Patent Publication Number: US-10780695-B2

Title: Ink jet printing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/521,481, filed on Apr. 24, 2017, which is a U.S. National Stage under 35 U.S.C. 371 of International Patent Application No. PCT/US2014/063184, filed Oct. 30, 2014, the disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Thermal ink jet printheads are fabricated on integrated circuit wafers. Drive electronics and control features are first fabricated, then the columns of heater resistors are added and finally the structural layers, for example, formed from photoimageable epoxy, are added, and processed to form the drop generators. The drop size for print heads is often uniform. However, this makes the high speed printing of documents problematic, as large drops, which can print at higher speed, do not resolve images as well. Printheads could be switched out by job, but a web press can have hundreds of printheads, making this option difficult. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain examples are described in the following detailed description and in reference to the drawings, in which: 
         FIG. 1  is a drawing of an example printing press that uses ink jet printheads to form images on a print medium; 
         FIGS. 2A and 2B  are block diagrams of an example of a printing system that may be used to form images using ink jet printheads; 
         FIG. 3  is a drawing of a cluster of ink jet printheads in an example print configuration, for example, in a printbar; 
         FIG. 4  is a top view of an example printhead showing adjacent nozzles over resistors; 
         FIG. 5  is a close up top view of two drop generators, showing the different nozzle designs; 
         FIGS. 6A and 6B  are drawings of the dot patterns from the nozzles described with respect to  FIG. 5 ; 
         FIG. 7  is a drawing of a pattern of HDW and LDW drop generators on a printhead; 
         FIG. 8  is a plot of ink densities for different ink tones, which can be used to linearize the rasters, e.g., to determine which drop generators to fire; 
         FIGS. 9A and 9B  are drawings showing the difference between pictures printed with only HDW drop generators versus only LDW drop generators; and 
         FIG. 10  is a process flow diagram of an example method to print a document using a printer that has HDW drop generators and LDW drop generators. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EXAMPLES 
     Ink jet printheads that are designed to produce two drop sizes, termed interstitial dual drop weight (iDDW), are described in examples herein. The ink jet printheads alternate the sizes of drop generators, including the heater resistors and nozzles. As used herein, a drop generator is an apparatus that ejects an ink drop at a print medium. The drop generator includes an inflow region comprising a flow chamber that fluidically couples an ink source with an ejection chamber. The ejection chamber has a heating resistor on a surface, and a nozzle disposed proximate the heating resistor. When a firing pulse is applied to the heating resistor, a steam or solvent bubble is formed within the ejection chamber, which forces an ink drop out the nozzle. 
     Each printhead has multiple columns, or arrays, of drop generators that alternate between high drop weight (HDW) and low drop weight (LDW). The HDW may be in the range of about 6-11 nanograms (ng), or about 9 ng, while the LDW may be in the range of about 3-5 ng, or about 4 ng. The drop generators share the same stack thickness for the fluidic, or ink flow, channels, and are centered on substantially the same pitch to assure correct drop placement, e.g., about 21.2 micrometers (μm) for 1200 dots per inch (dpi). 
     The ink jet printheads provide high speed printing for text and graphics and lower speed printing, with increased quality and reduced drop weight, for images. In an example, a control system may determine which type of drop generator to use depending on the input. The control system may use only the HDW drop generators for high speed printing of text and graphics, all LDW drop generators for high quality printing of images, or a mixture of LDW drop generators and HDW drop generators for general purpose use. 
     Further, in some examples, the printed drop shapes and printhead layout are improved by using a non-circular bore (NCB) for the nozzle of the HDW drop generator and a circular bore for the nozzle of the LDW drop generator. The NCB allows the bore area necessary for a HDW drop generator to fit within available space in the Y axis of the printhead while also reducing the drop tail length, which gives crisp edges to lines and text. The circular bore used on the nozzle of the LDW drop generator packs well between the adjacent NCBs of the nozzles for the HDW drop generators and produces a longer drop tail that splits into two, or more, smaller drops. These small drops are ideal for reducing grain in images. 
       FIG. 1  is a drawing of an example of a printing press  100  that uses ink jet printheads to form images on a print medium. The printing press  100  can feed a continuous sheet of paper from a large roll  102 . The paper can be fed through a number of printing systems, such as printing systems  104  and  106 . In the first printing system  104  a printbar that houses a number of printheads ejects ink drops onto the paper. Printheads in the second printing system  106  may be used to print additional colors. For example, the first system  104  may print black (K), while the second system  106  may print cyan, magenta, and yellow (CMY). The printing systems  104  and  106  are not limited to two, or the mentioned color combinations, as any number of systems may be used, depending, for example, on the colors desired and the speed of the printing press  100 . 
     After the second system  106 , the printed paper may be taken up on a take-up roll  108  for later processing. In some examples, other units may replace the take-up roll  108 , such as a sheet cutter and binder, among others. The printing press  100  may have a very high speed of operation and printing, and, thus, the design of the printheads may be important to achieving this speed. In one example, the paper, or other print medium, may be moving as fast as about 800 feet per minute, or about 244 meters per minute. Further, the printing press  100  may print about 129 million letter-sized images per month. The techniques described herein are not limited to the printing press  100  of  FIG. 1 , but may be used with any ink jet printing system, for example, from a personal printer to the printing press  100 . 
       FIGS. 2A and 2B  are block diagrams of an example of a printing system  200  that may be used to form images using ink jet printheads. As shown in  FIG. 2A , the printing system  200  includes a printbar  202 , which includes a number of printheads  204 , and an ink supply assembly  206 . The ink supply assembly  206  includes an ink reservoir  208 . From the ink reservoir  208 , ink  210  is provided to the printbar  202  to be fed to the printheads  204 . The ink supply assembly  206  and printbar  202  may use a one-way ink delivery system or a recirculating ink delivery system. In a one-way ink delivery system, substantially all of the ink supplied to the printbar  202  is consumed during printing. In a recirculating ink delivery system, a portion of the ink  210  supplied to the printbar  202  is consumed during printing, and another portion of the ink is returned to ink supply assembly. In an example, the ink supply assembly  206  is separate from the printbar  202 , and supplies the ink  210  to the printbar  202  through a tubular connection, such as a supply tube (not shown). In other examples, the printbar  202  may include the ink supply assembly  206 , and ink reservoir  208 , along with a printhead  202 , for example, in single user printers. In either example, the ink reservoir  208  of the ink supply assembly  206  may be removed and replaced, or refilled. 
     From the printheads  204  the ink  210  is ejected from nozzles as ink drops  212  towards a print medium  214 , such as paper, Mylar, cardstock, and the like. In some example, other media, such as treated papers that enhance adhesion, may be used. The nozzles of the printheads  204  are arranged in one or more columns or arrays such that properly sequenced ejection of ink  210  can form characters, symbols, graphics, or other images to be printed on the print medium  214  as the printbar  202  and print medium  214  are moved relative to each other. The ink  210  is not limited to colored liquids used to form visible images on paper. For example, the ink  210  may be an electro-active substance used to print circuits and other items, such as solar cells. Further, other types of materials, such as metallic or magnetic inks  210  may be used. In some examples, the printing system  200  may be used for other types of applications, such as three dimensional printing and digital titration, among others. In those examples, the inks  210  can encompass any number of other chemicals, such as acids, bases, plastic fluids, medical testing fluids, and the like. 
     In examples described herein, the printheads  204  have an iDDW design. In the iDDW design, one of two different sized ink drops  212  may be ejected from the printheads  204  depending on the types of images to be printed. It is desirable for the ink jet printing system  200  to maintain a high printing speed, and, thus, the printheads  204  may be designed to provide a similar speed for printing using each drop size. However, in some examples, the printing speed may be adjusted depending on the ratio of the types of drops, e.g., HDW to LDW. 
     A mounting assembly  216  may be used to position the printbar  202  relative to the print medium  214 . In an example, the mounting assembly  216  may be in a fixed position, holding a number of printheads  204  above the print medium  214 . In another example, the mounting assembly  216  may include a motor that moves the printbar  202  back and forth across the print medium  214 , for example, if the printbar  202  only included one to four printheads  204 . A media transport assembly  218  moves the print medium  214  relative to the printbar  202 , for example, moving the print medium  214  perpendicular to the printbar  202 . In the example of  FIG. 1 , the media transport assembly  218  may include the rolls  102  and  108 , as well as any number of motorized pinch rolls used to pull the paper through the printing systems  104  and  106 . If the printbar  202  is moved, the media transport assembly  218  may index the print medium  214  to new positions. In examples in which the printbar  202  is not moved, the media transport assembly  218  may move the print medium  214  continuously. 
     A controller  220  receives data from a host system  222 , such as a computer. The data may be transmitted over a network connection  224 , which may be an electrical connection, an optical fiber connection, or a wireless connection, among others. The data may include a document or file to be printed, or may include more elemental items, such as a color plane of a document or a rasterized document. The controller  220  may temporarily store the data in a local memory for analysis. The analysis may include determining timing control for the ejection of ink drops from the printheads  204 , as well as the motion of the print medium  214  and any motion of the printbar  202 . The controller  220  may operate the individual parts of the printing system over control lines  226 . Accordingly, the controller  220  defines a pattern of ejected ink drops  212  which form characters, symbols, graphics, or other images on the print medium  214 . For example, the controller  220  may determine when to use HDW drop generators and LDW drop generators for printing a particular image, as described further with respect to  FIG. 2B . 
     The ink jet printing system  200  is not limited to the items shown in  FIG. 2 . For example, the controller  220  may be a cluster computing system coupled in a network that has separate computing controls for individual parts of the system. For example, a separate controller may be associated with each of the mounting assembly  216 , the printbar  202 , the ink supply assembly  206 , and the media transport assembly  218 . In this example, the control lines  226  may be network connections coupling the separate controllers into a single network. In other examples, the mounting assembly  216  may not be a separate item from the printbar  202 , for example, if the printbar  202  is fixed in place. 
       FIG. 2B  is a block diagram of the controller  220  of  FIG. 2A . The controller  220  may have a processor  228  that is configured to execute stored instructions, coupled though a bus  230  to a storage device  232  that stores instructions that are executable by the processor  228 . The processor  228  can be a single core processor, a multi-core processor, a computing cluster, or any number of other configurations. As used herein, the storage device  232  is a non-transitory, machine readable medium. The storage device  232  may include both short term and long term storage. The short term storage may include random access memory (RAM), dynamic random access memory (DRAM), flash memory, or any other suitable memory systems, as well as any combinations thereof. The long term storage may include read only memory (ROM), a RAM drive, non-volatile RAM, a hard drive, an optical drive, a thumb drive, an array of drives, a remote array of drives, or any other suitable systems, as well as any combinations thereof. 
     A network interface controller (NIC)  234  may be coupled to the processor  228  through the bus  230 . The NIC  234  may couple the controller  220  to the host  222  through a network, such as a local area network (LAN), a wide area network (WAN), or the Internet, among others. 
     The storage device  232  may include a number of modules, or blocks of code, used to provide functionality to the ink jet printing system  200 . An image module  236  may direct the processor  228  to obtain and store an image, such as a document, from the host  222 . The image may be a picture, a text document, a portable document format (PDF) file, or any number of other files. 
     An RIP module  238  includes code to direct the processor to rasterize the image. The rasterization divides the image into layers, or rasters, wherein each raster represents a color of ink, that when combined, will give the initial image color. For example, one rasterization technique divides the image into CMYK rasters. CMYK represents cyan, magenta, yellow, and black rasters. The CMYK rasters may be used to represent all colors in a cost effective manner. Other raster schemes may be used, such as six plane schemes that use specialty colors to enhance image reproduction. For example, one such scheme, termed Hexachrome, adds orange and green inks to the standard CMYK palette to enhance the appearance of the printed document. 
     A linearization module  240  uses one dimensional tables to divide each raster into two planes, one plane representing the HDW drops, and one plane representing the LDW drops. The one dimensional table may be formed as described with respect to  FIG. 8 . 
     A halftoning module  242  uses a breakpoint table to convert the continuous color tone of each plane into individual drops. For example, the breakpoint table may represent intensity levels over a certain area of the plane that correspond to no ink drop, one ink drop, or two ink drops. 
     A masking module  244  divides the drops of the halftones planes among the printbar  202 , and printheads  204 . This creates a map of the print output. A printing module  246  then merges the LDW planes with the HDW planes for each color, and sends the resulting control data to the printbars  202  and printheads  204 . For example, the processor  228  may send the control data over a printer interface  248  coupled to the bus  230 . 
     The controller  220  for the ink jet printing system  200  is not limited to the configurations described with respect to  FIG. 2B , but may include any number of other configurations. For example, the code of the modules may be arranged in any number of other configurations while retaining the same general function. In another example, the modules may be shifted off of the controller  220 , and may be run remotely, such as by the host  222 . 
       FIG. 3  is a drawing of a cluster of ink jet printheads  204  in an example print configuration, for example, in a printbar  202 . Like numbered items are as described with respect to  FIG. 2 . The printbar  202  shown in  FIG. 3  may be used in configurations that do not move the printhead. Accordingly, the printheads  204  may be attached to the printbar  202  in an overlapping configuration to give complete coverage. Each printhead  204  has multiple nozzle regions  302 , such as columns of nozzles that alternate HDW drop generators and LDW drop generators. 
       FIG. 4  is a top view of an example printhead  400  showing adjacent nozzles  402  and  404  over resistors  406  and  408 , respectively. For simplicity, only a representative sample of each of the nozzles  402  and  404  and resistors  406  and  408  are labeled. A smaller nozzle  402  is located over a narrower resistor  406  to provide the LDW drop, for example, about 4 nanograms (ng) in weight. A larger nozzle  404  is located over a wider resistor  408  to provide the HDW drop, for example, about 9 ng in weight. An ink refill region  410  is coupled to each nozzle  402  and  404  through an inflow region  412 . To simplify the drawing, only a portion of the inflow regions are labeled. 
     The resistor pitch  414  may constant, for example, at about 21.1 μm in the y-direction  416 , corresponding to about 1200 dots per inch (dpi), in order to assure correct drop placement. An HDW drop generator includes a larger nozzle  404 , a wider resistor  408 , an ejection chamber located proximate to the nozzle and resistor, and an associated inflow region  412 . An LDW drop generator includes a smaller nozzle  402 , a narrower resistor  406 , an ejection chamber located proximate to the nozzle and resistor, and an associated inflow region  412 . 
     Although the HDW and LDW drop generators differ from conventional designs, the process of making the printhead  400  is similar to many inkjet printheads. The drive transistors and control electronics are first fabricated by conventional semiconductor processes. A layer of conductor is deposited over the wafer, and etched to form resistor windows. A layer of resistor material is deposited over the conductor layer and resistor windows, and is masked and etched to form traces and resistors  406  and  408 . After the formation of the traces and resistors  406  and  408 , protective layers may be deposited and then layers of photoimageable epoxy can be applied and imaged to form a base, flow channels, ejection chambers over the resistors  406  and  408 , and nozzles  402  and  404  over the ejection chambers. 
       FIG. 5  is a close up top view  500  of two drop generators, showing the different nozzle designs. Like numbered items are as described with respect to  FIG. 4 . In examples described herein, the layout of the top layer, e.g., the nozzles  402  and  404 , is used to create a printhead that can print multiple drop sizes on pitch. As described herein, the drop weight and drop velocity are dependent upon the interaction of the area of the resistors  406  and  408  and the bore, or area, of the nozzles  402  and  404 . For example, a bore for a 9-10 ng drop is in the range of between about 280 to 340 μm{circumflex over ( )}2 while a bore for a 3-4 ng drop is between about 160 to 200 μm{circumflex over ( )}2. If the nozzles were circular, the diameters would be about 19-20 μm and 12-14 μm respectively. As the wall between each drop generator is about 5 μm, the spacing for a 21.5 μm pitch would be about 32 μm. The diameters described above would not fit within this measurement. 
     However, the use of a two-lobed polynomial ellipse as a non-circular bore (NCB) for the nozzle  404  of the HDW drop generator reduces the extent of the bore in the y direction  416 , allowing the nozzle  404  to fit on the pitch. Further, the location of the smaller circular bore (CB) of the nozzle  402  for the LDW drop generator falls in a position that maximizes the space between the nozzles  402  and  404 . This increases the mechanical strength of the structure and limits fluidic interactions between the nozzles  402  and  404 . 
       FIGS. 6A and 6B  are drawings of the dot patterns from the nozzles described with respect to  FIG. 5 . Referring also to  FIG. 5 , the HDW nozzle  404  provides the drop pattern shown in  FIG. 6A . The NCB gives a large main drop  602  with small satellite drop  604 . This arrangement is desirable for text and graphics, since it may provide sharp edges to lines. An HDW drop produced by the NCB has much less relative ink volume in tail drops, providing better, sharper edges. Further, the thermal limit on printing speed is more a function of drops per second than ink volume per second. Thus, printing with the HDW drop generator gives more ink flux capability. 
     The LDW nozzle  402  provides the pattern shown in  FIG. 6B . The CB gives two dots  606  and  608  of similar size. This arrangement is desirable for images, as the smaller, less visible dots of the LDW drops cover more white space providing a smoother, more uniform image with less grain. However, more dots are used to make a specific tone. Further, at higher printer speeds, the head and tail of the LDW drops may become unacceptably distant, e.g., greater than about a 600 dpi pixel size, leading to blurring of text and images. As a result, the speed of the print medium may be controlled, at least in part, by the ratio of HDW drops to LDW drops used in the printing. For example, at high ratios of HDW drops to LDW drops the speed of the line may approach a design speed, such as about 1000 feet per minute (about 300 meters per minute) or higher. At low ratios of HDW drops to LDW drops, the speed may be decreased, for example, to 800 feet per minute (244 meters per minute) or lower. 
       FIG. 7  is a drawing of a pattern  700  of HDW drop generators and LDW drop generators on a printhead. The nozzles of the LDW drop generators are labeled cb 4 , and the nozzles of the HDW drop generators are labeled ncb 9 . The LDW nozzles and HDW nozzles are disposed opposite each other on opposite sides of an ink feed slot  702  in the direction of motion of the print medium. By arranging the design in this way, when only the HDW nozzles are used in high speed mode, the printed Y dot pitch  704  is about 1/1200 in ( 1/490 cm), as HDW nozzles from both sides of the ink feed slot  702  are used. The same is true for printing using only the LDW nozzles. Each two rows of drop generators placed on each side of an ink feed slot  702  may be termed an ink slot  706 . 
     The drop weight from a drop generator is determined for the most part by the areas of the resistor and the bore of the nozzle. Drop weight will increase as either is increased. However, the correct balance between the area of the resistor and the bore of the nozzle is necessary to obtain the correct drop velocity. 
     In some examples, the total pitch available for any of the LDW and HDW pairs going down a column of resistors is 21 μm. The space is partitioned between the resistor width for each drop generator and the spacing between the resistors. The spacing is determined by the minimum workable width for the epoxy that must separate the resistors of two adjacent drop generators. A minimum of 7 μm is needed for this material and thus the sum of the two resistor widths cannot exceed 28 μm. This parameter is combined with the area needed for each drop weight and the desired firing pulse, e.g., voltage and pulse width, in order to size the resistors. 
       FIG. 8  is a plot of ink densities for different ink tones, which can be used to linearize the rasters, e.g., to determine which drop generators to fire. The y-axis  802  represents the output ink density, e.g., the total amount of ink released from all of the drop generators. The x-axis  804  represents the input tone, for example, the depth of the color at each point. The example in  FIG. 8  is for a black raster. 
     Rules can be determined by the depth of the tone in the raster and the coverage provided by each of the drop generators. For example, in the light and mid tones, as indicated by line  806 , only the LDW drop generators may be used to provide smoother textures. 
     In dark tones, as indicated by line  808 , only the HDW drop generators may be used, as the grain is not visible due to white space coverage. Further, only the HDW drop generators may be used where edges are important, e.g., for dark text and lines. 
     In some regions, as indicated by line  810 , a combination of the LDW drop generators and HDW drop generators may be used. This may provide some advantages from both, e.g., more total ink may be provided by the HDW drop generators, while the LDW drop generators may lessen the impact of any visible grain. Because HDW drop generators and LDW drop generators are never used heavily at the same time, the average firing frequency for the whole ink slot  706  ( FIG. 7 ) is not higher than for one drop weight by itself. On average, the LDW drop generators may be used for about 60 to 70% of the printing on a page, while the HDW drop generators may be used for about 30 to 40% of the printing on a page. 
       FIGS. 9A and 9B  are drawings showing the difference between pictures printed with only HDW drop generators versus only LDW drop generators. The image in  FIG. 9A  was printed exclusively with the HDW drop generators, and shows more grain structure than the image in  FIG. 9B , which was printed exclusively with the LDW drop generators. 
       FIG. 10  is a process flow diagram of an example method  1000  to print a document using a printer that has HDW drop generators and LDW drop generators. Referring to  FIG. 2 , the method  1000  may be fully executed by the controller  220  in the ink jet printing system  200 . However, in some examples, some portion or even all of the method  1000  may be executed on the host  222 . The method  1000  begins at block  1002  with an input document. As described herein, the input document could be sent to the controller by the host, or may be provided by another system on a network. In some examples, a host or a controller may function as a queue, storing a number of input documents for sequential printing. At block  1004  the input document is rasterized to create color rasters  1006 . As described herein, each color raster  1006  is a color plane, or image, corresponding to an ink used by the printing system. 
     At block  1008 , the color rasters  1006  are linearized to create planes  1010  representing HDW printing and LDW printing. The linearization may be performed using rules developed from a plot of output ink density versus input tone, as described with respect to  FIG. 8 . 
     At block  1012 , the HDW and LDW planes  1010  may be processed to generate halftone planes  1014 . As described herein, the halftone planes  1014  represent the color intensity or tone at each position by printing 0, 1, or 2 drops of the associated drop weight, e.g., HDW drops or LDW drops. In some examples, the number of drops may be proportionally higher for the LDW drops. 
     At block  1016 , the HDW and LDW halftone planes  1014  may be masked to create HDW and LDW printhead maps  1018 , which map particular drops to particular printbars, printheads, and ink slots. At block  1020 , the HDW and LDW printhead maps  1018  are merged to create a single stream of print data, which is sent to the printheads  1022 . 
     The method  1000  described is not limited to the printhead designs shown, but may be used with other possible designs. For example, a first printhead that includes staggered rows of HDW drop generators may be in the line of motion of the print medium from a second printhead that includes LDW drop generators. In this example, each of the HDW drop generators in the first printhead may be on a dot pitch with a corresponding LDW drop generator in the second printhead. Although this arrangement, or other arrangements, would not be as desirable as the combined printheads described herein, the method  1000  could still be used to print a document in this arrangement. 
     The ink jet printheads described herein may be used in other applications besides two dimensional printing. For example, in three dimensional printing or digital titration, among others. In these examples, the different sizes of drop generators may be of benefit for other reasons. In digital titration, the HDW drop generator may be used to approach an end point quickly, while the LDW drop generator may be used to accurately determine the end point. 
     The present examples may be susceptible to various modifications and alternative forms and have been shown only for illustrative purposes. Furthermore, it is to be understood that the present techniques are not intended to be limited to the particular examples disclosed herein. Indeed, the scope of the appended claims is deemed to include all alternatives, modifications, and equivalents that are apparent to persons skilled in the art to which the disclosed subject matter pertains.