Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-01-19
2002-06-11
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06402280
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to a liquid ink printing apparatus and a method for gray scale printing. More particularly, the invention relates to an ink jet printhead having different size drop ejectors.
2. Description of Related Art
Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are ejected towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink.
An ink jet printhead can include one or more thermal ink jet printhead dies having an individual heater die and an individual channel die. The channel die includes an array of fluidic channels which bring ink into contact with the resistive heaters which are correspondingly arranged on the heater die. In addition, the die may also have integrated addressing electronics and driver transistors. Fabrication yields of die assemblies at a resolution on the order of 300-600 channels per inch is such that the number of channels per die is preferably in the range of 50-500 under current technology capabilities. Since the array of channels in a single die assembly is not sufficient to cover the length of a page, the printhead is either scanned across the page with a paper advance between scans or multiple die assemblies are butted together to produce a page width printbar. Because thermal ink jet nozzles typically produce spots or dots of a single size, high quality printing requires the fluidic channels and corresponding heaters to be fabricated at a high resolution on the order of 400-600 channels per inch.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers typically print information received from an image output device such as a personal computer. Typically, this received information is in the form of a raster scan image such as a full page bitmap or in the form of an image written in a page description language. The raster scan image includes a series of scan lines consisting of bits representing pixel information in which each scan line contains information sufficient to print a single line of information across a page in a linear fashion. Printers can print bitmap information as received or can print an image written in the page description language once converted to a bitmap consisting of pixel information.
In a printer having a printhead with equally spaced nozzles, each of the same size nozzles producing ink spots of the same size, the pixels are placed on a square first grid having a size S, where S is generally the spacing between the marking transducers or channels on the printhead as illustrated in a sample printing pattern of FIG.
2
. The nozzles
60
(schematically represented as triangles) traverse across a recording medium in the scan direction X as illustrated. The nozzles, which are spaced from one another a specified distance d, also known as the pitch, deposit ink spots or drops on pixel centers
62
on the grid having the grid spacing S in a direction perpendicular to the scanned direction, which is of course dependent on the spacing d. Typically, the nozzles and printing conditions are designed to produce spot diameters of approximately 1.414 (the square root of 2) times the grid spacing S. This allows complete filling of space, by letting diagonally adjacent pixels touch. A disadvantage of this printing scheme is that jaggedness may be objectionable at line edges, particularly for lines or curves at small angles to the scan direction as illustrated in
FIG. 2. A
first ellipse
64
located outside a second ellipse
66
in
FIG. 2
, indicate at what portions of the printed image the jaggedness would be most objectionable. In addition, print quality can be determined by 1) how much white space remains within the ring defined by the first and second ellipses, 2) how far the spots extend outside either the first or second ellipse, and 3) the amount of ink deposited on the recording medium.
One method of improving the line edge quality is to extend the addressability of the carriage to thereby allow dot placement at intermediate positions in the grid in the scanned direction. It is also possible to improve line edge quality by increasing the resolution. This, however, increases the complexity and cost of fabrication and typically slows down printing because of the additional number of spots to be printed.
The printheads and printing methods discussed above, and illustrated in
FIG. 2
for example, provide for the printing of ink jet images having sufficient quality, especially when the resolution is increased upwards to 600 channels per inch. These printheads and methods, however, do not always provide images having the desired quality especially when considering gray scale levels, ink saving print modes, and printing throughput.
A majority of thermal ink jet printers produce spots or drops of ink all having the same diameter, within approximately about 10 percent, and are therefore not capable of gray scale printing. Drop volume or spot size is determined by many factors, including the heater transducer area, the cross sectional area of the ink ejecting channel or nozzle, the pulsing conditions necessary to create an ink droplet and the physical properties of the ink itself, such as the ink temperature. Although spot diameter changes of approximately ±10 percent are possible by changing pulsing conditions or ink temperature during printing, the given spot size is nominally constant to the extent that deliberate spot size variations cannot span a large enough range to be useful in gray scale printing.
Another method of improving printing quality, especially gray scale printing quality is to use groups of different size nozzles, as disclosed in U.S. Pat. No. 5,745,131 to Kneezel et al., which is hereby incorporated by reference
Harrington Steven J.
Kneezel Gary A.
Mantell David Allen
O'Neill James F.
Tellier Thomas A.
Barlow John
Huffman Julian D.
Oliff & Berridg,e PLC
Xerox Corporation
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