Patent Publication Number: US-9833994-B2

Title: Inkjet head that circulates ink

Description:
RELATED APPLICATIONS 
     This non-provisional patent application is a continuation of U.S. patent application Ser. No. 15/007,105 filed on Jan. 26, 2016, which claimed priority to U.S. Pat. No. 9,272,514 filed on Apr. 24, 2014; both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The following disclosure relates to the field of printing, and in particular, to inkjet heads used in printing. 
     BACKGROUND 
     Inkjet printing is a type of printing that creates a digital image by propelling droplets of ink onto a medium, such as paper. The core of an inkjet printer includes one or more the print heads (referred to herein as inkjet heads) having a series of nozzles that are used to spray drops of ink. The structure of an inkjet head typically includes a housing, a series of plates, and a piezoelectric actuator. The housing has an opening for the piezoelectric actuator to pass through, and an inlet that connects to an ink supply (e.g., an ink cartridge). The inlet for the ink supply also connects to a groove in the housing that forms an ink supply channel for the inkjet head. 
     The plates of the inkjet head are attached to the housing and to one another to form a laminated structure. The laminated structure forms a plurality of ink channels that are each capable of dispersing ink. Each ink channel includes a nozzle, a chamber for ink, and a mechanism for ejecting the ink from the chamber and through the nozzle, which is typically a diaphragm. In order to form the ink channels, a common inkjet head includes a diaphragm plate, a restrictor plate, a chamber plate, and an orifice plate. The orifice plate includes a row of small holes that comprise the nozzles for the inkjet head. The chamber plate includes a row of openings that form chambers for the ink. The restrictor plate also includes a row of openings which form restrictors that fluidly connect the chambers to the ink supply and that control the flow of ink into the chambers. The diaphragm plate forms diaphragms over the chambers with a sheet of a semi-flexible material. The diaphragm plate also includes openings that allow ink to be drawn from the ink supply and into the chambers when the diaphragms vibrate. 
     The piezoelectric actuator includes a plurality of piezoelectric elements that attach to the diaphragm plate. Each piezoelectric element corresponds to one of the chambers formed in the chamber plate. When electrical signals are selectively applied to the piezoelectric elements, the elements expand and contract. This causes the diaphragms to vibrate over the chambers, which changes the volume of the chambers. The change in the volume of the chamber causes ink to be ejected from the chambers through the nozzles on the orifice plate. 
     One problem with inkjet heads is that the ink can dry in the nozzles or chambers when the head or individual nozzles are not in use. One or more of the ink channels can therefore become clogged within the head. 
     SUMMARY 
     Embodiments described herein provide for an inkjet head that circulates ink, or another material, through ink channels in the head. Circulation of ink through the ink channels provides advantages, such as automatically priming the ink channels with little waste, removing air bubbles near the nozzles, preventing heavy pigments from settling, and keeping ink from drying at the nozzles. To allow for circulation of ink, an additional restrictor plate is added to the head structure proximate to the nozzles of the inkjet head. The plates of the inkjet head also form a return manifold, where ink in the chambers of the head may flow through the additional restrictor plate and into the return manifold. With this configuration, ink may flow through the ink channels so that it is less likely to dry within the inkjet head and clog the nozzles. 
     One embodiment is an inkjet head comprising an orifice plate formed with a plurality of nozzles through which ink droplets are ejected. The inkjet head further includes first restrictor plate, and one or more chamber plates that form a plurality of chambers corresponding with the respective nozzles. The chamber plates also form a return manifold for circulating ink through the inkjet head. The head further includes a second restrictor plate, and a diaphragm plate that has a diaphragm for sealing the chambers. The first restrictor plate controls a flow of ink between the chambers and the return manifold. The second restrictor plate controls the flow of ink between a supply manifold and the chambers. 
     In another embodiment, the inkjet head further includes a plurality of piezoelectric elements attached to the diaphragm at positions opposite the chambers. 
     In another embodiment, the inkjet head further includes a housing that includes an opening for the piezoelectric elements to pass through to contact the diaphragm plate, and that includes a first groove on a surface facing the diaphragm plate that encompasses the opening for the piezoelectric elements to form the supply manifold. The housing may also include a second groove on the surface facing the diaphragm plate for the return manifold. 
     In another embodiment, the housing may include an inlet hole in the first groove that connects the supply manifold to a first reservoir, and an outlet hole in the second groove of the housing that connects the return manifold to a second reservoir. 
     In another embodiment, the pressure at the supply manifold (P_in) is positive, the pressure at the return manifold (P_out) is negative, and P_in+P_out is negative at the nozzles. 
     The above summary provides a basic understanding of some aspects of the specification. This summary is not an extensive overview of the specification. It is intended to neither identify key or critical elements of the specification nor delineate any scope particular embodiments of the specification, or any scope of the claims. Its sole purpose is to present some concepts of the specification in a simplified form as a prelude to the more detailed description that is presented later. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the present disclosure are now described, by way of example only, and with reference to the accompanying drawings. The same reference number represents the same element or the same type of element on all drawings. 
         FIG. 1  illustrates an exploded, perspective view of a conventional inkjet head. 
         FIG. 2  illustrates an exploded, perspective view of an inkjet head in an exemplary embodiment. 
         FIG. 3  illustrates a cross-sectional view of an ink channel within the inkjet head of  FIG. 2  in an exemplary embodiment. 
         FIG. 4  illustrates a cross-sectional view of ink circulating through the ink channel in an exemplary embodiment. 
         FIG. 5  illustrates an exploded, perspective view of an inkjet head in an exemplary embodiment. 
         FIG. 6  is a cross-sectional view of an ink channel in the inkjet head of  FIG. 5  in an exemplary embodiment. 
         FIG. 7  is a cross-sectional view of ink circulating through the ink channel in a reverse direction in an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The figures and the following description illustrate specific exemplary embodiments. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the embodiments and are included within the scope of the embodiments. Furthermore, any examples described herein are intended to aid in understanding the principles of the embodiments, and are to be construed as being without limitation to such specifically recited examples and conditions. As a result, the inventive concept(s) is not limited to the specific embodiments or examples described below, but by the claims and their equivalents. 
       FIG. 1  illustrates an exploded, perspective view of a conventional inkjet head  100 . Inkjet head  100  forms a plurality of ink channels that are each capable of dispersing ink. Each ink channel includes a nozzle, a chamber for ink, and a mechanism for ejecting the ink from the chamber and through the nozzle, which is typically a diaphragm. 
     In this example, inkjet head  100  includes a housing  102 , a series of plates  103 - 106 , and a piezoelectric actuator  108 . Housing  102  is a rigid member to which the plates  103 - 106  attach to form inkjet head  100 . Housing  102  includes an opening  110  for piezoelectric actuator  108  to pass through and interface with a diaphragm plate. Housing  102  further includes one or more grooves  112  on a surface facing plates  103 - 106  for supplying ink to the ink channels. Groove  112  includes one or more holes  113  that are in fluid communication with an ink reservoir. 
     The plates  103 - 106  of inkjet head  100  are fixed or bonded to one another to form a laminated plate structure, and the laminated plate structure is affixed to housing  102 . The laminated plate structure includes the following plates: an orifice plate  106 , a chamber plate  105 , a restrictor plate  104 , and a diaphragm plate  103 . Orifice plate  106  includes a plurality of nozzles  120  that are formed in one or more rows. Chamber plate  105  is formed with a plurality of chambers  121  that correspond with the nozzles  120  of orifice plate  106 . The chambers  121  are each able to hold ink that is to be ejected out its corresponding nozzle. Restrictor plate  104  is formed with a plurality of restrictors  122 . The restrictors  122  fluidly connect chambers  121  to the ink supply, and control the flow of ink into chambers  121 . Diaphragm plate  103  is formed with diaphragms  123  and filter sections  124 . Diaphragms  123  each comprise a sheet of a semi-flexible material that vibrates in response to actuation by piezoelectric actuator  108 . Filter sections  124  remove foreign matter from ink entering into the ink channels. 
     Piezoelectric actuator  108  includes a plurality of piezoelectric elements  130 ; one for each of the ink channels. The ends of piezoelectric elements  130  contact diaphragms  123  in diaphragm plate  103 . An external drive circuit (not shown) is able to selectively apply electrical signals to piezoelectric elements  130  which vibrate the diaphragm  123  for individual ink chambers. The vibration of diaphragms  123  changes the volume of the chambers  121 , which in turn changes the pressure in the chambers  121 . The change in pressure in a chamber  121  causes ink to be ejected from its corresponding nozzle  120 . Inkjet head  100  can therefore print desired patterns by selectively “activating” the ink channels to discharge ink out of their respective nozzles. 
     When inkjet head  100  is not in use for a period of time, or one or more of the ink channels is not in use during print operations for a period of time, the ink in the nozzles  120  and the chambers  121  can begin to dry. For example, ink that has a heavy pigment, magnetic ink, photopolymer materials used for three-dimensional (3D) printing, and the like can quickly begin to dry or harden in the inkjet head  100  when the ink channels are not used for printing. This can unfortunately clog inkjet head  100 , which may require cleaning before the head can be used again for printing. To avoid clogging of an inkjet head, the following embodiments describe an inkjet head that is able to circulate (or recirculate) ink or other printing liquids/fluids within the inkjet head. In order to circulate ink, a return manifold is formed in the inkjet head. The return manifold is fluidly connected to the chambers of the ink channels through an additional restrictor plate proximate to the nozzles. The additional restrictor plate controls a flow of ink from the chambers (near the nozzles) into the return manifold. With this configuration, ink may be circulated within the inkjet head from the supply manifold, through the chambers, and into the return manifold so that the ink is less likely to dry within the inkjet head and clog the nozzles. 
       FIG. 2  illustrates an exploded, perspective view of an inkjet head  200  in an exemplary embodiment. The inkjet heads as described herein, such as inkjet head  200 , may be used for two-dimensional (2D) printing or three-dimensional (3D) printing. Therefore, inkjet heads may be implemented in an apparatus for printing, such as an inkjet printer. In this embodiment, inkjet head  200  includes a plurality of ink channels that are each capable of dispersing ink. Each channel includes a nozzle, a chamber for ink, and a mechanism for ejecting the ink from the chamber and through the nozzle, which is typically a diaphragm. The term “ink” as defined herein comprises any material, fluid, or liquid that may be applied by an inkjet head to a medium. The term “ink” does not solely refer to liquids that contain pigments or dyes, but may also refer to liquids that contain plastic filaments, photopolymers, etc., which are used for 3D printing. 
     In this embodiment, inkjet head  200  includes a housing  202 , a series of plates  203 - 208 , and a piezoelectric actuator  209 . Housing  202  is a rigid member to which the plates  203 - 208  attach to form inkjet head  200 . Housing  202  includes an opening  210  for piezoelectric actuator  209  to pass through and interface with a diaphragm plate, which will be explained in more detail below. Housing  202  further includes a groove  212  on the surface facing plates  203 - 208  that encompasses or substantially surrounds opening  210 . Groove  212  includes one or more holes  213  that are in fluid communication with an ink reservoir, such as a supply reservoir. Therefore, groove  212  may represent a conduit for ink to travel from an ink reservoir to the individual ink channels in order to supply ink to the ink channels. The conduit (which includes groove  212 ) for supplying ink to the ink channels is referred to herein as a “supply manifold”. 
     Housing  202  further includes one or more grooves  215  on the surface facing plates  203 - 208  that are separate or isolated from groove  212 . Groove  215  includes one or more holes  216  that are in fluid communication with another ink reservoir, such as a return reservoir. Therefore, groove  215  may represent a conduit for ink to travel out of the ink channels in inkjet head  200  (instead of out of the nozzles of the head) in order to circulate ink through inkjet head  200 . The conduit (which includes groove  215 ) for removing ink from the ink channels during circulation is referred to herein as a “return manifold”. Although a supply reservoir and a return reservoir are described herein, a single reservoir may be used. 
     Plates  203 - 208  of inkjet head  200  are fixed or bonded to one another to form a laminated plate structure, and the laminated plate structure is affixed to housing  202 . The plate structure illustrated in  FIG. 2  is intended to be an example of a basic structure to show how circulation may be implemented in inkjet head  200 . There may be additional plates that are used in the plate structure that are not shown in  FIG. 2 . Also,  FIG. 2  is not necessarily drawn to scale. 
     In this embodiment, the laminated plate structure includes the following plates: an orifice plate  208 , a first restrictor plate  207 , chamber plates  205 - 206 , a second restrictor plate  204 , and a diaphragm plate  203 . Orifice plate  208  includes a plurality of nozzles  220  that are formed in one or more rows. Each nozzle  220  represents an individual ink channel in inkjet head  200  for ejecting ink. Although inkjet head  200  is shown as having two rows of nozzles in this embodiment, inkjet head  200  may have a single row of nozzles or more rows of nozzles in other embodiments. 
     Chamber plates  205 - 206  are each formed with a plurality of chambers  221  that correspond with the nozzles  220  of orifice plate  208 . Chambers  221  may be referred to as “supply chambers” or “pressure chambers”. Each chamber  221  is an opening in chamber plate  205 - 206 , and represents the portion of an ink channel that holds the ink which is ejected out its corresponding nozzle  220 . 
     Chamber plate  206  is also formed with elongated openings  222  that are parallel to the row of chambers  221 , which are referred to as “return openings”. Return openings  222  are slots that provide a further conduit for the ink to travel out of the ink channels in inkjet head  200  (instead of out of the nozzles of the head) in order to circulate ink through inkjet head  200 . Thus, return openings  222  are part of the return manifold for inkjet head  200 . Chamber plate  205  is formed with return openings  224  that are part of the return manifold for inkjet head  200 . The return openings  224  in chamber plate  205  are positioned off to the side of the rows of chambers  221 . When bonded as a laminate, the return openings  224  in chamber plate  205  will partially overlap with the return openings  222  in chamber plate  206 . The return openings  224  in chamber plate  205  will also correspond with grooves  215  in housing  202  to form the return manifold. 
     Restrictor plate  207  is sandwiched between orifice plate  208  and chamber plate  206 . Restrictor plate  207  is formed with a plurality of restrictors  223 . The restrictors  223  fluidly connect chambers  221  to the return manifold. When ink is circulated through inkjet head  200 , restrictors  223  control the flow of ink that circulates out of the chambers  221  and into the return manifold. 
     Restrictor plate  204  is sandwiched between chamber plate  205  and diaphragm plate  203 . Restrictor plate  204  is formed with a plurality of restrictors  225 . The restrictors  225  fluidly connect chambers  221  to the supply manifold, and control the flow of ink into chambers  221 . Restrictor plate  204  is formed with return openings  226  that are part of the return manifold for inkjet head  200 . The return openings  226  in restrictor plate  204  are positioned off to the side of the rows of restrictors  225 . When bonded as a laminate, the return openings  226  in restrictor plate  204  will correspond with grooves  215  in housing  202  to form the return manifold. 
     Diaphragm plate  203  is formed with diaphragms  227  and filter sections  228 . Diaphragms  227  each comprise a sheet of a semi-flexible material that extends longitudinally to correspond with the chambers  221 , and vibrates in response to actuation by piezoelectric actuator  209 . Filter sections  228  extend longitudinally to correspond with the supply manifold, and to remove foreign matter from ink flowing in the ink channels from the supply manifold. Although diaphragm plate  203  is shown as including both diaphragms  227  and filter sections  228  in this embodiment, diaphragms  227  and filter sections  228  may be implemented in separate plates in other embodiments. Diaphragm plate  203  is also formed with return openings  229  that are part of the return manifold for inkjet head  200 . The return openings  229  in diaphragm plate  203  are positioned off to the side of the rows of diaphragms  227 . When bonded as a laminate, the return openings  229  in diaphragm plate  203  will correspond with grooves  215  in housing  202  to form the return manifold. 
     Piezoelectric actuator  209  includes a plurality of piezoelectric elements  230 ; one for each of the ink channels. The ends of piezoelectric elements  230  contact diaphragms  227  in diaphragm plate  203  at positions opposite the chambers  221 . An external drive circuit (not shown) is able to selectively apply electrical signals to piezoelectric elements  230  which vibrate the diaphragm  227  for individual ink chambers. The vibration of diaphragms  227  changes the volume of chambers  221 , which in turn changes the pressure in chambers  221 . The change in pressure in a chamber  221  causes ink to be ejected from its corresponding nozzle  220 . 
       FIG. 3  is a cross-sectional view of an ink channel in inkjet head  200  in an exemplary embodiment. The view in  FIG. 3  is as if a slice where taken through the center of a nozzle  220  in head  200 . The slice is then oriented in  FIG. 3  with the nozzle  200  facing upward. Again, the plate structure illustrated in  FIG. 3  is intended to be an example of a basic structure to show how circulation may be implemented in inkjet head  200 . There may be additional plates that are used in the plate structure that are not shown in  FIG. 3 . Also,  FIG. 3  is not necessarily drawn to scale. 
     Beginning at the bottom of  FIG. 3 , the diaphragm plate  203  is shown as being connected to housing  202 . The filter section  228  of diaphragm plate  203  lines up with the supply manifold  302  formed by groove  212 . The diaphragm  227  of diaphragm plate  203  lines up with the chamber  221  of the ink channel. Restrictor plate  204  is sandwiched between diaphragm plate  203  and the chamber plates  205 - 206 . Restrictor plate  204  includes restrictor  225  that controls a flow of ink from the supply manifold  302  to the chamber  221  for the ink channel. 
     Chamber plates  205 - 206  form the chamber  221  for the ink channel. Chamber plate  206  also forms the return manifold  304  for the ink to circulate through the ink channel. Restrictor plate  207  is sandwiched between chamber plate  206  and orifice plate  208 . Restrictor plate  207  includes restrictor  223  that controls a flow of ink from the chamber  221  to the return manifold  304 . The top plate in  FIG. 3  is orifice plate  208  that has the nozzle  220  for the ink channel. 
       FIG. 4  is a cross-sectional view of ink circulating through the ink channel in an exemplary embodiment. The ink flow is illustrated by the arrows in  FIG. 4 . During a circulation, the ink flows into supply manifold  302 , as is illustrated by arrow points coming out of the page of  FIG. 4 . The ink then flows from supply manifold  302 , through the filter section  228  of diaphragm plate  203 , and through the restrictor  225  in restrictor plate  204  (see also  FIGS. 2-3 ). After passing through the restrictor  225 , the ink flows into the chamber  221  of the ink channel formed by chamber plates  205 - 206 . The ink then flows through the restrictor  223  in restrictor plate  207  (instead of exiting out of the nozzle  220  in orifice plate  208 ), and enters into return manifold  304  (see also  FIGS. 2-3 ). The ink will then flow out of return manifold  304 , as is illustrated by arrow tails going into the page of  FIG. 4 . As is evident from this figure, circulation of ink in inkjet head  200  is possible because return manifold  304  and an additional restrictor  223  has been added to the ink channel to allow ink to flow out of the chamber  221  of an ink channel instead of sitting in the chamber  221  and potentially drying or settling. The flow directions shown in  FIG. 4  are exemplary, and the actual flow of ink may depend on the position of the ink channel in the inkjet head  200 . 
     As is evident from  FIGS. 3-4 , restrictor  225  is formed on one end of chamber  221  toward the diaphragm  227 , and restrictor  223  is formed on the other end of chamber  221  toward the nozzle  220 . The vertical position of restrictor  225  in the stack generally corresponds with the vertical position of restrictor  223  in the stack, with the chamber plates  205 - 206  separating the restrictors. Because of the way restrictors  223  and  225  are formed in the laminated structure, the vertical position of return manifold  304  corresponds with the vertical position of the supply manifold  302  in the laminated structure (i.e., return manifold  304  is formed on top of supply manifold  302  with a layer between them). This is advantageous because the inkjet head  200  can be made narrow, but is still able to circulate ink to avoid clogging. 
     In order to circulate ink as illustrated in  FIG. 4 , the pressure in the supply manifold  302  and the return manifold  304  may be regulated. Drop-On-Demand (DOD) inkjet heads operate with slight negative pressure at their nozzles. This is to prevent ink from flowing out of the nozzles unintentionally. When inkjet head  200  is circulating ink, pressure at the supply manifold (P_in) and pressure at the return manifold (P_out) may be set as follows:
 
 P _in=positive
 
 P _out=negative
 
 P _in+ P _out=slightly negative at the nozzle(s)
 
 P _in− P _out=depends on the requirements (ink settling, drying prevention, and air removal, while still maintaining jetting stability).
 
     If a dual reservoir design is used, ink may be circulated by controlling the pressures for the reservoirs. The supply reservoir is regulated to have a positive pressure, while the return reservoir is regulated to have a negative pressure. The pressures are regulated in such a manner that the pressure at the nozzles are slightly negative. If a single reservoir design is used, then a pump may be placed in line with an inlet to the inkjet head to pump fluid into the head. Another pump may be placed in line with an outlet from the inkjet head to pump the fluid out of the head. The pumps may be used to regulate the positive pressure (inlets) and negative pressure (outlets) so that the pressure at the nozzles is slightly negative. 
     The flow direction in inkjet head  200  may also be reversed in other embodiments. Because restrictors  223  and  225  have similar designs, ink may flow in either direction through inkjet head  200 . Therefore, even though manifold  302  is referred to as a “supply” manifold and manifold  304  is referred to as a “return” manifold, the flow of ink through inkjet head  200  may be reversed to be the opposite of that shown in  FIG. 4 .  FIG. 7  is a cross-sectional view of ink circulating through the ink channel in a reverse direction in an exemplary embodiment. During a circulation in this embodiment, the ink first flows into return manifold  304 , and then through the restrictor  223  into chamber  221  of the ink channel. The ink then flows through the restrictor  225  in restrictor plate  204 , and enters into supply manifold  302 . The ink will then flow out of the supply manifold. If the flow of ink is reversed in this manner, another filter plate may be used to filter the ink that enters through return manifold  304 . 
     Example 
       FIG. 5  illustrates an exploded, perspective view of an inkjet head  500  in an exemplary embodiment. The structure illustrated in  FIG. 5  is just one particular example, and the embodiments described herein are not limited to the structure shown in the figure. In this example, inkjet head  500  includes a housing  501  and a series of plates  502 - 512  that are fixed or bonded to one another to form a laminated plate structure. Housing  501  includes an opening  520  for a piezoelectric actuator (not shown). Housing  501  further includes a supply groove  522  that encompasses or substantially surrounds opening  520 . Supply groove  522  forms the supply manifold for inkjet head  500 . Housing  501  also includes return grooves  523  that form the return manifold for inkjet head  500 . 
     Plate  502  is a filter plate that is porous (i.e., has many small holes that allow liquid to pass through), and removes foreign matter from the ink flowing in from the supply manifold. Filter plate  502  also includes an opening proximate to its center for the piezoelectric actuator to pass through. Plate  503  is a manifold plate that includes elongated supply openings  526  near its top and bottom for the supply manifold, and return openings  527  towards its ends (left and right in  FIG. 5 ) for the return manifold. Manifold plate  503  further includes elongated openings  528  toward its center for piezoelectric elements of the actuator to pass through. 
     Plate  504  is a diaphragm plate. Diaphragm plate  504  is formed with diaphragms  530  and filter sections  531 . Diaphragms  530  each comprise a sheet of a semi-flexible material that vibrates in response to actuation by a piezoelectric actuator. Filter sections  531  remove foreign matter from ink flowing from the supply manifold. Diaphragm plate  504  also includes return openings  532  towards its ends (left and right in  FIG. 5 ) for the return manifold. 
     Plate  505  is a support plate, and plate  506  is a restrictor plate. Support plate  505  is used in conjunction with restrictor plate  506  to control the flow of ink through restrictors. Restrictor plate  506  includes parallel rows of restrictors  538 . A restrictor  538  is formed as an opening or aperture (which is vertical in  FIG. 5 ), and one restrictor  538  from restrictor plate  506  corresponds with one ink channel for inkjet head  500 . Support plate  505  has openings  539  that correspond with the restrictors  538  in restrictor plate  506  to control the flow of ink through restrictors  538 . Support plate  505  and restrictor plate  506  each include return openings  540 - 541 , respectively, towards their ends (left and right in  FIG. 5 ) that form the return manifold. 
     Plate  507  is a chamber plate. Chamber plate  507  includes two parallel rows of chambers  544 . A chamber  544  is formed as an opening or aperture (which is vertical in  FIG. 5 ), and one chamber  544  in chamber plate  507  corresponds with one ink channel for inkjet head  500 . A chamber  544  represents the portion of an ink channel that holds the ink, and the pressure in the chamber  544  is changed to eject the ink out of its corresponding nozzle. Chamber plate  507  also includes return opening  546  towards its ends (left and right in  FIG. 5 ) that form the return manifold. 
     Plate  508  is also a chamber plate. Chamber plate  508  has a similar configuration as chamber plate  507  with parallel rows of chambers  548 . The return opening is different in chamber plate  508 , which has an elongated opening  550  near its top and bottom for the return manifold instead of just toward its ends as with chamber plate  507 . 
     Plate  509  is also a chamber plate. Chamber plate  509  is configured with parallel row of chambers  552 . The size of the openings for the chambers  552  in this plate is illustrated as smaller than the openings for the chambers  544 ,  548  in plates  507 - 508 . Chamber plate  509  also has an elongated return opening  554  near its top and bottom for the return manifold. 
     Plate  510  is another chamber plate. Chamber plate  510  includes parallel rows of chambers  556  like the other chamber plates. Chamber plate  510  also includes rows of manifold patterns  558 . The portion of manifold patterns  558  nearest the chambers  556  are partially etched to assist in controlling the flow of ink from the chambers into the return manifold (in conjunction with restrictors in another restrictor plate  511 ). The portion of manifold pattern  558  towards the top and bottom of chamber plate  510  are openings that form the return manifold. Although four chamber plates are illustrated in  FIG. 5 , more or less chamber plates may be used to form the ink chambers as desired. 
     Restrictor plate  511  includes parallel rows of restrictors  560 . A restrictor  560  is formed as an opening or aperture (which is vertical in  FIG. 5 ), and one restrictor  560  from restrictor plate  511  corresponds with one ink channel for inkjet head  500 . The partially-etched sections of the manifold pattern  558  in chamber plate  510  correspond with the restrictors  560  in restrictor plate  511  to control the flow of ink through restrictors  560  and into the return manifold. 
     Plate  512  is an orifice plate. Orifice plate  512  includes parallel rows of nozzles  566 . A nozzle is a small aperture in orifice plate  512  from which ink may be ejected. One nozzle  566  corresponds with one ink channel for inkjet head  500 . 
       FIG. 6  is a cross-sectional view of an ink channel in inkjet head  500  in an exemplary embodiment. The view in  FIG. 6  is as if a slice were taken through the center of a nozzle  566  in head  500 . The slice is then oriented in  FIG. 6  with the nozzle  566  facing upward. Again, the plate structure illustrated in  FIG. 6  is intended to be an example, as more or less plates may be used in other embodiments. Also,  FIG. 6  is not necessarily drawn to scale. 
     Beginning at the bottom of  FIG. 6 , filter plate  502  is sandwiched between the housing  501  and manifold plate  503 . Diaphragm plate  504  is shown as being connected to manifold plate  503 . The filter section  531  of diaphragm plate  504  lines up with the supply manifold formed by groove  522  in housing  501  (see  FIG. 5 ). The diaphragm  530  of diaphragm plate  504  lines up with the chamber  544  of the ink channel. 
     Next, support plate  505  is bonded to diaphragm plate  504 , and restrictor plate  506  is bonded to support plate  505 . Restrictor plate  506  includes a restrictor  538 , that when used in conjunction with support plate  505 , controls a flow of ink from the supply manifold to the chamber  544  for the ink channel. Following restrictor plate  506  are the chamber plates  507 - 510 . Chamber plates  507 - 510  form the chamber  544  for the ink channel. Chamber plates  508 - 510  also form the return manifold for the ink to circulate through the ink channel. 
     Restrictor plate  511  is sandwiched between chamber plate  510  and orifice plate  512 . Restrictor plate  511  includes a restrictor  560  that controls a flow of ink from the chamber  544  to the return manifold. As described in  FIG. 5 , chamber plate  510  has manifold pattern  558  that is partially-etched as indicated in  FIG. 6  to work in conjunction with the restrictor  560  in restrictor plate  517 . The manifold pattern  558  in chamber plate  510  also has an opening that forms the return manifold. The top plate in  FIG. 6  is orifice plate  512  that has the nozzle  566  for the ink channel. 
     To circulate ink through the ink channel shown in  FIG. 6 , the pressure at the supply manifold (P_in) is adjusted to a positive pressure, and the pressure for the return manifold (P_out) is adjusted to a negative pressure so that the overall pressure of the ink channel in slightly negative (P_in+P_out=slightly negative at nozzle  566 ). This will cause ink to circulate through the ink channel without being ejected from nozzle  566 . The ink flows from the supply manifold, and through the restrictor  538  in restrictor plate  506  into chamber  544 . The ink then flows through the restrictor  560  in restrictor plate  511  (instead of exiting out of the nozzle  566 ), and enters into the return manifold. The ink will then flow out of the return manifold, and into a return reservoir. This circulation of the ink prevents the ink from sitting in chamber  544  and potentially drying or settling. 
     In another embodiment, the flow of ink through inkjet head  500  may be reversed. During a circulation in this embodiment, the ink first flows into the return manifold. The ink then flows from the return manifold through the restrictor  560  closest to the nozzle  566  and into chamber  544  of the ink channel. The ink then flows through the other restrictor  538 , and enters into the supply manifold. The ink will then flow out of the supply manifold. 
     Although specific embodiments were described herein, the scope of the invention is not limited to those specific embodiments. The scope of the invention is defined by the following claims and any equivalents thereof.