Abstract:
A printer includes a printhead configured to eject high viscosity material and refill a reservoir in the printhead with high viscosity material. The printhead includes a transducer having an electroactive element and a member to which the electroactive element is mounted. An electrical signal activates the electroactive element to move the electroactive element and the member in the reservoir of high viscosity material. This movement thins the high viscosity material and enables the printhead to eject the thinned material while refilling the reservoir. The apertures through which the thinned material is ejected share a common manifold without separate chambers for each of the apertures.

Description:
TECHNICAL FIELD 
       [0001]    The device disclosed in this document relates to printheads that eject high viscosity materials and, more particularly, to printers that produce three-dimensional objects with such materials. 
       BACKGROUND 
       [0002]    Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes. The substrate is either supported on a platform that can be moved three dimensionally by operation of actuators operatively connected to the platform. Additionally or alternatively, the printhead or printheads are also operatively connected to one or more actuators for controlled movement of the printhead or printheads to produce the layers that form the object. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
         [0003]    In some three-dimensional object printers, one or more printheads having an array of nozzles are used to eject material that forms part of an object, usually called build material, and to eject material that forms support structures to enable object formation, usually called support material. Most multi-nozzle printheads contain cavities that are filled with the type of material to be ejected by the printhead. These cavities are pressurized to eject drops of material, but they can only print materials having a very limited range of viscosities. Typically, these materials have a viscosity in the 5-20 cP range. Some materials considered ideal for manufacturing objects have viscosities that greater than those of materials that can be used in currently known printheads. 
         [0004]    To overcome the limitations associated with high viscosity materials, single nozzle printheads have been used to eject materials to form objects. These single nozzle printheads are too large to be manufactured as arrays. Consequently, the productivity of the objects that can be produced by these printheads is limited. Printheads capable of enabling higher viscosity fluids to flow through the channels in a printhead and be ejected from the printheads would be advantageous. 
       SUMMARY 
       [0005]    A printhead that enables higher viscosity fluids to flow through the channels in the printhead and be ejected from the nozzles in the printhead includes a reservoir configured with at least one wall to hold a volume of a high viscosity material, at least one transducer having an electroactive element that is mounted to a member, and an electrical conductor electrically connected to the electroactive element of the at least one transducer to enable a controller to activate the at least one electroactive element with a first electrical signal and move the member in the high viscosity material adjacent to the electroactive element and the member to thin the high viscosity material and enable the thinned material to move away from the at least one transducer. 
         [0006]    A printer that incorporates the printhead that enables higher viscosity fluids to flow through the channels in the printhead and be ejected from the nozzles in the printhead includes a platen, a printhead positioned to eject material onto the platen to form an object, the printhead comprising: a reservoir configured with at least one wall to hold a volume of a high viscosity material, at least one transducer having an electroactive element that is mounted to a member, and an electrical conductor electrically connected to the electroactive element of the at least one transducer to enable a controller to activate the at least one electroactive element with a first electrical signal and move the member in the high viscosity material adjacent to the electroactive element and the member to thin the high viscosity material and enable the thinned material to move away from the at least one transducer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing aspects and other features of a printhead or printer that enables higher viscosity fluids to flow through the channels in the printhead and be ejected from the nozzles in the printhead are explained in the following description, taken in connection with the accompanying drawings. 
           [0008]      FIG. 1  is block diagram of a printhead and platen configuration in a three-dimensional object printer. 
           [0009]      FIG. 2  is a cross-sectional view of one of the printheads shown in of  FIG. 1 . 
           [0010]      FIG. 3  is a cross-sectional view of an alternative embodiment of a printhead in the configuration of  FIG. 1 . 
           [0011]      FIG. 4  is an illustration of how a single transducer can be configured to operate a member to eject material and facilitate replenishment of material in the vicinity of the member. 
           [0012]      FIG. 5  is an illustration of a plurality of transducers in a radial pattern within a material reservoir. 
           [0013]      FIG. 6  is an illustration of a double support beam configuration. 
           [0014]      FIG. 7  is an illustration of a single support beam configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    For a general understanding of the environment for the printhead and printer disclosed herein as well as the details for the printhead and printer, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
         [0016]      FIG. 1  shows a configuration of printheads, controller and a platen in a printer  100 , which produces a three-dimensional object or part on a platen  112 . The printer  100  includes a support platen  112  over which two printheads  104  are carried by a frame  108 . While the figure shows two printheads, a single printhead or more than two printheads can be used to configure a printer for forming three-dimensional objects. One of the printheads  104  can be operatively connected to a supply of building material and the other one operatively connected to a supply of support material. The frame  108  to which the two printheads  104  are mounted is operatively connected to actuators  116 , which are operatively connected to a controller  120 . The controller is configured with electronic components and programmed instructions stored in a memory operatively connected to the controller to operate the actuators and move the frame in an X-Y plane and a Z plane relative to the stationary platen. The X-Y plane is parallel to the surface of the platen  112  opposite the printheads  104  and the Z plane is perpendicular to the surface of the platen. Alternatively, the platen  112  can be operatively connected to the actuators  116  and the controller  120  to enable the controller to move the platen in the X-Y plane and the Z plane relative to the stationary frame  108  and printheads  104 . In yet another alternative embodiment, the frame  108  and the platen  112  can be operatively connected to different actuators to enable the controller  120  to move both the platen and the frame in the X-Y plane and the Z plane. 
         [0017]    While the platen  14  of  FIG. 1  is shown as a planar member, other embodiments of three-dimensional object printers include platens that are circular discs, an inner wall of a rotating cylinder or drum, or a rotating cone. The movement of the platen and the printhead(s) in these printers can be described with polar coordinates. The internal structure of the printheads discussed below that enable higher viscosity materials to be used in the printheads  104  can be used with any of the alternative platens. 
         [0018]    In the cross-sectional view of a portion of one of the printheads  104  provided in  FIG. 2 , a reservoir  204  with a wall  208  holds high viscosity material. As used in this document, “high viscosity material” refers to a material having a viscosity that is greater than 20 cP at the operating temperature of the printhead and that possesses the property called shear thinning. “Shear thinning” means that the viscosity of the material decreases in response to shear stress. A class of materials that exhibits shear thinning is pseudoplastics. The thinning of psuedoplastics is time independent. Additionally, many materials that can be used in object manufacturing processes are thixotropic, which indicates the thinning of the material is time dependent. That is, as the time to which the material is subjected to shear stress is increased, the viscosity of the material continues to decrease. 
         [0019]    With continued reference to  FIG. 2 , a transducer  210  includes an electroactive element  216 , a member  212  having a protrusion  224 . As used in this document, the term “electroactive element” means any material that responds to an electrical signal by changing its length in at least one dimension. The electroactive element  216  is electrically connected to an electrical conductor  220  to enable an electrical signal to be applied to the element  216 , which bends in response to the signal. The electroactive element  216  can be a piezoelectric element, a capacitive element, or the like. The member  212  is bonded to the electroactive element  216  and extends into the reservoir  204 . The member  212  can terminate prior to the wall  208  or the member  212  can be attached to wall  208 . In some embodiments, the member  212  has a bending modulus that is different than the bending modulus of the transducer  216  so the junction between the transducer and the floor acts as a bimorph. The member  212  moves in response to the bending of the electroactive element  216 . The protrusion  224  is part of the member  212  so member  224  moves in response to the movement of the member  212 . A controller, such as controller  120 , can be electrically connected to the conductor  220  to generate an electrical signal that activates the electroactive  216  so the member  212  and protruding member  224  of the transducer  210  move relative to the high viscosity material in the reservoir  204  to produce shear stress in the material. This shear stress decreases the viscosity of the material to levels that enable the material to flow within the reservoir. As shown in  FIG. 2 , the electrical signal provided on the electrical conductor  220  causes the electroactive element  216  to expand or contract in a transverse direction indicated by the arrows in the figure and causes the member  212  to bend. The high viscosity material adjacent the electroactive element  216  and the member  212  moves in response to the transducer  210  activation, while the material further away from the transducer does not move. This difference produces shear stress in the material adjacent the transducer. As the material adjacent to the transducer  210  decreases in viscosity in response to the shear stress, it flows away from the moving components of the transducer  210 . 
         [0020]    In one embodiment, the electroactive element is a piezoelectric material and the member  212  is a substrate of metal. In response to the activation of the electroactive element  216 , the portion of the member  212  between the element  216  and the member  224  acts as a cantilever and moves the protrusion  224  of the member  212  up and down. The up and down movement of the protrusion  224  operates as a hammer in the high viscosity fluid in reservoir  204 . This hammer action imparts shear stress to the high viscosity fluid over the protrusion  224  and decreases the viscosity of that fluid. This decrease in viscosity and the energy provided by the protrusion  224  ejects a portion of the thinned high viscosity material through a nozzle  232  in the substrate  228  that joins the wall  208  to enclose the reservoir  204 . The thinning of the high viscosity fluid in the vicinity of the electroactive element  216  and member  212  along with the thinning of the high viscosity fluid above the protrusion  224  causes the thinned material at the element  216  and member  212  to migrate toward the protrusion  224  to replace the thinned material ejected from the nozzle  232 . In effect, the thinning of the material in these two regions form a channel  230  ( FIG. 4 ) of thinned fluid that not only enables the ejection of material from the printhead, but the replenishment of material in the printhead as well. 
         [0021]      FIG. 3  illustrates another advantage that arises from the use of the shear stress produced by transducers to eject high viscosity materials from nozzles in a printhead. In  FIG. 3 , the electroactive elements  304  and  308  are mounted to member  312 . The substrate  316  that encloses the reservoir  320  includes two nozzles  324 . In previously known printheads, each electroactive element faces a pressure chamber that holds ink until the activation of the electroactive element urges a portion of the ink in the pressure chamber outwardly through a nozzle communicating with the pressure chamber. Each pressure chamber and transducer is mechanically insulated from adjacent pressure chambers and transducers by walls of a substrate in which the pressure chambers are formed. This mechanical insulation is important in previously known printheads because the movement of an electroactive element in low viscosity fluid could perturb ink in an area opposite an adjacent electroactive element and perhaps even eject ink from the nozzle opposite the adjacent element. Consequently, mechanically insulating structures were required between adjacent electroactive elements to prevent mechanical cross-talk between adjacent inkjet ejectors. Fully mechanically insulating structures are not required in printheads in which high viscosity material is ejected because the high viscosity material increasingly attenuates the shear stress as distance of the shear stress from the moving component decreases. Therefore, printheads ejecting high viscosity materials using the structures shown in  FIG. 2  and  FIG. 3  do not need the more complex mechanical insulating structures necessary in printheads ejecting low viscosity fluids. 
         [0022]    As shown in  FIG. 3 , the nozzles  324  in substrate  316  communicate pneumatically with each other with no mechanical structure insulating the nozzles from one another. In an example, electrical conductor  328  delivers an electrical signal to electroactive element  304 , but no electrical signal is delivered over electrical conductor  334  to electroactive  308 . Accordingly, the pressure wave produced by the expansion and contraction of electroactive element  304  is directed towards the nozzle  324  opposite the transducer to thin the high viscosity material between the element  304  and the nozzle  324  and eject a portion of the thinned material through that nozzle  324 . The high viscosity material in the portion of the reservoir  320  between the two electroactive elements and the two nozzles dampens any shear stress that emanates beyond the side boundaries of the bimorph formed by electroactive element  304  and the member  312 . Consequently, the high viscosity material between the electroactive element  308  and the nozzle  324  opposite that transducer is not thinned and no drop is ejected from that nozzle. Thus, the structure of a printhead configured for use with high viscosity material can be more mechanically simple than ink or other low viscosity fluid ejecting printheads since they do not require chambers enclosing each nozzle and containing a narrow fluidic inlet. 
         [0023]      FIG. 3  also depicts the two electroactive elements  304  and  308  with different top surfaces. Specifically, electroactive element  304  has a planar top surface, while electroactive element  308  has a concave top surface. The concave surface can focus the pressure wave produced by the expansion and contraction of the electroactive element  308  better than the flat surface of the electroactive element  304 . Other surface shapes and configurations are also possible. Additionally, the transducers shown in the figures have a tapered shape, although other shapes can be used. For example, the transducers can be circular, cylindrical, square, rectangular or the like. Additionally, a plurality of transducers can be configured in a radial pattern as shown in  FIG. 5 . In that figure, a plurality of electroactive elements  504  are mounted to member  512  in a radial pattern. Each electroactive element  504  has an electrical conductor  520  to enable each transducer to receive an electrical signal from the controller independently of the other transducers in the radial pattern. Additionally, member  512  includes protrusions  524 , which are positioned on the member  512  at a distance from a corresponding electroactive element  504  to operate as a hammer to thin and eject material through an aperture in another layer positioned above the protrusion  524 , but not shown in the figure. While the transducers are described above as being piezoelectric transducers, other transducer types can be used such as thermal, electrocapacitive or the like. 
         [0024]    As noted above, the member  212  can terminate prior to contacting wall  208  or it can join wall  208 .  FIG. 6  shows an embodiment of a protruding member  624  mounted to a member  612  that is joined to wall  608 . This configuration is called a double supported beam structure. In response to the activation of the electroactive element  604  by an electrical signal, the protruding member  624  modulates in a bowed pattern as indicated in the figure. In  FIG. 7 , the member  712  does not join a wall so the member  712  has a free end. The protruding member  724  is mounted to the member  712  at or near the free end of the member  712 . Consequently, activation of the electroactive element  704  causes the free end of the member  712  and protruding member  724  to swing in a pattern similar to an end of a diving board after a diver has left the board. This action thins the material between the protruding member  724  and the nozzle  732  in substrate  716  to enable a portion of the thinned material to be ejected through the nozzle  732 . This configuration is called a single support beam structure. 
         [0025]    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, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.