INK JET RECORDING APPARATUS

Abstract: The present invention relates to an ink jet recording apparatus comprising an ink jet head (1) having plural opening for discharging an ink, partial capping (8) means for covering a part of said plural discharging opening, and suction means (P) for sucking the ink from said part of discharging opening through said partial capping means.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ink jet recording apparatus for image recording by discharging of recording liquid (ink), and more particularly to an ink jet recording apparatus equipped with a recording head having plural orifices with an improved recovery system for orifice clogging caused by dust, or defective discharging from the orifice caused by viscosity increase of ink or presence of bubbles therein. Related Background Art Fig. 1 is a schematic perspective view showing an example of the recording head employed in the ink jet recording apparatus, wherein a discharging element 1 is provided with liquid paths in which respectively arranged are heat generating elements constituting means for generating thermal energy utilized for the ink discharging, discharging openings 10 provided at the front ends of said liquid paths and a common liquid chamber for storing ink to be supplied to said liquid paths, and discharges ink from said discharging openings to form recording liquid droplets. There are further shown a base plate 3 for fixing the discharging element 1 for example with an adhesive; a front plate 2 fixed on an end face of the discharging element 1 and the base plate 3 for example with bolts and having an aperture 2a for maintaining the discharging openings 10 in direct facing relationship to a recording medium; and members 15, 16, 17 constituting a part of an ink supply system, in which 15 is a connecting elbow pipe for introducing ink to the common liquid chamber in the discharging element 1, 17 is a filter unit provided in the ink supply path from an ink source such as an ink tank, and 16 is a supply pipe connecting the member 15 with the filter unit 17. Figs. 2 and 3 are respectively vertical and horizontal schematic cross-sectional view of the recording head shown in Fig. 1, wherein a cap 4 is pressed to the face of discharging openings of the discharging element 1 across the front plate 2 (omitted in Figs. 2 and 3) for the recovery of discharging failure. The liquid paths 12 respectively corresponding to plural discharging openings 10 communicate with a so-called canopy portion 13, which in turn communicates with a common liquid chamber 14. Energy generating means 11, for example composed of a heat generating element is provided in the liquid path 12 for the purpose of generating energy utilized for ink discharging. Inside the filter unit 17 there is provided a filter 100 composed of a mesh for eliminating small dusts and bubbles. Fig. 4 is a schematic view of a discharge failure recovery system in the conventional ink jet recording apparatus. In the normal recording state, a cap 4 is placed in a position not hindering the recording operation, and the ink is supplied from the ink tank 6 to the discharge element 1 by capillary action. When there is a failure to discharge ink, the cap 4 is fitted on the discharge element 1 in air-tight manner, and a pump 7 is actuated in this state to generate a negative pressure inside the cap 4 in comparison with the ink tank 6, thereby forcedly sucking the ink from the discharging opening 10. At the same time, the dusts, viscous ink, bubbles etc. responsible for the discharge failure are removed from the discharge element 1, together with the sucked ink. For example a minute bubble a that has migrated into a liquid path 12 as shown in Fig. 3 can be removed through the discharging opening 10 together with the ink, by the actuation of the pump 7. The ink removed from the discharging opening 10 is received by the cap 4 and guided to a used ink tank 5. For example see US-A-4 631 554. Fig. 5 is a circuit diagram showing a fluid mechanical equivalent circuit for the ink in the discharge failure recovery in the conventional apparatus. At the discharge failure recovery, there stands a relation: ΔP = qR1 + nq(RH + RC + RF + RS) (1) among the suction force ΔP, number of liquid paths n , fluid resistance R1 per each liquid path, fluid resistance RH of the canopy 13, fluid resistance RC of the common liquid chamber 14, fluid resistance RF of the filter 17, fluid resistance RS from the ink tank 6 to the common liquid chamber 14 except the filter 17, and flow rate q of the liquid path 12 in suction with a force ΔP. Thus: q = ΔP/{R1 + n(RH + RC + RF + RS)} (2) Usually the supply system is so designed as to obtain a relation R1 » RH + RC + RF + RS, but, in so-called full-line multiple head in which the discharging openings respectively communicating with the liquid paths are arranged by a number corresponding to the full recording width, the number n of the liquid paths becomes very large, so that the flow rate q per liquid path becomes very small. Also in case bubbles or dusts enter the liquid path 12 as shown in Fig. 3, the fluid resistance of said liquid path becomes higher. Consequently the flow rate in the liquid path with discharge failure becomes even lower than in the normal liquid path. Let us consider a case in which a bubble has entered a liquid path 12. Since the bubble usually sticks to the wall of the liquid path, there is required a pressure change or a flow rate in the liquid path 12, in order to peel the bubble off the wall. However, in such conventional structure, since the fluid resistance is smaller in the normal liquid paths, the pressure change obtained in the liquid path with discharge failure becomes even smaller. Fig. 6 is a schematic perspective view showing another example of the recording head employed in the conventional ink jet recording apparatus, and Figs. 7 and 8 are respectively a vertical and horizontal schematic cross-sectional views of the recording head shown in Fig. 6. The present example differs from the foregoing example in that the discharging element 1 is provided with two supply pipes 16, and that the recovery of discharge failure is conducted with a pressure applied to the ink in the supply pipes 16. In the present example, there is provided with a receiving member 4a for receiving the ink expelled from the discharging openings by the pressure. Fig. 9 is a schematic view of a discharge failure recovery system in the ink jet recording apparatus of the present example. In the normal recording state, a receiving member (cap) 4a is placed at a suitable position not hindering the recording operation, and a valve B2 is closed while valves B1, B3 are opened, whereby the ink is supplied from an ink tank 6 to a discharging element 1 through the valve B1 by capillary action. When there is a failure to discharge ink, the cap 4a is fitted on the discharging element 1, and the valve B1 is closed while the valves B2, B3 are opened. In this state a pump 7 is actuated to send the ink from the ink tank 6 to the ink supply path under pressure, thereby supplying the discharging element 1 with pressurized ink and forcedly ejecting ink from the discharging openings 10. At the same time, the dusts, viscous ink, bubbles etc. responsible for the discharge failure are removed from the discharging element 1, together with the ejected ink. For example a minute bubble a that has migrated into a liquid path 12 as shown in Fig. 8 can be removed through the discharging opening 10 together with the ink, by the actuation of the pump 7. The ink removed from the discharging opening 10 is received by the cap 4 and guided to a used ink tank 5. Now reference is made to Fig. 5 for explaining the fluid mechanical equivalent circuit in the present prior art. At the discharge failure recovery, there stands a relation: ΔP = qR1 + nq(RH + RC + RF + RS) (1) among the pressure ΔP, number n of liquid path, fluid resistance R1 per each liquid path 12, fluid resistance RH of the canopy 13, fluid resistance RC of the common liquid chamber, fluid resistance RF of the filter 17, fluid resistance RS from the ink tank 6 to the common liquid chamber 14 except the filter 17, and flow rate q of the liquid path 12 under the pressure ΔP. Thus: q = ΔP/{R1 + n(RH + RC + RF + RS)} (2) Usually the supply system is so designed as to obtain a relation R1 » RH + RC + RF + RS, but, in so-called full-line multiple head in which the discharging openings respectively communicating with the liquid paths are arranged by a number corresponding to the full recording width, the number n of the liquid paths becomes very large, so that the flow rate q per liquid path becomes very small. Also in case bubbles or dusts enter the liquid path 12 as shown in Fig. 8, the fluid resistance of said liquid path becomes higher. Consequently the flow rate in the liquid path with discharge failure becomes even lower than in the normal liquid path. Consequently the conventional recovery system for discharge failure has been often unable to restore the defective liquid path to the normal state, or has to repeat the recovery operation to realize such normal state. Also there has been required a large suction force or pressure in order to overcome such fluid resistance, and a large pump 7 has been required for this purpose. This has resulted in a larger consumption of ink, and particularly in case of recovery operation with an increased pressure, a need for an increased strength of junction for withstanding such pressure. SUMMARY OF THE INVENTION According to the present invention an ink jet apparatus of the kind comprising: ink jet head having a plurality of openings for discharging ink and capping means associated with the head is characterised in that the capping means is adapted to only sealingly cover some of the plurality of discharge openings; and suction or pressure generating means for either sucking the ink from the covered discharge openings through the capping means or pressurising the ink in the ink jet head thereby ejecting ink from the discharge openings not covered by the capping means. According to the present invention a recovery method for an ink jet head comprises the steps of: detecting whether or not ink is being discharged from each of a plurality of discharge openings of said ink jet head; capping either those openings which are clear or those which are blocked and either clearing the blocked and capped openings by suction or clearing the blocked but uncapped openings by pumping fluid through them. How the invention may be carried out will now be described by way of example only and with reference to the accompanying drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view showing an example of the recording head; Figs. 2 and 3 are schematic cross-sectional views of a conventional recovery system for discharge failure; Fig. 4 is a schematic view showing a conventional recovery system for discharge failure; Fig. 5 is an equivalent circuit diagram of a conventional recovery system for discharge failure; Fig. 6 is a schematic perspective view of another example of the recording head; Figs. 7 and 8 are schematic cross-sectional views of another conventional recovery system for discharge failure; Fig. 9 is a schematic view of another conventional recovery system for discharge failure; Fig. 10 is a schematic view of an embodiment of the principal portion of an ink jet recording apparatus of the present invention; Fig. 11 is a fluid circuit diagram equivalent to the embodiment shown in Fig. 10; Fig. 12 is a schematic perspective view of an example of the structure of the ink jet recording apparatus of the present invention; Fig. 13 is a schematic perspective view showing an example of a capping unit of the present invention; Figs. 14 and 15 are schematic cross-sectional views respectively showing a state in which one or the other cap of the capping unit is fitted with the recording head; Fig. 16 is a block diagram of an example of the control system of the present invention; Fig. 17 is a flow chart showing an example of control sequence for recording and discharge failure recovery in the control system shown in Fig. 16; Fig. 18 is a schematic view of another embodiment of the principal part of the ink jet recording apparatus of the present invention; Fig. 19 is a fluid circuit diagram equivalent to the embodiment shown in Fig. 18; Figs. 20 and 21 are schematic cross-sectional views respectively showing a state in which one or the other cap of the capping unit of another embodiment is fitted on the recording head; and Fig. 22 is a flow chart showing another example of sequence for recording and discharge failure recovery in the control system shown in Fig. 16. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is to achieve secure recovery of discharge failure by concentrating the force for recovery (suction force or pressure) on only a part of a plurality of discharge openings. In a preferred embodiment of the present invention, when a cap is fitted on the recording head and suction means is actuated for recovery of the discharge failure, the ink is sucked only from the discharging openings covered by the cap and including the opening showing discharge failure, whereby the cause of discharge failure is eliminated. In another preferred embodiment of the present invention, when a cap is fitted on the recording head and pressurizing means is actuated for recovery of the discharge failure, the ink is emitted only from the discharging openings covered by the cap and including the opening showing discharge failure, whereby the cause of discharge failure is eliminated. In the following there will be given detailed explanation on the embodiments of the present invention. In the following drawings showing the embodiments, the same parts as those in the foregoing prior art will be represented by same numbers and will not be explained further. [First embodiment] Fig. 10 is a schematic view showing a recovery system in the principal part of an ink jet recording apparatus of the present invention, wherein a partial cap 8 for covering a part of the array of discharging openings 10 (Fig. 1) is brought, at the recovery of discharge failure, to a position opposed to the discharging element 1 by suitable drive means, but, in the normal recording operation, is placed in a non-opposed position which does not hinder the recording operation (for example a position distant from the discharge element in a direction perpendicular to the plane of drawing). If the discharge failure occurs in the course of recording operation, the partial cap 8 is suitably moved and fitted on the discharge element so as to cover a portion including the discharging opening with said discharge failure, and the pump 7 is actuated in this state to effect the suction operation. In this operation the ink is ejected only from the discharging openings covered by the partial cap 8, and is recovered into the used ink tank 5 through an used ink tube 5A. A broken-lined member 40 is an entire cap which is similar to the cap 4 shown in Fig. 4 and which can be fitted on the discharging element 1 instead of the cap 8. In the present embodiment, detector means for discharge failure is provided in said entire cap 40. Fig. 11 is a circuit diagram showing a fluid mechanical equivalent circuit of the discharge element at the recovery of discharge failure, wherein m indicates the number of liquid paths corresponding to the discharging openings 10 covered by the partial cap 8, q′ is the flow rate per each of said liquid paths 12, and other symbols are same as already explained in relation to Fig. 5. In this equivalent circuit there stands the following relation: q′ = ΔP/{R1 + m(RH + RC + RF + RS)} (3) Since m < n, q′ > q (4) Thus, if the suction force of the pump 7 is maintained constant, in comparison with the conventional apparatus in which the recovery operation is conducted by sucking the ink from all the discharging openings 10, the efficiency of recovery becomes higher as m becomes smaller than n . On the other hand, if the flow rate is maintained same as in the conventional apparatus (q′ = q), there can be employed a pump 7 with a smaller suction force. In the recovery of discharge failure, the effect of recovery is larger as the number m of the discharging openings covered by the partial cap 8 decreases, but, if the discharge failure occurs in many liquid paths 12, there will be required a larger number of recovery operations, with accordingly increased time of recovery, as the number m decreases. In order to resolve this problem, the value of m can be selected in the following manner. The following relation stands for a flow rate q₀ capable of removing the cause of discharge failure such as bubbles in the liquid path 12: m = (ΔP - q₀R1)/q₀(RH + RC + RF + RS) (5) and an optimum value of m can be selected accordingly. The value of q₀ can be easily determined for example experimentally. In the following there will be explained the structure of the ink jet recording apparatus of the present embodiment. Fig. 12 is a schematic perspective view of an embodiment of the ink jet recording apparatus of the present invention, wherein shown are a recording head H comprising a discharging element 1 in which discharging openings 10 are arranged in the transversal direction of the recording medium over the entire width thereof; a capping unit 50 having a partial cap 8 and an entire cap 40, fixed on a wire 62 connected to a motor unit 60 and being vertically movable along unrepresented guides by the actuation of the motor unit 60; and a member 64 for regulating the tension of the wire 62. Sensors 110, 120, for example composed of photocouplers, for detecting the elevated position of the capping unit 50, detects when the partial cap 8 or the entire cap 10 is opposed to the recording head 10. A sensor 130, composed of a photocoupler for detecting the lowered position of the capping unit 50, is used for setting the capping unit 50 at a position not opposed to the recording head H in the recording operation. A light shield plate 51 is provided on the capping unit 50 for intercepting the light path of the sensors 110, 120 and 130. Fig. 13 is a schematic perspective view of an example of the capping unit 50, wherein shown are a motor unit 70 for fitting or detaching the partial cap 8 or entire cap 40 of the capping unit 50 with or from the recording head H; a sensor 72 composed of a photocoupler provided on the capping unit 50, for detecting the fitted or detached position of the partial cap 8 and the entire cap 40 with respect to the recording head H; and a light shield plate 74 moving integrally with the partial cap 8 and the entire cap 40 for intercepting the light path of said sensor thereby detecting the fitted or detached state of said caps. In the following there will be explained the structure for effecting the recovery operation by means of the partial cap 8 and the entire cap 40. Fig. 14 is a schematic cross-sectional view showing a state in which the partial cap 8 is positioned opposite to and fitted with the discharging element 1. The cap 8 of the present embodiment is provided with a cap member 81 of belt shape having a cap part 82 for covering the discharging openings of a number determined according to the aforementioned equation (5). The remaining openings may or may not be sealed by said cap member 81. There are also shown a tube 82A integral with the used ink tube 5A; and pulleys 83, 84 for supporting the cap member 81 movably in a direction W which is parallel to the direction of arrangement of the discharging openings 10. A pulley 83 is provided with a gear 83A. The rotation of a motor 85 is transmitted through a worm gear 86 provided on the shaft of said motor and the gear 83A to the pulley 83, thereby moving the cap member 81 in the direction W. There are further provided a slit plate 87 fixed to the partial cap 8, and a photocoupler 88 fixed on the cap member 81 for detecting the slit on the slit plate 87. The position of the cap member 81 can be identified from the detection signal. The partial cap 8 including the above-mentioned parts is provided in the capping unit 50, and is rendered integrally movable in a direction F for fitting or detaching with or from the discharging element 1 by means of the motor unit 70. Fig. 15 is a schematic cross-sectional view showing a state in which the entire cap 40 is fitted with the discharging element 1, wherein shown are a cap 42 capable of covering all the discharging openings of the discharging element 1, and a light-emitting element such as a semiconductor laser and a light receiving element such as a phototransistor 44, 46 which are fixed on suitable positions on the lateral face of the cap 42 whereby the light path L therebetween can be intercepted by the droplets emitted by each of the discharging openings 10. 42A is a used ink tube integral with the used ink tube 5A. Fig. 16 is a block diagram showing an example of the control system employed in the present embodiment, wherein a controller 90 for controlling various is also used as the controller for the recording operation in the ink jet recording apparatus. The controller 90 is provided with a CPU 90A for executing the control sequence shown in Fig. 12; a ROM 90B storing a program corresponding to the control sequence to be executed by the CPU 90A and other fixed data; and a RAM 90B serving as a work memory. A position setting unit 91 for setting the vertical position of the capping unit 50 includes, as shown in Fig. 12, the motor unit 60, sensors 110, 120, 130 etc. A position setting unit 92 for driving the capping unit 50 in the direction F shown in Figs. 14 and 15 in order to set said unit in a position fitted with or detached from the discharging element 1 includes, as shown in Fig. 13, the motor 70, the sensor 72 etc. A cap member driving unit 93, for driving the cap member 81 of the cap 8 in the direction W in Fig. 14 to drive the cap part 82 in a position opposed to the discharging opening with discharge failure, includes the motor 85, photocoupler 88 etc. shown in Fig. 14. A detector unit 94 for detecting the discharge failure includes the light-emitting element 44 and the light-receiving element 46 shown in Fig. 15. An alarm unit 95, composed of a display unit or a buzzer, is provided for giving alarm in case the discharge failure is not eliminated after a predetermined number of recovery operations. Fig. 12 is a flow chart showing an example of the control sequence for the recording and the discharge failure recovery in the present embodiment. At the start of sequence, at first a step S1 resets a counter N for counting the number of recovery operations, then a step S4 executes the recording for a predetermined amount or time, and a step S6 detects the discharge failure. In said detection, the cap 42 of the entire cap 40 is fitted on the discharging element 1 of the recording head H by means of the position setting units 91, 92 as shown in Fig. 15, and the light-emitting element 44 is actuated to emit light toward the light-receiving element 46. Then driving pulses of a predetermined frequency are supplied to the energy generating means in the liquid paths 12, in succession starting from the one positioned at the end. Each discharging opening communicating with the normal liquid path emits droplets, thus intercepting the light path L and causing a switching operation in the light receiving element 46. On the other hand, each opening communicating with the liquid path involving discharge failure either cannot achieve normal discharging or does not discharge droplet at all, so that the light-receiving element 46 shows unstable switching or is not switched. In this manner the discharge failure is detected, and the position of the liquid path of the energy generating means then actuated is stored in the RAM 90C for use in the succeeding recovery operation for the discharge failure. Said detecting operation can be completed in about 1.6 seconds if the discharges are conducted with a driving frequency of 2 KHz for the discharging openings arranged over the width of an A4-sized recording sheet. The sequence returns to the step S4 if the discharge failure is not detected. On the other hand, if the discharge failure is detected, a step S8 suitably drives the position setting units 91, 92 and the driving unit 93 to place the cap part 82 in a position close to the discharging opening 10 showing discharge failure, and to fit the partial cap 8 so as to cover said opening 10, and a step S12 actuates the pump 7. Thus a negative pressure is generated inside the cap part 82, whereby the ink is sucked only from the discharging openings covered by the cap part 82, thus removing the cause of the discharge failure. For example, in Fig. 14, the ink is sucked from the discharging openings 10 covered by the cap 82, whereby the bubble a present in the liquid path 12 is removed. After the continuation of this state for a predetermined period, identified for example by the arrival of the timer value t at a predetermined value y (step S14), a step S16 deactuates the pump 7, and a step S18 moves the partial cap 8 in the direction F shown in Fig. 14 thereby detaching it from the discharging element 1. A step S20 discriminates whether the-suction recovery process has been conducted on all the discharging openings 10 showing discharging failure, and, if completed, the sequence proceeds to a step S22. On the other hand, if not completed, the sequence returns to the step S8 for setting the cap 82 to another discharging opening 10 showing discharge failure and effecting the recovery operation for said discharging opening. If the discrimination in the step S20 turns out affirmative, the step S22 suitably actuates the position setting units 91, 92 and the driving unit 93 to fit the entire cap 10, instead of the partial cap 8, on the discharging element 1. Then a step S24 actuates the pump 7 in this state, thereby causing discharging of the ink from all the discharging openings 10. A step S26 continues this operation for a predetermined period (for example until the timer value t reaches a predetermined value z ), and a step S28 deactuates the pump 7. These steps S22 to S28 may be dispensed with in the present embodiment. Then a step S32 executes detection of discharge failure as in the step S6. If the result is negative, the sequence returns to the step S1 for preparing for a next recording operation. On the other hand, if the result is affirmative, a step S34 advances the count of the counter N by one, and a step S36 discriminates whether the count of the counter N has exceeded a predetermined value N₀, for example "1". If said discrimination turns out negative, the sequence returns to the step S8 for repeating the recovery operation for the discharge failure. On the other hand, if the step S36 provides an affirmative discrimination, indicating that the cause of discharge failure has not been removed even after a predetermined number of recovery operations, there is identified an abnormality, and the alarm unit 95 is actuated to inform the operator of this fact, for example by a display. As explained in the foregoing, the present embodiment removes the cause of discharge failure by sucking the ink only from a portion in which the discharge failure has occurred, so that it is rendered possible to select a smaller pump 7, or to increase the flow rate per liquid path, and to prevent the wasting of the ink. Besides the present embodiment allows to promptly detect abnormality in the recording head. [Second embodiment] In the second embodiment, the structure of the principal part of the ink jet recording apparatus, basic structure of the capping unit and the control system of the ink recording apparatus are substantially same as those already explained in relation to Figs. 12, 13 and 16 in the first embodiment, and will not, therefore, be explained again. Fig. 18 is a schematic view of another embodiment of the recovery system in the principal part of the ink jet recording apparatus of the present invention, wherein a partial cap 8, for maintaining a part of the array of the discharging openings 10 (see Fig. 6) in open state and maintaining the other in a sealed state, is set in a position opposite to the discharging element in the recovery operation of discharge failure, but is set, in the normal recording operation, in a non-opposed position, not hindering the recording operation, which is a position distant from the discharging element 1 for example in a direction perpendicular to the plane of drawing. If a discharge failure occurs in the course of recording operation, the partial cap 8 is so moved and fitted on the discharging element 1 that a portion including the discharging opening showing said discharge failure is maintained in the open state, and, in this state, the valve 81 is closed while the valves 82, 83 are opened and the pump 7 is actuated to send pressurized ink to the discharging element 1. The ink is discharged only from the openings 10 in the open state, and is recovered in the used ink tank 5 through the used ink tube 5A. A broken-lined member 40 is an entire cap similar to the cap 4a shown in Fig. 9, and can be fitted on the discharging element 1 in place for the partial cap 8. In the present embodiment, detector means for the discharge failure is provided in said entire cap 40. Fig. 19 is a circuit diagram showing a fluid mechanical equivalent diagram of the discharging element at the recovery operation for the discharge failure, in which the open and closed states of the discharging opening are respectively represented by the closed and open state of switches S. The symbol m indicates the number of liquid paths 12 corresponding to the liquid paths 12 in the open state, while q′ is the flow rate in each of said liquid paths 12, and other symbols are same as those in the foregoing description in relation to Fig. 5. The following relation stands in the present equivalent circuit: q′ = ΔP/{R1 + m(RH + RC + RF + RS)} (3) Since m < n; q′ > q (4) Thus, if the pressure of the pump 7 is maintained constant, in comparison with the conventional apparatus in which the recovery operation is conducted by discharging the ink from all the discharging openings 10, the efficiency of recovery becomes higher as m becomes smaller than n. Or, if the flow rate is maintained same as in the conventional apparatus (q′ = q), there can be employed a pump 7 with a smaller pressure. In the recovery of discharge failure, the effect of recovery is larger as the number m of the discharging openings maintained open by the partial cap 8 decreases, but, if the discharge failure occurs in many liquid paths 12, there will be required a larger number of recovery operations, with accordingly increased time of recovery, as the number m decreases. In order to resolve this problem, the value of m can be selected in the following manner. The following relation stands for a flow rate q₀ capable of removing the cause of discharge failure such as bubbles in the liquid path 12: m = (ΔP - q₀R1)/q₀(RH + RC + RF + RS) (5) and an optimum value of m can be selected accordingly. The value of q₀ can be easily determined for example experimentally. In the following there will be explained a structure for effecting the recovery operation by driving the partial cap 8 and the entire cap 40. Fig. 20 is a schematic cross-sectional view showing a state in which the partial cap 8 is fitted on the discharging element 1. The partial cap 8 is provided with a belt-shaped cap member 81, composed for example of rubber, capable of being fitted on and closing the discharging openings 10, further having a cap part 82 for maintaining the discharging openings of a number, determined according to the aforementioned relation (5), in open state. There are also shown a tube 82A communicating with the used ink tube 5A; and pulleys 83, 84 for supporting the cap member 81 movably in a direction W parallel to the direction of arrangement of the discharging openings 10. A pulley 83 is provided with a gear 83A. The rotation of a motor 85 is transmitted through a worm gear 86 provided on the shaft of said motor and the gear 83A to the pulley 83, thereby moving the cap member 81 in the direction W. There are further provided a slit plate 87 fixed to the partial cap 8, and a photocoupler 88 fixed on the cap member 81 for detecting the slit on the slit plate 87. The position of the cap member 81 can be identified from the detection signal. The partial cap 8 including the above-mentioned parts is provided in the capping unit 50, and is rendered integrally movable in a direction F into a capping or detecting position with or from the discharging element 1 by means of the motor unit 70. Fig. 21 is a schematic cross-sectional view showing a state in which the entire cap 40 is fitted with the discharging element 1, wherein shown are a cap 42 capable of covering all the discharging openings of the discharging element 1 in the open state, and a light-emitting element 44 such as a semiconductor laser and a light receiving element 46 such as a phototransistor which are fixed on suitable positions on the lateral face of the cap 42 whereby the light path L therebetween can be intercepted by the droplets discharged by each of the discharging openings 10. 42A indicates a used ink tube communicating with the used ink tube 5A. Fig. 22 is a flow chart showing an example of the control sequence for the recording and the discharge failure recovery in the present embodiment. At the start of sequence, at first a step S1 resets a counter N for counting the number of recovery operations, and a step S2 opens the valves B1 and B3 and closes the valve B2, thereby supplying the ink by capillary action to the discharging element 1 from the ink tank 6 and through the valve 81. A step S4 executes the recording operation in this state for a predetermined amount or time, and a step S6 detects the discharge failure. In said detection, the cap 42 of the entire cap 40 is fitted on the discharging element 1 of the recording head H by means of the position setting units 91, 92 as shown in Fig. 21, and the light-emitting element 44 is activated to emit light toward the light-receiving element 46. Then driving pulses are supplied with a predetermined frequency to the energy generating means in the liquid paths 12, in succession starting from the one positioned at the end. Each discharging opening communicating with the normal liquid path emits ink droplet, thus intercepting the light path L and causing a switching operation in the light receiving element 46. On the other hand, each opening communicating with the liquid path involving discharge failure either cannot achieve normal discharge or does not discharge droplet at all, so that the light-receiving element 46 shows unstable switching or is not switched. In this manner the discharge failure is detected, and the position of the liquid path of the energy generating then activated is stored in the RAM 90C for use in the succeeding recovery operation for the discharge failure. Said detecting operation can be completed in about 1.6 seconds if the discharges are conducted with a driving frequency of 2 KHz for the discharging openings arranged over the width of an A4-sized recording sheet. The sequence returns to the step S4 if the discharge failure is not detected. On the other hand, if the discharge failure is detected, a step S8 suitably drives the position setting units 91, 92 and the driving unit 93 to place the part 82 in a position close to the discharging opening 10 showing discharge failure, and to fit the partial cap 8 so as to maintain said discharging opening in the open state and others in the closed state. Then a step S10 closes the valve B1 and opens the valve B2, and a step S12 actuates the pump 7. Thus pressurized ink is introduced in the path leading from the pump 7 to the discharging element 1 through the valve 82, whereby the ink is discharged only from the discharging openings in the open state, thus removing the cause of the discharge failure. For example, in Fig. 20, the ink is discharged from the discharging openings 10 in the open state, whereby the bubble a present in the liquid path 12 is removed. After the continuation of this state for a predetermined period, identified for example by the arrival of the timer value t at a predetermined value y (step S14), a step S16 deactuates the pump 7, and a step S18 moves the partial cap 8 in the direction F shown in Fig. 20 thereby detaching it from the discharging element 1. A step S20 discriminates whether the pressurized recovery process has been completed on all the discharging openings 10 showing discharge failure, and, if completed, the sequence proceeds to a step S22. On the other hand, if not completed, the sequence returns to the step S8 for setting the cap 82 to another discharging opening 10 showing discharge failure and effecting the recovery operation for said discharging opening. If the discrimination in the step S20 turns out affirmative, the step S22 suitably activates the position setting units 91, 92 and the driving unit 93 to fit the entire cap 10, instead of the partial cap 8, on the discharging element 1. Then a step S24 actuates the pump 7 in this state, thereby causing discharge of the ink from all the discharging openings 10. A step S26 continues this operation for a predetermined period (for example until the timer value t reaches a predetermined value z), and a step S28 deactuates the pump 7. These steps S22 to S28 may be dispensed with in the present embodiment. Then a step S32 executes detection of discharge failure as in the step S6. If the result is negative, the sequence returns to the step S1 for preparing for a next recording operation. On the other hand, if the result is affirmative, a step S34 advances the count of the counter N by one, and a step S36 discriminates whether the count of the counter N has exceeded a predetermined value N₀, for example "1". If said discrimination turns out negative, the sequence returns to the step S8 for repeating the recovery operation for the discharge failure. On the other hand, if the step S36 provides an affirmative discrimination, indicating that the cause of discharge failure has not been removed even after a predetermined number of recovery operations, there is identified an abnormality, and a step S38 activates the alarm unit 95 to inform the operator of this fact, for example by a display. As explained in the foregoing, the present embodiment removes the cause of discharge failure by discharging the ink only from a portion in which the discharge failure has occurred, so that it is rendered possible to select a smaller pump 7, or to increase the flow rate per liquid path, and to prevent the wasting of the ink. Besides the present embodiment allows to promptly detect abnormality in the recording head. The present invention is effectively and easily applicable to any recording head with plural discharging openings, regardless whether the number or range of array thereof corresponds to the entire width of the recording medium or not, also regardless whether it is a line printer with a full-line multiple head or a serial printer, and regardless of the structure of the ink supply system. Also in the present invention, the energy generating means for generating energy for ink discharging be composed of the aforementioned electro-thermal converting element, or a heat generating element having a heat-generating resistor and electrodes connected thereto, or a piezoelectric element serving as an electro-mechanical converter. Also in the ink jet head to be employed in the present invention, the direction of ink supply to the heat generating portion of the heat generating element in the liquid path and the direction of ink discharging from the discharging opening may be substantially the same or different. For example said directions may be substantially perpendicular to each other. Furthermore the structure and the driving mode of the partial cap and the full width cap are naturally not limited to those in the foregoing embodiments. For example, in the present invention, the entire cap may be dispensed with. Furthermore, in the recovery of the discharge failure, the aforementioned recovery by partial suction and that by partial pressurizing may be applied simultaneously, alternately or in succession. Furthermore, the detection of the discharge failure or of the position thereof is not limited to the foregoing embodiments, but may be made visually by the operator in the course of normal recording or test recording, and, in such case, there may be provided switches for actuating the recovery operation and for entering the position of such discharge failure. Also the discharge failure may be detected with an image reading sensor on the result of test recording on the recording medium. In the foregoing embodiments, the detection and recovery of discharge failure are conducted after the recording operation, but it is also possible to conduct the recovery for the discharge failure immediately after the start of power supply or after a long pause in the recording operation. As detailedly explained in the foregoing, the present invention effects the recovery operation by suction or pressurizing in a part of the plural discharging openings, thereby significantly improving the reliability of the recovery operation. Also the amount of ink consumed in the recovery operation can be minimized, so that the running cost of the apparatus can be significantly lowered. Furthermore since the suction or pressurizing force of the pump can be reduced, so that the cost of this part can be lowered, and, particularly in case of pressurized recovery, it is possible to increase the freedom in the designing of strength in the supply system.

Claims: An ink jet recording apparatus of the kind comprising: ink jet head (1) having a plurality of openings (10) for discharging ink and capping means (4) associated with the head (1); characterised in that the capping means (8) is adapted to only sealingly cover some of the plurality of discharge openings (10); and suction or pressure generating means (7) for either sucking the ink from the covered discharge openings (10) through the capping means (8) or pressurising the ink in the ink jet head (1) thereby ejecting ink from the discharge openings (10) not covered by the capping means (8). Apparatus as claimed in claim 1 in which the capping means (8) is mounted on a belt (81) by which the capping means (8) can be moved to cover different ones of the said plurality of discharge openings (10). Apparatus as claimed in claim 1 or 2 in which in addition to the said capping means (8) adapted to only sealingly cover some of the plurality of discharge openings (10) there is also a full-width capping means adapted to be able to cover all the plurality of discharge openings (10). Apparatus according to any preceding claim, further comprising detection means for detecting the ink discharging from said discharge openings, and control means for moving the capping means (8) to cover either the blocked discharge openings or the clear discharge openings and to clear the blocked discharge openings either by the aforesaid suction or by the aforesaid pressure respectively. An apparatus according to any preceding claim, further comprising a tube communicating with said suction means. An apparatus according to any preceding claim, further comprising a reservoir communicating with said suction means. An apparatus according to any previous claim 10, in which the suction or pressure is provided by a pump. An apparatus according to any preceding claim, wherein said ink jet head comprises liquid paths communicating with said discharging openings. An apparatus according to claim 8, wherein said ink jet head comprises a common liquid chamber communicating with said liquid paths. An apparatus according to claim 9, having a supply pipe communicating with said common liquid chamber. An apparatus according to claim 10, wherein said supply pipe is provided with a filter. An apparatus according to any of claims 8 to 11, wherein said liquid paths are provided with energy generating means for generating energy utilized for ink discharging. An apparatus according to claim 12, wherein said energy generating means is adapted to generate thermal energy. An apparatus according to claim 13, wherein said energy generating means is an electro-thermal converter element. An apparatus according to claim 14, wherein said electro-thermal converter element comprises a heat generating resistor and electrodes connected thereto. An apparatus according to claim 12, wherein said energy generating means is an electro-mechanical converter element. An apparatus according to claim 16, wherein said electro-mechanical converter element is a piezo-electric element. An apparatus according to any of claims 9 to 17, wherein the direction of ink discharging from said discharging opening is substantially the same as the direction of ink supply to the position of said energy generating means in said liquid path. An apparatus according to any of claims 9 to 17, wherein the direction of ink discharging from said discharging opening is different from the direction of ink supply to the position of said energy generating means in said liquid path. An apparatus according to claim 19, wherein said two directions are substantially perpendicular to each other. An apparatus according to any preceding claim, wherein said plural discharge openings are provided corresponding to the width of the recording medium. A recovery method for an ink jet head comprising the steps of: detecting whether or not ink is being discharged from each of a plurality of discharge openings of said ink jet head; capping either those openings which are clear or those which are blocked and either clearing the blocked and capped openings by suction or clearing the blocked but uncapped openings by pumping fluid through them. A method according to claim 22, further comprising the step of either sucking the ink from all of said plurality of discharging openings, or pressurising the interior of the ink jet head to discharge ink from all of said plurality of discharge openings. A method according to claim 22 or 23, wherein applying suction to the blocked openings is conducted simultaneously with pressurizing the interior of the ink jet head to emit ink from the discharge openings. A method according to claim 22 or 23, wherein applying mention to the blocked openings is alternately conducted with the step of pressurizing the interior of the ink jet head to discharge the ink from the discharge openings.

Inventors: KOIZUMI, YUTAKA, MORI, TOSHIHIRO, NOZAWA, MINORU, SAITO, ATSUSHI
Applicants: CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: B41J2
IPC codes: B41J2