Abstract:
In general, in an aspect, an apparatus includes a body having a hollow ink refill chamber, a plate on a side of the body, the plate having a series of posts separating a series of hollow channels adjacent to the hollow ink refill chamber in the body.

Description:
This patent application claims the benefit of the priority date of U.S. Provisional Patent Application No. 61/606,709, filed on Mar. 5, 2012, and U.S. Provisional Patent Application No. 61/606,880 filed on Mar. 5, 2012, pursuant to 35 U.S.C. 119. These provisional applications are herein incorporated by reference in their entirety. This application also incorporates U.S. application Ser. No. 13/786,360, filed on the same day as this patent application, in its entirety. 
    
    
     BACKGROUND 
     This description relates to printhead stiffening. 
     SUMMARY 
     In general, in an aspect, an apparatus includes a body having a hollow ink refill chamber, a structure on a side of the body, the structure having a series of posts separating a series of hollow channels adjacent to the hollow ink refill chamber in the body. The series of posts support the body against compressive forces applied across the hollow ink refill chamber. 
     Implementations may include one or more of the following features. The plate is attached to the body to stiffen the body. The plate is attached to the body by mechanical bonding. The apparatus further includes a compliant element on an opposite side of the plate from the body and not in contact with the series of posts. The body includes carbon, the plate includes stainless steel and the compliant membrane includes polyimide. The apparatus further includes a cavity plate between the plate and the compliant element. The cavity plate includes a series of pumping chambers separated by lands. The plate is adjacent to the body. A width of each post of the series of posts in the plate is within ±10% of a width of a corresponding one of the lands in the cavity plate. A thickness of each post of the series of posts corresponds to a thickness of the plate. The apparatus further includes a second plate adjacent to the body, the second plate having a second series of posts separating a second series of hollow channels adjacent to the hollow ink refill chamber in the body. The apparatus further includes a second compliant element on an opposite side of the second plate from the body and not in contact with the second series of posts. The apparatus further includes a second cavity plate having a second series of pumping chambers each separated by lands, the second cavity plate being between the second plate and the second compliant element. 
     In general, in an aspect, an apparatus includes an assembly having a body that includes a hollow ink refill chamber and a plate on a side of the body. The plate has a series of posts separating a series of hollow channels adjacent to the hollow ink refill chamber in the body. The apparatus includes a compliant element on an opposite side of the plate from the body and not in contact with the series of posts. 
     Implementations may include one or more of the following features. The plate is attached to the body by mechanical bonding. The body includes carbon, the plate includes stainless steel and the compliant membrane includes polyimide. The assembly is a jetting assembly, the jetting assembly further includes a cavity plate between the plate and the compliant element. The cavity plate includes a series of pumping chambers separated by lands, and piezoelectric elements in contact with the compliant membrane. The apparatus further includes a collar, a descender plate, and a nozzle plate. The jetting assembly is held within the collar and is fluidically connected to the descender plate and the nozzle plate. The apparatus further includes a housing and flexible circuits connect the jetting assembly to an exterior of the housing. The jetting assembly is enclosed by the housing. 
     In general, in an aspect, mechanical support is provided to a body having a hollow ink refill chamber in a direction orthogonal to a length of the hollow ink refill chamber; and a force is applied in the direction to secure the body to an assembly positioned along the direction and under the body. 
     Implementations may include one or more of the following features. The assembly positioned along the direction and under the body is detached from the body and is thereafter attached an assembly under the body. The mechanical support is provided through a series of posts separating a series of hollow channels adjacent to the hollow ink refill chamber in the body. The body includes carbon and the series of posts includes stainless steel. The body and the assembly are held together under pressure. The body and the assembly are not glued together. Aligned ink flow paths are formed between orifices in the body and descender tubes in the assembly when the force is applied in the direction to secure the body to the assembly. 
     In general, in an aspect, a body having a hollow ink refill chamber is provided, the body is contacted with a plate on a side of the body, the plate having a series of posts separating a series of hollow channels adjacent to the hollow ink refill chamber in the body. 
     Implementations may include one or more of the following features. A compliant element is provided on an opposite side of the plate from the body, and during use of the compliant element, the compliant element does not contact the series of posts. The body is contacted with a second plate on a second side of the body, the second plate having a second series of posts separating a second series of hollow channels adjacent to the hollow ink refill chamber in the body. A force is applied in a direction along a height of the series of the posts; and the body is attached to an assembly positioned along the direction and under the body. The body is detached from the assembly. The assembly, which includes a nozzle plate, is cleaned. 
     These and other features and aspects, and combinations of them, can be expressed as systems, components, apparatus, methods, means or steps for performing functions, methods of doing business, and in other ways. 
     Other features, aspects, implementations, and advantages will be apparent from the description and the claims. 
    
    
     
       DESCRIPTION 
         FIGS. 1A-1C  are perspective, end, and magnified views of a nozzle plate assembly. 
         FIG. 1D  is a cross-sectional view of a printhead assembly. 
         FIG. 1E  is a perspective view of printhead assemblies on a print bar. 
         FIG. 1F  is a magnified view of a portion of  FIG. 1C . 
         FIGS. 2A-2B  are perspective and cross-sectional views of an inkjet array module. 
         FIG. 2C  is a perspective magnified view of an inkjet array module. 
         FIGS. 3A-3B  are top and front views of a carbon body. 
         FIGS. 4A-4B  are top views of a stiffener plate. 
         FIGS. 4C and 4D  is a perspective views of overlapped stiffener plate and cavity plate. 
         FIGS. 5A-5B  are top views of a cavity plate. 
         FIG. 5C  is a cross-sectional view of the cavity plate. 
         FIGS. 6A and 6B  are schematic perspective views of a nozzle plate. 
         FIGS. 7A-7C  show isometric views of a printhead assembly. 
         FIGS. 7D-7H  are views of a printhead assembly. 
         FIG. 8  is a side view of a carbon body. 
     
    
    
     As shown in  FIGS. 1A ,  1 B,  1 C,  1 D, and  1 F, a nozzle plate assembly (or collar assembly)  10  includes a collar  14 , an integrated recirculation manifold  15  separate from the collar  14 , a stainless steel descender plate  17 , a stainless steel nozzle recirculation plate  20 , and an electroformed nickel nozzle plate  21 . The collar, the recirculation manifold, the descender plate, the recirculation plate, and the nozzle plate all have the same peripheral size and shape. 
     A bottom surface  1012  of the collar  14  is joined using adhesives  1014  to an upper surface  1510  of the integrated recirculation manifold  15 . The integrated recirculation manifold  15  is affixed using adhesives, such as epoxies, to a laminated piece  23  that includes the descender plate  17  and the nozzle recirculation plate  20 . The lamination is done by gluing the descender plate  17  and the nozzle recirculation plate  20  together. The integrated recirculation manifold  15  integrates the flow paths of two recirculation systems. Details of the recirculation systems are described in [0297001], which is incorporated by reference in its entirety. A bottom surface  1018  of the recirculation plate  20  is then joined adhesively to the nozzle plate  21 . 
     The collar and the integrated recirculation manifold  15  may be made of carbon, while the nozzle plate  21  may be an electroformed plate made of nickel. A membrane  1641  (also termed a “rock trap”) has small holes  1643  at locations where the membrane  1641  covers corresponding descenders  194  in the manifold  15  (shown in  FIG. 1C ). Diameters of the small holes in the membrane  1641  are smaller than the diameters of the nozzles in order to prevent debris and other impurities from clogging the nozzles of the nozzle plate assembly  10 . 
     At opposite ends  16  and  17 , the collar  14  includes corresponding protrusions  140  and  141 . Protrusion  140  has two through-holes  142  and  143  through which two screws  130  and  131  can extend, while protrusion  141  has a single through-hole  144  (not shown) through which a screw  133  (not shown) can extend. The screws  130 ,  131 ,  132 , and  133  allow the nozzle plate assembly  10  to be mounted with other printhead components, on a print bar  1016  (shown in  FIG. 1E ), or other supports 
     As shown in  FIG. 1B , the collar  14  includes slots  161  and  162  which are separated by a wall  163  that extends along the length of the collar  14 . Two inkjet array modules  6  (one of which is shown, in an exploded perspective view, in  FIG. 2A ) can be mounted in each of the long rectangular slots  161  and  162  in the collar  14  such that a bottom edge  1640  of a carbon body  160  of the inkjet array module  6  contacts the upper surface  1510  of the integrated recirculation manifold  15  (see  FIG. 1C ). 
       FIG. 1C , which shows a partial cross-section view of a carbon body  160  of an inkjet module  6  mounted within the slot  161  of the collar  14 , is a magnified view of the area marked by a dotted rectangle in  FIG. 1B . A descender  192  is defined in the carbon body  160  for each nozzle opening  250  of the inkjet array module. Each descender  192  includes a  90  degree bend  193  joining an orifice  1644  defined on the lower portion of a face  162  of the carbon body  160  to an orifice  1642  defined on the bottom edge  1640  of the carbon body  160 .  FIG. 1F  shows a magnified view of  FIG. 1C . The integrated recirculation manifold  15  has a recirculation return manifold  19  defined on its lower surface. Details of the recirculation return manifolds  19  are provided in [ 0297001 ], which is incorporated by reference in its entirety. 
     Detailed views of the carbon body  160  are provided in  FIGS. 3A and 3B . There are two rows each having  128  orifices  1642  on the bottom surface  1640  of the carbon body because the face  162  and a face  163  opposite (into the plane of the drawing in  FIG. 3A ) the face  162  each has one row of the orifices  1644 . A spacing  164  between the orifices  1644  is the same as a spacing  165  between the orifices  1642 . The two rows of orifices are offset from one another along the length of the carbon body by a distance that is one half of the spacing between the orifices. In addition, the spacings  164  and  165  are also the same as the spacing between nozzle openings  250  (shown in  FIG. 7G ) in the nozzle plate  21 . The descender  192  is shown in  FIG. 1C  to align with the descender  194  defined in the integrated recirculation manifold  15 . Thus, an ink flow path is defined from the orifice  1641  through the 90° bend  193  through the hole in the rock trap and down the descender  194  to a descender  228  in the descender plate  17 . 
       FIG. 2A  shows the inkjet array module  6  having the carbon body  160 , and stiffener plates  211  and  212 , cavity plates  213 ,  214 , compliant membranes  1740 ,  1741 , and piezoelectric elements  1750  and  1751  assembled into stacks located next to opposite sides  1761 ,  1762  of the carbon body  160 . Four inkjet array modules  6  (i.e.,  6 A- 6 D) can be fitted within the slots  161  and  162  of the collar  14  in the nozzle plate array assembly to form a printhead assembly  100 . 
     A cross-sectional end view of the printhead assembly  100  is shown in  FIG. 1D . A vertical tube  184  in the center delivers ink to all of the inkjet array modules  6 A- 6 D. Integrated circuits  180  are mounted on each flex circuit  166 .  7 A- 7 D are metallic clamps that run the length of the array (i.e., into and out of the plane of the drawing in  FIG. 1D ) with screws  8 A- 8 D at each end of the metallic clamps  7 A- 7 D, respectively. Flexible conductors  1801  are part of the flex circuits and are connected to connectors  1805  to enable connection to the outside world. 
     It is useful for the nozzle plate assembly  10  (which is a relatively less valuable component) to be easily detachable from the printhead assembly  100  in order to perform routine maintenance (e.g., cleaning or replacement) of the nozzle plate assembly  10  that can prolong the operational lifetime of the printhead assembly  100  (which is relatively more expensive). In order to enable easy detachment of the nozzle plate assembly  10  from the printhead assembly  100 , the nozzle plate assembly  10  is not permanently bonded to the printhead assembly  100 . Instead, the nozzle plate assembly  10  is mechanically clamped to the printhead assembly  100 . A substantial clamping force  200  (shown in  FIGS. 1C and 3A ) in a direction perpendicular to the surface  1510  of the integrated recirculation manifold  15  is required to achieve a good mechanical seal between the nozzle plate assembly  10  and the inkjet modules  6 A- 6 D. However, such a clamping force cannot be evenly transmitted at all locations along the carbon body  160 , through the carbon body  160  of the inkjet modules  6 A- 6 D to the nozzle plate assembly  10 . This is due to a decrease in mechanical stiffness of the carbon body  160  along part  2101  of its length  210  (shown in  FIG. 3A ) caused by the presence of a hollow ink refill chamber  191  defined in the middle of the carbon body  160 . The hollow ink refill chamber would allow the carbon body to distort in the presence of a uniform force applied along the length of the top of the carbon body  160 , making it difficult to transmit the applied force uniformly along all positions at the bottom of the carbon body  160 . 
     To improve the evenness of the transmission of forces  169  from a top portion  161  of the carbon body  160  to forces  1691  at the bottom of the carbon body  160  towards the nozzle plate assembly  10 , two stainless steel stiffener plates  211  and  212  that are attached to and sandwich the carbon body  160  between them have a uniform series of stainless steel posts  330  (shown in  FIGS. 4A and 4B ) fabricated in a long hollow channel  320  adjacent to the ink refill chamber  191 . The posts provide stiffness on both sides of the ink refill chamber to stiffen the carbon body  160 , reduce the deformation of the carbon body, and enable it to transmit the clamping forces evenly from the top to bottom. In other words, the posts provide mechanical support to the carbon body  160  having a hollow ink refill chamber  191  in a direction orthogonal to a length of the hollow ink refill chamber; such that the clamping force  200  secures the carbon body  160  to the nozzle plate assembly  10  positioned along the direction and under the carbon body  160 . 
     The series of posts  330  define a corresponding series of hollow channels  310  in each of the stiffener plates  211 , adjacent to the ink refill chamber  191  of the carbon body  160 . These posts  330  and hollow channels  310  are also aligned between respective inkjet pumping chamber inlets  415  in the cavity plate  213 . In  FIG. 4C , the stiffener plate  211  lies above and overlaps the cavity plate  213 . The bottom half of the stiffener plate  211  is removed to show the underlying features on the cavity plate  213 . The posts  330  in the stiffener plate  211  line up with lands  426 , which separate two pumping chambers  220 , in the cavity plate  213 . The hollow channels  310  in the stiffener plate  211  are also lined up with the pumping chambers  220  in the cavity plate  213  to ensure that ink flows from the ink refill chamber  191  through the hollow channels  310  and into the pumping chambers  220 . When the carbon body  160  and the stiffener plates  211  and  212  are mechanically bonded together using an epoxy, the series of posts  330  in the stiffener plates  211  provide the needed mechanical stiffness in the direction marked with an arrow  2110 . 
     The distance between the centers of hollow channels  310  in the stiffener plates  211  and  212  is equivalent to the width of a gap  315 , which is also equal to a spacing between nozzle openings  250  in the nozzle plate  21 . The spacing between nozzle openings  250  in the nozzle plate  21  is the same as the spacing  341  between openings  340  in the stiffener plates. The dimensions of the hollow channels  310  between the posts  330  help to maintain a good volume of flow from the ink refill chamber  191  into each of the pumping chambers of  220  in the cavity plate  213  and  214  while the dimensions of the posts provide mechanical stiffness in the direction marked with arrow  2110 . The flow of ink leaves the ink refill chamber  191  and enters the stiffener plate through the hollow channels  310  between a pair of posts  330 . The dimensions of the posts also ensure that fluid resistance experienced by ink flowing out from the ink refill chamber through the hollow channels  310  is not too large such that the flow of ink from the ink refill chamber into the cavity plate is impeded. 
     The stiffener plate  211  can have a thickness  2111  (shown in  FIG. 2B ) of about 50 microns to 150 microns (for example, 127 microns). As shown in  FIG. 4B , a height  331  of the posts  330  can be, for example, less than about 4 mm, 3 mm, 2 mm, and/or greater than about 500 microns, 1 mm. A width  332  of the posts  330  can be for example, less than about 250 microns, 200 microns, 150 microns, 130 microns, and/or greater than about 100 microns, or 120 microns. The gap  315 , between the centers of two adjacent hollow channels  310  can be less than about 700 microns, 600 microns, 508 microns, and/or greater than about 350 microns, 450 microns, or 500 microns. 
       FIG. 5B  shows a magnified view of the pumping chambers  220  defined in the cavity plate  213 . The cavity plate  213  can have a thickness  2130  (shown in  FIG. 5C ) of about 50 to 150 microns (for example, 127 microns). A width  423  of the hollow pumping chambers  220  can be, for example, less than about 500 microns, 400 microns, 388 microns, and/or greater than about 250 microns, 300 microns, or 350 microns. A spacing  422  between the centers of two hollow pumping chambers  220  can be, for example, less than about 700 microns, 600 microns, 508 microns, and/or greater than about 400 microns, or 500 microns. 
     Ribs  424  and  425  each has about half the thickness of the cavity plate  213  and provides structural support, allowing the cavity plate  213  to be handled during assembly without damage to lands  426 , which are areas between pumping chambers  220 . The lands  426 , being narrow and thin, are fragile and vulnerable to bending, folding, or breaking before covers are mounted on the cavity plate  213 , which can then provide additional support. Covers are attached to each surface of the cavity plate  213  to form pumping chambers. The covers include compliant membranes  1740  and  1741  and the stiffener plates  211  and  212 . Due to the narrowness of lands  426 , the jetting assembly that includes the cavity plate  213  can therefore have a higher nozzle pitch and produce high resolution images. The dimensions of lands  426  can be, for example, less about 300 microns, 200 microns, 150 microns, 120 microns, and/or greater than 75 microns, or 100 microns. Further description is provided in U.S. Pat. No. 8,091,988, the entire content of which is incorporated herein by reference. 
     The posts  330  in the stiffener plate  211  are dimensioned to align with an (imaginary) extension  435  ( FIG. 4D ) in the ink refill passage  410  of the cavity plate  213 , the extension  435  being directly above the lands  426  between different pumping chambers  220 . 
     Two compliant membranes  1740  and  1741  that are parallel to the stiffener plates  211  and  212  are spaced by a distance not smaller than 118 micron (greater than 120 micron, greater than 150 micron, greater than 250 microns, and/or smaller than 400 microns, smaller than 300 microns) from the stiffener plates  211  and  212 , respectively, to handle the acoustic waves propagated in the ink properly by not contacting the posts  330  when the membranes are being deflected during operation. The compliant membranes help to reduce cross-talk between the pumping chambers  220 . 
       FIG. 2C  shows a perspective magnified view of the inkjet array module  6 . The posts  330  are stacked on the opposite faces  1761  and  1762  of the carbon body  160  to stiffen the carbon body along the hollow ink refill chamber  191 . The cavity plates  213  and  214  contain ribs  425  for structural support, as outlined above. Ink from the hollow ink refill chamber  191  flows between the posts  330  and enters the pumping chamber  220  through the pumping chamber inlet  415 . The compliant membranes  1740  and  1741  are stacked between the cavity plates and the piezoelectric elements  1750  and  1751 . 
     In one specific example, the following dimensions can be used for the parts discussed in the previous paragraph: the compliant membranes are each 25 micron thick, the stiffener plates and the cavity plates are each 127 micron thick, the posts  330  are 130 microns wide, 127 microns thick and 2 mm tall. 
     Crosstalk is unwanted fluidic interaction between ink flowing in and jetted from separate jets. A jet generally refers to the pumping chamber, the piezoelectric element, the fluid path to a nozzle, and the nozzle from which ink is ejected. Typically, it is desirable that there be no crosstalk between jets. When crosstalk is present, the firing of one or more jets may influence the performance of other jets by altering ink ejection velocities or the drop volumes jetted, for example. This can occur when unwanted energy is transmitted between jets. During operation of the inkjet module  6 , the piezoelectric elements  1750  and  1751  (e.g., PZT) expand and flex the compliant membranes  1740  and  1741 , which are attached to the piezoelectric elements. This in turns causes the compliant membranes to pull away from the cavity plates  213  and  214 , creating low pressure regions in the pumping chambers  220  due to the increase in volume of the pumping chambers, which causes ink  170  in the refill chamber  191  to be drawn into them, across the hollow channels  310  in the stiffener plates  211  and  212 , and into the ink fill passages  410  in the cavity plates  212  and  213 . 
     The increase in volume in the pumping chamber also causes the ink already present in the pumping chamber to launch a negative pressure wave which contains acoustic energy. This negative pressure starts in the pumping chamber and travels toward both ends of the pumping chamber  220  (towards an end  421  of the pumping chamber  220  and towards an ink fill passage  410  above the pumping chamber inlet  415 ). When the negative wave reaches the end of the pumping chamber and encounters the large area of the ink fill passage  410  (which can be approximated to a free surface), the negative wave is reflected back into the pumping chamber  220  as a positive wave, travelling towards the end  421  of the pumping chamber  220 . The effect of providing an ink fill passage with the equivalent of a free surface  441  (shown in  FIG. 5C ) is that more energy is reflected back into the pumping chamber at the pumping chamber inlet  415 , and less energy enters the ink fill passage  410  where the energy could travel down other pumping chambers and affect the performance of neighboring jets. Moreover, reflecting acoustic energy back into the pumping chamber  220  increases the pressure at the end  421  of the pumping chamber for a given applied voltage. 
     The compliance of the membranes  1740  and  1741  over the ink fill passage  410  also reduces crosstalk between jets by reducing the amplitude of pressure waves that enter the ink fill passage from firing jets. The compliant membrane  1740  and  1741  can for example, be a film of polyimide having a thickness of less than about 100 microns, 50 microns, 25 microns, and/or a thickness greater than about 10 microns, or 20 microns. In general, the more compliant (or less constrained) the membrane is, the better it reflects the negative pressure wave and attenuates any waste acoustic energy that may otherwise enter neighboring pumping chambers. The placement of the posts  330  in the stiffener plate  211  ensures that the compliant membrane can deflect sufficiently towards the cavity plate  223  and not be obstructed by the presence of posts  330 . In other words, during the operation of the printhead assembly  100 , the compliant membranes  1740  and  1741  do not contact the stiffener posts  330 . 
     After the piezoelectric element is held in the expanded state for a period of time, the piezoelectric element  1750  is deactuated so that it returns to its original position. The returning of the piezoelectric element to its original position creates a positive wave in the ink in the pumping chamber. The timing of the deactuation of the piezoelectric element is selected so that its positive wave and the reflected positive wave are additive when they reach the end  421  of the pumping chamber. This is discussed in U.S. Pat. No. 4,891,654, the entire content of which is incorporated herein by reference. 
     From the end  421  of the pumping chamber  220 , the ink leaves the pumping chamber  220  and is then pushed towards openings  340  defined in the stiffener plate  211  before entering the orifices  1641  in the carbon body  160 . The ink then negotiates the 90 degree bend of the descender  192  in the carbon body  160  and emerges from the carbon body  160  along the edge  1640  through orifices  1642  before continuing on the fluid path that leads to nozzle openings  250  in the nozzle plate  21 . Ink is the ejected from the printhead assembly  100  and gets deposited on a printing medium. 
     As shown in  FIG. 6A , a nozzle plate  600  has nozzle openings  601 . The nozzle plate  600  has an exposed surface  603  that faces a printing medium  604 ; each of the nozzle openings is at the exposed surface  603 , and ink droplets from each jet are ejected from the nozzle opening toward a substrate during printing. 
     As shown in  FIG. 6B , the nozzle opening for each jet lies at the end of a nozzle tube  607  in a nozzle plate  600 . At times when ink droplets are not being ejected from the nozzle opening, ink is held in the nozzle tube to prepare the nozzle for subsequent jetting of droplets. The ink in the nozzle tube then forms a meniscus  605  of ink  170  to define a liquid-air interface  606  within the nozzle tube  607  The meniscus  605  may have an outer rim  691  at the nozzle opening and a concave surface  693  caused by a negative pressure applied to the ink  170  upstream of the nozzle to keep it from leaking from the nozzle opening. (We often use the term nozzle interchangeably with the term nozzle tube.) The meniscus  605  extends over the diameter  608  of the nozzle opening  601  and is positioned within the nozzle tube  607  of the nozzle opening  601 , away from the exposed surface  603 . The ink, which can include pigments and solvents, may dry or undergo other changes in its characteristics at the nozzle opening  601  and within the nozzle tube, for example, when volatile solvents  609  evaporate from the ink through the liquid-air interface  606  of the meniscus  605 . Ink that is held in and flows through various parts of the inkjet array module is also subject to settling of pigments and to other changes in characteristics that can adversely impact the quality of the printing and the maintenance of the inkjet array module. To reduce these effects, ink can be recirculated continuously while the inkjet array module is in operation or in an idle state. For this purpose, recirculation can be carried out, for example, at a refill chamber  191  ( FIG. 7E ) of an inkjet array module  16 A ( FIG. 7E ), upstream of individual pumping chambers  220 . Several inkjet array modules can be installed in a printhead assembly  10 . 
     The refill chamber  191  houses a larger volume of ink  170  compared to the ink contained in individual pumping chambers  220 . Recirculating ink at the refill chamber  191  helps to prevent heavier pigments of inks  170  from settling there. Recirculating at the refill chamber  191  helps to ensure that ink having specific characteristics (for example, viscosity, temperature, amount of dissolved gases) is delivered to individual pumping chambers  220  for jetting. In addition, a deaerator can be arranged upstream of the refill chamber to remove gases from the ink supplied to the refill chamber  191 . In that way, inks having very low dissolved gas content can be supplied to pumping chambers  220  for jetting. Recirculating ink  170  at the refill chamber  191  also facilitates changing of inks because the refill chamber recirculation flow paths provide a fluid path for the ink  170  in the refill chamber  191  to be actively removed (using back pressure exerted from an external source  120 ) from the printhead assembly  10  in order for new inks to be introduced to the printhead assembly  10 . In the absence of the recirculation fluid paths, a particular ink would need to be flushed from the nozzles  249  before new ink can be introduced to the printhead assembly  10  (assuming that the printhead assembly  10  is not disassembled between changes of ink). Recirculation of ink also helps with priming and recovery. An empty printhead containing air can be primed by introducing a jetting fluid into the printhead such that a meniscus of the jetting fluid is formed at one or more nozzles of the printhead. Priming generally refers to the preparation of a meniscus at the nozzle. 
     In addition to recirculating ink at the refill chamber, recirculating ink  170  that is being held in and upstream of the nozzle  249  from which ink droplets are to be ejected helps to ensure that fresh ink, of the same characteristics (e.g., viscosity, temperature, and solvent content) as the ink that is in the refill chamber  191  is held in the nozzle  249 , for example, during the time when ink is not actually being jetted. Recirculation helps to ensure that, for example, the first droplet jetted from the nozzle opening  250  after a period of no jetting is of the same quality, size, and characteristics as other droplets that are jetted before and after the period of no jetting. This allows for better jetting performance. 
     For example, inks that contain volatile solvents may be dried out within the nozzle  249  when the meniscus  605  of the ink  170  at the ink-air interface  606  loses the volatile solvents  609  at the interface to the atmosphere, in the absence of recirculation. Some inks may absorb air through the ink-air interface  606  at the meniscus  605  when the ink is exposed to air. This absorption may cause bubble formation within the printhead assembly  10  that can render the printhead inoperable when these bubbles are trapped in ink passages in the printhead assembly  10 . 
     To recirculate ink that is held in the nozzle tube at times when the inkjet is not ejecting droplets from the nozzle opening can be done by providing a recirculation path that opens at one end into the nozzle tube and leads at its other end to a recirculation supply of ink. We describe such nozzle recirculation paths below. Note that, as shown in  FIG. 6B , the nozzle tube  607  includes not only the segment that lies within the nozzle plate but also a collinear segment within a nozzle recirculation plate  20 , and at least part of the nozzle recirculation path is provided in the nozzle recirculation plate, as described in more detail below. 
     Providing such recirculation paths from the nozzle tubes is not trivial due to space constraints in body in which the nozzles are formed. The inclusion of recirculation paths to closely spaced nozzles may also create cross talk between jets (explained in more detail below). Recirculation may also reduce efficiency of the jetting, because it draws some ink from the nozzle tube and reduces the ink pressure in the nozzle tube, which can reduce the amount of jetting fluid that is being ejected in a droplet from the nozzle opening onto the printing substrate. The recirculation flow also may perturb the meniscus pressure at the nozzle leading to a heightened sensitivity of the nozzle to the fluctuations in the recirculation pressure. 
     Ink flows at a nominal flow rate as it is ejected through each of the nozzle onto a substrate. Ink is held under a nominal negative pressure associated with a characteristic of a meniscus of the ink in the nozzle when ejection of ink from the nozzle is not occurring. Each flow path having a nozzle end at which it opens into one of the nozzles and another location spaced from the nozzle end that is to be subjected to a recirculation pressure lower than the nominal negative pressure so that ink is recirculated from the nozzle through the flow path at a recirculation flow rate. Each recirculation flow path has a fluidic resistance between the nozzle end and the other location such that a recirculation pressure at the nozzle end of the flow path that results from the recirculation pressure applied at the other location of the flow path is small enough so that any reduction in flow rate below the nominal flow rate when ink is being ejected is less than a threshold, or a change in the nominal negative pressure when ink is not being ejected is less than a threshold, or both. 
     In some inkjet heads, the ink  170  is split into two paths in a recirculation structure immediately upstream of the nozzle plate  21 . One of the paths conducts the ink to the nozzle plate  21 , from which ink is ejected. The other path provides a path for the ink to flow out of the printhead assembly  10  into an external ink reservoir  110 . 
     A recirculation flow rate for recirculation flow paths for nozzles of ink jets of an inkjet assembly is selected and a maximum external pressure to be applied to the recirculation flow paths is selected. A refill resistor having fluidic resistances to provide a fluid flow rate from the refill resistor that is similar to a sum of nozzle recirculation flow rates for the nozzles is designed. A portion of a fluid in a nozzle of an inkjet of an inkjet assembly flows from the nozzle through a recirculation path to a reservoir separate from the inkjet assembly. 
     In  FIG. 7A , an inkjet printhead assembly  10  has an ink inlet  11 , and an ink outlet  12 . The ink inlet  11  is connected to an external ink reservoir  110  through a tubing coupler  109  and piping  111  so that the ink reservoir  110  supplies ink  107  to the ink inlet  11  (in the direction indicated by arrow  103 ). The external ink reservoir  110  is also connected to the ink outlet  12  through a tubing coupler  105  and piping  112  and receives returned ink from the ink outlet  12  (in the direction indicated by arrow  101 ). The external ink reservoir  110  is connected to a vacuum source  120  through vacuum connections  121 . The vacuum source  120  can exert a vacuum pressure on the ink in the ink reservoir  110 . 
     The printhead assembly  10  includes a rigid housing  13  formed of two half-pieces  9  and  7 , which (when assembled) encapsulate components of the printhead assembly  10 . Examples of materials from which the two half-pieces of rigid housing  13  can be made include thermoplastics. The ink inlet  11  enters the housing  13  through a ring-shaped resilient support  156  that is captured in a round aperture  1001  formed on the upper wall of the housing  13  when the two half-pieces are mated. 
     Similarly, the ink outlet  12  leaves the housing  13  through a resilient ring support  155  that is captured in a round aperture  1004  formed in the upper wall of the housing  13  when the two half-pieces are mated. The bottom  1006  of the housing  13  has an inwardly projecting rim  1008  on both ends that mates with corresponding grooves  1010  on opposite ends of a collar  14 . The integrated recirculation manifold  15  is a separate piece from the collar, and integrates the flow paths of two recirculation systems. Details of the recirculation systems are described below. 
     The collar  14 , the integrated recirculation manifold  15 , the descender plate  17 , the nozzle recirculation plate  20  and the nozzle plate  21  jointly form a nozzle plate assembly  221 . 
     The housing  13  can be opened into two halves along a seam  150 . A multiple-contact electrical connector  157  at the top of the assembly can receive a mating connector of a signal cable to enable signals to be carried to and from actuation elements of the printhead assembly used to trigger jetting of ink from each inkjet, for example. Using the three mounting screws, the tubing couplings  105  and  109 , and the electrical connector  157 , the entire printhead assembly can be easily removed as a stand-alone assembly from the print bar  1016 , for maintenance, storage, or replacement. 
     As shown in  FIG. 7B , within the printhead assembly four inkjet array modules  16 A- 16 D are arranged in two pairs, each pair mounted in corresponding long rectangular slots  161  and  162  in the collar  14 . Each array module includes two flexible circuits  166  that are connected to circuitries mounted on a circuit board  158  supported within the housing  13 . A heater wire  195  is optionally included in some printhead assembly  10 . The heater wire  195  can be used to heat up the ink  107  that is supplied into each of the inkjet array modules  16 A- 16 D. 
     The ink inlet  11  is connected, as shown in  FIG. 7C , to the collar  14  at a throughhole  280  in the wall  163  by way of a piping  1100  and a coupler  1105 . The ink outlet  12  is connected to the collar  14  at a throughhole  122  in the wall  163  of the collar  14  through a coupler  1110  and a piping  1115 . A second return  1421  from the recirculation manifold is formed as a horizontal channel in the collar  14 . The four pairs of flexible circuits  166  are connected to electronic circuitries  171  arranged on the board  158 . 
       FIG. 7D  shows a cross-sectional end view of the printhead assembly  10 . Aluminum clamps  1184  span the length of each of the inkjet array modules  16 A- 16 D (into and out of the plane of the drawing). There is a screw  1185  at each end of the aluminum clamp  1184 , the screw having a screw head  1186  positioned above the clamp  1184 . Each of the array modules  16 A- 16 D includes a carbon body  190 , in which a refill chamber  191  is defined. All four refill chambers  191  for the array modules  16 A- 16 D are fluidically connected. The carbon body  190  is sandwiched between stiffener plates  210 ,  211  and cavity plates  212  and  213 . An enlarged view of the lower left portion of the printhead assembly (marked with a rectangle) is shown in  FIG. 7E . 
       FIG. 7E  shows two array modules  16 A and  16 B. The descender  192  extends through the integrated recirculation manifold  15  as a descender  194 . The integrated recirculation manifold has an upper surface  1510  and a lower surface  1515 . A total of eight recirculation return manifolds  19  are defined in the lower surface  1515 , of which five are shown in  FIG. 1E . An enlarged view of the lower middle portion of  FIG. 1E  is shown in  FIG. 1F . 
     The descender  194  defined in the integrated recirculation manifold  15  connects an end of descender  192  to a descender  220  defined in descender plate  17 . An enlarged view of the lower left portion of  FIG. 1F  is shown in  FIG. 1G . 
       FIG. 7G  shows a bottom up view (viewed from the nozzle plate  21 ) of a portion of the nozzle plate assembly  221 . The nozzle plate assembly includes the collar  14 , the integrated recirculation manifold  15 , the descender plate  17 , the nozzle recirculation plate  20  and the nozzle plate  21 . The nozzle plate  21  contains a number of nozzle openings  250 . The top portions of the figure shows the recirculation return manifold  19  defined in the lower surface  1515  of the integrated recirculation manifold  15 . Below the manifold  15  is the descender plate  17  in which a number of descenders  1220  and ascenders  1230  are defined. A void  240 , also known as a “glue sucker”, serves as an adhesive control feature by holding glue squeezed out between the recirculation manifold  15  and the descender plate  17  during assembly. The descenders  1220  are aligned with a port  22  in the nozzle recirculation plate  20 . The descender plate  17  is adhesively bonded to the nozzle recirculation plate  20  to form the laminate piece  23 . The port  22  in the nozzle recirculation plate  20  is connected via a V-shaped nozzle recirculation resistor or channel  24  to a port  23  which is aligned with the ascender  1230  in the descender plate  17  to the recirculation return manifold  19 . There are equal numbers of descenders  1220  and ascenders  1230  and the total number of descenders  1220  matches the total number of nozzle openings  250 . In other words, each nozzle opening  250  has its own dedicated nozzle recirculation resistor  24 . The nozzle recirculation resistor  24  is, for example, a fluidic channel. Elements  231  are cross sections of other V-shaped nozzle recirculation resistors  24  that belong to other nozzles  250  arranged into and out of the plane of the drawing in  FIG. 1G . The ink that is delivered to the recirculation return manifold  19  exits the printhead assembly  10  through the ink outlet  12 . 
       FIG. 7H  shows a similar view of the nozzle plate assembly  221 , but without the nozzle plate  21 . Each V-shaped nozzle recirculation resistor  24  is connected to a respective nozzle opening  250  via the port  22 , while the other end of the resistor  24  is connected to the port  23  which directs ink to the recirculation return manifold  19  through the ascender  230  in the descender plate  17 . 
     As shown in  FIGS. 1B and 1D , inkjet array modules  16 A-D are mounted within slots  161  and  162 . Each array module includes a carbon body  190  (shown in  FIG. 8 ) in which a refill chamber  191  is defined. A bottom edge  1640  of the carbon body  190  rests on the integrated recirculation manifold  15  when the array modules  16 A-D are assembled in the slots  161  and  162  of the collar  14 . The hashed portions of  FIG. 8  expose the subsurface features of the carbon body  190 . When the carbon body  190  of the inkjet array module is assembled within either slot  161  or  162  in the collar  14 , and contacts the top surface  1510  of the integrated recirculation manifold  15 , the opening of channel  1530  on the edge  1640  of the carbon body  190  lines up with the throughhole  44  of the integrated recirculation manifold  15 . In this way, the ink that leaves the top surface  1510  of the recirculation manifold  15  enters the channel  1530  in the carbon body  190  and is directed upwards into the ink refill chamber  191 . 
     Other implementations are also within the following claims.