Abstract:
Methods and apparatus are provided for preventing clogging in impulse ink jet printers. It has been found in accordance with the invention that ink clogging during quiescent periods can be prevented by providing an ink that has the property of forming a barrier of higher viscosity ink where the ink contacts the ambient air. As a result, the viscous barrier shields the remaining ink from the effects of air exposure during the quiescent period. Thereafter, the barrier is removed by a series of sub-pulses that re-homogenize the viscosity barrier and clear the nozzle.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to ink jet printers and, more particularly, to methods and apparatus for preventing ink clogging in such devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there preferably is no contact between the printer and the substrate with ink jet printing.  
           [0003]    Most of the ink jet printers known in the art may be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir.  
           [0004]    With so-called “impulse” or “drop-on-demand” ink jet printers, image formation is controlled by selectively energizing and de-energizing, for example, a piezoelectric transducer or solenoid rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated by print signals to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate.  
           [0005]    Because ink is ejected from impulse-type printers only periodically, these devices present a number of problems which typically are not encountered in continuous ink jet systems. These problems, which occur during the relatively short intervals between individual print signals during a single print cycle, include irregularly shaped drops and/or improper spacing of drops. The root cause of these problems may be attributable to movement of the ink meniscus at the time a print signal is generated, particularly where efforts are made to print at a frequency in excess of 3 KHz. One approach to these problems is presented by U.S. Pat. No. 4,266,232, in the name of Juliana, Jr., et al., which discloses an impulse printer wherein ink drops of substantially uniform size and spacing are generated by applying drive pulses in a mutually synchronous fashion at every one of predetermined equal intervals. The amplitude of the drive pulses is controlled so that the amplitude of the drive pulse is below that of a print signal when no drop is to be formed. An even better approach is presented by U.S. Pat. No. 4,459,601, in the name of Howkins, wherein a fill-before-fire mode of operation is disclosed, i.e., a pulse of predetermined length is used to initiate filling of the jet chamber and firing of a droplet occurs on the trailing edge of the pulse.  
           [0006]    Certain other problems associated with impulse ink jet printers relate to the considerably longer intervals between print cycles. Unlike continuous ink jet printers, impulse devices typically are maintained in stand-by or quiescent modes for relatively long intervals, sometimes on the order of seconds, minutes, and even hours. During these intervals, ink is allowed to stand, thicken due to evaporation of ink components, and possibly clog the nozzles of the print head. Impulse printers may begin a printing cycle with such thickened material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to clog the nozzles during quiescent periods. Ink clogging is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink and any such interruption is of considerably shorter duration. Even where ink is allowed to stand and solidify in a continuous ink jet printer, it is more easily purged due to the considerably higher pressures at which these devices operate.  
           [0007]    A number of methods and apparatus are known in the art for preventing clogging in ink jet printers during quiescent periods. For example, U.S. Pat. No. 4,970,527, in the name of Gatten, discloses an ink jet printer which prevents clogging by printing a few ink dots when the printer is idle. The method of Gatten, however, wastes both ink and printing substrate.  
           [0008]    U.S. Pat. No. 3,925,789, in the name of Kashio, discloses an ink jet recording device which comprises a timer for determining the length of a quiescent period and a means for preliminarily ejecting ink from a nozzle if the quiescent period exceeds a predetermined amount of time. The ejected ink is not directed to a printing substrate but, rather, to an ink collector. U.S. Pat. No. 4,540,997, in the names of Biggs, et al., discloses an ink jet printer wherein clogging is minimized by transporting the nozzles during quiescent periods to communicate with a wash station and then ejecting ink from the nozzles into the wash station if the printer has not functioned for a predetermined period of time.  
           [0009]    U.S. Pat. No. 5,329,293, in the name of Liker, discloses an ink jet printer apparatus wherein clogging is minimized by pulsing the ink in the nozzle during quiescent periods. The pulsing signal provided is less than the size of a pulse signal that would cause ink to eject from the nozzle. This techniques is referred to as sub-pulsing. The sub-pulsing method and apparatus are effective and efficient in preventing ink from clogging the nozzle. However, with some extremely fast-drying inks, the sub-pulsing leads to constant evaporation of solvents from the ink. As a result, all of the ink within the nozzle may suffer an increase in viscosity during the sub-pulsing period. Eventually the viscosity my increase too much and adversely effect the operation of the printer.  
           [0010]    Therefore, there exists a need for relatively simple methods and apparatus for preventing ink jet clogging with faster drying inks which do not waste ink or printing substrate and which do not require additional devices such as ink collectors and washing stations.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides methods and apparatus for preventing clogging in impulse ink jet printers. It has been found in accordance with the invention that ink clogging during quiescent periods can be prevented by providing an ink that has the property of forming a barrier of higher viscosity ink where the ink contacts the ambient air. As a result, this viscosity barrier shields the remaining ink from the effects of air exposure during the quiescent period. Thereafter, the barrier is removed by a series of sub-pulses that re-homogenize the viscosity barrier and thereby clears the nozzle.  
           [0012]    In a preferred embodiment, ink jet printers according to the invention comprise at least one nozzle having an orifice for ejecting ink droplets in response to a sequence of control signals, said sequence comprising firing signals and sub-firing signals, a chamber for containing an ink in fluidic communication with the orifice so that the ink forms a barrier of high viscosity ink at the orifice whenever the nozzle is in a quiescent state, control means for generating the sequence of control signals and for controlling the amplitude of the control signals. The control means generates a plurality of sub-firing signals after a predetermined period of quiescence. The sub-firing signals have amplitudes which are effective to remove the barrier from the orifice yet which are ineffective to eject droplets of ink therefrom. The control means generates a plurality of firing signals after the generation of sub-firing signals for a second predetermined period of time. The firing signals have amplitudes which are effective to eject droplets of ink from the nozzle; whereas the sub-firing signals have amplitudes which are effective to remove the barrier from the orifice yet which are ineffective to eject droplets of ink therefrom.  
           [0013]    This printer is operated by allowing the ink within the nozzle to be exposed to the ambient air during a quiescent period of a predetermined time period such that a barrier of higher viscosity forms in the ink near the orifice. Thereafter, before using the printer, a plurality of sub-pulsing signals are generated which are effective to remove the barrier yet which are ineffective to eject droplets of ink. After the barrier has been removed, a plurality of firing signals can be generated on demand to eject droplets of ink from said nozzle.  
           [0014]    One representative ink exhibiting the desired fast-drying properties comprises a colorant, propylene glycol methyl ether, diacetone alcohol, and at least one resin selected from the group consisting of polyester resins and styrene acrylic resins. The propylene glycol methyl ether comprise about 44% by weight of the ink. The diacetone alcohol comprises about 40.6% by weight of the ink. The polyester resin comprises about 6.3% by weight of the ink. And, the styrene acrylic resin comprises about 1.7% by weight of the ink. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures, in which:  
         [0016]    [0016]FIG. 1 is a diagram showing an impulse ink jet printing apparatus according to the present invention.  
         [0017]    [0017]FIG. 2 is a diagram showing a firing signal applied to a print head nozzle and the movement of ink within the nozzle in response to the signal.  
         [0018]    [0018]FIG. 3 is a diagram showing a sub-firing signal applied to a print head nozzle and the movement of ink within the nozzle in response to the signal.  
         [0019]    [0019]FIG. 4 is a diagram of the barrier formed on the meniscus of the ink within the nozzle;  
         [0020]    [0020]FIG. 5 is a flowchart of the sequence of firing and sub-firing signals. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The methods and apparatus of the present invention can be used in conjunction with virtually any impulse or “drop-on-demand” ink jet printer which is subject to stand-by or quiescent periods. Referring to FIG. 1, a representative printing apparatus according to the present invention is shown comprising a print head  10  having a plurality of nozzles  12  and control means  16  electrically coupled with the print head.  
         [0022]    Any of the wide variety of print heads known in the art may be employed in the present invention, so long as it comprises at least one nozzle which ejects ink droplets in response to control signals. It is preferred that the print head be of the piezoelectric type, more preferably an MICROCODER 32/16 liquid ink jet imaging print head, which is commercially available from Trident, Inc. of Brookfield, Conn.  
         [0023]    The control means  16  may be any of those known in the art to be capable of generating control signals. As shown in FIG. 1, control means  16  preferably comprises a power source  16   a , a voltage or current regulator  16   b , a signal generator  16   c , and a timing circuit  16   d  for determining the interval between firing signals. It is preferred that a voltage regulator be employed and that the signal generator generate signals initiated under software control. Control means amenable to the practice of this invention include computing devices such microprocessors, microcontrollers, capacitors, switches, circuits, logic gates, or equivalent logic devices. Preferred control means  16  include a personal computer coupled to a Trident 16-Channel Analog Driver Board, part number 016-7008-01, which is commercially available from Trident, Inc. The preferred driver board generates a control signal in the form of an RC time constant controlled waveform with a 14.5μ second leading pulse followed by a 1.5μ second off time and a 3.5μ second trailing pulse. U.S. patent application Ser. No. 08/823,718, filed Mar. 25, 1997 and entitled “High Performance Impulse Ink Jet Method and Apparatus,” which is incorporated herein by reference, discloses firing waveforms for ejecting ink from an ink jet nozzle and includes the preferred firing pulse of the present invention.  
         [0024]    As shown in FIGS. 1 and 2A- 2 E, one or more ink droplets  14   c  can be ejected from the nozzles  12  toward substrate  20  by selectively energizing and de-energizing piezoelectric transducers  13 . In preferred embodiments, each transducer  13  is attached to a membrane, sealant, or some other flexible member  15   a  in physical contact with a volume of ink  14   a  contained within chamber  15 . The transducers are energized and de-energized through application of control signals. Although the control signal waveform could be selected from many known ink droplet firing signals, for brevity and simplicity of understanding, the firing control signal is shown in FIG. 2A in the form of a square wave.  
         [0025]    As discussed in the background section above, sub-pulsing techniques are known in the art whereby a signal of lesser amplitude is provided during quiescent periods to prevent the nozzle from clogging. FIGS.  3 A- 3 E illustrated how the ink within a nozzle may react to a sub-pulse signal. As illustrated in FIG. 3A, the sub-pulse signal is typically of smaller amplitude and shorter duration than a full drop-ejecting pulse. As such, the pulse is sufficient to move the ink within the nozzle without ejecting it therefrom. Such a technique has been used when a printer is in a quiescent state to prevent fast drying solvent based inks from drying out and clogging the nozzle. (See for example, U.S. Pat. No. 4,459,601, which is incorporated herein by reference, for a sub-pulsing scheme that applies pulses to the nozzle that are sufficient to move the ink within the nozzle and prevent clogging, but which are insufficient to eject ink droplets). The inventors believe, the sub-pulsing operates by constantly mixing the ink within the nozzle to maintain consistent viscosity.  
         [0026]    In certain printing applications, it is advantageous to have an ink which has an extremely fast drying time. For some of the more fast drying inks, the mere application of sub-pulses would eventually increase the viscosity of the ink within the nozzle to a thick, unusable state. The present invention recognizes that allowing a fast-drying ink to dry in the nozzle forms a barrier of higher viscosity suspended solids between the nozzle orifice and the ink contained in chamber  15 . The ink jet industry has generally tried to avoid such an effect because such a barrier would become a thick plug that would cause the nozzle to clog and operate inefficiently. Contradistinctly, the present invention utilizes this previously undesirable trait of fast-drying inks and uses it to a distinct advantage. In essence, by proper ink formulation, the barrier form in such a manner as to advantageous control the evaporation of solvents within the ink. The result is ink within the chamber that maintains a relatively constant viscosity. According to an aspect of the invention, an ink is formulated to have extremely fast-dying properties so that during the quiescent period a viscosity barrier rapidly forms at the orifice of the nozzle. Solvent and resin based inks, as described more fully below, have demonstrated the desired properties.  
         [0027]    Preferred ink compositions comprise a glycol ether having a low boiling point, i.e. below 150° C., preferably a glycol alkyl ether having about 3 to 20 carbon atoms, more preferably 3-7 carbon atoms, and most preferably 4 carbon atoms. The preferred glycol alkyl ether is propylene glycol methyl ether. Preferably, the glycol alkyl ether comprises about 20 to 60% by weight of the ink composition with about 44% by weight being most preferred. Preferred ink compositions further comprise a ketone alcohol having about 1 to 10 carbon atoms. One preferred alcohol is diacetone alcohol. This component preferably comprises 20 to 60% by weight of the ink more preferably 35 to 45%, and most preferably about 40.6% by weight.  
         [0028]    The preferred ink compositions of this invention further comprise at least one resin selected from polyester resins and acrylic resins, such as styrene acrylic resin. Preferably, the ink composition comprises about 1 to 20% of an alcohol soluble polyester, more preferably about 5 to 10%, and most preferably 6.3%. One preferred alcohol soluble polyester is Prince 5180, manufactured by Lawter International, Northbrook, Ill. The ink composition preferably comprises about 1 to 10% of a styrene acrylic polymer, more preferably 1 to 3%, with 1.7% being the most preferred. One preferred styrene acrylic polymer is Joncryl 678, available from S.C. Johnson &amp; Son, Inc.  
         [0029]    Preferred ink compositions also comprise a colorant. The choice of colorant and its concentration principally depend on the solubility of the colorant and the intensity of its color for a particular application. Preferably, the colorant is selected to render the ink composition visible to the human eye or some mechanical data collection device, such as a bar code scanner or other type of optical character reader. A preferred colorant comprises a dye such as Orasol Black RLI, which is available from Ciba-Geigy Co. of Ardsley, N.Y.  
         [0030]    One particularly preferred ink that demonstrates the desired fast-drying properties has been prepared as follows:  
         [0031]    Propylene Glycol Methyl Ether 44.0%  
         [0032]    Diacetone Alcohol 40.6%  
         [0033]    Prince 5180 6.3%  
         [0034]    Joncryl 678 1.7%  
         [0035]    Orasol Black RLI 7.4%  
         [0036]    The ink composition of this invention may further comprise one or more of the ink additives known in the art, so long as incorporation of the additives does not change the key drying properties as described in further detail below.  
         [0037]    In accordance with the invention, the ink composition is selected so that a viscosity barrier of suspended solids, and which may actually become a solid, is allowed to form over the orifice of the nozzle during quiescent periods that exceeds predetermined time period, which is selected based on the ink formulation and other factors. Thereafter, when printing is requested, sub-pulsing is activated before printing can resume to remove the viscosity barrier by re-homogenizing it with fresh ink. While not wishing to be bound by any particular theory, it is believed that the theoretical explanation for the operation of the viscosity barrier is as described in further detail below.  
         [0038]    Without sub-pulsing, a solvent concentration gradient is rapidly formed at the surface of the ink in the nozzle orifice as the volatile solvent evaporates (see FIG. 4A). Formation of this viscosity gradient or barrier  18  dramatically slows the rate of solvent loss from nozzle  12 . Unlike many inks, with a properly selected ink, such as the formulation described above, re-homogenization readily occurs under perturbing action of the sub-pulse. Thus, by first sub-pulsing a nozzle containing the fast-drying ink, a nozzle  12  that has been dormant for an hour or more can be successfully fired again after a few seconds of sub-pulsing.  
         [0039]    By contrast, if a constant sub-pulsing system is applied to such a fast drying ink, such as the preferred ink described above, the ink viscosity within nozzle  12  will rise too high and produce poor print performance. In such a system, it is believed that the sub-pulsing generates eddy currents which agitate the ink in nozzle  12 , maintaining it homogeneity. As a result, fresh solvent is continuously being presented at the nozzle orifice and no viscosity gradient is formed. Over extended periods of sub-pulsing, the solvent level in nozzle  12  becomes extremely depleted causing the viscosity of the ink to rise significantly, especially at the edges of nozzle  12  where the sub-pulsing has the least effect (see FIG. 4B).  
         [0040]    The preferred sequence of steps to employ the viscosity barrier and the sub-pulsing are shown in the flow chart of FIG. 5. The startup sequence begins whenever a nozzle has been idle for an extended period of time (step  20 ). After startup, nozzle  12  is sub-pulsed for a predetermined period. Preferably, the sub-pulses have a pulse width of about 1.5μ seconds and have a frequency of about 5 kHz (step  22 ). The sub-pulse amplitude is selected to move the ink yet insufficient to eject the ink from nozzle  12 . The particular sub-pulse parameters were selected to operate effectively with the preferred ink. Other parameters may be substituted and produce similar results, particularly where the ink formulation is different from the preferred ink disclosed herein. The predetermined period of sub-pulsing time is the time required to ensure that the ink in the nozzle is re-homogenized with fresh ink from the chamber. It has been determined that 5 seconds is sufficient time to break-down the viscosity barrier that is formed by the preferred ink formulation described above. Of course, other re-homogenization periods could be used depending on the particular characteristics of the ink selected and the quality of operation desired. After the 5 second period (step  24 ), the print function of the nozzle is enabled (step  26 ). The nozzle is then available to eject droplets on demand until printing is complete and returns to a quiescent state (step  28 ).  
         [0041]    While the printer is active and for a period thereafter, sub-pulsing of the ink can continue to maintain a constant viscosity of the ink as with constant sub-pulsing system. That is, just as with constant sub-pulsing systems, the system disclosed herein contemplates that the printer can remain enabled with constant sub-pulsing on for some predetermined period of time without clogging the nozzle or raising the ink viscosity to an unusable level. However, if the printer remains idle for an extended period (about 15 minutes in the present example), the sub-pulsing should cease, allowing the viscosity barrier  18  to form over the orifice (see also FIG. 4A). Accordingly, a timer is set for about 15 minutes during which time printing can restart on demand (steps  30 ,  32 ). After that time has expired, sub-pulsing is stopped and restarting requires completion of the start-up sequence to re-homogenize the viscosity barrier (steps  34 ,  36 ).  
         [0042]    Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. For example, instead of generating control signals as in the present invention by modulating the amplitude of applied electric energy, it may be possible generate such signals by modulating applied light energy or heat. Moreover, other fast drying inks using completely different formulations could be used, so long as they exhibit the desired fast-drying properties. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.