Patent Publication Number: US-8529030-B2

Title: Check valve unit for solid ink reservoir system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and claims priority to co-pending application Ser. No. 12/177,649, filed on Jul. 22, 2008, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The following disclosure relates to image producing machines, and more particularly to solid ink machines that use a phase change ink melting and control apparatus. 
     In general, phase change ink image producing machines, such as printers, employ phase change inks that are in the solid phase at ambient temperature, but exist in the molten or melted liquid phase (and can be ejected as drops or jets) at the elevated operating temperature of the machine or printer. At such an elevated operating temperature, droplets or jets of the molten or liquid phase change ink are ejected from a printhead device of the printer onto a printing media. Such ejection can be directly onto a final image receiving substrate, or indirectly onto an imaging member before transfer from it to the final image receiving media. In any case, when the ink droplets contact the surface of the printing media, they quickly solidify to create an image in the form of a predetermined pattern of solidified ink drops. 
     An example of such a phase change ink image producing machine or printer, and the process for producing images therewith onto image receiving sheets is disclosed in U.S. Pat. No. 6,905,201, issued on Jun. 14, 2005, to Leighton et al., the disclosure of which is incorporated herein by reference. As disclosed therein, a high-speed phase change ink image producing machine, such as printer  10  shown in  FIG. 1 , includes a frame  11  to which are mounted directly or indirectly all its operating subsystems and components. One of the components is an imaging member  12  that is shown in the form of a drum, but can equally be in the form of a supported endless belt. The imaging member  12  has an imaging surface  14  that is movable in the direction  16 , and on which phase change ink images are formed. 
     The high-speed solid ink printer  10  also includes a phase change ink system  20  that has at least one source  22  of a single color phase change ink in solid form. When the printer  10  is a multicolor image producing machine, the ink system  20  includes four sources  22 ,  24 ,  26 ,  28 , representing four different colors CYMK (cyan, yellow, magenta, black) of phase change ink solid pieces, as shown in  FIG. 1 . The phase change ink system  20  also includes a solid phase change ink melting and control assembly or apparatus  100  ( FIG. 2A ) for melting or phase changing the solid form of the phase change ink into a liquid form, and for then supplying the liquid form to the printhead system  30 . The printhead system  30  includes at least one printhead assembly  32 , or in the case of a high-speed, or high throughput, multicolor image producing machine, four separate printhead assemblies  32 ,  32 ,  36  and  38 , as shown in  FIG. 1 . 
     The solid ink image producing printer  10  further includes a substrate supply and handling system, which may, for example, include multiple substrate supply sources  42 ,  44 ,  46 ,  48 . The substrate supply and handling system further includes a substrate treatment system  50  that has a substrate pre-heater  52 , substrate and image heater  52 , and a fusing device  60 . The phase change ink image producing printer  10  as shown may also include an original document feeder  70  that has a document holding tray  72 , document sheet feeding and retrieval devices  72 , and a document exposure and scanning system  76 . 
     Operation and control of the various subsystems, components and functions of the machine or printer  10  are performed with the aid of a controller or electronic subsystem (ESS)  80 . The ESS or controller  80  for example is a self-contained, dedicated mini-computer having a central processor unit (CPU)  82 , electronic storage  82 , and a display or user interface (UI)  86 . The ESS or controller  80  for example includes sensor input and control means  88  as well as a pixel placement and control means  89 . In addition the CPU  82  reads, captures, prepares and manages the image data flow between image input sources such as the scanning system  76 , or an online or a work station connection  90 , and the printhead assemblies  32 ,  32 ,  36 ,  38 . As such, the ESS or controller  80  is the main multi-tasking processor for operating and controlling all of the other machine subsystems and functions, including the machine&#39;s printing operations. 
     In operation, image data for an image to be produced is sent to the controller  80  from either the scanning system  76  or via the online or work station connection  90  for processing and output to the printhead assemblies  32 ,  32 ,  36 ,  38 . The controller determines and/or accepts related subsystem and component controls, for example from operator inputs via the user interface  86 , and accordingly executes such controls. As a result, appropriate color solid forms of phase change ink are melted and delivered to the printhead assemblies. Additionally, pixel placement control is exercised relative to the imaging surface  12  thus forming desired images per such image data, and receiving substrates are supplied by anyone of the sources  22 ,  22 ,  26 ,  28  and handled by means  50  in timed registration with image formation on the surface  12 . Finally, the image is transferred within the transfer nip  92 , from the surface  12  onto the receiving substrate for subsequent fusing at fusing device  60 . 
     In certain machines, the phase change ink system  20  includes a solid phase change ink melting and control apparatus  100  ( FIG. 2A ), including a pre-melter assembly  200  and a melter assembly  300 . The pre-melter assembly  200  is suitable for controllably supplying solid pieces of phase change ink from the sources  22 ,  22 ,  26 ,  28  ( FIG. 1 ) to the melter assembly  300  located below the pre-melter assembly  200 , and more particularly to the separate melters  300 A-D. A melted molten liquid ink storage and control assembly  400  is located below the melter assembly  300 . 
     In high throughput solid ink systems, the storage and control assembly  400  may incorporate a dual reservoir system corresponding to each of the individual melters  300 A-D for the various colors implemented in the solid in system. In this system, molten liquid ink is fed from a corresponding melter  300 A-D into an associated primary reservoir  404 A-D, which stores a first volume of melted ink for subsequent use. This reservoir is connected through a check valve or backflow prevention valve assembly  408  to a corresponding secondary reservoir  406  which stores a second volume of melted liquid ink. The liquid ink is ejected from the storage and control assembly  400  at an outlet array  410  and typically fed through a heated routing system to reach a respective printhead or printheads of the printhead assembly  30 . In systems of this type, pressure is applied at particular ones of the secondary reservoirs  406  to discharge ink through a corresponding outlet  410 , such as through a piston or pressurized air arrangement. The check valve assembly  408  prevents backflow of liquid ink from the now pressurized secondary reservoir back into the primary reservoir  404 . 
     In a typical prior art system, the check valve assembly  408  includes an individual check valve for each primary reservoir  404 A-D. Although each check valve is integrated into a common housing, the check valve itself is usually an off-the-shelf single valve that is supported within the housing. The check valves of the prior art are often ball valves, although needle and flapper or disc valves have also been used. The nature of the check valves used in prior art systems places significant limitations on the size of the valve assembly  408 . In other words, the nature of these prior check valve configurations requires a certain amount of space or a particularly large envelope so that a reduction in space requirements are not a viable option. Limitations on the “smallness” of the space requirements for the check valve assembly  408  cascades into limitations on the size and positioning of the first and second reservoirs  404 ,  406  served by the valve assembly. 
     Another difficulty with the check valve assemblies used in prior art machines is that they are typically formed of stainless steel. These stainless steel components require a warm-up time that is not conducive to a high speed, quick reacting printing machine. Since the printing machine utilizes molten ink, all of the components must be “at temperature” when the machine is operated to maintain ink in its molten state. Bringing the check valve assembly to temperature is particularly important since any partially solidified ink within the valve assembly can hold the check valve in an open or closed position, thereby destroying the functionality of the particular check valve. 
     There is a need for a check valve assembly that is readily scalable depending upon the nature of the printing machine. Such a check valve assembly should also be easy and inexpensive to manufacture, without sacrificing, and preferably improving, flow of molten ink through the valve. 
     SUMMARY 
     According to aspects illustrated herein, there is described a check valve unit for use in a high-speed phase change ink image producing machine having a printhead system and a system for feeding and controlling melted liquid ink provided to the printhead system, the printhead system having a plurality of printheads, the feeding and controlling system having a plurality of first storage reservoirs for receiving and holding a first volume of melted ink of a plurality of different colors delivered from a source and a like plurality of second storage reservoirs for holding a second volume of melted ink of the plurality of different colors to be delivered to ones of the plurality of printheads upon pressurization of corresponding ones of second storage reservoirs. The valve unit is coupled to the machine to be operable in an open position to control the flow of melted ink from each of the second storage reservoirs to the printhead system and in a closed position to prevent backflow of melted ink from the printhead system to the secondary reservoirs. 
     In certain embodiments, the valve unit comprises a lower valve housing defining a plurality of inlet passageways therethrough in communication with a corresponding one of a plurality of storage reservoirs, each inlet passageway defining a valve seat and a valve axis aligned with the valve seat. The valve unit further comprises a corresponding plurality of ball elements, each sized to seat on a corresponding valve seat in the closed position to prevent backflow through the corresponding inlet passageway. An upper valve housing mates with the lower valve housing to capture each of the plurality of ball elements. The upper valve housing defines a like plurality of discharge passageways in communication with at least one receiving unit, which may be a printhead of the printhead system, and aligned with a corresponding inlet passageway. In certain embodiments, each discharge passageway includes a first portion sized to receive a corresponding ball element when the ball element is unseated from the valve seat in the open position, the first portion aligned with the valve axis, and a second portion offset from the valve axis and communicating with the first portion at an intersection, the intersection sized to prevent passage of the ball element from the first portion into the second portion. 
     In another embodiment, the lower valve housing of the valve unit defining a plurality of inlet passageways, each inlet passageway defining a valve seat and a moat surrounding the valve seat. In a further embodiment, the inlet passageway includes an elongated bore having a first portion with a first diameter that defines a valve seat, and a second portion having a second diameter smaller than the first diameter. In manufacturing the lower housing with this feature, the second portion may be formed by drilling through the housing from a first surface of the housing. The first portion may then be formed by a higher precision drilling operation from an opposite surface of the housing. 
     In one embodiment, a check valve unit comprises a lower valve housing defining a plurality of inlet passageways therethrough in communication with a corresponding one of a plurality of storage reservoirs, each inlet passageway defining a valve seat; a corresponding plurality of ball elements, each sized to seat on a corresponding valve seat in the closed position to prevent backflow through the corresponding inlet passageway; and an upper valve housing mated to the lower valve housing to capture each of the plurality of ball elements between the upper and lower housings, the upper valve housing defining a like plurality of discharge passageways in communication with at least one receiving unit, which may be a printhead of the printhead system, and aligned with a corresponding inlet passageway. In this embodiment, the lower valve housing and the upper valve housing define mating surfaces, and each of the discharge passageways defines an end wall facing the valve seat, the end wall being offset from the mating surfaces by a depth greater than the diameter of one of the ball elements. In another embodiment, the end wall is angled to diverge from the mating surface toward a discharge opening in the upper housing. 
     In a further embodiment, the valve unit is provided with at least two alignment pins supported by one of the lower and upper valve housing, each of the alignment pins projecting from the mating surface and a surface opposite the mating surface. Alignment bores are defined in the other of the lower and upper valve housing, each alignment bore aligned to receive a corresponding one of the alignment pins. 
     A method for manufacturing a valve seat arrangement in a valve housing is provided that comprises providing at least one bore through the valve housing, positioning a ball element on each bore, the ball element formed of a harder material than the material of the valve housing at the bore, and pressing the ball element into the valve housing to deform the valve housing in the vicinity of the bore. Where two or more ball elements are provided, the method may include simultaneously pressing each ball element into a corresponding bore. 
     The valve unit disclosed herein is well-suited for use in a high throughput, high speed phase change ink image producing machine, such as a high speed solid ink printer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical schematic of a high-speed phase change ink image producing machine or printer. 
         FIG. 2A  is a perspective view of a phase change ink melting and control apparatus used in the machine shown in  FIG. 1 . 
         FIG. 2B  is a schematic view of an alternative phase change ink melting and control apparatus. 
         FIG. 3  is a side view of a check valve unit according to one embodiment disclosed herein. 
         FIG. 4  is an exploded perspective view of the check valve unit depicted in  FIG. 3 . 
         FIG. 5  is a top view of the check valve unit shown in  FIG. 3 . 
         FIG. 6  is a bottom view of the check valve unit shown in  FIG. 3 . 
         FIG. 7  is a bottom view of the upper housing of the check valve unit shown in  FIG. 3 . 
         FIG. 8  is a cross-sectional view of the upper housing shown in  FIG. 7 , taken along line  8 - 8  as viewed in the direction of the arrows. 
         FIG. 9  is a side cross-sectional view of the bottom housing of the check valve unit shown in  FIG. 3 . 
         FIG. 10  is an enlarged side cross-sectional view of a portion of the check valve unit illustrated in  FIG. 3 , shown with a ball element in its closed and open positions. 
         FIG. 11  is an enlarged bottom view of a discharge passageway of the upper housing shown with a ball element in its open position. 
         FIG. 12  is a side view of the bottom housing of the check valve unit disclosed herein, shown in one step of the process for manufacturing the housing. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     According to one embodiment, a check valve unit  450  is provided that can be integrated into a printing machine, such as the machine  10  shown in  FIG. 1 . The unit  450  is especially adapted for a phase change ink melting and control apparatus as depicted schematically in  FIG. 2B . In this unit, the melter assembly  300 ′ may incorporate an array of angled heated plates for each color solid ink stick. The melted ink drips from the plate into a primary, or low pressure, reservoir  404 ′ and the molten ink is fed to a secondary, or high pressure, reservoir  406 ′ through a passive check valve  408 ′. This valve  408 ′ may operate as described above to prevent backflow of ink from the secondary reservoir to the primary reservoir when the secondary reservoir  406 ′ is pressurized. The check valve unit  450  is interposed between the outlet of each secondary reservoir  406 ′ and an outlet array  410 ′. The outlet array supplies the molten ink to each printhead  30 ′ and may comprise an array of tubes feeding each printhead with multiple colors of molten ink. 
     In the modified apparatus shown schematically in  FIG. 2B  the printheads  30 ′ are located physically above the secondary reservoirs. Thus, molten ink fed to each printhead will tend to drain downward back into the secondary reservoirs after a printing operation has been completed. The check valve unit  450  operates as a non-return valve in that it prevents the molten ink from flowing back into the reservoir. When it is desired to charge a printhead with ink, pressure is applied to the ink in the secondary reservoir  406 ′, such as from an air pump through a dosing valve as depicted in  FIG. 2B . In this condition, the control valve unit  450  described herein opens to permit the flow of molten ink from the secondary reservoir to the printhead. 
     The unit  450  includes two housing halves, as shown best in  FIGS. 3-4 , an upper housing  452  and a lower housing  454  that are mated and sealed together to trap ball elements  456  within flow passageways defined in the housings. In a preferred embodiment, the housings mate at associated mating surfaces  460 ,  490 , which are manufactured to be held at a tolerance sufficient to ensure fluid tight sealing of the two halves together. In one embodiment, the valve housings  452 ,  454  are die cast of a material having a high thermal conductivity, such as aluminum, and then machined to incorporate the various flow passageway and valve features. The two housings may be sealed together by an adhesive. In some embodiments, the adhesive may be a thermally-cured silicone-based adhesive suitable for bonding aluminum and capable of accommodating potential thermal expansion of the housings. 
     As shown in  FIG. 4 , the housings  452 ,  454  are maintained in alignment by a pair of alignment pins  458  disposed between the housings. In one embodiment, each pin  458  includes a portion  458   a  that is press-fit within alignment bores  494  defined in the lower housing  454  from the mating surface  490  to the opposite lower surface  492  of the housing. The alignment bores are asymmetrically positioned at opposite sides of the housing, as illustrated in  FIG. 4 , so that the two housings cannot be misassembled. Each pin  458  includes an alignment portion  458   b  that projects above the mating surface  490  of the lower housing  454  when the pins are press-fit into their corresponding bores. The upper housing  452  includes an alignment bore  464  and an alignment slot  465  that are arranged to receive the alignment portion  458   b  of a corresponding pin when the two halves are mated. The slot  465  provides some tolerance between the relative positions of the alignment bores  494  in the lower housing  454  and the corresponding bore and slot  464 ,  465  in the upper housing  452 . 
     In a further feature, each alignment pin  458  includes an exterior alignment portion  458   c  that is configured to be received within alignment bores of the storage and control assembly  400  or some other component of the machine  10 . The alignment pins  458  thus provide a means for ensuring proper alignment and orientation of the control valve unit  450  when it is mounted within the machine. 
     As illustrated in  FIGS. 4-6 , the check valve unit  450  defines various openings and passageways for communicating molten ink from the secondary reservoirs  404 A-D to the outlet array  410 ′. Thus, the upper housing  452  defines a plurality of discharge openings  468  that may be fluidly connected to the outlet array in a conventional fashion. The lower housing  454  defines a like plurality of inlet passageways  496  that are fluidly coupled to a corresponding secondary reservoir  406 ′. In one embodiment, the lower housing defines a stem  504  through which each inlet passageway  496  passes, with each stem configured to be fluidly coupled to a mating component of the secondary reservoirs  406 ′. 
     The valve unit  450  includes eight ball elements  456  corresponding to eight inlet passageways  496  and eight discharge openings  468 . This particular unit may provide four ink colors to two different printheads, such as printheads  32 ,  34  in  FIG. 1 . An additional valve unit may be incorporated into the machine  10  to provide the four ink colors to two additional printheads, such as printheads  36 ,  38 . If the machine has more printheads, additional valve units may be provided. Moreover, it is understood that the eight valve elements can correspond to eight ink colors being fed to a single printhead. However, one benefit shared in any adaptation or use of the valve unit  450  is that the size the unit is kept to a minimum so that multiple units can be readily introduced into a larger multiple color, multiple printhead system. This feature of the valve unit  450  is accomplished in part by the manner in which the ink flow passageways are defined in the upper and lower housings of the unit. 
     In particular, as shown in the top view of  FIG. 5 , the discharge openings  468  are compactly arranged in an alternating fashion. As shown in the bottom view of  FIG. 6 , the inlet passageways  496  are also arranged in alternating fashion, although they are offset outboard relative to the discharge openings. This offset is achieved by the unique configuration of the discharge passageways  470  in the upper housing  452 , as best seen in  FIGS. 7-8 . In particular, the discharge passageways  470  include a first portion  472  that is aligned along an axis A 1  and a second portion  472  that is offset from the first portion along an axis A 2 . The two portions are in communication at an intersection  476  so that molten ink can pass freely from the first portion  472  into the second portion  474 . As seen in  FIG. 8 , the second portion  474  intersects the upper surface  462  at the discharge opening  468  that is configured to communicate with the outlet array  410 ′, as discussed above. 
     Looking next at  FIG. 9 , the construction of the lower housing  454  is illustrated. As previously explained, the lower housing includes a mating surface  490  that mates with the mating surface  460  of the upper housing  452 . The inlet passageways  496  open at this mating surface and are in communication with the first portion  472  of the discharge passageways  470  of the upper housing when the two housings are combined. Thus, the molten ink flows through the inlet passageway  496  into the first portion  472 , through the intersection  476  and into the second portion  494 , to be discharged through the discharge openings  468 . This flow of molten ink is controlled by the check valve features of the valve unit  450 , and in particular by the ball elements  456  disposed within the combined inlet and discharge passageways  496 ,  470 , respectively. (See,  FIG. 10 ). 
     The inlet passageway  496  includes a mating recess  497  at the mating surface  490  of the lower housing that is sized to coincide with the mating recess  480  at the mating surface  460  of the upper housing. This mating recess  497  opens into a ball chamber  498  within which the ball element  456  is disposed, at least in its closed position. In order to accommodate the check valve ball elements, the lower housing  454  defines a ball seat  500  in each ball chamber  498 . This ball seat projects above the base of the ball chamber to define a moat  506  around the seat  500 . 
     A central bore  502  is defined through the ball seat  500  ultimately defining an inlet opening  503  at the end of an inlet stem  504  that projects from the lower surface  492  of the lower housing. In one embodiment, the lower surface  492  further defines a mounting recess  508  around the collection of inlet stems  504  (see  FIG. 6 ). This mounting recess and the inlet stems can provide an interface to mount the check valve unit  450  to structure defining the secondary reservoirs  406 ′. It is understood that the configuration of these features can be modified as necessary for mounting the check valve unit  450  to a particular molten ink storage and control assembly. 
     As shown in  FIG. 10 , when the upper and lower housings are mated, the central bore  502  of the inlet passageways  496  are aligned along the axis A 1  and more particularly aligned with the first portion  472  of the discharge passageway  470 . The ball element is initially supported on the ball seat  500 , as represented by the ball element  456 ′, blocking or closing the central bore  502  and preventing liquid flow through the bore into the ball chamber  498  and into the discharge passageway  470 . In this position, the ball element acts as a check valve to prevent backflow of liquid ink back through the central bore  502 . As described above, liquid ink contained within the secondary reservoir  406  is discharged to a corresponding printhead by pressurizing the reservoir. This pressure forces molten ink out of the reservoir, with the majority of the ink traveling to the printhead. However, some portion of the ink will be forced back under pressure into the discharge passageway  470  of the check valve unit  450 . This pressurized backflow, along with gravity for a vertically oriented valve unit, will push the ball element  456  against the ball seat  500  and the ball element will prevent this backflow from entering the inlet passageway and the primary reservoir. 
     In the normal print cycle, once the secondary reservoir  406 ′ has been purged, it is refilled with liquid ink from the associated primary reservoir  404 ′. A corresponding secondary reservoir is then pressurized to force liquid ink through the inlet opening  503  and into the inlet passageway  496 , and more particularly into the central bore  502 . This pressurized flow of ink dislodges the ball element from the ball seat  500  to the position  456 ″ so that liquid ink can flow from the inlet passageway  496  into the discharge passageway  470 . When the ball element is unseated, it is forced under pressure upward into the first portion  472  of the discharge passageway  470 . As the pressurized flow carries the ball element with it, the ball element contacts the intersection  476  between the first and second portions to prevent passage of the ball element into the second portion  474  which would otherwise block fluid flow exiting the discharge opening  468 . Thus, as shown in the detail view of  FIG. 11 , the intersection  476  has a width dimension W that is less than the diameter D of the ball element. 
     In a typical check ball configuration, the fluid flow is entirely axial, which requires the liquid to flow around the check ball when it is unseated. In one feature of the check valve unit  450  disclosed herein, the inlet passageway  496  and discharge passageway  470  are configured to provide a flow path that “jogs” around the dislodged ball element in its open position  456 ″. This flow path is provided in part by the ball chamber  498  that has a larger diameter than the ball element. The mating recesses  497  and  480  can be included to provide an even larger flow area at the mating interface between the housings. In a further feature to enhance the fluid flow path, the first portion  472  of the discharge passageway includes an end wall  478  that is offset from the mating surface  460  a distance sufficient for the ball element to move substantially free of the ball chamber  498  in the inlet passageway  496 , as depicted in  FIG. 10 . This feature provides a flow path F that substantially circumvents the ball element so that this flow is not impeded by resistance that would otherwise be caused by the presence of the ball element in the flow path. This clear flow path allows the liquid ink to be quickly dispensed from the secondary reservoir  406 . This rapid response time permits faster printing operations which is especially valuable for high speed printing applications. 
     The end wall  478  of the first portion  472  of the discharge passageway may incorporate a further beneficial feature. As shown in  FIG. 10 , the end wall  478  is angled relative to the mating surface  460  of the upper housing  452 . In particular, the end wall  478  is angled to diverge from the mating surface toward the second portion  474  of the discharge passageway. This angled configuration provides a path for bubbles that may form during the pressurized charging and discharging of the secondary reservoirs. Any bubbles within the passageways will float upward within the first portion  472  and will continue to float upward along the angled end wall  478  into the second portion  474  where the bubbles can float freely through the discharge opening  468 . The bubbles can be thus eliminated from the valve unit thus preventing interactions with the valve performance that may result from meniscus forces. 
     It can be appreciated that the offset between the inlet passageway  496  defined along axis A 1  and the outlet opening  468  defined along the axis A 2  provides smooth, rapid response flow of molten ink from the secondary reservoirs to the outlet array. Referring back to  FIG. 7 , it can be seen that the discharge passageways  470  are compactly arranged while maintaining sufficient distance between openings in the mating surface  460  to prevent cross leakage between passageways. Thus, the distance D 1  between the two horizontal lines of second portions  474  can be kept to a minimum by offsetting the portions by the distance S 1 . This offset S 1  thus increases the spacing between portions  474  which reduces the risk of cross-leakage. The distance D 2  between the first portions  472  is minimized by orienting the discharge passageways at an angle relative to the horizontal line of second portions  474 . The angular orientation is configured to ensure an optimum spacing S 2  between the first portion  472  of one discharge opening and the second portion  474  of an adjacent discharge opening. This placement of discharge openings in the upper housing  452  thus provides a very compact arrangement without the risk of cross-leakage between discharge openings. The positioning of the first portions  472  of the discharge passageways also allows for compact positioning of the inlet passageways  496  in the lower housing  454 , as seen in  FIG. 5 . 
     In one embodiment, the housings  452 ,  454  of the check valve unit  450  may be initially cast from aluminum. Detailed features of the housings may be then machined in a conventional manner. The check valve unit incorporates certain features that enhance and simplify the manufacturing process for the check valve unit. For instance, the central bore  502  of the lower housing  454  includes a seat portion  502   a  and an inlet portion  502   b . The seat portion  502   a  is preferably held to tight tolerances because it provides the seat for the ball element  456 . It is thus preferred that the seat portion  502   a  define a sharp edge at its interface with the ball element. The seat portion is further preferably provided at a consistent radial wall thickness. However, the remainder of the central bore  502  does not require such precision. Thus, in one feature, the central bore  502  is initially formed by drilling from the lower surface  492  through the entire housing at the diameter of the inlet portion  502   b . If the ball chamber  498  is not cast into the initial form, the chamber can be machined in a conventional manner from the mating surface  490 , including the moat  506 . Then, the more critical feature, the seat portion  502   a  may be precision machined from the mating surface, ensuring sufficient wall thickness in the valve seat  500  around the seat portion  502   a  of the central bore and achieving the necessary sharp edge at the opening of the seat portion. 
     In one embodiment, the housing halves are mated using an adhesive joint. The check valve unit  450  incorporates certain features that facilitate assembling the upper and lower housings and the ball elements. In one feature, the end wall  478  is positioned at a height above the mating surface  460  so that the ball elements  456  can be held away from the mating surface when adhesive is applied. It is thus contemplated that the ball elements  456  are formed of a magnetic material, such as steel. A magnet laying on the upper surface  462  when the adhesive is applied and being cured will pull the ball elements upward against the end wall  478 , as shown in the position  456 ″ in  FIG. 10 . In this position, the ball elements are offset from the mating surfaces  460 ,  490  so any adhesive that may leak into the mating recesses  480 ,  497  cannot contact and compromise the ball elements. 
     The moat  506  in each inlet passageway  496  provides an additional region for capturing excess adhesive that may leak into the passageway when the housings are pressed together. The excess adhesive will drip down the wall of the ball chamber  498  and into the moat  506  surrounding the ball seat  500 . The moat  506  is sufficiently deep so that no excess adhesive can flow into the surface of the ball seat  500  or into the seat portion  502   a  of the central bore  502 . It can also be appreciated that the moat  506  helps make the valve unit more robust to contaminants that might otherwise rest on the valve seat itself. 
     In some instances the sharp edge contact with the spherical surface of the ball element provides sufficient sealing, particularly at lower pressures. However, in higher pressure or rapid, high volume applications, additional sealing capability is desirable. Additional sealing may be provided by increasing the area of contact between the ball element  456  and the ball seat  500 . This additional sealing area has been traditionally achieved by machining and polishing a chamfer at the opening to be engaged by the ball element. However, this approach requires precision machining. In order to maximize the sealing capabilities of the ball elements in the ball seats, certain embodiments contemplate a novel coining operation. In accordance with one embodiment, each ball element  456  is placed within the lower housing  454  on a respective valve seat  500  covering the corresponding seat portion  502   a  of the central bore, as shown in  FIG. 12 . In this position, a portion of the ball elements project above the mating surface  490  of the lower housing. A pressure plate R is placed on top of the ball elements, as shown in the figure. Pressure P is applied to the plate R which presses each ball element  456  into a corresponding ball seat  500 . In this embodiment, the housing is formed of a softer material than the ball element so only the ball seat  500  deforms under the applied pressure. The upper edge of the seat portion  502   a  of the central bore  502  is thus coined into a radiused chamfer that corresponds to the radius of the ball element  456 . This radiused chamfer thus provides an optimum sealing surface between the ball seat and the ball element. 
     The check valve unit  450  of the illustrated embodiments provides significant flexibility at a low cost. In particular, since multiple check valves are incorporated into a common housing, increasing the number of check valves does not entail an increase in components, other than additional ball elements. Since all of the inlet and discharge passageway features are formed in the upper and lower housings, adding check valves is achieved by forming additional passageways in the housings. In some applications, all of the passageway features may be formed by precision die casting. Even in applications in which certain features of the inlet and discharge passageways must be machined, these particular features readily lend themselves to single step machining on a single multi-tool machine. 
     As described above, it is contemplated in certain embodiments that the check valve unit will be mounted within the printing machine  10  to maintain the vertical orientation illustrated in  FIG. 10 . In this orientation, gravity will act on the ball elements  456  to return them to the closed or backflow prevention position  456 ′ when pressurized flow from the primary reservoirs ceases. This vertical orientation also takes full advantage of the angled back wall  478  feature of the discharge passageway portion  472  for dissipation of bubbles from within the liquid ink flow path F. 
     However, in some machines the structure of the storage and control apparatus  400  may not permit this vertical orientation. In such applications, a spring element may be incorporated between the ball element  456  and the back wall  478  so that the spring element can provide the restoring force to return the ball element to the ball seat. In some applications, the check valve unit  450  may be oriented with the ends vertically arranged. In this orientation, the check valve unit can be positioned so that the second portions  474  of the discharge passageways are vertically above the associated first portion  472 . In this orientation, the angled back wall  478  of the first portion will still assist in dispersing bubbles from the valve passageways. 
     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. 
     For instance, the arrangement of openings in the upper and lower housings may be modified to accommodate a particular printing machine. In addition, in embodiments in which the housings are not mated using adhesive, the moats  506  may be eliminated. Similarly, where precision machining is implemented to manufacture the lower housing  454 , the central bore  502  may be formed at a constant diameter. Likewise, where tight tolerances are not required, the need for precision machining of the seat portion  502   a  may not be necessary. 
     It should be appreciated that the relative dimensions among the components of the check valve unit  450  may be varied depending upon the particular application for the unit. For instance, the relative diameters of the inlet and discharge passageways may be modified depending upon the flow rates, liquid ink viscosity, etc., for the application. The check valve unit of the illustrated embodiments provides great flexibility to permit use of the disclosed features as needed for a particular application. 
     It can further be noted that the control valve unit  450  has been described as performing as a check valve between the outlet of each of the secondary pressurized reservoirs and the outlet array feeding the printheads of the machine  10 . The valve unit may be disposed between a plurality of ink reservoirs and at least one receiving unit through an appropriate outlet array. Thus, in the embodiments described above the receiving units correspond to the four printheads. A similar control valve unit may be positioned between the primary reservoir  404 ′ and the secondary reservoirs  406 ′ as the receiving units, in lieu of the passive check valves  408 ′. Additional or fewer ink colors and associated reservoirs may be accommodated in the valve unit  450  by changing the number of passageways defined in the valve body housings  452 ,  454 .