Abstract:
An apparatus controls dissipation of heat from melted ink within a component storing melted ink within a solid ink imaging device. The apparatus includes a housing, a passage within the housing that is configured to store melted ink, and a temperature control connector mechanically coupled to the housing and passage, the temperature control connector being configured to mitigate void formation in melted ink as the melted ink cools in the passage.

Description:
TECHNICAL FIELD 
       [0001]    The devices and methods disclosed below generally relate to solid ink imaging devices, and, more particularly, to solid ink imaging devices that permit melted ink to solidify in a print head of the solid ink imaging device. 
       BACKGROUND 
       [0002]    Solid ink or phase change ink printers conventionally receive ink in a solid form, either as pellets or as ink sticks. The solid ink pellets or ink sticks are typically inserted through an insertion opening of an ink loader for the printer, and the ink sticks are pushed or slid along the feed channel by a feed mechanism and/or gravity toward a melt plate in the heater assembly. The melt plate melts the solid ink impinging on the plate into a liquid that is delivered to an ink reservoir which maintains the ink in melted form for delivery to a print head for jetting onto a recording medium. 
         [0003]    One difficulty faced during operation of solid ink printers is the electrical energy consumed by the printer. In particular electrical energy is required for the melting device to convert the solid ink to melted ink and print heads also require electrical energy to maintain the melted ink in the liquid phase. In an effort to conserve energy, solid ink printers are operated in various modes that consume different levels of energy. In these various modes, one or more components that include heaters to maintain melted ink in the liquid phase may be shut off to enable the melted ink to “freeze” or return to the solid state. 
         [0004]    One problem that arises from the freezing of melted ink is the formation of bubbles in the solidified ink. These entrapped bubbles must be purged when electrical energy is coupled to the components to liquefy the solidified ink. The purging operation, however, results in the discarding of ink from the printing system. Customers generally view the loss of ink as being undesirable. Thus, enabling the solidification of melted ink without the formation of entrapped bubbles in the solidified ink would be useful. 
       SUMMARY 
       [0005]    An apparatus has been developed that enables melted ink in a print head to solidify with little or no formation of bubbles in the solidified ink. The apparatus includes a housing, a passage within the housing that is configured to store melted ink, and a temperature control connector mechanically coupled to the housing and passage, the temperature control connector being configured to mitigate void formation in melted ink as the melted ink cools in the passage. 
         [0006]    A print head has also been developed that enables melted ink in a reservoir of a print head to solidify with little or no formation of bubbles in the solidified ink. The print head includes a housing, a reservoir within the housing that is configured to store melted ink for ejection from the print head, and a thermal conductor that is thermally coupled to the melted ink within the reservoir to control solidification of the melted ink within the reservoir in response. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing aspects and other features of the present disclosure are explained in the following description, taken in connection with the accompanying drawings. 
           [0008]      FIG. 1  is a partial cross-sectional view of a print head housing containing multiple passages for ink; 
           [0009]      FIG. 2  is a cross-sectional view of an ink manifold housing; 
           [0010]      FIG. 3  is a partial cross-sectional view of a print head including a tapered passage and portion of a reservoir; and 
           [0011]      FIG. 4  is a cross-sectional view of an ink reservoir configured to convey ink to one or more print heads. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The term “printer” as used herein refers, for example, to reproduction devices in general, such as printers, facsimile machines, copiers, and related multi-function products. While the specification focuses on a system that controls the solidification process of phase-change ink in a printer, the system may be used with any phase-change ink image generation device. Solid ink may be called or referred to as ink, ink sticks, or sticks. The term “via” as used herein refers to any passage that conveys ink from one chamber to another chamber. 
         [0013]    An example of a print head housing that mitigates bubble formation in solidified ink held in the print head is depicted in the cross-sectional view of  FIG. 1 . The print head  100  has a housing  104 , typically made of a metal, such as stainless steel or aluminum, or a polymer material. Within the housing  104  are one or more chambers that hold ink as exemplified by chambers  108 A,  108 B, and  108 C. These chambers may be in fluid communication with one another through a passage not visible at the location of the cross-section. The chambers may have various shapes and sizes as determined by the requirements for ink flow through each print head  100 . In the print head of  FIG. 1 , various thermal conductors  112 A-C are disposed within and about the chambers  108 A-C. Each thermal conductor  112  passes through housing  104  and connects to the exterior of the housing  104 . The thermal conductors  112  act as temperature control connectors that control the rate of heat transfer from ink disposed within each chamber  108  to the exterior of housing  104 . As used herein, thermal conductor refers to a material having a relatively high coefficient of thermal conductivity, k, which enables heat to flow through the material across a temperature differential. In  FIG. 1 , the thermal conductors  112  are positioned so that the various regions of each chamber  108  have an approximately equal thermal mass. For example, thermal conductor  112 C bifurcates the surrounding ink channel in chamber  108 A, forming two regions with roughly equivalent thermal masses. Depending upon the desired rate of heat transfer, some or all of the thermal conductors  112  may connect to heat sinks (not shown) external to housing  104 . The heat sinks are typically metallic plates that may optionally have metallic fins that aid in radiating conducted heat away from print head  100 . 
         [0014]    Depending upon the desired heat conduction characteristics, thermal conductors may be of various shapes and sizes. In  FIG. 1 , thermal conductor  112 A is cylindrical in shape, while thermal conductor  112 B is also cylindrical with different diameter. Thermal conductors may also have a variety of shapes such as the oblique form of thermal conductor  112 C. A thermal conductor may be placed proximate to an ink chamber such as thermal conductor  112 A or placed within an ink reservoir as with thermal conductors  112 B and  112 C. The thermal conductors may be formed from various thermally conductive materials, with copper being one preferred material. In designing the thermal conductors, the particular material used may be influenced by the desired thermal conductivity for each thermal conductor, so alternative print heads may use other materials with differing thermal conductivity including different metals or thermoplastics, and may employ thermal conductors formed of two or more materials in a single print head housing. The precise size, shape, and position of thermal conductors are selected to affect either the time needed for a thermal mass to solidify, the direction in which solidification takes place, or both. Because the ink affects heat distribution in the print head, appropriate selection and placement of thermal conductors help to control the temperature of the ink so the ink is more likely to cool and solidify without forming voids. 
         [0015]    The following equation governs the characteristic time for conduction for a given thermal mass of ink: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                       eff 
                     
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                       α 
                     
                   
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                        
                       k 
                     
                     
                       ρ 
                        
                       
                           
                       
                        
                       
                         C 
                         p 
                       
                     
                   
                 
               
               
                 
                   Equation 
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                   1 
                 
               
             
           
         
       
     
         [0016]    In Equation 1, the characteristic time t eff  of thermal conduction for a thermal mass is expressed as the ratio of a characteristic dimension, L, to the thermal diffusivity, α, of the mass. The characteristic dimension, L, of the thermal mass is related to the volume to surface area ratio (V/A) of the thermal mass. For a sphere, V/A can be approximated by the radius or diameter, while for a cube it is the length of a side. Objects with large surface areas and small volumes have a small characteristic length for thermal conduction and cool much faster than objects with small surface areas and large volumes. As an example, the center of a sphere with radius  2 R takes roughly 4 times as long to reach a given temperature than the center of a sphere of radius R. Although modifying the heat capacity or the thermal conductivity of the ink or surrounding material can also affect the time to change temperature, using thermal conductors to alter the volume to surface area ratio is a more effective way of controlling heat distribution in a print head due to the nonlinear relationship between conduction path length and thermal response time. 
         [0017]    The thermal conductors are placed in a manner that produces a desired t eff  for each thermal mass of melted ink present in a print head. To be effective, thermal conductors need to be positioned to enable an effective cooling length of the thermal mass to be the same as the smallest characteristic dimension in a passageway leading into or out of the chamber. Likewise, as noted above, the thermal conductors may be used to alter the volume to surface area ratio appropriately. Alternatively, a thermal conductor needs to provide a local temperature that enables a thicker mass to cool equivalently as a smaller mass experiencing a higher temperature gradient. In the embodiment of  FIG. 1 , t eff  time values for the ink in the portions of the print head near the print head&#39;s narrow vias  116  are shorter than the t eff  time values in the chambers or the larger passages through the print head. Thus, the thermal conductors are positioned to equalize the thermal mass in the various portions of a chamber, to promote equalization of the time for the ink in the various portions of the print head  100  to solidify, or to encourage the freezing to occur in a direction that enables air bubbles or voids to be released from the solidifying ink. 
         [0018]    Continuing to refer to  FIG. 1 , one or more vias  116  convey ink to and from the chambers  108  in the print head  100 . The vias  116  in  FIG. 1  have a shape that is wider at the opening  120  at one end of the via  116  and which tapers to a narrower opening  124  at the other end of the via. The direction of the taper is selected to control how ink in the via  116  solidifies as it cools. The taper acts as a different form of temperature control connector, allowing the ink in the via  116  to cool in a predictable manner. The preferred selection is for the narrow end of each via to be disposed towards the portion of the print head where ink should solidify first, since the narrower portions of the via  116  have a lower thermal mass of ink that is likely to solidify before the ink in the wider portions of the via. 
         [0019]    An alternative structure for controlling heat transfer within a print head is depicted in  FIG. 2 . In  FIG. 2 , an ink manifold  200  includes an external housing  204  and reservoirs  208  that hold ink separately from one another. The manifold housing  204  is formed from a heat conductive material, such as a metal or a heat conductive thermoplastic. A heating element  212  acts as a heat source that heats ink stored in reservoirs  208 . The heating element  212  is typically an electrically resistive heating element that may be selectively controlled to maintain a desired temperature within the manifold  200 . The heating element allows for control over both the absolute temperature of the reservoirs and the rate of temperature change in the reservoirs  208 . This control enables more uniform and directional solidification of the ink starting from the narrow vias  216  and proceeding to the larger reservoirs  208 . 
         [0020]    Again referring to  FIG. 2 , an optional insulation layer  224  may also be placed around the housing  204 . The insulation layer  224  reduces differences in the rate of heat escape from the thermally conductive housing  204 , which leads to more uniform cooling. The insulation layer  224  operates as a temperature control connector that reduces “hot spots” and “cold spots” that could lead to ink solidifying in an uneven manner in the manifold reservoirs  208 . While the insulation layer  224  depicted in  FIG. 2  extends over the entire manifold housing  204 , the insulation may also be placed over selected portions of the manifold housing  204  in order to achieve a uniform rate of heat conduction. 
         [0021]      FIG. 2  also contains vias  216  that convey ink from reservoirs  208  to other chambers in the print head. As in  FIG. 1 , these vias have a shape that is wider at the opening  120  at one end of the via  116  and which tapers to a narrower opening  124  at the other end of the via. The direction of the taper is selected to control how ink in the via  216  solidifies as it cools. The taper acts as a different form of temperature control connector, allowing the ink in the via  216  to cool in a predictable manner. The preferred selection is for the narrow end of each via to be disposed towards the portion of the print head where ink should solidify first, since the narrower portions of the via  216  have a lower thermal mass of ink that solidifies prior to the wider portions of the via. 
         [0022]    An example of a tapered via used in the embodiments of  FIG. 1  and  FIG. 2  is depicted in  FIG. 3 . The via  300  has a wider opening  304  that tapers to a narrower opening  308 . In the example of  FIG. 3 , ink near the walls of the via solidify first forming solidifying fronts  312 A and  312 B. The tapered shape of the via means that the portions of ink proximate to the narrow opening  308  have a lower thermal mass and solidify more quickly. This shape enables directional solidification to start at the narrow opening  308  and move towards the wide opening  304 . Some forms of ink contract as they solidify, which can cause voids to form if no liquid ink is present to fill the voids. If contraction occurs in the structure of  FIG. 3 , the liquid ink in the reservoir  320  generates a positive back pressure that enables liquid ink to flow into the via  300  from the reservoir  320  to form a thermal mass  316  that fills voids between the solidified fronts  312 A and  312 B until the solidification process is complete. Because the reservoir  320  has a larger thermal mass than the narrow via  300 , the ink held in the reservoir solidifies after ink the in via  300 . Consequently, the reservoir  320  acts as a riser that provides additional liquid ink to fill any voids formed in via  300  during the solidification process. 
         [0023]    An ink reservoir and ink conduit adapted to supply liquid ink to the print heads of  FIG. 1  and  FIG. 2  is depicted in  FIG. 4 . The ink reservoir  404  holds ink  408  that may be solid or liquid depending upon the operational mode of the printer, with the example of  FIG. 4  depicting solidified ink. The reservoir  404  is connected to print heads  420  using a tapered connector  416 . In a similar manner to the via  300  depicted in  FIG. 3 , the tapered connector  416  promotes directional solidification of ink from the narrow end proximate to print heads  420  to the wide end proximate to ink reservoir  404 . The ink reservoir  404  holds a thermal mass that is larger than the thermal mass in the connector  416 . Thus, the ink reservoir  404  acts as a positive pressure generating riser that enables ink to flow into the tapered connector  416  to fill voids that may occur in the solidifying fronts forming the connector  416 . Consequently, the melted ink solidifies in a continuous mass free of voids or bubbles that rise to the surface of the mass inside the reservoir  404 . If any bubbles form, they form within the larger reservoir  404  as shown at  412 . In operation, bubbles in the reservoir  404  are eliminated when the solidified ink  408  is melted, preventing air bubbles from reaching the print heads  420 . 
         [0024]    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. A few of the alternative implementations may comprise various combinations of the methods and techniques described. 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.