THERMAL INK-JET HEAD STRUCTURE

Abstract: Off-setting the orifice (20) from the resistor (10) in a thermal ink-jet printhead provides improved print quality by controlling misdirection of first drops (26), second and subsequent drops (28) in multi-drop printing, and for satellite drop control.

Description: The present invention relates to thermal ink-jet printers, and, more particularly, to apparatus for improving the quality of printing from a thermal ink-ket printhead. Spray resulting from misdirected drops in frequently observed for thermal ink-jet heads. The problem is considerably worse for printing in the "multi-drop" mode, that is, printing groups of drops in bursts at 50 kHz drop rates. U.S. Patent 4,330,787 describes a thermal ink-jet printer in which the angle between the normals to the plane of the resistor and the plane of the orifice are between 0° and 90°. However, changing the angle between the resistor and orifice still does not provide the printing quality required. Normally, the orifice lies directly above the resistor. Experiments have shown that alignment between the resistor and orifice in the thermal head is a critical factor influencing the direction of existing drops. For heads employing the three-sided barrier structure, perfect alignment of the orifice over the resistor has been found not to be the ideal condition. In accordance with the invention, an appropriate off-set between the resistor and orifice of a thermal ink-jet printhead significantly improves drop directionality. Such improvement in drop directionality yields improved print quality. The extent of off-set depends on resistor and orifice sizes, as well as on other details of the head architecture. The off-set is an amount sufficient to maintain droplets of ink ejected from the orifice by a trajectory of less than about 0.5° from the normal to the orifice plate. To a first approximation, the center of the orifice is offset from the center of the resistor by about 1 to 25 µm. FIG. 1 is a top plan view, depicting a resistor associated with a three-sided barrier structure; FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 1, of an aligned resistor/orifice, illustrating misaligned firing of a drop of ink; FIG. 3 is a view similar to that of FIG. 2, illustrating an example of multidrop operation, with the trajectory of a second drop unacceptably different from the first drop as a consequence of employing an aligned resistor/orifice; FIG. 4 is a view similar to that of FIG. 2, but using a misaligned, or offset, resistor/orifice in accordance with the invention, illustrating proper firing of a drop of ink; FIG. 5 is a view similar to that of FIG. 4, illustrating an example of multidrop operation, with the trajectory of the second drop substantially the same as that of the first drop; FIG. 6 is a plot on coordinate axes of trajectory angle (in degrees) as a function of orifice-to-resistor off-set (in µm) for several conditions of resistor/orifice aspect ratios, showing data for the ejection of the first drop through the orifice; and FIG. 7 is a plot on coordinate axes of trajectory angle (in degrees) as a function of orifice-to-resistor off-set between the resistor and the orifice (in µm), showing data from the ejection of the second drop through the orifice. Referring now to the drawings wherein like numerals of reference designate like elements throughout, a resistor 10 is encompassed on three sides by a three-wall barrier structure 12, having side barriers 12a,b and rear barrier 12c. As is common, the resistor has square dimensions. Ink from a reservoir (not shown) enters from the fourth side, as indicated by arrow 14. An orifice plate 18, shown in FIG. 2, is provided with an orifice 20. The orifice 20 is positioned over the resistor 10. A substrate 22 supports the resistor 10 and the barrier structure 12. In operation, ink flows into the assembly 10 from the side opposite the rear barrier 12c from the ink reservoir. Upon appropriate application of a current to the resistor 10 from a voltage source (not shown) controlled by a microprocessor (not shown), the resistor emits a sufficient amount of heat to vaporize a thin layer of the ink, thereby forming a bubble 24. The rapid expansion of the bubble 24 causes a droplet 26 of ink to be propelled out through the orifice 20 toward a suitable printing medium, such as paper, transparency, and the like. Use of centered alignment of the orifice 20 over the resistor 10 causes misalignment of the trajectory (arrow B) of the droplet 26 with respect to the normal to the orifice plate 18 (arrow A). FIG. 2 is a line drawing of a photomicrograph, showing the shape of the droplet 26 and its misaligned trajectory. The ink is ejected at an angle ϑ with respect to the normal. In the multidrop mode, spray results from misdirected drops. To illustrate, FIG. 3 is a line drawing of a photomicrograph, depicting an example of multidrop operation at 50 kHz, where the trajectory of a second drop 28 is unacceptably different from that of the first drop 26. A significant angle of the second drop 28 relative to the orifice normal (A) as seen in the drawing is observed even when the orifice 20 is perfectly aligned with the resistor 10. It appears from studies that the first angle must be less than about 0.5° of the normal to the orifice plate 18 in order to have acceptable print quality. Thus, it is critical that the first drop, subsequent drops and any satellite drops be as close to the normal as possible. Centered alignment results in firing angles considerably greater than about 0.5° and hence are not useful either for single drops or for multiple drops at firing frequencies of 50 kHz and above, with up to about 10 drops per firing burst. In accordance with the invention, the orifice 20 is deliberately misaligned with respect to the resistor 10. Such an off-set assembly is depicted in FIG. 4, with the orifice 20 offset in the direction away from the rear barrier 12c by an amount designated X. Such offset provides a trajectory of the drop 26 substantially along the normal A. FIG. 5 depicts an example of multidrop operation at 50 kHz, showing that the trajectory of the second drop 28 is very close to that of the first drop 26 as a consequence of employing deliberate misalignment of the resistor 10 and orifice 20. FIG. 6 shows measurements of angular misdirection as a function of resistor/orifice offset for the first drop 26 ejected from a thermal ink-jet head with resistors 10 of various sizes and converging orifices 20 with various exit diameters. The measurement drop rate was below 1 kHz. In particular, the following curves reflect measurements for the below-listed resistor sizes and orifice openings: Curve Resistor Size, µm Bore, µm 30 60 x 60 65 32 50 x 50 65 34 65 x 65 45 36 50 x 50 45 It is clear that for the three-wall geometry, the orifice 20 should be offset at least about 1 µm further away from the third barrier 12c than the center of the resistor 10. The range of misalignment (X) is about 1 to 25 µm, preferably about 1 to 20 µm and most preferably about 2 to 10 µm. In FIG. 6, there is a dependence of trajectory error on offset for the first drop ejected. The curves do not go through the origin; therefore, at perfect alignment, there is a trajectory error. This error can be corrected with an offset, in accordance with the invention. For one set of head parameters, for a resistor measuring 50 µm x 50 µm and an orifice measuring 65 µm (Curve 32), that trajectory from the normal is as much as 1°. FIG. 7 shows a measurement of angular misdirection as a function of resistor/orifice off-set for the second drop 26 ejected from the ink-jet head with 63 µm x 63 µm resistors 10 and converging orifices 20 with an exit diameter of 50 µm (Curve 38). The measurement drop rate was 50 kHz. It is clear that for this geometry, the misalignment for the first drop benefits the trajectory of the second drop. In FIG. 7, for the second drop, the trajectories are at larger angles. For the case shown, perfect alignment would yield a trajectory from the normal of about 8°. However, at an offset of 9 µm, the second drop follows the first. It appears that the detailed break-off of the tail of ejected drops is related to the resistor/orifice alignment. For a properly "aligned" resistor/orifice (i.e., "properly off-set"), the tail will break-off at the center of the orifice, which will result in minimum spray due to misdirected satellite droplets. The orifice/resistor off-set disclosed herein appears to be critical in achieving the highest quality printing. Such off-set may range from 1 to about 25 µm to control misdirection of first drops, second and subsequent drops in multidrop printing, and for satellite drop control. Orifice/resistor off-set is expected to be used in constructing thermal ink-jet printheads for ink-jet printers. Thus, orifice/resistor off-set is thermal ink-jet printheads provides improved print quality. Many modifications and changes of an obvious nature may be made without departing from the spirit and scope of the invention, and all such modifications and changes are considered to fall within the scope of the invention, as defined by the appended claims.

Claims: A thermal ink-jet printhead for ink jet printing onto a print medium comprising laminar electrical heating means (10), the periphery of which is substantially surrounded by a barrier structure (12a-12c), and an orifice plate (18) which lies above the heating means (10), having an orifice (20) with the centre of the orifice (20) offset from the, centre of the heating means (10), and the offset between the centre of the orifice (20) and the centre of the heating means (10) producing droplets of ink that are ejected from the orifice (20) with a trajectory less than about 0.5° from a line normal to the line of the heating means (10). The thermal ink-jet printhead of claim 1 wherein the heating means (10) is a controlled resistor. The thermal ink-jet printhead of any preceding claim wherein the heating means (10) has four sides to its periphery of which the barrier structure (12a-12c) surrounds three. The ink-jet printhead of any preceding claim wherein the orifice plate (18) is substantially parallel to the heating means (10). The thermal ink-jet printhead of any preceding claim wherein the amount of offset ranges from about 1 µm to 25 µm. A method of fabricating a thermal ink-jet printhead comprising the steps of; forming laminar electrical heating means (10), forming a barrier structure (12a-12c) to partially surround the heating means, and forming an orifice plate (18) having an orifice (20) therein above the heating means with the centre of the orifice and the centre of the heating means offset to one another, wherein the amount of offset is selected to produce droplets that are ejected from the orifice with a trajectory less than about 0.5° from a line normal to the line of the heating means. The method of claim 6 wherein the amount of offset ranges from about 1 µm to 25 µm.

Inventors: DENLER, GORDON D., TAUB, HOWARD H.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: B41J2
IPC codes: B41J2