Abstract:
A system and method is provide for delivering molten ink to a printing mechanism including receiving molten ink in a receiving reservoir, and alternating which of a plurality of reservoirs is opened to the receiving reservoir to receive molten ink while at least one other of the plurality of reservoirs is opened to dispense molten ink to the printing mechanism, in response to the level of ink in a reservoir dispensing molten ink to the printing mechanism.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure generally relates to high speed printing machines which have one or more print heads that receive molten ink heated from solid ink elements. More specifically, the disclosure relates to improvements in pressurized ink transport. 
       BACKGROUND 
       [0002]    So called “solid ink” printing machines encompass various imaging devices, including printers and multi-function platforms, which offer many advantages over other types of document reproduction technologies, such as laser and aqueous inkjet approaches. These advantages often include higher document throughput (i.e., the number of documents reproduced over a unit of time), fewer mechanical components needed in the actual image transfer process, fewer consumables to replace, sharper images, and an eco-friendlier process. 
         [0003]    A typical solid ink or phase-change ink imaging device includes an ink loader which receives and stages solid ink elements that remain in solid form at room temperatures. The ink stock can be refilled by a user by simply adding more ink as needed to the ink loader. Separate loader channels are used for the different colors. For example, only black solid ink is needed for monochrome printing, while solid ink colors of black, cyan, yellow and magenta are typically needed for color printing. Solid ink or phase change inks are provided in various solid forms, and more particularly as pellets or as ink sticks. 
         [0004]    An ink melt unit melts the ink by raising the temperature of the ink sufficiently above its melting point. During a melting phase of operation, the solid ink element contacts a melt plate or heated surface of a melt unit and the ink is melted in that region. The melted ink is often retained in a melt reservoir, which is itself heated to keep the ink above its solidification temperature until a print operation is demanded. The liquefied ink is supplied to a single or group of print heads by gravity, pump action, or both. In accordance with the image to be reproduced, and under the control of a printer controller, a rotating print drum receives ink droplets representing the image pixels to be transferred to paper or other media. To facilitate the image transfer process, a pressure roller presses the media against the print drum, whereby the ink is transferred from the print drum to the media. The temperature of the ink can be carefully regulated so that the ink fully solidifies just after the image transfer. 
         [0005]    In higher throughput systems, the melted ink is pressurized for high speed delivery to the printheads. The throughput of such machines is ultimately controlled by the ability to maintain a constant supply of liquefied ink at the ready for delivery to the printheads. This ability is determined in part by the melt rate, i.e., the amount of solid ink that can be melted per unit time. In a typical ink stick system, the melt rates can vary between 6 and 16 gm/min. Higher melt rates can be often be achieved using solid ink pellets stored in a drum and fed to a high efficiency, high wattage melter. One such high volume melter is disclosed in co-pending and commonly-owned U.S. patent application Ser. No. 12/638,863 (the &#39;863 Application), filed on Dec. 15, 2009 and entitled “SOLID INK MELTER ASSEMBLY”, the disclosure of which is incorporated herein by reference in its entirety. Melters of this type can achieve melt rates of up to 250 gm/min with sufficient power to exceed the ink&#39;s heat of fusion and the latent energy required to raise the ink to the final setpoint temperature for moving to the printheads. 
         [0006]    There remains a need for a system capable of delivering ink to the print heads at a rate that can take full advantage of these high melt rates. 
       SUMMARY 
       [0007]    According to aspects disclosed herein there is provided an ink delivery system for delivering molten ink to a printing mechanism comprising a receiving reservoir for receiving molten ink and a reservoir system in fluid communication between the receiving reservoir and a molten ink outlet in communication with the printing mechanism. The reservoir system includes: a first reservoir having a first inlet in communication with the receiving reservoir and a first outlet in communication with the molten ink outlet; a separate second reservoir having a second inlet in communication with the receiving reservoir and a second outlet in communication with the molten ink outlet; a first valve assembly disposed between the first inlet and the first outlet and including a first seal member movable between a discharge position closing the first inlet and an intake position closing the first outlet; a separate second valve assembly disposed between the second inlet and the second outlet and including a second seal member movable between a discharge position closing the second inlet and an intake position closing the second outlet; and an actuator assembly operably coupled to the first and second valve assemblies and configured for coordinated movement of the first and second seal members so that one of the seal members is in the discharge position and the other of the seal members is in the intake position. In another aspect, the reservoir system is incorporated into a printing machine comprising a heating element for melting solid ink, a receiving reservoir for receiving ink melted by the heating element, and a printing mechanism coupled to the molten ink outlet to receive molten ink under pressure from the reservoir system. 
         [0008]    In a further aspect, a method for delivering molten ink to a printing mechanism is disclosed comprising: receiving molten ink in a receiving reservoir; preventing fluid communication between a first reservoir and the receiving reservoir while permitting fluid communication between the first reservoir and the printing mechanism; and substantially simultaneously permitting fluid communication between a second reservoir and the receiving reservoir while preventing fluid communication between the second reservoir and the printing mechanism. 
         [0009]    A further method for delivering molten ink to a printing mechanism, comprises: receiving molten ink in a receiving reservoir; and alternating which of a plurality of reservoirs is opened to the receiving reservoir to receive molten ink while at least one other of the plurality of reservoirs is opened to dispense molten ink to the printing mechanism, 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  is a perspective partial cut-away view of an ink delivery system according to the present disclosure. 
           [0011]      FIG. 2  is a side cross-sectional view of the ink delivery system shown in  FIG. 1 . 
           [0012]      FIG. 3  is an enlarged view of components of the ink delivery system shown in  FIG. 1 , with the components in a first state. 
           [0013]      FIG. 4  is an enlarged view of components of the ink delivery system shown in  FIG. 1 , with the components in a second state. 
           [0014]      FIG. 5  is an operational flowchart for the ink delivery system shown in  FIG. 1 . 
           [0015]      FIG. 6  are comparative graphs of ink levels in two reservoir components of the ink delivery system shown in  FIG. 1 . 
           [0016]      FIG. 7  are comparative graphs of ink levels in three reservoir components of the ink delivery system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , an ink delivery apparatus  10  includes a melting apparatus  11  configured to liquefy solid ink elements for eventual delivery to one or more printheads. In one embodiment, the solid ink elements are in pellet form. The melting apparatus  11  includes a pellet distributor  12  that receives solid ink pellets through an intake tube. The pellets may be obtained from an ink supply, such as a drum, by gravity feed or by a pressurized feed. The flow of solid ink pellets to the pellet distributor  12  may be regulated in a suitable manner to achieve optimum performance of the melting apparatus. 
         [0018]    The melting apparatus  11  further includes a high efficiency melter  15 . The melter  15  may be constructed as disclosed in the co-pending &#39;863 Application, the disclosure of which has been incorporated herein by reference in its entirety. Details of the structure and operation of the melter can be learned from the &#39;863 Application, the melter generally includes a plurality of heated fins onto which the solid ink pellets are dispensed. The pellets are continuously melted by the fins and drip between the fins into a low pressure reservoir  18 , as shown in  FIG. 1 . In the illustrated embodiment, the low pressure reservoir may be formed by a housing  16  and may include a drip pan positioned directly beneath the melter  15 , such as described in the &#39;863 Application. The low pressure reservoir or drip pan  18  is configured to direct the melted ink toward a collection region  19  where the melted ink can be conveyed to the high pressure reservoirs described below. 
         [0019]    The reservoir  18  is identified as “low pressure” because the reservoir is generally maintained at ambient pressure within the printing machine, or at a pressure less than the pressurized reservoirs described herein. Alternatively, the melting apparatus  11  may be slightly pressurized or maintained at atmospheric pressure. 
         [0020]    In accordance with one feature, the ink delivery apparatus is provided with multiple high pressure reservoirs that are used to provide a continuous uninterrupted supply of melted ink to the one or more printheads. In one embodiment, two such reservoirs are provided, namely reservoirs  20  and  22 , which are formed by a housing  17 . The housing  17  may be integral with or separate from the housing  16  forming the low pressure reservoir. For purposes of the present disclosure, the reservoirs may be referred to as the first and second reservoirs or as reservoir  1  and reservoir  2 . Like components of the reservoirs may also be designated with a subscript  1  or  2  to refer to the associated high pressure reservoir. 
         [0021]    The reservoirs  20 ,  22  are connected at inputs  24 ,  25  to a pressure source, which may be an air pressure supply that is controlled and regulated by a controller (not shown) of the printing machine. The pressure in the reservoirs  20 ,  22  is sufficient to feed high pressure jets of the one or more printheads, as is known in the art. As explained in more detail herein, the reservoirs  20 ,  22  are periodically pressurized as the ink supply is discharged to the printhead(s) and de-pressurized as a new supply of molten ink is introduced into the reservoir. 
         [0022]    Each high pressure reservoir  20 ,  22  may be provided with a corresponding ink level sensor  27 ,  28  that determines the volume or level of ink remaining in the reservoir. The sensors  27 ,  28  may be of any construction suitable for providing a signal indicative of the ink level and/or indicative of the ink level dropping to a threshold value. The sensor may be a mechanical float-type sensor or may be an electrical probe assembly such as the sensor assembly disclosed in co-pending and commonly-owned U.S. application Ser. No. 12/241,626, filed on Sep. 30, 2008 and entitled “INK LEVEL SENSOR”, the disclosure of which is incorporated herein in its entirety. 
         [0023]    Each high pressure reservoir  20 ,  22  may preferably include a heating element  30  that is operable to maintain the molten ink at a temperature above the solidification temperature of the ink. As shown in  FIG. 1 , the heating element  30  may include a plurality of spaced-apart heated fins to ensure a uniform heat distribution throughout the reservoir. 
         [0024]    As shown in  FIGS. 1-2 , liquid ink is supplied from the low pressure reservoir  18  to each of the high pressure reservoirs  20 ,  22  through an inlet opening  32  (or inlet openings  32   1 ,  32   2  depicted in  FIG. 2 ). Each reservoir also includes an outlet opening  36  (or openings  36   1 ,  36   2  shown in  FIG. 2 ) that communicate with a common outlet channel  37  (or openings  37   1 ,  37   2  shown in  FIG. 2 ). This outlet channel  37  is in communication with the printhead(s) and may incorporate a filter element  39  and a molten ink outlet  40  that feeds an outlet manifold (not shown) coupled to the printheads. 
         [0025]    In operation, pressurized liquid ink is forced from the outlet channel  37 , through the filter element  39  and outlet  40  to an array of tubing coupled to the printhead(s). The pressure in the outlet channel  37  is produced by pressure within an active one of the high pressure reservoirs  20 ,  22 . The ink delivery apparatus  10  disclosed herein provides a mechanism for alternately fluidly coupling one high pressure reservoirs to the outlet channel to discharge molten ink to the printhead(s) while the other high pressure reservoir is fluidly coupled to the low pressure reservoir  18  to be re-filled with liquid ink. The apparatus  10  thus comprises an ink delivery control mechanism  50  that includes a valve assembly  52 , a rocker assembly  54  and an actuator assembly  56 . 
         [0026]    Turning to  FIG. 2 , it can be seen that the valve assembly includes an assembly  52   1 ,  52   2  for each of the high pressure reservoirs. For the purposes of illustration, the valve assembly  52   2  will be described with the understanding that the valve assembly  52   1  may be substantially identically configured. The valve assembly  52   2  includes a valve seat body  60  disposed at or over the inlet opening  32   2 . The valve seat body  60  defines one or more flow openings  62  that communicate between the low pressure reservoir  18  and the inlet opening  32   2 . The valve seat body  60  may be provided with a mounting flange  63  that mates the body with the housing  17  defining the reservoir. The valve seat body  60  further includes a sealing hub  65  projecting from the mounting flange and configured to fit snugly within the inlet opening  32   2 . The sealing hub  65  may include sealing element, such as O-ring  66  or flat rubber face seal washer, between the hub and the housing  17  defining the reservoir and inlet opening. The sealing hub  65  defines a sealing face  68  facing the outlet opening  37   2 , as illustrated in  FIG. 2 . 
         [0027]    The valve assembly  52   2  further includes a seal body  70  disposed for translation within a chamber  61  aligned between the inlet opening  32   2  and the outlet opening  37   2 . The chamber  61  may be a portion defined by the housing  17  in the high pressure reservoir  22 , or may be defined by a number of walls that help align and guide the seal body  70 . In the latter case, the walls are preferably configured to ensure a constant supply of molten ink to the outlet opening  37   2  and sized to achieve max flow rate. 
         [0028]    The seal body  70  includes an upper seal  71  and a lower seal  73 . The upper seal is configured for sealed engagement with the sealing face  68  of the valve seat body  60  described above. The seal body  70   2  in  FIG. 2  is shown in sealed contact or engagement with the sealing face  68 —i.e., with the seal body in its uppermost position. One or both of the upper seal  71  and sealing face  68  may incorporate a compressible element and/or a recessed face operable to ensure a fluid and pressure tight seal with the seal body. In addition, the seal body and/or the upper seal may be configured for an enhanced fluid seal when pressure is applied behind the seal, such as when the high pressure reservoir  22  is pressurized to discharge molten ink to the printhead(s). 
         [0029]    The seal body  70  is movable to a position for sealing contact or engagement with the sealing face  38  at the outlet opening  36   2 . Thus, the seal body includes a lower seal  73  that is configured to achieve a fluid-tight seal with the sealing face. The seal body  70   1  on the left side of  FIG. 2  is shown in this sealed contact with the outlet opening. It can be appreciated from  FIG. 2  that the seal bodies  70   1 ,  70   2  forming part of the respective valve assembly  52   1 ,  52   2  may be substantially identical in construction, both bodies being configured to translate between an uppermost position sealing the inlet opening  32   1 ,  32   2 , and a lowermost position sealing the corresponding outlet opening  36   1 ,  36   2 . 
         [0030]    It can be appreciated that the length of the seal body  70  is less than the distance between the opposed inlet and outlet openings in each high pressure reservoir. The length of the seal body is calibrated so that when the seal body is sealing one opening (such as inlet opening  32   1 ) the body does not impede ink flow through opposite opening (such as outlet opening  32   2 ). At the same time, it is desirable that the travel distance of the seal body  70  between its two positions be limited so that the time delay between “unsealing” one opening and sealing the opposite opening is minimized—i.e., so that the valve assembly is quick and responsive to a command to changer high pressure reservoirs. In one specific embodiment, the length of the seal body  70  is about 80-90% of the distance between the inlet and outlet openings in a given high pressure reservoir. 
         [0031]    In order to accomplish this movement, each valve assembly  52  is driven by a corresponding rocker assembly  54 . The rocker assembly includes a control rod  75  that extends downward through the housings  16 ,  17 , and more particularly through the seal body  70 . The control rod  75  may be fastened or affixed to the seal body in various manners, including with an attachment pin extending transversely through the rod and seal body, as depicted in  FIG. 2 , to facilitate assembly. In the illustrated embodiment, the control rod  75  is sized to extend through the height of both the low pressure and high pressure reservoirs. The rod thus passes through a sealed bore  78  entering the low pressure reservoir, through a rod bore  78  in the valve seat body  60  and ultimately into a bore  82  defined by a rod support cup  81  at the base of the high pressure reservoir or reservoir housing  17 . The control rod  75  alignment is maintained by the rod bore  78  and the rod support cup  81  as the rod moves up and down between its two sealing positions. 
         [0032]    As shown in  FIG. 1 , the control rod  75  is coupled to a clevis  85  by a pivot pin  86 . The clevis  85  is pivotably mounted on an axle  89  supported on the ink delivery apparatus  10 . The clevis  85  includes a link arm  91  that is connected to an actuator rod  94  by a pivot pin  92 . The actuator rod  94  may be connected to a piston  95  of a pressure cylinder  97 . The cylinder  97  is a hydraulic cylinder, and most preferably a pneumatic cylinder to make use of the pneumatics within many solid ink printing machines. The pressure cylinder  97  is provided with inlet/outlet openings  98 ,  99  at opposite ends of the cylinder, and more particularly on opposite sides of the piston  95 . The pressure cylinder  97  is thus configured to drive the piston  95  upward or downward depending upon whether pressurized gas, such as air, is introduced through the lower opening  99  or upper opening  98 . 
         [0033]    It can be appreciated from  FIG. 1  that as the piston  95  is driven upward by air pressure through inlet  99 , the actuator rod  94  travels upward to pivot the link arm  91  clockwise about the axle  89 . This clockwise rotation of the link arm  91  and clevis  85  drives the control rod  75  and seal body  70  downward to the position shown in  FIG. 3 . In this position the lower seal  73  is sealed against the sealing face  38  about the outlet opening  36   1 . Conversely, when air pressure is released through air inlet  99  and introduced through inlet  98  at the top of pressure cylinder  97 , the piston  95  is driven downward, pulling the actuator rod  94  with it. This movement pivots the link arm  91  and clevis  85  counter-clockwise about the axle  89 , which in turn pulls the control rod  75  and seal body  70  upward until the upper seal  71  engages the sealing face  68 , as shown in  FIG. 4 . 
         [0034]    In lieu of providing pressurized air alternately to the two inlets  98 ,  99 , the piston  95  may be spring-biased to one position or the other (for instance biased upward) and a single inlet, such as inlet  98 , can be alternately pressurized to act against the spring bias or released to allow the piston to return under spring-bias. As a further alternative, the air cylinder can be replaced by other actuators such as a cam assy and stepper motor configured to drive the rocker arm into the two positions shown in  FIGS. 3 and 4 . 
         [0035]    In the position shown in  FIG. 3 , the outlet opening  36  from the high pressure reservoir is sealed by the lower seal  73  while at the same time the inlet opening  32  is open. In this position, the high pressure reservoir, for instance reservoir  20 , can be filled by ink that has been previously melted in the low pressure reservoir  18 . At the same time, pressure in the selected high pressure reservoir  20  is vented through its respective pressure input  24 . The molten ink in the low pressure reservoir may flow by gravity through the inlet opening  32  until the high pressure reservoir  20  is filled, or until the molten ink in the low pressure reservoir  18  has been depleted. It may be contemplated that the melter  15  may be deactivated and the intake tube  13  to the pellet distributor  11  closed while the current supply of molten ink is being fed to the high pressure reservoir. It may also be contemplated that the heating element  30  within the particular high pressure reservoir being filled may be activated to keep the ink in its molten state. 
         [0036]    While the high pressure reservoir  20  is being filled, the other high pressure reservoir  22  may be emptied by discharging its ink contents under pressure. The internal level of the ink inside the reservoir may be monitored via a low level sensor, such as the level sensor  28 , to prevent emptying the contents and driving air into the system. (Air must be prevented from entering the reservoir which can causes the ink heads to burp and spray onto the substrate during a refill operation.) The high pressure reservoir  22  will thus have the seal body  70  in the position shown in  FIG. 4  in which the upper seal  71  is sealed against the sealing face  68  to thereby close off the inlet opening  32 . When the seal body is in its uppermost position, the outlet opening  36  is unimpeded. The pressure input  25  for the second high pressure reservoir  22  is activated to pressurize the reservoir and supply the molten ink under pressure to the printhead(s). At the same time, the heating element  30  may be deactivated. The low level sensor continuously monitors the ink level in the active reservoir, in this case reservoir  22 , and generates a low level signal when the ink level drops to the threshold value. This low level signal initiates a switch of active reservoir from the reservoir  22  to the other reservoir  20 , which by this time has been filled with molten ink. 
         [0037]    It can be appreciated that the ink delivery control mechanism  50  disclosed herein provides a constant source of pressurized molten ink to be delivered to the printhead(s) by periodically switching between high pressure reservoirs  20 ,  22  feeding the molten ink. When one reservoir is “active” or “on-line”—i.e., supplying ink to the printhead(s)—the other reservoir can be re-filled from the low pressure reservoir. Once the ink in the active high pressure reservoir is at or near depletion, the control mechanism  50  can automatically open the other reservoir which has been filled with molten ink during its “inactive” or “off-line” state. The volumes in the chambers are sized so that the amount of ink buffered in both sides is sufficient to provide ink flow to meet the overall demand at maximum coverage on the substrate. 
         [0038]    The coordinated action of the actuator assemblies  56  of the ink delivery control mechanism  50 , the pressure inputs  24 ,  25  to the high pressure reservoirs, the melter  15  and the heating element  30  may be controlled by a suitable master control system (not shown). For instance, the master control system may control valves that either vent or supply pressurized air to the pressure inputs  24 ,  25 . Likewise, the master control system may control valves that alternately vent and pressurize the air inlets  98 ,  99  for the pressure cylinder  97  in the actuator assembly  56  associated with each high pressure reservoir  20 ,  22 . The master control system may be an electronic controller that is integrated into the printing machine and that may be operable to control other functions of the machine. The master control system may be programmable such as to change the ink level maximum and minimum thresholds, the air pressure provided to the actuator cylinders, any dwell in cylinder pressurization or de-pressurization, or other operating parameters of the ink delivery system. 
         [0039]    In one approach, this coordinated action is keyed to the ink level within the two high pressure reservoirs, based on signals generated by the ink level sensors  27 ,  28  as interpreted by the master control system. At start-up, solid ink is initially dispensed to the inlet distributor  11  and the high efficiency melter  15  activated. The first high pressure reservoir  20  is then charged by closing the outlet  36  and opening the inlet  32 . This step entails providing pressurized air to the air inlet  99  of cylinder  97  to drive the piston upward and the control rod  75  and seal body  70  downward to the position shown in  FIG. 3 . At the same time, the air inlet  98  to the other cylinder is pressurized to drive the corresponding piston downward, thereby pulling the control rod and seal body up to the position shown in  FIG. 4 . In this position, liquid ink will only flow to the first reservoir  20 . 
         [0040]    Once the first high pressure reservoir  20  is charged the control system may then implement a coordinated action as depicted in the flowchart of  FIG. 5 . On the first pass through series of steps, the reservoir “X” is the first reservoir  20 , while the reservoir “Y” is the second reservoir  22 . When a call is made for ink to be supplied to the printhead(s), the first step is depressurize the “inactive” reservoir, which in this first pass is the second reservoir  22 . The inlet of the “active” Reservoir “X”, in this case the first reservoir  20 , is then closed and the outlet of that reservoir opened. Substantially concurrently, the inlet of Reservoir “Y”, or in this case the second reservoir  22 , is opened and the outlet closed. In the next step, Reservoir “X” that is now in communication with the printhead(s) is pressurized and pressurized ink is jetted through the outlet  40  to the printhead(s) in a suitable manner. 
         [0041]    As the ink is being utilized by the printheads, the “offline” reservoir is being refilled. Consequently, in the next step, the melter  15  in the low pressure reservoir is activated and the intake tube  12  opened to begin melting the solid ink. Since the Reservoir “Y” is open to the low pressure reservoir, the melted ink is continuously fed to the inactive Reservoir “Y”. In one branch of the flowchart of  FIG. 5 , the control system continuously monitors the ink level in the Reservoir “Y”. Once the reservoir is full—i.e., when the ink level reaches a predetermined “full” threshold—the control system deactivates the melter and closes the intake tube to the pellet distributor. 
         [0042]    Concurrently, the control system also monitors the ink level in the “active” Reservoir “X”. When the ink level drops below a predetermined threshold indicative of a depleted or nearly depleted reservoir, the control system switches the two reservoirs and re-starts the sequence of steps to activate the previously inactive Reservoir “Y” and replenish or recharge the previously activated Reservoir “X”. It can be appreciated that the sequence of steps in the flowchart of  FIG. 5  may be continuously repeated as each newly recharged reservoir is depleted. In one embodiment, the timing of the steps is based on the ink level in the active reservoir so that switching of the reservoirs only occurs when the active reservoir is sufficiently depleted but prior to complete emptying of the active reservoir. It is contemplated that the low ink level threshold arises before all of the molten ink has been discharged from the active high pressure reservoir so that there will be only a negligible interruption in molten ink fed to the printhead(s), even for asynchronous printheads that do not demand ink flow all at the same time. 
         [0043]    The ink levels in a two reservoir system are illustrated in the graphs of  FIGS. 6 and 7 . As shown in  FIG. 7 , the molten ink in the first reservoir is being generally uniformly depleted while the ink in the inactive reservoir is generally uniformly recharged or replenished. It can be seen that the inactive reservoir becomes fully charged well prior to when the active reservoir reaches its depletion threshold. It can be appreciated that the slope of the “charging” line for the reservoirs can be calibrated in part by controlling the melter  15  feeding the low pressure reservoir  18 . The rate of charging may also be tuned to the usage rate of the active reservoir—i.e., a slower usage rate does not require rapid recharging of the inactive reservoir. 
         [0044]    As depicted in  FIG. 6 , the ink level Reservoir  1  was reduced to the threshold value at about the time 13 minutes. The control system thus commanded a switch (as indicated in  FIG. 5 ) and after a slight delay the second reservoir is activated to begin jetting molten ink to the printhead(s). There is a delay in supplying ink to the newly inactivated reservoir due to the need to warm up the melter  15 . Once warmed up, the melter begins to recharge the depleted reservoir. As can be seen in the graphs of  FIG. 7 , this cycle of depletion and recharging is uniformly cyclical and can continue indefinitely as long as solid ink is continuously fed to the melting apparatus  11 . It can also be seen that the ink level in the low pressure reservoir remains at or very near zero since solid ink is only melted when a high pressure reservoir requires recharging and since the inlet opening between the low pressure reservoir and high pressure reservoir is open throughout the melting process. 
         [0045]    In the illustrated embodiment, the seal body  70  is an elongated generally cylindrical body. The length of the seal body  70  is dictated in part by the distance between the inlet opening  32  and the outlet opening  36  in each high pressure reservoir  20 ,  22 . It is important that the seal body remain substantially clear of one opening when sealing the other opening so that the seal body does not adversely impact the flow of ink through the respective opening. The need for this sufficient gap is particularly important at the outlet opening  36  to avoid any turbulence as the ink is discharged under pressure. 
         [0046]    The seal body  70  is depicted in the present disclosure as a generally solid body. Alternatively, the seal body may constitute separate seals at the upper and lower positions on the control rod  75 , provided that the separate seals can exert sufficient sealing pressure against the respective sealing face  38 ,  68 , 
         [0047]    In the illustrated embodiment the seal bodies are moved upward and downward by the rocker assembly  54  and actuator assembly  56 . Other mechanisms are contemplated to achieve the coordinated movement of the seal bodies within the high pressure reservoirs  20 ,  22 . For instance, each control rod  75  may be an element of a linear actuator, without the rocker assembly  54 . In another alternative, the pressure cylinder  97  may be replaced by a mechanical actuator suitable to alternately translate the seal body  70  upward and downward. For instance, a cam and stepper motor may be configured to pivot the clevis  85  and link arm  91  or, alternatively, to directly reciprocate the control rods  75 . In this case, the control system would be operable to send electrical control signals to a motor driver to control the operation of the stepper motor. 
         [0048]    In certain applications individual control of the valve assemblies for the different high pressure reservoirs is needed. Alternatively, the movement of the seal bodies  70  within the reservoirs can be coordinated through a common actuator assembly. In this alternative, for instance, the control rods of two high pressure reservoir seal bodies can be attached at opposite ends of a single rocker arm. Pivoting the rocker arm alternately and simultaneously raises one control rod and seal body and lowers the other. In another alternative, the two rocker arms may be coupled to a single hydraulic cylinder so that upward movement of the piston pivots one rocker arm to a discharge position, for instance, while downward movement of the piston pivots the other rocker arm to the discharge position. As a further alternative, the relative movement of the seal bodies may be administered through a cam arrangement to, for instance, introduce a dwell period before raising or lowering a respective seal body. 
         [0049]    In the present disclosure, two high pressure reservoirs  20  and  22  are provided. The ink delivery control mechanism  50  may be modified to accommodate more than two reservoirs. Appropriate changes may be implemented in the master control system to account for the timing of movement of the seal bodies and pressurization/depressurization of each of the additional high pressure reservoirs, all with the goal of ensuring a constant supply of pressurized melted ink to the printhead(s). In the case of three or more high pressure reservoirs, it can be contemplated that the inactive reservoirs may be simultaneously re-filled with molten ink from the low pressure reservoir while their respective outlets are closed by the seal body. This configuration may require a larger low pressure reservoir to melt enough ink to fill more than one high pressure reservoir. 
         [0050]    It will be appreciated that various of the above-described features and functions, as well as other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.