Abstract:
An ink melt heater is disposed in a phasing printing system for heating a solid ink stick for melting the ink stick from a solid to a liquid phase. The heater includes a trace assembly having a plurality of power zones having different wattage densities respectively. The heat transfer plate is adhered to the trace assembly for mating engagement against the solid ink stick. The heater has a low thermal mass for enhanced and rapid heat transfer from the trace assembly through the transfer to the ink stick.

Description:
BACKGROUND 
     The present exemplary embodiments relate to printing systems and, in particular, printing devices which utilize a supply of color inks to be communicated to a print head for document printing. More particularly, the present embodiments utilize solid ink sticks as a supply ink, which must be heated to a liquid form before being capable of communication to the print head. Such systems are commercially available under the PHASOR® mark from Xerox Corporation. 
     The present embodiments concern the structure of the heater element that melts the solid ink stick to a liquid form. 
     The basic operation of such phasing printing systems comprises the melting of a solid ink stick, its communication to a reservoir for interim storage, and then a supply process from the reservoir to a print head for printing of a document. In the melting of the ink stick, a relatively large amount of thermal energy is needed to be applied in a very small area. Accordingly, the heating element itself needs a relatively high watt density for the efficient communication of the thermal energy to the ink stick. Any heater supporting structure for the heating elements will operate as an intermediate heat sink. The lower the mass of the support structure, the more efficient the communication of thermal energy there through. In addition, since the heating element must not only melt the ink stick, but assist in the melted ink&#39;s communication to the reservoir, the heater element needs to be formable, i.e., mechanically contourable to a shape to facilitate the supply of the melted ink to the reservoir, and such forming needs to be accomplished without large interface strain within the heater element assembly that could damage the heater element-to-support plate adhesion. Two thermal heaters zones are required in the heater assembly—one to regulate the ink melt rate of the solid ink stick and another higher temperature zone to reduce the ink viscosity for better fluid communication to the reservoir and improve ink filter efficiency. Efficient construction of a variable waft density heating assembly with a simple energy supply system is another need to be satisfied of the desired heating element assembly. Lastly, the solid ink stick needs to be mechanically secured to the heater to prevent ink detachment during shipping and to prevent the creation of pieces of solid ink particles which can cause marking of exterior surfaces of the printer. This last feature prevents the ink stick from breaking free from the heater plate, resulting in the creation of a large number of shards and ink particles that can then cosmetically mark the surfaces of the printer or potentially jam internal printer mechanisms. 
     The present exemplary embodiments satisfy these needs as well as others to provide a low thermal mass, variable watt density formable heater for a phasing printer application. However, it is to be appreciated that the present exemplary embodiments are also amendable to other like applications where the heating element construction requires a high waft density heater in a relatively small area. 
     BRIEF DESCRIPTION 
     An ink melt heater is provided for heating a solid ink stick for melting a heat stick from a solid to a liquid phase wherein the heater includes a plurality of power zones having different wattage densities respectively. The heater includes a heat transfer plate adhered to the trace assembly for mating engagement against the solid ink stick. The heater has a low thermal mass for enhancing rapid heat transfer from the trace assembly through the transfer plate to the ink stick. The heater has a formable construction for forming the heater into a non-planar configuration with an interface strain between the plate and trace assembly less than an amount that can damage the trace-to-plate adhesion. The plurality of power zones comprise a melt zone having a first trace assembly for melting the solid ink stick at a first preselected temperature, and a post-melt zone having a second trace assembly for raising the first preselected temperature of the melted ink to a second preselected temperature conducive to ink runoff of the liquid ink. A protrusion depends from the heat transfer plate and is disposed for engagement against the solid ink stick to form a mechanical lock of the ink stick to the heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view and partial section of a print head, ink stick and ink loader assembly, and power supply and control system therefor; 
         FIG. 2  is an end view of one embodiment of a heater melt plate; 
         FIG. 3  is an expanded cross-sectional stack up of one embodiment of heater material lay up; and 
         FIG. 4  is an elevated view of the heater assembly showing the opposite side from  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , the basic elements of an ink supply system in an ink “phase-changing” printing system can be seen. Ink loader assembly  10  includes a tray  12  for holding a solid phase ink stick  14 . An ink melt heater  16  is disposed at an open end  18  of the tray to contact the ink stick and to allow for egress of liquid phase ink during heating from the tray  10 . The heating plate  16  receives its heating energy from a power supply and control system  20 . The heating element includes an assembly with resistance traces thereon so that electrical energy supplied thereto can be converted to heat energy. 
       FIG. 1  shows an ink drip  40  falling from the tray  10  at ink drip point  70  and the heating element  16  assembly into a print head assembly  42 . Print head assembly  42  comprises a reservoir  44  to receive the melted ink and to communicate the ink to nozzles (not shown) within the print head assembly for printing on a document. It should be appreciated with reference to  FIG. 1  that the ink stick  14  is intended to engage the heat plate  16  as it is shown therein by being urged against the plate by gravity or a spring biased member (not shown) to enhance its contact between the stick  14  and the plate  16 . 
     With reference to  FIG. 2 , power pads  30  connect wires (not shown) from the power supply to the heater plate  16 . Plate  16  includes two thermal heater zones comprising an ink melt zone  32  and a liquid ink heat zone  34 . The ink melt zone  32  is intended to operate at about 100° C. when in heating contact with an ink stick and includes an assembly of resistive heating traces configured and disposed to achieve such a preselected temperature. The traces comprise a serpentine arrangement of a resistive material known for generating thermal energy from electric supply. INCOL® is preferably employed. The waft density in the ink melt zone is approximately 50 watts per square inch during the heating operation. A variable wattage distribution is included in each heater zone that is controlled by the heater trace design. In the middle of the first heater zone there is a deliberately created low wattage area to improve the ink flow during short heater duty cycles. 
     The liquid ink heat zone  34  allows the melted ink to run off and along its contour into the reservoir as is shown in  FIG. 1 . In this post melting heat zone, the ink is intended to be heated to a temperature of approximately 140° C., but this higher temperature can be effectively reached with a lower watt density due to the noncontact of this portion of heater with the solid ink stick. Accordingly, the heat traces in the post-melt heat zone are configured to generate approximately 25 watts per square inch (W/in 2 ) during the heating operation. In the preferred embodiment, the heat traces in the melt zone and in the post-melt zone are connected in series and the waft density is adjusted by varying the spacing of the traces, i.e., the traces in the post-melt heat zone are spaced farther apart than those in the ink melt heat zone, thereby adjusting the watt density per inch squared. 
     The heating element can reach relatively high temperatures (approximately 200° C.) during the melting process, for example, if there is no ink stick contact with the heater assembly, such as may occur in an ink stick jam, so the heater construction comprises high operating temperature adhesives and polymers. 
     With particular reference to  FIG. 3 , an exploded cross-section of the heater element is shown for illustrating the materials stack up. Because of the melt and “freeze on demand” requirements of the heater element (i.e., when the heater is not operating, it is desired to cool very quickly so that the ink stick may similarly cool when not being melted) a minimal thickness of aluminum sheet material (0.4 mm)  40  is used to provide the heat transfer mating surface for the ink melting, and to provide handling rigidity. Additionally, the thin sheet metal  40  allows for forming and bending to a non-planar shape the heater  16  without large interface strain that could damage the trace assembly to plate  40  adhesion. A first KAPTON® layer  44  and a second KAPTON layer  48  are adhered to the plate  40  with layers of PFA adhesive  42 ,  46 . KAPTON is an insulator. The heating traces  52  comprising INCONEL are adhered to the assembly with adhesive layers  48 ,  54  and sealed with an insulating layer of KAPTON  56 . 
     To keep the heater  16  from self-destruction during the startup temperature ramp (10° C./sec–20° C./sec) the layer of aluminum  40  provides a direct highly conductive path for the heater traces  52  to discharge their thermal energy. The rapid transfer of energy keeps the trace temperatures lower, thus creating lower thermal stresses with reduced chances of thermal buckling of the heater traces  52  that can cause interlayer delamination. Lower trace temperatures also enhance the life of the polymers, PFA, KAPTON, used in the heater construction. As noted above, the aluminum layer also allows a high watt density of 25 W/in 2 –50 W/in 2  or higher operation. 
     To keep the heater-to-heater variability low, as few as possible layers of insulation or aluminum are used to separate the ink stick from the heater traces  52 . To reduce process variability, automated manufacturing processes were selected so that a number of heater assemblies can be co-cured together in a controlled press and then punched and formed at the same time in one operation. Such process keeps the cost and between part-to-part variability low. 
     With particular reference to  FIGS. 2 and 4 , as noted above, a two-zone heater meets the dual performance requirements of a melt zone  32  for melting the ink and a post-melt zone  34  for raising the ink to the desired temperature prior to the ink dripping into the print head assembly. For minimizing costs, the traces  52  in both of these zones are connected in series and controlled by a single control circuit  20 . When the ink stick is not present against the assembly  16 , the melt plate temperature is controlled and limited by a thermistor (not shown) attached to the heater plate to avoid overheating. When an ink stick is presented to the upper melt zone  32 , the thermal load, supplied by the ink stick draws down the heater plate temperature in the zone occupied by the ink. The post-melting zone  34  does not receive a heavy thermal load; this allows the temperature of this zone to remain relatively unaffected or climb as more power is applied. The relative ratios of power between the zones determines the ink exit temperature and melt rate. 
       FIG. 4  illustrates the side of the heater assembly intended to contact the ink stick. A rigid metallic frame  60 , preferably also aluminum although other materials could certainly be used, includes a single narrow strip  62  in the middle of the melt zone  32  including protrusions  66  that embed into the partially melted ink stick  14  and provides the mechanical lock to keep the ink stick  14  from separating from the heater assembly  16 . This embedding feature is needed to prevent the ink stick  14  from breaking free from the heater plate  40 , resulting in the creation of a large number of shards and ink particles that can then mark the cosmetic surfaces of the printer or potentially jam internal printer mechanisms. The width of the aluminum strip  62  has to be as narrow as possible without compromising its design intent to reduce an undesirable variable thermal resistance between the heater assembly  16  and the ink stick  14 . The frame  60  is attached to the heater assembly by a single rivet  64 . A depending screen portion  66  comprises a lower part of the frame  60  to catch solid portions of the ink stick that may break off and to screen the liquid ink as it flows down towards the print head. 
     The exemplary embodiments have been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.