Ink jet printer and method of controlling it

Abstract: An ink jet printer for efficiently eliminating temporary ink ejection defects following print interruptions, and a control method for said ink jet printer, are disclosed. The idle time, i.e., the time lapsed between the previous current apply time and the present current apply time, is evaluated before printing. If the idle time is less than a first reference time (t1), an purging is accomplished with 0 or a first number of shots. If the time interval is between first and second reference times ( t1 = < IT < t2 ), purging is executed with a second number of shots. If the time interval is greater than or equal to the second reference time ( t2 = < IT ), purging is executed with a third number of shots that is less than the specified second number of shots.

Description: The invention relates to an ink jet printer and, more particularly, to an ink jet printer that executes a nozzle purging or refresh operation to prevent nozzle clogging. The invention also relates to a method of controlling the printer. 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 refresh operation. In a purging operation ink is ejected from the nozzles for the sole purpose of removing high viscosity ink and clear the nozzles. Usually a single purging operation involves a plurality of ink ejections (referred to as "shots" hereinafter) from each nozzle of the print head. Purging consumes ink which is wasted ink in the sense that it is not utilized for printing. There is a certain trade-off between an effective prevention of nozzle clogging on the one hand and an effective use of the ink, i.e., minimum amount of waste ink, on the other hand. The prior art has taken various approaches to find a good compromise between these two contradicting goals. According to EP-A-0 634 272, for instance, a counter is used for counting the idle time, i.e., the time period throughout which no ink is ejected from the nozzles. The counter is reset whenever printing is performed and after a purging operation. Whenever the idle time counted by the counter exceeds a predetermined reference time interval a purging operation is performed so as to keep the nozzles operative. EP-A-0 559 122 discloses a similar teaching additionally explaining that the reference time interval is fixed on the basis of a characteristic curve representing the relationship between the number of shots required for purging and the idle time obtained from experiments. While this characteristic curve depends on various factors like the structure of the print head, the composition of the ink and environmental conditions such as temperature and humidity it generally exhibits a linear relationship within a range of the idle time from 0 to about 6 h. Beyond 6 h there is a saturation with no further increase of the required number of shots. By selecting the reference time interval and determining the number of shots for purging in advance in accordance with the characteristic curve this prior art intends to avoid more ink than necessary being used for purging. Thus, while this prior art takes various factors into account for predetermining the number of shots for purging, the idle time is compared only with one reference time interval. As long as the idle time is shorter than the reference time interval no purging is executed while when the idle time equals or exceeds the reference time interval purging is performed always with the same number of shots. JP-A-1281950/89 discloses reducing the amount of ink required for purging by adaptively selecting the number of shots applied for purging in response to two factors, a time factor and a utilization factor. The time factor represents the elapsed time counted from the end of a predetermined time interval after the last printing operation until the time of the purging operation. The utilization factor represents the number of characters printed during the last printing operation. In this prior art the number of shots in a purging operation increases in proportion to idle time of the print head without considering characteristics of a particular ink etc.. EP-A- 0 442 438 discloses an ink jet recording apparatus having an ink jet head containing a plurality of nozzles for ejecting ink droplets onto a recording medium for printing, and control means responsive to a print command for driving the print head to perform said printing. The apparatus further comprises purging control means for purging the nozzles prior to printing by effecting a certain number of shots of ink ejection from the nozzles when the nozzles have been continuously idle for a certain time. The purging control means comprises a counter for measuring the idle time, resetting means for resetting the counter, storage means for storing a first reference time of six hours and a second reference time of three days. The counter is started to count when the main power source of the apparatus is turned off. In response to this main power source being turned on again, the purging control means compares the idle time counted by the counter during the apparatus was turned off with each of the reference times and to execute a purging operation by effecting a first number of shots when the idle time is smaller than the first reference time, a second number of shots when the idle time is between the first reference time and the second time or a third number of shots when the idle time is greater than the second reference time. In response to the purging operation having been completed, the counter is reset. Thus, the prior art purging is based on the assumed linear relationship between the number of shots required for purging and the idle time. The practice shows, however, that this prior art, while providing improvements, still suffers from ejection defects and does not ensure an optimal ink purging volume. So, the problem remains that either there is a relatively high amount of wasted ink or purging is insufficient resulting in well known problems as regards the printing quality. The object of the present invention is to provide an ink jet printer and a method of controlling it in which no ejection defects or less ejection defects compared to the prior art occur while at the same time the ink consumption for purging can be further decreased. This object is achieved with an ink jet printer as claimed in claim 1 and a method as claimed in claim 6. Preferred embodiments of the inventions are subject-matter of the dependent claims. The prior art is concerned with nozzle clogging resulting from high viscosity or dried ink. Accordingly, the prior art provides for purging only in case of relatively long idle times selecting the number of shots based on an assumed linear relationship between the number of shots and the idle time as explained in EP-A-0 559 122. More detailed studies on the relationship between the number of shots and the idle time of the print head surprisingly showed, however, that this relationship has a highly non-linear short term range calling for purging even after idle times in the range of seconds. According to the result of these studies, the short term range of the relationship is as represented by the characteristic curve shown in FIG. 2. In FIG. 2, the abscissa represents the idle time on a logarithmic scale and the ordinate represents the number of shots. Note that the term "number of shots" as used in this text means the number of shots required for effectively and sufficiently purge the nozzles while "idle time" refers to the time elapsed since the last ink ejection from the nozzles, i.e., the last printing operation or purging operation. FIG. 2 shows an initial sharp increase of the number of shots after an idle time as short as approximately 5 s (t 1 ), a maximum a few seconds later and a subsequent decrease to a minimum at about 10 min (t 2 ). Following the minimum, the number of shots gradually increases again. While various factors contribute to the initial sudden rise of the number of shots, tests using various ink compositions suggest that one such factor is a type of surfactant (surface active agent) used in the ink. More specifically, it is assumed that vibration of the ink causes the surfactant to collect at the nozzle tip after ejection and thereby to affect the nozzle function. While this phenomenon may not be what is conventionally understood by "nozzle clogging", since the effect is substantially the same it is included in "nozzle clogging" as the term is used in this text. The present invention has been conceived to optimize the purging operation taking into account the results of the studies explained above. More particularly, the idle times at which the number of shots changes are stored as reference times (time intervals). The optimum number of shots for each reference time is also stored. When a print command is received the actual idle time is compared with these reference times and, depending on the result of comparison, the corresponding one of the stored numbers of shots is selected. In this manner, not only can purging be effected when it is necessary to avoid ejection defects, even within the short term range of the idle time, but also can the number of shots be optimized in accordance with actual requirements. This ensures high quality printing with a minimum of wasted ink and, additionally, contributes to a high overall printing speed. When purging is performed in response to a print command but before the printing operation is commenced, the time required for the purging operation presents a delay time degrading the printing speed. The higher the number of shots the longer is the delay time, Thus, purging with a number of shots is no higher than required to achieve the desired result minimizes not only the amount of waste ink but also the delay time. The invention will be explained in more detail below with reference to the drawings illustrating preferred embodiments of the invention only. FIG. 1 is a block diagram of a control apparatus of an ink jet printer according to an embodiment of the invention. FIG. 2 is a graph showing the relationship between the idle time and the optimum number of shots for purging. FIG. 3 is a flow chart illustrating a first embodiment of a control method according to the invention. FIG. 4 is a simplified illustration of an ink jet printer. FIG. 5 is a flow chart illustrating a second embodiment of a control method according to the invention. FIG. 4 is a simplified illustration of known a serial ink jet printer comprising an on-demand type ink jet print head (see EP-A-0 634 272, FIG. 10) to which the present invention may be applied. Referring to FIG. 4, ink is supplied to print head 10 by means of tube 306 carrying ink from ink tank 301. A cap 304 is provided for covering the nozzles and collecting the ink ejected from the nozzles during the purging operation. This waste ink is pumped by pump 303 from cap 304 through tube 308 to waste ink tank 305. Note that instead of using tube 308, pump 303 and waste ink tank 305 a piece of felt or other ink absorbing member may be provided in cap 304 or immediately next to it to absorb the waste ink. Also shown in FIG. 4 are platen 300, carriage 302 on which print head 10 is mounted, and recording medium 105. Since the way in which printing and purging is performed with such serial ink jet printer is generally known further details will be omitted here. The printer shown in FIG. 4 has cap 304 for capping the print head and protecting the nozzles against nozzle clogging in case of a long-term non-use. Generally, on-demand type ink jet printers are available in two types, one with such a cap and another one without. As will be explained below the present invention is applicable to both types. Even printers having no cap for capping the print head have some means for collecting the ink ejected during the purging operation. Each time purging is to be performed the print head is moved to a purging position, i.e., the position of the cap or other ink collection means. FIG. 1 is a block diagram of a control apparatus of an printer according to a preferred embodiment of the invention. Referring to FIG. 1, printing mechanism 203 of the printer is represented by drive motor 202 and print head 10. Motor 202 is used for achieving a two-dimensional relative motion between print head 10 and recording medium 105, i.e., for moving print head and advancing recording medium 105; motor 202 is also used to cover or uncover the nozzles with cap 304. The control apparatus for controlling the printing mechanism comprises main controller 210, input means 207, storage means 211, motor controller 214, head controller 213, counter 204 and purge controller 206. Main controller 210 performs the overall control including controlling - via motor controller 214 and head controller 213 - of normal printing operations based on print data input via input means 207 from, e.g., a host computer. Main controller 210 and storage means 211 may be implemented as a microcomputer comprising a CPU, a RAM and a ROM and related peripheral circuit devices. In such case a built-in timer of the CPU may be used as counter 204. Since the control of normal printing operations may be conventional it will not be described in detail here. The purging operation according to the present invention will now be described in detail. As mentioned before, a detailed study of the optimum number shots for purging revealed a relationship between the number of shots and the idle time as illustrated by the characteristic curve in FIG. 2. According to this relationship, purging should be effected with a relatively high number of shots after short idle times of less than 1 min. Coagulation of the surfactant is absorbed and dispersed in the ink within approximately 7 - 10 minutes after which period the ink characteristics are equalized, so that when the idle time exceeds this particular duration, the number of shots can be reduced to between 1/2 and 1/3 the peak number. The present invention takes benefit from this characteristic. Specifically, the times at which the number of required ink purging operations changes are prestored as reference times in the storage means of the printer. The optimum number of shots for each reference time is also stored. Each time a print command is received instructing a printing operation to be performed, the idle time measured by means of counter 204 is compared with these reference times to determine to which part of the curve in FIG. 2 the idle time corresponds. Based on the result the ink purging operation is executed with the optimum number of shots for greatest efficiency. Counter 204 measures, as the idle time, the interval time from the last time current was applied to the print head to the moment current is again to be applied. Storage means 211 stores at least a first reference time and a second reference time the second reference time being longer than the first one. Time evaluation means which may be included in main controller 210 is provided for determining whether the idle time is within the first reference time, between the first reference time and the second reference time or greater than the second reference time and for selecting the appropriate number of shots. Purging controller 206 is responsive to the time evaluation means for performing purging, if required, with the number of shots selected by the time evaluation means. Assuming two reference times are prestored, the control is as follows: (1) if the idle time is less than the first reference time, purging is performed with a first predetermined number of shots (or is not performed, i.e., the first predetermined number may be zero); (2) if the idle time is between the first reference time and the second reference time, purging is performed with a second predetermined number of shots; and (3) if the idle time is greater than the second reference time, purging is performed with a third predetermined number of shots that is smaller than the second number. If purging is to be executed, purging controller 206 controls drive motor 202 via motor controller 214 to move print head 10 to the purging position and then applies a corresponding control signal to head controller 213. After purging or, if no purging is required, directly in response to a print command, main controller 210 applies print control signals to head driver 213 and motor driver 214 to perform printing in the usual manner. It should be noted that the printer of the present invention does not necessarily require a cap for covering the nozzles. However, when such a cap is provided, a capping controller can be included in main controller 210 to move the print head to a position in front of the cap and thereby cap the print head (nozzles) when the idle time reaches the second or a further reference time. FIG. 3 is a flow illustrating a first embodiment of the control method according to the invention. In this embodiment three reference times corresponding to t 1 , t 2 and t 3 in FIG. 2 are prestored. When one printing process (symbolized by step S10 and referred to as the "previous" process below) is completed, the counter 204 is reset (step S12) and starts measuring the idle time. Preferably, main controller 210 is structured (or software controlled) such that it counts separately for each nozzle the number of ink ejections made by the individual nozzle of print head 10. Depending on the pattern to be printed it may happen that one or some of the nozzles are either not used at all during a particular printing process or are used for a few ejections only. In other words, while the term idle time is used here as applying to all nozzles, the actual idle times may be different for each nozzle. In the present embodiment this is accounted for by resetting counter 204 in step S12 only if there is no nozzle that was not used in the previous printing process or was not used for a predetermined minimum number of ink ejections. The process then waits (step S14) for the next print command. When a print command is issued, whether or not purging is performed and, if it is performed, how many shots are used is determined based on the comparison between the idle time IT counted by counter 204 and the reference times in the following way: Condition (1): IT < t 1 , if at step S16 the idle time IT is less than the first reference time t 1 , no purging is executed (step S20) and the process proceeds to perform the printing process (step S30). Condition (2): t 1 = < IT < t 2 , if the idle time IT is between the first and the second reference time t 1 and t 2 , purging is executed with a first number of shots, i.e., 20 shots in this example (steps S18, S22). Condition (3): t 2 = < IT < t 3 , if the idle time IT is between the second and the third reference time t 2 and t 3 , purging is executed with a second number of shots, i.e., 10 shots in this example (steps S24, S26). Condition (4): t 3 = < IT, if the idle time IT is greater than or equal to the third reference time t 3 , purging is executed with a third number of shots, i.e., 30 shots in this example (steps S24, S28). As will be understood by those skilled in the art there are alternative ways of defining the conditions of (1) to (4) in that condition (2) may be any of t 1 = < IT < t 2 , t 1 < IT < t 2 , t 1 < IT = < t 2 and t 1 = < IT = < t 2 and this applies mutatis mutandis to condition (3) with conditions (1) and (4) respectively adapted.ln another alternative, step S20 may include purging with a minimum number of shots, e.g., 5 shots. Reference times t 1 , t 2 , and t 3 as well as the respective numbers of shots vary according to the head size, ink composition and nozzle shape. Their optimum values are therefore experimentally determined from, for example, a graph such as shown in FIG. 2 and prestored with the printer control program to the ROM or other storage means. The present embodiment can be applied to a printer having no nozzle cap. Even if such printers use a slow drying ink, the ink will still gradually dry therefore leading again to clogging. This tendency can also be read from the graph in FIG. 2 and can be accounted for by executing purging with an appropriate number of shots when nothing has been printed for an extended period of time, e.g., after an idle time of 4 - 5 h. The appropriate number of shots may be the same as that used for purging each time the printer is turned on. The third reference time is set for these cases, and when the idle time exceeds this third reference time, purging is accomplished with the third number of shots greater than the second number of shots. It goes without saying that even more than three reference times may be prestored and the control process correspondingly adapted if it is desired to adapt the purging operation more closely, i.e., in finer steps to the characteristic relationship between the number of shots and the idle time. FIG. 5 is a flow chart illustrating a second embodiment of the control method according to the invention. This embodiment is particularly suitable for a printer equipped with a cap. Note that like steps are identified by the same step number in FIG. 3 and FIG. 5, and further description thereof is omitted below. The control process waits at steps S14, S32 for the next print command. When a print command is issued, a purging is or is not executed (step S16, S20, S22) depending on the idle time, and the printing process is then executed in step S30 as in the flow chart shown in FIG. 3. However, if the idle time becomes greater than or equal to reference time t 2 (step S32), the capping controller (not shown, and assumed here to be part of main controller 210) causes the head to be transported to the capping position, and the cap is then driven to cap the nozzles (step S34). After the nozzles are capped, idle time IT counting is interrupted (step S36), and the control process waits for the next print command (step S38). If a print command is issued after the nozzles are capped, the cap is removed from the print head, purging with a particular number of shots, 10 in this example, is accomplished (step S26), and the print head is moved to the printing position and the printing process is then executed (step S30). While the present embodiment is applied to a printer comprising a cap for covering the nozzles, frequently capping the nozzles can result in reduced throughput (printing speed). Furthermore, the increase in ink viscosity at the nozzle tip caused by a sudden temporary coagulation of the surfactant as described above cannot be prevented by capping the nozzles, and until the second reference time capping the nozzles is not useful. In other words, it is more efficient during this time to maintain the print head in a print-ready state rather than moving it to the capping position, and maintaining this print-ready state can improve the actual printing speed (throughput). Applying the above control method to cap the nozzles only after a predetermined idle time is, thus, more efficient than capping the nozzles after each line. The ink viscosity rise and ink solidification are significantly delayed by capping the print head, and capping can thus be used to reduce the number of ink purging operations required when compared with not capping the print head. While this second embodiment has been exemplified with reference to two reference times t 1 and t 2 , it will be appreciated that more than two such reference times may be prestored. For instance, reference times t 1 and t 2 may be determined and used in substantially the same way as in the first embodiment whereas reference time t 3 may be used as the reference time in step S32. Also, reference times t 1 , t 2 and t 3 may be determined and used in substantially the same way as in the first embodiment whereas a fourth reference time may be used as the reference time in step S32. In this embodiment 10 shots are uniformly executed for purging when a print command is issued after capping the print head and interrupting the idle time count. It is also possible, however, to count the time the print head is capped and then once a print command is received determine the number of shots required for purging based on the elapsed time. It is possible, for example, to apply 10 shots if this capped state continues for less than 5 hours, 20 shots if the capped state continues for a time interval between 5 and 10 hours, and 30 shots if the capped state continues for more than 10 hours. This control method is suited to facsimile machines and similar devices that may be left for extended periods of time with the power on because a waste-free nozzle purging process considering changes in the ink at the nozzle tip over longer periods of time can be accomplished.

Claims: An ink jet printer having an ink jet print head (10) containing one or more nozzles for ejecting ink droplets onto a recording medium for printing, and control means (204, 206, 207, 210, 211, 213) responsive to a print command for driving the print head (10) to perform said printing and for purging said nozzles prior to printing by effecting a certain number of shots of ink ejection from said nozzles when the nozzles have been continuously idle for a certain time, said control means comprising: counting means (204) for measuring the idle time (IT), resetting means (210) for resetting the counting means in response to said printing, storage means (211) for storing a reference time (t 1 , t 2 , t 3 ), and purging means (206, 210) responsive to the print command for comparing the idle time (IT) as measured by said counting means (204) with said reference time (t 1 , t 2 , t 3 ) and for executing said purging when the idle time exceeds the reference time, characterized in that said storage means (211) is adapted to store a first reference time (t 1 ) and a second reference time (t 2 ) the second reference time being longer than the first reference time, and said purging means (206, 210) is adapted to compare said idle time (IT) with each of said reference times and to execute said purging operation by effecting a first number of shots when the idle time is smaller than the first reference time (t 1 ), said first number including zero, a second number of shots when the idle time is between the first reference time (t 1 ) and the second reference time (t 2 ), or a third number of shots when the idle time is greater than the second reference time (t 2 ), the third number being smaller than the second number. The printer according to claim 1 having plural nozzles and further comprising ejection counting means (210) for counting separately for each nozzle the number of ink ejections during printing, wherein said resetting means is responsive to the ejection counting means for resetting said counter (204) in response to said printing only if for all of the nozzles the value counted by the ejection counting means is equal to or greater than a predetermined value. The printer according to claim 1 or 2 wherein the third number of shots is smaller than or equal to one-half the second number of shots. The printer according to claim 1, 2 or 3 wherein said storage means (211) is adapted to store in addition to said first and second reference times (t 1 , t 2 ) a third reference time (t 3 ) longer than the second reference time, and said purging means (206, 210) is adapted to execute said purging by effecting a number of shots equal to or greater than said third number of shots when the idle time is greater than or equal to the third reference time. The printer according to claim 1 further comprising a capping controller for covering the nozzles with a cap (304) when the idle time is greater than or equal to said second reference time (t 2 ). A method of controlling an ink jet printer having an ink jet head (10) with one or more nozzles and adapted to perform, in response to a print command, a printing operation by ejecting ink droplets from the nozzles onto a recording medium (105), the method comprising: (a) measuring the idle time (IT) during which no ink is ejected from the nozzles, and resetting the idle time to zero each time ink is ejected from the nozzles, (b) comparing the idle time with a reference time (t 1 , t 2 , t 3 ) in response to a print command, (c) purging the nozzles by effecting a certain number of shots of ink ejection from the nozzles, when the idle time exceeds the reference time, and (d) performing the printing operation after step (c), characterized in that step (b) comprises comparing the idle time (IT) with a first reference time (t 1 ) and, if the idle time exceeds the first reference time, comparing the idle time with a second reference time (t 2 ) greater than said first reference time, and step (c) comprises purging the nozzles by effecting (c1) a first number of shots when the idle time is smaller than the first reference time (t 1 ), said first number including zero, (c2) a second number of shots when the idle time is between the first reference time (t 1 ) and the second reference time (t 2 ), or (c3) a third number of shots when the idle time is greater than the second reference time (t 2 ), the third number being smaller than the second number. The method according to claim 6 wherein step (b) further comprises comparing the idle time (IT) with a third reference time (t 3 ) greater than said second reference time if the idle time exceeds said second reference time (t 2 ), and step (c3) comprises purging the nozzles by effecting a fourth number of shots when the idle time is greater than the third reference time (t 3 ). The method according to claim 6 wherein step (b) comprises comparing the idle time (IT) with said first reference time (t 1 ) in response to a print command and comparing the idle time with said second reference time (t 2 ) without waiting for a print command and capping the nozzles when the idle time is greater than said second reference time (t 2 ), and step (c3) is performed in response to a print command received after said capping. The method according to claim 6 or 7 further comprising the steps of (e) comparing the idle time with a further reference time greater than said second (t 2 ) or third (t 3 ) reference time, respectively, without waiting for a print command, (f) capping the nozzles when the idle time is greater than the further reference time, and (g) purging the nozzles by effecting a further number of shots in response to a print command received after said capping. The method according to claim 8 or 9 wherein said measuring of the idle time (IT) is interrupted while the nozzles are capped. The method according to any one of claims 6 to 10 for a printer having plural nozzles, further comprising the step of (h) counting separately for each nozzle the number of ink ejections during printing, wherein step (a) comprises resetting the idle time (IT) to zero only if for all of the nozzles the value counted in step (h) is equal to or greater than a predetermined value. The method according to claim 11 wherein said predetermined value is less than or equal to said second number. The method according to any one of claims 6 to 12 wherein said third number of shots is equal to or less than one half said second number of shots. The method according to any one of claims 6 to 13 wherein said first reference time is in the range of a few seconds to 1 min.

Inventors: MINOWA MASAHIRO, NAKAZAWA CHIYOSHIGE, MINOWA, MASAHIRO, NAKAZAWA, CHIYOSHIGE
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1997
Main classification: B41J2
IPC codes: B41J2