Abstract:
An ink printing acoustic print head. The ink printing acoustic print head prints ink on a print medium. The ink printing acoustic print head has a fluid distribution manifold and a fluid distribution stack in fluid communication with the fluid distribution manifold. A front membrane has a print work area. The front membrane is sealed to the fluid distribution stack. An acoustic array having droplet emitters is fixedly located between the front membrane and the fluid distribution stack with the droplet emitters facing the print work area. Ink is transferred from the fluid distribution manifold through the fluid distribution stack and to the print work area. Droplets of ink are emitted from the print work area by selectively energizing the droplet emitters.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an acoustic ink printer and, more particularly, to an acoustic ink printer print head.  
           [0002]    AIP (Acoustic Ink Printing) is a method for transferring ink directly or via intermediate means such as a transfer belt, drum, or roll, to a recording medium with several advantages over other direct printing methodologies. Such advantages include that AIP does not need small nozzles and ejection orifices that have caused many of the reliability and picture element or pixel placement accuracy problems which conventional drop-on-demand and continuous-stream ink jet printers have experienced. Since AIP avoids the clogging and manufacturing problems associated with drop-on-demand, nozzle-based ink jet printing, it represents a promising direct marking technology. More detailed descriptions of the AIP process can be found in U.S. Pat. Nos. 4,308,547, 4,697,195, 4,801,953, 5,028,937, 5,229,793, 5,231,426 6,048,050, 6,200,491 and 6,217,151, all of which are incorporated herein by reference in their entirety. With AIP, bursts of focused acoustic energy emit droplets from the free surface of a liquid onto a recording medium. By controlling the emitting process as the recording medium moves relative to droplet emission sites, or as the droplet emission sites move relative to the recording medium, a predetermined image is formed. To be competitive with other printer types, acoustic ink printers need to produce high quality images at low cost. Print heads may be fabricated with a large number of individual droplet emitters using techniques similar to those used in semiconductor fabrication. While specific AIP implementations may vary, and while additional components may be used, each droplet emitter may include an ultrasonic transducer (attached to one surface of a body), a varactor for switching the droplet emitter on and off, an acoustic lens (at the opposite side of the body), and a cavity holding ink such that the ink&#39;s free surface is near the acoustic focal area of the acoustic lens. The individual droplet emitter is possible by selection of its associated row and column. Acoustic ink printing is subject to a number of manufacturing variables, including transducer piezo-electric material thickness, stress and composition variation; transducer loading effects due to wire bond attachment to the top electrode and top electrode thickness; ink channel gap control impacting acoustic wave focal point variations; aperture hole variations causing the improper pinning of the ink meniscus; RF distribution non-uniformity along the row electrodes, electromagnetic reflections on the transmission lines, variations in acoustic coupling efficiencies, and variations in the components associated with each transducer. Because of manufacturing constraints and where these variables cannot be sufficiently controlled, the variables can result in nonuniform print profiles. Acoustic ink printing requires precise positioning of the lenses with respect to each other on very closely spaced centers and precise positioning of the lenses with respect to the ink&#39;s free surface. The lenses may be chemically etched, molded or cast into materials or substrates such as alumina, silicon nitride and silicon carbide through the use of hot press or injection molding processes or otherwise. Accordingly, there is a desire to provide a print head design and manufacturing process where such arrays can be reliably and consistently manufactured to tight tolerances in large numbers and for reasonable cost.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with one embodiment of the present invention, an ink printing acoustic print head is provided. The ink printing acoustic print head is adapted to print ink on a print medium. The ink printing acoustic print head has a fluid distribution manifold and a fluid distribution stack in fluid communication with the fluid distribution manifold. A front membrane is provided having a print work area. The print work area provides an array of apertures that contain the fluid free surface. The front membrane is sealed to the fluid distribution stack. An acoustic array having droplet emitters is provided. The acoustic array is fixedly located between the front membrane and the fluid distribution stack with the droplet emitters facing the print work area. Ink is transferred from the fluid distribution manifold through the fluid distribution stack and to the print work area. Droplets of ink are emitted from the print work area by selectively energizing the droplet emitters.  
           [0004]    In accordance with another embodiment of the present invention, an ink printing acoustic ink print head is provided. The ink printing acoustic ink print head has a fluid distribution housing having a print work area. An acoustic array having droplet emitters is provided. The acoustic array is sealed to the fluid distribution housing with the droplet emitters facing the print work area. The fluid distribution housing has at least two different channels. The channels are adapted to distribute ink to at least two opposing sides of the print work area. Ink is transferred through the channels to the print work area. Droplets of ink are emitted from the print work area by selectively energizing the droplet emitters.  
           [0005]    In accordance with one method of the present invention, A method of manufacturing an acoustic ink print head is provided having a first step of providing an acoustic array having droplet emitters. A fluid distribution stack and a front membrane having a print work area are then provided. The acoustic array is then fixedly located relative to the front membrane with tooling features on the fluid distribution stack. The acoustic array and the front membrane are then coupled to the fluid distribution stack. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein:  
         [0007]    [0007]FIG. 1A is a side view of an acoustic ink print head assembly;  
         [0008]    [0008]FIG. 1B is an exploded front view of an acoustic ink print head;  
         [0009]    [0009]FIG. 2A is a top view of a top plate;  
         [0010]    [0010]FIG. 2B is a top view of a middle plate;  
         [0011]    [0011]FIG. 2C is a top view of a back plate;  
         [0012]    [0012]FIG. 3A is a view of stack assemblies on a tooling plate;  
         [0013]    [0013]FIG. 3B is an inverted top view of a stack assembly;  
         [0014]    [0014]FIG. 4A is a top view of a bridge plate;  
         [0015]    [0015]FIG. 4B is a top view of a stack assembly with a bridge plate and printed wiring board;  
         [0016]    [0016]FIG. 5A is an inverted bottom view of a manifold;  
         [0017]    [0017]FIG. 5B is an inverted bottom view of a manifold with adhesive;  
         [0018]    FIGS.  6 A- 6 B are respectively a detail elevation and bottom view of a manifold datum feature;  
         [0019]    [0019]FIG. 7A is a top view of an assembly fixture;  
         [0020]    [0020]FIG. 7B is an isometric view of a top plate barb feature;  
         [0021]    [0021]FIG. 8 is a front exploded view of a stack to manifold assembly and assembly fixture;  
         [0022]    [0022]FIG. 8A is a schematic cross sectional view of the stack to transducer plate assembly and another assembly fixture;  
         [0023]    [0023]FIG. 9 is a bottom view of a print head assembly with spacers;  
         [0024]    [0024]FIG. 10 is a front view of a print head showing ink flow distribution;  
         [0025]    [0025]FIG. 11A is a top view of a print head showing ink distribution;  
         [0026]    [0026]FIG. 11B is a front view of a print head showing ink distribution;  
         [0027]    [0027]FIG. 11C is a side view of a print head showing ink distribution;  
         [0028]    [0028]FIG. 11D is a section view of a print head. 
     
    
     DETAILED DESCRIPTION  
       [0029]    Referring to FIG. 1A, there is shown, a side view of an acoustic ink print head assembly  11  incorporating features of the present invention. Although the present invention will be described with reference to the embodiments shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used.  
         [0030]    Acoustic ink print head assembly  11  generally comprises housing  14  with ink supply barb  16 , ink supply barb  18 , electrical control interface  20  and print head work area  22 . In order to form a predetermined image, the emitting process is controlled at electrical control interface  20  such that a predetermined droplet emission pattern is emitted from the droplet emission sites of print head work area  22  as the recording medium moves relative to print head  11 , or as print head  11  move relative to the recording medium. The description above is merely intended to be exemplary. More or less features could also be provided. For example, any suitable ink supply connection or reservoir could be provided instead of barbs  16  and  18 . As a further example, different shape or orientation of the print head work area or otherwise may be provided.  
         [0031]    Referring to FIG. 1B, there is shown, an exploded front view of an acoustic ink print head assembly  11  incorporating features of the present invention. Acoustic ink print head assembly  11  generally comprises a manifold  26 , bridge  28 , stack assembly  30 , substrate or acoustic array  32  and front plate, membrane or LLC  34 . Manifold  26  is disposed in housing  14  and has passages  36 ,  38  (shown in dotted line) that allow ink to flow through manifold  26  from barbs  16 ,  18 . Bridge plate  28  provides a mounting interface for interconnect  20  such that interconnect  20  may be electrically or otherwise connected to substrate or acoustic array  32 . Stack  30  locates substrate  32  and LLC  34  relative to each other in addition to providing a fluid path from passages  36 ,  38  to substrate  32  and LLC  34 . Stack assembly  30  has top plate  40 , middle plate  42  and back plate  44 . The description above is merely intended to be exemplary. More or less features could also be provided. For example, more or less plates, paths or different sized components may be provided.  
         [0032]    Referring now also to FIG&#39;S.  2 A,  2 B and  2 C, there is shown top views of top plate  40 , middle plate  42  and back plate  44 . The sheet metal stack  30  consists of plates  40 ,  42  and  44  which are manufactured as a sheet metal laminate which is readily adaptable to form reentrant structures such as fluid passages, etc. Fine blanking, precision punching, or chemical milling can produce inexpensive parts with high quality which may be bonded together to make such structures. In alternate embodiments, other suitable materials or manufacturing operations may be used such as with molded polymers or otherwise. External features on these structures can be used to provide datum edges that may, for example, locate the structure to other parts of the print head, locate the parts in automated production tooling, locate the print heads in the final product or otherwise. Stack assembly  30  (see FIG. 1B) may be a brazed stack where only middle plate  42  needs to be plated with brazing material. Top plate  40 , middle plate  42  and back plate  44  made be made from materials with the same or similar coefficients of thermal expansion. In alternate embodiments, more or less plates may be provided or alternative manufacturing processes may be employed such as utilizing adhesives or utilizing one piece molding or casting processes. In alternate embodiments, materials such as plastics, ceramics, composites or other suitable materials may be used.  
         [0033]    Referring now to FIG. 2A, there is shown a top view of top plate  40 . Top plate  40  has opening  50  and barbs  52  and  54 . Opening  50  has passages  56  and  58  and locating surfaces  60 ,  62 ,  64 ,  66 ,  68  and  70 .  
         [0034]    Referring now to FIG. 2B, there is shown a top view of middle plate  42 . Middle plate  42  has opening  76  and openings  78  and  80 . Opening  76  has passages  82  and  84  and locating surfaces  86 ,  88 ,  90 ,  92 ,  94 , and  96 . Recessed surfaces  98 ,  100  and  102  are also provided.  
         [0035]    Referring now to FIG. 2C, there is shown a top view of back plate  44 . Back plate  44  has opening  110  and openings  112  and  114 . Back plate  44  has passages  116 ,  118 ,  120  and  122  locating datum surfaces  124 ,  126  and  128  are also provided. In the embodiment shown, locating datum surfaces  124 ,  126  and  128  are provided on back plate  44  such that these datum surfaces provided for drop in installation of stack  30  (see FIG. 1B) in a printer carriage. In alternate embodiments, more or less datum surfaces may be provided in alternate locations on the parts or on alternate parts. In alternate embodiments, datum surfaces may also be used for tooling. When stack  30 , shown in FIG. 1B, is assembled, recessed surfaces  98 ,  100  and  102  of middle plate  42  are recessed sufficiently to ensure that datum surfaces  124 ,  126  and  128  can contact tooling or locating features.  
         [0036]    Referring now to FIG. 3B is an inverted top view of the stack assembly  30 . As noted before, the sheet metal stack  30  consists of plates  40 ,  42  and  44 . Stack assembly  30  may be a brazed stack where only middle plate  42  (see FIG. 2B) may be plated with brazing material. The tabs  52 ,  54  in conjunction with the slots or openings  112 ,  114  (and  78 ,  80  see FIG. 2B) provides for ease of assembly and repeatable location of plates  40 ,  42  and  44  relative to each other. Additionally, plates  40 ,  42  and  44  may be symmetrical as shown such that the parts may be assembled in a failsafe manner. The tabs  52 ,  54  in conjunction with the slots or openings  112 ,  114  (and  78 ,  80  on plate  42 ) also provides for self tooling of stack  30  in the brazing process such that the location of plates  40 ,  42  and  44  are held in position relative to each other during brazing of the laminated stack. Top plate  40 , middle plate  42  and back plate  44  may be made from materials with the same or similar coefficients of thermal expansion. When the stack  30  is heated for brazing, the tab and opening or slot design allows the stack  30  to expand or contract on its own without interference of any external tooling.  
         [0037]    Referring now to FIG. 3A there is shown a view of stack assembly  30  along with an array of stack assemblies similar to stack assembly  30  on a tooling plate. Tooling plate  136  may be used for a process such as brazing of stack  30  or a plurality of such stacks. Tooling plate  136  may have holes  138 ,  140  such that stack  30  is located by tabs  52 ,  54  (projecting down in FIG. 3A) in holes  140 ,  138  respectively. Holes  138 ,  140  may be over sized relative to tabs  52 ,  54  allowing for free expansion or contraction of stack  30  during the brazing process. Tooling plate  136  may be fabricated out of high temperature material such as graphite or ceramic and can be drilled and ground flat providing for easy loading and providing for flat assemblies during and after the brazing process. Tooling plate  136  may be fabricated out of a material that has a different coefficient of thermal expansion than top plate  40 , middle plate  42  or back plate  44  to prevent tool bonding. In alternate embodiments, stack  30  may be assembled with adhesives instead of by brazing.  
         [0038]    Referring now to FIG. 4A there is shown a top view of bridge plate  28 . Bridge plate  28  has openings  144 ,  146  and  148  and slots or openings  150  and  152 . Recessed surfaces  154 ,  156  and  158  are also provided. The bridge plate  28  supports the electrical interface between the glass or substrate or acoustic array  32  (see FIG. 1B) and other electrical components located in the manifold body or attached to the or part of the print head.  
         [0039]    Referring now to FIG. 4B there is shown a top view of stack assembly  30  with bridge plate  28 , glass or substrate or acoustic array  32  and interconnect  20  assembled. Circuit board or interconnect  20  is mounted to bridge plate  28  by bonding or otherwise. Bridge plate  28  may be bonded with adhesive or otherwise to stack  30  locating on tabs  52 ,  54  as shown. Interconnect  20  may be electrically or otherwise connected to glass or substrate or acoustic array  32  by wire bonding or otherwise. End  162  of circuit board  20  may be bent ninety degrees out of the page about axis  160 - 160  such that the assembly may be coupled to manifold  26  (see FIG. 1B).  
         [0040]    Referring now to FIG. 5A there is shown an inverted bottom view of manifold  26 . Referring also to FIG. 5B there is shown an inverted bottom view of manifold  26  with adhesive  166  dispensed thereon. Manifold  26  has passages  36 ,  38  and  37 ,  39  (see FIG. 1B) that allow ink to flow through manifold  26  from barbs  16 ,  18  (not shown) to stack assembly  30  (not shown). Protruding datum features  168 ,  170 ,  172  and  174  may be provided in conjunction with holes  176 ,  178  to allow location of stack assembly  30  relative to manifold  26 . The protruding datum features may be substantially similar to each other. Protruding datum features  168 ,  170 ,  172  and  174  may be pressed against the stack  30  during the assembly process. Prior to assembly of stack  30  to manifold  26 , adhesive  166  may be dispensed as shown in FIG. 5B on manifold  26  in order to further enable fastening of stack  30  to manifold  26  and/or sealing of stack  30  to manifold  26  such that passages  38  and  39  may be in fluid communication with openings or passages of stack  30  to allow ink flow (see FIG. 11B). Adhesive  166  may be dispensed by robotic means or otherwise using a thixotropic adhesive or other suitable adhesive. Adhesive  166  may become the fluid seal between the manifold  26  and stack  30 . In alternate embodiments, other suitable methods of fastening or sealing may be provided. Holes  176 ,  178  may be provided to allow tabs or barbs  16 ,  18  to be fastened to manifold  26  by ultrasonic welding or other suitable means. The stack/bridge plate assembly is placed in tooling with the circuit board  20  bent up ninety degrees as previously described in order to pass through opening  180  through manifold  26  upon assembly. The manifold is then placed in the tooling and ultrasonics or other suitable fastening and locating means may be used to drive the parts together as well as define additional datum surfaces  187  (see FIG. 5A) as will be described further below.  
         [0041]    Referring now to FIG&#39;S.  6 A and  6 B there is shown respectively a detail elevation and bottom view of a manifold datum feature  168  on manifold  26 .  
         [0042]    Referring now to FIG. 7A there is shown a top view of an assembly fixture  173 . Assembly fixture  173  is adapted to accept the print head prior to ultrasonic deformation. Assembly fixture  173  has a fixed nest  177  and a moveable portion  179  that allows part loading. Assembly fixture  173  additionally has datum surfaces  180  and  182 . Datum surfaces  180  and  182  are provided to generate datum surfaces  187  on the print head as will be described further below (such as due to ultrasonic deformation of datum features  168 ,  170 ,  172 ,  174  and of holes  176 ,  178 ) to provide precise print head to print head uniformity.  
         [0043]    Referring now to FIG. 7B, there is shown an isometric view of top plate  40  having a top plate barb feature  54  with protrusions, barbs or serrations as shown. The top plate barb feature  54  may be designed for interference fit into the respective holes  176 ,  178  of manifold  26  (see FIG. 5A).  
         [0044]    Referring now to FIG. 8, there is shown a front exploded view of a stack to manifold assembly with ultrasonic horn  186  and assembly fixture  173  used to show the ultrasonic assembly process. The forward assembly  30 ,  28 ,  20 ,  32  is placed in the assembly fixture  173  with reference feature or surface  184  against datum  180  and the barbs  52 ,  54  facing up. The molded manifold  26  is placed in the fixture with interference fit holes  176 ,  178  engaging over tabs  52 ,  54 . The ultrasonic horn  186  moves down capturing the tabs  52 ,  54  in holes  176 ,  178 , melting the interference material in the holes around the barbed feature, and melting the protruding datum points  168 ,  172 ,  170  and  174  (see FIG. 5A) against the stack  30  until reference feature or surface  187 - 187  rests against datum  182  of the nest. Alternately, hot plate, pin point, or other forms of plastic heating, joining or otherwise may be used as alternatives to the ultrasonic process. In this manner, precise location of features on the manifold or otherwise may be provided relative to the front face of the print head assembly, even where part thickness variations, such as in the stack  30 , are to be accommodated. In this manner, secondary operations, such as costly grinding or machining, are avoided. In this manner, datum surfaces on the print head are provided to allow precise print head to print head uniformity. Print head to print head uniformity may be important where multiple image on image print heads are used, for example, in a printer cartridge with multiple colors where color to color alignment of ink drops is critical. The process described herein may be adapted for automated assembly and may be adaptable to any CRU or module that requires precision alignment.  
         [0045]    Referring now to FIG. 8A, there is shown a schematic cross-sectional view of the stack assembly  30  to glass plate  32  (transducer array) assembly and another assembly fixture  310 . Assembly fixture  310  may be included as part of fixture  173  (see FIG. 7) or may be an independent fixture as desired. As seen in FIG. 8A, assembly fixture  310  generally has first reference surface  314  and a second reference surface  312 . Reference surfaces  314 ,  312  of fixture  310  are displaced from each other by an offset e. Reference surfaces  312 ,  314  are formed by any suitable means, such as by machining or precision molding for example, to provide a precise offset e. Offset e is established so that the desired gap d (shown in FIG. 11D) is established upon mounting of the LLC  334  to the acoustic ink print head assembly as will be described in greater detail below. As seen in FIG. 8A, during mounting of the glass plate  32  to the stack assembly  30 , the stack assembly may be positioned onto fixture  310  so that front surface/datum  40 F is seated on reference surface  314 . The glass plate  32  is rested on reference surface  312  of the fixture  310 . FIG. 8A shows the glass plate  32  being mounted to the stack  30  before the stack  30  and manifold  26  are coupled to each other (as described previously) for example purposes only. In alternate embodiments, the glass plate may be coupled to the stack after mating the stack to the manifold.  
         [0046]    A bead of suitable adhesive  300  is dispensed into the stack assy  30 , specifically onto the inside surface  44 L of the backplate  44 . The transducer/glass  32  is placed by means of fixture into the cavity of the stack assembly  30  and seated against surface  44 L. Fiducials may be provided on surface  44 L to aid locating the glass plate  32  in the stack assembly cavity and ensure the transducer is disposed accurately with the work area of the print head. The adhesive can be cured in place by various methods, or tacked in place with a secondary adhesive such as UV cure with a post cure (batch) of the main adhesive. The adhesive absorbs tolerance variations between stack assembly  30  and glass plate  32 , and provides a solid bed for mounting and ink sealing.  
         [0047]    In order to mount the LLC  34  to the stack assembly  30  a very light coating of adhesive is applied to the outside surface  40 F of stack assembly  30 , by means of adhesive transfer or other precise application. This adhesive layer is thin, possibly a few microns, and may be of a thermosetting type formulation. The adhesive may dry to the touch after application, but is not yet cured. The LLC  34  (see FIG. 11D) is brought into contact using a fixture (not shown) that allows for alignment. The apertures or fiducials (not shown) of LLC  34  are aligned to the lenses of glass plate  32  prior to or after contact depending on the actual techniques employed. Machine vision or manual guidance may be used to provide the position feedback as desired. Once satisfactory alignment and contact is made, time, temperature and/or secondary tacking can be employed to immobilize the parts, join them together and form a seal for ink to flow within during operation.  
         [0048]    Referring now to FIG. 9, there is shown a bottom view of a print head assembly  11  with spacers. The dimension d between the LLC  34  and the substrate  32  (shown in FIG. 11D for example) must be held tightly in order to have effective use of the work area  22  of the print head  11 . Holding this gap tightly is difficult as ink in the gap is at negative pressure which may cause the LLC  34  to bow. Such bowing may degrade the quality of the print output. The amount of bow will vary with pressure, material thickness, material property, gap, and other manufacturing variables making it difficult to control. Precision spacers or combs  190 ,  192  may be added to control the gap between LLC or membrane  34  and the substrate  32  before LLC  34  is fastened to front plate  40  by adhesives or as part of the ultrasonic bonding process or otherwise. With this distance tolerance controlled, the complexity of related hardware may be simplified resulting in a reduction of cost. Spacers or combs  190 ,  192  may be retained in features  194 ,  196  or otherwise retained or fastened to the assembly. Spacers or combs  190 ,  192  may have fingers  198  that are staggered in order to prevent fluid flow or pressure loss anywhere in the work area of the print head. Spacers or combs  190 ,  192  may be precision molded from plastic or other suitable material or otherwise manufactured from suitable materials. By way of example, in the embodiment shown, the open span for membrane deflection is reduced from unsupported distance A (such as for example, about 0.770 inches) to unsupported distance B (such as for example, about 0.312 inches) in order to reduce the deflection of membrane or LLC  34 . In alternate embodiments, other unsupported distances may be provided.  
         [0049]    Referring now to FIG. 10 there is shown a front section view of a print head showing a nozzle effect at the hole to slot interface. Manifold  26  has fluid intake hole  36  drilled or otherwise molded in order to intersect a narrow radiused slot  38  in the manifold. This provides a bi-directional nozzle which allows a smooth transition of the ink or fluid  200  from a round to a wide flat fan shaped plenum in order to smoothly transition into the openings or slots  116 ,  118 ,  120  and  122  in the stack  30  (see FIG. 2C). The radiused shaped plenum  38  delivers a smooth flow of ink over the entire length of the printing area and also preventing potential for air entrapment. This aids in priming the head assembly with ink. By way of example, in the embodiment shown, a first time flow through of ink may be six seconds or less with bubbles purged. Attached to the plenum is a front piece ink delivery area  191  formed by the interface between the plenum  38  of manifold  26  to stack  30 , LLC  34  and substrate  32  (see FIG. 11C also).  
         [0050]    Referring now to FIG. 11A, there is a top view of a print head showing ink distribution. Referring also to FIG. 11B is a front view of a print head showing ink distribution. Referring also to FIG. 11C is a side section view of a print head showing ink distribution. Referring also to FIG. 11D is a enlarged section view of a print head showing the assembly. A secondary narrowing or constriction of the ink flow path as shown at  204  is provided after the ink delivery area. This narrowing or constriction is desirable to spread ink out and limit the fluid volume, equalize pressure, and enhance the uniformity of the ink flow over each nozzle. The secondary narrowing or constriction of the ink flow path as shown at  204  is formed as part of stack  30  where the passages  122 ,  116 ,  120 ,  118  of back plate  44  are offset from the passages  82  and  84  of middle plate  42 . In this manner, the re-entrant features of stack  30  are provided by simply narrowing a dimension in the middle plate  42 .  
         [0051]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.