Patent Publication Number: US-7588305-B2

Title: Dual drop printing mode using full length waveforms to achieve head drop mass differences

Description:
BACKGROUND 
   Dual-drop printing is achieved using two or more full length waveforms and a predetermined jet geometry that generates two or more different drop masses from each jet. 
   Dual-drop mode refers to the ability of the printhead to generate two or more different drop masses. However, only one of these masses is typically used in a given image. This is accomplished with the use of separate full length waveforms that achieve different drop masses from an individual jet nozzle. For example, the Phaser 340, available from Xerox Corporation, used this to achieve a 110 ng drop and a 67 ng drop by firing one of the two waveforms depending on a mode of operation. In order to achieve the smaller drop with the same jet geometry, the smaller drop waveform was run at a lower frequency. 
   Drop-size-switching (DSS) refers to the ability of a jet to generate a multitude of drop masses (two, for example) on-the-fly. This can be accomplished by fitting two half (½) length waveforms into the jetting time 1/fop. Here “fop” refers to “frequency of operation”, which is the frequency at which drops eject from each jet of a print head when firing continuously. The electronics select one of the two waveforms according to one or more patterning methodologies to print a page length document. This achieves printing from individual jet nozzles of either a large drop or a small drop. 
   As shown in  FIG. 1 , a printhead driver  200  incorporates two separate waveforms (waveform  1  and waveform  2 ) into a single print firing period (1/fop). One of the two waveforms is selected “on the fly” by driver  200  to drive individual jets of printhead  100  based on specific image criteria or image quality. Printhead  100  includes an aperture plate  110  and a diaphragm plate  120 . A piezoelectric transducer  130  is provided on the diaphragm plate  120 . Between the two plates  110 ,  120  are defined ports  140 , feed lines  150 , manifold  160 , inlet  170 , body  180 , outlet  185 , and apertures  190 . An example of this type of “on the fly” printhead is further described in U.S. Pat. No. 5,495,270 to Burr et al., the disclosure of which is hereby incorporated herein in its entirety. 
   This concept was introduced in the Phaser 850 Enhanced Mode, also available from Xerox Corporation. Both a 51 ng and a 24 ng drop size could be generated “on the fly.” However, in this design, the printhead ran at the slower frequency of the small drop. Because the smaller drop ran at a lower frequency, it could not be printed at high speed. However, because the large drop was available to allow an overall reduction in resolution while maintaining appropriate total solid coverage, the dual-drop mode worked and was beneficial. 
   SUMMARY 
   There is always a quality/speed consideration that must be made when setting the dropmass of a printer. Large drops are needed in solid fill regions to increase color saturation at lower resolutions that afford higher print speeds, and small drops are needed in light fill regions to reduce graininess. Printing with multiple drop sizes on each image improves the image quality for a given speed and/or increases the speed for a given image quality because large drops fill solid color regions quickly while small drops reduce graininess in lighter shaded regions. 
   The primary limitation of the Phaser 850 method of dual-drop printing is the need to fit both a small drop waveform and a large drop waveform in a single firing period (1/fop). As newer jet designs operate at higher frequencies (increased fop), the associated period (1/fop) becomes too short to fit two waveforms. Accordingly, there is a need for an improved printing architecture and method that can address this limitation. 
   In accordance with various aspects, a printer architecture uses a modified DSS mode “Soft DSS” that allows smaller drops in light fill areas to decrease graininess in the image, while also allowing larger drops in solid fill areas to increase color saturation at lower resolutions to improve print quality at either extreme. 
   In accordance with various other aspects, a printer architecture uses a Soft DSS mode having full length waveforms, which are easier to develop and implement than half length waveforms. That is, they are much simpler to design and implement robustly within required product time cycles. An additional benefit of these “Soft DSS” modes is to maximize print speed because there will not be the wait time between pulses inherent in an “on the fly” dual-drop mode system using partial length waveforms that require slower print frequencies. 
   In accordance with exemplary embodiments, a Soft DSS mode printer architecture provides a page output with an alternating pattern of small and large drop sizes. In one exemplary arrangement, the pattern achieves alternating columns of large and small drops. In another exemplary embodiment, the pattern achieves alternating rows of large and small drops. In various exemplary embodiments, the pattern layout is for an entire page. In further exemplary embodiments, the pattern can change down the page, such as by printing in a checkerboard pattern, or changed in consecutive passes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments will be described with reference to the drawings, wherein: 
       FIG. 1  illustrates a cross-sectional view of a conventional single geometry ink nozzle driven by one of two known dual-drop half-frequency waveforms to achieve either a large or small drop mass size; 
       FIG. 2  illustrates a cross-sectional view of an exemplary ink nozzle array driven by one of two dual-drop full frequency waveforms to achieve either a large or small drop mass size; 
       FIG. 3  illustrates a perspective view of an exemplary fluid ejection device; 
       FIG. 4  illustrates a schematic block diagram showing the exemplary fluid ejection device of  FIG. 3  having an apparatus used to generate the piezoelectric drive waveforms of  FIG. 2 ; 
       FIG. 5  illustrates a top pictorial view showing a printhead mounted to a shaft for translational X-axis movement while an adjacent drum supporting an intermediate transfer surface is rotated about a Y-axis; 
       FIG. 6  illustrates an exemplary flowchart showing a method for generating a page output from a printer having an alternating pattern of large and small ink drops; 
       FIG. 7  illustrates a flowchart of a specific exemplary embodiment for generating a page output from a printer having an alternating pattern of large and small ink drops arranged in alternating rows; 
       FIG. 8  illustrates consecutive printhead cycles or rows of printheads driven by the method of  FIG. 7 ; 
       FIG. 9  illustrates an exemplary dual drop printing output in accordance with the method of  FIG. 7  and printhead configuration of  FIG. 8  in which every other line (row) is printed with small drops; 
       FIG. 10  illustrates an exemplary waveform diagram according to the method of  FIG. 7 ; 
       FIG. 11  illustrates an exemplary dual drop printing output in accordance with a modified version of the method of  FIG. 7  in which a multiple number of rows of large drops are alternated with rows of small drops; 
       FIG. 12  illustrates a flowchart of a specific exemplary embodiment for generating a page output from a printer having an alternating pattern of large and small ink drops arranged in alternating columns; 
       FIG. 13  illustrates consecutive printhead cycles or rows of printheads driven by the method of  FIG. 12 ; 
       FIG. 14  illustrates an exemplary dual drop printing output in accordance with the method of  FIG. 12  and printhead configuration of  FIG. 13  in which every other column is printed with small drops; 
       FIG. 15  illustrates a flowchart of a specific exemplary embodiment for generating a page output for a printer having an alternating pattern of large and small drops arranged in alternating columns; 
       FIG. 16  illustrates a first printhead cycle, during a first rotation of an intermediate drum, in a full width printhead driven by the method of  FIG. 15 ; 
       FIG. 17  illustrates a second printhead cycle, during a subsequent rotation of the intermediate drum, in a full width printhead driven by the method of  FIG. 15 ; and 
       FIG. 18  illustrates an exemplary dual drop printing output in accordance with the method of  FIG. 15 . 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   In accordance with exemplary embodiments, a printer architecture with a Soft DSS mode provides a page output with an alternating pattern of small and large drop sizes. This is suitable for use in many fluid ejection devices, such as ink jet printers. However, it is particularly beneficial when used with a phase-change, offset solid ink printer. 
   In the exemplary embodiment of  FIG. 2 , printhead  100  of a printer  400  (shown in  FIGS. 3-4 ) includes an aperture plate  110  and a diaphragm plate  120 . A piezoelectric transducer  130  is provided on the diaphragm plate  120 . An array of apertures  190  forming individual fluid nozzles is defined on the aperture plate  110 . The array is closely and uniformly spaced with a predetermined spi (spot per inch) resolution. The apertures  190  are connected to a fluid source through various channels. 
   A suitable fluid, such as a phase-change solid ink that has been heated to liquid form, flows to an ink manifold  160  from an inlet port  140  through feed line  150 . Ink from manifold  160  flows through an inlet  170  to a pressure chamber  180  where it is acted on by transducer  130 , such as a piezoelectric transducer. Piezoelectric transducer  130  is driven by a printhead driver  300 , which applies a particular waveform that deforms transducer  130  to displace an amount of ink within the pressure chamber  180  through outlet  185 . Ultimately this amount of ink is forced through apertures  190  to eject a predetermined mass of ink from the printhead  100 . Reverse bending of transducer  130  following ejection causes a refill of ink into the pressure chamber  180  to load the chamber for a subsequent ejection cycle. 
   In certain exemplary embodiments, the geometry of each aperture  190  and outlet  185  of each nozzle in the printhead  100  is common to all fluid nozzles. However, by application of a repeating sequence of two different full wavelength waveforms, a pattern of two different drop sizes can be produced from this common printhead nozzle geometry. In other exemplary embodiments, a pattern of different drop sizes can be achieved through application of a common full length waveform and different printhead nozzle geometries. In other exemplary embodiments, a pattern of different drop sizes can be achieved through interlacing of consecutive passes using a different waveform for each pass. 
   Printhead  100  can be manufactured as known in the art using conventional photo-patterning and etching processes in metal sheet stock or other conventional or subsequently developed materials or processes. The specific sizes and shapes of the various components would depend on a particular application and can vary. The transducer can be a conventional piezoelectric transducer. One common theme in all exemplary embodiments is that a pattern of alternating drop sizes is formed globally on a page or sub-page output through suitable selection of full length drive waveform and nozzle geometry. 
   An exemplary printer is a solid-ink offset printer  400  shown in  FIGS. 3-5 . In an offset printing system, the printhead  100  jets a fluid, such as phase-change solid ink, onto an intermediate transfer surface, such as a thin oil layer on a drum  450 . A final receiving medium, such as a sheet of paper P, is then brought into contact with the intermediate surface where the image is transferred. In a typical offset printing architecture, the printhead  100  translates in an X-direction, as better shown in  FIG. 6 , while the drum rotates perpendicularly along a Y-axis. Typically, the printhead  100  includes multiple jets configured in a linear array to print a set of scan lines on the drum  450  during each rotation of the drum. Precise movement of the X-axis and Y-axis translation is required to avoid unnecessary artifacts. This can be achieved, for example, using a print head drive mechanism such as the ones described in U.S. Pat. No. 6,244,686 to Jensen et al. and U.S. Pat. No. 5,389,958 to Bui et al., the subject matter of which is hereby incorporated herein by reference in its entirety. 
   Ejecting ink drops having dual controllable volume/mass is achieved by printhead driver  300 , which is better illustrated in  FIG. 4 . Driver  300  is provided within printer  400  and includes a waveform generator  310  capable of generating multiple waveform patterns. As shown in  FIG. 2 , exemplary embodiments provide at least two selectable full wavelength patterns (waveform  1  and waveform  2 ). Transducer  130  responds to the selected waveform by inducing pressure waves in the ink that excite ink fluid flow resonance in outlet  185 . A suitable waveform is selected using selector  330 , based on criteria to be described later in more detail. The waveform selected is fed to amplifier  320 . From amplifier  320 , an amplified signal is delivered to the piezo transducer of printhead  100 , driving one or more rows of jets in the printhead. Movement of the piezo transducer causes ejection of a suitable volume of fluid, such as ink, from printhead  100  of printer  400  based on image signals received from a source (such as a scanner or stored image file) in image data input  420  and controlled by CPU  410  of the printer. 
   Ink is provided in a storage area  430  and supplied to printhead  100  through an ink loader  440 . In an exemplary embodiment, printer  400  is a solid ink printer that contains one or more solid ink sticks in storage area  430 . The solid ink sticks are melted and jetted from ink jet nozzles of the printhead  100  onto the intermediate transfer surface on drum  450 , which may be rotated one or several revolutions to form a completed intermediate image on the transfer surface on the drum. At that time, a substrate, such as paper, can be advanced along a paper path that includes roller pairs  460 ,  470  and between a transfer roller  470  and drum  480 , where the image is transferred onto the paper in a single pass as known in the art. 
   A different resonance mode may be excited by each full wavelength waveform to eject a different drop volume/mass in response to each selected mode. In the  FIG. 2  example, one waveform (waveform  1 ) may provide a small drop size, while the other waveform (waveform  2 ) may provide a large drop size when driving jet nozzles having the same nozzle geometry. The waveform design chosen would be based on the design constraints of the fluid pathway, the transducer operating parameters, the meniscus parameters of the fluid, and the like. Selection of modal properties can be determined by empirical modeling or experimentation based on known governing principles. For example, details of the equations governing fluid dynamics relevant to fluid ejection can be found in U.S. Pat. No. 5,495,270 to Burr et al., the subject matter of which is hereby incorporated herein by reference in its entirety. From these and other conventional teachings, one of ordinary skill can select appropriate full length waveforms to produce a desired droplet size. 
   Alternatively, different drop volume/mass may be achieved by use of one of the two waveforms and nozzles in the array having different geometries, such as aperture size, shape, etc. Thus, by creating the array with nozzles that are arranged in a pattern so that first and second drop sizes are formed when applied with the same full wavelength waveform, the same effect can be achieved. However, because the nozzle geometry cannot be changed readily without replacement of the array, this alternative cannot have the resultant pattern changed as easily as embodiments that use a common nozzle geometry and simply change the pattern through selection of different drive waveforms. 
   An important aspect of the disclosure is in the control of the full length waveforms globally on a page or partial page basis so that printhead  100  drives various rows of nozzles with a particular pattern of alternating large and small ink drops on a page to achieve benefits of each size drop. That is, a whole page does not need to be printed using only a single drop size, but instead achieves a pattern incorporating both drop sizes so that advantages to use of each size can be realized. 
   Various different patterning techniques are disclosed. For example, the embodiments of  FIGS. 7-11  achieve alternating rows of large and small drops on a page or sub-page basis. The embodiments of  FIGS. 12-14  and  FIGS. 15-17  achieve alternating columns of large and small drops. In various exemplary embodiments, the pattern layout is for an entire page. In further exemplary embodiments, the pattern can change on a sub-page basis or in consecutive passes. 
   A basic generalized method of printing using the printhead and driver of  FIGS. 2-5  will be described with reference to  FIG. 6 . The process starts at step S 500  and advances to step S 510  where selector  330  of driver  300  selects appropriate full length waveform pattern(s) to drive the nozzle array with to achieve a predetermined pattern of first and second drop sizes on a page. From step S 510 , flow advances to step S 520  where page image data is received. From step S 520 , flow advances to step S 530 , where driver  300  drives the nozzle array based on the page image data and based on the predefined waveform(s) selected to output an image in which the page globally forms an alternating pattern of first and second drop sizes on the page output. The process then ends at step S 540 . 
   Alternatively, the step of receiving image data can be performed prior to selection of waveform pattern by selector  330 . This could, for example, take into account global properties of the received image and use this information to determine which global page-based or sub-page based pattern of large and small drops would produce better image quality. For example, if the image data is determined to be primarily solid fill, one pattern with a more dominant mix of large drops may be better than another pattern. Likewise, an image with a lot of light fill areas may have better print quality if a pattern with more dominant small drops is present. Moreover, based on the image and resolution details, it may be preferable to have the pattern aligned in rows or columns to take into account x-resolution or y-resolution problems with a particular printer architecture. Thus, although certain embodiments have a 1:1 ratio of large to small drops globally, various patterns may have differing proportions, such as 2:1; 3:1; 5:3, etc. More specific examples of these will be described with reference to the following embodiments. 
   A first specific embodiment will be described with reference to  FIGS. 7-11  and achieves printing of an image with a pattern of small and large drops arranged in horizontal rows. It is achieved using an ink jet nozzle array having common nozzle geometry and use of two different full length waveforms to achieve the different drop size. 
   For simplicity, the process will be discussed in terms of generating a solid fill image. This will demonstrate the global dropmass grid of which the printer imaging will know and will utilize in the actual color image formation. The process starts at step S 600  and flows to step S 610  where a waveform pattern is selected to achieve alternating rows of at least two different drop sizes (large and small). From step S 610 , flow advances to step S 620  where page image data is received that corresponds to a specific input image to be reproduced. From step S 620 , flow advances to step S 630  where select printhead nozzles in row X are each driven using the same full wavelength waveform  1  to form a row X of first sized ink drops. For example, as shown in  FIGS. 8-9 , a single array of nozzles  190  provided on printhead  100  can have a common nozzle geometry and be driven in a first cycle such that all nozzles corresponding to the image are driven with waveform  1  to achieve a row X of small ink drops  510 . 
   From step S 630 , flow advances to step S 640 , where row X+i is driven using full length waveform  2  to form row X+i having second, different size drops  420 . For example, in  FIG. 8 , during a second cycle, the single array  190  of printhead  100  is driven with waveform  2  such that all nozzles corresponding to the image are driven to achieve a row X+i of large drops. From step S 640 , flow advances to step S 650 , where additional rows are printed using the pattern of waveforms so that alternating rows of first and second ink drops are formed on a page output  500  as better shown in  FIG. 9 . 
   This method can also be performed using a two-dimensional array of nozzles that are driven at the same time. This is achieved by driving each individual row of nozzles with one of the two waveforms sequentially to achieve a desired pattern of alternating rows of large or small drops. 
   Printing with this method can be performed to achieve one-half the print area with small drops and one-half the print area with large drops. Such patterning achieves benefits of using each drop size, and does not suffer the problems associated with using only a single drop size. That is, by alternating between two different waveforms in a predetermined pattern over the entire image print frequency can be maximized to improve print speed and full length waveforms can be used. Moreover, by using both drop sizes on a page in this alternating manner, benefits attributed to each drop size can be realized to improve image quality at both solid fill and light fill regions of an image. Thus, the quality/speed tradeoff can be lessened. 
   As shown in  FIG. 10  for an individual nozzle of the array driven in consecutive cycles, each nozzle would be driven by alternating waveforms to produce a small drop  510 , a large drop  520 , a small drop  510 , and a large drop  520  in sequence. This method offers a substantially different set of design opportunities compared to those available when only considering ½ length waveforms. Moreover, because the pattern of large and small drops is globally set, image processing can be simplified, while the patterning of large and small drops achieves advantages to use of each size to images across the page. 
     FIG. 11  shows a modified version of the method of  FIG. 7  in which a multiple of sequential rows are printed with a same drop size so that the pattern is more dominant with either the first drop size or the second drop size. In the  FIGS. 8-9  example, there is a 1:1 ratio of large to small drops. However, it may be desirable to adjust the ratio so that one size is more dominant. An example of this is shown in  FIG. 11 , where a 2:1 ratio of large to small drops is achieved by printing row  1  in cycle  1  using the small droplet waveform  1  while both rows  2  and  3  are driven by waveform  2  to provide two consecutive rows of large drops. Then, cycle  4  repeats to provide a row of small drops. Other ratios of 3:1, 4:1, 5:2, etc. can be substituted and can be dominant with either the small drop size or the large drop size. The ratio does not necessarily have to remain the same over the entire image, but must remain set for each drum revolution. Therefore, depending on the jet spacing and resolution, even hybrid patterns composed of columns of the pattern in  FIG. 9  and other columns of the pattern shown in  FIG. 11  are possible. The actual implementation of which would be optimized to achieve various benefits. For example, a higher ratio of small drops may improve printing of light fill images, whereas a higher ratio of larger drops may improve solid fill dropout. Additionally, modifying the pattern in a second direction (say a slightly offset pattern for every other column) could be used to additionally reduce some repetitive patterning if banding and/or modeling of the image is discovered. Such things must typically be determined empirically, but can be readily performed by anyone skilled in the art. 
   Another embodiment will be described with reference to  FIGS. 12-14  and achieves printing of an image with a pattern of small and large drops arranged in vertical columns. The process starts at step S 1200  and flows to step S 1210  where a waveform pattern is selected to achieve alternating columns of at least two different drop sizes (large and small). From step S 1210 , flow advances to step S 1220  where page image data is received that corresponds to a specific input image to be reproduced. From step S 1220 , flow advances to step S 1230  where select printhead nozzles in rows X and X+i are driven using the selected full wavelength waveform (waveform  1  or waveform  2 ) to form alternating first and second drop sizes for the rows. For example, an array of nozzles provided on printhead  100  can be driven with a same waveform. However, as shown in  FIG. 13 , alternating nozzles in the array have a different nozzle geometry. For example, nozzle  190 A has a smaller nozzle diameter than nozzle  190 B. Because of this difference in geometry, even when applied with the same full wavelength waveform, the output from the array achieves a row of alternating small and large ink drops as shown in  FIG. 14 . From step S 1230 , flow advances to step S 1240 , where the process ends. 
   This process achieves the output image shown in  FIG. 14  in which the small drops and large drops are aligned vertically into alternating columns. As with the previous embodiment, it is possible to alter the ratio to be other than a 1:1 ratio of large and small drops. This can be achieved, for example, by replacing the array with an array having a different distribution of large and small nozzles. 
   A third exemplary embodiment will be described with respect to  FIGS. 15-18 . In this embodiment, a full width offset printer  400  is provided that uses line interlacing to create an image on intermediate transfer surface on drum  450  with an alternating pattern of large and small drops. For a more detailed description of line interfacing, see U.S. Pat. No. 5,734,393 to Eriksen and U.S. Pat. No. 5,949,452 to Jones, the subject matter of which is hereby incorporated herein by reference in its entirety. 
   In this embodiment, printhead  100  includes an array of nozzles  190  that are spaced in the X-direction by a value nX, where n is an integer and X is a pixel width. During printing, drum  450  rotates in the direction of arrow Y ( FIG. 5 ). As the drum rotates, the printhead translates along the X-axis and a plurality of ink jets eject ink onto the intermediate transfer surface supported by drum  450 . One rotation of the drum and simultaneous translation of the printhead  100  while firing the jets results in the deposition of a set of very slightly angled vertical scan lines on the intermediate transfer surface on drum  450 . One scan line has an approximate width of one pixel. A set of scan lines corresponds to one rotation of the drum  450  (one line for each jet in the array). Therefore, the inter-jet spacing nX dictates the number of rotations of the drum that must occur to create a full image at a given resolution. For example, in the illustrative  FIGS. 15-17 , an inter-jet spacing of 2X is provided. Thus, two rotations are needed to form a complete solid fill image. However, other interlacing could be used. For example, an inter-jet spacing of 10X would require 10 rotations of the drum to produce a solid fill image. 
   Each column could contain a single nozzle, in the case of a monochrome printer, or four nozzles as shown in the case of a color printer (one for each of cyan, magenta, yellow and black). Although only six columns are shown, the array would extend the full width of the drum and in actuality would contain a much larger number of columns. 
   In this embodiment, driver  300  is capable of driving the array with a different full width wavelength during each rotation of intermediate drum  450 . For example, during a first rotation shown in  FIG. 16 , waveform  1  can be applied to each driven nozzle to form a series of small ink drops  710  shown in  FIG. 18 . Then, during a second rotation as shown in  FIG. 16 , waveform  2  can be applied to each driven nozzle to form a series of large ink drops  720  shown in  FIG. 18 . Because the printhead  100  is translated in the X direction, the second rotation produces drops that are laterally displaced relative to drops ejected during the first rotation. This could be achieved by incremental translation in the X-direction during rotation of the drum in the Y-direction. Alternatively, translation can occur in a single step at the end of each drum revolution, such as while the printhead is over an interdocument region of the drum  450 . Thus, in this simple example with an inter-jet spacing of two pixels, alternating between waveform  1  and waveform  2  for consecutive revolutions of the drum  450  results in alternating columns of small and large drops as shown in  FIG. 18 . For a given inter-jet spacing, ratios of large to small drops can be varied to values other than 1:1, through careful selection of which waveform to use during each drum revolution. This selection would change depending on the resolution and interlace, but is known a priori. As described in previous embodiments, this would allow for adjustments to make either the large or small drops more dominant to adjust image quality. In a preferred embodiment, adjustment to the waveform (i.e., changing between waveform selections), would take place during an interdocument spacing zone on the drum when no printing occurs. 
   An exemplary method of printing using the offset printer  400  will be described with respect to  FIG. 15 . The process starts at step S 1600  and proceeds to step S 1610  where a waveform profile is selected to be used during a first revolution of the offset printing drum to drive the array of nozzles  190 . From step S 1610 , flow advances to step S 1620  where page image data is received. At step S 1630 , a column (typically a series of spaced columns) of first size ink drops is printed on the drum during a first revolution of the drum by driving the nozzle array using the selected full wavelength profile. From step S 1630 , flow advances to step S 1640  where a different waveform profile is selected for use during a subsequent revolution of the offset printing drum to drive the nozzles to produce second, different size ink drops. From step S 1640 , flow advances to step S 1650  where the printhead is translated in the X-direction by a specified amount. From step S 1650 , flow advances to step S 1660  where a column of second size ink drops is formed on the offset printing drum laterally offset from the previously formed column to form a pattern of alternating columns of first and second ink drop sizes. From step S 1660 , the process advances to step S 1670  where the image formed on the offset printing drum is transferred to a paper substrate, preferably in a single pass. From step S 1670 , flow advances to step S 1680  where the process stops. 
   The specific drop size used for the large and small drops would depend on various criteria, including the resolution of the printhead, properties of the ink and transfer process, etc. A large drop in exemplary embodiments useful in a monochrome or color solid ink-based piezo fluid ejector or printer is set to about 31 ng or higher, but would depend on several considerations, including a desired small drop size, ink dye loading, etc. 
   A small drop requirement should be less than about 24 ng, and preferably in the range of around 10-20 ng. Therefore, in preferred embodiments using solid ink-based fluid ejectors, the nozzle geometry and/or waveform(s) selected would be chosen to provide and alternating pattern of large and small ink drops where the large drop is set to be about 31 ng, and the small drop is set to be less than 24 ng, preferably 10-20 ng. This combination of drop size has been found to achieve acceptable text quality, improve light fill areas and reduce graininess as well as improve image transfer and maximize print speed. 
   It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.