Print window improvement for continous ink jet printer

Abstract

 A system and method are provided for improving the operating latitude for the charging and deflection voltage of an ink jet printhead, by catching non-print drops (16) with a wall impinging catcher to form a fluid layer in a face (10) of the catcher; and lowering drag on the fluid layer on the catcher face. This is accomplished by providing a drop generator for producing ink drops and a catcher/charge plate assembly for catching and recirculating non print drops. A printhead frame maintains the alignment of the drop generator and the catcher/charge plate assembly. Temperature induced distortion of these components is eliminated by heating them to a constant temperature.

Description

Technical Field

[0001] The present invention relates to the filed of continuous ink jet printers and, more particularly, to means to improve the operating latitude thereof.

Background Art

[0002] In continuous ink jet printers, ink drops are produced by a drop generator. These drops are formed in the vicinity of a charge plate which can selectively charge and deflect the ink drops. Drops which have been so charged and deflected are caused to impact a catcher. These ink drops form a fluid layer on the face of the catcher which flows down the catcher face and enters the catcher throat. From there the ink is claimed by the fluid system for reuse. Drops which have not been charged or deflected pass the catcher and strike the print media to form part of an image.

[0003] As ink jet printers are pushed to higher resolutions, the dot size must be reduced. This involves reducing the drop size. With the reduced drop momentum associated with the smaller drop size, the fluid layer thickness on catchers such as described in U.S. Patent No. 4,757,328 has been noted to nominally increase and be more unstable. This fluid layer has been found to adversely affect the operating latitude or print window for the charging and deflection voltage of the printhead.

[0004] It seen, then, that what is needed is a means to improve the operating latitude for the charging and deflection voltage of the printhead.

Summary of the Invention

[0005] This need is met by the means for improving the print window, according to the present invention, wherein the operating latitude of continuous ink jet printers is improved.

[0006] In accordance with one aspect of the present invention, a system and method are provided for improving the operating latitude for the charging and deflection voltage of an ink jet printhead, by catching non-print drops with a wall impinging catcher to form a fluid layer in a face of the catcher; and lowering drag on the fluid layer on the catcher face. This is accomplished by providing a drop generator for producing ink drops and a catcher/charge plate assembly for catching and recirculating non print drops. A printhead frame maintains the alignment of the drop generator and the catcher/charge plate assembly. Temperature induced distortion of these components is eliminated by heating them to a constant temperature, greater than 40C, and preferably approximately 43C.

[0007] Accordingly, it is an advantage of the present invention that it improves the operating latitude for the charging and deflection voltage of the print head in a continuous ink jet printing system.

[0008] Other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.

Brief Description of the Drawing

[0009] Figs. 1 and 2 are side views of a catcher face of a continuous ink jet print head,
illustrating fluid flow down the face, to illustrate the problem solved by the technique of the present invention.

Detailed Description of the Preferred Embodiments

[0010] In accordance with the present invention, ink is heated to an operational temperature which is higher than all expected ambient temperature. This is done to lower the ink viscosity to improve the fluid flow on the catcher. The ink temperature must be kept below the temperature at which the ink breaks down. This heated ink is further used to heat up the critical components of the printhead to a defined temperature so that thermal expansion induced distortions which might affect operating latitude of the printhead are eliminated.

[0011] Referring to Fig. 1, there is illustrated a side view of a wall-impingement catcher face 10 of a continuous ink jet print head, illustrating ink drops 16 striking the face of the catcher and the fluid 12 flow down the face 10. In order to improve the stability of the fluid film 12 (i.e. increasing the film velocity, thereby reducing the film thickness) on the catcher face 10, the resistance to fluid film flow on the catcher face must be reduced. As the ink flows down the catcher face, the viscous drag of the fluid moving down the catcher face causes the ink to slow down. As the fluid slows down, the fluid layer must get thicker to maintain the same flow rate, as shown in Fig. 1.

[0012] With smaller drops, the viscous drag causes the fluid film to slow more rapidly with large drops. As a result, smaller drops actually produces a thicker and slower layer of ink than is produced with larger ink drops. If the fluid layer get too large, the bulging fluid layer can interfere with uncharged drops intended as print drops 14. When print drops collide with the ink bulge and get absorbed into it, an area of the print media fails to be printed as it should be. The resulting print defect is called pick out or white defect.

[0013] This problem is further complicated by surface waves on the layer of ink on the catcher face. If the fluid velocity is higher than the wave velocity of the surface wave, waves produced by drop impacts move down the catcher in an orderly manner. If however the fluid velocity is slower than the wave velocity, surface waves can propagate up and across the catcher face. As these surface waves collide and interact, they can produce much larger wave crests. The large wave crests can then interfere with the undeflected drops as they pass in front of the catcher face.

[0014] The print window of a printhead is defined as the charge voltage range which gives proper print. At low voltages some catch drops 16 receive charge from charge plate 18 as a result of being adjacent to uncharged print drops 14, and are not properly caught, as shown in Fig. 2. The resulting print defect is known as dark defect. At high voltages the uncharged print drops, adjacent to charged catch drops, cannot properly clear the bulging fluid layer 12 on the catcher 10, as illustrated in Fig. 2. The resulting print defect is known as pick-out. As a result of the viscous drag, higher resolution printheads having smaller drop size tend to have reduced print windows.

[0015] To reduce this problem, it is desirable to reduce the drag on the fluid flowing down the catcher. One way to reduce the viscous drag of the fluid on the catcher face is to reduce the surface roughness of the catcher face. It has been found that the polishing or buffing the catcher face to produce a smoother finish is effective at improving the print window, as a result of the reduced drag. A surface finish of less than 6 microinch is preferred. Electropolishing is one effective means for providing the desired smooth surface.

[0016] An additional means to reduce the drag on the fluid is to increase the radius at the bottom of the catcher. As the fluid rounds the radius, the adhesion forces which keep the ink attached to the radius cause the fluid to decelerates. By increasing the radius the deceleration or drag is reduced. While a radius of about .03 inch works provides acceptable print windows for printheads having holes diameters of about 0.0012 inches, for hole diameters of less than 0.0011 inches a radius of about 0.06 inches is preferred.

[0017] An additional method for reducing the viscous drag on the ink is by reducing the viscosity of the ink. While this might be achieved by means of different formulations of ink, it is generally not a workable option. For water based inks, the viscosity of the ink will always be greater than that of water. The need for permanence on the print media 20 generally requires the addition to the ink of components which raise the viscosity.

[0018] A second means to lower the ink viscosity is to raise the ink temperature. As the viscosity of normal fluids, including inks, decrease exponentially with increasing temperature, it is desirable to operate the printhead at a high temperature. Care must be taken however to avoid heating the ink to the point that it begins to break down. Therefore, in a preferred embodiment of the present invention, a typical ink or fluid ejected from drop generator 24 is heated to a preferred ink temperature of about 43 degrees C. It has been found that heating the ink in this manner makes a significant improvement in the print windows as a result of the reduced drag.

[0019] The use of heated ink has an additional advantage for long arrays. By always operating at a constant temperature, in particular, a temperature of approximately 43C, the printhead parts can be fabricated and aligned without having to deal with thermal expansion differences warping the parts or causing the alignment to shift. For longer arrays, the thermal expansion induced distortion of the critical components can be severe on systems where the printhead temperature is not controlled. In a preferred embodiment for a long array ink jet printhead each of the critical printhead assemblies is heated by the ink. This includes the drop generator assembly, catcher/charge plate assembly, and the printhead frame which maintains the alignment of the other two components. The drop generator and the catcher/charge plate assembly are naturally heated by the ink as a result of carrying out their ink jet functions. The printhead frame is heated by using it as a fluid manifold through which the heated ink passes. By heating the main printhead part in this manner, the print window is not degraded or shifted by thermal induced distortions of shifts in the printhead.

[0020] An additional benefit of heated the ink to a constant temperature is that temperature dependent fluid properties no longer need to be compensated for. By using a constant operating temperature for the ink, it is no longer necessary to deal with temperature compensation for the stimulation amplitude, ink pressure, concentration sensor, or drop charging phase.

[0021] The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that modifications and variations can be effected within the spirit and scope of the invention.

Claims

1. A method for improving the operating latitude for the charging and deflection voltage of an ink jet printhead, comprising the steps of:

catching non-print drops with a wall impinging catcher to form a fluid layer in a face of the catcher; and

lowering drag on the fluid layer on the catcher face.

2. A method as claimed in claim 1 wherein the step of lowering drag on the fluid layer comprises the step of improving surface finish of the catcher face to less than 12 microinch.

3. A method as claimed in claim 1 wherein the step of lowering drag on the fluid layer comprises the step of using a catcher radius of greater than 0.050 inch.

4. A method as claimed in claim 1 wherein the step of lowering drag on the fluid layer comprises the step of lowering the ink viscosity or increasing the ink temperature.

5. A method for improving the operating latitude for the charging and deflection voltage of an ink jet printhead comprising the steps of:

providing a drop generator for producing ink drops;

providing a catcher/charge plate assembly for catching and recirculating non print drops;

providing a printhead frame for maintaining the alignment of the drop generator and the catcher/charge plate assembly;

eliminating temperature induced distortion of these components by heating them to a constant temperature.

6. A method as claimed in claim 5 further comprising the step of providing a drop generator having orifice diameters of less than 0.0011 in.

7. A method as claimed in claim 5 further comprising the step of lowering drag on the fluid layer

8. A method as claimed in claim 7 wherein the step of lowering drag comprises the step of improving surface finish of the catcher face by electrochemical means.

9. A method as claimed in claim 7 wherein the step of lowering drag of the fluid comprises the step of increasing ink temperature.

10. A method as claimed in claim 9 wherein the step of increasing ink temperature comprises the step of increasing the ink temperature to a temperature above 40 degrees Celsius.

Drawing