Thermal ink-jet head and recording apparatus

Abstract: A channel wafer has a plurality of nozzle flow channels (5,5a,5b,5c) and a common ink reservoir (7). Pits (2,2a,2b,2c) communicating with the ink reservoir from above heating elements (1,1a,1b,1c) are formed in a polyimide layer (9) on a heater wafer (8). Each of the pits has a throttled portion in the rear of the heating element and the terminal of a nozzle flow channel is situated on the throttled portion so as to form the minimum sectional area portion of the flow channel. Stable ink discharge characteristics are attained by means of the flow channel resistance of the minimum sectional area portion and bubble pressure is prevented from being relieved toward the ink reservoir. The pressure propagated to the ink reservoir is made to attenuate internally, so that no crosstalk is produced.

Description: BACKGROUND OF THE INVENTION The present invention relates to a thermal ink-jet head according to the preamble of claim 1. From document EP 0 594 369 A1 two designs of a thermal ink-jet head are known. In a first design an ink reservoir is directly connected to a plurality of ink supply ports. The ink flow has to overcome an obstacle within the rear flow channel before it is supplied to the bubble-producing pit above a heated resistor pad. In the second design the flow channel shows no obstacle, so that the ink reservoir is directly connected to the pit, producing the air bubbles. A small partitioning wall is separating the ink reservoir from an elongated ink flow channel which provides in its front section the bubble-producing heater. From document JP-A-7101059 a thermal ink-jet head is known, having a plurality of heater elements which are formed on a heater wafer 10. These wafer elements are positioned in a pit of a heater substrate and additionally to said heater pit the heater substrate is showing a by-pass pit separated from said heater pit. Compared to the prior art higher operating speed and higher image quality have been sought for in recording apparatus. Ink-jet recording apparatus for recording data by discharging ink from nozzles are no exception. In order to meet such requirements, technology applicable to thermal ink-jet heads includes raising printing speed by increasing repetitive response capability and making ink drops respond to frequency with stability to ensure that they reach the surface of recording paper. In case ink drops are jetted unstably, the time required for the ink drops to reach the surface of recording paper and the direction in which they streak tend to vary widely. Consequently, characters are not recorded in position and this results in lowering image quality. Further, the technology of improving image quality includes increasing density and integration; more specifically, technology necessary for contributing to increasing image quality from now on is to arrange nozzles at a pitch corresponding to a dot density and decreasing the nozzle pitch. However, the problem is how to deal with nozzle-to-nozzle crosstalk. When ink is jetted under pressure, nozzles will be badly affected if the space between pressure sources is set smaller because the ink becomes jetted unstably. The pressure applied to adjoining nozzles is naturally propagated through a common flow channel behind the nozzles. In order to suppress such crosstalk in the first place, bubble pressure for use in jetting ink is propagated to the nozzle efficiently so as to reduce the pressure propagation through the common flow channel. Therefore, an ink flow channel is preferably structured so that the backward pressure relief from a pressure source is minimized with respect to the nozzle. Unexamined Patent Publication No. 226978/1994, for example, discloses an attempt made to increase energy directed to the nozzle side by placing a conductance regulating wall in an ink cavity. However, the provision of such a conductance regulating wall simultaneously interferes with a refill of ink due to an increase in flow channel resistance and incurs the lowering of response frequency, which results in making printing unstable. A second method of suppressing such crosstalk is to ease the pressure propagated backward from the nozzle by contriving a proper flow channel structure. Unexamined Patent Publications No. 210872/1994 and No. 191030/1994, for example, discloses a configuration including a buffer chamber with a gas enclosed therein so as to control impedance. With this configuration, however, no desired effect is attainable because it tends to become complicated in structure and because there arises a new instability factor originating from handling gas. As proposed in Patent Application No. 307221/1994, further, a communicating flow channel is provided between a nozzle flow channel and an ink reservoir together with the provision of grooves extending from a heating element up to the communicating flow channel, and connecting the ink reservoir and the communicating flow channel so as to secure response capability by promoting a refill of ink and to catch dust creeping into the flow channel. This construction has made attainable a high frequency response capability and the function of trapping dust but failed to obviate crosstalk because pressure is propagated via the communicating flow channel situated in the rear of a heating element. With the progress of manufacturing technology in the meantime, dust has completely been prevented from creeping in the flow channel, whereby the necessity for installing filters in chip flow channels has been saved. It may be taken into consideration to lower ink flow channel resistance as one method of promoting a refill of ink. However, printing defects will conversely be produced if the flow channel resistance is extremely lowered. Fig. 16 is a diagram illustrating a dropout defect and Fig. 17 is an enlarged view of a dropout defect portion. As shown in Fig. 16, a dropout defect in the form of a white line may occur in the vicinity of the head of a solidly printed portion when solid printing is carried out at a high frequency. The observation of the printing condition has revealed that, as shown in Fig. 17, the appearance of such a white line is not caused by the dropout of dots due to non-jet and the like but by the shifting of dot positions. The white line is detected when several dots are shifted in the beginning of printing before dots are stably printed; this is the mechanism of allowing the white line to occur when high-frequency printing is carried out. Therefore, this image quality defect can be avoided by stabilizing the ink flow in the beginning of printing and avoiding such a. defect is accomplishable by putting fluid vibration quickly to the static condition after ink is jetted. In other words, it is possible to decrease the image quality defect by providing sufficient flow channel resistance to suppress the ink vibration. Thus a refill of ink and the ink flow need to be stabilized by controlling the flow channel resistance suitably. Further, the flow channel resistance is usable for suppressing the backward pressure propagation as stated above and the flow channel structure should be determined by taking the foregoing problems into consideration. Moreover, the flow channel structure thus determined should be formed uniformly. As described in Unexamined Patent Publication No. 238904/1994, for example, a method of forming a flow channel through a multi-stage process has been proposed as that of uniformly forming such a flow channel. Nevertheless, this method incurs a cost hike as the process of manufacture is complicated; the process of manufacture is desired to be simple. As disclosed in Unexamined Patent Publications No. 155030/1993, No. 183002/1994 and No. 270404/1994, further, attempts have been made to improve performance by providing a plurality of kinds of grooves or recesses in a thick-film synthetic resin layer between a channel substrate and a heating substrate, the recesses having different functions and objects. However, strict precision is required to form such grooves and recesses so as to satisfy reliability of them and this also incurs an increase in cost. SUMMARY OF THE INVENTION An object of the present invention made in view of the foregoing problems is to provide a thermal ink-jet head so structured and adapted as to improve frequency resonse capability without causing crosstalk and an increase in manufacturing costs while keeping a chip small in size, and a recording apparatus in that the sectional area of the ink reservoir starting with from the ink supply port toward the nozzle flow channel may be decreased. This object is achieved by the subject matter of claim 1. Preferred embodiments are subject of dependent claims 2 to 5 and an apparatus using such a thermal ink-jet head as claimed with dependent claim 6. According to a preferred embodiment of the invention, the ink reservoir may be provided for use common to the plurality of nozzle flow channels. According to another preferred embodiment of the invention, the sectional area of the groove between the upper portion of the bubble-producing resistor and the ink reservoir may be reduced in the direction in which the nozzle is orientated. At this time, the nozzle flow channel may have a tilted side extended in a direction perpendicular to the direction in which the nozzle flow channel is orientated and the direction in which the nozzle flow channel is extended, and the ink-reservoir-side terminal of the tilted side may be situated above the portion of the groove where its sectional area is reduced. According to a further embodiment of the invention, a thermal ink-jet head comprises a heater substrate having bubble-producing resistors and a channel substrate having a plurality of nozzle flow channels, an ink reservoir and a plurality of ink supply ports and is characterized in that the channel substrate is formed with at least the plurality of nozzle flow channels each passed on the bubble-generating resistors and formed up to almost nearly the end portions of the bubble-producing resistors, the ink supply ports and the ink reservoir for use common to the plurality of nozzle flow channels, the sectional area of the ink reservoir communicating with the ink supply ports being increased from the ink supply port toward the nozzle flow channel; a synthetic resin layer is provided on the heater substrate; and a groove is provided in the heater substrate, the groove being long enough to cover the distance from the upper portion of the bubble-producing resistor up to a place where the groove is coupled to the ink reservoir formed in the channel substrate, the sectional area of the groove being reduced in the direction in which the nozzle flow channel is orientated within the distance from the upper portion of the bubble-producing resistor up to the ink reservoir. According to still an other embodiment of the invention, a recording apparatus uses the thermal ink-jet head as in any one of the preceding aspects 1 to 5. According to the present invention, the nozzle flow channel formed in the channel substrate is passed on the bubble-generating resistor and formed up to almost nearly the end portion of the bubble-producing resistor; the groove is at least provided in the heater substrate, the groove being long enough to cover the distance from the upper portion of the bubble-producing resistor up to a place where the groove is coupled to the ink reservoir. Further, the sectional area of the ink flow channel formed with a partition wall between the nozzle flow channel and the ink reservoir formed in the channel substrate and with the groove provided in the heater substrate is minimized. Therefore, the bubble pressure produced on the bubble-producing resistor acts favorably on the nozzle side since the sectional area of the flow channel in the rear of the bubble-producing resistor is minimized, whereby the backward propagation of the pressure can be reduced. As the bubble pressure is efficiently utilized for the discharge of ink drops, sufficient ink-jetting force is secured and the operation is stabilized. Thus improvement in the drive frequency and image quality is accomplishable. Although it is feared that a refill of ink is impeded in the portion where the sectional area is minimized, the ink is only caused to linearly move from between the groove and the ink reservoir after the bubble dies out as the groove on the bubble-producing resistor is extended up to the ink reservoir. Consequently, a refill of ink is conducted on the bubble-producing resistor and the ink is resupplied speedily and satisfactorily to ensure a high frequency response capability. Thus the bubble pressure is efficiently used to discharge the ink without impeding a refill of ink by placing the least sectional area portion of the flow channel in the rear of the bubble-producing resistor to provide proper flow channel resistance. Since the ink reservoir side has a sufficient impedance component, not only the attraction of air from the nozzle due to the backward pressure propagation caused after the jetting of ink but also the disturbance based on the correlation between the rear component of the pressure at the time of high-frequency printing and the bubble-producing pressure is quickly suppressible. Moreover, image quality is made improvable by precisely controlling the dot position as the discharge of ink is stabilized. Further, even the pressure propagated via the groove to the ink reservoir is diffused and absorbed into the ink reservoir, whereby crosstalk is reducible. Notwithstanding the provision of the ink reservoir for use common to the plurality of nozzle flow channels, pressure to be propagated to another nozzle is extremely low, so that the influence of crosstalk is obviated. The aforementioned arrangements are materializable through the conventional process of manufacture only by altering the mask pattern. Therefore, the effects stated above is achievable without any change in cost. As the length of the flow channel is reducible, the number of heads to be laid out per wafer can be increased, which will result in cost reduction. Although a plurality of holes have heretofore been provided in a flow channel corresponding to one nozzle in a heater substrate, only one hole is needed according to the present invention. As a result, not so greater hole-to-hole precision than before is required and this production easier. With the arrangement of providing a synthetic resin layer for a hole the thin synthetic resin layer tends to constitute a factor of trouble such as the jutting-out of ink. However, it is intended to minimize an unstable manufacturing factor to decrease the number of holes according to the present invention and this is also led to improving reliability. With the arrangement of decreasing the sectional area starting with the ink supply port toward the nozzle flow channel in reference to the structure of the ink reservoir the diffusion and absorption of the pressure propagated to the ink reservoir are promoted. When the thermal ink-jet head is installed in a recording apparatus, moreover, an ink supply means for supplying ink from an ink tank to an ink supply port is joined to the head. This construction makes it possible to not solely increase the joint area above but also form an airtight ink flow channel satisfactorily. The sectional area of the groove provided in the heater substrate and extended between the upper portion of the bubble-producing resistor and the ink reservoir may be reduced in the direction in which the nozzle is orientated. Therefore, the shape of the bubble produced on the bubble-producing resistor is regulated in the reduced portion of the groove while the bubble is growing and the bubble pressure is prevented from being relieved backward, whereby the bubble pressure is efficiently utilizable for ink to be jetted. As the nozzle flow channel has the tilted side extended in a direction perpendicular to the direction in which the nozzle flow channel is orientated and the direction in which the nozzle flow channel is extended, the bubble pressure produced on the bubble-producing resistor can be directed to the opening of the nozzle because of the tilted sided with the effect of making the pressure utilizable with efficiency. Further, the ink-reservoir-side terminal of the titled side is situated above the portion of the groove where it sectional area is reduced, whereby the sectional area of the ink flow channel is reducible so as to decrease the relief of the bubble pressure toward the ink reservoir. Since the titled side is positioned close to the bubble-producing resistor or in contact therewith, the bubble can be formed into good shape and the bubble pressure is also efficiently utilizable. The groove provided in the synthetic resin layer of the heater substrate contributes the realization of the aforementioned function. The use of the thermal ink-jet head capable of functioning as set forth above makes it possible to put a recording apparatus operating at high speed and offering good image quality to practical use. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view of a thermal ink-jet head embodying the present invention. Fig. 2 is a schematic sectional view of the thermal ink-jet head according to the present invention. Figs. 3A to 3D are trihedral view showing the structure of a flow channel in the thermal ink-jet head according to the present invention. Fig. 4 is a partial enlarged view of a pit in the thermal ink-jet head according to the present invention. Fig. 5 is an enlarged perspective view of a portion near the pit in the thermal ink-jet head according to the present invention. Figs. 6A and 6B are partial enlarged view of a design polyimide mask pattern by way of example. Figs. 7A and 7B are illustration of the formation of bubbles by way of example. Fig. 8 is a graph showing frequency response capability when a pattern is printed dot to dot in the thermal ink-jet head according to the present invention. Fig. 9 is a graph showing frequency response capability at the time of solid printing in the thermal ink-jet head according to the present invention. Fig. 10 is a graph showing the relationship between the internal head pressure and print frequencies resulting in producing printing defects in the thermal ink-jet head according to the present invention. Fig. 11 is a graph showing the relationship between the print frequency and the appearance of a white line in the front position at the time of solid printing in the thermal ink-jet head according to the present invention. Fig. 12 is a graph showing the measured results of ink discharge rates in the respective nozzles of one head. Fig. 13 is a block diagram of an exemplary control unit embodying the present invention. Figs. 14A and 14B are schematic diagram illustrating another thermal ink-jet head embodying the present invention. Figs. 15A and 15B are schematic diagram illustrating still another thermal ink-jet head embodying the present invention. Fig. 16 is a diagram illustrating a dropout defect. Fig. 17 is an enlarged view of a dropout defect portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a schematic perspective view of a thermal ink-jet head embodying the present invention; Fig. 2 a sectional view of the same; Figs. 3A to 3D a trihedral view showing the structure of a flow channel thereof; Fig. 4 a partial enlarged view of a pit thereof; and Fig. 5 an enlarged perspective view of a portion near the pit thereof. In Figs. 1 to 5 inclusive, reference numerals 1, 1a, 1b, 1c denote heating elements; 2, 2a, 2b, 2c, pits; 3, 3a, 3b, 3c, polyimide walls; 4, a channel wafer; 5, 5a, 5b, 5c, nozzle flow channels; 6, the forward portion of an ink reservoir; 7, an ink reservoir; 8, a heater wafer; 9, a polyimide layer; 10, a protective film; and 11, a channel pressure wall. Fig. 4 is an enlarged view of the circular portion enclosed with a dotted line of Figs. 3A to 3D. The thermal ink-jet head is made up by sticking the channel wafer 4 and the heater wafer 8 having the polyimide layer 9 together. The heater wafer 8 is made of, for example, Si and formed with a plurality of heating elements 1a, 1b, 1c,..., a common electrode (not shown), a discrete electrode and the like. Moreover, the protective film 10 for protecting the electrodes is formed on the heater wafer 8, and the polyimide layer 9 as a synthetic resin layer is formed on the protective film 10. Further, the pits 2a, 2b, 2c,... coupled to the forward ink-reservoir portion 6 from above the heating elements 1a, 1b, 1c are formed by etching or the like in the polyimide layer 9. On the other hand, the channel wafer 4 is also made of Si, for example, and the ink reservoir 7 having the nozzle flow channels 5a, 5b, 5c,... and the forward ink-reservoir portion 6 is formed by ODE, for example. The nozzle flow channel formed by the ODE is in the shape of a trihedron. The ink reservoir 7 is formed by the ODE conducted twice; that is, the ink reservoir 7 is formed in the shape of a through-hole in the channel wafer 4 by the first ODE and the forward ink-reservoir portion 6 is then formed by the second ODE. Thus an ink supply port as a hole formed in the channel wafer 4 has a small aperture, whereby an area where the port makes contact with an ink supply means (not shown) is enlarged. Needless to say, the ink reservoir 7 may be formed by the first ODE in this case without providing the forward ink-reservoir portion 6. As shown in Figs. 3A to 3D, a small part of the polyimide layer 9 ahead of the heating element 1 in the pit 2 is cut out. Moreover, the pit is so structured as to form a throttled portion by horizontally throttling the flow channel in the rear of the heating element 1. The configuration like this can easily be accomplished by designing the mask pattern of the polyimide layer 9 along the contour of the pit 2. The positional relationship between the throttled portion and the nozzle flow channel 5 is such that the endmost portion of the channel pressure wall 11 of the nozzle flow channel 5, that is, the minimum closed portion of the flow channel because of the channel pressure wall 11 is positioned above the throttled portion. The throttled portion is configured in such a manner that it is gradually narrowed from the side of the ink reservoir 7 toward the heating element 1 and horizontally minimized in size right after the heating element 1. Consequently, the flow channel resistance in ink is reduced when a refill of ink is conducted, whereas the bubble pressure produced on the heating element 1 is prevented from being relieved backward. The growth of a bubble is thus controlled from the standpoint of configuration. The polyimide wall 3 formed at the joint between the pit 2 and the ink reservoir 7 is semicircular. The endmost of the stretched pit 2 obviously acts as a pressure reflective wall against the bubble pressure produced in the heating element 1 and by making this portion what is of pressure-wave absorption structure, a reduction in crosstalk is made accomplishable. When the circular structure is actually designed, a polygonal structure is employed for a polyimide mask pattern. Figs. 6A and 6B are partial enlarged view of a design polyimide mask pattern by way of example. Figs. 6A and 6B show the simplest mask pattern and another example which art triangular and pentagonal, respectively. Therefore, such a mask pattern is not necessarily a complete semicircle but designed as what is decaoctagonal according to the present embodiment of the invention. Due to the restriction of resolution, the polyimide wall 3 that has actually resulted remains substantially semicircle. On the other hand, the unetched portion between the nozzle flow channel 5 and the ink reservoir 7 is disposed so that its end on the side of the nozzle flow channel 5 is located above the throttled portion of the pit 2. The ink flow channel formed with the unetched and throttled portion corresponds to what has the minimum sectional area of this head. Because of the flow channel resistance in this portion, ink vibration is suppressed when printing is started as illustrated in Figs. 16 and 17 as mentioned above, whereby any defect such as a dropout in an image can be prevented. Moreover, pressure passing through that portion and propagating toward the ink reservoir can be minimized. The flow channel resistance varies with the position of the end of the unetched portion on the side of the nozzle flow channel 5. Moderate flow channel resistance can be set by controlling that position. The tilted channel pressure wall 11 is formed at the terminal of the nozzle flow channel 5 formed by the ODE. As shown in Fig. 5, the channel pressure wall 11 is used to form the flow channel having the minimum sectional area in the throttled portion of the pit 2, the channel pressure wall being then capable of expanding the flow channel three-dimensionally. Consequently, the total sectional area of the flow channel is increased. As the channel pressure wall 11 is extended up to the vicinity of the end portion of the heating element 1, it not only controls the growth of the bubble produced on the heating element 1 but also functions as what reflects the bubble pressure toward an ink discharge port. As shown in Figs. 2 and 3, the ink flows from ink reservoir 7 via the pit 2 to the nozzle flow channel 5. The ink that has flowed into the pit 2 is passed through the throttled portion before being supplied onto the heating element 1. At this time, the ink is caused to pass through the minimum section area under the unetched portion between the nozzle flow channel 5 and the ink reservoir 7. Then there develops proper flow channel resistance, which suppresses the ink vibration when the ink is driven at a high drive frequency. Since the flow channel has been enlarged three-dimensionally by the channel pressure wall 11 ahead, the total sectional area of the flow channel is increased thereby, so that the flow channel resistance remains unchanged. When the bubble produced on the heating element 1 dies out, the ink is allowed to linearly flow into the nozzle flow channel 5 along the channel pressure wall 11 and consequently supplied into the nozzle flow channel 5 along the channel pressure wall 11. Although the flow channel resistance is present when the ink passes through the minimum sectional area under the unetched portion, the ink flows smoothly. As a refill of ink is accomplished satisfactorily, the frequency response capability of the ink is never deteriorated. When a bubble is produced on the heating element 1, a good bubble is formable because of the aforementioned configuration of the pit 2 around the heating element 1. Figs. 7A and 7B illustrate the formation of bubbles by way of example. In the case of a conventional thermal ink-jet head, for example, what is described in Patent Application No. 269899/ 1993, it has been arranged that the pits 2a, 2b, 2c,... are directly coupled to the common slit from above the heating elements 1a, 1b, 1c,... For this reason, the forward bit wall is used to control the growth of the bubble and the rear side of the heating element is set free. As a result, the bubble grows backward as shown in Fig. 7B, that is, the pressure is caused to be relieved toward the rear side. According to the aforesaid embodiment of the invention, the front of the heating element 1 is slightly cut out, whereas the rear thereof is throttled, whereby the growth of the bubble can be controlled in such a way that it is substantially turned in the direction in which the ink is discharged. Further, the bubble is grown along the channel pressure wall 11 and the pressure generated by the growth of the bubble is caused to act toward the ink discharge port. Therefore, the bubble pressure is efficiently utilizable. The backward propagation of the pressure beyond the throttled portion of the pit 2 is then minimized by the throttled portion of the pit 2 and the channel pressure wall 11. The pressure propagated beyond the throttled portion of the pit 2 collides with the semicircular polyimide wall 3 of the pit 2 and attenuates. Further, the pressure propagating in the direction of the forward ink-reservoir portion 6 after turning its direction at that point attenuates after diffusing along the forward ink-reservoir portion 6 and the whole tilted side of the ink reservoir 7. The pressure kept propagating from the throttled portion of the pit 2 is almost canceled when it attenuates in the ink reservoir 7 because the volume of the ink reservoir is far greater than that of the nozzle flow channel 5. In consequence, the pressure is prevented from not only propagating into the adjoining nozzle flow channels 5 but also causing crosstalk. A description will subsequently be given of a specific thermal ink-jet head by reference to Figs. 2 to 4. The width of the pit 2, that is, the width g of the heat generating area may be set to approximately 6 µm, the flow channel width d of the throttled portion where the nozzle-channel-side end of the unetched portion between the nozzle flow channel 5 and the ink reservoir 7 is located to approximately 36 µm, one side e of the throttled quantity of the throttled portion to approximately 14 µm, and the length f of the throttled portion to approximately 30 µm. At this time, the minimum section area of the flow channel is the product of the flow channel width d and the thickness of the polyimide wall, which makes 36 x 25 µm. The configuration of the polyimide wall 3 of the pit 2 may be decaoctagonal as stated above and close to semicircular. Further, the length c of the cut-out portion ahead of the heating element 1 of the pit 2 may be set to, for example, 10 µm. Further, the width b of the bottom of the nozzle flow channel 5 in the form of a triangular prism may be set to approximately 52 µm and set slightly smaller than the width g of the heat generating area. Further, the length a of the nozzle of the nozzle flow channel 5 may be set to approximately 30 µm. Since the tilted side of the nozzle flow channel 5 is formed by the ODE, it forms an angle of 54.7° with the bottom thereof. These ink flow channels may be disposed at a density of approximately 300 spi, for example. The minimum length h of the unetched portion between the nozzle flow channel 5 and the ink reservoir 7 may be approximately 35 µm. The ink reservoir 7 may be formed by the ODE conducted twice as stated above. In the case of the first ODE, etching is carried out to form a through-hole with an etching mask whose size is determined by the ink supply port. The thickness j of the channel wafer 4 is approximately 500 µm. In the case of the second ODE, an etching mask having an opening greater than that of the etching mask used initially, so that the nozzle flow channel 5 together with the ink reservoir 7 is formed. The etching depth i by means of the second ODE is determined by the chip size and may be approximately 60 µm, the depth being adjustable in accordance with the etching time. The length of the forward ink-reservoir portion 6 is reducible to substantially zero and the ink reservoir 7 is formable by the first ODE in this case; even though this portion is conversely set longer, it remains unaffected as far as the flow channel resistance is incomparably lower than that right behind the heater. The whole length k of the thermal ink-jet head thus prepared with the aforementioned dimensions is approximately 2,000 µm. In other words, the flow channel length according to the present invention can be reduced by over 100 microns in comparison with any prior system. Therefore, availability is improvable in a ratio of one to 20 chips in a case where a chip of approximately 2,000 microns is employed. Figs. 8 and 9 are graphs showing frequency response capability in the case of the thermal ink-jet head according to the present invention. Fig. 8 shows the relationship between the print frequency and the number of defects produced in a case where a pattern is printed dot to dot. On the other hand, Fig. 9 shows the relationship between the print frequency and the number of defects produced in the case of solid printing. Image quality has been affected by crosstalk in the conventional head when a pattern is printed dot to dot even the print frequency is low. Even in the case of solid printing, printing defects such as dropouts have been produced as the print frequency becomes higher when a refill of ink is conducted. As shown in Figs. 8 and 9, however, the thermal ink-jet head according to the present invention allows no defects to be produced and is capable of maintaining image quality even at high print frequencies heretofore resulting in producing defects. Therefore, it becomes possible to improve the defect-producing frequency greatly in the half-tone and solid printing portions that have posed a serious problem on the conventional head. More specifically, the thermal ink-jet head according to the present invention can be operated up to levels of about 10 - 12 kHz practically without any trouble. When characters and the like are printed, a print frequency of approximately 20 kHz is possible in such a character mode because a high ink flow rate is not required for a graphic pattern, that is, for solid, half tone and so forth. Fig. 10 is a graph showing the relationship between the internal head pressure and print frequencies resulting in producing printing defects. When a bubble dies out after ink is discharged, the quantity of ink corresponding to what has been discharged is required to flow onto the heating element 1; in other words, a refill of ink is conducted. Then the ink in the nozzle flow channel 5 is drawn into the nozzle when the absolute value of the negative pressure in the ink-jet head is large and caused to introduce air from the tip of the nozzle. A phenomenon of this kind will never occur if the ink is supplied from the ink reservoir 7 to the heating element 1 satisfactorily. If, moreover, the absolute value of the negative pressure is large, the propellent force of discharging the ink will be necessitated. Therefore, a nonconforming discharge may be brought about when the absolute value of the negative pressure becomes large unless the bubble pressure is utilized efficiently. As shown by a broken line of Fig. 10, the conventional ink-jet head has developed printing defects generally at low print frequencies because a refill of ink is not accomplished satisfactorily. As bubble pressure has been utilized insufficiently, printing defects tend to become conspicuous as the absolute value of the negative pressure increases; that is, such printing defects are produced even at low print frequencies. On the other hand, no printing defects are produced generally even at high print frequencies according to the present invention as shown by a solid line of Fig. 10 and even when the absolute value of the negative pressure increases, no printing defects are produced. In other words, a refill of ink is conducted efficiently in the ink-jet head according to the present invention and moreover the bubble pressure is seen to be utilized with efficiently. Fig. 11 is a graph showing the relationship between the print frequency and the appearance of a white line in the front position at the time of solid printing in the thermal ink-jet head according to the present invention. As stated above in reference to Figs. 16 and 17, such a white line appears in the front position when solid printing is made at high print frequencies. As shown in Fig. 11, a white line is seen to appear roughly at 6 kHz in the conventional example, whereas any white line defect is not found substantially up to approximately 9 kHz according to the present invention. Fig. 12 is a graph showing the measured results of ink discharge rates in the respective nozzles of one head. As shown by black circles of Fig. 12, approximately 0.5 m/sec variation in standard deviation σ has conventionally been observed in the case of head having 128 nozzles. However, the standard deviation σ in the thermal ink-jet head according to the present invention has been improved up to roughly 0.2 as shown by x of Fig. 12. Variations in the ink discharge rate have so far been known as the results of reflection of variations in the workmanship of finished product of heads. Since the flow channel resistance as a resistance component sufficient for pressure propagation in a moment in which a bubble develops is situated in the rear of the heating element 1 without interfering with the refill flow, a structure not only insensitive to variation in such workmanship in the rear of the heating element 1 but also capable of determining a jet flow rate solely by the bubble forming condition has been attained actually. As a result, variations in the ink discharge rate are considered to be lowered as shown in Fig. 12. This means that image quality is improvable because an error in the dot position in the direction in which a carriage moves is reduced. Fig. 13 is a block diagram of an exemplary control unit embodying the present invention, wherein reference numeral 21 denotes a 4-bit shift register; 22, 23, latch circuits; 24, a 32-bit bidirectional shifter register; 25, AND circuits; and 26, a heater driving circuit. The aforementioned heating element 1 is controlled by a drive control unit as shown in Fig. 13. In this case, the drive control unit is used for sequentially driving the blocks with four nozzles as one block. A DAT/DIR signal is a signal for indicating printing data or a scanning direction; a BIT SHIFT signal is for shifting the 4-bit shift register 21; and a FCLR signal is for resetting the 4-bit shift register 21 and the 32-bit bidirectional shift register 24 and for latching the latch circuit 23. Further, an ENABLE signal is a timing signal for driving the nozzles, namely, 128 nozzles. The AND circuits 25 are provided so as to correspond to the respective heating element 1 and its output is used to control the heater driving circuit 26. Since the blocks are sequentially driven with four nozzles as one block according to this embodiment of the invention, each of the output terminals Q1,..., Q32 is connected to four AND circuitS 25. The 4-bit shift register 21 and the 32-bit bidirectional shift register 24 are reset by the FCLR signal and when these registers rise, the latch circuit 23 latches the DIR signal, whereby the shifting direction of the 32-bit bidirectional shift register 24 is determined. Then image data is output as the DAT/DIR signal and the BIT SHIFT signal is input as a clock signal for the 4-bit shift register 21. For example, the image data are sequentially taken into 4-bit shift register 21 when the BIT SHIFT signal rises. When the 4-bit image data is taken in, it is latched in the latch circuit 22 when the ENABLE signal rises. The image data thus latched is fed into the AND circuit 25. With the ENABLE signal as a clock signal, on the other hand, the 32-bit bidirectional shift register 24 is shifted and an output from any one of the output terminals Q1,..., Q32 is input to the AND circuit 25. Therefore, only the four AND circuits 25 in that one block selected by the 32-bit bidirectional shift register 24 are driven in response to the image data. Then the heater driving circuit 26 is driven only during the "H" period of the ENABLE signal, so that the heating element 1 is actuated. The heat generated from the heating element 1 makes a bubble grow on the heating element 1 of the pit 2. The pressure generated when the bubble grows causes an ink drop to be discharged for the purpose of printing a character. Thus the output terminals of the 32-bit bidirectional shift register 24 is shifted from one to another each time the ENABLE signal is input and the heating elements 1 are sequentially driven every four out of 32 blocks. Figs. 14A and 14B are schematic diagram illustrating another thermal ink-jet head embodying the present invention: Fig. 14A is a sectional view of a portion near the pit; and Fig. 14B a top view of the pit. The polyimide wall 3 shown in Figs. 3A to 3D is linearly formed according to this embodiment of the invention. It is unnecessary to form the polyimide wall 3 into a substantially semicircle as shown in Figs. 3 and 6 because crosstalk can be lowered sufficiently with the throttled portion and the forward ink-reservoir portion 6 in this structure. In consequence, the linear polyimide wall shown in Figs. 14A and 14B is readily manufactured. Figs. 15A and 15B are schematic diagram illustrating still another thermal ink-jet head embodying the present invention: Fig. 15A is a sectional view of a portion near the pit; and Fig. 15B a top view of the pit. Since an adhesive agent is used to join the channel wafer 4 and the synthetic resin layer 9 together, it may jut out on the heating element 1. In order to secure stability during the process of manufacture, a margin area needs setting accordingly because of the jutting-out of the adhesive agent. As shown in Figs. 15A and 15B, a non-heating area approximately 10 µm wide is provided in the rear of the heating element 1 and a back portion of the throttled portion of the synthetic layer 9 is shifted backward and besides the length of the nozzle flow channel 5 is increased by 10 microns accordingly. In this case, the length h of the unetched portion is set to 50 microns, that is, to the extent that the flow channel resistance becomes lower than what is shown in Figs. 3A to 3D and 14. Consequently, the adhesive agent is prevented from jutting out on the heating element 1, which results in reducing variations during the process of manufacture. Not only a higher frequency response capability but also a reduction in crosstalk is secured by the throttled portion of the pit 2 and the unetched portion as stated previously even in this case. The aforesaid thermal ink-jet head is, as shown in each embodiment of the invention, designed to make the ink supply port of the channel wafer 4 communicate with an ink tank to facilitate the ink flow by bonding an ink supply means (not shown) to the ink supply port thereof. While the thermal ink-jet head or recording paper is kept moving, the heating element 1 is supplied with power to generate heat according to the image data and ink is caused to be discharged from the nozzles for recording data by means of the ink supply means fitted to the recording apparatus. Thus the recording apparatus furnished with the thermal ink-jet head according to the present invention is capable of making obtainable stable, high-quality printed images at all times. As is obvious from the description given of the present invention, the ink jetting force is improved as the bubble energy is utilizable for the discharge of ink with certainty and the printing operation can stably be performed by dealing with external disturbance such as the drying of the nozzle and ink leakage. Further, pit-to-pit crosstalk is obviated and the discharge of ink is stabilized without relying on the print pattern. In addition, high-speed printing is made feasible by high frequency response capability. Further, the location of an optimum flow channel in the rear of the heating element results in stabilizing the ink flow in the vicinity of the heating element and thus providing a print head far free from poor image quality. Further, the head can be made small-sized and inexpensive as the whole flow channel length is reduced. Since the formation of only one opening is required for each nozzle in the synthetic resin layer, variations during the process of manufacture are reduced with the effect of manufacturing products with stability.

Claims: A thermal ink-jet head comprising: a heater substrate (8) having bubble-generating resistors (1) ; a channel substrate (4) having a plurality of nozzle flow channels (5), an ink reservoir (7), and a plurality of ink supply ports (2), wherein said nozzle flow channel (5) formed in said channel substrate (4) is passed over said bubble-generating resistor (1), and the ink flows from the ink reservoir (7) via the ink supply ports (2) to the nozzle flow channels (5), a groove provided in said heater substrate, wherein said groove being long enough to cover the distance from the front end portion of said bubble-generating resistor (1) at the side of the nozzle flow channels (5) along its length and up to a place where the groove is coupled to said ink reservoir (7), a partitioning wall (11) formed in said channel substrate (4), wherein the sectional area of an ink flow channel formed with said partitioning wall (11) between said nozzle flow channel (5) and said ink reservoir (7) formed in said channel substrate (4) and with the groove provided in said heater substrate is minimized; characterized in that said partitioning wall (11) is provided in the vicinity of an end portion of said bubble-generating resistor (1) at a side of said ink reservoir (7), and that said groove provided in said heater substrate (8) has a throttled portion (e) in the ink flow direction between said partitioning wall (11) and said bubble-generating resistor (1) to suppress ink vibrations and crosstalk. The thermal ink-jet head of claim 1, wherein the sectional area of the forward portion (6) of said ink reservoir (7) starting with said ink supply port toward said nozzle flow channel (5) is decreased. The thermal ink-jet head of claim 1, wherein said ink reservoir (7) is provided for use common to said plurality of nozzle flow channels (5). The thermal ink-jet head of claim 1, wherein the sectional area of the groove between the upper portion of said bubble-producing resistor (1) and said ink reservoir (7) is reduced in the direction in which said nozzle (5) is orientated, said nozzle flow channel (5) has a tilted side (11) extended in a direction perpendicular to the direction in which said nozzle flow channel (5) is orientated and the direction in which said nozzle flow channel (5) is extended, and the ink-reservoir-side terminal of the tilted side (11) is situated above the portion of the groove where sectional area thereof is reduced. A thermal ink-jet head according to claim 1 further comprising: said channel substrate (4) formed with said plurality of nozzle flow channels (5) each passed on said bubble-generating resistors (1) and formed up to almost nearly the end portions of said bubble-producing resistors (1), wherein the ink supply ports and said ink reservoir are used in common for said plurality of nozzle flow channels (5) and the sectional area of said ink reservoir (7) is communicating with said ink supply ports which is increased from said ink supply port toward said nozzle flow channel (5), and a synthetic resin layer (9) provided on the heater substrate (8). A recording apparatus using the thermal ink-jet head of any one of claims 1 to 5.

Inventors: FUJII MASAHIKO, FUJIMURA YOSHIHIKO, HAMAZAKI TOSHINOBU, ISOZAKI JUN, KOIZUMO YUKIHISA, MORITA NAOKI, FUJII, MASAHIKO, FUJIMURA, YOSHIHIKO, HAMAZAKI, TOSHINOBU, ISOZAKI, JUN, KOIZUMO, YUKIHISA, MORITA, NAOKI, Fujii, Masahiko, c/o Fuji Xerox Co., Ltd, Fujimura, Yoshihiko, c/oFujii Xerox Co., Ltd, Hamazaki, Toshinobu, c/o Fuji Xerox Co., Ltd, Isozaki, Jun, c/o Fuji Xerox Co., Ltd, Koizumo, Yukihisa, c/oFujii Xerox Co., Ltd, Morita, Naoki, c/o Fuji Xerox Co., Ltd
Applicants: FUJI XEROX CO LTD, FUJI XEROX CO., LTD.
Application year: 1996
Main classification: B41J2
IPC codes: B41J2