The invention relates generally to the field of ink jet printing and more particularly to ink jet printing with a print head capable of depositing multiple drop sizes.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner onto an image recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the recording element to yield the desired image. In one process, known as drop-on-demand ink jet printing, individual ink droplets are projected as needed onto the recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation using heat actuators. With regard to heat actuators, a heater placed at a convenient location within the nozzle or at the nozzle opening heats the ink in selected nozzles and causes a drop to be ejected to the recording medium in those nozzles selected in accordance with image data. With respect to piezoelectro-actuators, piezoelectric material is used which piezoelectric material possesses the property such that when electrical field is applied to the material a mechanical stresse is induced therein reducing the volume of the nozzle and causing a drop to be selectively ejected from the nozzle selected. The image data applied to the print head determines which of the nozzles are selected for ejection of a respective drop from each nozzle at a particular pixel location on a receiver sheet. Some drop-on-demand ink jet printers described in the patent literature use both piezoelectric actuators and heat actuators.
In another process, known as continuous ink jet printing, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the recording element, while unimaged droplets are caught and returned to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and pictorial imaging to short run printing and industrial labeling.
A typical ink jet printer reproduces an image by ejecting small drops of ink from a print head containing an array of spaced apart nozzles, where the ink drops land on a receiver medium (typically paper) to form round ink dots. In some printers, all drops are the same size, and therefore, all dots are the same size. Normally, these drops are deposited with their respective dot centers on a rectilinear grid, a raster, with equal spacing, p, in the horizontal and vertical directions. Therefore, to achieve full coverage of the ink it is necessary for the dots, 10, to have at least diameter p*sqrt(2), as shown in FIG. 3. Some printer designs may allow for even bigger dots in order to compensate for unwanted variations in the placement of the drops.
Modem inkjet printers may also possess the ability to vary (over some range) the amount of ink that is deposited at a given location on the page. Ink jet printers with this capability are referred to as xe2x80x9cmultitonexe2x80x9d or gray scale ink jet printers because they can produce multiple density tones at each pixel location on the page. Some multitone ink jet printers achieve this by varying the volume of the ink drop produced by the nozzle by changing the electrical signals sent to the nozzle or by varying the diameter of the nozzle. See for example U.S. Pat. No. 4,746,935. Other multitone ink jet printers produce a variable number of smaller, fixed size droplets that are ejected by the nozzle (or by plural nozzles during different passes of the nozzle array), all of which are intended to merge and land at the same pixel location on the page. See for example U.S. Pat. No. 5,416,612. These techniques allow the printer to vary the size or optical density of a given ink dot or pixel, which produces a range of density levels at each dot or pixel location, thereby improving the image quality. Thus printing methods that require multiple drop sizes usually depend upon the way the drops are generated by the print head. As noted above some print heads have multiple size nozzle diameters, others have circuitry in which the individual ink chambers acccept changing electrical signals to instruct each chamber how much ink to eject. Still other print heads have nozzles that eject a variable number of small, fixed size droplets that are intended to merge then land in a given image pixel location. Printing methods that deposit more than one drop in a pixel location are typically carried out by multiple printing passes wherein the print head prints a row of pixels pixels multiple times, the image data to the print head changing in accordance with each pass so that the correct number of total droplets deposited at any pixel location is commensurate with the density required by the processed image data.
The exact relationship between drop size and dot size depends on many factors. However, in many cases it can be approximated by the equation
d=a*vb,xe2x80x83xe2x80x83(Equ 1)
where d is the diameter of the dot, v is the volume of the drop, a is a positive constant whose magnitude depends on the units of d and v, and b is a positive constant in the range 0.0 to 1.0. This means that the ratio of dot size to volume is given by
d/v=a*v(bxe2x88x921).xe2x80x83xe2x80x83(Equ 2)
Therefore, as drop volume goes up the ratio of dot size to drop volume goes down, which generally means that increasing drop volume provides diminishing returns in terms of dot size.
Note, however, to achieve full coverage with a multitone ink jet printer it is still necessary that the largest dot have at least a diameter of p*sqrt(2), and that this largest drop be deposited at each addressable location on the raster.
The time required for an ink jet print to dry can be directly related to the volume of ink deposited on the media. The maximum volume of ink is determined by the dot size required to achieve full coverage. In the case of a binary or multitone printer writing on a raster the dot size per pixel required to achieve full coverage has already been shown in FIG. 3 to be one dot with diameter p*sqrt(2).
In the field of inkjet printing it is also well known that if ink drops placed at neighboring locations on the page are printed at the same time, then the ink drops tend to flow together on the surface of the page before they soak into the page. This can give the reproduced image an undesirable grainy or noisy appearance often referred to as xe2x80x9ccoalescencexe2x80x9d. It is known that the amount of coalescence present in the printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thereby improving the image quality. There are many techniques present in the prior art that describe methods of increasing the time delay between printing adjacent dots using methods referred to as xe2x80x9cinterlacingxe2x80x9d, xe2x80x9cprint maskingxe2x80x9d, or xe2x80x9cmultipass printingxe2x80x9d. There are also techniques present in the prior art for reducing one-dimensional periodic artifacts or xe2x80x9cbands.xe2x80x9d This is achieved by advancing the paper by an increment less than the printhead width, so that successive passes or swaths of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. See, for example, U.S. Pat. No. 4,967,203 and 5,992,962. The term xe2x80x9cprint maskingxe2x80x9d generically means printing subsets of the image pixels in multiple passes of the printhead relative to a receiver medium.
There is a need for improvement over the prior art in ink jet printing to achieve full coverage with a minimum amount of ink so as to minimize the dry time required for an ink jet print. The prior art utilized large dots with excessive amounts of overlap in order to achieve full coverage. This invention provides a method for achieving full coverage with less dot overlap and with smaller drops, thereby achieving faster dry times.
An object of the present invention is to achieve full coverage with less total ink volume, thereby minimizing dry time. This invention relies on multitone printing capability in combination with a printer that can print on the shifted raster. As used herein a xe2x80x9cshifted rasterxe2x80x9d implies a subsidiary grid of printing locations that provides dot or pixel locations that are not on the primary or reference raster and wherein spacing between pixel locations on the shifted raster and the reference raster pixel locations are always less than the nominal spacing between centers of recording elements on the printhead.
This invention provides a method of printing and a printer apparatus for reducing the dry time required for an inkjet print by reducing the amount of ink required to achieve full coverage. The method is implemented in the controller which prepares data for ink jet printing and in the controller which controls the positioning of the ink jet head and the position of the ink receiver, or media.
The invention and its objects and advantages are achieved in accordance with a first aspect of the invention by an ink jet printer, comprising an ink jet print head having an array of nozzles, each nozzle being capable of selectively producing when actuated at least two ink drop sizes including a larger ink drop size and a smaller ink drop size; and a controller providing, in response to each pixel density signal of maximum density value at a respective pixel location, a signal to a nozzle to print an ink drop of the larger ink drop size at the respective pixel location on a reference raster and a signal to a same or different nozzle to print an ink drop of the smaller ink drop size at a pixel location adjacent to the respective pixel location but on a shifted raster.
In accordance with a second aspect of the invention there is provided an ink jet printer, comprising an ink jet print head having an array of nozzles, each nozzle being capable of selectively producing when actuated an ink drop; and a controller responsive to a pixel density signal for controlling the print head for printing, in response to a signal calling for a maximum density value at a respective pixel location, a larger ink dot at the respective pixel location on a reference raster and a smaller ink dot at a pixel location adjacent to the respective pixel location but on a shifted raster and wherein the larger and smaller ink dots are of respective sizes such that for a 2xc3x972 set or cluster of adjacent ink dots, each of the larger ink dot size and each at adjacent pixel locations of the reference raster, a gap is left in the center of the cluster, and the smaller ink dot on the shifted raster when in the center of the 2xc3x972 set of adjacent larger ink dots is of a size to cover the gap.
In accordance with a third aspect of the invention there is provided an ink jet printer, comprising:
a) an ink jet print head having an array of nozzles arranged in at least one row, each nozzle being capable when actuated of selectively producing a larger drop or a smaller drop at a pixel location on a receiver medium;
b) an ink jet print head drive for moving the ink jet print head in a fast scan direction perpendicular to the row of nozzles;
c) a print media driver for moving the receiver medium past the print head in a slow scan direction orthogonal to the fast scan direction;
d) a controller for controlling the size of drops from the nozzles, the print head drive and the print media drive for depositing ink drops in a reference raster, whereby a 2xc3x972 cluster of four adjacent large ink drops in the reference raster leaves a gap in the center of the cluster, and for depositing a smaller ink drop in a shifted raster in response to a print signal for printing a maximum density dot on the reference raster whereby the smaller ink drop covers the gap in the cluster.
In accordance with a fourth aspect of the invention there is provided a method of printing with an ink jet printer, comprising the steps of providing an ink jet print head having an array of nozzles, each nozzle being capable of selectively producing when actuated at least two ink drop sizes including a larger ink drop size and a smaller ink drop size; and in response to each pixel density signal of maximum density value at a respective pixel location on a receiver printing an ink drop of the larger ink drop size at the respective pixel location on a reference raster and printing an ink drop of the smaller ink drop size at a pixel location adjacent to the respective pixel location but on a shifted raster.
In accordance with a fifth aspect of the invention there is provided a method of printing with an ink jet printer, comprising the steps of providing an ink jet print head having an array of nozzles, each nozzle being capable of selectively producing when actuated an ink drop; and printing, in response to a signal calling for a maximum density value at a respective pixel location, a larger ink dot at the respective pixel location on a reference raster and a smaller ink dot at a pixel location adjacent to the respective pixel location but on a shifted raster and wherein the larger and smaller ink dots are of a respective size such that for a 2xc3x972 set or cluster of adjacent ink dots each of the larger ink dot size and each on the adjacent pixel locations of the reference raster a gap is left in the center of the cluster, and the smaller ink dot on the shifted raster in the center of the 2xc3x972 set of adjacent larger ink dots is of a size to cover the gap.
The invention is particularly suited for an ink jet printer that has a maximum dot size capability wherein a square of four pixels printed by the printer will not provide ink coverage for the center between the four pixels.
It will be shown that the combination of a drop with dot diameter less than p*sqrt(2) on the reference raster and a second drop with smaller diameter on the shifted raster can achieve full coverage with less total ink volume than either: a) a single drop with dot diameter p*sqrt(2) on the reference raster, or b) two drops each with dot diameter p, one each on the reference and shifted rasters.