Patent Publication Number: US-9849677-B2

Title: Muzzle for printhead assembly

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/292,319, filed May 30, 2014, entitled MUZZLE FOR PRINTHEAD ASSEMBLY, the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present invention relates generally to inkjet printers, and more particularly, to printhead assemblies for inkjet printers. 
     BACKGROUND 
     An ink jet printer typically includes a printhead and a carrier. The ink jet printhead can comprise a printhead body, nozzles, and corresponding ink ejection actuators, such as heaters on a printhead chip. The actuators cause ink to be ejected from the nozzles onto a print medium at selected ink dot locations within an image area. The carrier moves the printhead relative to the medium, while the ink dots are jetted onto selected pixel locations, such as by heating the ink at the nozzles. 
     In some such systems, the ink reservoir comprises a removable or separable tank, such that the tank can be separated from the printhead, and replaced or refilled, when the ink is low. The printhead components can then be re-used. In such ink tank systems, a separable fluid connection between the tank and the printhead body is needed, in contrast to systems where the printhead body is integral with the ink reservoir. The connection permits ink to flow to the nozzles from the tank, but is separable such that the ink tank can be removed when empty. The printhead assembly can also include a filter within an ink passageway leading from the ink reservoir to the nozzles, for isolating any contaminants or debris from the ejectors and nozzles. 
     In the industrial market, the proliferation of digital printing is underway. This proliferation provides a unique opportunity for thermal inkjet technology, due to low cost points associated with the bill of materials (BOM) and manufacturing of thermal inkjet printers. The printhead requirements for the industrial market is different and more challenging due to the non-traditional inks being used. The ink chemistries, which are solvent UV curable and latex based, are formulated to wet, penetrate and adhere to non-porous medias (examples of the various substrates are mentioned above). Solvents that are typically used generally have lower surface tension compared to water and will wet lower surface energy surfaces/substrates. Another property that the solvent system provides is the ability of the solvent to cause interfacial diffusion of ink into the substrate allowing for improved adhesion and durability. This is critical due to the non-porous nature of the various substrates used in the industry and the fact that the printed media will be subjected to various environments. Ketones and acetates such as methyl ethyl ketone (MEK) or ethyl acetate are some of the most aggressive solvents used in solvent ink formulations. Currently MEK based inks provide a significant advantage over alcohol-based inks because of its ability to wet and adhere to various plastic (polyolefin base substrates) in a variety of packaging applications/markets. 
     SUMMARY OF THE INVENTION 
     Currently, there is not a thermal inkjet printhead that can withstand the aggressive nature of MEK. Accordingly, an object of the present invention is to provide an inkjet printhead that can store and deliver MEK based inks to a substrate. 
     Due to the nature of the design of the MEK jetting printhead of the present invention, there is a need to completely seal the printhead during shipping so as to prevent leakage of the solvent into the shipping materials. Thus, another object of the present invention is to provide an inkjet printhead that exhibits a good seal during normal shipping environments. 
     Other features and advantages of embodiments of the invention will become readily apparent from the following detailed description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of exemplary embodiments of the present invention will be more fully understood with reference to the following, detailed description when taken in conjunction with the accompanying figures, wherein: 
         FIG. 1  is a perspective view of a conventional inkjet printhead; 
         FIG. 2  is a perspective view of a conventional inkjet printer useable with the inkjet printhead assembly according to an exemplary embodiment of the present invention; 
         FIG. 3  is an exploded perspective view of a printhead assembly according to an exemplary embodiment of the present invention; 
         FIG. 4  is a cross-sectional view taken along the line A-A of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along the line B-B of  FIG. 3 ; 
         FIG. 6  is an exploded perspective view of an ink reservoir according to an exemplary embodiment of the present invention; 
         FIG. 7A  is a perspective view of a muzzle cap according to an exemplary embodiment of the present invention; 
         FIG. 7B  is another perspective view of the muzzle cap of  FIG. 7A ; 
         FIG. 8  is a cross-sectional view of the muzzle cap of  FIG. 7A  taken along the line A-A; 
         FIG. 9  is a perspective view of a vent seal according to an exemplary embodiment of the present invention; 
         FIG. 10  is a perspective view of a nozzle plate seal retainer molded into a muzzle body according to an exemplary embodiment of the present invention; 
         FIG. 11  is a perspective view of a nozzle plate seal according to an exemplary embodiment of the present invention; 
         FIG. 12  is a perspective cross-sectional view of the nozzle plate seal of  FIG. 11  taken along the line B-B; 
         FIG. 13  is a perspective view of a nozzle plate seal retainer according to an exemplary embodiment of the present invention; 
         FIG. 14  is a cross-sectional view showing a nozzle plate seal and nozzle plate seal retainer assembled with a muzzle body according to an exemplary embodiment of the present invention; 
         FIG. 15  is another cross-sectional view showing a nozzle plate seal and nozzle plate seal retainer assembled with a muzzle body according to an exemplary embodiment of the present invention; 
         FIG. 16  is a cross-sectional view of a muzzle cap attached to a printhead assembly according to an exemplary embodiment of the present invention; and 
         FIGS. 17-19  are cross-sectional views showing assembly of a muzzle cap onto a printhead assembly according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the words “may” and “can” are used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including but not limited to. To facilitate understanding, like reference numerals have been used, where possible, to designate like elements common to the figures. 
       FIG. 1  shows an inkjet printhead generally designated by reference number  1101 . The printhead  1101  has a housing  1127  formed of a lid  1161  and a body  1163  assembled together through attachment or connection of a lid bottom surface and a body top surface at interface  1171 . The shape of the housing varies and depends upon the external device that carries or contains the printhead, the amount of ink to be contained in the printhead and whether the printhead contains one or more varieties of ink. In any embodiment, the housing or body has at least one compartment in an interior thereof for holding an initial or refillable supply of ink and a structure, such as a foam insert, lung or other, for maintaining appropriate backpressure in the inkjet printhead during use. In one embodiment, the internal compartment includes three chambers for containing three supplies of ink, especially cyan, magenta and yellow ink. In other embodiments, the compartment contains black ink, photo-ink and/or plurals of cyan, magenta or yellow ink. It will be appreciated that fluid connections (not shown) may exist to connect the compartment(s) to a remote source of bulk ink. 
     A portion  1205  of a tape automated bond (TAB) circuit  1201  adheres to one surface  1181  of the housing while another portion  1211  adheres to another surface  1221 . As shown, the two surfaces  1181 ,  1221  exist perpendicularly to one another about an edge  1231 . The TAB circuit  1201  has a plurality of input/output (I/O) connectors  1241  fabricated thereon for electrically connecting a heater chip  1251  to an external device, such as a printer, fax machine, copier, photo-printer, plotter, all-in-one, etc., during use. Pluralities of electrical conductors  1261  exist on the TAB circuit  1201  to electrically connect and short the I/O connectors  1241  to the bond pads  1281  of the heater chip  1251  and various manufacturing techniques are known for facilitating such connections. It will be appreciated that while eight I/O connectors  1241 , eight electrical conductors  1261  and eight bond pads  1281  are shown, any number are embraced herein. It is also to be appreciated that such number of connectors, conductors and bond pads may not be equal to one another. 
     The heater chip  1251  contains at least one ink via  1321  that fluidly connects to a supply of ink in an interior of the housing. Typically, the number of ink vias of the heater chip corresponds one-to-one with the number of ink types contained within the housing interior. The vias usually reside side-by-side or end-to-end. During printhead manufacturing, the heater chip  1251  preferably attaches to the housing with any of a variety of adhesives, epoxies, etc. well known in the art. As shown, the heater chip contains four rows (rows A-row D) of fluid firing elements, especially resistive heating elements, or heaters. For simplicity, dots depict the heaters in the rows and typical printheads contain hundreds of heaters. It will be appreciated that the heaters of the heater chip preferably become formed as a series of thin film layers made via growth, deposition, masking, photolithography and/or etching or other processing steps. A nozzle plate, shown in other figures, with pluralities of nozzle holes adheres over or is fabricated with the heater chip during thin film processing such that the nozzle holes align with the heaters for ejecting ink during use. Alternatively, the heater chip is merely a semiconductor die that contains piezoelectric elements, as the fluid firing elements, for electro-mechanically ejecting ink. As broadly recited herein, however, the term heater chip will encompass both embodiments despite the name “heater” implying an electro-thermal ejection of ink. Even further, the entirety of the heater chip may be configured as a side-shooter structure instead of the roof-shooter structure shown. 
       FIG. 2  shows an external device in the form of an inkjet printer for containing the printhead  1101 , generally designated by reference number  1401 . The printer  1401  includes a carriage  1421  having a plurality of slots  1441  for containing one or more printheads. The carriage  1421  is caused to reciprocate (via an output  1591  of a controller  1571 ) along a shaft  1481  above a print zone  1431  by a motive force supplied to a drive belt  1501  as is well known in the art. The reciprocation of the carriage  1421  is performed relative to a print medium, such as a sheet of paper  1521 , that is advanced in the printer  1401  along a paper path from an input tray  1541 , through the print zone  1431 , to an output tray  1561 . 
     In the print zone, the carriage  1421  reciprocates in the Reciprocating Direction generally perpendicularly to the paper Advance Direction as shown by the arrows. Ink drops from the printheads are caused to be ejected from the heater chip  1251  ( FIG. 1 ) at such times pursuant to commands of a printer microprocessor or other controller  1571 . The timing of the ink drop emissions corresponds to a pattern of pixels of the image being printed. Often times, such patterns are generated in devices electrically connected to the controller (via Ext. input) that are external to the printer such as a computer, a scanner, a camera, a visual display unit, a personal data assistant, or other. A control panel  1581  having user selection interface  1601  may also provide input  1621  to the controller  1571  to enable additional printer capabilities and robustness. 
     To print or emit a single drop of ink, the fluid firing elements (the dots of rows A-D,  FIG. 1 ) are uniquely addressed with a small amount of current to rapidly heat a small volume of ink. This causes the ink to vaporize in a local ink chamber and be ejected through the nozzle plate towards the print medium. The fire pulse required to emit such ink drop may embody a single or a split firing pulse and is received at the heater chip on an input terminal (e.g., bond pad  1281 ) from connections between the bond pad  1281 , the electrical conductors  1261 , the I/O connectors  1241  and controller  1571 . Internal heater chip wiring conveys the fire pulse from the input terminal to one or many of the fluid firing elements. 
     In order to operate within industrial printers, a printhead according to exemplary embodiments of the present invention must be able to accommodate ketone, acetate and alcohol based inks. For example, certain materials that are compatible with such inks may be selected for the body and lid of the printhead and internal features and the back pressure system of the printhead may be altered as compared to conventional printheads. 
       FIG. 3  is an exploded perspective view and  FIGS. 4 and 5  are cross-sectional views of a printhead assembly, generally designated as reference number  1 , according to an exemplary embodiment of the present invention. The printhead assembly  1  includes an ink cartridge body  10 , filter  20 , filter cap  30 , gasket  40 , in reservoir  50 , fill ball  60  and lid  70 . The ink cartridge body  110  includes a datum surface  13 . The ink cartridge body  10  has a chamber  12  that is sized and configured to receive the ink reservoir  50 . Although only one ink reservoir  50  is shown in the figures, it should be appreciated that multiple ink reservoirs may be provided to accommodate one or more color inks. The ink reservoir  50  includes an exit port  52  for delivery of the ink, once installed in the chamber  12 , and the port  52  can include an interface structure as appropriate, such as a lip or extension. The exit port  52  can be sealed using a removable seal, which can be removed at the time of installation. 
     Attached to the ink cartridge body  10  is a print head chip  11  including a plurality of nozzles for delivery of the ink to the print medium. In other embodiments, the nozzles are provided on a structure separate from the chip. The ink flows from the exit port  52  of the ink reservoir  50  through channels in the lower portion of the body  10 . The ink then flows within the body  10  to a manifold in the print head chip  11 , from which it is drawn to the nozzles for ejection onto the print medium, such as by using heater elements or piezoelectric elements formed in the chip  11 . The system  1  is moved relative to the print medium, such that the nozzles drop ink at one or more desired locations on the medium. 
     The lower portion of the ink cartridge body  10  includes a tower  14 . The tower  14  may include any appropriate extension, structure, port, or interface for receiving ink for printing. The tower  14  of this example includes a raised tubular extension, or standpipe, having one or more openings  15  through which the ink may flow. Other tower configurations are also possible as will be readily apparent to one of ordinary skill in the art. 
     As shown in  FIGS. 4 and 5 , the filter cap  30  engages the tower  14 , and in particular may be welded to an upstanding outer perimeter wall of the tower  14 . The filter cap  30  includes a conduit or guide component for providing a passage between the ink cartridge body  10  and the ink reservoir  50 . In this example, the filter cap  30  includes an inner passage  32  for providing ink therethrough, the passage  32  being defined by a smaller diameter upper passage portion  34  at the ink reservoir end and a larger diameter lower passage portion  36  at the ink cartridge body end. The filter cap  30  may be made of a polyamide, such as, for example, nylon, or other suitable materials that can provide a fluid resistant seal against the tower  14 , ink cartridge body  10 , and/or ink reservoir  50 . 
     The upper passage portion  34  of the filter cap  30  engages a corresponding exit port  52  of the ink reservoir  50  to allow ink to flow from the ink reservoir  50  to the passage  32  of the filter cap  30 . A sealing member is disposed adjacent the filter cap  30  and assists in sealing between the filter cap  30  and the ink reservoir  50 . In this example, the sealing member includes the gasket  40  that engages the upper passage portion  34 , so as to create a fluidic seal to control fluid and evaporative losses from the system, and prevent air from entering the system to maintain back pressure. The gasket  40  may be made of a suitable elastomer material, or other material with good sealing properties. 
     The filter  20  filters contaminants in the ink from reaching the printhead chip. The filter  20  can also provide capillary functions to allow ink to pass upon demand to the printhead chip and to prevent air passage into the printhead chip. The filter  20  can be made of a metal weave, a polymer weave, or other mesh, screen, or weave materials. For instance, a stainless steel dutch twill or a stainless steel random weave material may be used to form the filter  20 . The filter  20  may be insert injection molded in the tower  14 , or otherwise disposed in the ink cartridge body  10 . As another example, the filter  20  may be heat staked to the ink cartridge body  10 . 
     The material used to form the ink cartridge body  10  and associated lid  70  may be, for example, nylon (e.g., Nylon 6,6, Nylon 6, Nylon 6,12), polyethersulfone, polypropylene, polyethylene, polyoxymethylene or other materials that are compatible with ketone, acetate and alcohol based inks. Since these materials exhibit vapor loss through permeation, a secondary boundary may be provided in the form of the ink reservoir  50 . In this regard, the ink reservoir  50  may be made of polypropylene and/or polyethylene based materials so as to create a sufficient permeation barrier. The ink reservoir  50  is also provided to serve as a back pressure device since conventional back pressure devices are made of foam or felt materials, which are easily attacked by ketone, acetate and alcohol based inks. The ink reservoir  50  provides the primary permeation boundary for the ink cartridge body  10  and when the ink reservoir  50  is attached internally to the ink cartridge body  10  and lid  70 , a tortuous vent path is created having a high length to area ratio. This tortuous path allows air to move through it, while maintaining a high humidity environment, which reduces evaporative losses and greatly reduces permeation from the system. 
       FIG. 6  is an exploded perspective view of the ink reservoir  50 . The ink reservoir  50  is made up of a peripheral frame  51 , spring  53 , side plates  54 , and side walls  55 . The frame  51  is generally rectangular shaped and is open on both sides. The frame  51  may be made of a polypropylene and/or polyethylene based material. An ink fill hole  56  is disposed at the top of the frame  51 . In this regard, the lid includes an opening  72  that corresponds with the ink fill hole  56  of the frame  51 , as well as an air vent opening  74  and indent  76  for locking an associated muzzle cap in place (as described in more detail below). The fill ball  60  may be disposed within the ink fill hole  56  to allow for passage of ink into the ink reservoir  50  while preventing leakage of ink out of the ink reservoir  50 . The spring  53  may be made from 316 stainless steel or other compatible material, and is used to deliver force to the side plates  54 , to generate a back pressure. The side plates  54  may be made of 316 stainless steel or other comparable material, and act as the rigid surface area that generates the back pressure in the system. The side plates  54  may be attached to the spring  53  at either end. In an exemplary embodiment, the side plates  54  may be attached to the side walls  55 , though they need not be. The side walls  55  are made of multi-layer polymeric films that are thermally formed and then welded to the sides of the frame  51  to create the chamber needed to store the ink. The polymeric film used to form the side walls  55  may be, for example, thermally formed polypropylene and/or polyethylene film. 
     During printing, ink is ejected out of the nozzles, causing an increase in negative pressure under the filter  20 . This negative pressure pulls ink from above the filter  20  and into the tower  14 . Since the ink reservoir  50  is in direct fluid connection with the tower  14 , the negative back pressure inside the ink reservoir  50  increases as well. The negative back pressure pulls against the side walls  55  and side plates  54 , which causes the spring  53  to collapse further. The spring  53  is what maintains and dictates the static back pressure in the system. 
     During shipping any inkjet printhead can see temperature and atmospheric changes that can change the internal back pressure in the printhead, which in turn may lead to leaks. With water based inks this can lead to unhappy customers that have ink on their hands when they open the shipping bag, but when solvent based inks are introduced, an added danger exists in that combustible vapors may be released when a bag is opened. In this regard, a muzzle cap according to exemplary embodiments of the present invention keeps the printhead completely sealed during shipping and maintains the pressure inside the printhead cavity equalized with the surrounding atmosphere upon removal of the muzzle cap to minimize the risk of drooling or air ingestion into the printhead. Drooling would produce an unhappy customer from the standpoint of ink dripping everywhere, and in the case of air ingestion, poor print quality. To this end, the muzzle cap according to exemplary embodiments of the present invention seals the nozzle plate, covering each and every nozzle, without causing damage to the nozzle plate, and also seals the atmospheric vent in the printhead to prevent air pressure changes from reaching the back pressure device. The opening of these seals is done in a particular order in order to prevent problems from occurring. In particular, the atmospheric vent must be opened first in order to equalize the internal pressure in the printhead prior to the opening of the nozzles. 
       FIGS. 7A and 7B  show perspective views and  FIG. 8  is a cross-sectional view of a muzzle cap, generally designed by reference number  100 , according to an exemplary embodiment of the present invention. The muzzle cap  100  includes a main body  110 , vent seal  120 , nozzle plate seal  130  and nozzle plate seal retainer  140 . The main body  110  may be a unitary member including a side wall  112 , a top wall  114  and a bottom wall  116 . The main body  110  is made of a plastic that is compatible with the ketone and acetate based inks that is being jetted, so as to not degrade in the presence of the ink. For example, the main body  110  may be made of nylon (e.g., Nylon 6,6, Nylon 6, Nylon 6,12), polyethersulfone, polypropylene, polyethylene, polyoxymethylene or other materials that are compatible with ketone, acetate and alcohol based inks. The main body  110  includes guiding and locking elements, such as protrusions  113  and snap-locking element  119 . As described in further detail below, these features locate the vent seal  120  and nozzle plate seal  130  relative to the printhead assembly  1  as accurately as possible so as to cover openings in the printhead assembly  1  and lock the muzzle cap  100  in place relative to the printhead assembly  1 . The main body  110  may also contain datum features  152  that directly address datum features  13  on the printhead in order to minimize tolerance stack-ups. In this regard, the main body  110  may include a datum biasing element  154  ( FIG. 19 ) that applies force to push the datum feature  13  of the printhead body  10  into engagement with the datum feature  152  on the main body  110 . 
       FIG. 9  is a perspective view of the vent seal  120 , which is preferably made of a thermoset elastomer, such as a peroxide cured ethylene propylene diene monomer (EPDM) material, so as to reduce compression set over time and provide maximum resistance to the ketone and acetate solvent inks. The vent seal  120  closes the opening in the printhead assembly  1  that is in direct communication with the atmosphere, where the opening otherwise allows air to enter the internals of the printhead assembly  1  during printing as ink is displaced. The vent seal  120  is a generally cylindrical element portions of which have different diameters from one another. In particular, the vent seal  120  includes a sealing surface portion  122  that interfaces with the air vent opening  74  in the lid  70 , where the opening has a raised rim around it. This seals the opening with minimal force. A compression locking portion  124 , which has a smaller diameter than the sealing surface portion  122 , compresses into an opening  115  in the top wall  114  of the main body  110  of the muzzle cap  100  so as to create an interference fit between the compression locking portion  124  and the opening  115 . The assembly lead in portion  126 , which has a smaller diameter than the compression locking portion  124 , allows the vent seal  120  to be grabbed with a tool to pull the vent seal  120  into place. 
     As shown in  FIG. 10 , the nozzle plate seal retainer  140  may be molded into the muzzle body  110  so as to reduce tooling and component costs, and eliminate the need to track an additional component. Prior to use of the muzzle cap  100 , the nozzle plate seal retainer  140  is twisted out of the muzzle body  110  and pressed into the nozzle plate seal  130 . 
       FIG. 11  is a perspective view and  FIG. 12  is a cross-sectional view of the nozzle plate seal  130 . The nozzle plate seal  130  is a generally open-bottomed cuboid shaped element including a top portion  131  and a bottom portion  134 . The nozzle plate seal  130  is preferably made of a thermoset elastomer, such as a peroxide cured ethylene propylene diene monomer (EPDM) material, so as to reduce compression set over time and provide maximum resistance to the ketone and acetate solvent inks. The top surface of the top portion  131  of the nozzle plate seal includes an elevated portion that forms a sealing surface  132 . The sealing surface  132  has a smooth finish that allows good sealing to the nozzles. The perimeter of the bottom portion  134  locates the nozzle plate seal  130  in a corresponding opening  117  in the muzzle body  110 , and, as described in further detail below, when used in conjunction with the nozzle plate seal retainer  140 , centers the nozzle plate seal  130  in the muzzle body  110 . The bottom surface of the top portion  131  forms a flexing floor  136  that flexes to reduce the force applied to the nozzle plate and to also provide a uniform distribution of force on the nozzle plate to aid in sealing. The top portion  131  of the nozzle plate seal  130  extends over the bottom portion  134  so as to form a retaining lip  138  that acts as a stop for the nozzle plate seal  130  once assembled. 
       FIG. 13  is a perspective view of the nozzle plate seal retainer  140 , which is preferably made of a plastic material, such as nylon. The nozzle plate seal retainer  140  includes a top surface having an elevated portion  142 . Teeth-like locking projections  144  are arranged around the perimeter of the elevated portion  142 . The top surface also includes an elevated perimeter forming a rim  146 . 
     As shown in  FIGS. 14 and 15 , in order to assemble the muzzle cap  100 , after the nozzle plate seal  130  is disposed within the opening  117 , the nozzle plate seal retainer  140  is engaged with the nozzle plate seal  130  by sliding the elevated portion  142  of the nozzle plate seal retainer  140  into the open bottom of the nozzle plate seal  130 . The locking projections  144  “bite into” the elastomer material of the nozzle plate seal  130  to retain the nozzle plate seal  130  in place, while naturally centering the nozzle plate seal  130  in the muzzle body opening  117 . 
       FIGS. 16-18  show assembly of the muzzle cap  100  onto the printhead assembly  1 . The muzzle cap  100  is placed on the printhead assembly  1  in a manner such that the nozzle plate seal  110  engages and seals the nozzle plate before the vent seal  120  engages and seals the air vent opening in the lid  70  of the printhead assembly  1 . In particular, as shown in  FIG. 16 , as the muzzle cap  100  is assembled to printhead assembly  1 , the datums in the printhead are guided by guides  113  and biased to a datum pad in the muzzle body  110  to provide proper alignment to the nozzle plate seal  130 . As shown in  FIGS. 17 and 18 , the sequence of steps taken to place the muzzle cap  100  on the printhead assembly  1  may include a first step of engaging the bottom portion of the muzzle cap  100  with the bottom portion of the printhead assembly  1 , and then sliding the snap locking element  119  onto the lid  70  so that the snap locking element  119  engages with the indent  76 , thereby locking the muzzle cap  100  in place relative to the printhead assembly  1 . Engagement of the snap locking element  119  with the indent  76  in the lid  70  ensures proper placement of the vent seal  120  over the air vent opening  74  and also causes the nozzle plate seal  130  to deflect into tight engagement with the nozzle plate  13 , thereby preventing damage to the nozzle plate  13  and maintaining a uniform force across the nozzle plate  13 . When removing the muzzle cap  100 , the snap locking element  119  must first be disengaged from the lid  70 . This allows internal air pressure in the printhead to equalize to atmosphere prior to removal of the nozzle plate seal  130 , thereby minimizing drooling due to pressure differentials. 
     While particular embodiments of the invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications may be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.