Patent Publication Number: US-6655789-B2

Title: Ink jet print head

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division application of U.S. patent application Ser. No. 09/736,466, filed on Dec. 14, 2000, and issued on May 21, 2002 as U.S. Pat. No. 6,390,607, which is a continuing application of U.S. patent application Ser. No. 09/211,681, filed on Dec. 14, 1998 and issued on Sep. 18, 2001 as U.S. Pat. No. 6,290,338, which is a continuing application of U.S. patent application Ser. No. 08/549,053, filed on Oct. 27, 1995 and issued on Mar. 9, 1999 as U.S. Pat. No. 5,880,761 (reissued Ser. No. 09/548,651, filed on Apr. 13, 2000 and issued on Mar. 9, 1999 as U.S. Pat. RE37453. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an ink jet print head for discharging ink drops from ink outlets by use of thermal energy. 
     DESCRIPTION OF THE RELATED ART 
     Recently, in contrast with the wire dot printing methods, non-impact recording method is attracting interest because the recording noise level is negligible. In particular, an ink jet recording method is attractive as it permits high-speed recording on ordinary paper without the need of a deposition treatment on the paper side. In the field, therefore, aiming at an optimal ink discharge performance, various approaches have been made, with associated implementations. 
     In the ink jet recording method, a recording is effected with discharged droplets of recording liquid, called “ink” deposited on a recordable material. This method is categorized into several systems according to the manner in which the drops of recording liquid are formed. 
     FIG. 1 illustrates a bubble jet recording system as a conventional example. The conventional system includes a substrate  32  provided with a heating resistor  30 , a channel plate member  36  for defining an ink supply path  34 , and an orifice plate  40  formed with an orifice as an ink outlet  38  communicating with the ink supply path  34 . The heating resistor  30  rapidly heats to vaporize a volume of ink supplied on a heating zone surrounding the resistor  30 , causing ink bubbles  42  to grow, exerting pressures therearound so that an ink drop is discharged from the ink outlet  38 , with trailing droplets  50 ,  52  as shown in FIG.  2 . 
     Grown bubbles  42  become deflated as they are cooled by surrounding ink, and fade out with ink vapour therein condensed to be liquidated. 
     A consumed volume of ink by the discharge is supplemented from an ink pool through the ink supply path  34 , due to capillary forces acting on an ink meniscus  44  retreating inside the ink outlet  38 . 
     To permit a high-speed recording, it is desirable to repeat a discharge of an ink drop in a short period, supplementing at a high speed a volume of ink during every discharge through the ink outlet  38 . 
     In a conventional implementation, the diameter of the ink outlet  38  is reduced to have an increased capillary force, and the channel resistance of the ink supply path  34  is reduced. 
     Thus, ink is supplemented at an increased speed, and with an increased momentum, which causes, as shown in FIG. 1, an elongated ink pillar  46  to project from the ink outlet  38 , before it deforms into an ink drop. In the deformation, the elongated ink pillar  46  is broken so that a leading upper portion is changed into a main drop  48  and a trailing lower portion is separated into a number of relatively large low-speed satellites  50 ,  52  such as in FIG.  2 . Such satellites adversely affect the printing. 
     Moreover, as a volume of ink is supplemented with an increased momentum, as shown in FIG. 3, an ink meniscus  44  at a top end of the ink outlet  38  has an increased tendency to convex outside and concave inside of the outlet  38 . The meniscus  44  thus vibrates with a reduced damping ratio. That is, the vibration of the meniscus  44  is not readily stopped. 
     As the ink discharge is repeated in a short period, a subsequent discharge occurs immediately after the supplement of ink, so that it may occur when the ink meniscus  44  starts convexing above the ink outlet. This causes an undesirable deformation of an ink drop and an undesirable development of low-speed satellites, resulting in a reduced quality of recording. 
     Further, some volume of ink may flood over a surface area around the ink outlet  38 , causing an ink drop to be discharged in an oblique direction, or bubbles to be involved, stopping the discharge, with a reduced reliability of recording. 
     A probable solution to such problems may include entering subsequent discharge after a sufficient damping of vibration, which however is inconsistent with an intended high-speed recording. 
     The present invention has been achieved with such points in mind. 
     SUMMARY OF THE INVENTION 
     It therefore is an object of the present invention to provide an ink jet print head with criteria such as on an sectional area of an ink outlet and a fluid resistance of an ink supply path to achieve an optimal high-speed ink discharge with an increased reliability and an improved cost effect, without additional elements. 
     In the present invention, an ink outlet is tapered, with a gradually reduced diameter, toward an orifice plate surface. Supposing a straight aperture of a diameter, it is typical that the quantity Q of an ink drop discharged from a print head of an identical resolution is substantially identical, as well as the volume of a void defined by an ink outlet and an ink meniscus drawn back therein just after a discharge of an ink drop, i.e., the quantity Q, of ink to be supplemented. 
     Letting t r  be a time for the drawn back ink meniscus to restore to an exit level of the ink outlet, and v be a mean flow velocity in the ink outlet, 
     v=Q r /(A·t r ). 
     Letting ρ be an ink density, and M be a mean momentum per unit volume, 
     M=ρQ r   2 /(A·t r ) 
     Thus, the larger the diameter of the ink outlet is, the smaller the mean momentum becomes, with a reduced frequency of occurrence of an overshooting ink meniscus. 
     As the overshooting meniscus convexes like a paraboloid of revolution, letting Q o  be an overshooting volume of ink and h be an overshooting height or projection of ink, 
     h=2·Q o /A. 
     Thus, the larger the diameter of the ink outlet is, the smaller the overshooting volume of ink becomes. 
     For a quantity of ink supplemented in a time, the larger the diameter of the ink outlet is, the overshooting ink might have the smaller projection h. However, experiments showed that the projection h of an ink overshoot depends on a sectional configuration of the ink supply path, i.e., a channel resistance or flow resistance thereof. 
     This fact means that an optimized relationship between an ink outlet sectional area and a channel resistance permits a high-speed recording without low-speed satellites. 
     The inventors found that a subsequent discharge of ink immediately after a concaved meniscus of the ink has restored to an exit level of an ink outlet can be free from an undesirable deformation of a drop of the ink, when an overshooting height or projection h of the ink falls within a range such that: 
     
       
         0&lt; h&lt; 0.3 ( q/A ),  
       
     
     where q is a quantity in volume of the ink drop, and A is a sectional area at the exit level of the ink outlet. 
     The present invention is based on this fact. 
     Thus, to achieve the object, a genus of the present invention provides an ink jet print head comprising a substrate member formed with a heating resistor, an ink path defining member provided on the substrate member, for defining an ink supply path including a heating zone in a vicinity of the heating resistor, and an orifice plate member formed with an ink outlet communicating with the ink supply path and laminated on the substrate member, with the ink path defining member interposed therebetween, the ink jet print head generating heat from the heating resistor to discharge a drop of ink from the ink outlet, the ink supply path having a fluid resistance so that a relationship is established such that 0&lt;h&lt;0.3 (q/A), where q is a quantity of the drop of the ink, A is a sectional area at an exit level of the ink outlet, and h is a maximumal projection that a meniscus of the ink has when it projects from the ink outlet after it has restored the exit level from a retreat position it had after the drop of the ink had been discharged. 
     According to a species of the genus of the invention, the relationship is established such that: 
     
       
         π{(3 q )/(4π)} 2/3   ≦A≦π {(3 q )/(2π)} 2/3 .  
       
     
     This is because of the following reason. 
     An undesirable overshooting height becomes smaller as the ink outlet has an increased diameter. The ink outlet diameter may preferably be increased. 
     If the ink outlet has a small diameter, a volume of ink extruded to be discharged therefrom constitutes an elongated ink pillar, which has a reduced tendency to be deformed to constitute an ink drop due to surface tensile forces of the ink so that it is ruptured into droplets, thus causing satellite drops to degrade a print quality. 
     To avoid such a rupture, letter d be a diameter of an ink outlet and D be a diameter of an unexpected ink drop, it is preferable that: 
     
       
         d≧D  (a).  
       
     
     Letting q be a volume of the ink drop, 
     
       
           q= (4π/3)( D/ 2) 3    
       
     
     Thus, letting A be a transverse sectional area of the ink outlet, it so follows that: 
     
       
           A=π ( D/ 2) 2   (c).  
       
     
     From the expressions (a), (b) and (c), 
     
       
           A≧π {(3 q )/(4π))} 2/3   (d).  
       
     
     On the other hand, if the ink outlet diameter is excessively large, a discharged ink drop has a reduced velocity with a reduced momentum susceptible to disturbances, causing an ink flying direction to be deviated or an air bubble to be involved. 
     To ensure a normal ink flying direction, it is preferable for the ink outlet to function as a nozzle for extruding a volume of ink in a direction normal to an orifice plate so that a lateral side of the extruded ink is perpendicular to a top surface of the orifice plate, which means the extruded ink has a volume q equivalent to or larger than a volume of a hemisphere having the same diameter d as the ink outlet. 
     It thus so follows that: 
     
       
           q≧ (4π/3)( d/ 2) 3 ×½  (e).  
       
     
     From the expressions (c) and (e), 
     
       
           A≦π {(3 q )/(2π))} 2/3   (f)  
       
     
     Thus, from the expressions (d) and (f), 
     
       
         π{(3 q )/(4π))} 2/3   ≦A≦π {(3 q )/(2π))} 2/3   (g).  
       
     
     The ink outlet may have an arbitrary sectional form other than a circle, e.g. it may have a polygonal section. The expression (g) is applicable also to such an arbitrary form, as it has a mean sectional area when assumed as a circle equivalent in area. 
     According to another species of the genus of the invention, another relationship is established such that 0.9×t 1 &lt;t min &lt;1.1×t 1 , where t 1  is a time for the meniscus of the ink to restore to the exit level from the retreat position, and t min  is a minimal period by which the ink jet print head discharges the drop of the ink. 
     According to the present invention, after an ink drop is discharged from an ink outlet by grown bubbles, the projection of a meniscus at ink refill is kept small by optimizing the channel resistance value of an ink supply path. Therefore, the periodic damping time of the meniscus becomes short, and the unfavorable effect to be caused at the subsequent discharge is avoided. 
     Moreover, in the case in which the sectional area of the ink outlet is set large within the predetermined range, the projection of the meniscus at ink refill becomes small, and the damping time of the vibration of the meniscus becomes short. 
     Further, in a constitution in which the relation between the restoring time of the meniscus and the minimum driving period of a print head is set within the predetermined range, no dead time exists before the subsequent discharge without an undesirable deformation of the discharged ink drop, and the discharge interval becomes short. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, features and advantages of the present invention will become more apparent from consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross section of a prior art discharge device showing the state before a discharge of an ink drop; 
     FIG. 2 is a cross section of the prior art device of FIG. 1 showing the state after a discharge of an ink drop; 
     FIG. 3 is a cross section of the prior art device of FIG. 1 showing the state after a discharge of an ink drop; 
     FIG. 4 is a perspective view of an ink jet print head according to an embodiment of the present invention; 
     FIG. 5 is a cross section view showing an embodiment of the present invention; 
     FIG. 6 is a plan view of the device of FIG. 4 in which the orifice plate is removed; 
     FIG. 7 is a cross section of discharge device according to the present invention showing the state before a discharge of an ink drop; 
     FIG. 8 is a cross section of the device of FIG. 7 showing the state after a discharge of an ink drop; 
     FIG. 9 is a cross section of the device of FIG. 7 showing the maximal projection of an ink meniscus; 
     FIG. 10 is a graph showing a damping state of an ink meniscus; and 
     FIG. 11 is a table describing the difference between the embodiment shown in FIG.  5  and the conventional example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Detailed below is a preferred embodiment of the present invention, with reference to FIGS. 4 to  11 . Like members are designated by like reference characters. 
     Referring now to FIG. 4, an ink jet print head  1  according to an embodiment of the invention comprises a substrate  6  formed with a plurality of heating resistors (hereafter collectively “heating resistor”)  4  and a central recess or groove  6   a  used as an ink pool or trunk path an ink path defining resin sheet member  10  defines a plurality of channels as branched ink supply paths (hereafter collectively “ink supply path”) each including a heating zone. A orifice plate  12  is formed with a plurality of orifices or nozzles as ink outlets  14  (hereafter collectively “ink outlet”). 
     FIG. 5 shows a cross sectional view of a nozzle of an ink jet print head (multi-nozzle) identical to the print head  1 . FIG. 6 shows a plan view of the ink jet print head with an orifice plate removed. 
     An ink jet print head  2  is provided with a substrate  6 , a heating resistor  4 , an ink path defining member  10  defining an ink supply path  8 , and an orifice plate  12  laminated on the substrate  6  with the ink path defining member  10  interposed therebetween. An ink outlet  14  is formed on the orifice plate  12  at the position opposing the heating resistor  4 . Further, the ink supply path  8  includes a heating zone  16  in a vicinity of the heating resistor  4 , surrounding te resistor  4 . Although not shown, the heating resistor  4  is connected to a power supply electrode and to a common electrode so that it can be heated by exterior driving pulses. 
     Next, a discharge operation of the ink jet print head  2  will be described with reference to FIGS. 7 to  9 . 
     In the embodiment, the channel resistance of the ink supply path  8  is set so that a relationship is established between a quantity q of a discharged ink drop, a sectional area A of the ink outlet, and a maximal projection h that a meniscus of the ink has when it projects from the ink outlet  14  after it has restored to the exit level from a retreat position it had after the drop of the ink had been discharged (FIG.  9 ), such that 0&lt;h&lt;0.3 q/A, more specifically, h=0.2 q/A. 
     Further, between the quantity q of an ink drop and the sectional area A of the ink outlet  14  a setting is made such that: 
     
       
         π{(3 q )/(4π)} 2/3   ≦A≦π {(3 q )/(2π)} 2/3 .  
       
     
     When a driving pulse is applied between the individual electrode and the common electrode to heat the heating resistor  4 , the ink above the heating resistor  4  is rapidly heated and boiled, and as a result, a bubble  18  (as a collective term of bubbles) is developed from vapours of ink components as shown in FIG.  7 . The bubble  18  extrudes the ink above it out from the ink outlet  14 , thereby forming an ink pillar  20 . 
     The condition “π{(3q)/(4π)}⅔≦A” means that the diameter of the ink outlet  14  is larger than that of an ink drop which provides an intended ink drop quantity. Therefore, the ink pillar  20  will not elongate, but is formed into a combination of an ink drop  22  and negligible droplets, as shown in FIG.  8 . Accordingly, an excellent printing quality without dispersion or scattering is obtained. 
     If the diameter of the ink outlet  14  is unnecessarily large, the flow velocity of the ink at a discharge of an ink drop becomes slow. Therefore, the velocity or the momentum of an ink drop becomes slow or small, with increased influences of disturbances. Further, as the nozzle action for the discharge of ink drops from the ink outlet  14  becomes less effective, the discharge direction of an ink drop becomes irregular, which causes an irregular deposition of ink drops on the target paper. As a result, a deterioration is caused in the printing quality. 
     However, since the present embodiment operates under the condition “A≦π{(3q)/(2π)} 2/3 ”, that is, the diameter of the ink outlet  14  is smaller than that of a hemisphere which provides an expected ink drop quantity, the side surface of the ink pillar  20  becomes right angled to the surface of the orifice plate  12 . Consequently, the ink outlet  14  functions as a nozzle free of irregularities in the spattering direction of an ink drop. 
     After formation of the ink pillar  20 , the internal pressure and temperature begin to fall due to the cooling effect of the adiabatic expansion and surrounding ink, and the bubble  18  starts to contract, as shown in FIG.  8 . As described above, the ink pillar  20  is changed into an ink drop  22  to be discharged toward the recording medium, while the ink meniscus  24  is drawn inside the ink outlet  14 . 
     Then, the ink meniscus  24  recovers, heading toward the exit level of the ink outlet  14 , driven by a capillary force, which is a resultant force of the surface tension. Due to the inertial force of the ink, the ink meniscus  24  reaches the exit level of the ink outlet  14 , and although the capillary force is gone, the meniscus  24  does not stop instantly but projects out from the exit level of the ink outlet  14 , as shown in FIG.  9 . 
     However, since the channel resistance value is set as to meet the condition “h=0.2 q/A” in the present embodiment, although the ink is supplemented at high-speed, the projection h is considerably smaller as compared to the prior art. Therefore, even though a sequential discharge is executed immediately after the arrival of the ink meniscus  24  to the exit level, there is neither an undesirable deformation of an ink drop nor development of low-speed satellites. Moreover, since an ink overflow hardly occurs, there is no deterioration in the printing quality caused by the irregularity in the discharge direction, and also, no discharge error is caused by a bubble. 
     Further, since the projection h of the ink meniscus  24  is considerably smaller as compared to the prior art, the periodic damping time of the meniscus  24  becomes extremely short as shown in FIG. 10, and thereby the unfavorable effect of the vibration to be caused at the subsequent discharge is avoided. In the figure, t 1  represents the time for the ink meniscus  24  to reach the exit level of the ink outlet  14 . 
     Therefore, by driving the ink jet print head  2  under the condition of 0.9×t 1 &lt;t min   &lt;1.1×t   1 , in which t 1  represents a time for the ink meniscus  24  to reach the exit level of the ink outlet  14  and t min  represents a minimum operation period of the ink jet print head  2 , the discharge interval is minimized with the excellent discharge performance kept, and as a result, a high-speed printing is achieved. 
     With the constitution described above, the measurement data of the present embodiment is compared with that of the conventional case as shown in FIG.  11 . 
     As it is clear from FIG. 11, according to the ink jet print head of the present embodiment, a printing without dispersion is permitted in high-speed, which is almost twice the speed of the conventional print head. 
     While each associated member is illustrated in a particular shape in the embodiment, the present invention is not to be restricted by them, for example, the shape of the ink outlet  14  can be defined to a polygon or other. As long as the above-mentioned condition is satisfied, the elements are permitted to be properly selected and exchanged. 
     According to the present invention, there is provided an excellent ink jet print head which enables a non-dispersed high-speed printing without additional members or devices. 
     While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by this embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.