Patent Publication Number: US-9849689-B1

Title: Water-vapor assist for filling aqueous print heads

Description:
TECHNICAL FIELD 
     This disclosure relates to aqueous print heads, more particularly to the initial filling of the print head. 
     BACKGROUND 
     Printing systems typically include a print head, a supply of ink, a controller and associated electronics that guide the production of images from the ink onto print substrates. The print head may take many forms, including a jet stack, which is a series of thin, metal or polymer plates stacked together to form manifolds, pressure chambers, and an array of nozzles to deposit ink on the print substrates. Regardless of the configuration of the print head, it must undergo an initial fill. 
     The initial filling typically occurs at the manufacturing plant. A problem arises during the fill process because air, in the form of bubbles, becomes trapped in the print head. The presence of air bubbles interferes with the ability of the actuators to transmit pressure waves through the print head efficiently and reliably, where the pressure waves drive the ink out of the print head. 
     Historically, eliminating these bubbles involves a painful process of pushing large volumes of liquid through the print head. This typically dislodges large bubbles. Small bubbles become absorbed into the ink after a long process of patiently waiting. This can take several hours per print head. 
     SUMMARY 
     An embodiment is a method of filling a print head that includes drawing a vapor into an empty print head to displace any air in the print head and filling the print head with ink. 
     Another embodiment is a method of filling a print head that includes connecting a print head to a vacuum pump using a first valve, to a liquid supply using a second valve, and to an ink supply using a third valve, opening the first and second valves, applying vacuum to the print head and liquid supply using the vacuum pump, causing the liquid to generate a vapor that moves into the print head, closing the first valve and the second valve, and opening the third valve to move ink into the print head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of an apparatus used to fill a print head with ink 
         FIG. 2  shows an embodiment of an apparatus as a print head fills with vapor. 
         FIG. 3  shows an embodiment of an apparatus after a print head fills with vapor. 
         FIG. 4  shows an embodiment of an apparatus after a final vacuum has been reached. 
         FIG. 5  shows an embodiment of an apparatus after an ink valve has been opened. 
         FIG. 6  shows an embodiment of an apparatus after a print head has been filled. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows an embodiment of an apparatus used to fill a print head. The apparatus consists a vacuum pump  12  connected to the print head  10 . An ink supply  14  connects through the print head to the vacuum pump, as does a liquid supply  16 . In the examples shown here, the liquid consists of water, but may consist of other liquids discussed later. One should note that the ink fills the channel with no air leading into the print head. In contrast, a pocket of air or gas resides above the liquid in the liquid chamber. 
     The term ‘ink’ as used here comprises any liquid that is being jetted or deposited by the print head. It may consist of aqueous (water based ink), non-aqueous ink or any other jettable solution. Similarly, the term liquid may consist of any liquid, including water. Further, the ‘liquid’ supply may consist of a gas, and the liquid repository may be a gas chamber. 
     After connecting the print head to the vacuum pump, ink and liquid supplies, the valves  20  and  24  are opened and the vacuum pump  12  turns on. When the vacuum pump  12  lowers the pressure in the print head  10  and the liquid chamber  16  below the vapor pressure of the liquid, the liquid will begin to ‘boil’ and vapor from the liquid will be sucked into the print head as shown in  FIG. 2 . 
     The liquid vapor, in the example above, water vapor, displaces the vast majority of the air within the print head. The fill station continues in this state for a time to ensure maximum air displacement. Tests may be run to determine the necessary length of time to cause the vapor to displace the maximum amount of air based upon the pressure settings of the pumps. 
     Once the print head has been substantially purged of air and filled with water vapor, valve  24  is closed, as shown in  FIG. 3 . When this valve closes, the boiling of the water ceases and the vacuum pump can a draw a deeper vacuum. The vapor pressure of the liquid in the supply  16  no longer limits the vacuum. Experiments have shown that it is possible to reach 0.5 Tor or lower in this portion of the process. 
     Once the final vacuum has been reached, valve  20  closes, as shown in  FIG. 4 . Valve  22  also opens, shown in  FIG. 5 . Because of the low pressure in the print head, the ink is sucked into the head. As the ink comes into the head, the vacuum level inside the print head returns to ambient pressure. When the vacuum level rises above the vapor pressure of the liquid, the vapor will condense into liquid and disperse into the ink. Once the filling of the head completes, valve  22  is closed, as shown in  FIG. 6 . 
     In this manner, once the fill of the print head is complete, there is a much reduced volume of gas in the print head. The fill process has much higher efficiency and takes much less time than previous processes. 
     Variations and modification exist for this process. For example, the fluid used may consist of water, as discussed above, or alcohol, or other fluids that are compatible with the inks being used in the print head. Alcohol provides an attractive alternative because it readily absorbs into water and has a low vapor pressure. This allows the alcohol to readily absorb into the aqueous ink. Another variation would be to perform the process inside a humidity chamber rather than using the vapor-creation of the vacuum pump. While this may be better than current practices, it may not be as advantageous as using a vacuum pump because the volume of air in the chamber that mixes with the vapor making it less efficient in the print head. Another variation would be to replace the chamber  16  liquid with a “gas chamber” containing a gas that is highly soluble in the ink to be used in the print head. The fundamental principle is to replace the air with a gas, of which water vapor is just an example, that can be readily absorbed into the ink being used within the print head. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.