Ink jet printer and ink priming method therefor

Abstract: An ink jet printer reliably prevents printing defects resulting from bubbles remaining in the ink path after ink priming. An ink suction mechanism (6) of the ink jet printer performs a post-priming process for sucking ink from the nozzles when an hour has passed after the ink priming process. This process sucks a large volume of ink from the ink nozzles, and can thus reliably expel bubbles from the ink path. By performing this process at a sufficiently long time interval after ink priming, enough time has elapsed for bubbles that are formed by the ink path filter during ink priming and collect in offsets in the ink path to grow to a size where the bubbles protrude from the offset into the ink path. Bubbles that are thus freed into the ink path can therefore be reliably expelled from the nozzles. Printing defects resulting from bubbles left by ink priming can thus be reliably prevented.

Description: The present invention relates to an ink jet printer, and relates more particularly to an ink priming method therefor, and to an ink jet print head recovery process applied after ink priming. Once a user has purchased an ink jet printer and installs for the first time an ink cartridge into the printer in order to use it, a so-called ink priming is required to charge the ink path from the cartridge to the nozzles of the printer's print head with ink from the ink cartridge. Ink priming is also required after the ink jet print head needed replacement and a new ink jet print head has been installed. Methods accomplishing this with an ink suction mechanism have been proposed. JP-A-8-267785, for example, teaches an ink jet printer comprising such an ink suction mechanism. As is well known (see, e.g., EP-A- 0 803 359), when ink jet printers, in particular on-demand type ink jet printers, are left unused for a certain (idle) time the viscosity of the ink in the nozzles of the print head increases due to evaporation. The increasing viscosity tends to cause what is known as nozzle clogging. Nozzle clogging is commonly used to describe the state that high viscosity or dried ink in the minute nozzles seriously affects the nozzle function such that ink ejection is now longer possible or at least irregular (ejection defects occur). This is a well known problem and various means have been proposed to prevent such nozzle clogging. The principle common to all solutions known so far is a so called purging or head recovery. In such head recovery ink is ejected and/or sucked from the nozzles for the sole purpose of removing high viscosity ink and clear the nozzles. Immediately after ink priming, a large number of air bubbles is left in the ink path due to the ink passing a filter disposed in the ink path. The bubbles tend to accumulate at offsets in the walls of the ink path, such as that formed where an ink supply tube connects to the print head. While the bubbles are sufficiently small, they are trapped in the corners of these offsets and are, thus, not in the path of ink flow produced by suction in the head recovery. These small bubbles thus do not reach the nozzles, and therefore do not cause nozzle failure, that is, ink ejection problems. Small bubbles that have been captured by such offsets gradually grow as a result of, for example, a rise in ink temperature after the printer is turned on, and an increase in surface tension resulting from aggregation of numerous small bubbles. Eventually the bubbles grow to a size at which they protrude from the offset into the flow of ink through ink path and are affected by the flowing ink. As shown Fig. 12, a bubble B may even block the ink path (note that Fig. 12 is an enlarged partial view of the connection between elements 34 and 97 in Fig. 4 explained later). A bubble large enough to protrude into the ink path can be pulled into an ink chamber in the print head at the flow rate produced by a normal suction type head recovery, but will not be expelled from the nozzle. Small bubbles trapped at such offsets in the ink path immediately after the ink priming cannot be expelled from the nozzles no matter how much the ink flow rate is increased for the suction type head recovery. Furthermore, bubbles that become attached to an inside wall of the ink path during the ink priming and remain there are gradually freed from the wall into the ink path as the surface tension increases and the wetness of the inside walls increases over time. As a result, these bubbles are carried toward the print head and are left in the ink nozzle(s) by the ink suction operation used for a regular head recovery. When the print head is then driven for printing, bubbles in the nozzles cause printing problems such as non-firing nozzles. The size of small bubbles resulting from the ink priming also gradually increases as a result of an increase in printer temperature when the ink jet printer is left turned on. Temperature rises by approximately 10° C in the first hour after the printer has been turned on, and continues to gradually rise thereafter. Some conventional ink jet printers therefore run the suction type head recovery immediately following ink priming in an attempt to remove these small bubbles, but this results in no more than wasted ink. After ink priming, when the bubble size has grown to a point at which it affects the ink flow, some conventional ink jet printers apply a normal suction type head recovery using a small amount of ink. With this method, however, bubbles are only transported into the print head's pressure chamber. As a result, any attempt to pressurize the pressure chamber simply in order to eject ink only compresses the large bubbles (Pascal's law). Pressure is therefore not transmitted to the ink, and ink cannot be ejected from the nozzles. An ink priming method and apparatus according to the pre-characterizing portion of claim 1 is known from EP-A-0 778 140. This prior art performs step a) for a first predetermined time interval (T 2 ), starts step b) immediately after having stopped step a) and performs step b) for a second predetermined time interval (T 3 ). EP-A-0 427 202 discloses an ink jet printing method and apparatus that avoid printing defects by applying an on-demand ink recovery process to purge the ink nozzles. Two reasons are mentioned that may cause printing defects, namely particles and dust clogging an ink nozzle (light defects) and air bubbles in an ink nozzle (heavy defect). Fewer ink needs to be sucked from an ink nozzle in a recovery operation applied to recover from a light printing defect as compared to the amount required for recovering from a heavy defect. A recovery button is provided and pressed to initiate a recovery process. The amount of ink sucked from an ink nozzle is controlled in accordance with whether a light or a heavy defect occurred. The distinction between the light and heavy defects is made depending on either the interval of operation of the recovery button or the amount of printing performed in that interval. This prior art silent about a priming process such as defined in the pre-characterizing portion of claim 1. It is an object of the present invention is to overcome the aforementioned problems of the prior art and to provide a control method for an ink jet printer and an ink jet printer allowing to reliably avoid unnecessary waste of ink accompanying an ink suction head recovery process as well as ink ejection defects as a result of bubbles left in the ink path during ink priming. These objects are achieved with a method as claimed in claim 1 and a printer as claimed in claim 4. Preferred embodiments of the invention are subject-matter of the dependent claims. According to the present invention small bubbles that are formed during the ink priming process and later grow to larger bubbles can be reliably expelled from the nozzles of the print head. The post-priming process is performed a specific time interval after the ink priming process. This time interval is selected such that small bubbles, that are formed during ink priming, accumulate in offsets in the ink path where the ink path diameter changes, have grown to a certain size, and are free in the ink flow so that they can be moved in the ink path and reliably expelled from the ink nozzles. As a result, waste from ink suction accompanying a head recovery process can be prevented. The post-priming process suctions a larger volume of ink than does a normal head recovery process. Bubbles that remain in the ink path after ink priming can thus be reliably expelled from the ink nozzles and prevented from stopping in the pressure chamber. Printing defects attributable to a head recovery process can therefore be avoided. Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description of preferred embodiments taken in conjunction with the accompanying drawings. Fig. 1 is a perspective view of an ink jet printer embodying the present invention; Fig. 2 is a perspective view showing major components of the ink jet printer shown in Fig. 1; Fig. 3 is a perspective view of the ink supply mechanism in the ink jet printer shown in Fig. 1; Fig. 4(A) is a sectional view along line X-X in Fig. 3, Fig. 4(B) is a sectional view along line Y-Y in Fig. 3; Fig. 5 is a flow chart of an ink suction control process in the ink jet printer shown in Fig. 1; Fig. 6 is a flow chart of an alternative ink suction control process in the ink jet printer shown in Fig. 1; Fig. 7 is a flow chart of the ink suction process shown in Fig. 6 forced by a head recovery process command; Fig. 8 is a flow chart of the ink suction process shown in Fig. 6 initiated by the passage of a specific time period; Fig. 9 is a flow chart of the ink suction process shown in Fig. 6 performed immediately after the printer power is turned on; Fig. 10 is a block diagram of the present invention; Fig. 11 is a flow chart used to describe a further alternative embodiment of the present invention; and Fig. 12 illustrates occlusion(block) of the ink path by a bubble. Overall configuration Fig. 1 is a perspective view of an ink jet printer according to a preferred embodiment of the invention. Fig. 2 is another perspective view showing the major internal components of the same printer but viewed from the side opposite to that of Fig. 1. As shown in these figures, printer 1 comprises an ink jet print head 3 with nozzles 31 for ejecting ink droplets, a carriage 2 on which print head 3 is mounted, a carriage moving mechanism 4 for reciprocating carriage 2 in a main scanning direction indicated by an arrow A, and an ink supply mechanism 10 (see Fig. 3) for supplying ink to print head 3. Print head 3 has a rectangular nozzle surface 32 which is exposed through a rectangular opening formed in carriage 2. Two rows of nozzles, each having a plurality of the nozzles 31 arrayed in a line, are formed in this nozzle surface 32. As shown in Fig. 2, carriage moving mechanism 4 comprises a guide rail 45 extending along the main scanning direction A, a timing belt 41 engaging a drive pulley 43 and a follower pulley 44, and a carriage motor 42 for rotationally driving drive pulley 43. The bottom part of carriage 2 is slidably supported on guide rail 45, and is linked to timing belt 41. Driving carriage motor 42 therefore causes carriage 2 to move in the main scanning direction along guide rail 45. A recording medium 14 is transported in a subscanning direction, perpendicular to the main scanning direction A, past a position defined by the range of motion of the nozzle surface 32. Printing on the surface of recording medium 14 can, thus, be effected by ejecting ink droplets from nozzles 31 onto the surface of recording medium 14 while moving carriage 2 in the main scanning direction over the surface of recording medium 14 and feeding the recording medium in the subscanning direction. For printing, carriage 2 and, thus, print head 3 is reciprocated within a printing area denoted as B in Fig. 2. Outside of this printing area B and to the left of it as viewed in Fig. 2, is the print head's home position HP. When in its home position, the print head is positioned just opposite a head maintenance unit 5 described in more detail further below. Ink supply mechanism As shown in Fig. 1 and Fig. 2, ink supply mechanism 10 comprises an ink cartridge 7, which is installable to and removable from an ink cartridge holder 13 accommodated in a main housing 12 of printer 1; a pressure compensator 9 mounted on carriage 2; and an ink supply tube 8 connecting ink cartridge 7 to pressure compensator 9. As shown in Fig. 2, Fig. 3, and Fig. 4(A), ink cartridge 7 comprises a flat, box-like rigid case 71, and a flexible ink tank 72. Ink tank 72 is housed inside case 71 and is filled with ink. Ink tank 72 has an ink outlet 73, which projects to the outside of case 71. Note that reference sign 11 in the figures denotes the total ink path connecting the nozzles 31 to ink tank 72. Ink supply tube 8 comprises a tube 81, and a needle 82 attached to one end of tube 81. The other end of tube 81 is connected to pressure compensator 9. Needle 82 is inserted into and removed from outlet 73 of ink cartridge 7 as ink cartridge 7 is installed in and removed from printer 1, respectively. As shown in Fig. 3 and Fig. 4, pressure compensator 9 comprises a flat, cup-shaped, rigid case 91 with a substantially octagonal shape in section as shown in Fig. 3; a soft film 92 attached to case 91 so as to close the open part of case 91; and a leaf spring 93 affixed to the inside surface of soft film 92. An ink pressure attenuation or compensation chamber 90 is thus formed between case 91 and soft film 92. An ink inlet 94 and an ink outlet 95 are formed in case 91. One end of tube 81 is connected to ink inlet 94; ink outlet 95 communicates with one end of an ink outflow channel 96 formed in case 91. The other end of ink outflow channel 96 is a large diameter print head connector 97. Print head 3 comprises an ink inlet tube 34 and an ink chamber 33 for holding ink introduced through ink inlet tube 34. An end of ink inlet tube 34 is inserted into and held sealed in print head connector 97. Ink held in the ink chamber 33 is subsequently ejected from nozzles 31. As shown in Fig. 4(B), a specific quantity of ink 30 is held inside chamber 90. As the pressure inside chamber 90 increases and decreases, soft film 92 deforms flexibly to the outside or inside, thus changing the volume of the chamber 90. This displacement of soft film 92 holds the internal pressure of chamber 90 constant, i.e., it compensates for any pressure fluctuations inside the ink supply system. As a result, the ink supply pressure of ink supplied from ink outlet 95 to print head 3 remains constant even when the ink pressure at ink inlet 94 changes. Ink suction mechanism As mentioned before and shown in Fig. 2, head maintenance unit 5 is disposed opposite to the print head's home position and comprises an ink suction mechanism 6 for sucking high viscosity ink and residual bubbles off the nozzles 31 of print head 3. The mechanism 6 has a nozzle cap 62 for covering nozzle surface 32 of print head 3 when print head 3 is located at its home position. Either in response to the carriage movement or by means of a separate motor 52 attached to a casing 50 of head maintenance unit 5, nozzle cap 62 can be moved between a retracted position where it is retracted into the casing 50 and a capping position where it projects from the casing 50 toward print head 3. Nozzle cap 62 is in its retracted position when print head 3 is within the printing area B. When print head 3 reaches its home position, nozzle cap 62 is moved to its capping position so as to cover the nozzle surface 32 of print head 3 as indicated by phantom lines in Fig. 4(A). The pressure inside the cavity thus formed and sealed by nozzle surface 32 and nozzle cap 62 is then lowered by means of a pump 61, which is driven by motor 52. Ink can therefore be suctioned out from the nozzles 31 by operating the pump 61 while the nozzle cap 62 covers the nozzle surface 32. Control of the ink suction mechanism As shown in Fig. 2, printer 1 also includes a drive control unit 63 for performing the overall control of the printer including control of ink suction mechanism 6. Fig. 10 is a functional block diagram of printer 1 illustrating drive control unit 63 and mechanisms controlled by it in more detail (note that Fig. 10 corresponds to the third embodiment using timer 106 rather than timers 66 and 67). The drive control unit 63 comprises a microcomputer 103, which runs a control program stored in a ROM (not shown) to control recording medium transportation (not shown in Fig. 10), printing by the print head, and the ink suction operation of the ink suction mechanism 6. Both a switch 64 for manually initiating head recovery, and host computer 65 are connected to drive control unit 63. Ink suction mechanism 6 can be operated to perform a forced head recovery either by operating switch 64 or in response to an appropriate command from the host computer 65. The drive control unit 63 further comprises a first timer 66 and a second timer 67; both timers can be implemented either by hardware or software. The first timer 66 counts the time elapsed after the last ink priming process, which is described in further detail below. The second timer 67 counts the time elapsed after the last ink suction process performed by ink suction mechanism 6. The drive control unit 63 may also have a counter Co for counting the number of head recovery processes (which are described in detail below) performed. An ink cartridge detector 101 detects whether or not an ink cartridge 7 is installed in printer 1. An ink end detector 102 detects whether any ink is left in ink tank 72. CPU 103 controls pump 61, and receives the signals output from ink cartridge detector 101 and ink end detector 102. When ink cartridge detector 101 is unable to detect an ink cartridge 7 being installed, a "no ink cartridge" warning is issued . When ink end detector 102 determines that there is no ink or not sufficient ink in ink tank 72, a "no ink" warning is issued. Various methods can be used for issuing these two warnings, including, for example, turning an LED on or flashing an LED, or sounding an audible alarm. When CPU 103 receives print data from host computer 65 through an interface 104, it drives print head 3 using a print buffer 105 to print the received data. An EEPROM 108 is provided for storing a priming flag Fl and a post-priming flag SCL. Another type of nonvolatile memory could be used for storing these flags. When CPU 103 detects that switch 64 is on or a corresponding command has been received from the host computer 65, it performs a forced ink suction process by sucking a certain volume of ink off the nozzles. In this and other ink suction processes described later, the desired ink suction volume to be sucked off by means of pump 61 is achieved by controlling the number of steps through which step motor 52 for driving pump 61 is turned to control the amount of ink suctioned by pump 61. Alternatively to controlling the number of steps of motor 52, CPU 103 can control the amount of sucked ink based on the time period pump 61 is driven as counted by a timer (not shown). A power switch 109 for controlling the on/off state of the main power supply to printer 1 is also provided. CPU 103 also controls the location of print head 3 (carriage 2). It can thus detect whether print head 3 is in its home position HP, i.e., at the position of nozzle cap 62, and if it is not, move print head 3 to that position if necessary. First embodiment of ink suction control Fig. 5 is a flow chart of the operation of the ink jet printer 1, showing primarily the ink suction control. In this first embodiment, ink suction processes performed by means of ink suction mechanism 6 include: (1) an ink priming process (ink suction process L) which is performed as required; (2) a post-priming process (ink suction process S) which is performed a predetermined first time interval T0 after the ink priming process; during this post-priming process ink is suctioned with a force greater than that during a normal head recovery process; (3) a first head recovery process (ink suction process A) which is performed every time a predetermined second time interval T1 has passed since the last ink suction process was performed; a small amount of ink is suctioned during this process; and (4) a second head recovery process (ink suction process M) which is performed when switch 64 is operated or a corresponding command received from the host computer 65 to force head recovery when desired . The timing of these processes is described in detail below with reference to the flow chart in Fig. 5. When the printer is turned on (step ST1), it is detected in step ST2 whether ink priming process L has been completed by reading the state of the priming flag Fl. If flag Fl is reset (OFF), ink priming process L is determined to have not been completed, and is, therefore, performed using ink suction mechanism 6 (step ST3). The ink suction volume V0 of this ink priming process L is set to 15 cm 3 in this exemplary embodiment. After ink priming process L is completed, flag Fl is set (ON) (step ST4), and the first timer 66 begins counting the time Tp elapsed since ink priming (step ST5). Next, it is detected in step ST8 whether first time interval T0 has elapsed by reading the first timer 66 (Tp ≥ T0). Until a print command is received in step ST9 or time interval T0 has elapsed in step ST8, the process loops through steps ST8 and ST9. If a print command is detected in step ST9 the process advances to step ST10 to execute the printing process. Once the printing process is completed, control loops back to step ST8, and the loop ST8 - ST9 continues. In this preferred embodiment of the invention time interval T0 is set to one hour. As a result, the loop through steps ST8 - ST9 - (ST10) - ST8 continues for one hour after the ink priming process L is completed. If the printer is turned off while this loop is being executed and is then turned on again, step ST2 detects that flag Fl is set, and then branches to step ST6, which determines whether time interval T0 has elapsed since the ink priming process L (Tp ≥ T0). There are various possibilities how this determination may be performed. One way (not shown in the figure) is by setting the flag SCL after step ST11 and checking that flag SCL in step ST6 rather than checking whether Tp ≥ T0. Another way is that timer 66 is made to continue counting even while the printer is turned off. If timer 66 does not continue counting, its count value may be stored immediately before the printer in turned off and the timer preset to the stored count value upon the printer being turned on again. If time interval T0 has not elapsed, the first timer 66 resumes counting (step ST7), and the loop of steps ST8 - ST9 - (ST10) - ST8 is re-entered. When time interval T0 has passed, the post-priming process S (step ST11) is performed. The ink suction volume V1 of this post-priming process S is 3 cm 3 in this exemplary embodiment. After post-priming process S is performed, the first timer 66 stops and second timer 67 starts counting the time elapsed since the post-priming as a time Te (step ST12), and step ST13 waits for a print command. When a print command is received, the printing process is performed (step ST14), and control loops back to step ST13. If no print command is received (ST13 = "N"), step ST15 determines whether the second time interval T1 has elapsed since the last ink suction process was performed (Te ≥ T1). In this case the last ink suction process was the post-priming process S. In this exemplary embodiment time interval T1 is set to ten hours (10 h). A first head recovery process A is performed when time interval T1 has passed since the last ink suction process (step ST16). At the same time, timer 67 is re-starts counting time Te. Note, that the ink suction volume V2 of this first head recovery process A is set to 0.1 cm 3 . As a result, 0.1 cm 3 of ink is suctioned to clean and recover the print head every time interval T1 after the post-priming process S is performed. If the printer is turned off and then on again while in this loop counting Te, step ST6 passes control to step ST13 because time interval T0 has elapsed since ink priming process L was completed and post-priming process S has been performed. The loop from ST13 to ST16 is, therefore, resumed. In this preferred embodiment the ink suction volume V1 of the post-priming process S is greater than the ink suction volume V2 of the first head recovery process A. Any residual bubbles in the ink path 11 can therefore be reliably expelled from the nozzles 31 by performing just a single post-priming process S. In addition, if time interval T1 is suitably set to, e.g., one hour the post-priming process S is performed after bubbles adhering to the inside walls of the ink path 11 after ink priming have separated from the walls and can, therefore, be reliably expelled from the ink path. It is therefore possible to reliably prevent printing defects arising from bubbles remaining in the ink path 11 after ink priming. Second embodiment of ink suction control Figs. 6 to 9 are flow charts of a second embodiment of the ink suction control according to the present invention. In this second embodiment, ink suction processes performed by means of ink suction mechanism 6 include: (1) the ink priming process (ink suction process L) which is performed as required; (2) the post-priming process (ink suction process S); (3) the first head recovery process (ink suction process A); (4) the second head recovery process (ink suction process M); and (5) a third head recovery process (ink suction process B). The ink priming process, the post-priming process as well as the first and the second head recovery processes may be the same as in the first embodiment. In this second embodiment, however, the post-priming process S is performed k times immediately after ink priming. k is a predefined value ( k = 3 in this exemplary embodiment). During normal operation, the first head recovery process A in which a small volume of ink is expelled is performed at a regular first time interval T1 (T1 = 10 h in this exemplary embodiment) after the last ink suction process, but if a relatively long second time interval T2 (T2 = 1 week in this exemplary embodiment) or more, has elapsed since the last ink suction process, the high volume third head recovery process B whereby a large volume of ink is expelled is performed. Other aspects of the control of this second embodiment are basically the same as those of the first embodiment illustrated in Fig. 5 and described above. Referring to Fig. 6, when the printer is turned on (step ST21), it is checked in step ST22 whether the priming flag Fl is set (ON) to detect whether the ink priming process L has been completed. If flag Fl is not set (i.e., is OFF), the ink priming process L has not been performed, and is, therefore, started (step ST23). The ink suction volume V0 of this ink priming process L is 15 cm 3 in this exemplary embodiment. When the ink priming process L ends, flag Fl is set (ON) (step ST24), and a counter Co is preset to k (step ST25). Step ST27 then checks for a print command, and passes control to the printing process (step ST28) when a print command is detected. If no print command is detected, step ST29 detects whether there is a command for a forced head recovery, i.e., whether switch 64 has been operated or a corresponding command received from the host computer 65. If no command for a forced head recovery is detected, step ST31 detects whether the time Te elapsed since the last ink suction process equals or exceeds time interval T1. This elapsed time Te is counted by the second timer 67 which is re-started every time an ink suction process has been completed. If time interval T1 has not passed (Te < T1), step ST31 loops back to step ST27, and the above-described process is repeated. If the answer at step ST29 is "Y" (yes), the post-priming process S or the second head recovery process M is executed in step ST30 as illustrated in more detail in Fig. 7. Referring to Fig. 7, step ST30 first detects whether the value of counter Co is greater than zero (ST51). If it is, the post-priming process S is performed (step ST53), the counter Co is decremented by one (step ST54), and step 30 ends. However, if the value of counter Co is 0, the second head recovery process M is performed (step ST52) and step 30 then ends. Thus, when a forced head recovery is initiated by manual operation of switch 64 or by the host computer 65, the post-priming process S is executed if it has not already been performed k times. If the post-priming process S has already been performed k times, the second head recovery process M is performed instead. It should be noted that the ink suction volume V1 of the post-priming process S is 3 cm 3 , and the ink suction volume V3 of the second head recovery process M is 1 cm 3 in this exemplary embodiment. If the answer in step ST29 is "N" (no) but time interval T1 has elapsed since the last ink suction process in step ST31 (Te ≥ T1), the post-priming process S or the first head recovery process A is executed in step ST32 as illustrated in more detail in Fig. 8. Referring to Fig. 8, step ST32 starts similar to step ST30 by evaluating the value of counter Co (step ST61). If the value of counter Co is greater than zero, the post-priming process S is performed (step ST63), the counter Co is then decremented by one (step ST64), and step ST32 ends. However, if the value of counter Co is 0 or less, the first head recovery process A is performed instead (step ST62) and step ST32 then ends. The post-priming process S is therefore performed when time interval T1 has passed after the last ink suction process unless the post-priming process S has already been performed k times. Once the post-priming process S has been performed k times, the first head recovery process A is performed. It should be noted that the ink suction volume V2 in this first head recovery process A is set to 0.1 cm 3 in this exemplary embodiment. Returning to Fig. 6, if ink priming process L has already been performed when the printer is turned on, i.e., flag Fl is found ON in step ST22, step ST26 finds the type of ink suction process, if any, to be performed next as shown by the flow chart in Fig. 9. Referring now to Fig. 9, the first step in this routine is to evaluate the value of counter Co (step ST41). If the value of counter Co is zero, the post-priming process S has already been performed k times. Step ST42 therefore determines whether the elapsed time Te since the previous ink suction process as counted by second timer 67 equals or exceeds the second time interval T2 (Te ≥ T2). If time interval T2 has not elapsed (Te < T2), step ST43 determines whether the elapsed time Te equals or exceeds time interval T1. If the elapsed time since the last ink suction process is less than T1 (Te < T1), step ST26 ends. However, if the answer in step ST42 is "Y" (Te ≥ T2), the third head recovery process B is performed (step ST45). If a week or more has passed since the last ink suction process, viscous ink or a large number of residual bubbles will be present in the ink path 11. Therefore, if the ink is not purged from the ink path 11, it may not be possible to restore the ink in the print head 3 to a normal condition. The ink suction volume V4 of this third head recovery process B is therefore 7 cm 3 in this exemplary embodiment, that is, greater than the ink suction volume V1 of the post-priming process S. Furthermore, if the elapsed time Te is less than T2 but equal to or greater than T1 (T1 ≥ Te < T2), the first head recovery process A described above is performed (step ST44). If the value of the counter Co is greater than zero in ST41, the post-priming process S has still not been performed three times. Step ST46 therefore determines whether the elapsed time Te counted by the second timer 67 equals or exceeds T2. If Te < T2, the post-priming process S is performed (step ST48), the counter Co is decremented by one (step ST49), and the procedure ends. If the elapsed time Te is equal to or greater than T2 (Te ≥ T2), however, the third head recovery process B is performed (step ST47) to suction a large volume of ink. The counter Co is then decremented by one, and the procedure returns. Any residual bubbles in the ink path can thus be reliably purged using the ink suction control of this exemplary embodiment because the post-priming process suctioning a relatively large volume of ink is repeatedly performed after ink priming. In addition, a third head recovery process is performed to suction a large volume of ink if the printer has been left without printing for a week or more since the last ink suction process. It is therefore possible to resume printing without suffering from print defects even when printing is first resumed after leaving the print head unused for an extended period of time. Third embodiment of ink suction control In this third embodiment, timer 106 is employed for measuring the elapsed time Te and power supply to the timer 106 is maintained by a backup battery 107 even when the power switch 109 is turned off and the main power supply to the ink jet printer is interrupted after ink priming. It is then possible to initiate the post-priming process once an hour has elapsed following ink priming after the power is turned on again. It is preferable to perform the post-priming process about an hour after ink priming because the fine bubbles trapped in parts of the ink path as described above tend to accumulate and combine into a number of large bubbles within an hour. Bubble size grows gradually after ink priming. After about an hour, the bubbles will grow to a size sufficient to block the ink path, thus stopping ink supply to the print head, and causing missed dots (nonfiring nozzles). See Fig. 12. It is assumed below that the ink supply tube diameter is 1.4 mm and the largest diameter at an offset in the ink path is 3.3 mm. Assuming these common parameters, bubble size will grow gradually after ink priming as shown in the following table. Bubble growth over time after ink priming Time (min) Bubble size (mm) Bubble count Bubble state 0 0.2 - 0.3 numerous in offsets; not in flow path 10 1.0 10> as above 20 1.4 2 - 3 in both offsets and flow path 30 2.0 2 as above 40 2.4 2 as above 50 2.8 2 as above 60 3.3 1 Completely occluding flow path 70 3.3 1 as above 80 3.3 1 as above As noted in the above table, the bubbles gather in offsets in the ink path while the bubble size is on the order of 0.2 mm, and are not picked up by the ink flow. Bubbles of this size therefore do not reach the nozzles or ink chamber, and therefore do not prevent ink from being ejected from the nozzles, that is, print defects due to bubbles in the ink path do not occur at this stage. Furthermore, even if some of these small bubbles do appear in the ink path, the bubbles float in the ink path and do not reach the ink chamber. However, when the bubbles coalesce into a single 3.3 mm bubble, a bubble trapped in an ink path offset protrudes into the flow path, completely blocking the entire ink supply tube. The bubble thus completely occludes the ink flow path, and prevents ink from being supplied to the nozzles. A low volume ink suction process at this time can carry the bubble from the ink path to the ink chamber. However, a normal low volume ink suction process will allow the bubble to stop in the ink chamber and will not expel the bubble from a nozzle. The bubble thus becomes trapped and cannot be expelled from the ink chamber of an ink jet head like that used according to the present invention wherein ink is ejected from the nozzles by changing the volume of a pressure chamber. A process as shown in Fig. 11 and described below is another way to prevent this. Note that the flow chart in Fig. 11 illustrates a third embodiment of the present invention. The priming flag Fl and the post-priming flag SCL are cleared (reset) before the process shown in the flow chart in Fig. 11 begins, that is, before the ink jet printer is shipped from the factory. As noted above, the timer 106 is backed up by a backup battery 107 so that the timer 106 continues operating even when the main power supply is off. The timer 106 begins a separate timer count after manual cleaning operations and normal ink suction processes. There are several known possibilities how a plurality of parallel time counts can be realized with timer 106; for instance, timer 106 may be a real-time clock which together with the CPU implements a plurality of time counters for measuring a corresponding plurality of times. According to step ST101 in Fig. 11 nothing happens until the printer is turned on. When the printer is turned on (ST101 = "Y"), the process starts and step ST102 detects whether an ink cartridge 7 is correctly installed. If not (ST102 = "N"), a "no cartridge" warning is issued (ST103). If an ink cartridge 7 is installed (ST102 = "Y"), ink end detector 102 detects whether there is any ink in the ink tank (ST104). If there is substantially no ink left in the tank (ST104 = "N"), a "no ink" warning is issued (ST105). It should be noted that steps like steps ST102 to ST105 are preferably included in the control according to the first and second embodiments even though they have not been described there. If there is ink in the ink tank (ST104 = "Y"), step ST106 determines whether the priming flag Fl is set. If the priming flag Fl is not set (ST106 = "N"), the ink priming process L is initiated (ST110). This ink priming process L in ST110 pumps just enough ink from the ink cartridge to fill the ink path to the nozzles, that is, approximately 15 cm 3 in this exemplary embodiment. Step ST111 waits for the end of this priming process L. The ink suction volume is substantially proportional to the time period during which the ink is sucked to the nozzles. The required amount of ink can therefore be suctioned to the nozzles by controlling the operating time period of pump 61 using an appropriately set timer. Because pump 61 is driven by stepping motor 52, the ink suction volume is also substantially proportional to the number of steps the stepping motor is driven. Thus, the required amount of ink can alternatively be suctioned to the nozzles by controlling the number of steps stepping motor 52 is driven. Step ST111 can therefore determine whether the priming process L is completed by monitoring the value of a preset timer (which may be one of the time counters implemented by means of timer 106), or detecting whether the number of rotary steps of the stepping motor 52 has reached a predetermined step count. When ink priming is completed (ST111 = "Y"), the priming flag Fl is set (ST112), and the timer 106 starts counting for measuring the elapsed time Te since ink priming (ST113). The timer 106 continues counting the elapsed time Te (ST115) even if the printer is turned off (ST114 = "Y") before Te reaches one hour. In this situation, when the printer is turned on again, flag SCL in step ST107 is still not set, so that the process gets via steps ST107 and ST109 to step ST116. Any print command detected in step ST109 (ST109 ="Y") will be carried out and the process then return to step ST116. In step ST116 it is checked whether one hour has passed since ink priming. If this hour has already passed while the printer was off and the printer is then turned on again, unless a print command is detected in step ST109, the process immediately proceeds to step ST117 and the post-priming process S is performed. If the hour has not passed yet the process loops through steps ST109 and ST116 until one hour has passed, upon which the post-priming process S is performed in step ST117. On the other hand, if ink priming is completed (ST111 = "Y") and an hour passes with the printer power remaining on (ST116 = "Y"), the same post-priming process S (ST117) is performed. This process suctions 3 cm 3 of ink, a volume sufficient to eject ink bubbles that have grown from the ink nozzles. The post-priming flag SCL is then set (ST118), and control returns to the normal printing process (ST119). When the printer is next turned on after it was turned off, the printer is initialized (ST102, ST104), and step ST106 detects whether flag Fl is set. If flag Fl is set (ST106 = "Y"), step ST107 detects whether flag SCL is set. As flag SCL is now set (ST107 = "Y"), control passes to the normal printing process (ST108). The ink suction volumes sucked off from the nozzles 31 in this exemplary embodiment are as follows: (1) For ink priming in the ink priming process, 15 cm 3 of ink is suctioned from the ink tank to fill the ink path to the nozzles. (2) For the post-priming process S performed one hour after ink priming process L, 3 cm 3 of ink is sucked off the nozzles. (3) The elapsed time Te from each ink suction process is counted by timer 106 even when the printer is turned off (the main power of the printer is off); when a week or more has passed since the last ink suction process, 7 cm 3 of ink is suctioned from the nozzles in ink suction process B. (4) When the elapsed time Te as counted by timer 106 is ten or more hours and less than one week, 0.1 cm 3 of ink is suctioned from the nozzles in ink suction process A. (5) When switch 64 is turned on for forced head recovery, 1 cm 3 of ink is suctioned from the nozzles in ink suction process M regardless of the current count of timer 106. The same applies when a forced head recovery is instructed by means of a corresponding command from the host computer. It should be noted that ink suction processes B, A, and M noted in (3), (4), and (5) above have no relationship to the problem of bubble formation and growth during ink priming, and the passage of bubbles from ink path offsets into the ink flow, and further details of these processes are, thus, omitted here. Generally, these processes may be performed under the conditions and control corresponding to those explained for similar ink suction processes in the context of the first and/or second embodiment of the invention. Different from the foregoing description, the post-priming process S can be alternatively performed immediately after ink priming and at regular intervals thereafter. It is to be noted that the ink suction volumes and in particular that used for the post-priming process are not limited to the aforementioned exemplary values but should be optimized according to the specific volume of the ink path to which the process is being applied. The time elapsed after ink priming shall also obviously not be limited to one hour as described in the above exemplary embodiments of the present invention, and should be optimally set according to the various parameters of the ink jet printer in which the ink priming method of the invention is applied. Benefits of the invention An ink priming method for an ink jet printer according to the present invention can thus reliably expel the large number of bubbles that are present in the ink path immediately following ink priming. As a result, printing defects resulting from such residual bubbles can be reliably avoided. Bubbles remaining in the ink path after ink priming can also be reliably expelled from the nozzles, and printing defects attributable to such residual bubbles can be reliably prevented, because a post-priming head recovery process as described above is performed after a specific time has elapsed following ink priming. Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.

Claims: An ink priming method for a print head of an ink jet printer, comprising: a) performing an ink priming process so as to fill, for the first time, an ink path connecting an ink reservoir (7, 72) with one or more nozzles (31) of the print head with the ink from the ink reservoir; and b) subsequently performing a post-priming process by sucking ink off said one or more nozzles; characterized by c) measuring the time that has elapsed after step a) and comparing the measured time with a predetermined time interval (T0), and d) performing step b) when step c) indicates said measured time is equal to or longer than said time interval (T0), wherein said time interval (T0) is selected such as to allow bubbles left in the ink path at the end of step a) to grow to a size sufficient that the bubbles can be moved and expelled from said one or more nozzles (31). The method of claim 1, wherein the ink volume sucked off in step b) is greater than that sucked off in a normal head recovery process. The method of claim 1 or 2, wherein said post-priming process is executed k times, wherein k is equal to or greater than 2. An ink jet printer comprising: a print head (3) having one or more nozzles (31) for ejecting ink droplets; an ink tank (7, 72) for storing ink; means (13) for replaceably mounting said ink tank (7, 72); means (8) for supplying ink from the ink tank (7, 72) to said one or more nozzles (31); and an ink suction unit (6) comprising a pump (61) and a nozzle capping member (62) for suctioning ink from the ink nozzle; characterized by means (63) adapted to perform the method according to any one of claims 1 to 3.

Inventors: HANAOKA YUKIHIRO, NISHIOKA ATSUSHI, YODA SATOSHI, HANAOKA, YUKIHIRO, NISHIOKA, ATSUSHI, YODA, SATOSHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1999
Main classification: B41J2
IPC codes: B41J2