Abstract:
Heat is uniformly conducted to print media in an ink-jet printer in conjunction with the uniform application of vacuum pressure to the media for supporting the media as it is conveyed on a heated belt through the printer. The heat is applied to the media by conduction, in a manner that does not overheat the print head of the printer nor interfere with the trajectory of the droplets expelled from the print head. The heat is applied to the media in the print zone as well as regions on either side of the print zone where the media enters and exits the print zone. The amount of heat applied to each of these regions is independently controlled, and can be related to the physical characteristics of the particular type of print media or inks that are used.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a divisional of copending application Ser. No. 09/412,842 filed on Oct. 5, 1999, now U.S. Pat. No. 6,336,722 which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This invention relates to the heating of print media that is advanced through an ink-jet printer. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     An ink-jet printer includes at least one print cartridge that contains liquid ink within a reservoir. The reservoir is connected to a print head that is mounted to the body of the cartridge. The print head is controlled for ejecting minute droplets of ink from the print head to a print medium, such as paper, that is advanced through the printer. 
     Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the droplets onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed. 
     Oftentimes, especially for color images, the carriage is scanned more than once across the same swath. With each such scan, a different combination of colors or droplet patterns may be printed until the complete swath of the image is formed. One reason for this multi-scan print mode is to enable the ink of one color to dry on the media before printing a second color pattern that abuts the first pattern. This print mode thus prevents color bleeding that might otherwise occur if two abutting, different-colored droplets were printed at the same time. 
     The speed with which the print media is moved through a printer is an important design consideration, called “throughput.” Throughput is usually measured in the number of sheets of print media moved through the printer each minute. A high throughput is desirable. A printer designer, however, may not merely increase throughput without considering the effect of the increase on other print quality factors. 
     For instance, one important factor affecting the print quality of ink-jet printers is drying time. The print media movement must be controlled to ensure that the liquid ink dries properly once printed. If, for example, sheets of printed media are allowed to contact one another before ink is adequately dried, smearing can occur as a result of that contact. Thus, the throughput of a printer may be limited to avoid contact until the sheets are sufficiently dry. This potential for smearing is present irrespective of whether ink is applied by a scanning technique as discussed above or by other methods, such as stationary print head arrangements that effectively cover an entire width of the print media. 
     Scanning type ink-jet printers must have their throughput controlled so that separate scans of the carriage are spaced in time by an amount sufficient to ensure that no color bleeding occurs as mentioned above. 
     In addition to throughput, an ink-jet printer designer must be concerned with the problem of cockle. Cockle is the term used to designate the uncontrolled, localized warping of absorbent print media (such as paper) that occurs as the liquid ink saturates the fibers of the paper, causing the fibers to swell. The uncontrolled warping causes the paper to move toward or away from the print head, changing both the distance and angle between the print head and the paper. These unpredictable variations in distance and angle reduce print quality. A predictable and constant distance and angle are desired to assure high print quality. Even if the occurrence of cockle does not affect this aspect of print quality, the resultant appearance of wrinkled print media is undesirable. 
     Heat may be applied to the print media in order to speed the drying time of the ink. Heat must be applied carefully, however, to avoid the introduction of other problems. For example, if the heat is not uniformly applied to the printed media, the resultant uneven drying time of a colored area of an image can produce undesirable variations in the color&#39;s hue characteristic. 
     Another problem attributable to improperly applied heat can be referred to as “buckling.” Normally, print media carries at least some moisture with it. For example, a sealed ream of standard office paper comprises about four and one-half percent moisture. High amounts of moisture in the media, such as paper, may be present in humid environments. As heat is applied to part of the paper, uneven drying and shrinkage occurs. The uneven shrinkage causes the paper to buckle in places, which undesirably varies the distance between the paper and the print head, as occurs with the cockle problem mentioned above. 
     Some print media, such as polyester-based transparency print media, will carry insignificant amounts of water and, therefore, will not buckle as a result of uneven shrinkage. Such media, however, may buckle if all or portions of it are overheated. Thus, uniform, controlled heating of the media is important for high print quality, irrespective of the type of print media. 
     If heat is applied to the media, it is useful to have it applied in the print zone of the printer. The print zone is the space in the printer where the ink is moved from the print head to the print media. Thus, the media is moved through the print zone during a printing operation. Heating the media in the print zone rapidly drives off (evaporates) a good portion of the liquid component of the ink so that cockle is unable to form, or at least is minimized, and so that the time between successive scans of the same swath can be minimized. 
     When one attempts to heat the media in the print zone, it is important to ensure that the applied heat is not directed to the print head of the cartridge. If the print head overheats, droplet trajectory and other characteristics of the print head can change, which reduces print quality. Also, the heat should not be applied in a way (as by convection) that may directly alter the droplet trajectory. The heat should be applied in a cost-efficient manner. 
     Another printer design consideration involves the support of media in the printer for precise relative positioning and movement relative to the print head of the cartridge. Vacuum pressure may used to support print media for rapid advancement through the printer. One method of supporting a sheet of print media is to direct it against an outside surface of a moving carrier such as a perforated drum or porous belt. Vacuum pressure is applied to the interior of the carrier for holding the sheet against the moving carrier. The carrier is arranged to move the sheet through the print zone. 
     The vacuum pressure or suction (Here the term “vacuum” is used in the sense of a pressure less than ambient, although not an absolute vacuum.) must be applied at a level sufficient for ensuring that the sheet of print media remains in contact with the carrier. Moreover, a uniform application of vacuum pressure to the media will help to eliminate the occurrence of cockle in the sheet because the vacuum pressure helps overcome the tendency of the media fibers to warp away from the surface of the carrier that supports the media. 
     With the foregoing in mind, the present invention may be generally considered as a technique for heating print media in an ink-jet printer. As one aspect of this invention, heat is uniformly applied to the media in conjunction with mechanisms for uniformly applying vacuum pressure to the media for supporting the media as it moves through the printer. 
     The heat is efficiently applied to the media by conduction, in a manner that will not overheat the print cartridge print head nor interfere with the trajectory of the droplets expelled from the print head. The hardware for applying the heat has high thermal transfer efficiency and low thermal mass. As a result, there is less likelihood of overheating the print cartridge or other printer components through heat radiation from the heating components after the paper is moved from the print zone. 
     In a preferred embodiment, the heat is applied to the media in the print zone as well as regions on either side of the print zone, where the media respectively enters and exits the print zone. The entry region is sized and heated by an amount that ensures that media is sufficiently dry before entering the print zone so that shrinkage and buckling does not occur in the print zone, thus ensuring that a constant distance and angle is maintained between the media and the print head. 
     The amount of heat applied to each of the entry and exit regions and to the print zone is independently controlled. The amount of heat applied can be related to the physical characteristics of the particular type of print media or inks that are used. Also, the thermal transfer efficiency of the heater mechanisms provides a quick temperature rise time so that the paper can be heated quickly, thus permitting high throughput. 
     Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the primary components of an ink-jet printer that may be adapted for conductive heating of print media in accordance with the present invention. 
     FIG. 2 is a diagram showing a preferred embodiment of the present invention, including mechanisms for heating and supporting print media in an ink-jet printer. 
     FIG. 3 is an enlarged detail view of a portion of the preferred embodiment of FIG.  2 . 
     FIG. 4 is a top plan view of mechanisms for supporting and heating the print media in the printer. 
     FIG. 5 is a section view taken along line  5 — 5  of FIG.  4 . 
     FIG. 6 is a top plan view of another preferred embodiment of the present invention. 
     FIG. 7 is a cross sectional view of the embodiment of FIG.  6 . 
     FIG. 8 is a cross section view of another preferred embodiment of the present invention, showing heaters and rollers for respectively heating and facilitating movement of the print media. 
     FIG. 9 is a detail view of a portion of a roller that is part of the embodiment of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The diagram of FIG. 1 shows an ink-jet print cartridge  20 , which may be mounted to a printer by conventional means such as a movable carriage assembly (not shown). For illustrative purposes, only one cartridge is shown in the figures, although it is contemplated that more than one cartridge may be employed. For instance, some color printers use four cartridges at a time, each cartridge carrying a particular color of ink, such as black, cyan, yellow, and magenta. In the present description, the term “cartridge” is intended to mean any such device for storing liquid ink and for printing droplets of the ink to media. Preferred cartridges are available from Hewlett Packard Co. of Palo Alto, Calif., http://www.hp.com. The cartridges may be connected to remote sources of ink that supplement the ink supply that is stored in each cartridge. 
     The carriage assembly supports the cartridge  20  above print media, such as a sheet of paper  22 . A print head  24  is attached to the underside of the cartridge. The print head  24  is a planar member and has an array of nozzles through which the ink droplets are ejected. The cartridge  20  is supported so that the print head is precisely maintained at a desired spacing from the paper  22 , such as, for example, between 0.5 mm to 1.5 mm from the paper. Also, the array of nozzles in the print head is maintained in substantially parallel relationship with the portion of the paper  22  underlying the print head. 
     The paper  22  is advanced though the printer, and the cartridge print head  24  is controlled to expel ink droplets to form an image on the paper. In the vicinity of the cartridge  20 , the paper  22  is supported on a support surface of a moving carrier  26 , such as a drum or conveyor belt. A flat carrier is shown in FIG. 1. A drum-type carrier would, of course, appear curved. The carrier  26  moves the paper  22  through the printer&#39;s print zone  28 . As noted above, the print zone  28  is the space in the printer where the ink is moved from the print head  24  to the paper  22 . Two imaginary boundaries of the print zone  28  are shown in dashed lines in FIG.  1 . 
     For the purposes of this description, one can consider the space that is adjacent to the print zone (to the left in FIG. 1) as an entry zone  30  through which the paper  22  is conveyed before entering the print zone  28 . The space that is on the opposite side of the print zone is the exit zone  32 , through which the paper is conveyed as it passes out of the print zone  28  on its way to a collection tray or the like. 
     In accordance with the present invention there is hereafter described a technique for heating the paper  22  as it is moved through the printer. Heat is uniformly applied to the paper in conjunction with mechanisms for uniformly applying vacuum pressure to the paper (or any other media) to support the paper as it moves through the printer. 
     Preferably, the heat is applied to the paper  22  while the paper is in the print zone  28 . Also provided are mechanisms for heating the paper as it moves through the entry zone  28  and the exit zone  32 . 
     With particular reference to FIGS. 2-4, a preferred embodiment of the present invention includes a media handling system  40  for heating and supporting the media in an ink-jet printer. The system includes a platen  42  that generally provides support for media, such as paper sheets  22 , that are directed through the print zone of the printer. 
     The platen  42  is a rigid member, formed of a heat conductive material such as stainless steel. In this embodiment, vacuum pressure is employed for drawing the paper against the platen to support the paper as it is advanced through the printer. Thus, the platen  42  has ports  44  formed through it. The platen  42  also forms the top of a vacuum chamber or box  46  that is inside the printer. 
     The vacuum box  46  includes a body  49  to which the platen  42  is attached. The box  46  is thus enclosed but for the ports  44  in the platen  42  and a conduit  48  to a vacuum source  50 . The vacuum source is controlled to reduce the pressure in the interior of the box  46  so that suction or vacuum pressure is generated at the ports  44 . 
     The platen  42  has a planar support surface  52  (FIG. 3) that faces the print head  24 . The ports  44  in the platen open to the support surface  52 . As best shown in FIG. 4, the ports are preferably formed in uniform rows across the support surface. The ports  44  are sized and arranged to ensure that vacuum pressure is uniformly distributed over the platen surface  52 . In a preferred embodiment, the ports are circular where they open to the surface  52 . The circles are 3.0 mm in diameter and spaced apart by 6.0 mm to 6.25 mm. This arrangement of ports thereby provides a platen support surface having more than 33% of its area covered with vacuum ports. Of course, other port sizes and configurations can be used to arrive at an equivalent distribution of ports over the support surface of the platen. 
     The ports  44  of the platen communicate vacuum pressure to whatever is supported on the support surface. For instance, if the platen were part of a rotating drum or carousel, sheets of paper could be loaded directly onto the platen support surface  52  and moved by the rotating drum through the print zone  28  as the vacuum pressure secures the paper to the platen. The paper in such a system could be heated in accordance with the present invention as described below. A preferred embodiment of the invention, however, contemplates a stationary platen used in combination with a porous transport belt for moving the paper through the print zone as described next. 
     A suitable transport belt  60  is configured as an endless loop between a fixed drive roller  62  and tension roller  64  (FIG.  2 ). In the figures, the belt  60  is shown rotating clockwise, with a transport portion  66  of the belt (FIG. 3) sliding over the support surface  52  of the platen  42 . The return portion of the belt  60  underlies the vacuum box  46 . Paper  22  is directed onto the transport portion by conventional pick and feed roller mechanisms (not shown). 
     The belt  60  conducts heat to the paper  22  (or other type of print media) that is carried on its transport portion  66 . Moreover, the belt permits a uniform communication of vacuum pressure to the underside of the paper  22 . To this end, the belt is porous and made of heat conductive material. 
     In a preferred embodiment the belt is formed of a stainless steel alloy, commonly known as Invar, having a thickness of about 0.125 mm. The belt  60  has a width that is sufficient to cover all but the margins of the platen  42  (FIG.  4 ). The belt  60  is heated by conduction. In one preferred embodiment, the conductive heating of the belt is accomplished by the use of heaters  70  that are attached to the support surface  52  of the platen  42  as best shown in FIG.  4 . 
     The heaters  70  are comprised of an array of linear, resistive heating elements  72  (preferably, eight elements  72  for each heater  70 ). The heating elements  72  extend between the rows of vacuum ports  44  that are defined on the support surface  52  of the platen. At the edges of the support surface  52  the individual elements  72  are joined (as at reference numeral  74 ) and the termini of the heaters are enlarged into two contact pads  76  for connecting to a current source and ground as explained more below. 
     The heaters  70  are arranged so that one heater, a “print region heater,” resides on the central portion of the platen  42  immediately underlying the print zone  28 . As shown in FIG. 4, the region on the platen support surface underlying the print zone is designated with the reference number  128  and is hereafter referred to as the print region  128  of the platen. Thus, in addition to a uniform distribution of vacuum ports  44  in the print region  128 , the platen is configured to have a uniform distribution of heating elements  72  for uniform application of heat to the paper  22 . In particular, a heating element  72  is located to extend between each row of ports  44 . 
     In the embodiment depicted in FIG. 4, there are also two heaters  70  in the entry region  130  of the platen surface (that region corresponding to the above-described entry zone  30 ). These heaters will be referred to as the entry region heaters. Similarly, two “exit region heaters” are provided in the exit region  132  of the platen surface (the region corresponding to the above-described exit zone  32 .) Thus, in this embodiment, twice as much platen support surface area is heated in the entry region  130  or exit region  132  as compared to print region  128 . 
     The heaters  70  are of the thick-film type. The heaters include a ceramic base layer that is silk-screened onto the support surface  52  of the platen in the pattern depicted in FIG.  4 . Resistive paste layers are then deposited between vitreous dielectric layers, which are dried and fired to produce an integrated heating element  72 . The heating elements  72  are about 1.5 mm wide (as measured left to right in FIG. 3) and protrude slightly above the support surface  52  as shown (although exaggerated) in FIG.  3 . In a preferred embodiment, the heating elements  72  protrude by about 0.05 to 0.10 mm above the support surface  52  of the platen  42 . 
     The underside  61  of the transport belt  60  slides over the top surfaces of the heating elements  72  as the belt is driven to move paper  22  through the print zone. Preferably, the underside of the belt is thinly coated with a layer of low-friction material, such as Dupont&#39;s polytetrafluoroethylene sold under the trademark Teflon. 
     The protruding heating elements  72  are advantageously employed for distributing the vacuum pressure that is communicated to the belt  60  via the ports  44  in the platen. As can be seen in FIG. 3, the space between adjacent heating elements  72  and between the belt  60  and support surface  52  of the platen defines an elongated channel  45  that is continuous with the each port in a row of ports  44 . Thus, each channel  45  distributes vacuum pressure across the entire width of the porous belt  60 . 
     As depicted in FIG. 5, the contact pads  76  of each heater  70  are connected, as by leads  78 , to a heater controller  80 . In a preferred embodiment, the heater controller  80  is connected to at least three temperature sensors  82  (only one of which appears in FIG.  5 ). One sensor is attached to the undersurface  84  of the platen, centered in the print region  128  and between a row of ports. The other two sensors are similarly located to underlie, respectively, the entry region  130  of the platen surface and the exit region  132  of the platen surface. The sensors  82 , which can be embodied as thermistors, provide to the heater controller  80  an output signal that is indicative of the temperature of the platen. 
     The heater controller  80  is also provided with control signals from the printer microprocessor  86 . (For illustrative purposes, the heater controller is shown as a discrete component, although such heater control may be incorporated into the overall printer control system.) Such signals may provide an indication of the type of media about to be printed. 
     The heater controller  80  identifies the corresponding range of temperatures that should be read on the sensors  82  to ensure that an optimal amount of heat is being applied to the given type of media in the region corresponding to that sensor. The corresponding heater  70  is then driven with the appropriate current for achieving the correct sensor temperature. In one preferred embodiment, the heater in the print region  128  is normally driven by a current sufficient to establish a temperature of about 150° C. at the transport portion  66  of the belt, which contacts the paper  22 . 
     The identification of the desired temperature range can be carried out, for example, by resort to a look-up table stored in read only memory (ROM) of the heater controller  80  and that is made up of an empirically derived range of temperatures correlated to many different media types. For instance, if the printer operator selects a transparency-type of print media, the range of temperatures to be detected on sensor  82  in the print region  128  of the platen (hence applied via conduction to the media) would likely be lower than such temperatures for paper media. 
     Irrespective of the relative size of the heated entry, print, and exit regions, it is desirable to control those heaters separately from one another. To this end, separate control leads are provided from the heater controller  80  to the contact pads  76  of the heaters  70  located in each surface region. The separate control of the heating regions affords a degree of customization for heating the print media, depending, for example, on the physical characteristics of the media used. 
     For instance, if the printer operator employs transparency-type media (which contains practically no moisture), the heater(s) in the entry region  130  may be controlled to provide little or no heat, although the heaters in the print region  128  and exit region would be operated to dry the ink as soon as it is applied. 
     As another example, the amount of heat applied to the print media  22  by the exit region heaters may be boosted relative to the entry region or print region heaters in instances where the printer microprocessor  86  provides to the heater controller  80  a control signal indicating that a particularly large amount of ink is to be printed onto the media sheet that next reaches the platen. The extra heat in the exit region  132  would facilitate timely drying of the large amount of ink. 
     FIG. 5 depicts one method for assembling a vacuum box  46  using a platen  42  as described above. Preferably the portion of the platen  42  that defines the entry region  130 , print region  128 , and exit region  132  is a separate module that is fastened to the body  49  of the vacuum box. This module also defines the support surface  52  and is formed from flat stainless steel of about 1.0 mm thick. At the edge of the module, there are integrally attached flanges  90  that extend downwardly, perpendicular to the surface  52 . The flanges are joined at each corner of the module and provide stiffening support to the plate surface to ensure that the surface does not bend out of its plane. This helps to ensure that the distance between the print head  24  and paper  22  that is carried by the support surface remains constant even as the platen is heated and cooled. 
     The lowermost edges of the flanges  90  seat in correspondingly shaped grooves formed in the vacuum box body  49 . A gasket is provided to seal this junction. The undersurface  84  of the platen  42  also includes a number of evenly spaced, internally threaded studs  92 . Three studs appear in FIG.  5 . The studs receive the threaded shafts of fasteners  94  that pass through the vacuum box body  49  to thus fasten together the platen  42  and the body  49 . 
     As an alternative, the platen comprising the support surface may be formed of a thin sheet of ceramic material to provide a robust platen as respects, especially, the ability of the platen to maintain its planar shape despite heating and cooling cycles. Flanges, configured as those appearing at  90  in FIG.  5  and formed of thermally insulating material, are used in this embodiment as support for the ceramic surface and to maintain spacing to define the vacuum box underlying the platen. 
     The platen  42 , including the entry, print, and exit regions, may be sized to define the entire support surface that underlies the transport portion  66  of the belt  60 . Alternatively, this platen module may be attached to the valve box body between non-heated extensions of the platen surface that may or may not include vacuum ports (and associated fluid communication with the interior of the box  46 ) for securing the media, depending primarily upon the physical characteristics of the media that is accommodated by the printer. 
     It will be appreciated that a number of other platen configurations may be employed for uniformly heating and supporting print media in accord with the present invention. One alternative embodiment is depicted in FIGS. 6 and 7. Those figures show a platen  142  that, like platen  42  in the earlier described embodiment, forms the top of a vacuum chamber or box that is inside the printer. In this regard, the cross section of FIG. 7 shows the body  149  of a vacuum box  146  that matches the box  46  described earlier in that the box  146  is enclosed but for ports  144  in the platen  142 , and a conduit to a vacuum source (not shown). The vacuum source is controlled to reduce the pressure in the interior of the box  146  so that suction or vacuum pressure is generated at the ports  144 . 
     The platen  142  of this embodiment includes two parts: a rigid top plate  143  that mates with a bottom plate  145 . The top plate  143  is formed of a heat conductive material such as an aluminum alloy or copper and includes a planar support surface  152  that faces the print head  24 . The ports  144  in the platen top plate open to the support surface  152 . As best shown in FIG. 6, the ports  144  are preferably formed in uniform rows across the support surface. The ports  144  are sized and arranged to ensure that vacuum pressure is uniformly distributed over the platen surface  152 . In this embodiment, the ports are rectangular where they open to the surface  152 . There the ports are 2.0 mm wide and 6.0 mm long. The ports  144  are aligned with their short sides being parallel to the direction of paper movement over the platen  142  (left to right in FIG.  6 ). 
     Each row of ports  144  is closely spaced relative to an adjacent row, thereby to ensure uniform distribution of vacuum pressure at the support surface  152  of the platen  142 . In a preferred embodiment, the space between adjacent rows of ports is 2.0 mm, preferably no larger than 3.0 mm. Put another way, the space between the rows is no larger than one and one-half times the width of the ports. Of course, other port sizes and configurations can be used to arrive at an equivalent distribution of ports over the support surface  152  of the platen  142 . 
     Apertures  151  are formed through the top plate  143  of the platen  142 , one aperture for each port  144 . These apertures extend from the base of the rectangular portion of the port to the underside  153  of the platen top plate. An air space  155  is defined beneath that underside  153  and the upper surface  157  of the bottom plate  145  of the platen, as will be explained more below. 
     The bottom plate  145  of the platen  142  is formed of rigid, high-temperature plastic such as the polyetherimide sold by General Electric under the trademark Ultem. In a preferred embodiment the bottom plate includes a peripheral frame  159  that surrounds the top plate  143  and includes a groove  161  into which fits the edge of the top plate (FIG.  7 ). The otherwise flat upper surface  157  of the bottom plate is interrupted with an array of cylindrical heater support posts  163  that project upwardly from the surface  157 . Those posts are evenly spaced in an array of seven rows and five columns across the area of the bottom plate (one row of posts being depicted in FIG.  7 ). 
     The upper ends of each column of support posts  163  are bonded to the underside of an elongated substrate  165  that is part of a heater  170 . In this embodiment, there are five such heaters  170 . The heaters fit into correspondingly shaped grooves that are formed in the underside  153  of the platen  142  at spaced-apart locations across the width of the platen  142  as shown in FIG.  6 . 
     The substrate of each heater is comprised of ceramic material. Upon the substrate is attached a resistive heating element  172  (FIG.  7 ), preferably formed of conventional thick-film resistive paste. The heating elements are terminated in contact pads  176  (FIG.  6 ), which, like the pads  76  of the earlier described embodiment permit the individual heaters to connect with and be controlled by a heater controller as explained above. 
     One of the heaters  170  underlies the print region  228  (which functionally corresponds to the print region  128  of the earlier embodiment) in the platen surface  152 , as shown in FIG.  6 . In this regard, the posts  163  are sized so that the heating elements  172  of the heaters are pressed against the heat conductive top plate  143  so that heat is conducted through the top plate and to the transport portion  266  (FIG. 6) of a transport belt  260  that matches the construction of the above described transport belt  60 . 
     In this embodiment, the belt  260  is driven to slide directly across and in contact with the support surface  152  of the platen  142  (that is, the heaters  170  are remote from, and thus do not protrude from, that support surface). Both the belt  260  and the support surface  152  are thus thinly coated with a layer of low-friction material, such as Dupont&#39;s polytetrafluoroethylene sold under the trademark Teflon. 
     As was the case in the earlier embodiment, a pair of heaters  170  are attached to the platen adjacent to an entry region  230  of the support surface  152 , and another pair of heaters  170  are attached to the platen adjacent to an exit region  230  of that surface. As before, these heaters are separately controlled. 
     It is also contemplated that the heaters of one region may be somewhat isolated from the heater(s) of another region. In this regard, FIGS. 6 and 7 depict an example of a restriction or notch  177  formed in the surface of the platen to limit the conduction of heat through the platen between the print region  228  and the exit region  232 . This restriction limits or chokes the transfer of heat through the platen cross section at the notch since the cross section there is much reduced relative to the remainder of the platen. As a result, most of the heat generated by an operating print region heater will not flow into the adjacent exit region  232 . Such a restriction is useful where, for example, print quality requirements are such that the exit region heaters should be substantially cooler than the print zone heater. 
     The bottom plate  145  also includes through apertures  154  that are axially aligned with the apertures  154  in the top plate  143 . As a result, the vacuum pressure developed in the vacuum box  149  is communicated though the bottom plate apertures  154 , through the air space  155 , through the top plate apertures  151  to the ports  144  on the surface of the platen. Thus, the uniform distribution of vacuum pressure is present across the platen support surface  152 . 
     It is noteworthy that no top plate apertures  151  are provided in the platen above the heaters  170 . In these locations, vacuum port extensions  148  are provided in the surface  152 . These extensions  248  are recesses formed in the surface  252  to extend from a port  144  (which has a connecting aperture  151 ) to the surface area overlying the heater so that the vacuum pressure provided to the connected port  144  is distributed via the extensions  148  to the surface area over the heaters  270 . This permits the uniform distribution of the pressure over the entire platen support surface  252 . 
     The embodiment of FIGS. 8-9 is primarily directed to conductive heating of the heat conductive belt  260  (which generally matches the belt  60  of the earlier described embodiment) while supporting the belt above the surface  252  of the platen  242 , thereby to minimize friction between the belt and platen. In this embodiment, heaters  270 , which are constructed like those heaters  170  of the embodiment of FIGS. 6 and 7, are mounted to spaced-apart pads  273  of rigid, high-temperature plastic such as the polyetherimide sold by General Electric under the trademark Ultem. These heater support pads  273  are located in grooves formed in the support surface  252  of the platen that extend in a direction perpendicular to the direction of movement of media through the print zone. 
     Alternative structures for supporting the heaters include elongated strips that fill the bottom of the grooves and have upwardly protruding, thin edges that support the heater and thus include between those edges a thermally insulating air gap. This structure, as well as the foregoing pads  273 , may be formed of open-cell silicon foam, for more insulating effect. This foam could also be applied between the pads  273  or to fill the just described air gap. 
     The substrate  265  and heating element  272  of each heater are stacked onto the support strip. The uppermost surface of the heater  270  protrudes above the support surface  252  and contacts the underside  261  of the heat conductive belt. 
     Support members are mounted to the platen at closely spaced locations along the support surface  252 . In a preferred embodiment, the support members are elongated, cylindrical rollers  281  that extend between each heater  270 . As best shown in FIG. 9, the lower half of each roller fits in a correspondingly shaped, semi-cylindrical recess  285  made in the support surface  252  of the platen. The recess  285  is slightly larger that the roller  281 , thus a gap  287  is present around the outer surface of the roller. 
     The ends of each roller are formed into a small diameter spindle  283  that fits into a slot  289  made in the surface  252  at opposite ends of each recess. Preferably, the opening of the slot  289  at the surface  252  is slightly narrower than the diameter of the spindle so that the spindle can be snap fit into the slot, free to rotate in the slot, but not able to move out of the slot in the absence of a sufficient force applied to remove the roller. 
     The upper sides of the rollers  281  provide rolling support for the belt  260  as it is driven across the platen in contact with the heaters  270 . It will be appreciated that the embodiment depicted in FIGS. 8 and 9 provides an enhanced low-friction approach to moving the belt relative to the platen. Moreover, the uniform distribution of vacuum pressure to the belt is also provided in this embodiment. 
     Specifically, each gap  287  that surrounds a roller  281  has a number of spaced-apart apertures  290  opening to it. Each aperture  290  communicates with the vacuum pressure developed in the vacuum box that underlies the platen. As a result, the gaps  287  serve as vacuum ports in the support surface of the platen, thereby to facilitate the uniform distribution of vacuum pressure to the transport belt  260 . 
     Although preferred and alternative embodiments of the present invention have been described, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.