Patent Publication Number: US-8529038-B2

Title: System and method for pressure control of an ink delivery system

Description:
TECHNICAL FIELD 
     This disclosure relates generally to printing devices having one or more print heads that eject liquid ink onto an image receiving member, and more particularly, to printing devices that use pressure to supply ink to the one or more printheads. 
     BACKGROUND 
     Phase-change ink printing systems include various imaging devices that 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, and sharper images. 
     A typical solid ink or phase-change ink imaging device includes an ink loader, which receives and stages solid ink for delivery to a melting device. The ink supply can be replenished by a user inserting more solid ink into the loader. The loader has separate loader channels, each of which supplies a different color of ink to a melting device. For example, four loader channels are provided in an imaging device that uses cyan, magenta, yellow, and black (CMYK) ink to form ink image images. Solid ink is supplied in a variety of forms that include blocks, sticks, pellets, and pastilles, for example. 
     Solid ink imaging devices melt the solid ink to a liquid phase for imaging operations. In a typical embodiment, a melting device heats the solid ink to a temperature at which the solid ink enters a liquid phase. The one or more printheads in an imaging device receive the liquid ink and eject liquid drops of the ink through a plurality of inkjets onto an image receiving member, such as paper, or an indirect receiver, such as a rotating drum or endless belt. Many printer embodiments maintain a supply of liquefied ink in an ink reservoir that is fluidly coupled to one or more printheads for printing onto the image receiving member. 
     In some printers, gravity urges ink in the reservoir to flow to the printheads. In other printers, a pumping system applies pressure to liquid ink in the reservoir to urge the ink to the printheads. Continuous feed printers form images on an elongated media web that moves through the printer at a high speed for high-volume production. These continuous feed printers often consume ink at a high rate and require a pressurized ink reservoir to maintain a uniform supply of ink to the printheads. In one type of continuous printer, two separate delivery reservoirs supply ink to a common set of printheads using an alternating operating technique. In the alternating operating technique, one of the two delivery reservoirs is connected to a low pressure reservoir to enable ink to flow from the low pressure reservoir to the connected delivery high pressure reservoir side while the other delivery reservoir is disconnected from the low-pressure reservoir and the high pressure reservoir side is pressurized to deliver ink to the printheads. When the level of ink in the pressurized delivery reservoir drops below a predetermined fluid level, the pressurized delivery reservoir is disconnected from the pressure source, a double-ended seal piston is toggled to enable ink to flow from the low pressure reservoir. The other ink delivery reservoir is disconnected from the low-pressure reservoir and pressurized to deliver ink to the printheads. In one implementation of the alternating process, each of the ink delivery reservoirs includes a piston. The two pistons are mechanically linked so that one piston seals one ink delivery reservoir from the low-pressure reservoir, while the other piston simultaneously opens the other ink delivery reservoir to receive ink from the low-pressure reservoir. The alternating arrangement of the two delivery ink reservoirs enables a substantially continuous supply of ink to the printheads during printing operations. 
     One challenge facing a pressurized reservoir system involves the depressurization of the ink delivery reservoirs. The challenge arises from the pressurization of an air pocket positioned over the ink held in the ink delivery reservoir. When the level of ink in the pressurized reservoir drops below the predetermined threshold and the ink delivery reservoir is placed in fluid communication with the low-pressure reservoir, the pressurized air exits the delivery reservoir and enters the low-pressure reservoir through the valve seat opening. This pressurized air exits through the valve seat opening to cause ink held in the low-pressure reservoir to splatter and aerosol some of the ink in the low-pressure reservoir. Ink droplets resulting from the splatter may escape the low-pressure reservoir through openings in the low-pressure ink reservoir. Some of the escaped ink may contaminate the air supply vents and cause premature failure by blocking the air flow in the vent line. Consequently, improvements to the operation of the pressurized ink delivery system to prevent ink contamination would be beneficial. 
     SUMMARY 
     In one embodiment, an ink delivery system in a phase change ink printer has been developed. The system includes a housing forming a first ink reservoir and a second ink reservoir, a first ink inlet formed in the housing to enable liquid ink to enter the first ink reservoir, a first ink outlet formed between the first ink reservoir and the second ink reservoir in the housing to enable liquid ink to move from the first ink reservoir to the second ink reservoir, a second ink outlet formed in the housing to fluidly couple the second ink reservoir to at least one printhead, a member positioned within the housing and configured to move between a first position and a second position, the member forming a seal with the second outlet opening to enable liquid ink to enter the second reservoir through the first ink outlet in the first position, and the member forming another seal with the first ink outlet to enable ink to exit the second ink reservoir through the second ink outlet in the second position, a conduit formed through the housing, the conduit having an inlet configured to receive pressurized air and an outlet in fluid communication with the second ink reservoir to enable pressurization of air in the second ink reservoir when the member is in the second position, and an orifice formed in the conduit, the orifice being configured to enable the pressurized air in the second ink reservoir to exit the second ink reservoir through the conduit when the member is in the second position. 
     In another embodiment, a method of operating an ink delivery system in a phase change ink printer has been developed. The method includes receiving liquid ink in a first ink reservoir, moving a member to a first position that enables the liquid ink in the first ink reservoir to exit the first ink reservoir through a first ink outlet and enter a second ink reservoir, moving the member to a second position that seals the first ink outlet and enables the ink in the second ink reservoir to exit the second ink reservoir through a second ink outlet that is fluidly coupled to at least one printhead, activating a source of pressurized air to supply pressurized air to the second ink reservoir through a conduit fluidly coupled to the second ink reservoir, deactivating the source of pressurized air to enable pressurized air in the second ink reservoir to exit the second ink reservoir through an orifice formed in the conduit, and moving the member from the second position to the first position to enable ink in the first ink reservoir to enter the second ink reservoir through the first ink outlet after deactivation of the source of pressurized air. 
     In another embodiment, an ink delivery system in a phase change ink printer has been developed. The system includes a housing forming a first ink reservoir, a second ink reservoir, and a third ink reservoir, a first ink inlet formed in the housing to enable liquid ink to enter the first ink reservoir, a first ink outlet formed between the first ink reservoir and the second ink reservoir to enable the liquid ink to move from the first ink reservoir to the second ink reservoir, a second ink outlet formed between the first ink reservoir and the third ink reservoir to enable the liquid ink to move from the first ink reservoir to the third ink reservoir, a third ink outlet formed in the housing to fluidly couple the second ink reservoir to at least one printhead, a fourth ink outlet formed in the housing to fluidly couple the third ink reservoir to the at least one printhead, a first member positioned within the housing and configured to move between a first position and a second position, the first member forming a seal with the third outlet opening to enable liquid ink to enter the second ink reservoir through the first ink outlet in the first position, and the first member forming another seal with the first ink outlet to enable ink to exit the second ink reservoir through the third ink outlet in the second position, a second member positioned within the housing and configured to move between a third position and a fourth position, the second member forming a seal with the fourth outlet opening to enable liquid ink to enter the third ink reservoir through the second ink outlet in the third position, and the second member forming another seal with the second ink outlet to enable ink to exit the second ink reservoir through the fourth ink outlet in the fourth position, a first conduit formed through the housing having a first inlet configured to receive pressurized air and a first outlet in fluid communication with the second ink reservoir to enable pressurization of air in the second ink reservoir when the first member is in the second position, a second conduit formed through the housing having a second inlet configured to receive pressurized air and a second outlet in fluid communication with the third ink reservoir to enable pressurization of air in the third ink reservoir when the second member is in the fourth position, a first orifice formed in the first conduit, the first orifice being configured to enable pressurized air in the second ink reservoir to exit the second ink reservoir through the first conduit when the first member is in the second position, and a second orifice formed in the second conduit, the second orifice being configured to enable pressurized air in the third ink reservoir to exit the third ink reservoir through the second conduit when the second member is in the fourth position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of an ink delivery system that controls a pressure applied to a pressurized ink reservoir are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  is a perspective partial cut-away view of an ink delivery system according to the present disclosure. 
         FIG. 2  is a side cross-sectional view of the ink delivery system shown in  FIG. 1 . 
         FIG. 3  is an enlarged view of components of the ink delivery system shown in  FIG. 1 , with the components in a first state. 
         FIG. 4  is an enlarged view of components of the ink delivery system shown in  FIG. 1 , with the components in a second state. 
         FIG. 5  is a partial cutaway of a pressure input and orifice of the ink delivery system shown in  FIG. 1 , cut along line  5 - 5  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, or the like. 
     Referring to  FIG. 1 , an ink delivery apparatus  10  includes a melting apparatus  11  configured to liquefy solid ink for delivery to one or more printheads. In one embodiment, the solid ink is in pellet form, although solid ink sticks, blocks, or pastilles may be used in other embodiments. The melting apparatus  11  includes a pellet distributor  12  that receives solid ink pellets through an intake tube  13 . 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. 
     The melting apparatus  11  further includes a high efficiency heater  15 . The heater  15  generally includes a plurality of heated fins onto which the solid ink pellets are dispensed. The pellets are continuously melted by the fins and the melted ink drips between the fins into a low pressure reservoir  18 . In the illustrated embodiment, the low pressure reservoir is formed within a housing  16  that includes a slanted floor positioned directly beneath the heater  15 . The slanted floor of the low pressure reservoir is configured to direct the melted ink received through heater  15  toward a collection region  19  where the melted ink can be conveyed to the high pressure reservoirs described below. 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 high pressurized reservoirs described herein. Alternatively, the melting apparatus  11  may be slightly pressurized or maintained at atmospheric pressure. 
     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 one or more printheads. In one embodiment, two such reservoirs are provided, namely reservoirs  20   A  and  20   B , which are formed within a housing  17 . In one embodiment, the housing  17  is integral with the housing  16 , while in other embodiments the housing  17  is separate from the housing  16 , which forms the low pressure reservoir  18 . For purposes of the present disclosure, the high pressure reservoirs  20   A ,  20   B  may be referred to as the first and second reservoirs or as reservoir A and reservoir B, respectively. Like components of the reservoirs may also be designated with a subscript A or B to refer to the associated high pressure reservoir. 
     The reservoirs  20   A ,  20   B  are connected by pressure 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   A ,  20   B  is sufficient to feed high pressure inkjets of the one or more printheads, as is known in the art. As explained in more detail herein, the high pressure reservoirs  20   A ,  20   B  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. Each high pressure reservoir  20   A ,  20   B  is provided with a corresponding ink level sensor  27 ,  28 , which determine 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. Each high pressure reservoir  20   A ,  20   B  includes 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. 
     As shown in  FIGS. 1-2 , liquid ink is supplied from the low pressure reservoir  18  to each of the high pressure reservoirs  20   A ,  20   B  through an inlet opening  32   A ,  32   B . Each reservoir also includes an outlet opening  36   A ,  36   B  that communicates with outlet channels  37   A ,  37   B  respectively. Each outlet channel  37   A ,  37   B  is in fluid communication with at least one printhead and may incorporate a filter element  39  and a molten ink outlet  40  that feeds an outlet manifold (not shown). 
     In operation, pressurized liquid ink is forced from the outlet channels  37   A ,  37   B , through the filter element  39  and outlet  40  to an array of tubing (not shown), which is coupled to the printhead(s). The pressure in the outlet channel  37   A ,  37   B  is produced by pressure within the high pressure reservoir  20   A ,  20   B  that is currently pressurized. The ink delivery apparatus  10  disclosed herein provides a mechanism for alternately fluidly coupling one of the high pressure reservoirs to the ink outlet  40  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 valve assemblies  52   A ,  52   B , rocker assemblies  54   A ,  54   B , and an actuator assembly  56 . 
     For the purposes of illustration, the valve assembly  52   B  is described with the understanding that the valve assembly  52   A  is substantially identically configured. The valve assembly  52   B  includes a valve seat body  60  disposed at or over the inlet opening  32   B . 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   B . The valve seat body  60  is provided with a mounting flange  63  in one embodiment 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   B . The sealing hub  65  includes a sealing element  66 , such as an O-ring or flat rubber face seal washer, between the hub  65  and the housing  17 . The sealing hub  65  defines a sealing face  68   B  facing a seal member  70   B , as illustrated in  FIG. 2 . 
     The seal member  70   B  is disposed for translation within a chamber  61  aligned between the inlet opening  32   B  and the outlet opening  36   B . The chamber  61  is partially defined by the housing  17  in the high pressure reservoir  20   B  in one embodiment and is defined by a number of walls that help align and guide the seal body  70   B  in other embodiments. In the latter case, the walls are configured to ensure a constant supply of molten ink to the outlet opening  36   B  and sized to achieve an optimal flow rate. Seal members  70   A ,  70   B  forming part of the respective valve assembly  52   A ,  52   B  are substantially identical in construction, both bodies being configured to translate between an uppermost position sealing the inlet opening  32   A ,  32   B , and a lowermost position sealing the corresponding outlet opening  36   A ,  36   B . As shown in  FIG. 2 , the seal members  70   A ,  70   B  are arranged such that when seal member  70   B  seals the inlet opening  32   B , the other seal member  70   A  seals the outlet opening  36   A . The seal members  70   A ,  70   B  are configured to alternate positions, such that one seal member seals the corresponding outlet opening while the other seal member seals the corresponding inlet opening. 
     The seal member  70   B  includes an upper seal  71   B  and a lower seal  73   B . The upper seal is configured for sealed engagement with the sealing face  68   B  of the valve seat body  60  described above. The seal member  70   B  in  FIG. 2  is shown in sealed contact or engagement with the sealing face  68   B —i.e., with the seal body in its uppermost position. One or both of the upper seal  71   B  and sealing face  68   B  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 member  70   B  and/or the upper seal  68   B  may be configured for an enhanced fluid seal when pressure is applied behind the seal, such as when the high pressure reservoir  20   B  is pressurized to discharge molten ink to the printhead(s). The seal member  70   B  is movable to a position for sealing contact or engagement with the sealing face  38   B  at the outlet opening  36   B . Thus, the seal body includes a lower seal  73   B  that is configured to achieve a fluid-tight seal with the sealing face  38   A . In  FIG. 2 , seal member  70   A  is shown in sealed contact with the corresponding outlet opening  36   A . 
     The length of the seal members  70   A ,  70   B  are less than the distance between the opposed inlet  32   A ,  32   B  and outlet openings  36   A ,  36   B  in each high pressure reservoir  20   A ,  20   B . The length of the seal members  70   A ,  70   B  are calibrated such that when the seal member is sealing one opening (such as inlet opening  32   B ) the member does not impede ink flow through opposite opening (such as outlet opening  36   B ). At the same time, the travel distance of the seal members  70   A ,  70   B  between the first position, sealing the outlet opening, and the second position, sealing the inlet opening, is preferably limited so that the time delay between unsealing one opening and sealing the opposite opening is minimized. In one specific embodiment, the length of the seal members  70   A ,  70   B  are about 80-90% of the distance between the inlet and outlet openings in a given high pressure reservoir. 
     In order to translate the seal members  70   A ,  70   B , each valve assembly  52   A ,  52   B  is driven by a corresponding rocker assembly M A , M B . Rocker assembly M B  includes a control rod  75   B  that extends downward through the housings  16 ,  17 , and the seal body  70   B . The control rod  75   B  is affixed to the seal member  70   B  by an attachment pin  76  extending transversely through the rod  75   B  and seal member  70   B , as depicted in  FIG. 2 . In the illustrated embodiment, the control rod  75   B  is sized to extend through the height of both the low pressure  18  and high pressure  20   B  reservoirs. The rod  75   B  thus passes through a bore  78  entering the low pressure reservoir, through a rod bore  79  in the valve seat body  60  and ultimately into a receptacle  82  of rod support cup  81  at the base of the high pressure reservoir  20   B  and reservoir housing  17 . Alignment of the control rod  75   B  is maintained by the bore  78 , the rod bore  79 , and the rod support cup  81  as the rod  75   B  moves up and down between the two sealing positions. 
     As shown in  FIG. 1 , the control rod  75   B  is coupled to a clevis  85  by a pivot pin  86 . The clevis  85  pivots 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 . 
     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 . The clockwise rotation of the link arm  91  drives the control rod  75   A  and seal member  70   A  downward to the position shown in  FIG. 3 . In this position the lower seal  73   A  is sealed against the sealing face  38   A  about the outlet opening  36   A . At the same time, the clevis  85  is pivoted counter-clockwise around the axle  89  as the pivot link arm  91  pivots clockwise, causing the control rod  75   B  and seal member  70   B  to move upwardly causing upper seal  71   B  to seal with sealing face  68   B , as shown in  FIG. 2 . 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  down. This movement pivots the link arm  91  counter-clockwise and the clevis  85  clockwise about the axle  89 . Control rod  75   A  and seal member  70   A  are pulled upwardly until the upper seal  71   A  engages the sealing face  68   A , as shown in  FIG. 4 , while control rod  75   B  and seal member  70   B  are driven downwardly, sealing lower seal  73   B  with the sealing face  38   B . 
     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 upwardly) 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 assembly and stepper motor configured to drive the rocker arm into the two positions shown in  FIGS. 3 and 4 . 
     The embodiment of  FIG. 1  includes one actuating assembly  56  configured to operate both rocker assemblies  54   A ,  54   B  and both valve assemblies  52   A ,  52   B . Alternatively, the system may include a separate actuating assembly to operate each rocker and valve assembly. 
     In the position shown in  FIG. 3 , the outlet opening  36   A  from the high pressure reservoir  20   A  is sealed by the lower seal  73   A  while at the same time the inlet opening  32   A  is open. In this position, the high pressure reservoir  20   A  can be filled by ink that has been previously melted in the low pressure reservoir  18 . At the same time, pressure in the high pressure reservoir  20   A  is vented through pressure input  24 . The molten ink in the low pressure reservoir flows by gravity through the inlet opening  32   A  until the high pressure reservoir  20   A  is filled or until the molten ink in the low pressure reservoir  18  is depleted. 
     While the high pressure reservoir  20   A  is being filled, the other high pressure reservoir  20   B  is discharging the ink contents of the reservoir  20   B  under pressure. The internal level of the ink inside the reservoir is monitored by the low level sensor  28  to prevent emptying the contents and driving air into the system. The high pressure reservoir  20   B  thus has the seal member  70   B  in the position shown in  FIG. 2  in which the upper seal  71   B  is sealed against the sealing face  68   B  to close off the inlet opening  32   B . When the seal member  70   B  is in its uppermost position, the outlet opening  36   B  is unimpeded. The pressure input  25  for the second high pressure reservoir  20   B  is activated to pressurize the reservoir and supply the molten ink under pressure to the printhead(s). The low level sensor  28  continuously monitors the ink level in the reservoir  20   B  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  20   B  to the other reservoir  20   A , which by this time has filled with molten ink. 
       FIG. 5  shows a partial cutaway view of pressure input  24 . The pressure input  24  includes a conduit  110 , having an inlet  112  and an outlet  114 . The inlet  112  receives pressurized air from the pressure source. The pressurized air exits the conduit  110  through the outlet  114 , where the air enters high pressure reservoir  20   A . An orifice  100  is bored through the conduit  110  and housing  16  to fluidly connect the low pressure reservoir  18  with the conduit  110  and high pressure reservoir  20   A . The orifice  100  has a precision inlet  102 , which for one configuration is 0.028 inches in diameter. The size of the precision inlet  102  may be larger or smaller in other configurations, depending on the desired airflow through the orifice  100 . The orifice  100  also includes an outlet  104 , which discharges air into the low pressure reservoir  18 . The orifice outlet  104  of the illustrated embodiment is larger than the precision inlet  102 , although in other embodiments the outlet  104  is substantially equal to the size of the precision inlet  102 . Pressure input  25  is configured substantially identical to pressure input  24 , including a second conduit having an inlet, an outlet, and an orifice, which includes a second precision inlet. 
     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   A ,  20   B  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. 
     When the system switches ink supply to the printheads from high pressure reservoir  20   A  to high pressure reservoir  20   B , the pressure source of pressure input  24  ceases providing pressurized air. The pressure in high pressure reservoir  20   A  decreases according to a known pressure decay curve as the air escapes from the high pressure reservoir  20   A  through the orifice  100  and into the low pressure reservoir  18 . In one embodiment, the seal member  70   A  remains in a sealing position with inlet opening  32   A  for approximately six seconds after the pressure input ceases, to allow the pressure in the high pressure reservoir  20   A  to substantially equalize with the low pressure reservoir  18 . In other embodiments, the time required to substantially equalize the pressure between high pressure reservoir and low pressure reservoir may be less than or greater than six seconds, depending on the size of the precision inlet of the orifice and the pressure difference between the high pressure reservoir and low pressure reservoir. In some embodiments, the high pressure reservoir  20   A  includes a pressure sensor to monitor the pressure in the high pressure reservoir  20   A , and the delay time after the pressure input ceases is determined from the pressure sensor. The printheads in the system are configured to continue to operate with ink stored in the printhead units, lines, and manifold for the time required to reduce the pressure in the high pressure reservoir and switch the pressure source to the other high pressure reservoir. 
     Once the pressure between the high pressure reservoir  20   A  and the low pressure reservoir  18  is substantially equal, the seal member  70   A  is translated downwardly into a seal with outlet opening  36   A  as the seal member  70   B  is translated upwardly into a seal with inlet opening  32   B . The pressure source of pressure input  25  is then activated to pressurize high pressure reservoir  20   B . High pressure reservoir  20   B  begins supplying ink to the printheads through outlet opening  36   B  and outlet channel  37   B . High pressure reservoir  20   A  is fluidly connected to low pressure reservoir  18  through inlet opening  32   A  and begins filling with molten ink. 
     Because the pressure in the low pressure reservoir  18  and the high pressure reservoir  20   A  are substantially equal when the sealing member  70   A  switches from sealing the inlet opening  32   A  to the outlet opening  36   A , no pressurized air bubble transfers from the high pressure reservoir  20   A  to the low pressure reservoir  18 . Therefore, ink inside the low pressure reservoir  18  is less likely to be perturbed and escape through holes and vents in the low pressure reservoir  18 . By dropping the high pressure reservoir to atmosphere and then switching the seal actuators, the spray of ink is avoided. 
     When active, the pressure source(s) of pressure inlets  24 ,  25  are configured to operate at input pressures sufficient to compensate for the air escaping through the orifice  100 , while maintaining the desired ink pressure to the corresponding high pressure reservoir  20   A ,  20   B  and the printheads. In one embodiment, the pressure through the pressure inlet is nine psi (62 kPa) above atmospheric pressure, resulting in ink pressure to the high pressure reservoir of eight psi (55 kPa). The pressure input may vary in other embodiments depending on the size of the orifice and the desired pressure in the high pressure reservoirs. 
     The coordinated action of the actuator assembly  56  of the ink delivery control mechanism  50 , the pressure inputs  24 ,  25  to the high pressure reservoirs, the heater  15 , and the heating element  30  are controlled by a suitable master control system in one embodiment (not shown). For instance, the master control system controls valves that supply pressurized air to the pressure inputs  24 ,  25 . Likewise, the master control system controls valves that alternately vent and pressurize the air inlets  98 ,  99  for the pressure cylinder  97  in the actuator assembly  56  associated with the high pressure reservoirs  20   A ,  20   B . In some of the embodiments, the master control system is an electronic controller that is integrated into the printing machine and is operable to control other functions of the machine. The electronic controller is programmable to enable changes to 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. 
     It will be appreciated that various of the above-disclosed and 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.