Patent Application
20140036002
The invention relates generally to the field of liquid ejection systems, and in particular to ejection using a type of thermal inkjet ejector having greatly improved reliability for drop ejection of liquids that have poor latency using conventional thermal inkjet ejectors.
Drop on demand liquid ejection systems include a liquid supply fluidically connected to a liquid ejector that is capable of ejecting individual droplets of the liquid as needed. A familiar type of drop on demand liquid ejection system is an inkjet printer, where liquid ink is provided to an ejector, such as a piezoelectric ejector or a resistive heater ejector. Other types of liquid ejection systems are used for precise metering of liquids, or patternwise deposition of liquids in non-imaging applications, for example, to form electronic or optical devices or structural members.
A piezoelectric ejector includes a chamber for holding a small quantity of liquid and one or more piezoelectric elements, which change the volume of the chamber when an electrical pulse is applied in order to eject a droplet through a nozzle associated with the chamber. A resistive heater ejector includes a chamber holding a small quantity of liquid and a resistive heater in contact with the liquid. When an electrical pulse is applied to the resistive heater, the heater and the liquid near the heater are heated up so that a portion of the liquid is vaporized, forming an expanding bubble that propels a droplet of liquid through a nozzle associated with the chamber. Resistive heater ejectors (which are used in thermal inkjet printheads) have advantages of simple and economical fabrication at high ejector resolution, but they typically do not have as wide a latitude for jetting different types of liquids as piezoelectric ejectors.
Liquids in liquid ejection systems typically include a material of interest and a carrier fluid. In an inkjet printing system, the material of interest is typically a colorant, and the carrier fluid is typically water-based. Additional components are included in an ejectable liquid for reliable jetting or to promote desirable properties of the ejected droplets, including their interaction with a medium onto which they are ejected.
For printing applications, ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment inks can have inferior durability after printing, especially under conditions where abrasive forces have been applied to the printed image and especially at short time intervals from immediately after printing to several minutes while the inks are drying.
Pigment-based inks must be reliably ejected from a printhead for numerous individual firing events during the lifetime of a printer. This includes situations where the printhead is left idle or uncapped for long periods of time and then is actuated again to eject ink. In some instances, the idle printhead nozzles can partially clog or crust with ink components thereby degrading the ability of the printhead to eject properly. For example, the ink can be misdirected from the partially clogged nozzles or the drop velocity can be greatly diminished. In some instances, a nozzle can become permanently clogged and in other instances a lengthy and costly maintenance operation may be required to recover the nozzle back to a usable state of operation. This phenomenon is known in the art of inkjet printing as latency or decap. An ink having good latency performance will exhibit a useful drop velocity after long decap intervals. A longer latency is highly desirable as the ink can reside in the idle printhead for a longer time without adversely affecting the ink ejection performance. Inkjet printers typically include a cap or other reservoir for ejecting maintenance droplets periodically, so that droplets ejected as part of an image will be reliably and accurately ejected for good image quality. Printing throughput is adversely affected if it is required to eject maintenance droplets too frequently.
Formulation of ejectable liquids, such as inkjet inks, involves balancing desirable jetting properties of the liquid through the associated liquid ejector with properties of the material of interest in the ejected droplets. For example, in a pigment-based ink, polymeric dispersants can be added to keep the pigments in suspension in the carrier fluid, and polymeric binders can be added to improve durability of an image on a recording medium onto which the droplets have been ejected.
Pigment-based inks formulated with polymeric dispersants and binders can be difficult to jet through inkjet printheads having small nozzle diameters especially by the thermal inkjet printing process. This is especially true of pigment-based inks, which are formulated with humectants or penetrants that lower dynamic surface tension. In recent years, thermal inkjet printers have moved to higher jetting frequencies to provide faster printing speeds. Thermal inkjet printers are now capable of printing at jetting frequencies in excess of 10 kHz. However, this high frequency firing can come at the cost of variability in the drop velocity, which can lead to poor image quality in the final printed image.
Polyurethane binders have been used as durability enhancing additives in dye-based and pigment-based inkjet inks. U.S. Pat. No. 6,136,890 discloses a pigment-based inkjet ink wherein the pigment particles are stabilized by a polyurethane dispersant. U.S. Patent Application 2004/0242726 discloses a pigment dispersed by a cross-linking step between a resin having a urethane bond and a second water-soluble polymer. U.S. Patent Application 2004/0229976 discloses polyurethane/polyurea resins for pigmented inks where the weight fraction of a polyurethane urea part is at most 2.0 wt % to the urethane resin.
Although polyurethanes are known for their excellent durability, they also have a number of drawbacks. For example, not all polyurethane polymers are conducive to jetting from a thermal inkjet head. In particular, water-dispersible polyurethane particles, such as those disclosed in U.S. Pat. Nos. 6,533,408 and 6,268,101, Statutory Invention Registration U.S. H2113H, and published U.S. Patent Applications 2004/0130608 and 2004/0229976 are particularly difficult to jet from a thermal inkjet printhead at high firing frequencies. The molecular weight of the polyurethane binder plays an important role in the ink performance and durability of the resulting printed images. For example, molecular weights below about 8,000 generally do not provide highly durable images. On the other hand, molecular weights above about 20,000 can be detrimental to firing performance from a thermal inkjet printhead, especially for inks having high solids content, i.e. a content of more than about 5% by weight of pigment particles and polymers. The acid number of the polyurethane or other binder polymer also creates limitations for use in an inkjet printing system. If the acid number of the binder polymer is too high the resulting abrasion resistance of the image can become degraded, especially under conditions of high temperature and high humidity. If the acid number of the binder polymer is too low, a substantial amount of particulate polymer will exist and jetting can become degraded.
Both the ejector design and the liquid formulation have an impact on the latency, i.e. on how long a time interval between ejecting droplets through an ejector can be while still providing reliable ejection of the next droplet. In the context of inkjet printing, it is desired to provide deposited drops on the recording medium having small spot size of uniform pigment loading to reduce image graininess, high intensity of color for wide color gamut, fade resistance, and good adhesion to the recording medium. It is also important to provide interaction between the ejected ink and the recording medium, without causing undesirable changes, such as extensive curling, in the recording medium after printing. For jetting reliability, it is important to keep the viscosity at a sufficiently low level, enable high frequency ejection, and provide long latency. It can be difficult to provide desirable marking and jetting properties, particularly for a printhead having small nozzles, and for liquids having high solids content or high molecular weight polymers.
Although the use of pigments and polymer binders have found use in liquid ejection systems such as inkjet printers, there remains the need to identify an resistive heater ejector design that is capable of providing a greater latitude for ejecting inks or other liquids having desirable properties over the required range of operating conditions. This is especially true for inks or other liquids having high solids content above about 5 percent by weight, as well as for inks or other liquids including a significant loading of polymers having high molecular weight. It is therefore an object of this invention to identify a liquid ejector design having a demonstrated significant improvement in latency relative to conventional liquid ejectors that have poor latency for ejecting such liquids having high solids content or significant loading of polymers having high molecular weight
A method of printing an image on a recording medium comprising: supplying a pigmented ink to an inkjet printhead having an array of dual feed thermal inkjet ejectors, wherein the pigmented ink includes an aqueous carrier with a pigment particle loading of at least 4 percent by weight and a polymer loading of at least 1 percent by weight; ejecting a plurality of maintenance drops of the pigmented ink from the array of dual feed thermal inkjet ejectors prior to a start of printing the image on the recording medium; printing the image swath by swath by ejecting printing drops of the pigmented ink on the recording medium as a carriage moves the printhead back and forth in a carriage scan direction across the recording medium between successive advances of the recording medium, wherein a plurality of printing swaths are required in order to complete the printing of the image; and ejecting a plurality of maintenance drops of the pigmented ink from the array of dual feed thermal inkjet ejectors after a completion of printing the image on the recording medium, wherein no maintenance drops are ejected between the start and the completion of the printing of the image.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
U.S. Pat. No. 7,857,422, incorporated by reference herein in its entirety, discloses a dual feed liquid drop ejector, some configurations of which are described below relative to
Referring to
Referring to
Referring to
In
The flow patterns of
An important aspect of the dual feed liquid ejector 20 is that each chamber 30 is supplied with liquid by the first liquid feed channel 38 that is connected to a segment 37 or 39 of the segmented liquid inlet 36 located on one side of the chamber 30, and by the second liquid feed channel 40 that is connected to a segment 39 or 37 of the segmented liquid inlet 36 located on the opposite side of the chamber 30. That is different from a conventional liquid ejector (not shown) having a chamber that is bounded typically on three sides by walls, with the fourth side being open and facing a single ink inlet.
In
Each first segment 37 of the segmented liquid inlet 36 includes ends 46 that are substantially in line with ends 48 of each second segment 39 of the segmented liquid inlet 36. In
One or more posts 52 can be disposed in the chamber 30, the first liquid feed channel 38, the second liquid feed channel 40, or combinations thereof. As discussed in more detail below, the posts 52 can be symmetrically or asymmetrically disposed about the nozzle orifice 32 and within one or both of the liquid feed channels 38, 40. The posts 52 can have the same cross sectional area or different cross sectional areas when compared to each other. The posts 52 can also have same shapes or different shapes when compared to each other.
Referring to
Having described the basic components of the dual feed liquid ejector 20, the operation of a dual feed liquid ejector 20, as embodied in a thermal inkjet printhead, will be described so that the advantages and reasons for those advantages become more apparent. Ink enters the printhead die 18 through the segmented liquid inlet 36 and passes through the first and second liquid feed channels 38 and 40 from opposite directions to enter the fluid chamber 30. In a conventional thermal inkjet printhead, the chamber 30 is filled with ink through a single liquid feed channel from only one direction. When the chamber 30 of the dual feed liquid ejector 20 is filled with ink, the resistive heating element 34, which is positioned below the nozzle orifice 32, is in thermal contact with the pool of ink in the chamber 30. A particular configuration of the resistive heating element 34 is shown that includes two parallel legs of a resistive material 33, joined at one end by a conductive shorting bar 35. Electrical leads 56 are connected to each leg 33 at the opposite end from the shorting bar 35. However, other configurations of the resistive heating element 34 are possible.
With reference to
For accurate firing of jets, it is preferable for the droplet to be ejected at a velocity of approximately 6 to 20 meters per second, depending somewhat on the size of the droplet. In order to increase the drop velocity (and increase the energy efficiency, which is the energy of the drop divided by the energy input into the resistive heating element 34), it is helpful to preferentially direct the expansion of the bubble toward the nozzle. This is one of the functions of the posts 52, which act as a source of lateral fluid impedance, so that a greater amount of the bubble expansion is directed toward the nozzle orifice 32.
The posts 52 also restrict the amount and momentum of liquid flow away from the chamber 30, so that the refill of the chamber 30 is able to occur more quickly. Refill of the chamber 30 is typically the rate limiting step for how quickly the same chamber can be fired again. After the drop is ejected, liquid must feed in from the segmented liquid inlet 36 through the first and second liquid feed channels 38 and 40 and into the chamber 30. The dual feed configuration inherent in this invention increases refill rate (and hence printing throughput speeds) for several reasons. As mentioned above, the posts 52 restrict the backflow of ink so that the reversal of ink flow can happen more quickly. Another important factor promoting faster refill is the existence of the two liquid feed channels 38 and 40 rather than a single feed channel, thereby increasing the rate of flow of ink back into the chamber 30. In addition, compared to conventional liquid ejectors, which are fed from one side of the chamber 30, but have a fluidic dead-end at the opposite side of the chamber 30, the dual feed liquid ejector 20 described herein is fed from two opposite sides of the chamber 30. As a result, the ink-air interface possesses symmetric curvature relative to the chamber 30 during refill, which enhances the pressure differences that drive refill, so that refill occurs more rapidly. Computer simulations of flow, as well as testing of the dual feed configuration indicate that refill rate is approximately twice as high as for a conventional single feed configuration for a comparably sized drop.
As can be seen in
A lower magnification top view of a portion of the liquid ejection printhead die 18 is shown in
Although there are various configurations of the dual feed liquid ejector 20, the essential features of the dual feed liquid ejector 20, as defined herein with application to thermal inkjet include a structure defining the chamber 30, the chamber 30 including a first surface and a second surface, the first surface including the nozzle orifice 32; the resistive heating element 34 located on the second surface of the chamber 30 opposite the nozzle orifice 32; the first liquid feed channel 38 and the second liquid feed channel 40 being in fluid communication with the chamber 30; and the segmented liquid inlet 36, the first segment 37 of the segmented liquid inlet 36 being in fluid communication with the first liquid feed channel 38, and the second segment 39 of the segmented liquid inlet 36 being in fluid communication with the second liquid feed channel 40. Such dual feed liquid ejectors 20 having a resistive heating element 34 that functions as the drop forming element are also sometimes called a dual feed thermal inkjet ejector herein. For an array of the dual feed liquid ejectors 20, as seen in the example described above relative to
The initial primary motivation for the design of the dual feed liquid ejector 20 was to provide faster refill and higher drop ejection frequency to enable faster printing throughput as described above, and that predicted improved performance was verified by experiment. However, in testing the ejection of a range of different liquid compositions, including a variety of ink formulations, a surprising result was found. In particular, the dual feed thermal inkjet ejector 20 was found to provide much better latency than a conventional single feed thermal inkjet drop ejector when ejecting inks or other liquids that tend toward poor latency. In other words the dual feed thermal inkjet ejector 20 is able to consistently eject a drop of a latency challenged liquid after a waiting interval since the previously ejected drop that is at least several times longer, and up to more than an order of magnitude longer, than can be done with a conventional single feed thermal inkjet drop ejector. Some amount of improvement in latency with a dual feed thermal inkjet ejector could be expected due to having two sources of liquid feeding the chamber 30 rather than one source. Typically, as carrier fluid (such as water) evaporates near the nozzle, the less volatile components increase in viscosity, making it difficult to eject a drop. With two sources of liquid connected to the chamber 30 in a dual feed thermal inkjet ejector 20, more carrier fluid can diffuse toward the chamber 30. However, the large extent of the improvement in latency for a dual feed thermal inkjet ejector was unexpected.
Factors in Inks or Other Liquids that Influence Latency
U.S. Pat. No. 8,044,115, included by reference herein in its entirety, describes a number of factors that influence latency of a liquid, as summarized below.
Many inkjet inks are aqueous-based inks. By aqueous-based it is meant that the ink comprises mainly water as the carrier fluid for the remaining ink components. Pigment-based aqueous inks are defined as inks containing at least a dispersion of water-insoluble pigment particles. Dye-based inks are defined as inks containing at least a colored dye, which is soluble in the aqueous carrier. Colorless inks are defined as inks, which are substantially free of colorants such as dyes or pigments and as such, are not intended to contribute to color formation in the image forming process.
An ink set is defined as a set of two or more inks. The ink sets may contain inks of different colors, for example, cyan, magenta, yellow, red, green, blue, orange, violet or black. For example, a carbon black pigmented ink is used in an ink set comprising at least three inks having separately, a cyan, a magenta and a yellow colorant. Useful ink sets also include, in addition to the cyan, magenta and yellow inks, complementary colorants such as red, blue, violet, orange or green inks. In addition, the ink set can include light and dark colored inks, for example, light cyan and light magenta inks. It is possible to include one or more inks that comprise a mixture of different colorants in the ink set. An example of this is a carbon black pigment mixed with one or more colored pigments or a combination of different colored dyes in the same ink. An ink set can also include one or more colored inks in combination with one or more colorless inks. An ink set can also include at least one or more pigment-based inks in combination with additional inks that are dye-based ink.
Many pigment-based inks include pigment particles dispersed in the aqueous carrier using a polymeric dispersant. The pigment particles can be prepared by any method known in the art of inkjet printing. Useful methods commonly involve two steps: (a) a dispersing or milling step to break up the pigments to primary particles, where primary particle is defined as the smallest identifiable subdivision in a particulate system, and (b) a dilution step in which the pigment dispersion from step (a) is diluted with the remaining ink components to give a working strength ink.
Typically, polymeric dispersants are copolymers made from hydrophobic and hydrophilic monomers. In this case, the copolymers are designed to act as dispersants for the pigment by virtue of the arrangement and proportions of hydrophobic and hydrophilic monomers. The pigment particles are colloidally stabilized by the dispersant and are referred to as a polymer dispersed pigment dispersion. The pigment dispersions useful in pigment-based ink compositions desirably have a median particle diameter of less than 200 nm and more preferably less than 100 nm.
Typically, the weight average molecular weight of the copolymer dispersant has an upper limit such that it is less than about 50,000 Daltons. Desirably the weight average molecular weight of the copolymer preferably less than 10,000 Daltons. The molecular weight of the copolymer has a weight average molecular weight lower limit such that it is greater than about 500 Daltons.
Particularly useful polymeric pigment dispersants are further described in U.S. Publication 2006/0012654 and 2007/0043144, the disclosures of which are incorporated herein by reference.
Pigments suitable for use in an inkjet ink include, but are not limited to, azo pigments, monoazo pigments, disazo pigments, azo pigment lakes, β-Naphthol pigments, Naphthol AS pigments, benzimidazolone pigments, disazo condensation pigments, metal complex pigments, isoindolinone and isoindoline pigments, polycyclic pigments, phthalocyanine pigments, quinacridone pigments, perylene and perinone pigments, thioindigo pigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthrone pigments, dioxazine pigments, triarylcarbonium pigments, quinophthalone pigments, diketopyrrolo pyrrole pigments, titanium oxide, iron oxide, and carbon black.
The pigment particles can be dispersed by a dispersant in an amount sufficient to provide stability in the aqueous suspension and subsequent ink. The amount of dispersant relative to pigment is a function of the desired particle size and related surface area of the fine particle dispersion. The ratio of pigment to dispersant can range from about 10:1 to about 1:1, and more preferably from about 5:1 to about 2:1. It is understood that the amount of polymer and relative ratios of the monomer constituents can be varied to achieve the desired particle stability and ink firing performance for a given pigment, as it is known that pigments can vary in composition and affinity for the dispersant.
Inkjet inks also optionally include self-dispersing pigments that are dispersible without the use of a dispersant. Pigments of this type are those that have been subjected to a surface treatment such as oxidation/reduction, acid/base treatment, or functionalization through coupling chemistry. The surface treatment can render the surface of the pigment with anionic, cationic or non-ionic groups. Examples of self-dispersing type pigments include, but are not limited to, Cab-O-Jet® 200 and Cab-O-Jet® 300 (Cabot Corp.) and Bonjet® Black CW-1, CW-2, and CW-3 (Orient Chemical Industries, Ltd.).
Ink compositions typically include one or more humectants to help retain water in the ink. Glycerol is an effective humectant for pigment-based inks and provides stable vapor bubble formation in a thermal inkjet printhead. Glycerol is a desirable ingredient in a thermal inkjet ink since it aids in maintaining the heater surface which leads to long term printhead lifetimes. Inks formulated with glycerol as a humectant typically tend toward good latency performance.
Inks are formulated not only to have good jetting performance, but also for desirable properties of the ejected drops on the recording medium 24 (
The latency performance of inks comprising glycerol and 1,2-alkanediols can be significantly improved by the additional presence of a pyrrolidinone compound. Preferred pyrrolidinone compounds include, 2-pyrrolidinone, 1-(2-hydroxyethyl)-2-pyrrolidinone, and 1-methyl-2-pyrrolidinone. The pyrrolidinone can be used alone or as a mixture of two or more such compounds. A particularly preferred combination of pyrrolidinones is a mixture of 2-pyrrolidinone and 1-(2-hydroxyethyl)-2-pyrrolidinone.
In order to help make the pigment particles adhere to the recording medium 24, ink compositions can also include at least one water-dispersible polymer binder, such as a polyurethane compound or an acrylic compound. By water-dispersible it is meant to include individual polymer molecules or colloidal assemblies of polymer molecules, which are stably dispersed in the ink without the need for a dispersing agent.
Preferred polymer binders have a sufficient amount of acid groups in the molecule to have an acid number from about 50 to about 150 in the case of a polyurethane binder, and around 300 for an acrylic binder. If the acid number of the binder polymer is too high, the resulting abrasion resistance of the image can become degraded, especially under conditions of high temperature and high humidity. If the acid number of the binder polymer is too low, a substantial amount of particulate polymer will exist and jetting can become degraded. The acid number is defined as the milligrams of potassium hydroxide required to neutralize one gram of polymer. The acid number of the polymer may be calculated as follows:
Acid number=(moles of acid monomer)*(56 grams/mole)*(1000)/(total grams of monomers), where moles of acid monomer is the total moles of all acid group containing monomers that comprise the polymer, 56 is the formula weight for potassium hydroxide and total grams of monomers is the summation of the weight of all the monomers, in grams, comprising the target polymer.
For excellent image durability on the recording medium 24, a polymeric binder, such as polyurethane, in an aqueous based pigmented ink preferably has a minimum molecular weight of at least 15,000. Polymeric binders such as polyurethane in an inkjet ink preferably have a maximum molecular weight of 150,000. Latency tends to decrease particularly for significant loading (1% or greater) of polymers having a molecular weight of greater than 15,000, especially where the ink also includes relatively high loading of pigment particles. Latency can be especially low for significant loading of polymers having a molecular weight of at least 20,000, and especially for higher acid numbers. The polyurethane dispersions useful as a binder preferably have a mean particle size of less than 100 nm and more preferably less than 50 nm.
Surfactants may be added to adjust the surface tension of the ink to an appropriate level, for example to control intercolor bleed between the inks. The surfactants can be anionic, cationic, amphoteric or nonionic and used at levels of 0.01 to 5% of the ink composition. A typical surfactant for an inkjet ink is Surfynol.
An anti-curl agent can be added to the ink to interact with the recording medium 24 such that the recording medium 24 does not curl up extensively after being printed upon. A particular type of anti-curl agent that has been demonstrated to be very effective in preventing curl, but also tends to cause the ink to have poor latency when using a conventional single feed thermal inkjet drop ejector is a branched, polyethylene glycol ether of at least 0.5 percent by weight. Such branched polyethylene glycol ether materials include those based on glycerol, such as the Liponic or Glycereth materials, and also those based on pentaerythritol, such as the pentaerythritol ethoxylates and propoxylates.
A biocide (0.01-1.0% by weight) can also be added to prevent unwanted microbial growth which may occur in the ink over time. Additional additives which can optionally be present in an inkjet ink composition include thickeners, conductivity enhancing agents, anti-kogation agents, drying agents, waterfast agents, dye solubilizers, chelating agents, binders, light stabilizers, viscosifiers, buffering agents, anti-mold agents, stabilizers and defoamers.
The dual feed liquid ejectors 20 can also be used to eject liquids other than inkjet inks that are used in the printing of images. For example, in the field of functional printing, devices, circuitry or structures can be fabricated on a substrate (analogous to recording medium 24) by ejecting one or more liquids in patternwise fashion. Liquids for making such devices, circuitry or structures can include electrically conductive particulate or polymeric material for making a conductive portion, resistive material for making a resistive portion, insulating material for making an insulating portion, semiconducting material for making a semiconducting portion, magnetic material for making a magnetic portion or structural materials such as polymers for making a structural member. In order to make a conductive member with suitably high conductivity, it can be advantageous to use a particle loading of metal particles, such as silver particles, of at least 4 percent by weight. In order to bind the conductive particles to the substrate it can be advantageous to have a polymer loading of at least 1 percent by weight.
Although many of the ink compositions and other liquids described herein can be ejected through a conventional single feed thermal inkjet drop ejector, such as the liquid ejector described in U.S. Pat. No. 7,600,856, it has been found that when certain components or combinations of components are included at high enough loading levels, the latency of the ink or other liquid can be adversely affected. As a result it becomes necessary to eject maintenance drops as often as every few seconds so that the liquid ejector is consistently able to eject drops as needed for printing an image or forming a device or other structure. Short latency times adversely impact ejection productivity and also waste ink or other ejection liquids.
Latency of an ejection liquid in a liquid ejector can be characterized relative to a maximum time interval between reliably ejecting a drop and a previous drop. The longer the time interval, the better the latency is. Desirable latency times depend upon the application. For example, a desktop carriage inkjet printer can print a swath of an image in less than a second, but it can require five seconds or more to print a letter-sized color image, and thirty seconds or more to print a high quality photographic image in a multi-pass print mode. For a wide format printer, the swath time can be greater than two seconds, and the total print time can be several minutes. The printhead needs to eject maintenance drops (typically into a cap or spittoon outside of the printing region) frequently enough that the poorest latency ink in the ink set continues to be reliably ejectable over the range of temperatures and humidities that can be encountered in the printer. Latency times that are less than a few seconds can significantly slow down printing throughput.
An additional consideration is how many maintenance drops are required after the time interval in order to ensure continued reliable ejection. It has been observed that if the ink or other liquid in a liquid ejector has increased in viscosity in the nozzle region, multiple firing attempts can be required to restore desirable jetting performance. For this reason, rather than firing only a single maintenance drop from each liquid ejector, it is more typical to pulse each liquid ejector multiple times, for example 5 to 20 times, while the printhead is at the cap or spittoon. The first firing, or the first several firings, may not even result in ejection of a drop at all. When drops begin to be ejected, they can have slow velocity or otherwise poor performance. As the time interval between ejecting a drop and the previous drop increases, more and more maintenance drops can be required to restore jetting performance. For sufficiently long time intervals, as many as 50 maintenance drop firings can be required. It is sometimes considered not to be practical to use time intervals that require attempting to eject more than about 50 maintenance drops.
A new testing method and metric have been devised to characterize latency performance of different liquids in different liquid ejectors based on how many failed ejections occur at various wait time intervals. A printhead or other liquid ejector is mounted in a jetting fixture having a drop detection device, such as an optical sensor. The ink or other liquid of interest is connected to the inlet of the liquid ejector and primed to fill the chambers near the nozzles. The liquid ejector is then pulsed multiple times while monitoring the ejected drops until stable jetting performance is observed. Then a sequence of pulsing groups of firing pulses with each group separated by successively increasing wait times is run while monitoring the ejected drops. For example, each group of firing pulses can include 50 pulses for the liquid ejector. Successive wait times can include 1 second, 2 seconds, 5 seconds, 10 seconds, 20 seconds, 30 seconds, 50 seconds, 75 seconds, 100 seconds, 200 seconds and 500 seconds. A new latency metric called the latency score LS is defined below in equation (1):
where E=ejection observed (1 or 0);
a=number of jetting attempts (1 to 50);
tW=wait time in seconds (1, 2, 5, 10, 20, 30, 40, 50, 75, 100, 200, and 500); and
LSmax is the maximum value of the double summation if each E equals 1.
A perfect latency score is LS=1.0, and the higher the latency score the better. For the wait times tW listed above, LSmax˜4647.7, which is used to normalize the latency score. The rationale for the latency score calculation is that an ink or other liquid has better latency if drops can be successfully ejected (E=1) even for long wait times tW. Relatively few unsuccessful jetting attempts (E=0) at a given wait time is also preferred.
The latency score provides a compact comparison of different inks or other liquids being ejected from different types of ejectors without getting into the details of exactly which drops failed to fire. To understand what various ranges of latency scores imply, Table 1 lists the calculated latency score for various numbers of failed drops at different wait times. The examples in Table 1 are selected based on observing that the typical behavior is that for comparatively short wait times all drops are ejected successfully. For successively longer wait times, more and more of the initial attempted firings fail as wait time is increased. Note in the first several entries in the table, due to the heavy weighting on weight time tW, especially for the initial attempt (a=1), the latency score drops fairly rapidly from the perfect score of 1 due to relatively few initial drop failures at long wait times of 500 seconds or 200 seconds.
Qualitative ratings are indicated in the leftmost column of Table 1. It is important to note that the qualitative ratings depend on context. For example, because of the longer wait time required when printing with a wide format printer as compared to a desktop carriage printer, fair latency for a desktop printer might be poor latency for a wide format printer. In addition, the latency score is based upon whether a drop was successfully ejected or not. It does not take into account the quality of the ejected drop. For example, after several failed attempts at a given wait time a particular drop might be ejected, but the first successfully ejected drop or drops at a given wait time might have poor velocity and directionality, and thereby not satisfactory for high quality printing. The latency score is a compact comparative indicator of the performance of various inks and other liquids in different ejectors using a simple measurement technique. However, it does not take the place of printing experiments within an actual printer over its entire range of operating temperatures and humidities to determine an actual maintenance algorithm.
The rationale for some of the qualitative ratings is as follows. If there are no failed ejections for wait times of over 1 minute, and if even at wait times of 100 seconds, 200 seconds and 500 seconds the ejector successfully ejects drops after a number of attempts that is consistent with typical maintenance routines (spitting 5-20 drops), then the latency of that ink or other liquid with that ejector is outstanding. Thus, according to Table 1, a latency score of 0.44 or greater is consistent with outstanding latency performance. On the other hand, if the first few drops fail to eject at a wait time of 5 seconds, and successively more drops fail to eject at longer wait times, the latency is poor. If the first few drops fail to eject at a wait time of 1 second, and successively more drops fail to eject at longer wait times, the latency of that ink or other liquid and ejector type is probably unusable. From the table below, poor latency is characterized by a latency score between 0.003 and 0.014. The ratings are intended to provide guidelines for comparison, not to specify maintenance routines.
Table 2 summarizes experimental data and the corresponding latency scores for a variety of pigmented inks having a range of total solids content (percent by weight of pigment plus percent by weight of polymer) when ejected from a conventional single feed thermal inkjet ejector (for example, the drop ejector described in U.S. Pat. No. 7,600,856) versus a dual feed thermal inkjet ejector as described above with reference to
From the results listed in Table 2, although latency ratings with the single feed thermal inkjet ejector range from poor to outstanding for the different inks and temperatures, the latency ratings using the dual feed thermal inkjet ejector are consistently outstanding. Comparing the latency scores LS with the examples in Table 1, it is evident that the wait times that a dual feed thermal inkjet ejector can experience and still eject drops of the inks listed in Table 2 can be over an order of magnitude longer than the wait times that a single feed thermal inkjet ejector can experience and still eject drops.
It is also evident from Table 2 that latency scores typically decrease as the total solids content increases. Still, for the entire range studied here, whether the solids content was greater than 5%, 6%, 7%, 8%, 9% or 10%, the latency rating for ejecting the various inks through the dual feed thermal inkjet ejector was consistently outstanding and significantly improved relative to the single feed thermal inkjet drop ejector.
Pigment particle loading is especially important for some inks. In particular, in order to achieve a sufficiently wide color gamut using presently available pigments on a wide range of recording media, it is required to have a magenta pigment loading of at least 4% by weight in the magenta ink. As can be seen from Table 2, a dual feed thermal inkjet ejector has no latency issues for ejecting inks with a magenta pigment particle loading of at least 4% or even higher by weight, while latency for a conventional single feed thermal inkjet liquid ejector is typically marginal, especially at the higher end of temperatures encountered in a printer.
With regard to the portion of solids content that is due to polymers, it is found to be advantageous for an aqueous based pigmented ink if the dispersant polymer loading is at least 10% of the pigment loading by weight (i.e. at least 0.4% by weight in a magenta ink having a magenta pigment loading of 4% by weight). For durability of the printed image on the recording medium it is also advantageous for the binder polymer loading to be at least 1% by weight in the ink. Thus, for a magenta ink having a magenta pigment loading of 4% by weight, the solids content is preferably at least 5.4% by weight.
Each of the aqueous based pigmented inks represented in Table 2 includes the same amounts of glycerol, 1,2-hexanediol, 2-pyrrolidinone, and Surfynol. For each of the inks in Table 2, the binder polymer is a water-dispersible polyurethane having a molecular weight of 17,600. Molecular weight of the polymeric dispersant had a weight average of less than 15,000. The magenta pigment was the same for all of the magenta inks and the cyan pigment was the same for all of the cyan inks. Thus, although the solids loading is varied in the experiments listed in Table 2, the other ink components were held constant.
A set of experiments was also run to determine latency scores for dual feed thermal inkjet ejectors versus single feed thermal inkjet ejectors (each sized for a nominal drop volume of 3 picoliters) using a set of aqueous based pigment inks, including high solids content with significant loading of polymers having molecular weights of 20,000 and above, as well as a range of acid numbers. The results are listed below in Table 3. Each of the inks in the test included constant amounts of glycerol, 1-2 hexanediol, 1-(2-hydroxyethyl)-2-pyrrolidinone, and Surfynol. Each of the test inks also included magenta pigment at a loading of 5% by weight. The polymer loading was 2 percent by weight of a series of different molecular weight (MW) water-dispersible polyurethanes.
From the results listed in Table 3, it is evident that latency scores tend to decrease as molecular weight of the polyurethane binder polymer increases, and also as acid number increases. Comparing the latency ratings between a conventional single feed thermal inkjet ejector and a dual feed thermal inkjet ejector, the dual feed thermal inkjet ejector almost always has significantly better latency. However, comparing the last pair of entries in the table when both the molecular weight (88,000) and the acid number (120) are high, the latency is significantly affected even for a dual feed thermal inkjet ejector, so that there is only marginal improvement relative to a conventional single feed thermal inkjet ejector, particularly when the ejector is sized to eject drops as small as 3 picoliters.
It was noted above that a particular type of anti-curl agent that has been demonstrated to be very effective in preventing curl in printed documents, but also tends to cause the ink to have poor latency when using a conventional single feed thermal inkjet drop ejector is a branched, polyethylene glycol ether of at least 0.5 percent by weight. A set of experiments was run to determine latency scores for dual feed thermal inkjet ejectors versus single feed thermal inkjet ejectors (each sized for a nominal drop volume of 3 picoliters) using a set of aqueous based pigment inks including high solids content plus various amounts of Liponic EG-1. Liponic EG-1 is also called glycerth-26 and is an example of a branched polyethylene glycol ether. The results are listed below in Table 4. Each of the inks in the test included constant amounts of glycerol, 1-2 hexanediol, 1-(2-hydroxyethyl)-2-pyrrolidinone, and Surfynol. Each of the test inks also included magenta pigment at a loading of 5% by weight. The polymer loading included 2 percent by weight of a water-dispersible polyurethane having a molecular weight of 20,300.
From the results listed in Table 4, it is evident that latency scores decrease for the conventional single feed thermal inkjet ejector with increasing amounts of Liponic EG-1, but the latency score ratings are consistently outstanding when using a dual feed thermal inkjet ejector. Thus while the effect on latency performance of an ink containing such an anti-curl agent can cause the designers of an inkjet printing system to omit this ink component when using a printhead having conventional single feed thermal inkjet ejectors, the anti-curl agent can be included even in high solids content inks if a printhead having dual feed thermal inkjet ejectors is used. Thus, not only are prints provided more quickly, they also have a more pleasing appearance and flat shape.
Viscosities of the test inks in Tables 2 through 4 range between 2 and 7 centipoise. Inks having viscosities ranging from 2 to 10 centipoise can be jetted using dual feed thermal inkjet ejectors, although at viscosities above 5 cps a drop ejector sized for nominal drop volumes of greater than 3 picoliters can be more appropriate, especially for high solids content liquids. The viscosity ranges referred to herein refer to the viscosity of the ink or other liquid that is supplied to the liquid ejector. As water or other carrier fluid evaporates near the nozzle during extended wait times before firing, the local viscosity near the nozzle increases further, but that increased local viscosity is not what is referred to in the viscosity measurements or viscosity ranges herein.
It was noted above that dual feed thermal inkjet ejectors can also be used to eject liquids other than inkjet inks that are used in the printing of images. For example, in the field of functional printing, devices, circuitry or structures can be fabricated on a substrate (analogous to recording medium 24) by ejecting one or more liquids in patternwise fashion. Conductive polymers are one class of polymers that are becoming increasingly important and new ways of applying such polymers are correspondingly important. A particular conductive material of great interest is PEDOT, which stands for the polymerization of 3,4-ethylenedioxythiophene to Poly(EthyleneDiOxyThiophene). PEDOT is difficult to solubilize, so it is formed as a dispersion using poly(styrene sulfonate) or PSS as a carrier polymer. The PSS is typically very high molecular weight. In the case of the Heraeus Clevios™ materials PH1000 and FEK, the molecular weight of the PSS is at least 200,000. Furthermore it has an ionizable group on each monomer unit making it very water soluble, but also causing the viscosity to build rapidly at low solids content. Some information is copied below from the Heraeus website on their highly conductive Clevios™ materials:
“Generally speaking, polymers are insulators. However, there is a special class of polymers—the intrinsically conductive polymers—that have conductivity levels between those of semiconductors and metals (from 10−4 to 103 S/cm). The combination of metal and polymer properties opens up new opportunities in many applications, particularly in the electronics industry. With PEDOT (poly(3,4-ethylenedioxythiophene))—available under the trade name Clevios™—Heraeus has developed the latest generation of conductive polymers which are characterized by outstanding properties: high conductivity, high transparency, high stability, and easy processing. For high conductive coatings Clevios™ PH 1000 or its ready to use formulation, Clevios™ FE-T can be used. These materials offer not only high conductivities but also exceptional levels of transparency. A conductivity of 900-1000 S/cm (approx. 200 Ohm/sq) can be reached by using Clevios™ PH 1000 together with a conductivity enhancement agent such as DMSO or ethylene glycol. The ready to use formulation CLEVIOS™ FE-T is water-based and contains a polyester dispersion for force dry applications. Coating formulations have been optimized for individual substrates, such as A-PET, PET, polycarbonate, glass for different wet film thicknesses and surface resistivities. Coating can be achieved by standard printing processes, such as slit die, flexographic, screen or gravure methods. Also brushing, spraying, spin-coating or roller coating can be used.”
Clevios™ PH 1000 is a dispersion of the PEDOT with the PSS in a ratio of 1:2.5. In other words, in PH 1000 the high molecular weight PSS component is about 71% of the polymer loading. Clevios™ PH 1000 is supplied as a 1.3 wt % solids in water and is diluted as specified to make ink formulations typically below 1% solids. The high molecular weight and high degree of ionization of the PSS causes the viscosities to be high at relatively low solids content. FEK is a custom material similar to the FE-T material referred to on the Heraeus website. The specifications are proprietary.
A set of experiments was run to determine latency scores for dual feed thermal inkjet ejectors versus single feed thermal inkjet ejectors (each sized for a nominal drop volume of 3 picoliters) using a set of aqueous based test fluids including Clevios™ PH 1000 or Clevios™ FEK. Each of the test fluids included ethylene glycol and also included either Surfynol or Capstone FS-35 as a surfactant. The results are listed in Table 5 below.
From the results listed in Table 5, it is evident that jetting performance and latency scores vary widely for the conventional single feed thermal inkjet ejector depending primarily upon whether PH 1000 or FEK is the Clevios™ material being ejected. In fact, whether Capstone FS-35 or Surfynol was used as a surfactant, FEK was not jettable using the conventional single feed ejector, while for the dual feed thermal inkjet latency scores were very good or outstanding respectively. At a content of 0.5% PH 1000, latency scores were outstanding for both the single feed and the dual feed thermal inkjet ejectors. However, as the content of PH 1000 is increased to 0.75%, the latency score drops somewhat for the single feed thermal inkjet ejector. Thus, for better jetting performance and latency, the dual feed thermal inkjet ejector has a wider latitude for ejecting these conductive polymer materials and at higher concentrations. Although it can seem surprising that even the single feed ejector has excellent to outstanding latency scores for the PH 1000 test liquids, this illustrates that it is not necessarily the viscosity of the liquid provided to the ejector by the liquid supply that determines the latency, but rather how much the viscosity increases near the nozzle when water is lost by evaporation.
Inkjet Printing System with Dual Feed Thermal Inkjet Ejectors
Also shown in
The printhead 250 is mounted in the carriage 200, and a multi-chamber ink supply 262 and a single-chamber ink supply 264 are mounted in the printhead 250. The mounting orientation of the printhead 250 is rotated relative to the view in
A variety of rollers are used to advance the recording medium 24 through the printer as shown schematically in the side view of
The motor that powers the paper advance rollers is not shown in
Toward the left side of the printing mechanism 307, in the example of
Toward the rear of the printing mechanism 309, in this example, is located the electronics board 390, which includes cable connectors 392 for communicating via cables (not shown) to the printhead carriage 200 and from there to the printhead 250. Also on the electronics board 390 are typically mounted motor controllers for the carriage motor 380 and for the paper advance motor, a clock for measuring elapsed time, a processor and other control electronics (shown schematically as the controller 14 in
Printing with a printhead having dual feed thermal inkjet ejectors can be particularly advantageous in a wide format carriage printer (not shown). Desktop carriage printers, such as the example shown in
Having described typical inkjet printing systems with a printhead having dual feed thermal inkjet ejectors, a context has been provided for describing a method of printing an image using inks that tend to have latencies that typically require frequent maintenance ejection operations when using a printhead having conventional single feed thermal inkjet ejectors. The method of printing an image on a recording medium includes supplying a pigmented ink to an inkjet printhead having an array of dual feed thermal inkjet ejectors, where the pigmented ink includes an aqueous carrier with a pigment particle loading of at least 4 percent by weight and a polymer loading of at least 1 percent by weight; ejecting a plurality of maintenance drops of the pigmented ink from the array of dual feed thermal inkjet ejectors prior to a start of printing the image on the recording medium; printing the image swath by swath by ejecting printing drops of the pigmented ink on the recording medium as a carriage moves the printhead back and forth in a carriage scan direction across the recording medium between successive advances of the recording medium, such that a plurality of printing swaths are required in order to complete the printing of the image; and ejecting a plurality of maintenance drops of the pigmented ink from the array of dual feed thermal inkjet ejectors after a completion of printing the image on the recording medium, where no maintenance drops are ejected between the start and the completion of the printing of the image.
A plurality of printing swaths are specified above in the method of printing because for some images, such as a title page document, the entire document can be printed in a single swath without requiring maintenance drops being ejected between the start and the completion of the printing of the image even with conventional single feed thermal inkjet ejectors and latency-challenged inks because the printing time is so short. What the dual feed thermal inkjet ejectors can enable for such latency-challenged inks is a time interval of greater than or equal to 10 seconds, or even greater than or equal to 20 seconds between ejecting the plurality of maintenance drops prior to the start of printing the image and ejecting the plurality of maintenance drops after the completing of the image. In this way, even time consuming prints, such as large documents or high quality photographic images printed in multiple passes, can be printed without stopping to eject maintenance drops between swaths of printing.
In some instances, the latency time using a printhead with dual feed thermal inkjet and latency-challenged inks can be sufficiently long that it is not necessary to eject maintenance drops after the end of each sheet of recording medium. Instead, after discharging the first sheet of recording medium after the completion of printing a first image, a second sheet of recording medium can be advanced to a position near the inkjet printhead and a second image can be printed swath by swath on the second sheet as the carriage moves the printhead back and forth in the carriage scan direction across the second sheet between successive advances of the second sheet, such that ejection of maintenance drops is not done immediately following printing the first image on the second sheet, but rather occurs after the printing of the second image on the second sheet. In some cases, several sheets can be printed before it is required to move the printhead to the cap or spittoon to eject maintenance drops, thereby further improving printing throughput.
In addition to improving printing throughput there are other advantages to the improved latency performance using a printhead with dual feed thermal inkjet ejectors. Because fewer maintenance drops are required, there is less ink that is invested in maintenance and more that is available for printing, thereby making the printing system more cost efficient. Also, because there is less ink ejected into the cap or spittoon, there is less ink to accommodate in a waste pad. This is true of both the volatile components that are subsequently evaporated and the solids content that can accumulate and interfere with efficient dispersion of ink from subsequent maintenance operations.
The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.
Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. ______ (K001189), concurrently filed herewith, entitled “Ejector with Improved Jetting Latency for High Molecular Weight Polymers” by Thomas Brust, et al.; and co-pending U.S. patent application Ser. No. ______ (K001159), concurrently filed herewith, entitled “Ejector with Improved Jetting Latency for High Solids Content” by James Blease, et al., the disclosures of which are herein incorporated by reference.
