Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-08-03
2001-06-12
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S094000
Reexamination Certificate
active
06244694
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to computer controlled printers that expel ink drop-by-drop to form images and, more particularly, to methods and apparatus for improving the operation of such printers.
BACKGROUND OF THE INVENTION
Computer controlled printers and in particular ink-jet and piezoelectric printers have been commercially available since at least the late 1980's. Their general construction is also well known, being the subject of numerous patents world-wide. An example of this technology can be found in U.S. Pat. No. 5,455,613 entitled “Thin Film Resistor Printhead Architecture for Thermal Ink-Jet Pens” by Canfield et al. issued on Oct. 3, 1995.
In a computer controlled printer, the ink is expelled drop-by-drop in a controlled manner. In a thermal ink-jet printer a firing resistor is electrically pulsed which in turn generates a drive bubble. The drive bubble expands in the firing chamber and expels a drop of ink from the chamber. In a piezoelectric printer a piezoelectric transducer is electrically pulsed which in turn expels a drop of ink from the chamber. In both, a region of vibration in the ink in the chamber is formed by the process of expelling the drop of ink. In addition, in both, the ink in the chamber bulges out of the orifice and a generally convex meniscus across the orifice results. The meniscus is not uniformly curved; the meniscus is actually oblate and also sloshes back and forth under the influence of the vibration of the ink in the chamber. The meniscus responds to a surface tension phenomenon. The ink in the chamber and the meniscus act much like a classical mass-spring-dashpot system.
Referring to
FIG. 1
, reference numeral
12
generally indicates a drop
14
of ink being expelled from an orifice plate
16
on the wall of a chamber
17
. Reference numeral
18
indicates the generally convex meniscus resulting after the expulsion of the drop.
Before expelling the next drop of ink, the chamber should be refilled. Refilling the chamber with ink as fast as possible is a very desirable design goal. However, if ink flows into the chamber too fast, the ink will flow out of the orifice and leak into the printer. On the other hand, refilling too slowly will cause the printer to operate unnecessarily slowly and the media throughput of the printer will be adversely affected.
In addition, before expelling the next drop from the chamber, both the vibration in the chamber must be damped out as much as possible and the meniscus flattened, or the trajectory of the next drop will be adversely affected. Specifically, if the next drop is prematurely expelled, the drop will not travel along its designed path and the quality of the resulting image will be degraded.
The effects of less than optimum damping and refilling are best shown in the graph,
FIG. 2
, which illustrates how the weight of the drops expelled from an ink-jet print head vary as the frequency of a firing resistor is changed. The geometry of the chamber and the chemical properties of the ink remain unchanged in
FIGS. 2 and 3
. The optimum firing frequency for the resistor is indicated by reference numeral
20
. The chamber overshoots and is not being damped sufficiently in the area indicated by reference numeral
21
.
Heretofore, to properly damp the vibration in the chamber and to achieve optimum refilling times, five hydraulic resistance variables have been optimized either through computer modeling or trial and error or both. The two parameters for ink are viscosity and surface tension, and the three geometric parameters of the print head are the length, width, and height of the ink inlet channel to the chamber.
FIG. 3
illustrates a fully damped, prior art chamber in which the problem of being under damped, i.e., overshooting, was eliminated. Reference numeral
22
indicates the optimum firing frequency for this chamber. Typically to achieve this prior damping solution, the length of the inlet channel to the chamber was lengthened and the width and the height of the channel were decreased. However, although overshooting was eliminated, the optimum firing frequency
22
was reduced as compared to the optimum firing frequency
20
in FIG.
2
. The net effect was that the printer ran slower and the output of media per minute was reduced.
It will be apparent from the foregoing that although there are well known ways of dampening the vibration in the ink in printers, there is still a need for an approach that allows the printer to operate as fast as possible while tolerating the maximum hydraulic under-damping that achieves acceptable print quality.
SUMMARY OF THE INVENTION
Briefly and in general terms, an apparatus according to the invention includes a means for expelling a liquid from a chamber drop-by-drop in a controlled manner, two flow conduits connected to the chamber, and means for sweeping the vibration, produced by the expulsion of a drop, out of the chamber and into one of the flow conduits.
In operation according to the invention, the apparatus expels a drop of liquid from the chamber and thereby creates a region of vibration in the liquid remaining in the chamber. The flow of liquid through the chamber sweeps the region of vibration out of the chamber, thereby hydraulically dampening the vibration.
The principal advantage of the invention is that by dampening the vibration in the chamber in the manner described, a printer can be operated at higher speeds and thus have a greater throughput of printed media, i.e., produce more printed pages per minute.
Further, the traditional mass-dashpot-spring damping system for the chamber is replaced by a new form of hydraulic compliance. Now instead of a “ringing” in the chamber that must be damped out and a convex meniscus forming at the orifice of the chamber that must be controlled, the flowing liquid entrains the vibration and its flow flushes the region of vibration out of the chamber via a second flow conduit.
The smaller hydraulic resistance of the chamber compared to the larger hydraulic resistances of the two flow conduits form a venturi that lowers the pressure in the chamber compared to the pressures in the two flow conduits. This lower pressure in the chamber decreases the curvature of the meniscus and lessens the likelihood of liquid flowing out of the orifice plate and into the printer.
The flow of liquid through the chamber results in several other benefits. The overall reliability of the firing resistor in a thermal inkjet print cartridge improves because after firing, the drive bubble is swept away from the resistor before collapsing and cavitation damage to the resistor is reduced. The flow also sweeps any entrapped air bubbles out of the chamber and out of the liquid flow path and onto other regions more suited to warehouse them without affecting the operation of the print head, thereby removing another source of drop trajectory instability. Also, the flow of ink through the print head carries off the heat generated in the print head by expelling drops.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 4496960 (1985-01-01), Fischbeck
patent: 4612553 (1986-09-01), Kohler
patent: 5455613 (1995-10-01), Canfield et al.
patent: 5459498 (1995-10-01), Seccombe et al.
patent: 5755032 (1998-05-01), Pan et al.
patent: 5815173 (1998-09-01), Silverbrook
patent: 5818485 (1998-10-01), Rezanka
Maker, II Edward
Weber Timothy L
Barlow John
Hewlett--Packard Company
Maker, II Edward
Stephens Juanita
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