INK JET APPARATUS AND METHOD OF OPERATING THE SAME

Abstract: Method and apparatus for an ink jet, in which ink is maintained in a solid-state in a reservoir and in a liquid state in an imaging system, during a standby mode, the ink is heated in the reservoir to change the phase of the ink to the liquid phase, and the ink in the reservoir and in the imaging system is maintained in a liquid state during the ink droplet ejection mode.

Description: This invention relates to an ink jet wherein the ink within the jet is of the phase change type which may be referred to as hot melt ink. The phase change or hot melt ink of the type utilized in an ink jet is characteristically solid at room temperature. When heated, the ink will melt to a consistency so as to be jettable. The hot melt may be jetted from a variety of apparatus, such as the ink dispenser described in European Patent Application No. 0 109 754. It has been found that extended or continous heating of hot melt ink to a temperature such that the ink is in a liquid state can actually degrade the ink. In other words, the application of heat at an elevated temperature will adversely affect the characteristics of the ink such that both the performance of the ink jet as well as the characteristics of the jetted ink will vary. Such a degradation can adversely affect quality of printing achieved by an ink jet or an array of ink jets. Because of this degradation of the ink, it has been found to be desirable to cool the ink when the ink jet is in a standby mode, i.e., the ink jet is not being called upon to print. However, hot melt ink will contract upon a phase change from the solid state to the liquid state. Such a contraction can result in the depriming of the ink jet which is, of course, undesirable. According to the invention from one aspect there is provided a method of operating an ink jet apparatus employing phase change ink, said apparatus comprising imaging means including a chamber, an orifice for ejecting ink droplets from the chamber and an inlet to the chamber, and an ink reservoir for storing feed ink for said inlet, in which phase change ink is heated to change the phase of the ink from a solid state to a liquid state for ejection from said orifice, characterized in that said method comprises: maintaining the ink in the reservoir in the solid state in a standby mode; maintaining the ink in the imaging means in the liquid state during the standby mode; heating the ink in the reservoir so as to melt the ink into a liquid state in the reservoir; and maintaining the ink in the imaging means and the reservoir means in the liquid state during the droplet ejection mode; and cooling the ink in the reservoir after droplet ejection to return the ink in the reservoir to the solid state in the standby mode. With at least some embodiments of the invention, it is possible to provide a hot melt ink jet method and apparatus wherein degradation of the ink due to extended heating is minimized and/or depriming of the ink jet is minimized. In one arrangement, the ink in the liquid state in the imaging means is heated so as to elevate the temperature of the ink in the droplet ejection mode over the temperature of the ink in the standby mode. Another possibility is for the temperature of the ink in the imaging means in the droplet ejection mode is to be kept greater than the temperature of the ink in the reservoir in the droplet ejection mode. Moreover, the temperature of the ink may be maintained substantially constant in the reservoir during the standby mode, substantially constant in the imaging means during the standby mode, substantially constant in the reservoir during the droplet ejection mode and substantially constant in the imaging means during the droplet ejection mode. A portion of the ink may be maintained in the reservoir adjacent the inlet to the ink jet chamber in the liquid state in the standby mode. In order to accomplish the foregoing, the invention provides, according to a second aspect thereof, ink jet apparatus comprising: imaging means comprising at least one ink jet including a chamber, an orifice for ejecting ink droplets from the chamber and an inlet to the chamber; ink reservoir means for storing hot melt feed ink for said inlet characterized by a solid state below a predetermined temperature and a liquid state above said temperature, characterized in that said ink jet apparatus comprises means for substantially independently heating said imaging means and said reservoir means so as to maintain said ink in the liquid state in said imaging means while causing said ink to change on demand from the solid state to the liquid state and vice versa in said reservoir means. The means for substantially independently heating the imaging means and the reservoir may include a first heater closely thermally coupled to the imaging means and a second heater closely thermally coupled to the reservoir means. A high thermal resistance path or barrier may be used between the imaging means and the reservoir means so as to permit independent heating. In order to maintain the ink at the inlet in a liquid state in a standby mode, the inlet may comprise a substantially thermally conductive material. Preferably, the imaging means as well as the reservoir means comprises a thermally conductive material having a thermal conductivity in excess of approximately 12.55 W/(m.k) [0.03 g cal/sec cm²(°C/cm)] and preferably in excess of approximately 83.68 W/(m.k) [0.2 g cal/sec cm²(°C/cm).] The invention will be better understood from the following description given by way of example and with reference to the accompanying drawings, wherein:- Fig. 1 is a sectional view of an ink jet apparatus according to one embodiment of this invention; Fig. 2 is a schematic representation of the temperature of the imaging means or head and the reservoir as a function of time; Fig. 3 is a flow diagram depicting the operation of the system of Fig. 1 to achieve the temperatures as a function of time as depicted in Fig. 2; and Fig. 4 is a sectional view taken along line 4-4 of Fig. 1. Referring to Fig. 1, an ink jet apparatus comprises an imaging head 10 including at least one ink jet 12 and a reservoir 14. The ink jet apparatus is adapted to jet hot melt or phase change ink. As shown, a block of solid state ink 16 is juxtaposed to an opening in a trough 18. When the pellet 16 drops into the trough 18, the pellet 16 proceeds to melt in response to heat generated by a heater 20 located at the base of the reservoir 14 below a sloping surface 22. As shown in Fig. 1, the ink jet 12 includes a chamber 24 having an orifice 26 for ejecting droplets of ink and an inlet 28 extending toward the lowermost extremity 30 of the reservoir 14 adjacent the sloping bottom 22. The head 10 is provided with an independent heater 32 located between a thermal resistance barrier 34 and a head member 36. By providing the heater 32 which is independent of the heater 20, it is possible to maintain the head 10 at a different temperature from the reservoir 14. This allows the reservoir 14 and the ink within the reservoir to be cooled in the standby mode, thereby avoiding cooking of and resulting degradation of a large volume of ink while at the same time maintaining the ink within the imaging head 10 in a liquid state so as to prevent depriming. With respect to the prevention of depriming, it will be appreciated that inlet 28 passes through the head member 36 which is highly conductive such that heat is conducted to the end 38 which maintains a pool of ink 40 in the liquid state in the immediate vicinity of the end 38 while the remainder of the ink within the reservoir 14 is able to cool to the solid state when the system is in a standby mode. As shown in Fig. 1, the pool 40 of liquid ink is maintained in an otherwise solid state mass 42 of ink extending up to a level 44. As also shown in Fig. 1, a transducer 46 is juxtaposed to the end of the chamber 24. The transducer which is provided with electrodes is energized by a signal provided through a printed circuit board 48 located above the member 36. The transducer 46 and the printed circuit board 48 are then housed within head members 50 and 52. As also shown in Fig. 1, the head includes a chamber plate 54 forming the chamber 24, which is in communication with a foot 56 located at the end of the transducer 46. As the transducer changes state in response to signals applied, the position of the foot 56 varies so as to expand and compress the volume within the chamber 24. The inlet 28 supplies a manifold 58 located in a plate 60 which is coupled to restricted inlets to the jet 12. Actually, the manifold 58 serves a plurality of restricted inlets in an array of ink jets identical to the jet 12 shown in Fig. 1. The reservoir 14 as shown in Fig. 1 also includes a filter 62 which is located below a port 64 which is adapted to be opened and closed by a needle valve 66. The needle valve 66 is employed to close the port 64 during priming. Ink is delivered to the reservoir 14 by means of a cartridge 68. As also shown in Fig. 1, the insulating barrier 34 which provides a high thermal resistance path is sealed against portion 70 of the reservoir 14, using an O-ring 72 which is also characterized by adequate insulating properties. Preferably, the member 36 which extends down into the reservoir 14 toward the lowermost portion 30 is slightly spaced from portion 70 of the reservoir 14. This spacing assures an adequate thermal barrier and high thermal resistance path so as to permit independent heating of the ink within the head as compared to the heat within the reservoir 14. Finally, Fig. 1 shows low and out-of-ink level sensing elements 74 and 76, which may comprise thermistors, RF level sensing or other electrical sensor means. Baffles with slots 78 are also provided in the reservoir 14. From the foregoing, it will be appreciated that the ink within the reservoir 14 may be maintained in a solid state during the standby mode while the ink within the head 10 is maintained in the liquid state. However, when it is desirable to operate in a droplet ejection mode, the temperature of the ink within the reservoir 14 may be elected so as to undergo a phase change from a solid state to a liquid state. Of course the ink within the head 10 remains in the liquid state where it is even elevated further in the droplet ejection mode. Referring to Fig. 4, a by-pass between the valve 66 and the filter 54 is shown. The by-pass comprises a stand-pipe 78 and a channel 80. The by-pass is necessary since the filter 54 may not permit the passage of air. In addition, Fig. 4 discloses a means by which the ink jets may be primed in the event of depriming. This may be accomplished by means of a bulb 82 which communicates with a one-way valve 84 in the upper region of the reservoir 14. When a door 86 covering the bulb 82 is opened, the door 86 pivots about a point 88. The opening of the door 86 exposes the bulb 82 with the valve 66 closed so as to permit a forcing of air into the reservoir 14 through the one-way valve 84 until such time the ink jet is properly primed. In order to achieve this independent heating of the ink within the reservoir 14 and the ink within the head 10, it is necessary to assure adequate conductivity to heat the ink. Therefore, substantially all portions of the reservoir 14 and associated parts in contact with the ink (directly or indirectly through a non-reactive coating) preferably have a thermal conductivity factor in excess of approximately 12.55 W/(m.K) [0.03 g cal/sec cm²(°C/cm)] and preferably in excess of approximately 83.68 W/(m.K) [0.2 g cal/sec cm²(°C/cm)]. The same is true with respect to the head 10. Suitable materials include stainless steel and aluminium. On the other hand, it is desirable to provide a thermal insulating factor or thermal resistance so as to permit the independent heating of the reservoir 14 as compared with the head 10. Of course, the inlet 28 to the head 10 is of a thermal conductivity in excess of approximately 12.55 W/(m.K) [0.03 g cal/sec cm²(°C/cm) preferably in excess of approximately 83.68 W/(m.K) [0.2 g cal/sec cm²(°C/cm)] as is the remainder of the head so as to achieve an always liquid state of the pool 40 even in the standby mode, thereby preventing depriming. Referring to Fig. 2, the relative temperature of the imaging head 10 and the reservoir 14 are depicted with temperature on the vertical axis and time on the horizontal axis. As shown, the head standby temperature while the head is in the standby mode between times T₁ and T₂ is above the ink melting temperature but rises even higher to a head operating temperature during the droplet ejection mode between times T₂ and T₃. Then, when the ink jet apparatus returns to the standby mode between times T₃ and T₄, the head temperature drops back down to the head standby temperature. In contrast, the reservoir standby temperature between times T₁ and T₂ and times T₃ and T₄ is below the melting point. However, at time T₂, the reservoir temperature is raised, in this particular embodiment, up to substantially the same level as the head standby temperature, i.e., above the ink melt temperature. It will be noted that the temperature of the ink in both the reservoir and the head is substantially constant during the ejection mode so as to provide uniformity. Of course, by maintaining the head standby temperature and the head operating temperature above the melting point at all times, depriming of the head is avoided. On the other hand, by allowing the reservoir to cool to a temperature just above room temperature and well below the ink melting point during the standby mode, extended cooking and degradation of the supply of ink is avoided. Control of the imaging apparatus of Fig. 1 to achieve the relative temperature shown in Fig. 2 will now be described with reference to the flow diagram of Fig. 3. As shown, the reservoir 14 is maintained at a first temperature below the melting point of the ink in the standby mode as depicted by block 100. At the same time, the head is maintained at a second standby temperature above the melting point of ink as depicted by block 102. Just prior to time T₂ as shown in Fig. 3, start-up is initiated as depicted by block 104 of Fig. 3, which begins the elevation of the temperature of the ink in the head as well as the reservoir as depicted by blocks 106 and 108. The temperature of the reservoir and the head is monitored as depicted by blocks 110 and 112. When the temperature in the reservoir reaches a third temperature which is substantially equal to the head standby temperature and the temperature of the ink in the head reaches a fourth temperature representing the head operating temperature, droplet ejection for printing may proceed as depicted by block 114. After droplet ejection and printing has been completed, shut-down may be initiated as depicted by block 116 whereupon the temperature of the ink in the reservoir as well as the temperature of the ink in the head is decreased as depicted by blocks 118 and 120. The temperature of the ink in both the reservoir and the head is then monitored as depicted by blocks 122 and 124 until such time as the reservoir and the head reach the standby temperature whereupon these temperatures are maintained as depicted by blocks 100 and 102. The disclosed embodiment of the invention may utilize ink as disclosed in U.S. Patent No. 4,390,369, assigned to the assignee of the present application. Reference is made throughout the specification and the appended claims to temperatures of the ink at a particular location, e.g., within the reservoir. It will be appreciated that slight gradients will exist within the ink and its surroundings, e.g., the walls of the reservoir. However, these gradients are substantially minimized and the vast majority of ink at a particular location is at substantially the same temperature.

Claims: A method of operating an ink jet apparatus employing phase change ink, said apparatus comprising imaging means including a chamber (24), an orifice (26) for ejecting ink droplets from the chamber (24) and an inlet (28) to the chamber (24), and an ink reservoir (14) for storing feed ink for said inlet (28), in which phase change ink is heated to change the phase of the ink from a solid state to a liquid state for ejection from said orifice, characterised in that said method comprises: maintaining the ink in the reservoir (14) in the solid state in a standby mode; maintaining the ink in the imaging means in the liquid state during the standby mode; heating the ink in the reservoir (14) so as to melt the ink into a liquid state in the reservoir (14); maintaining the ink in the imaging means and the reservoir means (14) in the liquid state during the droplet ejection mode; and cooling the ink in the reservoir (14) after droplet ejection to return the ink in the reservoir (14) to the solid state in the standby mode. A method according to claim 1, including the step of heating the ink in the liquid state in the imaging means so as to elevate the temperature of the ink in the droplet ejection mode over the temperature of the ink in the standby mode. A method according to claim 1 or 2, wherein the temperature of the ink in the imaging means in the droplet ejection mode is greater than the temperature of the ink in the reservoir (14) in the droplet ejection mode. A method according to any preceding claim, wherein the temperature of the ink in the reservoir (14) in the standby mode is above room temperature. A method according to any preceding claim, wherein the temperature of the ink is maintained substantially constant in the reservoir (14) during the standby mode, substantially constant in the imaging means during the standby mode, substantially constant in the reservoir (14) during the droplet ejection mode, and substantially constant in the imaging means in the droplet ejection mode. A method according to any preceding claim, including the step of maintaining a portion of the ink in the reservoir (14) adjacent the inlet (28) in the liquid state in the standby mode. Ink jet apparatus comprising: imaging means comprising at least one ink jet (12) including a chamber (24), an orifice (26) for ejecting ink droplets from the chamber (24) and an inlet (28) to the chamber (24); and ink reservoir means (14) for storing hot melt feed ink for said inlet (28), said ink characterized by a solid state below a predetermined temperature and a liquid state above said temperature, characterized in that said ink jet apparatus comprises means for substantially independently heating said imaging means and said reservoir means (32, 34 and 20) so as to maintain said ink in the liquid state in said imaging means while causing said ink to change on demand from the solid state to the liquid state and vice versa in said reservoir means (14). Apparatus according to claim 7, wherein said means for substantially independently heating includes a first heater (32) closely thermally coupled to said imaging means and a second heater (20) closely thermally coupled to said reservoir means (14). Apparatus according to claim 7 or 8, wherein said means (32, 20) for substantially independently heating said imaging means and said ink reservoir means (14) comprises thermal resistance means between said imaging means and said reservoir means (14). Apparatus according to claim 7, 8 or 9, wherein said inlet (28) extends into said reservoir (14), said inlet (28) comprising a substantially thermally conductive material so as to maintain the ink in said reservoir (14) adjacent said inlet (28) in the liquid state.

Inventors: DEBONTE, WILLIAM JOHN
Applicants: DATAPRODUCTS CORPORATION
Application year: 1985
Main classification: B41J2
IPC codes: B41J2