Abstract:
Among other things, an apparatus for use in fluid jetting is described. The apparatus comprises a printhead including a flow path and a nozzle in communication with the flow path that has a first end and a second end. The apparatus also includes a first container fluidically coupled to the first end of the flow path, a second container fluidically coupled to the second end of the flow path, and a controller. The first container has a first controllable internal pressure and the second container has a second controllable internal pressure. The controller controls the first internal pressure and the second internal pressure to have a fluid flow between the first container and the second container through the flow path in the printhead according to a first mode and a second mode. In either mode, at least a portion of the fluid flowing along the flow path is delivered to the nozzle when the nozzle is jetting. The first mode has the first internal pressure higher than the second internal pressure and the second mode has the second internal pressure higher than the first internal pressure. The fluid flows from the first container to the second container according to the first mode and flows from the second container to the first container according to the second mode.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure generally relates to fluid circulation in a fluid ejector. 
       BACKGROUND 
       [0002]    An ink jet printer typically includes an ink path from an ink supply to an ink nozzle assembly that includes nozzles from which ink drops are ejected. Ink drop ejection can be controlled by pressurizing ink in the ink path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. A typical printhead has a line of nozzles with a corresponding array of ink paths and associated actuators, and drop ejection from each nozzle can be independently controlled. In a so-called “drop-on-demand” printhead, each actuator is fired to selectively eject a drop at a specific pixel location of an image, as the printhead and a printing media are moved relative to one another. 
         [0003]    A printhead can include a semiconductor printhead body and a piezoelectric actuator. The printhead body can be made of silicon, which is etched to define ink chambers. Nozzles can be formed in the silicon body, or defined by a separate nozzle plate that is attached to the silicon body. The piezoelectric actuator can have a layer of piezoelectric material that changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. 
         [0004]    Printing accuracy can be influenced by a number of factors, including the uniformity in size and velocity of ink drops ejected by the nozzles in the printhead and among the multiple printheads in a printer. The drop size and drop velocity uniformity are in turn influenced by factors, such as the dimensional uniformity of the ink paths, acoustic interference effects, contamination in the ink flow paths, and the uniformity of the pressure pulse generated by the actuators. Contamination or debris in the ink flow can be reduced with the use of one or more filters in the ink flow path. 
       SUMMARY 
       [0005]    In one aspect, the disclosure describes an apparatus for use in fluid jetting. The apparatus comprises a printhead including a flow path and a nozzle in communication with the flow path. The flow path has a first end and a second end. The apparatus also includes a first container fluidically coupled to the first end of the flow path, a second container fluidically coupled to the second end of the flow path, and a controller. The first container has a first controllable internal pressure and the second container has a second controllable internal pressure. The controller controls the first internal pressure and the second internal pressure to have a fluid flow between the first container and the second container through the flow path in the printhead according to a first mode and a second mode. In either mode, at least a portion of the fluid flowing along the flow path is delivered to the nozzle when the nozzle is jetting. The first mode has the first internal pressure higher than the second internal pressure and the second mode has the second internal pressure higher than the first internal pressure. The fluid flows from the first container to the second container according to the first mode and flows from the second container to the first container according to the second mode. 
         [0006]    Implementations may include one or more of the following features. The fluid flowing from the first container to the nozzle in a direction opposite to the direction in which the fluid flows from the second container to the nozzle. The first internal pressure and the second internal pressure are both lower than the atmospheric pressure. A difference between the first and second internal pressures is larger than a difference between the atmospheric pressure and the first or second internal pressure. The controller controls a rate of the fluid flow between the first and second containers to be higher than the rate of the fluid delivery from the first or second container to the nozzle when the nozzle is jetting. For a given period of time, an amount of the fluid flown between the first and second containers is at least 10 times an amount of fluid jetted by the printhead when the printhead is jetting a fluid. A rate of the fluid flow through the flow path is about 5% or less of a velocity of a fluid droplet ejected from the nozzle. The apparatus also includes a sensor to sense a fluid level in each of the first container and the second container. The controller controls the first and second internal pressures to be in the first mode when the sensed fluid level in the second container is below a predetermined value. The controller controls the first and second internal pressures to be in the second mode when the sensed fluid level in the first container is below a predetermined value. The first container is in a first chamber and the second container is in a second chamber, and the first and second containers are flexible and contain substantially no air. Each of the first and second chambers is connected to a vacuum source to provide adjustment to the first and second internal pressures. The flow path is about 1 micron to about 30 microns upstream of the nozzle, e.g., measured along a path in which the fluid flows. The first and second containers are self-contained fluid reservoirs. The first and second containers are mounted on a housing that is connectable to the printhead. The connection between the housing and the printhead is switchable between a first state in which the first and second containers are in fluid communication with the flow path and a second state in which the first and second containers are fluidically disconnected from the flow path. 
         [0007]    In another aspect, the disclosure features a method for use in fluid jetting. The method comprises delivering a fluid at a controlled flow rate from a first container to a second container along a flow path in a printhead along a first direction and delivering the fluid at a controlled flow rate from the second container to the first container along the flow path in the printhead along a second direction opposite to the first direction. A portion of the fluid flowing in the flow path is delivered to a nozzle in communication with the flow path when the nozzle is ejecting the fluid. A portion of the fluid flowing in the flow path is delivered to the nozzle in communication with the flow path when the nozzle is ejecting the fluid. 
         [0008]    Implementations may include one or more of the following features. The fluid flowing from the first container to the nozzle in a direction opposite to the direction in which the fluid flows from the second container to the nozzle. A pressure difference between an internal pressure of the first container and an internal pressure of the second container is maintained. Each internal pressure of the first and second containers is maintained to be lower than an atmospheric pressure. The pressure difference between either internal pressure of the first and the second containers and the atmospheric pressure is maintained to be smaller than the pressure difference between the internal pressure of the first container and the internal pressure of the second container. The first and second containers are flexible and the pressure difference is maintained by applying different pressures to exterior surfaces of the flexible first and second containers. A fluid level in the first and second containers is sensed and a fluid delivery direction from the first and second directions is selected based on the sensed fluid level. Delivering the fluid in the selected direction comprises adjusting the internal pressures of the first and second containers. The controlled flow rate is about 5% or less of a velocity of a fluid droplet ejected by the nozzle. 
         [0009]    In another aspect, the disclosure features an apparatus for use in fluid jetting. The apparatus comprises a printhead including a flow path and a nozzle in communication with the flow path, the flow path having a first end and a second end; a first container fluidically coupled to the first end of the flow path, the first container having a first controllable internal pressure; a second container fluidically coupled to the second end of the flow path, the second container having a second controllable internal pressure; and a controller to control the first internal pressure and the second internal pressure to have a fluid flow between the first container and the second container through the flow path in the printhead. At least a portion of the fluid flowing along the flow path is delivered to the nozzle when the nozzle is jetting, the first internal pressure being higher than the second internal pressure. 
         [0010]    Implementations may include one or more of the following features. The fluid flowing from the first container to the nozzle in a direction opposite to the direction in which the fluid flows from the second container to the nozzle. The first internal pressure and the second internal pressure are both lower than atmospheric pressure. The first container is in a first chamber and the second container is in a second chamber, and the first and second containers are flexible and contain substantially no air each of the first and second chambers is connected to a vacuum source to provide adjustment to the first and second internal pressures. The first and second containers are self-contained fluid reservoirs. The first container contains the fluid and the second container is empty before use. 
         [0011]    Implementations may include one or more of the following advantages. An assembly having a printhead module attached to a cartridge containing self-contained fluids can be used for testing operations, such as test printing. The cartridge can include two separate chambers each enclosing a fluid container capable of providing the fluid to nozzles of the printhead module to be jetted. The fluid can be recirculated between the two fluid containers to prevent the fluid from drying along one or more fluid paths in the system or at the nozzles. Particles in fluid can be kept in suspension in the fluid to maintain the quality of the fluid. For example, the fluid can have a high uniformity. Further, air bubbles along the fluid paths can be removed by the recirculation flow. The fluid recirculation can be performed during the fluid jetting. The entire assembly can be disposed of following the testing operation, avoiding having to flush clean a printhead module between tests. 
         [0012]    Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages may be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a printing system. 
           [0014]      FIG. 1A  is a schematic diagram of a fluid meniscus in a nozzle. 
           [0015]      FIG. 2  is a flow diagram describing operations of a controller. 
           [0016]      FIG. 3A  is a perspective view of a printing system. 
           [0017]      FIGS. 3B-3D  are cross-sectional views of a printing system. 
           [0018]      FIG. 4  is a schematic perspective view of a printhead body. 
           [0019]      FIG. 5  is a cross-sectional view of a printhead body. 
           [0020]      FIG. 6  is a perspective view of a portion of a printhead body. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A printhead module generally includes a printhead body with multiple nozzles that are in fluid communication with an external fluid supply to allow for a continuous printing operation. In certain applications, a printhead module that can be effectively operated using a relatively small volume of a fluid, e.g., for a fluid testing operation, is desirable. The printhead module can include a fluid supply assembly designed for a relatively small volume of a printing fluid, and the fluid supply assembly can be attachable to the printhead body. In some implementations, the fluid supply assembly is a non-refillable fluid supply assembly, e.g., a single-use printing fluid supply cartridge. Such a device is described in U.S. Pat. No. 7,631,962, which is incorporated by reference. 
         [0022]    After use, the printhead body and the fluid supply assembly can be discarded. For example, when testing printing fluids of different colors or qualities, each type of fluid is contained within a fluid supply assembly and printed using a printhead body that is not used to print any other types of printing fluids. There would be no need to flush clean the fluid supply assembly or the printhead body when testing different printing fluids. 
         [0023]    Referring to  FIG. 1 , an assembled system  10  (or a printhead module  10 ) for use, e.g., in test printing, includes a printhead body  16  and a fluid supply assembly  12 , e.g., in the form of a cartridge  12  that can be attached to the printhead body  16 . The fluid supply assembly  12  contains two fluid containers  14   a ,  14   b  to supply a fluid to a printhead body  16 . One or more nozzles  18  (only one nozzle shown in the figure) of the printhead body  16  can be activated to eject fluid drops  20  to form a pattern on a substrate (not shown). The pattern can be studied to evaluate the quality of the fluid, the image effect of the printing, or the design of the printhead module  16 . 
         [0024]    The two fluid containers  14   a ,  14   b  each can be a self-contained fluid reservoir that communicates with each other through a fluid path  24  extending from each fluid container  14   a ,  14   b , and passing through the printhead body  16 . In this context, self-contained means that during the printing operation, fluid is not supplied into the reservoir from a source outside the fluid containers  14   a ,  14   b . Rather, the fluid to be used is the fluid contained within the self-contained fluid containers  14   a ,  14   b . For convenience, we name the fluid path  24  from the fluid container  14   a  and outside the printhead module  16  as  24   a , the fluid path  24  from the fluid container  14   b  and outside the printhead module  16  as  24   b , and the fluid path  24  within the printhead module as  24   c . The fluid path  24   c  can be formed in an MEMS die (see  FIGS. 5 and 6  below) and is upstream of the nozzle  18 . The fluid can flow back and forth through the flow path  24  between the two fluid containers  14   a ,  14   b  to recirculate the fluid between the two containers. During the flow, a portion of the fluid is directed to the nozzle  18  when needed, e.g., when fluid droplets  20  are being jetted. The fluid to be jetted by the printhead module  16  can be delivered from either of the fluid containers  14   a ,  14   b.    
         [0025]    The recirculation (or circulation) of the fluid between the two containers  14   a ,  14   b  can improve printing quality, e.g., by preventing the fluid from drying at any location along the fluid path or approximate the nozzle  18 . Particles in the fluid can be kept in suspension in the fluid without substantial coagulation to maintain the quality, e.g., uniformity of viscosity and/or avoidance of large particles that could clog the fluid path or nozzle, of the fluid. In some implementations, air bubbles generated along the fluid path  24  can be carried with the flow and be removed at the containers  14   a ,  14   b , e.g., by rising to the surface of the fluid in the containers  14   a ,  14   b . The test printing results from the system  10  contain few artifacts generated by fluid drying, air bubbles, or fluid quality variations. The system  10  resembles a real printing system (that is not used only for testing), and the test printing results can provide a true representation of the elements that are being tested, e.g., the quality of the fluid. 
         [0026]    In the assembled system  10 , to prevent the fluid from automatically flowing out of an inactivated nozzle  18  and control the fluid flow between the containers  14   a ,  14   b  (explained in more detail below), the fluid pressure in each fluid container  14   a ,  14   b  is controlled. In the example shown in  FIG. 1 , the fluid containers  14   a ,  14   b  each includes a flexible wall  36   a ,  36   b  that transfers the pressure in each chamber  22   a ,  22   b  of the cartridge  12  to the fluid inside the containers  14   a ,  14   b . Each chamber  22   a ,  22   b  encloses a respective fluid container  36   a ,  36   b . The pressure within each chamber  22   a ,  22   b  can be adjusted using a pressure control device  28 , e.g., one or more pumps or vacuum sources, connected to the chambers through openings  30   a ,  30   b , respectively. The chambers  22   a ,  22   b  are sealed from each other and the pressure in each chamber can be independently adjusted by the pressure control device  28 . 
         [0027]    In some implementations, the amount of fluid in the containers  14   a ,  14   b  is small and the fluid pressures within the containers  14   a ,  14   b  are substantially the same as the fluid pressures in the chambers  22   a ,  22   b , respectively. Each container  14   a ,  14   b  can be air-free or under a vacuum before the fluid is filled into the container. In some implementations, a system  10  can have one of the fluid containers  14   a ,  14   b  filled with a desired amount of fluid, e.g., 0.25 ml to 10 ml, 0.5 ml to 3 ml, or 1.5 ml, and the other one of the fluid containers empty and airless. In some implementations, the fluid containers  14   a ,  14   b  may contain some air. In some implementations, the fluid containers contain a gas but do not contain oxygen gas. The fluid path  24  can be controlled to be airless or free of oxygen. An airless system or a system free of oxygen can prevent air or oxygen dissolving into the fluid to affect the quality of printing or quality of the fluid. In some implementations, the system  10  can be assembled under an inert atmosphere. 
         [0028]    The fluid in each containers  14   a ,  14   b  is maintained at a selected negative pressure, e.g., −0.5 inch of water to −20 inches of water or −6 inches to −7 inches of water, depending on factors such as size of the orifice or nozzle  18 . When the nozzle  18  is not activated to eject droplets  20 , the negative pressure prevents the fluid from automatically seeping out of the nozzle  18  and at the same time prevents air from being drawn into the printhead module  16  from the nozzle  18 . Referring to  FIGS. 1 and 1A , the negative pressure in the fluid balances the combined forces of fluid source pressure (produced by the height location of the fluid containers  14   a ,  14   b  relative to the printhead module  16 , which can be positive or negative), capillary action, and atmospheric pressure to maintain a meniscus  34  on an fluid-air interface at the nozzle  18 . When the nozzle  18  (or the pumping chamber) is activated, the meniscus  34  can allow the fluid to be jetted out of the nozzle  18  readily. Such a negative pressure in the fluid is maintained during the flow circulation between the containers  14   a ,  14   b , and also during fluid jetting from the nozzle  18 . During fluid jetting, the fluid pressure in the vicinity of the nozzle  18  (e.g., upstream of the nozzle  18  and in a pumping chamber (not shown)) can be changed by an actuator, e.g., a piezoelectric actuator. 
         [0029]    The direction of fluid flow along the fluid path  24  is controlled by a difference between the fluid pressures in the fluid containers  14   a ,  14   b . For example, when the fluid pressure in the container  14   a  is higher than the fluid pressure in the container  14   b , the fluid flows from the container  14   a  towards the container  14   b  (as an arrow  32  shows). The pressure control device  28  maintains the negative pressure in the fluid (in the containers  14   a ,  14   b  or at the printhead body  16 ) and, e.g., at the same time, generates the pressure difference between the pressures within the chambers  22   a ,  22   b . The rate of the fluid flow can be affected by the value of the pressure difference and other factors, such as the dimensions of the flow path  24 . 
         [0030]    The amount of recirculation fluid between the two fluid containers can be about 1/1000 to about 10 times the maximum amount of fluid jetted by the print body  16  in a given time period. The recirculation fluid flow rate (i.e., the amount of recirculation fluid passing by a cross-section of the flow path  24  per second) can be selected based on the need of the system. In some implementations, the ratio of the recirculation fluid flow rate to the amount of fluid jetted depends on the duty cycle of the printing or percentage of the jetting nozzles per unit time period, e.g., be lower when the printing is operating at a higher duty cycle. The recirculation fluid flow velocity can be controlled to prevent effects on, e.g., errors in, fluid jetting trajectories because the recirculation fluid is in communication with the nozzle  18 , e.g., flows past the nozzle  18 . 
         [0031]    The value of the pressure difference between the two fluid containers can be chosen based on the desired flow rate, the characteristics of the fluid, e.g., viscosity, the design of the flow path  24 , and other factors. In some implementations, the value of the pressure difference is pre-chosen based on the assembly  10  and the fluid while the direction of the pressure difference can be changed dynamically. The assembly  10  switches the direction of the pressure difference to drive the fluid flow in the desired direction. For example, when the pressure in the fluid container  14   a  is higher than the fluid pressure in the fluid container  14   b , the fluid flows from the fluid container  14   a  to the fluid container  14   b . When the direction of the pressure difference is reversed (i.e., the fluid container  14   b  has a higher pressure than the fluid container  14   a ), the flow direction is reversed. In some implementations, the value of the pressure difference is about 0.1 inch of water up to 100 inches of water. 
         [0032]    A controller  26  determines the direction of fluid flow based on the fluid levels in each container  14   a ,  14   b , and instructs the pressure control device  28  to form a desired pressure difference between the two containers to drive the fluid flow. In some implementations, the fluid levels are sensed by fluid level sensors  36   a ,  36   b  located within the containers  14   a ,  14   b , respectively. Examples of the sensors  36   a ,  36   b  can include contact sensors that touch the fluid containers  14   a ,  14   b . Other sensors (not shown) suitable for use can include optical sensors, which can be placed outside of the containers  14   a ,  14   b , proximity sensors, or magnetic sensors, such as reed switches. The sensors  36   a ,  36   b  can communicate with the controller  26  through a wire (not shown) or wirelessly. In some implementations, the sensors  36   a ,  36   b  and the controller  26  are connected by one or more optical fibers for communication, e.g., data delivery. 
         [0033]    The controller  26  can be programmed to store criteria for use in forming the instructions to the pressure control device  28  or other associated devices, e.g., the printhead body  16 , based on the sensed fluid levels in the containers  14   a ,  14   b . For example, the criteria can be a minimum fluid level. Under some stored criteria, the controller  26  can function as shown in  FIG. 2 . Upon receiving  50  the sensed fluid levels in the containers  14   a ,  14   b  from the sensors  36   a ,  36   b , the controller  26  compares the sensed fluid levels with the stored criteria. The controller  26  first determines  52  whether the sensed fluid levels in both containers  14   a ,  14   b  are both lower than a predetermined minimum level (PML). If yes, the controller  26  instructs  54  the printhead module  16  to stop printing because the sensed fluid levels indicate that the fluid in the containers  14   a ,  14   b  is running out. In addition, the controller  26  can also provide a signal to the user to indicate that the fluid level is low and the cartridge  12  may be discarded or needs to be refilled (discussed later). The pressure control device  28  can also be instructed to stop working, although maintaining the negative pressure for the fluid meniscus at the nozzle  18  may be desirable so that the fluid does not leak. If no, then the controller  26  determines  56  whether the sensed fluid levels are both higher than the predetermined minimum level. If yes, the fluid flow conditions, e.g., direction or rate, along the fluid path  24  between the two containers  14   a ,  14   b  do not need to be changed. The controller  26  keeps receiving  50  the sensed fluid levels and monitors the fluid flow. If no, the controller  26  further determines  58  whether the current flow direction in the fluid path  24  is from the container having the high flow level to the container having the low flow level. If yes, the fluid flow conditions do not need to be changed, and the controller  26  keeps receiving  50  the sensed fluid flow levels and monitors the fluid flow. If no, then the controller  26  instructs  60  the pressure control device  28  to reverse the pressure difference between the two containers  14   a ,  14   b  so that the fluid flow direction is reversed. 
         [0034]    The controller  26  can also use other criteria and function in ways different from that described in  FIG. 2  to control the fluid flow between the two containers  14   a ,  14   b . The criteria can be set at the controller  26  when the system  10  is manufactured or can be set/reset by any user of the system  10 . The criteria can be selected practically, e.g., how much fluid needs to be in the system  10  to allow the printhead body  16  to effectively print, or how much fluid is initially filled in the containers  14   a ,  14   b . For example, when one of the fluid containers is fully filled, and the other one is partially filled, the criteria (e.g., the predetermined minimum level) have to be reasonably high because not all fluid in the full container can be circulated into the partially filled container. The predetermined minimum level can also be affected by the sensitivity and reliability of the sensors  36   a ,  36   b  for sensing the ink levels in the two containers  14   a ,  14   b . Examples of the predetermined minimum level can be 0.1 ml to about 0.2 ml. The predetermined minimum level can also be a percentage, e.g., 5%-20%, of the total initial fluid amount in each container or in both containers. 
         [0035]    The controller  26  can be implemented with circuitry, e.g., a programmable microcontroller, or other hardware, software, firmware, or combinations. The controller  26  can also communicate with a controller (not shown) controlling the fluid jetting of the printhead module  16 . In some implementations, the controller  26  can control both the pressure control device  28  and the fluid jetting. The controllers can be powered by one or more batteries (not shown) in the system  10  and can coordinate to control the fluid jetting and the fluid flow for fluid recirculation, e.g., simultaneously. Fluid recirculation in a printhead is also discussed in U.S. Pat. No. 7,413,300, U.S. Pat. No. 5,771,052, U.S. Pat. No. 6,357,867, U.S. Pat. No. 4,891,654, U.S. Pat. No. 7,128,406, and U.S. patent application Ser. No. 12/992,587, the entire contents of which are incorporated herein by reference. 
         [0036]    The system  10  can be implemented as an assembly  70  shown in  FIGS. 3A-3D . The controller  26  and the pressure control device  28  can be separate from the assembly  70  and be attached to the openings  72   a ,  72   b . The assembly  70  includes a fluid supply assembly  74  attached to a printhead housing  76 . A printhead body  78  is connected to the printhead housing  76 . The fluid supply assembly  74  includes two fluid containers  80   a ,  80   b  in two separate chambers  74   a ,  74   b  to supply a jetting fluid to the printhead body  78 . The fluid supply assembly  74  can be similar to the cartridge  12  of  FIG. 1 , the fluid containers  80   a ,  80   b , and the chambers  74   a ,  74   b  can have similar features to those of the fluid containers  14   a ,  14   b , and the chambers  22   a ,  22   b . The printhead body  78  can have features, e.g., flow path and nozzles, like the flow path  24   c  and the nozzles  18  of  FIG. 1 . Each chamber  74   a ,  74   b  includes an opening  72   a ,  72   b  to be connected to a pressure control device (such as the pressure control device  28  of  FIG. 1 ). The fluid contained in the containers  74   a ,  74   b  is recirculated between the containers and supplied to the printhead body  78  in a manner, e.g., through flow paths  80   a ,  80   b , similar to that described in  FIG. 1 . 
         [0037]    In particular,  FIGS. 3B and 3D  are cross-sectional perspective views of the assembly  70  depicted in  FIG. 3A  taken along line  3 B- 3 B.  FIG. 3C  is a cross-sectional perspective view of the assembly  70  taken along line  3 C- 3 C. The fluid supply assembly  74  includes the self-contained fluid containers  80   a ,  80   b , at least one of which containing a small volume of a fluid, such as ink. Like the containers  14   a ,  14   b , the fluid containers  80   a ,  80   b  are flexible containers, similar to bags, and shall be referred to as fluid bags, although other forms of self-contained fluid containers can be used. The fluid bags  80   a ,  80   b  can be filled with the fluid before or after the fluid supply assembly  74  is attached to the printhead housing  76 . In some implementations, the total amount of fluid filled in the fluid bags  80   a ,  80   b  does not exceed the capacity of one fluid bag  80   a  or  80   b . For example, the fluid bag  80   a  can be fully filled with the fluid while the fluid bag  80   b  is empty. In some implementations, up to about 75% of the total capacities of the two fluid bags  80   a ,  80   b  can be filled with the fluid. The unfilled capacity in either one or both of the fluid bags  80   a ,  80   b  provides room for the fluid to be recirculated between the two bags. 
         [0038]    The fluid bags  80   a ,  80   b  can be sealed after the fluid is filled into the bags. The fluid remains in the fluid bags until it is used. Seals  84   a ,  84   b , e.g., O-rings, form seals between the fluid bags  80   a ,  80   b  and the printhead housing  76 . Referring particularly to  FIGS. 3B and 3D , the embodiments depicted include a double snap-fit connection, whereby the fluid supply assembly  74  can be first attached to the printhead housing  76  in position A, the closed position ( FIG. 3B ). In the closed position, the fluid paths  82   a ,  82   b  are closed and the fluid bags  74   a ,  74   b  are not in fluid communication with the printhead body  78 . Prior to commencing a printing operation, the fluid supply assembly  74  is moved into position B, the open position ( FIG. 3D ). In the open position, the fluid bags  74   a ,  74   b  are in fluid communication with the printhead body  78  via the open fluid paths  82   a ,  82   b.    
         [0039]    To connect the fluid supply assembly  74  to the printhead housing  76  in the closed position A, a user aligns the male connectors  115  protruding from the fluid supply assembly  74  with the corresponding female connectors  117  formed in the printhead housing  76  and exerts enough force to engage the male connectors  115  with the female connectors  117  at position A ( FIG. 3B ), but not too much force so as to engage the female connectors  117  at position B ( FIG. 3D ). The user should receive enough tactile feedback when mating the fluid supply assembly  74  to the printhead housing  76  to determine when position A has been reached. 
         [0040]    To move the fluid supply assembly  74  into the open position B with respect to the printhead housing  76 , a user exerts additional force to engage the male connectors  115  with the female connectors  117  at position B. The male connectors  115  have enough flexibility to bend under pressure to disengage from the female connectors  117  at position A and snap into engagement at position B. The female connectors  117  can be configured to facilitate this movement, for example, by having angled faces as depicted that encourage the similarly angled male connectors  115  to slide out of engagement upon the exertion from a downward force. The above describes one implementation of a double snap-fit connection. Other configurations of a double snap-fit connection can be used, as well as other types of connections that allow for a closed and an open position. 
         [0041]    The fluid paths  82   a ,  82   b  are opened or closed based on the relative position of the fluid supply assembly  74  and the printhead housing  76 . The fluid paths  82   a ,  82   b  include upper portions  81   a ,  81   b  within the fluid supply assembly  74  and extending from respective fluid bags  80   a ,  80   b . The upper portions  81   a ,  81   b  ends at the bottom surfaces of outlet heads  118   a ,  118   b  of the fluid supply assembly  74 . The fluid paths  82   a ,  82   b  also include lower portions  124   a ,  124   b  formed in the printhead housing  76 . When the fluid supply assembly  74  is in the position A of  FIG. 3B , the upper portions  81   a ,  81   b  and the lower portions  124   a ,  124   b  do not connect. Instead, the seal  84   a ,  84   b  are in contact with the bottom surface of the outlet heads  118   a ,  118   b  and close the flow paths  82   a ,  82   b . A spring  114  in the outlet head  118  exerts a downward force compressing the seal  110 . The fluid in the fluid bags  80   a ,  80   b  cannot flow past the bottom surface of the outlet heads  118   a ,  118   b . When the fluid supply assembly  74  is in the position B of  FIG. 3D , the bottom of the outlet heads  118   a ,  118   b  contact the lower portions  124   a ,  124   b , which can compress the spring  114  within the outlet heads  118   a ,  118   b . The seals  84   a ,  84   b  are positioned past the distal end of the lower portions  124   a ,  124   b  of the fluid paths  82   a ,  82   b  and are not in contact with the bottom of the outlet head  118 . The flow paths  82   a ,  82   b  are no longer blocked by the seal  110 . The fluid can thereby flow from the fluid bags  80   a ,  80   b  to the printhead body  78 . Detailed designs of the fluid path to enable such flow control are discussed, e.g., in U.S. Pat. No. 7,631,962, the entire content of which is incorporated herein by reference. 
         [0042]    In some implementations, the fluid supply assembly  74  is permanently attached to the printhead housing  76 , i.e., cannot be detached without breaking a component of the assembly  74  or housing  76 . Once the fluid contained within the fluid bags  80   a ,  80   b  has been used, the assembly  70  can be discarded. The fluid bags  80   a ,  80   b  are filled via the outlet heads  118   a ,  118   b  before attaching the fluid supply assembly  74  to the printhead housing  76 . The assembly  70  thereby provides a self-contained disposable testing unit that uses only a small volume of test liquid. Because the assembly  70  is only used once, testing can occur without flushing clean printhead modules between tests. 
         [0043]    The system  10  of  FIG. 1  can also be implemented in assemblies different from those shown in  FIGS. 3A-3D . For example, the control of the flow path  82   a ,  82   b  between the fluid bags  80   a ,  80   b  and the printhead body  78  ( FIGS. 3A-3D ) can be differently performed using different structures and/or mechanisms. Some sample structures are described in U.S. Pat. No. 7,631,962. 
         [0044]    The printhead body  16  in the system  10  can be any type of printhead body. Referring to  FIG. 4 , a printhead body  100  includes a fluid ejection module, e.g., a quadrilateral plate-shaped printhead module, which can be a die  103  fabricated using semiconductor processing techniques. The fluid ejector further includes an integrated circuit interposer  104  over the die  103  and a lower housing  322  discussed further below. A housing  110  supports and surrounds the die  103 , integrated circuit interposer  104 , and lower housing  322  and can include a mounting frame  142  having pins  152  to connect the housing  110  to a print bar. A flex circuit  201  for receiving data from an external processor and providing drive signals to the die can be electrically connected to the die  103  and held in place by the housing  110 . Tubing  162  and  166  can be part of the fluid paths  24   a ,  24   b  of  FIG. 1  and are to be connected to the cartridge  12  of  FIG. 1  to supply a fluid to the die  103 . 
         [0045]    Referring to  FIG. 5 , the die  103  includes a substrate  122 , e.g., a silicon-on-insulator (SOI) wafer and the integrated circuit interposer  104 . Within the substrate  122 , fluid paths  242  are formed to recirculate the fluid along the M direction (single arrow) or along the N direction (double arrow) between an inlet  176  and an outlet  172  (e.g., of the tubing  162 ,  166  of  FIG. 4 ) while delivering the fluid to a pumping chamber  174  to be jetted from a nozzle  126 . In implementations, the inlet  176  can be connected to the fluid container  14   a  and the outlet  172  can be connected to the fluid container  14   b  of  FIG. 1 . In the example shown in the figure, the pumping chamber  174  is part of the flow path  242 . Each fluid path  242  includes an inlet channel  176  leading to the pumping chamber  174 , and further to both the nozzle  126  and the outlet channel  172 . The fluid path  242  further includes a pumping chamber inlet  276  and a pumping chamber outlet  272  that connect the pumping chamber  174  to the inlet channel  176  and outlet channel  172 , respectively. The fluid path can be formed by semiconductor processing techniques, e.g., etching. In some embodiments, deep reactive ion etching is used to form straight walled features that extend part way or all the way through a layer in the die  103 . In some embodiments, a silicon layer  286  adjacent to an insulating layer  284  is etched entirely through using the insulating layer as an etch stop. The pumping chamber  174  is sealed by a membrane  180  and can be actuated by an actuator formed on the surface of the membrane  180  opposite to the pumping chamber  174 . The nozzle  126  is formed in a nozzle layer  184 , which is on an opposite side of the pumping chamber  174  from the membrane  180 . The membrane  180  can be formed of a single layer of silicon. Alternatively, the membrane  180  can include one or more layers of oxide or can be formed of aluminum oxide (AlO 2 ), nitride, or zirconium oxide (ZrO 2 ). 
         [0046]    The actuators can be individually controllable actuators  401  supported by the substrate  122 . Multiple actuators  401  are considered to form an actuator layer, where the actuators can be electrically and physically separated from one another but part of a layer, nonetheless. The substrate  122  includes an optional layer of insulating material  282 , such as oxide, between the actuators and the membrane  180 . When activated, the actuator causes the fluid to be selectively ejected from the nozzles  126  of corresponding fluid paths  242 . Each flow path  242  with its associated actuator  401  provides an individually controllable MEMS fluid ejector unit. In some embodiments, activation of the actuator  401  causes the membrane  180  to deflect into the pumping chamber  174 , reducing the volume of the pumping chamber  174  and forcing fluid out of the nozzle  126 . The actuator  401  can be a piezoelectric actuator and can include a lower electrode  190 , a piezoelectric layer  192 , and an upper electrode  194 . Alternatively, the fluid ejection element can be a heating element. 
         [0047]    The integrated circuit interposer  104  includes transistors  202  (only one ejection device is shown in  FIG. 5  and thus only one transistor is shown) and is configured to provide signals for controlling ejection of fluid from the nozzles  126 . The substrate  122  and integrated circuit interposer  104  include multiple fluid flow paths  242  formed therein. 
         [0048]    Referring to  FIG. 6 , the fluid can flow from a fluid supply, e.g., one of the fluid containers  14   a ,  14   b  of  FIG. 1 , through the lower housing  322  of the printhead body  100  ( FIG. 4 ), through the integrated circuit interposer  104 , through the die  103 , and out of the nozzles  126  in the nozzle layer  184 . The lower housing  322  can be divided by a dividing wall  130  to provide an inlet chamber  132  and an outlet chamber  136 . The fluid from the fluid supply can flow into the fluid inlet chamber  132 , through fluid inlets  101  in the floor of the lower housing  322 , through fluid inlet passages  476  of the lower housing  322 , through the fluid paths  242  of the die  103 , through fluid outlet passages  472  of the lower housing  322 , out through the outlet  102 , into the outlet chamber  136 , and to the fluid return, e.g., the other one of the fluid containers  14   a ,  14   b  of  FIG. 1 . During fluid recirculation, the flow direction can also be opposite to what is described above. A portion of the fluid passing through the die  103  can be ejected from the nozzles  126 . 
         [0049]    Each fluid inlet  101  and fluid inlet passage  476  is fluidically connected in common to the parallel inlet channels  176  of a number of MEMS fluid ejector units, such as one, two or more rows of units. Similarly, each fluid outlet  102  and each fluid outlet passage  472  is fluidically connected in common to the parallel outlet channels  172  of a number of MEMS fluid ejector units, such as one, two or more rows of units. Each fluid inlet chamber  132  is common to multiple fluid inlets  101 . And each fluid outlet chamber  136  is common to multiple outlets  102 . The terms “inlet” and “outlet” do not indicate the flow directions. In other words, the fluid can be provided to the pumping chambers in the die  103  from the inlets  101  or from the outlets  102 , depending on the flow direction between the two fluid supplies. Printhead modules are discussed in U.S. patent application Ser. No. 12/833,828, the entire content of which is incorporated herein by reference. 
         [0050]    In other implementations, each fluid container  14   a ,  14   b  can include a fluid refill port so that the system  10  can be reused. For example, when the fluid in the containers is substantially used up, the same fluid can be refilled into the containers through the refill port. In some implementations, the used containers can be cleaned and a different fluid can be filled into the containers for test printing. The fluid container  14   a ,  14   b  can be the same as the chambers  22   a ,  22   b . In other words, the fluid can be directly stored in the chambers  22   a ,  22   b  without the containers  14   a ,  14   b . The pressure of the fluid in different chambers  22   a ,  22   b  can be similarly controlled using the pressure source  28  and the controller  26 , as explained previously. The flow paths  24   a ,  24   b ,  24   c  each may correspond to multiple flow paths in implementations. 
         [0051]    In other implementations, the fluid containers  14   a ,  14   b  do not include any sensing devices to determine the fluid levels in the containers. The system  10  can be programmed to stop printing when a full bag of fluid is emptied by recirculation and jetting. No fluid flows back from a second bag back to the emptied bag. Such a design can reduce the cost of the system  10 . Generally, in this embodiment, one of the fluid containers, e.g., container  14   a , is full and the other container, e.g., container  14   b , is empty before jetting. To fully use the fluid contained in the fluid container  14   a , the print head body  16  can be programmed to jet until no fluid is left in the fluid container  14   a.    
         [0052]    The fluid can include ink of various colors and properties. A food grade printing fluid can also be used. In some implementations, the fluid can also include non-image forming fluids. For example, three-dimensional model pastes can be selectively deposited to build models. Biological samples can be deposited on an analysis array. Circuitry forming materials can also be used. 
         [0053]    All publications, patent applications, patents, and other references mentioned herein are incorporated by reference herein in their entirety. Other embodiments are within the scope of the following claims.