Patent Abstract:
Protrusions are positioned on the inner surfaces of a channel within a printhead body or member to control the size and location of bubble formation. An inkjet printhead includes a member having a first opening and a second opening to enable melted ink to enter the channel at the first opening and flow through the channel to the second opening. At least one protrusion extends from the member into the channel to position a portion of the protrusion into melted ink in the channel to form a dominant stress concentration in the melted ink.

Full Description:
TECHNICAL FIELD 
     This disclosure relates generally to printheads for inkjet printers, and more particularly, to systems and methods for the control of the size and location of air bubbles that form in a liquid path for ink in a printhead. 
     BACKGROUND 
     Air bubbles in ink flow paths of inkjet printers can impact the performance of the printers. In printers that use solid ink, air bubbles are formed during the freezing and melting of the solidified ink. Typically, when a solid inkjet printer is not operating, melted ink in the ink flow paths solidifies. 
       FIG. 3A  is a cross-sectional view of fluid paths, a pressure chamber, and air vents in a prior art inkjet in a printhead  500 , and  FIG. 3B  is a top plan view of an exemplary nozzle plate  550  in a printhead that includes the inkjet of  FIG. 3A . The exemplary print head  500  is configured for use in an inkjet printer. While  FIG. 3A  and  FIG. 3B  depict a single inkjet for illustrative purposes, existing printhead embodiments include multiple inkjets, including arrays of hundreds or thousands of inkjets in some embodiments. 
     In  FIG. 3A , the printhead  500  includes a substrate  520 , a silicon wafer  530  on an upper surface of the substrate  520 , an ink passage  540  through the substrate  520  and silicon wafer  530 , a tube  545  connecting the ink passage  540  of the print head  500  to an ink supply reservoir, and a nozzle plate  550  mounted on the structure. An electrostatically actuated membrane  560  is formed on the silicon wafer  530  as shown. A pressure chamber  565  receives liquid ink through the fluid ink passage  540 . A nozzle hole  570  and a matrix of purge vents  590  ( FIG. 3B ) can be formed in the nozzle plate  550 . The purge vents  590  in  FIG. 3A  and  FIG. 3B  are formed as a group of small nozzle holes formed through the nozzle plate  550 . Air enters and leaves the pressure chamber  565  during operation of the print head  500  through the group of purge vents  590 . The purge vents  590  are large enough to enable air to escape from the pressure chamber  565  as ink fills the pressure chamber, and to admit air when liquid ink in the pressure chamber solidifies in embodiments of the printhead  500  that use a phase-change ink. 
     In the print head  500 , the membrane  560  is an electrostatically actuated diaphragm, in which the membrane  560  is controlled by an electrode  562 . The membrane  560  can be made from a structural material such as, for example, polysilicon, as is typically used in a surface micromachining process. An air vent  564  between membrane  560  and wafer  530  can be formed using typical techniques, such as by surface micromachining. The electrode  562  acts as a counterelectrode and is typically either a metal or a doped semiconductor material, such as polysilicon. Alternative inkjet embodiments include a piezoelectric actuator or a thermal actuator. 
     During operation of an electrostatic or piezoelectric actuator, the electrode  562  receives an electrical signal and the membrane  564  deflects into the pressure chamber  565 . The deformation generates pressure on the ink in the pressure chamber  565  and the pressure urges an ink drop, such as the ink drop  582 , through the nozzle  570 . In some configurations, the membrane  560  deflects toward the electrode  562  prior to deflection into the pressure chamber  565  to draw ink into the pressure chamber  565  for ejection through the nozzle  570 . In a thermal inkjet, the electrical signal generates heat in the pressure chamber and the heat produces an air bubble that urges ink in the pressure chamber  565  through the nozzle  570  to eject an ink drop in a similar manner to the arrangement of  FIG. 3A . 
     The purge vents  590  in the nozzle plate  550  have diameters that are typically smaller than the diameter of the nozzle  570 , and are sufficiently narrow to prevent ink from passing through the nozzle plate  550  at a location other than the nozzle  570  during operation of the printhead  500 . During operation, a meniscus of liquid ink forms across the opening to each of the purge vents  590  from the nozzle plate  550  to the pressure chamber  565 . The strength of the meniscus enables ink to remain in the pressure chamber  565  and to be ejected through the nozzle  570  without being ejected or otherwise leaking through the purge vents  590 . In one embodiment, each of the purge vents  590  is formed with a diameter of approximately 3 to 5 microns. In comparison, the diameter of the nozzle  570  is approximately 27.5 microns in the embodiment of  FIG. 3A . The small size of the purge vents  590  minimizes the impact of the vents on the flow of liquid ink to the inkjet, which enables the ink to flow to the pressure chamber  565  with sufficient liquid pressure to supply the inkjet with ink during printing operations. 
     In the prior art embodiment, the vents  590  enable air bubbles to escape from liquid ink in the fluid path  540  and pressure chamber  565 . Some air bubble, however, may be formed in portions of the printhead where the air bubbles are unable to be vented easily. For example, in the printhead  500  an air bubble that forms near the nozzle  570  does not escape through the vents  590 , but instead escapes through the nozzle  570  where the air bubble disrupts the process of ejecting ink drops. Additionally, while small air bubbles that form near the vents  590  can escape from the printhead  500 , larger air bubbles formed within the channel  540  and the pressure chamber  565  can interrupt the flow of ink to the pressure chamber  565  for a longer period of time before escaping from the printhead  500 . What is needed is a printhead design that mitigates the formation of air bubbles in locations that are difficult to purge, and mitigates the formation of large air bubbles. 
     SUMMARY 
     An inkjet printhead has been developed that facilitates the removal of air bubbles from ink flow paths in a printhead and helps reduce the size of air bubbles formed in the ink flow paths. The inkjet printhead includes a member having a channel through the member with a first opening and a second opening to enable melted ink to enter the channel at the first opening and flow through the channel to the second opening, and at least one protrusion extending from the member to position a portion of the protrusion into melted ink in the channel to form a dominant stress concentration in the melted ink. 
     A method of making an inkjet printhead has been developed that facilitates the removal of air bubbles from ink flow paths in a printhead and helps reduce the size of air bubbles formed in the ink flow paths. The method includes providing a vent in a member having a channel with a first opening and a second opening to enable melted ink to enter the channel at the first opening and flow through the channel to the second opening, and providing at least one protrusion extending from the member into the channel to position a portion of the protrusion into melted ink in the channel to establish a dominant stress concentration in the melted ink for forming air bubbles at a predetermined location in the channel. 
     The inkjet printhead and method can be used in an inkjet printer to facilitate the removal of air bubbles from ink flow paths in a printhead and help reduce the size of air bubbles formed in the ink flow paths. The inkjet printer includes a printhead having a body, a reservoir, a channel within the printhead body that is fluidly connected to the reservoir, the channel having a first opening and a second opening to enable melted ink to enter the channel at the first opening and flow through the channel to the second opening, and at least one protrusion extending from the printhead body into the channel to position a portion of the protrusion into melted ink in the channel to enable air bubble formation at the protrusion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a fluid path for use in a printhead that includes protrusions to control the formation of air bubbles within the fluid path when the fluid path is filled with a liquid ink. 
         FIG. 1B  depicts the fluid path of  FIG. 1A  when the fluid path contains solidified ink. 
         FIG. 1C  depicts the fluid path of  FIG. 1A  and  FIG. 1B  after solidified ink in the fluid path returns to a liquefied state. 
         FIG. 2A  depicts another fluid path for use in a printhead that includes a protrusion to control the formation of air bubbles within the fluid path when the fluid path is filled with a liquid ink. 
         FIG. 2B  depicts the fluid path of  FIG. 2A  when the fluid path contains solidified ink. 
         FIG. 2C  depict the fluid path of  FIG. 2A  and  FIG. 2B  after solidified ink in the fluid path returns to a liquefied state. 
         FIG. 3A  depicts a cross-sectional view of a prior art inkjet in a prior art printhead. 
         FIG. 3B  depicts a plan view of a prior art nozzle plate in the printhead of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION 
     Protrusions can be arranged in a printhead flow path to mitigate the formation of large air bubbles that are difficult to remove.  FIG. 1A-FIG .  1 C depict a printhead channel  300  within a member or body of the printhead that enables ink to flow through the printhead and a plurality of protrusions formed in the channel to control the locations of bubble formation within the channel  300 . Referring to  FIG. 1A , the printhead channel  300  provides a flow path  304 . The flow path  304  has two opposite ends  306  and  308 . The flow path  304  is filled completely with melted solid ink  310 , which flows from the end  306  to the end  308  during normal operation. However, unlike the pressure chamber  565  in  FIG. 3A , the printhead channel  300  includes protrusions  312 . The protrusions  312  are arranged along the wall  302 , and extend from the wall  302  into the flow path  304  and, accordingly, into the solid ink  310 . The protrusions  312  modify the nominal stress concentrations as the melted solid ink  310  solidifies by establishing dominant stress concentrations at each of the protrusions  312 . As used in this document, the term “dominant stress concentration” refers to a local maximum in the average force per unit area that particles of a body exert on adjacent particles of the body. The dominant stress concentrations promote the cracking of the solid ink  310  at their locations when the solid ink  310  solidifies. The dotted lines represent the expected cracking points in the solid ink  310  as the ink shrinks during cooling and freezing.  FIG. 1B  depicts the printhead channel  300  of  FIG. 1A  in which the solid ink  310  has cooled and solidified within the flow path  304 . As the solid ink  310  solidifies, cracks form in the solid ink and voids  314  are formed in the solid ink. However, the dominant stress concentrations at the protrusions  312  enable the voids  314  to form in a predictable and distributed manner.  FIG. 2   c  depicts the printhead channel  300  of  FIG. 2   b  in which the solidified solid ink  310  has been warmed to a temperature that enables the solidified solid ink to melt within the flow path  304 . The voids  314  have turned into air bubbles  316 . The air bubbles  316  are small and are distributed across the length of the flow path  304 , thereby enabling the air bubbles  316  to be removed from the flow path  304  with less ink purged from the path. In this way, protrusions can be strategically arranged within a printhead flow path for the purpose of mitigating the formation of large and difficult to remove air bubbles. Smaller air bubbles can be forced out of the purge vents with less ink flow than larger air bubbles, reducing waste. Protrusions can be any of a variety of shapes such as conical, spherical, cylindrical, rectangular, and the like. The shapes and sizes of the protrusions are governed by the geometry of the channel, ambient conditions surrounding the printhead, processes for warming and cooling the printhead, active and passive thermal gradients, imposed pressure gradients, ink properties and the like. 
     In addition to controlling the size of air bubble formation, protrusions can also be strategically arranged to control the location of air bubble formation.  FIG. 2A-FIG .  2 C depict a printhead channel  400  within a member or body that defines a flow path  404  for ink. The flow path  404  has two opposite ends  406  and  408 . The flow path  404  is completely filled with melted solid ink  410 , which flows from the end  406  to the end  408  during normal operation. The flow path  404  has a narrow region  412 . Purge vents  414  are arranged along the wall  402  near the end  408  and downstream of the narrow region  412 . The narrow region  412  can cause the melted solid ink  410  to crack in the narrow region  412  as the ink solidifies. The printhead channel  400  includes a protrusion  416 , which is positioned on the wall  402  near the narrow region  412  and substantially opposite the purge vents  414 . The protrusion  416  extends into the flow path  404  to establish a dominant stress concentration near the narrow region  412  but angled slightly toward the end  408  and the purge vents  414 . The dotted line represents the expected cracking point in the solid ink  410  as the ink shrinks during cooling and freezing. 
       FIG. 2B  depicts the printhead channel  400  of  FIG. 2A , wherein the solid ink  410  has cooled and solidified within the flow path  404 . As the ink  410  cools and solidifies, the volume of the ink contracts and shrinks compared to the volume of the equivalent mass of ink in the liquid state. The shrinkage of the ink during the transition from the liquid state to the solid state of the ink  410  produces cracks and voids, such as the void  418 . However, the dominant stress concentration established by the protrusion  416  near the narrow region  412  angles the void  418  slightly toward the end  408  and the purge vents  414 .  FIG. 2C  depicts the printhead channel  400  of  FIG. 2B , in which the solidified solid ink  410  has been warmed to a temperature that enables the solidified ink to melt within the flow path  404 . The void  418  has turned into an air bubble  420 . However, because the void  418  was angled slightly toward the end  408  and the purge vents  414 , the air bubble  420  buoyed toward the purge vents  414  on one side of the narrow region  412 , rather than possibly migrating toward the end  406 , which does not have any purge vents. This bubble placement facilitates the removal of the bubble through the vents  414  during a purging process. Accordingly, the amount of ink required to purge the printhead channel  400  is significantly reduced over previously known printheads. 
     The protrusions disclosed in this document can be used to mitigate the size of air bubbles formed during a solidifying/melting cycle as well as to control the locations where air bubbles are formed. By applying these concepts to different printhead geometries, printhead designers can establish predictability in the size and locations of air bubble formation. This predictability can be exploited to optimize the size, quantity, and locations of purge vents. An efficient purge vent layout in which air bubbles are properly staged near purge vents and extraneous purge vents are removed, results in a reduction of the amount of ink lost during purges and overall ink costs. Furthermore, the predictability allows printhead geometries to be scaled without substantially altering air bubble purging strategies. 
     These concepts are of even greater use for complex printhead geometries that can accommodate purge vents in fewer locations than simple geometry printheads. Protrusions can be arranged to control air bubble formation in such a way as to promote the formation of air bubbles in preferable areas, such as those were purge vents can be accommodated, while mitigating the formation of air bubbles in undesirable locations, such as those that will not accommodate a purge vent. 
     The geometries of the printhead channels shown in  FIG. 1A-FIG .  1 C and  FIG. 2A-FIG .  2 C are exemplary and have been greatly simplified for the purposes of promoting an understanding of the principles of the protrusions and their placements. Although typical printhead geometries are much more complex than those shown in the exemplary figures and embodiments, the principles can be applied to any printhead geometry for strategic control of both the size and the locations of air bubble formation. 
     It will be appreciated that variants of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.

Technology Classification (CPC): 1