Method and apparatus for forming and moving ink drops

Abstract: A method of forming and moving ink drops across a gap between a print head (14) and a print medium (20) in a marking device includes generating an electric field, forming the ink drops adjacent the print head (14) and controlling the electric field. The electric field is generated to extend across the gap. The ink drops are formed in an area adjacent the print head (14). The electric field is controlled such that an electrical attraction force exerted on the formed ink drops by the electric field is the greatest force acting on the ink drops.

Description: This invention relates to ink forming and moving ink drops. Conventional ink drop printing systems use various methods to form and impact ink drops upon a print medium. Well-known devices for ink drop printing include thermal ink jet print heads, piezoelectric transducer-type ink jet print heads and bubble jet print heads. Each of these print heads produces approximately spherical ink drops having a 15 to 100 µm diameter. Acoustic ink jets can produce drops that are less than 15 µm in diameter. These smaller ink drops lead to increased resolution. Conventional print heads impart a velocity of approximately four meters per second on the ink drops in a direction toward the print medium. Actuators in the print heads produce the ink drops. The actuators are controlled by a marking device controller. The marking device controller activates the actuators in conjunction with movement of the print medium relative to the print head. By controlling the activation of the actuator and the print medium movement, the print controller directs the ink drops to impact the print medium in a specific pattern, thus forming a desired image on the print medium. Conventionally, the actuators also impart an impulsive force to propel the ink drops across a gap separating the print head and the print medium. A significant amount of energy is required to form and propel the ink drops. Moreover, some types of actuators are very inefficient. For example, the efficiency of piezoelectric devices is approximately 30%. In acoustic ink jet printing, approximately 95% of the energy input to form and expel the ink drops is lost in the form of excess heat. Such excess heat is undesirable because it raises the operating temperature of the surrounding components, such as the print head. This leads to thermal stresses that decrease the long-term reliability of the device. Copending European Patent Application No. 96304090.2, which is commonly assigned, discloses providing an electric field to assist in directing ink drops toward the print medium in a desired manner, e.g., by selectively deflecting the ink drops slightly to enhance the resolution of the image produced by a given print head configuration. The ink jet actuators form and impart an initial velocity on the ink drops. The charged ink drops are then steered by electrodes such that the drops alternately impact upon the print medium at positions slightly offset from positions directly opposite the apertures of the print head. Although this method increases the resolution of the image formed on the print medium, it does not address the problem of controlling the operating temperature of the print head. As a result, the high print head operating temperature shortens the usable life of the device. Further, this method does not address the problem of satellite drops. Satellite drops are formed due to imperfections in the formation of primary ink drops. Satellite drops are much smaller than primary drops, and thus tend to be more influenced by environmental conditions, e.g., air currents in the gap. In conventional devices, the satellite drops decelerate rapidly due to higher air drag. At some point, the satellite drops return and impact on the print head. Other drops that cross the gap produce undesirable printing artifacts due to the result of air currents that reduce the print quality. This result is undesirable because the accumulation of satellite drops on the print head can decrease its performance over time. The invention addresses the problems of actuator efficiency, energy consumption, and print head temperature control, described above. The invention alleviates these problems by forming ink drops with an initial velocity of approximately zero, then providing an electric field to accelerate the ink drops from rest to move across the gap. This approach is advantageous because it significantly reduces actuator energy consumption and improves drop formation efficiency. As a result, the actuator and surrounding components can operate at a reduced temperature, extending print head life and device reliability. Previous systems which are capable of forming ink drops with an initial velocity of approximately zero are described in EP-A-0437062 and DE-A-3211345. The invention also addresses the problem of satellite drops, described above. The invention alleviates this problem by providing an electric field that provides approximately the same travel time from print head to print medium for primary and satellite ink drops, which therefore impact the print medium at approximately the same time. The electric field further serves to polarize charge within a pre-drop plume by induction. Therefore, the resulting primary drop and its satellite drops are all charged and are therefore all accelerated by the field, so that no initial velocity component toward the print medium is necessary. This approach is advantageous because it prevents satellite drop accumulation on the print head without reducing resolution. This approach is applicable even to actuators that form ink drops of less than 15 µm. In a first aspect, the invention consists in a method of forming and moving ink drops across a gap from an ink surface of a print head in a marking device to a print medium, the method including the steps of: forming an ink drop with an initial velocity of approximately zero adjacent the print head by imparting energy to the ink beneath the ink surface with an acoustic wave; and moving the ink drop across the gap to the print medium by providing an electric field to exert an electrical force to accelerate the formed ink drop from rest to move across the gap; characterised in that the step of forming the ink drop includes focusing the acoustic wave with a Fresnel lens In a further aspect, the invention consists in apparatus for forming and moving ink drops across a gap from an ink surface of a print head in a marking device to a print medium, the apparatus including drop formation means for forming an ink drop with an initial velocity of approximately zero adjacent the print head by imparting energy to the ink beneath the ink surface with an acoustic wave; and drop moving means for moving the ink drop across the gap to the print medium with an electric field which exerts an electrical force to accelerate the formed ink drop from rest to move across the gap; characterised in that the drop formation means also includes a Fresnel lens which focuses the acoustic wave. The generating step can include biasing the print support medium with a voltage source. Further, the generating step can include charging the print head, e.g., setting the print head to ground. The ink drops can be formed by exerting an ink drop forming force slightly greater than a threshold surface tension force that acts in a direction opposite the drop forming force. The electric field can be controlled to maintain a field strength of approximately 1.0 V/µm. The electric field can also be controlled such that a travel time from the print head to the print medium is approximately the same for the primary and satellite ink drops that are smaller than the primary ink drops. The ink drops can be formed to have a radius of at least approximately 1 µm and not greater than 15 µm. Forming the ink drops can include producing a plume of ink extending in a direction from the print head toward the print medium and separating an end portion of the plume to form the ink drops. The electric field can be generated by a voltage source. The drops can be formed by an acoustic ink jet-type actuator. The gap between the print head and the print medium is preferably approximately 1 millimeter. In one embodiment, an apparatus includes an ink jet marking device having a print head for forming an image on a print medium. The print head is separated from the print medium by a gap. The marking device includes a generating device that generates an electric field across the gap, a drop forming device that forms drops of ink adjacent the print head and a controller coupled to the generating device and the drop forming device for controlling the electric field such that an electrical attraction force exerted on the formed ink drops is greater than other forces acting on the ink drops. The drop forming device is coupled to the generating device. The ink jet marking device can also include a print medium support positioned on a side of the print medium opposite the print head. The print medium support is coupled to the generating device such that the generating device produces a voltage on the print medium support. Preferably, the generating device is a voltage source. The drop forming device preferably forms drops of ink by exerting a drop forming force slightly greater than a threshold surface tension force acting in an opposite direction. Preferably, the drop forming device includes an acoustic ink jet-type actuator. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram showing general features of a printing device; Fig. 2 is a top view of a marking device; Fig. 3 is a graph showing the ink drop radius versus the ink drop impact velocity for two different field strengths; Fig. 4 is a graph showing the ink drop radius versus the electric field strength for various times of flight; Fig. 5 is a graph showing the ink drop radius versus the required ink drop charge for various times of flight; and Fig. 6 is a graph showing the ink drop radius versus the ink drop impact velocity for various times of flight In Fig. 1, a voltage source 10 is shown coupled to a print head 14 and to a print medium support 18. A marking device controller 12 directly communicates with and is coupled to the print head 14. The marking device controller 12 controls a print medium movement mechanism (not shown) that moves a print medium 20 relative to the print head 14. The print medium 20 is preferably a sheet or roll of paper, but can also be transparencies or other materials. In one embodiment, the print head 14 is a page-width print head and the print medium 20 is moved relative to the print head 14. Alternatively, the print head 14 can be configured as a scanning print head to move relative to either a stationary or a movable print medium. The print head 14 includes a drop forming device 16. The drop forming device 16 is an acoustic ink drop actuator. As shown in Fig. 2, an electric field F is established between a print medium support 18 and a front surface 32 of the print head 14 by the voltage source 10. The print medium support 18 is made of a conductive material, usually metal. A dielectric coating 21 about 1 mil (25.4 µm) thick is coated onto the print medium support 18. The print medium 20 is positioned between the front surface 32 of the print head 14 and the print medium support 18 in contact with the dielectric coating 21. A gap G between the front surface 32 and the print medium 20 is approximately 1 mm. The print head 14 includes a series of apertures 22, two of which are shown, through which ink exits the print head 14. The print head 14 also includes one or more drop forming devices 16 that impart energy into the surrounding ink to form drops at an ink surface 30 adjacent the front surface 32. In an acoustic ink drop actuator drop forming energy is provided at radio frequencies (RF) and is therefore referred to as RF energy. The drop forming device 16 is of the acoustic actuator-type. In an acoustic actuator-type drop forming device, a transducer is excited to produce an acoustic wave in the ink. The wave is focused through a Fresnel lens to a point just below the ink surface 30. The focused acoustic energy creates a pressure difference that causes an ink plume 28 to form, as shown in the left side of Fig. 2. The drop forming force D is a liquid jet which acts in a direction opposite the ink surface 30 and the drop forming device 16. The drop forming force D increases and eventually exceeds a threshold surface tension force S. The plume 28 breaks to form a primary drop 24, as shown in the right side of Fig. 2. The plume 28 extends outward from the ink surface 30 by a distance proportional to a radius of the resulting drop formed when the plume 28 breaks. Due to the biased field, plume 28 is inductively charged with a polarity opposite the field and drops formed when plume 28 breaks all have a net charge so that the field accelerates them toward print medium 20. In a conventional ink jet apparatus, the primary drop 24 is influenced by an additional expulsion force component that propels the primary ink drop 24 across the gap G to the print medium 20. In the device according to copending European Patent Application No. 96 304 090.2, this expulsion force is further supplemented by a force due to an electric field established across the gap G. In the present embodiment, however, the drop forming force D is only slightly greater than the threshold surface tension force S that acts in the opposite direction. Therefore, the drop forming force D is only sufficient to form the primary drop 24. A satellite drop 26 may also be formed due to imperfections in the formation of the primary drop 24. In conventional devices, satellite drops 26 tend to return toward and impact upon the front surface of the print head 32, which is undesirable. According to the present embodiment, satellite drops 26 are controlled to have the same flight time as primary ink drops. The electric field F exerts a Coulomb force C on the primary and satellite ink drops. The ink drops are formed without being forcibly expelled. The Coulomb force is the greatest force acting on the ink drops. Accordingly, the Coulomb force is greater than the other forces acting on the ink drops, which include the drag force due to friction between the ink drops and the air through which they travel. Fig. 3 shows the effect of the drag force due to air. For drops that are ejected at 4 meters per second, which is within the range of conventional devices, drops having a radius of less than 4.6 µm are retarded by the drag force and fail to cross the gap, as indicated by the left-hand portion of the lower curve. The retarded ink drops return to the front surface 32, which contaminates the print head 14. As shown by the upper curve, a field strength of 1.5 V/µm ensures that ink drops of all sizes move under the Coulomb force C across the gap. Within the electric field F, a finite charge is induced in the plume 28 proportional to the net voltage difference between the tip of the plume 28 and the front surface 32, the radius R of the ink drop and the voltage difference across the gap G (i.e., the field strength). By controlling the field strength, the amount of induced charge can be controlled. Correspondingly, by controlling the field strength, the dynamics of how quickly the ink drops travel across the gap (i.e., the "time of flight") can be controlled. Therefore, the electric field F both charges and accelerates the ink drops. Referring to Fig. 4, a required field strength is determined by setting a simulation constraint such that drops having a range of radii traverse the gap within specified times of flight. Surprisingly, the times of flight for drops of different sizes are approximately the same for a field strength of 1.0 V/µm as shown by the flat portion of the lowest curve. The times of flight for satellite droplets in the lower range of radii and primary droplets in the upper range of radii are approximately the same. More generally, for a given field strength, approximately equal flight times for primary and satellite drops can be obtained by adjusting other parameters. Referring to Fig. 5, the required drop charge to traverse the gap in the specified times of flight can be determined. In the range from approximately 1 to 8 µm as shown, the required level of charge can be obtained with aqueous inks. In particular, because this range of radii is in the transition region, neither the Coulomb force (which is inversely proportional to R) nor the drag force (which is inversely proportional to R 2 ) dominates. Referring to Fig. 6, the impact velocities corresponding to the range of electric field strengths can be shown. In particular, ink drops may accelerate from a velocity of zero to impact the print medium at a velocity of several meters per second by using a 1.0 V/µm field. Accordingly, by controlling the drop formation energy and the strength of the electric field. repeatable and controllable printer performance is possible. Test results show that an embodiment of the present invention requires 25% less energy to operate than a conventional device.

Claims: A method of forming and moving ink drops across a gap (G) from an ink surface (30) of a print head (14) in a marking device to a print medium (20), the method including the steps of: forming an ink drop (24) with an initial velocity of approximately zero adjacent the print head (14) by imparting energy to the ink beneath the ink surface (30) with an acoustic wave; and moving the ink drop (24) across the gap (G) to the print medium (20) by providing an electric field (F) to exert an electrical force to accelerate the formed ink drop (24) from rest to move across the gap; characterised in that the step of forming the ink drop (24) includes focusing the acoustic wave with a Fresnel lens. The method of claim 1, in which the step of forming an ink drop further comprises: exerting an ink drop forming force (D) slightly greater than a threshold surface tension force (S) that acts in a direction opposite the ink drop forming force (D). The method of claim 1 or claim 2, in which the step of forming an ink drop further comprises forming a satellite ink drop (26) smaller than the ink drop (24); and in which the step of moving the ink drop further comprises: providing an electric field (F) with a field strength that accelerates the ink drop (24) and the satellite ink drop (26) to move across the gap (G) to the print medium (20) in approximately the same travel time. The method of any one of the preceding claims, in which the step of moving the ink drop further comprises providing an electric field (F) with a field strength of approximately 1.0 V/µm. The method of any one of the preceding claims, in which the step of forming an ink drop further comprises producing a plume (28) of ink extending in a direction from the print head (14) toward the print medium (20); and separating an end portion of the plume (28) to form the ink drop (24). The method of any one of the preceding claims, in which energy is imparted to the ink beneath the ink surface (30) at radio frequencies (RF). A apparatus for forming and moving ink drops (24) across a gap (G) from an ink surface (30) of a print head (14) in a marking device to a print medium (20), the apparatus including drop formation means (16) for forming an ink drop (24) with an initial velocity of approximately zero adjacent the print head (14) by imparting energy to the ink beneath the ink surface (30) with an acoustic wave; and drop moving means (10) for moving the ink drop (24) across the gap (G) to the print medium (20) with an electric field (F) which exerts an electrical force to accelerate the formed ink drop (24) from rest to move across the gap (G); characterised in that the drop formation means (16) also includes a Fresnel lens which focuses the acoustic wave. The apparatus according to claim 7, wherein the drop formation means (16) exerts an ink drop forming force (D) slightly greater than a threshold surface tension force (S) that acts in a direction opposite the ink drop forming force (D). The apparatus according to claim 7 or claim 8, in which the drop formation means (16) further forms a satellite ink drop (26) smaller than the ink drop (24); the drop moving means (10) providing an electric field (F) with a field strength that accelerates the ink drop (24) and the satellite ink drop (26) to move across the gap (G) to the print medium (20) in approximately the same travel time. The apparatus according to any one of claims 7 to 9, in which the drop formation means (16) produces a plume (28) of ink extending in a direction from the print head (14) toward the print medium (20) and separates an end portion of the plume(28) to form the ink drop (24), the electric field (F) provided by the drop moving means (10) polarizing the plume (28) before the end portion separates to provide a charge on the ink drop (F). The apparatus according to any one of claims 7 to 10, in which the drop formation means (16) comprises an acoustic ink jet-type actuator. The apparatus according to any one of claims 7 to 11, wherein the drop formation means (16) imparts energy to the ink beneath the ink surface (30) at radio frequencies (RF).

Inventors: LEAN MENG H, ROY JOY, SMITH DONALD L, STEARNS RICHARD G, LEAN, MENG H., ROY, JOY, SMITH, DONALD L., STEARNS, RICHARD G.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1997
Main classification: B41J2
IPC codes: B41J2