Abstract:
A method for assembling an inkjet jet print head enables piezoelectric transducers to be bonded to an inkjet ejector without closing inlets to a pressure chamber within the inkjet ejector. The method includes bonding a polymer layer to a diaphragm layer having a plurality of openings, bonding piezoelectric transducers to the diaphragm layer with a thermoset adhesive, placing thermoset polymer in areas between the piezoelectric transducers on the diaphragm layer, and drilling inlets through the thermoset polymer and the diaphragm at the openings in the diaphragm.

Description:
PRIORITY CLAIM 
     This document claims priority to U.S. patent application Ser. No. 12/638,573, which was filed on Dec. 15, 2009 and is entitled “A Print Head Having A Polymer Layer To Facilitate Assembly Of The Print Head.” The application issued as U.S. Pat. No. 8,303,093 on Nov. 6, 2012. 
     TECHNICAL FIELD 
     This disclosure relates generally to inkjet ejectors that eject ink from a print head onto an image receiving surface and, more particularly, to print heads having inkjet ejectors comprised of multiple layers. 
     BACKGROUND 
     Drop on demand inkjet technology has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by the selective activation of inkjets within a print head to eject ink onto an ink receiving member. For example, an ink receiving member rotates perpendicular to a print head assembly as the inkjets in the print head are selectively activated. The ink receiving member may be an intermediate image member, such as an image drum or belt, or a print medium, such as paper. An image formed on an intermediate image member is subsequently transferred to a print medium, such as a sheet of paper, or a three dimensional object, such as an electronic board or bioassay. 
       FIGS. 3A and 3B  illustrate one example of a single inkjet ejector  10  that is suitable for use in an inkjet array of a print head. The inkjet ejector  10  has a body  48  that is coupled to an ink manifold  12  through which ink is delivered to multiple inkjet bodies. The body also includes an ink drop-forming orifice or nozzle  14  through which ink is ejected. In general, the inkjet print head includes an array of closely spaced inkjet ejectors  10  that eject drops of ink onto an image receiving member (not shown), such as a sheet of paper or an intermediate imaging member. 
     Ink flows from the manifold to nozzle in a continuous path. Ink leaves the manifold  12  and travels through a port  16 , an inlet  18 , and a pressure chamber opening  20  into the body  22 , which is sometimes called an ink pressure chamber. Ink pressure chamber  22  is bounded on one side by a flexible diaphragm  30 . A piezoelectric transducer  32  is rigidly secured to diaphragm  30  by any suitable technique and overlays ink pressure chamber  22 . Metal film layers  34  that can be coupled to an electronic transducer driver  36  in an electronic circuit can also be positioned on both sides of the piezoelectric transducer  32 . 
     Ejection of an ink droplet is commenced with a firing signal. The firing signal is applied across metal film layers  34  to excite the piezoelectric transducer  32 , which causes the transducer to bend. Upon actuation of the piezoelectric transducer, the diaphragm  30  deforms to force ink from the ink pressure chamber  22  through the outlet port  24 , outlet channel  28 , and nozzle  14 . The expelled ink forms a drop of ink that lands onto an image receiving member. Refill of ink pressure chamber  22  following the ejection of an ink drop is augmented by reverse bending of piezoelectric transducer  32  and the concomitant movement of diaphragm  30  that draws ink from manifold  12  into pressure chamber  22 . 
     To facilitate manufacture of an inkjet array print head, an array of inkjet ejectors  10  can be formed from multiple laminated plates or sheets. These sheets are configured with a plurality of pressure chambers, outlets, and apertures and then stacked in a superimposed relationship. Referring once again to  FIGS. 3A and 3B  for construction of a single inkjet ejector, these sheets or plates include a diaphragm plate  40 , an inkjet body plate  42 , an inlet plate  46 , an outlet plate  54 , and an aperture plate  56 . The piezoelectric-transducer  32  is bonded to diaphragm  30 , which is a region of the diaphragm plate  40  that overlies ink pressure chamber  22 . 
     One goal of print head design is to provide increasing numbers of inkjet ejectors in a print head. The more inkjet ejectors in a print head, the greater the density of the ink ejected and the perceived quality of the image. One approach to increasing inkjet ejector density in a print head is to locate the manifold external of the inkjet ejector. One way of implementing this approach includes providing an inlet in the diaphragm layer for each ejector. Coupling the inlet to the manifold to receive ink for ejection from the ejector, however, requires an opening in the piezoelectric-transducer layer to enable ink flow from the manifold to the inlet and then into the pressure chamber in the inkjet body plate. Each opening in the piezoelectric-transducer layer is located in a polymer portion in the interstices between the piezoelectric transducers. 
     In the assembly of previously known layered print heads having piezoelectric actuators, also known as piezoelectric transducers, the process of mounting the layer containing the piezoelectric actuators and polymeric interstitial material to the diaphragm layer requires the use of a liquid thermoset polymer prior to curing. This thermoset polymer spreads and enters the openings in the piezoelectric-transducer layer and the inlets in the diaphragm layer and then cures. When the polymer is subsequently cured, it can partially block the ink flow path at the inlet or body regions. Removal of the cured thermoset polymer from the ink inlets is difficult. A print head assembly method that enables the layer containing the piezoelectric actuators to be mounted to a diaphragm layer and that prevents the flow of uncured polymers into undesired locations of the ink path would be useful. 
     SUMMARY 
     A method for assembling an inkjet jet print head enables piezoelectric transducers to be bonded to an inkjet ejector without partially blocking or closing inlets to a pressure chamber within the inkjet ejector. The method includes bonding a polymer layer to a diaphragm layer having a plurality of openings formed in the diaphragm layer, bonding piezoelectric transducers to the diaphragm layer with a thermoset adhesive, placing thermoset polymer in areas between the piezoelectric transducers on the diaphragm layer, and drilling inlets through the thermoset adhesive and the polymer layer at pre-existing holes in the diaphragm layer. In one embodiment, the drilling is done with a laser. 
     The method produces piezoelectric print heads in which the location of thermoset adhesive has been controlled by the presence of the polymer layer covering pre-existing holes in the diaphragm layer. The blocking polymer layer is on the side of the diaphragm opposite the piezoelectric transducers and the thermoset polymer. The piezoelectric print head includes a body layer in which a plurality of pressure chambers is configured, a diaphragm plate having a plurality of openings, a polymer layer interposed between the body layer and the diaphragm plate, a plurality of piezoelectric transducers bonded to the diaphragm plate, thermoset polymer filling a region between the piezoelectric transducers bonded to the diaphragm plate, and openings through the polymer layer and thermoset polymer that align with the openings in the diaphragm plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of forming inlets through a polymer layer and thermoset polymer filling interstitial space between piezoelectric transducers are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1A  is a profile view of a partially completed inkjet print head including a diaphragm layer, body layer, and a polymer layer. 
         FIG. 1B  is a profile view of the same partial inkjet print head of  FIG. 1A  additionally including piezoelectric transducers bonded to the diaphragm layer. 
         FIG. 1C  is a profile view of the same partial inkjet print head of  FIG. 1B  further including thermoset polymer filling an interstitial area between the piezoelectric transducers. 
         FIG. 2A  is a profile view of the completed assembly of  FIG. 1C  after the assembly is bonded to an electrical circuit board and ink channels have been ablated. 
         FIG. 2B  is a profile view of a complete inkjet head including an outlet plate attached to the body layer and an ink manifold attached to a rigid or flexible electrical circuit layer. 
         FIG. 3A  is a schematic cross sectional side view of a prior art embodiment of an inkjet. 
         FIG. 3B  is a schematic view of the prior art embodiment of the inkjet of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that performs a print outputting function for any purpose, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, biological assays, printed organic electronics, mask making, 3D structure building, etc. The word “ink” can refer to wax-based inks known in the art but can refer also to any fluid that can be driven from the jets including water-based solutions, solvents and solvent based solutions, and UV curable polymers. The word “polymer” encompasses any one of a broad range of carbon-based compounds formed from long-chain molecules including thermoset polyimides, thermoset adhesives, thermoplastics including thermoplastic polyimides, resins, polyetherether ketone, polyetherimide, polysulfone, polycarbonates, and many other compounds known to the art. The word “metal” may encompass either single metallic elements including, but not limited to, copper, aluminum, or titanium, or metallic alloys including, but not limited to, stainless steel or aluminum-manganese alloys. A “transducer” as used herein is a component that reacts to an electrical signal by generating a moving force that acts on an adjacent surface or substance. The moving force may push against or retract the adjacent surface or substance. 
       FIG. 1A  is a profile view of a partially completed inkjet print head including a diaphragm layer  104 , body layer  111 , and a thermoplastic polymer layer  108 . The diaphragm layer  104  may be formed from a metal, ceramic, glass, or plastic sheet that has one or more ink ports  116  that extend through the layer, with one ink port corresponding to each pressure chamber  120  in the body layer  111 . The diaphragm plate should be thin enough to be able to flex easily, but also resilient enough to return to its original shape after it has been deformed. The diaphragm layer is bonded to a polymer layer, which is bonded as an unbroken sheet. DuPont ELJ-100® is an example of a material that is suitable to form the polymer layer. The polymer layer may also be formed from a polyimide material or other polymers including polyetherether ketone, polysulfone, polyester, polyethersulfone, polyimideamide, polyamide, polyethylenenaphthalene, etc. The polymer layer can be a self-adhesive thermoplastic or have a thin layer of adhesive deposited on the side of the polymer layer that is placed in contact with the body layer  111 . Alternatively, another thermoplastic or thermoset adhesive could be used to bond the polymer layer to the diaphragm. In yet further alternatives the adhesive could be a dispensed or transfer film of liquid adhesive. 
     The body layer is bonded to the opposite side of the polymer layer. The fluid path layer may be formed from one or multiple metal sheets that are joined via brazing as shown here as the body plate  111  and the outlet plate  112 . The fluid path layer could also be made from a single structure molded, etched or otherwise produced. The fluid path layer contains openings or channels through the various layers that form paths and cavities for the flow of ink through the finished print head. A pressure chamber is structured with diaphragm layer  104  and polymer layer  108  forming the top portion, the body plate  111  and the outlet plate  112  forming the fluid body layer and providing the lateral walls and base for the pressure chamber. The chamber base has an outlet port  124  that allows ink held in the pressure chamber to exit the body layer when the diaphragm is deformed by a piezoelectric transducer (not shown). 
       FIG. 1B  is a profile view of the same partial inkjet print head of  FIG. 1A  additionally including bonded piezoelectric transducers. In this view, a piezoelectric transducer  132  has been bonded to the diaphragm plate  104  in alignment with the pressure chamber  120 . In order to bond the piezoelectric transducers to the appropriate locations, they are first arranged on a carrier plate (not shown) with the sides opposite the diaphragm plate temporarily affixed to the carrier plate. Then, a thermoset polymer, typically an epoxy, is deposited on the surface of the diaphragm sheet. The carrier plate is aligned with the diaphragm plate, and pressure and heat are applied until the thermoset polymer has bonded the piezoelectric transducers to the diaphragm plate. The carrier plate is then released using known techniques from the piezoelectric transducers. The pressure from the bonding process squeezes excess thermoset polymer  128  from under the piezoelectric elements, leaving residual adhesive on the exposed diaphragm, some of which may flow into the ink ports  116 . Flow of the bonding adhesive is stopped at the polymer bonding layer  108 . The piezoelectric transducers are now rigidly bonded to the diaphragm plate so that when one of the piezoelectric transducers deforms, the diaphragm plate deforms in the same direction. 
       FIG. 1C  is a profile view of the same partial inkjet print head of  FIG. 1B  further including an interstitial polymer layer  136  formed between the piezoelectric transducers. This layer fills in the spaces between piezoelectric transducers including the pre-existing openings in the diaphragm layer. The interstitial polymer can be deposited as an uncured liquid by a number of means including flowing, dispensing or capillary filling. To ensure filling of the interstitial space, the thermoset polymer is added until it covers or partially covers the transducer upper surface. The thermoset polymer is then cured. In some embodiments, a thin sheet of non-stick polymer, such as polytetrafluoroethylene (commonly referred to as PTFE and sold commercially as Teflon®), may be applied to the upper surface of the thermoset polymer before curing to planarize the surface. This PTFE layer is then removed after curing. Alternatively, a UV curable polymer could be used for the interstitial fill and then a UV light used to cure the polymer. After curing of the thermoset polymer and the removal of the PTFE sheet, if used, the piezoelectric transducers are electrically isolated by the cured thermoset polymer alone or the cured thermoset polymer and non-stick coating. The piezoelectric transducers are cleaned via laser ablation or reactive ion etching to remove the polymer film from upper transducer surface  140 . The ink inlet holes are then drilled through the multiple polymer layers and through the pre-existing openings in the diaphragm. 
       FIG. 2A  is a profile view of the completed assembly of  FIG. 1C  after the inkjet ejector is bonded to an electrical circuit board (ECB)  252  and the ink inlets have been ablated. In one embodiment, a laser is used to drill the ink passages  262  through the polymer layer  208 , any interstitial polymer  236 , and an electrical standoff layer  244 . Many laser drilling processes can be used to form the ink passages through these layers. In one process an excimer laser illuminates a lithography mask with transparent regions corresponding to one or several of the ink passages that are to be drilled through the polymers. The laser illuminated mask openings are positioned on an exposed layer on the print head in alignment with the locations for the desired openings in the layer. The mask is then imaged onto the exposed surface. The substrate is then moved under a laser imaging system in a step and repeat process. Excimer lasers at 248 nm or 308 nm with laser fluence of 250 mJ/cm 2 -800 mj/cm 2  are suitable parameters though other laser wavelengths and fluencies may be used. Alternatively, a scanned laser beam may be used to drill individual ink passages. In this alternative process, the laser can be scanned with galvanometer-driven mirrors and focused onto the substrate with a scan lens. The ink passages can be generated with a beam at a fixed position to produce each hole or it can be scanned in a circle or other shape to form each ink passage through the polymer layers. Preferred lasers for the scanned laser drilling include a solid state laser or a fiber laser at 355 nm or a CO 2  laser having a 9.4-10.6 μm wavelength. 
     In  FIG. 2A , another layer of electrical insulator material, or standoff layer  244 , has been bonded to the piezoelectric layer  210 . The standoff layer has gaps  246  in its surface that correspond to the locations of the piezoelectric transducers  232 . These gaps allow the piezoelectric transducers to expand in a direction away from the pressure chamber  220 . A flexible, electrically conductive epoxy  248  is placed into the gaps to connect the electrically conductive traces  256  etched in the ECB  252  to the piezoelectric transducer surface electrodes  240 . Pre-existing holes  263  in the ECB  252  are larger than the ink passages  262  and aligned with the ink passages so that the ink path is not interrupted by the circuit board  252 . In another embodiment, the circuit board can be replaced by a flexible circuit having electrical pads aligned to the array of piezoelectric elements similar to the ECB. For the flexible circuit pre-existing holes for ink passages can exist, or in one embodiment, the ink passages are formed in the laser drilling process that forms the ink passage  262 . As further described below, the full printhead assembly and order of layer processing can happen in many different orders so long as the polymer layer  208  is attached to the diaphragm  204  prior to the piezoelectric elements  232  and interstitial polymer  236  being added to the assembly. 
       FIG. 2B  is a profile view of a complete inkjet head including an aperture plate  272  attached to the outlet plate  212  by aperture plate adhesive  268 . The manifold  264  acts as an ink reservoir supplying ink to the inlets of one or more pressure chambers, and each pressure chamber has a dedicated ink inlet connected to the manifold. The body layer  211  is attached to an outlet layer  212  to form a portion of each pressure chamber. The aperture plate adhesive  268  includes an outlet channel  270  corresponding to each pressure chamber. The aperture plate  272  may be formed from metal or a polymer and has apertures or nozzles  274  extending through the plate to allow ink to exit the print head as droplets. 
     Other embodiments may have different numbers of layers or combine several functions into a single layer such as having a thin adhesive layer directly on the aperture plate that permits attachment of the aperture plate to the outlet plate  212 . Other assembly and processing orders are also possible. For instance, polymer layer  208  can be bonded to the diaphragm  204  followed by the bonding of the piezoelectric elements  232  to the diaphragm and the adding and curing of the interstitial polymer  236 . The inlets  262  can then be drilled prior to the bonding of a completed fluid stack consisting of a diaphragm  204 , polymer layer  208 , body plate  211 , outlet plate  212 , aperture plate adhesive  268 , and aperture plate  272 . Finally the electrical interconnection  248 ,  252 ,  256  can be completed and the manifold  264  added. Other combinations of these assembly orders are also possible. 
     In operation, ink flows from the manifold through ECB channel  263  and the inlet port  262  into the pressure chamber  220 . An electrical firing signal sent to the piezoelectric transducer  232  via conductive traces  256  and conducting epoxy  248  or other means of producing the electrical connection  248  causes the piezoelectric transducer to bend, deforming the diaphragm  204  and polymer layer  208  into the pressure chamber. This deformation urges ink out the outlet port  224 , into the outlet channel  270 , and through the nozzle  274  where the ink exits the print head as a droplet. After the ink droplet is ejected, the chamber is refilled with ink supplied from the manifold with the piezoelectric transducer aiding the process by deforming in the opposite direction to cause the concomitant movement of the diaphragm and polymer layers that draw ink from the manifold into the pressure chamber. 
     It will be appreciated that various 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.