Abstract:
Disclosed is a process for preparing an ink jet printhead which comprises: (a) providing a diaphragm plate having a plurality of piezoelectric transducers bonded thereto with an adhesive; (b) placing an encapsulant thin film on the piezoelectric transducers; and (c) applying heat, pressure, or a combination thereof to the encapsulant thin film to a degree sufficient to cause the encapsulant to encapsulate the adhesive.

Description:
BACKGROUND 
     Disclosed herein are piezoelectric ink jet printheads and methods for making them. 
     Ink jet systems include one or more printheads having a plurality of jets from which drops of fluid are ejected towards a recording medium. The jets of a printhead receive ink from an ink supply chamber or manifold in the printhead which, in turn, receives ink from a source, such as an ink reservoir or an ink cartridge. Each jet includes a channel having one end in fluid communication with the ink supply manifold. The other end of the ink channel has an orifice or nozzle for ejecting drops of ink. The nozzles of the jets can be formed in an aperture or nozzle plate having openings corresponding to the nozzles of the jets. During operation, drop ejecting signals activate actuators in the jets to expel drops of fluid from the jet nozzles onto the recording medium. By selectively activating the actuators of the jets to eject drops as the recording medium and/or printhead assembly are moved relative to one another, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium. 
     Piezoelectric ink jet printheads typically include a flexible diaphragm and a piezoelectric transducer attached to the diaphragm. When a voltage is applied to the piezoelectric transducer, typically through electrical connection with an electrode electrically coupled to a voltage source, the piezoelectric transducer deflects or bends, causing the diaphragm to flex which expels a quantity of ink from a chamber through a nozzle. The flexing further draws ink into the chamber from a main ink reservoir through an opening to replace the expelled ink. 
     Piezoelectric transducers are bonded to the diaphragm with an adhesive. If exposed to oxygen, this adhesive can degrade over time and compromise the bond integrity between the piezoelectric transducer and the diaphragm, thus impeding or preventing drop ejection. Adhesives that are both robust to oxidation and suitable for this application are difficult to obtain. 
     Other proposed solutions have additional difficulties. For example, filling the piezoelectric transducer area with a liquid adhesive or epoxy and subsequently curing it would require multiple steps and additional time, and would also require mixing and dispensing the liquid adhesive or epoxy, which frequently introduces air bubbles into the adhesive that would need to be removed before curing. Another possible solution, creating a perimeter seal around the entire piezoelectric transducer area, would require additional capital machinery in the form of dispense robots and expertise. 
     SUMMARY 
     Disclosed herein is a process for preparing an ink jet printhead which comprises: (a) providing a diaphragm plate having a plurality of piezoelectric transducers bonded thereto with an adhesive; (b) placing an encapsulant thin film on the piezoelectric transducers; and (c) applying heat, pressure, or a combination thereof to the encapsulant thin film to a degree sufficient to cause the encapsulant to encapsulate the adhesive. Also disclosed herein is a process for preparing an ink jet printhead which comprises: (a) providing a diaphragm plate having a plurality of piezoelectric transducers each having a plurality of surfaces, said piezoelectric transducers being bonded to the diaphragm plate on at least one surface with an adhesive, said piezoelectric transducers having at least one surface unbonded to the diaphragm plate, the unbonded surfaces of multiple piezoelectric transducers defining interstices therebetween; (b) placing an encapsulant thin film on at least one surface of the piezoelectric transducers not bonded to the diaphragm plate; and (c) applying heat, pressure, or a combination thereof to the encapsulant thin film to a degree sufficient to cause the encapsulant to flow into the interstices and encapsulate the adhesive. Further disclosed herein is an ink jet printhead comprising: (a) a diaphragm plate; (b) a plurality of piezoelectric transducers mounted on the diaphragm plate with an adhesive; (c) an encapsulant material encapsulating the adhesive; (d) a plurality of nozzles corresponding to the piezoelectric transducers and operatively connected thereto; and (e) an electrical circuit board operatively connected to the piezoelectric transducers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross sectional side view of an embodiment of an ink jet printhead. 
         FIG. 2  is a schematic view of the embodiment of the ink jet printhead of  FIG. 1 . 
         FIG. 3  is a profile view of a partially completed ink jet printhead, including a diaphragm layer, body layer, and a polymer layer. 
         FIG. 4  is a profile view of the same partial ink jet printhead of  FIG. 3  additionally including piezoelectric transducers bonded to the diaphragm layer. 
         FIG. 5  is a schematic exploded profile view of a partial ink jet printhead in a stack press during the manufacturing process. 
         FIG. 6  is a profile view of the completed assembly prepared as described in  FIG. 5  after the assembly is bonded to an electrical circuit board and ink channels have been ablated. 
         FIG. 7  is a profile view of a complete ink jet head including an outlet plate attached to the body layer and an ink manifold attached to a rigid or flexible electrical circuit layer. 
         FIG. 8  is a graph of surface depth across the width of the printhead prepared in Example III. 
         FIG. 9  is a profile view of the same partial ink jet printhead of  FIG. 4  additionally including encapsulant material pressed into the interstitial areas between the piezoelectric transducers. 
     
    
    
     Drawings are not to scale. 
     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, multi-function machine, or the like. Devices of this type can also be used in bioassays, masking for lithography, printing electronic components such as printed organic electronics, and making 3D models among other applications. The word “polymer” encompasses any one of a broad range of carbon-based compounds formed from long-chain molecules, including thermoset polyimides, thermoplastics, resins, polycarbonates, epoxies, or related compounds known to the art, as well as mixtures thereof. The word “ink” can refer to wax-based inks or gel-based inks known in the art and can also refer to any fluid that can be driven from the jets, including water-based solutions, solvents and solvent-based solutions, or UV-curable polymers, as well as mixtures thereof. The word “metal” encompasses single metallic elements, including those such as copper, aluminum, titanium, or the like, or metallic alloys, including those such as stainless steel alloys, aluminum-manganese alloys, or the like, as well as mixtures thereof. 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. 
       FIGS. 1 and 2  illustrate one example of a single ink jet ejector  10  suitable for use in an ink jet array of a printhead. The ink jet ejector  10  has a body  48  coupled to an ink manifold  264  through which ink is delivered to multiple ink jet bodies. The body also includes an ink drop-forming orifice or nozzle  274  through which ink is ejected. In general, the ink jet printhead includes an array of closely spaced ink jet 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  264  and travels through a port  116 , an inlet  262 , and a pressure chamber opening  120  into the ink pressure chamber  122 . Ink pressure chamber  122  is bounded on one side by a flexible diaphragm  30 . A piezoelectric transducer  132  is rigidly secured to diaphragm  30  by any suitable technique and overlays ink pressure chamber  122 . 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  132 . 
     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  132 , which causes the transducer to bend. Upon actuation of the piezoelectric transducer, the diaphragm  30  deforms to force ink from the ink pressure chamber  122  through the outlet port  124 , outlet channel  270 , and nozzle  274 . The expelled ink forms a drop of ink that lands onto an image receiving member. Refill of ink pressure chamber  122  following the ejection of an ink drop is augmented by reverse bending of piezoelectric transducer  132  and the concomitant movement of diaphragm  30  that draws ink from manifold  264  into pressure chamber  122 . 
     To facilitate manufacture of an ink jet array printhead, an array of ink jet 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. The embodiments shown in the Figures are illustrative, and sometimes more or fewer layers are employed to accomplish fluidic routing in a similar manner. 
     Referring once again to  FIGS. 1 and 2  for construction of a single ink jet ejector, these sheets or plates include a diaphragm plate or layer  104 , an ink jet body plate  111 , an inlet plate  46 , an outlet plate  112 , and an aperture plate  272 . The piezoelectric transducer  132  is bonded to diaphragm  30 , which is a region of the diaphragm plate  104  that overlies ink pressure chamber  122 . 
       FIG. 3  is a profile view of a partially completed ink jet printhead including a diaphragm plate or layer  104 , body layer  111 , and a thermoplastic polymer layer  108 . The diaphragm plate  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  122  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 can 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 printhead. 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. 4  is a profile view of the same partial ink jet printhead of  FIG. 3  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  122 . In order to bond the piezoelectric transducers to the appropriate locations, they are first arranged on a carrier plate (not shown in  FIG. 4 ) with the sides opposite the diaphragm plate temporarily affixed to the carrier plate. Then, an adhesive such as 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 adhesive 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. 
     The piezoelectric transducers have a plurality of surfaces, at least one of which is bonded to the diaphragm plate. Four surfaces (i.e., a tetrahedron) is the least number of surfaces a three-dimensional object can have. Typically, the piezoelectric transducers will have six surfaces, although other configurations and configurations with more surfaces are also possible. In a specific embodiment, the piezoelectric transducers are cube-shaped or tile-shaped (i.e., have six surfaces, or approximately so if the edges of the cubes or tiles are not perfectly sharp) and one surface thereof is bonded to the diaphragm plate with the adhesive. 
       FIG. 5  is a schematic exploded profile view of a partial ink jet printhead in a stack press during the manufacturing process. Piezoelectric transducers  132  have been bonded to diaphragm plate  104 , which in turn is situated on body plate  111 , by the method shown in  FIG. 4 . Encapsulant thin film  140  is situated on top of piezoelectric transducers  132 . Spacers  150  are situated on diaphragm plate  104 , and are of approximately the same height as piezoelectric transducers  132  so that encapsulant thin film  140  lies in an approximate plane across the array. The arrangement is situated in a stack press, having stack press lower cassette  160  and stack press upper platen  170 . A sacrificial layer of protective material  165 , such as TEFLON® or the like, is situated between stack press lower cassette  160  and the printhead. Disposable block  180 , part of the stack press, is situated between stack press upper platen  170  and encapsulant thin film  140 . Disposable block  180  is coated with mold release agent  185  or other suitable means for preventing adhesion thereto. 
     Encapsulant thin film  140  is formulated of a thin film encapsulant material, such as an oligomer, a polymer, or other suitable material. Examples of suitable encapsulant materials include polyamideimide resins, such as HITACHI KS6600, a siloxane modified polyamideimide resin available from Hitachi Chemical Co., Japan, or the like. 
     By “thin film” is meant a thin, continuous material in the form of a sheet or membrane. It can be oligomeric or polymeric or of another suitable material. 
     The thin film can be of any desired or effective thickness. In one embodiment, the thin film is roughly approximate in thickness to the thickness of the piezoelectric transducers. For example, if the piezoelectric transducers are 50 μm thick, the thin film is in one embodiment at least about 38 μm, in another embodiment at least about 43 μm, and in yet another embodiment at least about 48 μm, and in one embodiment no more than about 62 μm, in another embodiment no more than about 57 μm, and in yet another embodiment no more than about 52 μm. 
     The encapsulant material has a Young&#39;s modulus sufficiently low to minimize mechanical coupling or crosstalk between adjacent piezoelectric transducers, in one embodiment 2 gigaPascals or less, and in another embodiment 1 gigaPascal or less. 
     In one embodiment the encapsulant material is a thermoset material, curable at temperatures of in one embodiment at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 500° C., in another embodiment no more than about 400° C., and in yet another embodiment no more than about 300° C. 
     In another embodiment, the encapsulant material can be a thermoplastic material, particularly when the operating temperature of the printhead is below the melting point of the thermoplastic material. In this embodiment, the thermoplastic material can be subjected to temperatures similar to those suitable for curing the thermoset material. 
     The encapsulant material can be a gel, crystalline, semicrystalline, or amorphous, and mixtures of suitable materials can also be used; accordingly, suitable melting points, softening points, and glass transition points for specific embodiments will be provided. 
     The encapsulant material can have a melting point of in one embodiment at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 500° C., in another embodiment no more than about 400° C., and in yet another embodiment no more than about 300° C. 
     The encapsulant material can have a reflow point of in one embodiment at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 500° C., in another embodiment no more than about 400° C., and in yet another embodiment no more than about 300° C. 
     The encapsulant material can have a softening point of in one embodiment at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 500° C., in another embodiment no more than about 400° C., and in yet another embodiment no more than about 300° C. 
     The encapsulant material can have a glass transition temperature (T g ) of in one embodiment at least about 25° C., in another embodiment at least about 50° C., and in yet another embodiment at least about 100° C., and in one embodiment no more than about 500° C., in another embodiment no more than about 400° C., and in yet another embodiment no more than about 300° C. 
     The stack press is operated at a temperature and pressure and for a period of time sufficient to effect desirable flow characteristics of the encapsulant material. The temperature and pressure will depend on the specific material used as the encapsulant, and are generally provided with the encapsulant manufacturer&#39;s instructions. The temperature is generally above the Tg in the case of an amorphous material, and is below the melting point of a thermoplastic material. An example of suitable time, temperature, and pressure conditions for HITACHI KS6600 are 200 pounds per square inch (PSI) at 290° C. for 30 minutes. 
     Subsequently, the encapsulant can be cured. For example, the encapsulant can be cured at in one embodiment at least about 50 psi, in another embodiment at least about 150 psi, and in yet another embodiment at least about 180 psi, and in one embodiment no more than about 300 psi, in another embodiment no more than about 250 psi, and in yet another embodiment no more than about 220 psi. 
     The encapsulant can be cured at, for example, in one embodiment at least about 50° C., in another embodiment at least about 150° C., and in yet another embodiment at least about 180° C., and in one embodiment no more than about 350° C., in another embodiment no more than about 250° C., and in yet another embodiment no more than about 220° C. 
     The encapsulant can be cured for, for example, in one embodiment at least about 10 minutes, in another embodiment at least about 20 minutes, and in yet another embodiment at least about 30 minutes, and in one embodiment no more than about 200 minutes, in another embodiment no more than about 100 minutes, and in yet another embodiment no more than about 50 minutes. 
     For purposes of subsequent manufacturing steps, it is sometimes desirable that the encapsulant material fill the interstices between the piezoelectric transducers to a substantial extent, leaving relatively shallow valleys or no valleys between the transducers. The adhesive used to apply subsequent layers, such as the circuit (a flexible circuit in some embodiments) can then further fill these shallow remaining valleys without impairing the planar structure of the printhead. In these embodiments, the maximum remaining depth of the interstitial area between piezoelectric transducers after being filled with the encapsulant material is in one embodiment 25 μm or less, in another embodiment 15 μm or less, and in yet another embodiment 10 μm or less. 
       FIG. 6  is a profile view of the completed assembly prepared as described in  FIG. 5  after the ink jet 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  108 . 
     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  108  is attached to the diaphragm  104  prior to the piezoelectric elements  132  being added to the assembly. 
       FIG. 7  is a profile view of a complete ink jet head including an aperture plate  272  attached to the outlet plate  112  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  111  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 printhead as droplets. 
     Other embodiments may have different numbers of layers or combine several functions into a single layer. Other assembly and processing orders are also possible. 
     In operation, ink flows from the manifold through ECB channel  263  and the inlet port  262  into the pressure chamber  122 . An electrical firing signal sent to the piezoelectric transducer  132  in piezoelectric layer  210  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  104  and polymer layer  108  into the pressure chamber. This deformation urges ink out the outlet port  124 , into the outlet channel  270 , and through the nozzle  274  where the ink exits the printhead 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. 
       FIG. 9  is a view similar to that of  FIG. 4  showing the same partial ink jet printhead after the encapsulant material  140  has been pressed into the interstitial areas between piezoelectric transducers  132 . Note that a small amount of encapsulant material  140  is present on top of piezoelectric transducers  132 . 
     Specific embodiments will now be described in detail. These examples are intended to be illustrative, and the claims are not limited to the materials, conditions, or process parameters set forth in these embodiments. All parts and percentages are by weight unless otherwise indicated. 
     Example I 
     A partial printhead array was provided comprising a body plate of 316L stainless steel, a diaphragm layer of 316L stainless steel, and piezoelectric transducers in 20×84 array bonded to the diaphragm layer with lead zirconate titanate. 
     The partial printhead array was placed in a stack press in the configuration illustrated in  FIG. 5  and a thin film of HITACHI KS6600 38 μm thick was laid on top of the piezoelectric transducers. Heat and pressure were applied at 200 PSI at 290° C. for 30 min according to the manufacturer&#39;s recommended bonding parameters for high flow. No bubbles or voids were observed. The surface topography was not characterized. The tops of all of the piezoelectric transducer tiles were covered by the encapsulant material. 
     Example II 
     The process of Example I was repeated except that ADWILL D-624 ultraviolet release tape 90 μm thick, obtained from Lintec of America, was used instead of the HITACHI KS6600 as the encapsulant. Heat and pressure were applied at 100 PSI at 190° C. for 30 min. The material exhibited high flow characteristics and no bubbles or voids were observed. The surface topography was not characterized. The tops of all of the piezoelectric transducer tiles were covered by the encapsulant material. 
     Example III 
     The process of Example I was repeated except that ADWILL G-65 release tape 90 μm thick, obtained from Lintec of America, was used instead of the HITACHI KS6600 as the encapsulant. Heat and pressure were applied at 100 PSI at 190° C. for 30 min. The material exhibited high flow characteristics and no bubbles or voids were observed. The surface topography was characterized, and is illustrated in  FIG. 8 .  FIG. 8  is a graph of surface depth (y-axis) across the width of the printhead subsequent to application of the encapsulant. As  FIG. 8  indicates, dips of only 5 μm were observed in the narrow interstices, and dips of 8 μm were observed in the wide interstices. Since wide interstices are not present in commercially fabricated printheads, it is believed that dips of 5 μm will be the maximum observed in commercially fabricated printheads. The tops of all of the piezoelectric transducer tiles were covered by the encapsulant material. 
     Other embodiments and modifications of the present invention may occur to those of ordinary skill in the art subsequent to a review of the information presented herein; these embodiments and modifications, as well as equivalents thereof, are also included within the scope of this invention. 
     The recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefor, is not intended to limit a claimed process to any order except as specified in the claim itself.