A drum encoder mounted at the end of the drive spindle  312  provides timing signals to the image processing apparatus  10 . 
     As shown in FIG. 4, the vacuum spindle  318  is provided with a central vacuum opening  320 . The central vacuum opening  320  is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame  202  (see FIGS.  1  and  2 ). The vacuum fitting has an extension, which extends within but is closely spaced from the vacuum spindle  318 , thus forming a small clearance. With this configuration, a slight vacuum leak is provided between the outer diameter of the vacuum fitting and the inner diameter of the central vacuum opening  320  of the vacuum spindle  318 . This assures that no contact exists between the vacuum fitting and the imaging drum  300  that might impart uneven movement or jitters to the imaging drum  300  during its rotation. 
     The opposite end of the vacuum fitting is connected to a high-volume vacuum blower (not shown), which is capable of producing 50-60 inches of water at an air flow volume of 60-70 CFM. The vacuum blower provides vacuum to the imaging drum  300 . The vacuum blower provides the various internal vacuum levels required during loading, scanning and unloading of the thermal print media  32  and the dye donor sheet materials  36  to create the intended image. With no media loaded on the imaging drum  300 , the internal vacuum level of the imaging drum  300  is preferably approximately 10-15 inches of water. With just the thermal print media  32  loaded on the imaging drum  300 , the internal vacuum level of the imaging drum  300  is preferably approximately 20-25 inches of water. This level is desired so that when a dye donor sheet material  36  is removed, the thermal print media  32  does not move and color to color registration is maintained. With both the thermal print media  32  and dye donor sheet material  36  completely loaded on the imaging drum  300 , the internal vacuum level of the imaging drum  300  is approximately 50-60 inches of water in this embodiment. 
     In operation, vacuum is applied through the vacuum holes  306  extending through the drum housing  302 . The vacuum supports and maintains the position of the thermal print media  32  and dye donor sheet material  36  as the imaging drum  300  rotates. The ends of the imaging drum are preferably enclosed by the cylindrical end plates, which are each provided with a centrally disposed spindle  318 . The spindles extend outwardly through support bearings and are supported by the scanning frame. The drive end spindle extends through the support bearing and is stepped down to receive the motor armature, which is held on by a nut. The stator is held by the scanning frame, which encircles the armature to form the reversible, variable speed DC drive motor for the imaging drum. An encoder mounted at the end of the spindle provides timing signals to the image processing apparatus. The central vacuum opening  320  on the opposite spindle  318  is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame  202 . The vacuum fitting has an extension extending within the vacuum spindle and forming a small clearance. A slight vacuum leak between the outer diameter of the vacuum fitting and the inner diameter of the opening of the vacuum spindle assures that no contact exists between the vacuum fitting and the imaging drum, which might impart uneven movement or jitters to the imaging drum during its rotation. 
     Referring to FIG. 5, the outer surface of the imaging drum  300  is provided with an axially extending flat  322 , which preferably extends approximately 8 degrees of the drum  300  circumference. The imaging drum  300  is provided with donor support rings  324 , which form a radial recess  326  (see FIG.  4 ). This recess extends radially from one side of the axially extending flat  322  around the imaging drum  300  to the other side of the axially extending flat  322 , from approximately one inch from one end of the imaging drum  300  to approximately one inch from the other end of the drum  300 . Although a preferred embodiment herein does include an axially extending flat and a radial recess, the present invention need not include either. 
     The imaging drum axially extending flat has two main purposes. First, it assures that the leading and trailing ends of the dye donor sheet material are somewhat protected from the effect of air during the relatively high speed rotation that the imaging drum undergoes during the imaging process. Here, the air will have less tendency to lift the leading or trailing edges of the dye donor sheet material. The axially extending flat also ensures that the leading and trailing ends of the dye donor sheet material are recessed from the periphery of the imaging drum. This reduces the chance of the dye donor sheet material coming into contact with other parts of the image processing apparatus, such as the printhead. Such contact could cause a jam and possible loss of the intended image, or even catastrophic damage to the image processing apparatus. 
     The imaging drum axially extending flat also acts to impart a bending force to the ends of the dye donor sheet materials as they are held onto the imaging drum surface by vacuum from within the interior of the imaging drum. When the vacuum is turned off to that portion of the imaging drum, the end of the dye donor sheet material will tend to lift from the surface of the imaging drum. Thus turning off the vacuum eliminates the bending force on the dye donor sheet material, and is used as an advantage in the removal of the dye donor sheet material from the imaging drum. 
     As shown in FIGS. 6A through 6C, the thermal print media  32  when mounted on the imaging drum  300  is seated within the radial recess  326 . Therefore, the donor support rings  324  have a thickness which is substantially equal to the thickness of the thermal print media  32  seated therebetween. In this embodiment, this thickness is 0.004 inches. The purpose of the radial recess  326  on the imaging drum  300  surface is to eliminate any creases in the dye donor sheet material  36 , as the materials are drawn down over the thermal print media  32  during the loading of the dye donor sheet material  36 . This ensures that no folds or creases will be generated in the dye donor sheet material  36 , which could extend into the image area and seriously adversely affect the intended image. The radial recess  326  also substantially eliminates the entrapment of air along the edge of the thermal print media  32 , the vacuum holes  306  in the imaging drum  300  surface cannot always ensure the removal of the entrapped air. Any residual air between the thermal print media  32  and the dye donor sheet material  36  can also adversely affect the intended image. 
     An alternate and also preferred embodiment of the present invention is illustrated in FIG.  7 : a laser thermal printer proofer. The laser thermal printer proofer comprises generally a material supply assembly  90 , a sheet cutter assembly  82 , a sheet transport assembly  91 , an imaging drum  300 , a printhead assembly  99 , and exit transport systems  22 ,  24 , which are all described herein above or below. 
     Referring to FIGS. 7 and 8, the material supply assembly  90  comprises a carousel assembly  100  mounted for rotation about a horizontal axis  102  on bearings  37  at the upper ends of vertical supports  38 . The carousel assembly comprises a vertical circular plate  40  having a plurality of material supporting spindles  42  cantilevered outwardly from and equispaced about the front fact of the circular plate. Each of the spindles  42  is arranged to carry a roll supply  30  of material for use on the imaging drum  300 . 
     The carousel  100  is rotated counterclockwise in FIG. 7 by means of a drive motor  64  driving a sheave  66 , which engages a belt  68  that is tensioned around the periphery of the carousel circular plate  40 . A brake assembly  70  is arranged to hold the carousel stationary when it is not being driven by motor  64 . 
     Continuing with FIG. 7, the sheet cutter assembly  82  is disposed adjacent the material supply carousel  100  at the material feed location and is arranged to receive the end of web material as it is fed by the material feed assembly  46 . The sheet cutter assembly comprises a mating pair of cutter blades  84  through which the web material is moved by the material feed assembly  46 . A material metering drum  86  and mating endless drive belt  88  cooperate to engage the web material as it is driven between the cutter blades  84 , to assist the feeding thereof, and to change its path from substantially horizontal to a generally vertical direction. The metering drum  86  and a sensor are arranged to sense the end of the web material being fed and to determine when the desired length of the sheet has been fed between the cutter blades  84 . At that point the metering drum  86  and the cooperating belt  88 , as well as the drive assembly, are stopped and the cutter blades are actuated to chop a sheet member from the end of the web material. The web metering arrangement is capable of providing sheets having two different lengths for cutting the receiver material and the donor material. It is desired to form the donor material with a greater length than the receiver material, so that it overlies the leading and trailing ends of the receiver material when they are superposed upon the imaging drum. The material metering drum  86  and its mating drive belt  88  gently engage the material being transported, and do not scratch or otherwise damage the sensitive surface of the material being fed. 
     Although FIG. 8 illustrates a carousel having six spindles, a greater or lesser number of spindles may be provided depending upon the needs of the user. Each of the material supply spindles is provided with a corresponding material feeder assembly  46 , only one of which is illustrated in FIG.  7 . Each of the material feeder assemblies is arranged to withdraw the end of the roll material from the rolls  30  carried on the spindles  42 . 
     The roll material  30  is provided to the apparatus  10  on flangeless cores to economize cost and weight. The flanges for the rolls  30  are part of the spindles  42  of the carousel. The weight of the roll of material is sufficient to keep the roll from telescoping, clockspringing, or unwinding unless the material is driven by the drive roller  48  (see FIG.  8 ). 
     The carousel  100  is rotated about its axis to bring a selected roll supply of material into opposition with the sheet cutter assembly  82  where the material is removed from the roll supply  30 , is fed through the cutter assembly  82 , is measured, and is then cut. 
     The sheet transport assembly  91  is also illustrated in FIG.  7 . The sheet transport assembly  91  comprises an upwardly directed air table  94  open through into an air chamber  92  beneath. The air chamber  92  is provided with a source of pressurized air. The air escapes through a plurality of holes in the air table  94 . A sheet is thus supported on the air table. The supply of air to the air chamber is controlled by a damper valve (not shown) in the inlet to the air chamber. A plurality of wire guides  95  are suspended above the surface of the air table  94  to limit the upward movement of the sheet due to air flow from the air table. One edge of the air table  94  is provided with an edge guide, which depends from a plate that lies in substantially the same plane as the wire guides  95 . As shown in FIG. 7, three pairs of soft, flexible drive rollers  104  are disposed along the lateral edge of the air table  94 . The drive rollers  104  form a nip with sheet transport rollers  103  and are arranged to gently engage the lateral edges of the sheets being transported to drive the sheet toward the imaging drum and urge it against the edge guide to provide lateral registration of the sheet with respect to the drum axis. The rollers  103 ,  104  are supplied with power from a motor. 
     The printhead, or writehead, assembly  99  comprises the printhead  500 , translation stage member  220 , and laser diodes  402 , as shown in FIG.  7  and described above. The optical fibers extend from the end of the printhead assembly through a protective sheath  242  to the diode lasers  402  (see FIG.  7 ). 
     With regard to the exit transport systems of the present invention, the dye donor sheet material  36  is removed from the imaging drum without disturbing the thermal print media beneath it after the intended image has been written on the thermal print media  32 . The exit transport systems comprise a waste sheet exit transport  22 , and an image sheet exit transport  24 . The dye donor sheet material  36  is transported out of the image processing apparatus  10 , and additional dye donor sheet materials  36  are sequentially superimposed with the thermal print media  32  on the imaging drum. Then they are imaged onto the thermal print media until the intended image is complete. The completed image on the thermal print media is then unloaded from the imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport. 
     Referring to FIG. 7, the sheet material exit transport  22  includes a sheet material waste exit and an imaged sheet material exit. The dye donor sheet material exit transport includes a waste dye donor sheet material stripper blade  410 , which is disposed adjacent to the upper surface of the imaging drum  300 . The donor stripper blade  410  is movable between an unloading position, where it is in contact with the sheets on the imaging drum surface, and an inoperative position, where it is moved up and away from the surface of the imaging drum. In the unloading position, the donor stripper blade  410  contacts the waste dye donor sheet material on the imaging drum surface. When not in operation, the stripper blade is moved up and away from the surface of the imaging drum  300 . A driven waste dye donor sheet material transport belt  412  is arranged substantially horizontally to carry the waste dye donor sheet material, which is removed by the stripper blade  410  from the surface of the imaging drum, to an exit  414  from the image processing apparatus. A waste bin  18  for waste/dye donor sheet materials is separate from the image processing apparatus  10  (see FIG.  1 ). 
     The image sheet exit transport  24  comprises a stationary image exit blade  416  disposed adjacent to the top surface of the imaging drum  300  substantially opposite from the movable stripper blade  410 , as shown in FIG.  7 . An image sheet transfer belt  418  is arranged for cooperation with a vacuum table  420  to deliver a receiver sheet with an image formed thereon to an exit tray  422  in the exterior of the apparatus. 
     As shown in FIG. 7, the material supply assembly  90 , sheet cutter assembly  82 , sheet transport assembly  91 , imaging drum  300 , load roller  350 , printhead assembly  99 , and exit transport systems  22 ,  24  are preferably enclosed by a proofing printer cabinet  26 . 
     An imaging drum  300  and a magnetic load roller  350  of the present invention are shown in partial schematic view in FIGS. 9 and 10. The magnetic load roller  350  is shown in an unloaded position in FIG. 9, with the load roller  350  detached from the surface of the imaging drum  300 , and a loaded position in FIG. 10, with the magnetic load roller  350  articulated into the imaging drum. In the loaded position, the load roller  350  is in contact with the dye donor sheet material  36 , which is in place over the thermal print media  32  on the imaging drum  300 . When the apparatus  10  is in operation, the magnetic load roller  350  is in the loaded position. When the apparatus  10  is not in use, the load roller  350  is in the unloaded position. 
     In the preferred embodiment shown in FIGS. 9 and 10, the load roller  350  is comprised of three layers. The outermost layer is an elastic or elastomeric layer  370  or coating for cushioning the outside of the roller. The second layer is a magnetic layer  380 . Beneath the magnetic layer is the load roller core  390 , which is most preferably made of steel. The magnet layer is layered around the load roller core  390 , and the elastomeric layer  370  is wrapped around the magnet layer  380 . The imaging drum  300  is coated with a ferrous coating  360 . 
     In operation, once the thermal print medium receiver sheet material  32  is in place, the dye donor sheet material  36  is positioned on the imaging drum  300  in registration with the thermal print media  32 . The process for loading the thermal print media  32  to the imaging drum  300  is as described herein. The dye donor sheet material  36  now rests atop the thermal print media  32 , with a narrow gap between the two. The narrow gap is created by micro-beads embedded in the surface of the thermal print media  32 . The load roller  350  is moved into contact with the dye donor material  36 . Surprisingly, when magnets are embedded in the surface of the load roller, or when the load roller is itself a magnet, the load roller aligns itself as it approaches the imaging drum. The imaging drum  300  has a ferrous coating  360  so that it attracts the magnetic load roller  350  when it is nearby. The imaging drum must be ferrous-coated or otherwise “magnet-attracting”, so that it attracts the magnetic load roller when the load roller is in the vicinity of the imaging drum. The imaging drum  300  is then rotated counterclockwise, with the magnetic load roller  350  engaged, until the magnetic load roller  350  is at the end of the dye donor sheet material  36 . 
     The direction of rotation of the imaging drum  300  is then reversed until the magnetic load roller  350  is passed to the opposite end of thermal print medium receiver sheet material  32 , and over embedded magnets  395  in the imaging drum  300 . The opposing force of the embedded magnets in the imaging drum forces the magnetic load roller  350  away from the surface of the imaging drum  300 . 
     Referring to FIG. 11, a cutaway of the imaging drum  300  shows an unload drum magnet  395  embedded in the surface of the imaging drum. In this preferred embodiment, a thin line (e.g., about {fraction (1/16)}-1 inch wide) or row of at least one, and preferably a plurality of adjacent, unload drum magnets  395  extends the length of the imaging drum  300 . The unload magnet  395  is charged with a North polarity. The external magnetic load roller  350  is shown in an unloading position. Once the imaging drum  300  rotates around so that the load roller  350  comes into contact with the line of magnets in the imaging drum, the North pole of the load roller  350  faces toward the North pole of the magnet  395  in the imaging drum  300 . Since like forces repel, the opposing force between the magnetic fields of the imaging drum  300  and the load roller  350  force the magnetic load roller  350  away from the surface of the imaging drum  300 . 
     This is in contrast to the loading position, where the magnetic North pole of the load roller  350  faces away from the North pole of the imaging drum  300 , as shown in FIG. 9, and the load roller is attracted to the imaging drum. 
     Alternatively, brute force generated by the motor can be used to pull the load roller off the imaging drum once the task is complete. 
     There are several ways to prepare or manufacture the magnetic load roller. A ferromagnetic or stainless steel coating may be sprayed on the load roller. The load roller may be plasma coated or vacuum coated with a ferromagnetic coating. Individual or strip magnets may be cast into the load roller or epoxied onto the load roller. Magnets may be wrapped around the load roller. Alternatively, the load roller itself may be magnetized. 
     In a more difficult and therefore less preferred embodiment herein, the ferrous coating could be sprayed on the load roller  350 , and the imaging drum  300  could be magnetized as described herein instead of the load roller. In the same manner as described herein, the load roller would thus be attracted to the imaging drum. 
     In the past, deflection caused a load roller to bow outward in the center over time in contact with the imaging drum. In an effort to remedy this, the ends of the load roller were crowned, so that the load roller was smaller in diameter at its ends than toward the inside of the roller. With the present invention, crowning is not necessary. This results in cost and time savings during manufacture of the load rollers. The present invention allows even distribution of pressure against the magnetic load roller. Magnets are preferably evenly distributed along the surface of the load roller, or the magnet layer is distributed evenly within the load roller, depending on the particular embodiment. The magnetic load roller  350  is evenly attracted along its length to the ferrous-coated imaging drum, except for the magnetic “unload” line  395  along the imaging drum. As described herein, the magnetic load roller  350  also provides uniform loading to the imaging drum. 
     Although a vacuum imaging drum is employed in this preferred embodiment, other types of imaging drums or similar surfaces may also be employed herein. The magnetic load roller of the present invention could in fact be utilized in any mechanical apparatus that requires a load roller to remove entrained air between the media and the operating surface, such as a lamination roller. For example, a magnetic load roller  350  may be used in an imaging apparatus having an ink jet head rather than a laser printer to remove entrained air between a receiver sheet and the surface on which the image is formed. 
     Also, a magnetic load roller according to the present invention can be used with a platen instead of an imaging drum, so long as the platen is coated with or contains a material, which is attractive to the magnetic field of the load roller  350 . The platen can be, for example, coated with a ferrous coating, or it can be made of plastic or rubber that is manufactured or coated with a ferrous material. 
     Referring to FIG. 12, another alternative embodiment herein is a magnetic load roller  350  positioned on the inside of a hollow imaging drum  300  rather than outside the drum. Such a magnetic load roller  350  may be used, for example, with an internal drum scanner or an internal drum writer to eliminate entrained air. In this embodiment, the magnetic load roller  350  is articulated away from the surface of the imaging drum  300 , as shown in FIG. 12, when it is not in use. When in use, the magnetic load roller  350  is attracted to the ferrous coating of the inside drum wall  348 . Once the thermal print media  32 , or other suitable type of media, is loaded on the inside drum wall  348 , the magnetic load roller  350  presses against the thermal print media sheet  32  on the inside wall  348  of the imaging drum  300 . As the imaging drum  300  rotates in either direction, any entrained air between the thermal print media sheet  32  and the inside drum wall  348  is eliminated by the action of the magnetic load roller  350 . Once the magnetic load roller  350  comes into contact with the row of magnets  395  embedded in the surface of the inside drum wall  348 , it disengages from the inside drum wall  348 . 
     In general, a preferred imaging apparatus for forming images on a print media, comprises: a) an imaging head, most preferably a printhead  500 ; b) a magnet-attracting imaging surface; c) print media, most preferably a thermal print media sheet  32 , removably mounted on the imaging surface, the imaging head being positioned to move over the print media on the imaging surface; and d) a magnetic load roller  350 . Preferably, the imaging apparatus is an ink jet printer or, most preferably, a laser printer. The imaging surface is preferably a platen comprising a ferrous material, or an imaging drum internally or externally coated with a ferrous material so that it attracts (hence the term “magnet-attracting”) the magnetic load roller when it is nearby. The load roller preferably comprises a plurality of magnets embedded in its surface, which provide its magnetic field. 
     An alternate, less preferred embodiment herein comprises: (a) a magnetic, rotatable imaging drum; (b) an imaging head which is movable along the longitudinal axis of the imaging drum at a speed synchronous with the rotation of the imaging drum; (c) thermal print media removably mounted on the imaging drum, the imaging head being positioned to systematically travel over the thermal print media on the imaging drum; and (d) a load roller coated with a ferrous material. 
     The operational sequence of one embodiment, a laser thermal printer proofer, according to the present invention will now be described with reference to FIGS. 7,  10 ,  11 ,  13   a-h ,  14   a-c , and  15 . This preferred embodiment includes a drum flat and a sheet sensor. Other embodiments herein, which are also preferred, though, do not have a drum flat or sheet sensor and employ other devices for, or means of, positioning the media. 
     In the following description of the operation, the sequence step order is indicated at the beginning of each step as a number in a bracket [#], which also corresponds to the sequence number indicated in the chart illustrated in FIGS. 14 a-c . FIGS. 13 a-h  show the loading and unloading of material to and from the imaging drum  300  at various selected steps in the process. FIGS. 14 a-c  show tabulations of drum operating conditions at each of the steps shown in FIGS. 13 a-h . FIG. 15 illustrates an imaging drum  300  and magnetic load roller  350  in various operating positions. The drum centerline positions noted in FIG. 15 are illustrated schematically in FIGS. 13 a-h.    
     [1] With the imaging drum  300  located with the centerline  269  of the drum flat  352  located in the “Home” position, as shown in FIG. 15, the carousel  100 , imaging drum  300 , metering roll  86 , and the sheet drive rollers  103 ,  104  are all stationary. At “Home”, no material is superposed on the imaging drum  300  and the vacuum pump connected thereto is off. The magnetic load roller  350  is disengaged from the imaging drum  300 , as is the donor stripper blade  410 . To begin, the carousel  100  is rotated until the supply roll  30  of the receiver material  32  is located adjacent to the sheet cutter assembly  82 . The carousel is stopped and the edge guide is disposed in a first, receiver, position. The air to the air table  94  is turned off by closing the damper valve. The sheet feed roll  48  is driven by the drive, feeding the end of the receiver web into the sheet cutter assembly  82 . There it is engaged by the metering roll  86  and drive belt  88  and advanced until the proper length of material is determined. The cutter blades  84  are actuated. The condition of the overall system at this point is illustrated in FIG. 13 a.    
     [2] The imaging drum is then rotated in a first, clockwise direction. Where the drum has a drum flat, the imaging drum  300  is stopped with the centerline  269  of the drum flat  352  disposed under the sheet sensor  101  at position H (see FIGS.  14  and  15 ). All other conditions are the same as Step 1. 
     [3] The sheet transport assembly  91  is then actuated and the thermal print media sheet  32  is driven up until its leading edge is sensed by the sheet sensor  101  and the transport assembly drive rollers and the sheet are stopped. As the thermal print media sheet  32  is driven into engagement with the imaging drum  300 , the sheet sensor  101  detects the lead edge of the sheet  32  by detecting the reflection from the surface of the sheet and the sheet  32  is stopped with its edge at the sheet sensor  101  centerline at position “A” (see FIG.  15 ). 
     [4] The imaging drum is then rotated counterclockwise to the thermal print media load position “B” (see FIG. 15) and is stopped. 
     [5] Where the imaging drum is a vacuum drum, the vacuum to the imaging drum  300  is then actuated by actuating the vacuum pump. All of the vacuum openings and vacuum chambers in the imaging drum are supplied with vacuum at this point. A vacuum blower  331 , which supplies vacuum to the vacuum drum, is shown in FIGS. 13 a-h.    
     [6] The magnetic load roller  350  is moved into engagement with the end of the thermal print media sheet  32 . The vacuum operates to hold the thermal print media sheet  32  to the drum surface. The condition of the overall system at this point is illustrated in FIG. 13 b . Since the load roller  350  is magnetic, it is attracted to the ferrous-coating  360  of the imaging drum  300  (see FIG.  11 ). It is therefore not necessary to mechanically press the magnetic load roller  350  against the imaging drum  300 . 
     [7] The imaging drum  300  is then rotated counterclockwise until the trailing edge of the thermal print media sheet  32  is under the magnetic load roller  350 . The magnetic load roller  350  facilitates the removal of the air from between the thermal print media sheet  32  and the drum surface. Since the magnetic load roller  350  is so effective, it is not necessary to rotate the imaging drum clockwise again until the lead edge of the thermal print media sheet  32  is beneath the load roller  350 . The magnetic load roller therefore saves a step in the imaging process. 
     [8] The magnetic load roller  350  is then moved away from the imaging drum  300 . Preferably, the imaging drum  300  is then rotated counterclockwise until the line of magnets  395  on the imaging drum  300  nears the magnetic load roller  350 . The force of the repulsion moves the load roller to a disengaged, or unload, position, which is spaced apart from the surface of the imaging drum. Alternatively, the magnetic load roller  350  is mechanically pulled away from the imaging drum  300 . 
     [9] The carousel  100  is rotated and stopped at the appropriate donor supply location, and the edge guide is moved to the second, donor position so the dye donor sheet will properly overlap the thermal print media sheet  32  when the dye donor sheet  36  is superposed therewith on the imaging drum  300 . The sheet feed roll  48  is driven by the drive, feeding the end of the donor web into the sheet cutter assembly where it is engaged by the metering roll  86  and belt  88  and advanced until the proper length is reached. The cutter blades  84  are actuated to cut off a dye donor sheet. The imaging drum is rotated clockwise to the donor sheet loading position “H”. 
     [10] The vacuum to a donor chamber is turned off by engaging a valve actuator cam actuated by a motor. The chamber is opened to atmospheric pressure because, since the thermal print media sheet has previously been superposed on the majority of the imaging drum surface, closing off the majority of the vacuum openings therethrough, the vacuum now available at the lead edge of the donor sheet is sufficiently strong that it might prevent the movement of the sheet over the drum surface were that portion of the vacuum holes not isolated from the vacuum. 
     [11] The dye donor sheet  36 , which is now located on the sheet transport assembly  91 , is driven upward and stopped with the leading edge of the dye donor sheet at the sheet sensor  101 . The sheet sensor  101  has previously checked the location of the thermal print media sheet  32  to assure that it does not overlie a registration indicia and, if it does, to generate a fault signal to stop the sequence. The condition of the overall system at this point is illustrated in FIG. 13 c.    
     [12] The vacuum to the donor chamber is turned back on by disengaging the valve actuator cam. Vacuum is thus reapplied to the leading edge of the dye donor sheet  36 . 
     [13] The magnetic load roller  350  is then moved to the engaged position (see FIG.  10 ), which forces the dye donor sheet  36  into engagement with the drum flat  352 . 
     [14] The imaging drum  300  is rotated counterclockwise until the trailing edge of the donor sheet  36  is under the magnetic load roller  350 . The condition of the overall system at this point is illustrated in FIG. 13 d.    
     Importantly, reversing the imaging drum  300  and re-rolling the dye donor sheet  36  into contact with the thermal print media sheet  32  was necessary prior to the present invention because some air often still remained after the first pass of the load roller. Such residual entrained air went unnoticed until the image was formed. Areas of low density arc were caused by the entrained air. A second pass of the load roller was necessary to eliminate the remaining entrained air. With the present invention, the attraction between the imaging drum and the magnetic load roller  350  applies greater pressure on the media during the first pass. Since residual entrained air is eliminated during the first pass, a second pass is not necessary. This saves time and reduces complication. Thus, a method of rolling out sheet material so as to remove air entrapment between the sheet media and the drum or platen, or between sheets of media, is provided herein. 
     [15] The imaging drum  300  is rotated in an opposite direction, until the row of magnets  395  in the imaging drum  300  disengages the magnetic load roller  350  from the imaging drum (see FIG.  11 ), or until the magnetic load roller is mechanically disengaged. The rotation of the imaging drum  300  is then accelerated in the counterclockwise direction to image writing speed, and the imaging process commences. 
     After the image has been written onto the thermal print media sheet from the first donor sheet, the first donor sheet must be removed from super-position with the thermal print media sheet without moving the thermal print media from its location on the imaging drum surface. The donor sheet must be removed without disturbing the thermal print media sheet. This is accomplished by the following sequence of steps: 
     [16] The imaging drum is stopped at the donor unload position indicated by position “F” in FIG.  15 . 
     [17] The donor stripper blade  410  is actuated to the position against the imaging drum surface. 
     [18] The vacuum chamber valve actuator cam  398  is actuated to engage the valve actuators to both the donor vacuum chamber  362  and the receiver vacuum chamber  364 . At this point, since the vacuum under the leading edge of the donor sheet has been turned off, and since this portion of the donor sheet has been wrapped around the leading edge of the drum flat, the beam strength of the sheet material tends to lift the leading edge of the donor sheet from the drum flat  352 , as illustrated in FIG. 13 e . Although the vacuum to the trailing edge of the thermal print media sheet has also been turned off, the trailing edge of the  29  thermal print media sheet is still held by the superposed trailing edge of the donor sheet. 
     [19] The imaging drum is now rotated counterclockwise until approximately 1 inch of the leading edge of the donor sheet, which has raised up away from the flat on the surface of the imaging drum, engages the donor stripper blade, substantially as illustrated in FIG. 13 f.    
     [20] The donor stripper blade  410  is then moved to its disengaged position, with the leading edge of the donor sheet supported thereon. The donor exit drive belt  412  is energized at this point. 
     [21] The valve actuator cam is moved to its inoperative position, reapplying vacuum to all of the vacuum chambers in the vacuum drum. 
     [22] The imaging drum  300  is rotated counterclockwise to completely strip the donor sheet  36  from superposition with the thermal print media sheet  32  and to drive it to the waste exit  414  of the apparatus (see FIG.  7 ). 
     The imaging drum is now ready for the superposition of the next donor sheet with the thermal print media material already registered thereon and containing a first image recorded from the first donor sheet. The second donor is then loaded onto the imaging drum by repeating Steps [8-15] from the above loading sequence, and the next image is written onto the thermal print media  32 . That donor sheet is then removed according to, the foregoing unloading sequence of Steps [16-22]. This sequence continues, utilizing as many donor material sheets as the operator or program calls for. The apparatus is then ready to unload the receiver sheet bearing the finished image. 
     To unload the finished receiver, the following sequence is employed: 
     [23] The imaging drum  300  is stopped at the receiver unload position “D”, as shown in FIG.  15 . 
     [24] The valve actuator cam is engaged, which reduces vacuum in the imaging drum  300 . This releases the trailing end of the thermal print media sheet  32 , which is no longer held down by a superposed donor sheet  36 , and the exit transport belt  412  and vacuum are activated. 
     [25] The imaging drum  300  is rotated clockwise until the trailing end edge of the thermal print media sheet  32  is engaged by and lifted from the imaging drum  300  by the receiver sheet exit guide  416 . The condition of the overall system at this point is illustrated in FIG. 13 h.    
     [26] The valve actuating cam is disengaged, permitting vacuum to be reapplied to all of the imaging drum  300 . 
     [27] The imaging drum  300  is rotated clockwise driving the thermal print media sheet  32  onto the receiver sheet exit guide on the receiver exit transport belt  412 . Even though the vacuum has been reapplied to the imaging drum and the thermal print media sheet is being peeled from the surface of the imaging drum by the receiver sheet exit guide, the number of vacuum holes open to the atmosphere is progressively increasing as the thermal print media sheet is removed, so that less and less vacuum hold down is provided to the thermal print media sheet remaining on the imaging drum, with only an amount of vacuum remaining sufficient to retain the “leading” end of the thermal print media sheet in position until the imaging drum has rotated sufficiently that the entire thermal print media sheet has been removed therefrom. The finished thermal print media sheet is exited from the machine. 
     [28] The imaging drum  300  is then rotated counterclockwise to the “Home” position, the vacuum is turned off, and the apparatus is ready to generate the next proof. 
     A preferred process for loading thermal print media and/or donor material onto an imaging drum according to the present invention, then, includes the following steps: 
     a) rotating an imaging drum  300  in a first direction of rotation (see Step [2]); 
     b) actuating a sheet transport assembly  91  and driving a sheet of thermal print media  32  to the imaging drum  300  until a leading edge of the thermal print media sheet  32  engages the imaging drum  300 , and then stopping the sheet transport assembly (see Step [3]); 
     c) rotating the imaging drum  300  in a second direction of rotation and stopping the imaging drum at a first media load position (see position “G” in FIG.  15  and Step [4]); 
     d) moving a magnetic load roller  350  into engagement with the leading edge of the thermal print media sheet  32  (see Step [6]); 
     e) rotating the imaging drum  300  in a second direction of rotation until a trailing edge of the thermal print media sheet  32  is under the magnetic load roller  350 , and then stopping rotation of the imaging drum (see Step [7]); and 
     f) moving the magnetic load roller  350  away from the imaging drum  300  (see Step [8]). 
     The process preferably includes a step g): repeating Steps a) through g) using a donor material  36 . Steps subsequent to Step f) preferably include the following: 
     h) continuing to rotate the imaging drum  300 , while actuating the sheet transport assembly  91  and driving a sheet of donor media  36  to the imaging drum  300  (see Steps [9, 11]); 
     i) stopping the imaging drum  300  at a donor sheet loading position (see position “H” in FIG.  15 ), and overlapping a leading edge of the donor sheet  36  onto the thermal print media sheet  32  (see Steps [9, 11]); 
     j) engaging the magnetic load roller  350  (see Step [13]); 
     k) rotating the imaging drum  300  in the second direction until a trailing edge of the donor sheet  36  is under the magnetic load roller  350  (see Step [14]); and 
     1) disengaging the magnetic load roller  350  from the imaging drum  300  (see Step [15]). 
     These steps, with slight modification, are also appropriate for embodiments of the present invention that do not include a drum flat or a sheet sensor. Where the imaging surface is an imaging drum, the first direction of rotation is preferably clockwise, and the second direction of rotation is preferably opposite to the first direction, and counterclockwise. Where the imaging surface in alternate embodiments is a platen, though, the second direction need not be counterclockwise, or even a direction opposite to the first direction. Movement, for example, may occur in a right or left direction, or in an up and down direction. 
     Preferably, Step (c) further comprises creating a vacuum in the vacuum drum, where the imaging drum is a vacuum drum. 
     Preferably, Step f) comprises rotating the imaging drum  300  until the magnetic load roller  350  contacts a row of magnets  395  in the imaging drum  300 , and opposing magnetic forces between the load roller  350  and the magnets  395  in the drum push the magnetic load roller away from the media  32 ,  36  on the imaging drum to a more remote, disengaged position. Alternatively, the magnetic load roller  350  is mechanically pulled away from the imaging drum  300  to a disengaged position. 
     Preferably, in Step k) the donor media  36  is superposed on the thermal print media  32 . 
     Preferably, Step l) comprises rotating the imaging drum  300  in the first direction, until the magnetic load roller  350  is disengaged by nearing at least one unload magnet  395  embedded in the surface of the imaging drum  300 . Alternatively, Step l) comprises moving the magnetic load roller  350  to a disengaged position by motor-driven mechanical means (e.g., the linear drive motor drives an arm attached to the magnetic load roller). Step l) is preferably followed by Step m): accelerating the imaging drum  300  in a second direction to image writing speed and writing the image. Step m) is followed by Step n): removing the donor sheet  36  (see Steps [16-22] above), and Step o): unloading the finished thermal print media sheet  32  (see Steps [23-28] above). 
     Preferably, Step a) is preceded by the steps of: 1) rotating a carousel assembly  100  until a feed roll  48  of thermal print media material is adjacent to a sheet cutter assembly  82 ; 2) stopping the carousel assembly  100 ; 3) driving the media feed roll  48  and feeding an end of the feed roll into the sheet cutter assembly  82 ; 4) engaging the end of the media feed roll with a metering roll  86  and drive belt  88 , and advancing the feed roll  48  until a predetermined length of thermal print media material is determined; and 5) actuating cutter blades  84  in the sheet cutter assembly  82  and cutting a thermal print media sheet from the media feed roll. 
     Preferably, Step h) is preceded by the steps of: (1) rotating the carousel  100  until a feed roll  48  of donor material is located adjacent to a sheet cutter assembly  82 ; (2) stopping the carousel  100  at a donor supply location; (3) driving the sheet feed roll  48 , and feeding the end of a donor roll into the sheet cutter assembly; (4) engaging the end of the donor feed roll by the metering roll  86  and drive belt  88  until a pre-determined length of donor material is determined; and (5) actuating cutter blades  84  in the sheet cutter assembly  82 , and cutting a donor sheet from the donor feed roll. 
     While the preferred embodiment has been described with respect to an apparatus that employs a rotating imaging drum, many of the features and advantages thereof can be incorporated in a process and apparatus employing a driven platen to carry the superposed thermal print media and donor materials. 
     While certain preferred operating conditions and ranges have been set forth above, it will be understood that the apparatus can use other operating conditions and ranges. For example, the writing laser diodes may operate with a variable power range of 160-500 mw each, at wavelengths in the range of 800-880 nm (nanometers), and the imaging drum can write at a resolution in the range of 1200-2400 dpi at speeds of 250-1200 rpm. While the focusing beam of the preferred embodiment preferentially has a wavelength of 960 nm, it will be appreciated that alternative wavelengths may be chosen so long as they are sufficiently different from the predominant wavelength of the writing beam as to be readily distinguishable therefrom. 
     A further alternative to the preferred embodiment may be found in the surface chosen from which to reflect the focus beam. While the reflective surface of the receiver element is preferred, it is possible to reflect the focus beam from the surface of the drum member, particularly if the receiver element is transparent, or if the imaging drum surface is particularly reflective. Other surfaces of the writing element may also be chosen as the surface from which to reflect the focus beam. 
     Additional variations in the present invention relate to the placement of the photodetector. For example it may be located outside, but adjacent to the writing head so that the reflected portion of the focusing beam need not pass through the focusing assembly. Further, it is possible to locate the photodetector behind a transparent surface of the support member so that it responds to the direct impingement of the focusing beam without requiring any reflection thereof. 
     Accordingly, the present invention provides a process and apparatus for consistently, quickly and accurately generating an image utilizing such an imaging process to create high quality, accurate, and consistent proof images, which process and apparatus is substantially automated to improve the control, quality and productivity of the proofing process while minimizing the attendance and labor necessary. Moreover, the writing apparatus is capable of not only generating this high quality image consistently, but is capable of creating a multi-color image which is in registration regardless of how the various individual images are supplied to the element comprising the final image. Thus, the present invention provides both a process and apparatus in which the various donor material sheets are sequentially superposed with a single thermal print media sheet and then removed. 
     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 as described hereinabove and as defined in the appended claims by a person of ordinary skill in the art, without departing from the scope of the invention. While preferred embodiments of the invention have been described using specific terms, this description is for illustrative purposes only. It is intended that the doctrine of equivalents be relied upon to determine the fair scope of these claims in connection with any other person&#39;s product which fall outside the literal wording of these claims, but which in reality do not materially depart from this invention. 
     PARTS LIST 
       10 . Image processing apparatus 
       12 . Image processor housing 
       14 . Image processor door 
       16 . Donor ejection chute 
       18 . Donor waste bin 
       20 . Media stop 
       22 . Waste sheet exit transport 
       24 . Image sheet exit transport 
       26 . Printer cabinet 
       30 . Roll media 
       32 . Thermal print media 
       34 . Dye donor roll material 
       36 . Dye donor sheet material 
       37 . Carousel bearing 
       38 . Vertical support 
       40 . Vertical circular plate 
       42 . Spindle 
       46 . Material feed assembly 
       48 . Driven roll 
       50 . Sheet material trays 
       50   a . Lower sheet thermal print material tray 
       50   b . Upper sheet input image material tray 
       52 . Media lift cams 
       52   a . Lower media lift cam 
       52   b . Upper media lift cam 
       54 . Media rollers 
       54   a . Lower media roller 
       54   b . Upper media roller 
       56 . Media guide 
       58 . Media guide rollers 
       60 . Media staging tray 
       64 . Drive motor 
       66 . Sheave 
       68 . Sheave belt 
       70 . Brake assembly 
       80 . Transport mechanism 
       82 . Sheet cutter assembly 
       84 . Cutter blades 
       86 . Material metering drum 
       88 . Drive belt 
       90 . Material supply assembly 
       91 . Sheet transport assembly 
       92 . Air chamber 
       94 . Air table 
       95 . Wire guides 
       99 . Printhead assembly 
       100 . Media carousel 
       101 . Sheet sensor 
       102 . Horizontal axis 
       103 . Sheet transport rollers 
       104 . Sheet drive rollers 
       110 . Media drive mechanism 
       112 . Media drive rollers 
       120 . Media knife assembly 
       122 . Media knife blades 
       198 . Master lathe bed scanning engine 
       200 . Lathe bed scanning subsystem 
       202 . Lathe bed scanning frame 
       204 . Entrance passageway 
       206 . Rear translation bearing rod 
       208 . Front translation bearing rod 
       210 . Linear translation subsystem 
       220 . Translation stage member 
       242 . Protective sheath 
       250 . Lead screw 
       252 . Threaded shaft 
       254 . Lead screw drive nut 
       258 . Translator drive linear motor 
       260 . Axial load magnets 
       260   a . Axial load magnet 
       260   b  Axial load magnet 
       262 . Circular-shaped boss 
       264 . Ball bearing 
       266 . Circular-shaped insert 
       268 . End cap 
       269 . Centerline of drum flat 
       270 . Hollowed-out center portion 
       272 . Radial bearing 
       300 . Imaging drum 
       302 . Drum housing 
       304 . Hollowed-out interior portion 
       306 . Vacuum hole 
       308 . Vacuum end plate 
       310 . Drive end plate 
       312 . Drive spindle 
       318 . Vacuum spindle 
       320 . Central vacuum opening 
       322 . Axially extending flat 
       324 . Donor support ring 
       326 . Radial recess 
       331 . Vacuum blower 
       332 . Vacuum grooves 
       348 . Inside wall of imaging drum 
       350 . Magnetic load roller 
       352 . Drum flat 
       360 . Ferrous coating 
       370 . Elastic layer 
       380 . Magnetic layer 
       390 . Load roller core 
       395 . Magnet in unload position 
       400 . Laser assembly 
       402 . Laser diodes 
       404 . Fiber optic cables 
       406 . Distribution block 
       410 . Stripper blade 
       412 . Waste transport belt 
       414 . Exit 
       416 . Exit blade 
       418 . Image sheet transfer belt 
       420 . Vacuum table 
       422 . Exit tray 
       454 . Optical centerline 
       500 . Printhead