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A print head assembly for an ink jet printer includes an ink flow path configured to allow passage of a phase-change ink. A pressure unit is fluidically coupled to the ink flow path to apply a pressure to the ink. The applied pressure is controlled by a control unit during a time that the ink in the

A print head assembly for an ink jet printer includes an ink flow path configured to allow passage of a phase-change ink. A pressure unit is fluidically coupled to the ink flow path to apply a pressure to the ink. The applied pressure is controlled by a control unit during a time that the ink in the ink flow path is undergoing a phase change. During the phase change, a portion of the ink in a first region of the ink flow path is in liquid phase and another portion of the ink in another region of the ink flow path is in solid phase. A constant or variable pressure can be applied at least to the liquid phase portion of the ink during a phase transition from a liquid phase to a solid phase or from a solid phase to a liquid phase.

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1. A print head assembly for an ink jet printer, comprising; an ink flow path, the ink flow path configured to allow passage of a phase-change ink along the ink flow path;a pressure unit configured apply pressure to the ink; anda control unit configured to control the pressure applied to the ink and

1. A print head assembly for an ink jet printer, comprising; an ink flow path, the ink flow path configured to allow passage of a phase-change ink along the ink flow path;a pressure unit configured apply pressure to the ink; anda control unit configured to control the pressure applied to the ink and to coordinate the pressure applied to the ink with temperature of the ink during a time that the ink in the ink flow path is undergoing a phase change, wherein coordination of the pressure applied to the ink with the temperature of the ink is configured to push air out of a free surface of the ink flow path. 2. The print head assembly of claim 1, wherein the phase change involves a transition from a solid phase to a liquid phase. 3. The print head assembly of claim 1, wherein the phase change involves a transition from a liquid phase to a solid phase. 4. The print head assembly of claim 1, further comprising one or more thermal elements configured to heat or cool the ink. 5. The print head assembly of claim 4, wherein the thermal elements are active thermal elements controlled by the control system. 6. The print head assembly of claim 5, wherein the pressure unit is configured to apply a variable pressure to the ink. 7. The print head assembly of claim 4, wherein the control unit is configured to control the thermal elements to create a thermal gradient along at least a portion of the ink flow path during the time that the ink is undergoing the phase change, the thermal gradient causing one portion of the ink in the ink flow path to be in solid phase and a second portion of the ink in the ink flow path to be in liquid phase. 8. The print head assembly of claim 1, further comprising: one or more temperature sensors positioned on components defining the ink flow path, the temperature sensors configured to generate electrical signals modulated by temperature of the ink; andthe control unit is configured to receive the electrical signals and to control the pressure applied to the ink in response to the electrical signals. 9. A method of operating an ink jet printer, comprising; applying pressure to ink in an ink flow path of the ink jet printer; andcoordinating the pressure applied to the ink with temperature of the ink during a time that the ink in the ink flow path is undergoing a phase change; andforcing air out of the ink through a free surface of the ink flow path in response to coordinating the pressure applied to the ink with the temperature of the ink. 10. The method of claim 9, wherein the phase change involves a transition from a solid phase to a liquid phase. 11. The method of claim 9, wherein the phase change involves a transition from a liquid phase to a solid phase. 12. The method of claim 9, further comprising controlling the temperature of the ink. 13. The method of claim 9, wherein applying the pressure to the ink comprises applying a variable pressure to the ink. 14. The method of claim 9, further comprising controlling the temperature of the ink to create a thermal gradient along at least a portion of the ink flow path during the time that the ink is undergoing the phase change, the thermal gradient causing one portion of the ink in the ink flow path to be in solid phase and a second portion of the ink in the ink flow path to be in liquid phase. 15. An ink jet printer, comprising: a print head assembly comprising: a print head with ink jets configured to selectively eject ink toward a print medium according to predetermined pattern;an ink flow path, the ink flow path configured to allow passage of a phase-change ink along the ink flow path to the ink jets;a pressure unit configured apply pressure to the ink; anda control unit configured to control the pressure applied to the ink and to coordinate the pressure applied to the ink with temperature of the ink during a time that the ink in the ink flow path is undergoing a phase change, the coordination of the pressure applied to the ink with the temperature of the ink configured to push air out of a free surface of the ink flow path; anda transport mechanism configured to provide relative movement between the print medium and the print head. 16. The printer of claim 15, further comprising thermal elements configured to create a thermal gradient along at least a portion of the ink flow path during the time that the ink is undergoing the phase change, the thermal gradient causing one portion of the ink in the ink flow path to be in solid phase and a second portion of the ink in the ink flow path to be in liquid phase. 17. The printer of claim 16, further comprising one or more temperatures sensors configured to sense the temperature of the ink at one or more locations, wherein the control unit is configured to control the thermal elements to create the thermal gradient based on the sensed temperature of the ink. 18. A print head assembly for an ink jet printer, comprising: one or more components that define an ink flow path of the ink jet printer, the ink flow path configured to allow passage of a phase-change ink along the ink flow path;a pressure unit configured to apply pressure to the ink; anda control unit configured to coordinate the pressure applied to the ink with temperature of the ink during a time that the ink is transitioning form a liquid phase to a solid phase and a portion of the ink in the ink flow path is in a solid phase and another portion of the ink in the ink flow path is in a liquid phase, wherein coordination of the pressure applied to the ink with the temperature of the ink is configured to push air out of a free surface of the ink flow path. 19. The assembly of claim 18, further comprising one or more thermal elements, wherein the control unit is configured to control the thermal elements. 20. The assembly of claim 18, further comprising one or more temperature sensors configured to sense temperature of the ink, wherein the control unit is configured to coordinate the pressure with the sensed temperature of the ink. 21. The assembly of claim 18, further comprising one or more thermal elements, wherein the control is configured to control the thermal elements to create a temperature gradient along at least a portion of the ink flow path. 22. The assembly of claim 21, wherein the components include at least a reservoir and a print head and the temperature gradient includes a higher temperature at the reservoir and a lower temperature at the print head. 23. A method of reducing voids in the ink of an ink jet printer, comprising: determining temperature of ink in an ink flow path of the ink jet printer during a time the ink is undergoing a transition from a liquid phase to a solid phase, wherein a portion of the ink in the ink flow path is in liquid phase and another portion of the ink in the ink flow path is in solid phase;coordinating pressure applied to the ink with the temperature of the ink during the transition; andforcing air out of the ink through a free surface of the ink flow path in response to coordinating the pressure applied to the ink with the temperature of the ink. 24. The method of claim 23, wherein coordinating the pressure comprises coordinating a variable pressure as a function of the temperature. 25. The method of claim 23, further comprising controlling one or more thermal elements to create a thermal gradient in the ink during the transition. 26. The method of claim 25, further comprising coordinating the pressure with the thermal gradient. 27. An ink jet printer configured to implement the method of claim 25. 28. The method of claim 23, wherein the controlling the thermal elements comprises applying phased zoned heating to the ink flow path, the phased zone heating including: heating a first zone of the ink flow path; andafter heating the first zone, heating a second zone of the ink flow path. 29. An ink jet printer configured to implement the method of claim 23.