a conveyer arranged to convey print media sheets one after another in a conveying direction;

a print head disposed at the conveyer and having an array of dot printing elements, the array extending in a direction Z normal to the conveying direction, the print head being arranged to print images onto the media sheets in accordance with actuation instructions sent to the print head; and

a maintenance manager adapted to analyze the actuation instructions and to decide on a maintenance operation for the print head on the basis of the actuation instructions and

- a controller adapted to convert print instructions that specify images to be printed onto the media sheets into the actuation instructions for the print head,

wherein the conveyer is arranged to convey sheets with different formats which differ in their dimension in the direction Z, and the maintenance manager is adapted to decide on the maintenance operation, whenever the format of the sheets to be conveyed to the print head changes from a narrower format to a wider format with a dimension in Z direction larger than that of the narrower format, on the basis of actuation instructions for printing elements located within the width range of the wider format but outside of the width range of the narrower format. Printers of this type are disclosed in US 2007/132799 A1 and US 2009/122106 A1 .

US 7 036 897 B2 discloses a printer, more specifically an ink jet printer in which the printing elements are formed by nozzles and actuators associated therewith for expelling ink droplets from the nozzles. When the nozzles are not active, the ink in the nozzle orifices starts to dry out, so that the nozzle orifice is contaminated and eventually clogged with dried ink, which disturbs the drop generation process and may eventually lead to a complete failure of the nozzle.

In the known printer, a maintenance operation is performed at the start of a new print job. The maintenance operation consists in activating the nozzle for several hundreds of times so as to purge the nozzle orifice with liquid ink, thereby to remove any contaminants and to clear the nozzle orifice. Conceivably, when this maintenance operation would be performed for all the nozzles of the print head, a considerable amount of ink would be wasted for the maintenance operation. For this reason, it is proposed in the cited document to perform the maintenance operation only for those nozzles that will actually be needed for printing in the print job.

On the other hand, printers, especially ink jet printers are known which are capable of processing print media sheets in different formats in random succession. As long as the format of the sheets does not change, it is possible to control the print head such that all nozzles that are located within the width range of the sheets are activated so frequently that the time in which an individual nozzle has been idle, the so-called nozzle open time, is kept below a certain limit which, when exceeded, would necessitate a maintenance operation. However, when the format of the sheet changes from a narrow format with a certain dimension in the direction Z to a wider format, in which the dimension of the sheet in the direction Z is larger, then the print process may involve nozzles that have not been actuated for a considerable time period, larger than the admissible nozzle open time, because these nozzles were located outside of the width range of the sheets that have been processed previously, so that the droplets ejected by these nozzles would not have hit the recording medium but would have stained the conveyer. It has therefore been common practice to perform a maintenance operation each time when such a format change occurs.

In a known printer, the maintenance operation is performed for the entire print head which, to this end, is lifted off from the conveyer and moved aside into a maintenance station where the maintenance operations are performed. For example, the

maintenance operations may comprise purging the nozzles and wiping the surface in which the nozzle orifices are formed. It is an object of the invention to provide a multi-format printer which permits a higher productivity.

In order to achieve this object, the invention provides a printer of the type indicated above, wherein the maintenance manager is adapted to decide on a maintenance operation on the basis of whether or not the actuation instructions for the dot printing elements located outside of the width range of the narrow format but inside the width range of the wider format specify that at least one of these dot printing elements is needed for printing a part of the image. The invention is based on the consideration that, when the format of the sheets changes from a narrower format to a wider format, this does not necessarily mean that a maintenance operation is actually needed. For example, there may be cases in which the nozzles that are located outside of the width range of the narrower format that has been processed previously are not needed for printing an image on the sheets of the wider format, neither. For this reason, the invention involves an analysis of the actuation instructions for the printing elements, which permits to decide whether the nozzles located outside of the width range of the previous format will actually be needed.

As a consequence, the invention permits to avoid unnecessary but time-consuming maintenance operations, so that the productivity of the printer can be increased and less ink, to the extent needed by the maintenance operations, will be spoiled.

This is especially the case for printers in which a maintenance operation is performed for the entire print head and/or involves a movement of the print head from an operative position to a maintenance station and then back into the operative position.

More specific optional features of the invention are indicated in the dependent claims.

As long as wide format sheets are being processed, the actuation instructions for all printing elements on the entire width of this format include spitting instructions assuring that all nozzles will be actuated in time intervals smaller than the admissible nozzle open time. The maintenance manager may therefore preferably perform an analysis of the actuation instructions, including the spitting instructions, for a number of sheets that have been processed earlier, in order to determine the nozzle open times.

Further, the maintenance operations may include different types of operations which differ in their cleaning efficiency, and the maintenance manager may decide on the basis of the nozzle open times not only whether a maintenance operation is to be performed at all, but also which of the available maintenance operations shall be performed. An embodiment example will now be described in conjunction with the drawings, wherein:

Fig. 1 is a block diagram of a printer according to the invention; and

Fig. 2 is a flow diagram of a printing method according to the invention.

The printer shown in Fig. 1 comprises a conveyer 10 arranged to convey print media sheets 1 - 9 one after another in a conveying direction X, and a print head 12 disposed at the conveyer 10 and having an array 14 of dot printing elements, the array extending in a direction Z normal to the conveying direction. The print head is arranged to print images 16, 18 onto the media sheets 1 - 9 in accordance with actuation instructions 20 sent to the print head.

The printer further includes an electronic controller 22 which controls the overall operation of the printer. Among others, the controller 22 receives print instructions 24 from a print job file that is transmitted to the controller 22 via a network or has been temporarily stored on a storage medium that is connected to the controller 22. On the basis of the print instructions 24, the controller 22 generates the actuation instructions 20 for the print head 12.

The print instructions 24 may further include format instructions specifying a format of the sheets 1 - 9 on which the pages of the print job shall be printed. A feeder 26 is disposed at an upstream end of the conveyer 10 and arranged to receive instructions from the controller 22 so as to withdraw cut sheets with the required format from one of a plurality of bins (not shown) and to feed these sheets in the desired sequence to the conveyer 10.

The print instructions 24 may further comprise instructions specifying whether the images 16, 18 are to be printed only on one side of each sheet (simplex printing) or on both sides (duplex printing). In case of duplex printing, the sheets with a first image printed on one side thereof are re-circulated via a duplex loop 28 back to the feeder 26, and they are fed to the conveyer 10 in reverse orientation, so that another image will then be printed on the second side of the sheet. The print head 12 is disposed above the conveyer 10 in a position in which the array 14 of printing elements extends over the entire width of the conveyer 10. In the example shown, the printer is an ink jet printer, and the printing elements are constituted by nozzles from which ink droplets are expelled onto the media sheets 1 - 9 in accordance with the actuation instructions 20. To create the droplets, each nozzle is associated with an actuator, e.g. a piezoelectric actuator that generates a pressure wave in the liquid ink, such that the pressure wave may propagate to the nozzle to cause a droplet to be formed and expelled.

When a nozzle is not actuated for more than a specified time period, e.g. 40 s, the ink in the nozzle orifice may dry out, so that a certain amount of dried ink is deposited in the nozzle orifice and will disturb the subsequent droplet generation or will clog the nozzle orifice completely, causing the nozzle to fail. In such a case, a maintenance operation is required for cleaning the nozzle again. For that purpose, the print head 12 is disposed on a manipulator 30 that is arranged to lift the entire print head 12 off from the conveyer 10 and to move it laterally to a maintenance station 32 where all the nozzles of the print head are purged with either ink or a specific cleaning liquid. The maintenance operation may further include a wiping step in which a surface of the print head 12 that includes the nozzle orifices is wiped, e.g. with a wet sheet, so as to remove any remnants of dried ink. When the cleaning process has been completed, the manipulator 30 moves the print head 12 back to the operative position shown in Fig. 1.

The maintenance station 32 and the manipulator 30 are controlled by a maintenance manager 34 which is capable of analysing the print instructions 24 as well as the actuation instructions 20 being generating in the controller. The controller 22 - including the maintenance manager 34 - is arranged to schedule all operations of the printer in advance for a certain number of pages, e.g. 200 pages to be printed in accordance with the print instructions 24.

The time interval for which an individual nozzle has not been active since the last actuation is called the nozzle open time. Based on the actuation instructions 20 that are generated in the controller 22, the maintenance manager 34 is capable of monitoring the nozzle open times of each individual nozzle and to compare these nozzle open times to certain limits which indicate the necessity of a maintenance operation. The printer is capable of performing different types of maintenance operations, and each of these types of maintenance operations is associated with a specific limit. A maintenance operation will be performed when the nozzle open time exceeds the associated limit but does not exceed a higher limit that is associated with another (more intense or efficient) maintenance operation.

For example, a maintenance operation with a relative low open time limit is a so-called pre-fire operation which can be performed by the print head 12 itself without using the maintenance station 32. In this pre-fire operation, the actuators for the pertinent nozzles of the print head are energized to cause vibrations in the liquid ink in the nozzle orifice with an amplitude that is not sufficient for expelling an ink droplet. Yet, these vibrations will help to remove or dissolve possible contaminations in the form of dried ink in the nozzle orifice, especially, when the ink is just about to dry out and has not yet dried completely.

Only when the nozzle open time has been so long that it exceeds a higher second limit will it be necessary to perform the purge and wipe maintenance operation in the maintenance station 32.

As long as the sheets to be supplied to the print head 10 have all the same format, it can be assumed that most nozzles of the print head 12 will be actuated frequently in order to print the desired images 16, 18, so that the nozzle open times will be safely below the lowest open time limit. However, there may be nozzles, e.g. those located in a white margin region of the sheet, which are not needed for printing, so that their nozzle open time may exceed the limits. A known method for keeping the nozzles operative in these cases is the so-called spitting method, wherein an ink droplet is expelled from the nozzle before a nozzle open time limit for that nozzle is exceeded, even though the print instructions 24 do not command a dot to be printed in the pertinent position on the sheet. Consequently, the ink droplet will be "spit" onto a portion of the sheet which should remain white according to the print instructions. However, since a single ink dot is so small that it is not or hardly visible with the human eye, the spitting process does not degrade the quality of the printed image but nevertheless helps to keep the nozzle open times low. It is prerequisite for the spitting operation, however, that the nozzle that performs the spitting operation is located within the contour of the sheet, because otherwise the ink droplet would be spit onto the surface of the conveyer 10 and would stain the conveyer (and consequently the back sides of the sheets and the images printed thereon in the duplex mode).

The spitting instructions are generated in the controller 22 on the basis of the monitored nozzle open times and can be considered as a specific type of actuation instructions. Thanks to the spitting mechanism described above, it can be taken for granted that none of the nozzles that are located within the width range of the sheets to be printed will exceed an open time limit as long as the format of the sheets is not changed.

However, when the sheet format changes from a narrow format with a width range w1 , such as that of the sheets 2, 4, 5, 7 and 8 in Fig. 1 , to a wider format with a width range w2, such as that of the sheets 1 , 3, 6 and 9 in Fig. 1 , there may be nozzles that are located in a margin area 36 of the wide format sheets (shown for the sheet 6 in Fig. 1 ) which is outside of the width range w1 of the narrow format sheets. Since no spitting process was possible for these nozzles when printing the previous sheet or sheets in the narrower format, an open time limit may be exceeded. Nevertheless, it is possible that these nozzles will now be needed for printing an image 16 on the next wide format sheet.

For this reason, whenever such a format change occurs, the maintenance monitor 34 checks whether or not a maintenance operation is necessary and, if yes, which maintenance operation should be performed. The decision is taken on the basis of the actuation instructions 20, including the spitting instructions, for the pertinent nozzles. Among others, the maintenance manager 34 will derive from the actuation instructions, at least for those nozzles that will be required for printing the next sheet, the nozzle open times that will have elapsed when the nozzle is actuated for printing on the next wide format sheet as scheduled.

Fig. 2 is a flow diagram illustrating the operations that the maintenance manager 34 will perform for each sheet that is being scheduled for printing. In step S1 , it is decided whether the sheet (sheet n) that is being scheduled for printing has a width, i.e. a dimension in the direction Z, which is smaller than or equal to the width of the last sheet (n-1 ) that has been scheduled before. If that condition is fulfilled (Y), then the sheet n is scheduled for printing in step S2, which means that no maintenance operation shall be performed immediately before that sheet n reaches the print head 12.

If the condition specified in step S1 is not fulfilled (N), then the maintenance manager 34 checks the nozzle open times for all nozzles that will be used for sheet n but were not used for sheet n-1 . In this step, the "nozzles that will be used for sheet n" include only those nozzles that are actually needed for printing the desired image, but not the nozzles that will only be scheduled for spitting. On the other hand, the nozzles that were "not used for sheet n-1 " will include the nozzles that have been used neither for printing the desired image nor for spitting. Consequently, all the nozzles of the print head 12 that are located in the width range w1 of the narrow format sheets (sheets 2, 4, 5, 7 and 8 in Fig. 1 ) may be excluded from consideration in step S3 because these nozzles will have been used at least for spitting.

The nozzle open times to be checked in step S3 for each of the remaining nozzles is counted from the last instant when the nozzle has been used either for printing an image dot (firing) or for spitting until the time instant when the nozzle is scheduled for being actuated again, either for printing actual image information or for spitting.

It should be observed that a nozzle may be scheduled for spitting as soon as the leading edge of a sheet has reached the X position of the nozzle. However, there may be a large number of nozzles that need spitting at that time, so that a faint line would be printed on the leading edge of the sheet. In order to avoid this, it is preferred to distribute the spitting operations over a certain time period, even though this means that the nozzle open times for certain nozzles will be somewhat extended. In step S4, the maintenance manager 34 checks for each of the nozzles which have not been used for sheet n-1 , whether the nozzle open time is larger than a first limit T1 that is associated with the purge and wipe maintenance operation to be performed in the maintenance station 32. If that is the case (Y), this maintenance operation is performed in step S5. This means that the print production process has to be interrupted for the duration of the maintenance operation. Subsequently sheet n is planned in step S8. When the limit T1 has not been exceeded in step S4 (N), it is checked in step S6 whether the nozzle open time for at least one of the nozzles that will be needed for the sheet N exceeds a lower second limit T2. If that is the case (Y), the maintenance manager schedules, in step S7, a pre-fire maintenance operation which is less efficient than the purge and wipe maintenance operation performed in step S5 but is sufficient if the nozzle open time was not too long. The pre-fire maintenance operation has the advantage that it may be performed with the print head 12 remaining "in-situ", i.e. in the operative position shown in Fig. 1. In continuation from step S7 the sheet is planned in step S8.

As a consequence, the time consuming purge and wipe maintenance operation can be avoided under this condition, so that the productivity of the print process is increased. If it is found in step S6 that the nozzle open time for the nozzles in consideration does not even exceed the lower limit T2 (N), then the sheet n is scheduled for printing without any maintenance operation at all, so that even the less time-consuming maintenance operation in step S7 can be spared. In an alternative embodiment step S6 is left out from the method and from step S4 (N) the method continues with step S7.

In the example shown in Fig. 1 , the sheet 1 (n=1 ) is a wide-format sheet. The next sheet 2 (n=2) is a narrow format sheet, which means that some nozzles of the print head 12, i.e. those located outside the range w1 , cannot be used for printing and cannot be actuated for spitting, neither.

The next sheet 3 (n=3) is again a wide-format sheet, and the image 16 to be printed thereon includes image parts near the left and right edges, so that, for printing these image parts, nozzles are needed that have not been actuated during the time when the sheet 2 was printed. When this sheet 3 reaches the print head 12, the format changes from the narrow format of the sheet 2 to the wide-format of the sheet 3 and,

consequently the maintenance manager 34 decides on a maintenance operation. In this case, depending upon the length of the sheet 2, the maintenance manager my find in step S6 that the nozzle open times for the nozzles, even in the above-mentioned image parts near the edges of sheet 3, exceed only the smaller limit T2 but not the larger limit T1. Consequently, the maintenance manager will decide to perform a pre-fire maintenance operation before printing the sheet 3. Next, when the sheets 4 and 5 are printed, the nozzles near the left and right ends of the array 14 are idle again. This time, the nozzle open time exceeds the larger limit T1 because the number of narrow format sheets to be printed was larger. Then, when sheet 6 (n=6) reaches the print head and the maintenance manager 34 has to decide again whether a maintenance operation is necessary, it is found in step S3 that the nozzles for which the limit T1 would be exceeded are not needed for printing the sheet 6 because the image 18 to be printed on that sheet is so narrow that it does not overlap with the margin areas 36 of the sheet. Consequently, the answers in both, steps S4 and S6 will be "No" (N) and the sheet 6 will be scheduled for printing in step S8 without any maintenance operation being performed.

It would be possible to perform spitting operations for the nozzles in the margin areas 36 of the sheet 6. However, since a satisfactory condition and reliable operation of these nozzles cannot be relied upon (because of the large nozzle open time), the spitting operations would not make sense and will therefore preferably be omitted.

When printing the next narrow format sheets 7 and 8, the nozzle open time will again exceed the higher limit T1 , and then a decision on a maintenance operation has to be made for the next wide format sheet 9. This time, however, the image 16 to be printed on the sheet 9 overlaps the margin areas 36, so that nozzles will be needed for which the limit T1 has been exceeded. Consequently, the maintenance manager 34 will in this case decide in step S4 that the time-consuming purge and wipe maintenance operation in the maintenance station 32 cannot be avoided.

If the sheets 1 - 9 shown in Fig. 1 are to be printed in the duplex mode, the sheets will be re-circulated through the duplex loop 28 and back to the feeder 26. In this case, if the time required for re-circulating the sheets through the duplex loop is larger than the time needed for printing the sheets 1 - 9 on the first side, there may be a time gap in the printing sequence, and this gap will add to the nozzle open times that are checked in step S3. Moreover, when the first sheet after the gap is being scheduled, the nozzle open time should be checked for all the nozzles of the print head including those within the width range w1 of the narrow format, because there is actually no sheet that immediately precedes the sheet to be scheduled (no sheet n-1 ), and consequently no nozzles have been used at all.

When the nozzle open times exceed T1 , the purge and wipe maintenance operation in step S5 may of course be scheduled for a time so early that the time gap in the print sequence may be utilized for performing at least a part of the maintenance operation, thereby to enhance productivity.

On the other hand, when the gap in the print sequence that is caused by sheets being returned through the duplex loop is larger than the time required for performing the purge and wipe maintenance operation, then it may be preferable to perform such maintenance operation in any case, irrespective of the result of the check of the nozzle open times, because no extra time will be needed for the maintenance operation, anyway.