Patent Publication Number: US-8540356-B2

Title: Printing machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing machine including an ink supply that has a degasser. 
     2. Description of Related Art 
     There are known printing machines provided with an inkjet head for propelling out ink droplets, and an ink supply as a measure for supplying ink to the inkjet head. In such printing machines, the inkjet head has a set of piezoelectric elements or like elements exerting pressures on flux of ink, when printing images. This propels out droplets of ink onto a print sheet or such, to make the printing. 
     In such printing machines, ink contains dissolved gases that, if contained much, have tendencies to form bubbles depending on ink temperature and pressure variations while printing. Formed bubbles may absorb substantial pressures exerted on associated flux of ink, resulting in a failure to discharge ink, as an issue. To this point, there are known printing machines including a degasser for reducing contents of gases dissolved in ink. 
     There has been an inkjet printing machine disclosed in Japanese Patent Application Laying-Open Publication No. 2007-190703, including an ink circulation system for circulating ink, a dissolved gas amount acquirer for acquiring information on an amount of dissolved gases in ink, a three-way valve installed on an ink line in the ink circulation system for route selection among branched ink routes, a degasser installed on one of the ink routes branched at the three-way valve, and a pressure loss compensator for compensating a pressure of ink caused by the degasser. 
     This printing machine has been adapted to work after initiation of a printing, to operate upon a determination made on an excessive amount of gases dissolved in ink, to control the three-way valve, to conduct flux of ink through the ink route having the degasser installed thereon. This has afforded to supply flux of degassed ink to an inkjet head, there being a pressure loss caused in flux ink flowing through the degasser. The pressure loss compensator has been operated to make a pressure adjustment of such flux of ink. 
     SUMMARY OF THE INVENTION 
     However, the printing machine described has employed a method of following an initiation of a printing to determine a degassing to be made or not, sometimes leading to a delayed initiation of the degassing after a determination on an excessive amount of dissolved gases in ink, resulting in a failure in discharge of ink, as an issue. 
     The present invention has been devised in view of such issues. It therefore is an object of the present invention to provide a printing machine allowing for a suppressed failure in discharge of ink due to the delay of degassing. 
     To achieve the object described, according to an aspect of the present invention, there is a printing machine comprising an inkjet head including a piezoelectric element set configured to vibrate to propel out droplets of ink, a degasser configured to degas flux of ink to be supplied to the inkjet head, a memory configured to store therein a threshold table having mutually associated a threshold set of a determination parameter preset for a determination on a degassing process and a domain set of a control parameter of a physical amount constituting a cause of bubble formation in ink, and a controller configured to control driving the degasser, the controller being adapted to calculate a value of the determination parameter from a set of image data of a print job, and operate as the value is equal to or greater than a threshold in the threshold table read out of the memory, to drive the degasser. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall schematic diagram of a duplex printing machine according to a first embodiment. 
         FIG. 2  is a schematic diagram of an ink circulation system of the duplex printing machine shown in  FIG. 1 . 
         FIG. 3  is an explanatory block diagram of a control system of the duplex printing machine shown in  FIG. 1 . 
         FIG. 4  is an example of threshold table listing print time thresholds associated with ink temperatures. 
         FIG. 5  is an explanatory flowchart of a printing process according to the first embodiment. 
         FIG. 6  is an explanatory flowchart of a printing process according to a second embodiment. 
         FIG. 7  is an example of threshold table listing print time thresholds associated with numbers of vibration cycles of piezoelectric elements. 
         FIG. 8  is an explanatory flowchart of a printing process according to a third embodiment. 
         FIG. 9  is an example of threshold table listing print time thresholds associated with combinations of vibration cycles of piezoelectric elements and ink temperatures. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     There will be described an aqueous ink addressing inkjet type duplex printing machine according to a first embodiment of the present invention, with reference to the drawings. As used herein, the aqueous ink involves the concept of a moisture containing ink encompassing an O/W (Oil in Water) type and a W/O (Water in Oil) type emulsion ink. 
       FIG. 1  is an overall schematic diagram of the duplex printing machine according to the first embodiment.  FIG. 2  is a schematic diagram of an ink circulation system.  FIG. 3  is an explanatory block diagram of a control system of the duplex printing machine.  FIG. 4  is an example of threshold table listing print time thresholds associated with ink temperatures. The following description presumes a user standing at an obverse sheet side as a front side of  FIG. 1 , having upward, downward, rightward, and leftward directions marked in  FIG. 1  in consistent with upward, downward, rightward, and leftward directions seen from the user, respectively. 
     In  FIG. 1 , those paths depicted by bold lines are mutes of a transfer system adapted for transfer of print sheets. Among the transfer routes, those depicted by solid lines and dashed lines constitute a normal route RC. Among the transfer routes, those depicted by chain lines constitute a reversing route RR. Among the transfer routes, those depicted by two-dot chain lines constitute a system of feed routes RS. 
     As illustrated in  FIG. 1 , the duplex printing machine  1  includes a sheet feeder  2 , a printer  3 , a sheet dryer  4 , a sheet discharger  5 , a sheet reverser  6 , and a controller  7 . 
     The sheet feeder  2  is configured to feed a print sheet PA. The sheet feeder  2  constitutes an upstream end of the transfer system. The sheet feeder  2  includes a set of feed racks  11 , and a set of pairs of feed rollers  12 . Paired feed rollers  12  work to transfer a print sheet PA from any feed rack  11 , along a feed route RS to the printer  3 . 
     The printer  3  is configured to transfer a print sheet PA, printing images on the print sheet PA. The printer  3  is disposed downstream of the sheet feeder  2 . The printer  3  includes a pair of register rollers  15 , a belt transfer section  16 , a combination of four ink circulation systems  17 , and a heat exchanger  18 . 
     The pair of register rollers  15  works to transfer a print sheet PA fed from the sheet feeder  2  or the sheet reverser  6 , to the belt transfer section  16 . The belt transfer section  16  is configured to hold thereon by sucking a print sheet PA sent thereover from register rollers  15 , to transfer to the sheet dryer  4 . 
     Four ink circulation systems  17  are each configured to circulate a supplied aqueous ink  50  in one direction of a circulation loop, propelling out droplets of aqueous ink  50  as necessary to print images. The ink circulation systems  17  circulate different colors (e.g. black, cyan, magenta, and yellow) of aqueous ink  50 , respectively. 
     As illustrated in  FIG. 2 , each ink circulation system  17  includes an inkbottle  21 , a lower tank  22 , a circulation pump  23 , a degasser  24 , a filter  25 , an upper tank  26 , and an inkjet head  27  provided with a temperature sensor  29 , and a cap  30 , as well as a number of tube lines  28  constituting flow paths for aqueous ink  50 . There is an ink supply configured with the inkbottle  21 , the lower tank  22 , the circulation pump  23 , the filter  25 , the upper tank  26 , and associated ink lines  28 . 
     The inkbottle  21  serves for storage of aqueous ink  50  to be supplied. 
     The lower tank  22  is configured for temporary storage of aqueous ink  50  supplied from the inkbottle  21  through an ink line  28 . The lower tank  22  is adapted for temporary storage of aqueous ink  50  returned from the inkjet head  27  through an ink line  28 . The lower tank  22  is disposed lower than both the upper tank  26  and the inkjet head  27 . The lower tank  22  has a relief valve  22   a  operable to relieve an inner pressure to the atmosphere. 
     The circulation pump  23  is adapted to send flux of aqueous ink  50  from the lower tank  22 , where it has been stored, to the upper tank  26  through associated ink lines  28 . This causes flux of aqueous ink  50  to flow in a flow direction along the circulation route shown by arrows A in  FIG. 2 , for the supply to the inkjet head  27 . 
     The degasser  24  is configured for degassing flux of aqueous ink  50  being supplied to the inkjet head  27 , to remove dissolved gases therein. The degasser  24  has a degassing pump  24   a . With such configuration, the degasser  24  is adapted to have part of the ink circulation route pressure-reduced for degassing aqueous ink. 
     The filter  25  serves to remove lint or the like in flux of aqueous ink  50 . 
     The upper tank  26  is configured for temporary storage of aqueous ink  50  before the supply to the inkjet head  27 . The upper tank  26  is filled with a combination of aqueous ink  50  and air. The upper tank  26  is adapted to supply flux of aqueous ink  50  that has been stored therein, to the inkjet head  27  through an ink line  28 . The upper tank  26  has a relief valve  26   a  operable to relieve an inner pressure to the atmosphere. 
     The inkjet head  27  is configured to propel droplets of aqueous ink  50  onto a print sheet PA, printing images thereon. The inkjet head  27   2  is disposed lower than the upper tank  26 . The inkjet head  27  has a set of piezoelectric elements each operable for exertion of pressures to propel out a droplet of aqueous ink  50 . During circulation of ink, there is flux of aqueous ink  50  stored in the inkjet head  27 , and returned to the lower tank  22  through ink line  28 . 
     The temperature sensor  29  is configured to detect an ink temperature T of flux of aqueous ink  50  in the inkjet head  27 . The temperature sensor  29  is adapted to output a detected ink temperature T to the controller  7 . 
     The cap  30  is adapted for sealing a downside of the inkjet head  27 . The cap  30  is configured to cover the downside of the inkjet head  27 . The cap  30  is displaced to locate in position as shown in  FIG. 2  in each course of printing, and to locate close to the downside of inkjet head  27  in each non-printing course. 
     The heat exchanger  18  is configured to exchange heat between streams of aqueous ink  50  in the four ink circulation routes  17 . The heat exchanger  18  serves to average temperatures of four colors of aqueous ink  50 . 
     The sheet dryer  4  is configured to transfer a printed print sheet PA, while drying. The sheet dryer  4  is disposed downstream of the printer  3 . The sheet dryer  4  includes a drying duct  31 , a triple of pairs of transfer rollers  32 , and a heating blower or fan  33 . 
     The drying duct  31  is configured to guide a printed print sheet PA, while accumulating heat of air sent from the heating blower or fan  33 . The drying duct  31  has a transfer space (non-depicted) defined therein as part of normal route RC for transfer of print sheet PA. Paired transfer rollers  32  are adapted to transfer the printed print sheet PA in the drying duct  31 . 
     The sheet discharger  5  is configured to discharge a printed print sheet PA in a stacking manner. The sheet discharger  5  is disposed downstream of the sheet dryer  4 . The sheet discharger  5  constitutes a downstream end of the normal route RC. The sheet discharger  5  includes a route selector  35 , a pair of pairs of discharge rollers  36 , and a stacking rack  37 . 
     The route selector  35  is configured to select a transfer route of print sheet PA between the normal route RC and the reversing route RR. Paired discharge rollers  36  are adapted to discharge a print sheet PA onto the stacking rack  37 . 
     The sheet reverser  6  is configured to reverse a one-side printed print sheet PA, to transfer to the printer  3 . The sheet reverser  6  includes a set of pairs of reversing rollers  41 , a flipper  42 , and a switchback section  43 . 
     Some pairs of reversing rollers  41  are cooperative to receive a one-side printed print sheet PA from the sheet dryer  4 , once bringing inside the switchback section  43 . Some pairs of reversing rollers  41  are cooperative to bring back the print sheet PA outside the switchback section  43 , to transfer to the printer  3  through the flipper  42 . 
     The controller  7  is adapted to govern entire control of the duplex printing machine  1 . As shown in  FIG. 3 , the controller  7  includes a CPU  51  configured to execute various programs, a RAM  52  adapted for temporary storage of associated data, a ROM  53  adapted for storage of base programs and the like, an HDD  54  adapted for storage of a printing program, a degassing program, and the like, and an I/O port  55  adapted to implement an I/O interface. 
     In the HDD  54 , as shown in  FIG. 4 , there is stored a threshold table Tb 1  listing a number of domain intervals of ink temperature ΔT n  (n=1, 2, . . . ) (=T 1 ˜T 2 , T 2 ˜T 3 , T 3 ˜T 4 , . . . ) and a number of thresholds of print time Th n  (n=1, 2, . . . ) related thereto in a one-to-one corresponding manner. According to the first embodiment, the controller  7  is adapted to associate an ink temperature T with a threshold Th n  of print time PT to determine a degassing to be performed or not. This is a criterion introduced for determination in consideration of dissolved gases in ink that: on one hand, become less dissoluble, having increased tendencies to form bubbles, as the ink temperature T increases with accumulated heat dissipation of a set of associated piezoelectric elements, as the piezoelectric element set has an increased total number of vibration cycles N, with a longer print time PT than an associated threshold Th n ; and on the other hand, have increased tendencies to form bubbles, as an associated ink chamber has a decreased ink pressure due to vibrations of the piezoelectric element set, with an increased print time TP. In either case, there should be a determination to execute a degassing process or treatment. As used herein, the total vibration cycle number N means a total number of vibration cycles of piezoelectric elements in the inkjet head  27  as necessary to execute a single time of print job, that is, an associated total number of droplets of aqueous ink propelled out of the inkjet head  27 . It is noted that in the HDD  54  there is stored a set of thresholds ThV of degas time VT each prepared as a criterion to determine a completion of degassing. In the HDD  54  there is stored a combination of previous print time PTA and print completion time PTE. 
     The I/O port  55  adapted to work as an I/O interface is connected with the sheet feeder  2 , sheet dryer  4 , sheet discharger  5 , and sheet reverser  6 . The I/O port  55  is connected to an external device such as a personal computer (non-depicted) to input image data. 
     The I/O port  55  is connected to the register rollers  15  and the belt transfer section  16  of the printer  3 . The controller  7  is thereby adapted to control a print speed related to the speed of transfer of a certain print sheet PA. The controller  7  is configured to work when degassing, to perform an associated printing by a print speed (referred to as a first print speed) slower than a normal print speed (referred to as a second print speed), before returning to a printing by the normal print speed. 
     The I/O port  55  is connected with the four ink circulation systems  17 . More specifically, the I/O port  55  is connected, in each ink circulation system  17 , to its circulation pump  23 , degassing pump  24   a , inkjet head  27 , and temperature sensor  29 . The controller  7  is thereby adapted to control the circulation pump  23  for circulation of aqueous ink  50 . 
     The controller  7  is configured to work while printing images, to control the degassing pump  24   a  for degassing aqueous ink  50 . More specifically, the controller  7  is configured to work for ink temperatures T within an associated ink temperature interval ΔT n , to operate for print limes PT equal to or longer than an associated print time threshold Th n , for driving the degassing pump  24   a  to degas flux of aqueous ink  50 , while printing images. In this regard, the controller  7  is adapted to operate for print times PT shorter than the print time threshold Th n , to print images without driving the degassing pump  24   a.    
     The controller  7  is configured to work at a respective timing corresponding to a set of image data to be printed, to send a corresponding set of prescribed ink discharge signals to a driver (non-depicted) at the inkjet head  27 . The inkjet head  27  is thereby driven to propel an array of droplets of aqueous ink  50  onto a print sheet PA. It is noted that  FIG. 3  has omitted three ink circulation systems  17  out of the four ink circulation systems  17  being common in configuration and connection. 
     (Printing Process) 
     Description is now made of printing actions of the duplex printing machine  1  according to the first embodiment.  FIG. 5  is an explanatory flowchart of a printing process of the duplex printing machine. 
     As shown in  FIG. 5 , first, at a step S 1 , the controller  7  operates for calculation to determine a total number of vibration cycles N from a set of image data in a current input print job. Further, the controller  7  calculates a required print time PT for the current print job on bases involving the calculated total vibration cycle number N. More specifically, the controller  7  calculates the print time PT depending on combination of the total vibration cycle number N calculated from image data of the print job, and a set of print parameters the duplex printing machine  1  has set up inclusive of transfer speeds VP of associated print sheets PA, and discharge speeds VI of aqueous ink  50  at associated nozzles. 
     At a step S 2 , the controller  7  operates for calculation to determine an interval of time ΔPT between previous and current print jobs, using a previous print completion time PTE stored in the HDD  54  and a current clock time CT. 
     Next, at a step S 3 , the controller  7  operates to determine whether or not the time interval ΔPT is equal to or longer than a threshold of interval time ThD stored in the HDD  54 . If the time interval ΔPT is determined as being the interval time threshold ThD or more (Yes at the step S 3 ), then the control flow goes to a step S 5 , where the controller  7  enters a designated process. On the other hand, if the time interval ΔPT is determined as being shorter than the interval time threshold ThD (No at the step S 3 ), then the control flow goes to a step S 4 , where the controller  7  operates for calculation to add the previous print time PTA to the current print time PT, to determine a sum of them to be set as a new print time PT. To this point, it is noted that for short time intervals ΔPT exceeding a prescribed value, there is circulation of aqueous ink  50  repeated in consideration of properties of aqueous ink  50 , such as viscosity. This is because of the concept in favor of regarding the previous print job as having been continued to the current print job, in order for failures in discharge of aqueous ink  50  to be suppressed safe against any interval of time ΔPT that is so short as being smaller than the threshold ThD, as a lapse of time after completion of the previous print job. 
     Next, at the step S 5 , the controller  7  operates to drive the circulation pump  23 . This causes flux of aqueous ink  50  to flow, as shown in  FIG. 2 , in the flow direction indicated by arrows A, along the circulation route including the lower tank  22 , circulation pump  23 , degasser  24 , filter  25 , upper tank  26 , and inkjet head  27 , with associated ink lines  28  inclusive. 
     Next, at a step S 6 , the controller  7  operates to extract, from the threshold table Tb 1  stored in the HDD  54 , a threshold Th n  of print time PT corresponding to an ink temperature interval ΔT n  covering an ink temperature T input from the temperature sensor  29 , and set up the same. This is to hold out formation of bubbles tending to exert influences at different lengths of print time PT depending on the ink temperature T. In the example of threshold table Tb 1  shown in  FIG. 4 , there is a threshold Th 2  of print time PT to be set for ink temperatures T belonging to an ink temperature interval ΔT 2 =T 2 ˜T 3 . 
     Next, at a step S 7 , the controller  7  operates to determine whether the print time PT is the threshold Th n  or more, or not. 
     If the print time PT is determined as not being the threshold Th n  or more (No at the step S 7 ), then the control flow goes to a step S 8 , where the controller  7  operates to execute a normal image printing process without driving the degassing pump  24   a . Here, the length of print time PT not being the threshold Th n  or more refers to an extent of status of aqueous ink  50  to be developed within the short print time PT and substantially free of dissolved gases activated to form bubbles, needing no degassing. 
     Description is now made of actions for print to be executed in the normal image printing process (at the step S 8 ) under control of the controller  7  through implements including the sheet feeder  2  to the sheet reverser  6 . Initially, there is a non-printed print sheet PA being fed from any one of the feed racks  11  by associated feed rollers  12  along a feed route RS to the printer  3 . The print sheet PA fed to the printer  3  is registered by the register rollers  15 , and set in position on the belt transfer section  16 . The print sheet PA on the belt transfer section  16  is carried at a normal print speed, when the print sheet PA has images printed thereon by droplets of aqueous ink  50  propelled out as necessary from inkjet heads  27  of ink circulation systems  17 . The print speed is controlled by the controller  7  controlling revolution numbers of associated feed rollers  12 , register rollers  15 , and transfer rollers  32 . The print sheet PA thus printed is still carried by the belt transfer section  16 , to forward along the normal transfer route RC into the drying duct  31  of the sheet dryer  4 . 
     At the sheet dryer  4 , the print sheet PA is transferred by transfer rollers  32 , while being guided by wall of the drying duct  31 , to forward through a transfer space defined inside the drying duct  31  filled with heating air. The print sheet PA having been moist with aqueous ink  50  is thus dried in the drying duct  31 . Then, the print sheet PA is forwarded outside the drying duct  31 . 
     For one-side printing, the print sheet PA is transferred to the sheet discharger  5 . At the sheet discharger  5 , the print sheet PA is guided by the route selector  35 , and carried by discharge rollers  36 , to discharge onto the stacking rack  37 . 
     For duplex printing, the print sheet PA is guided by the route selector  35  into the reversing route RR of the sheet reverser  6 . At the sheet reverser  6 , the print sheet PA is forwarded by associated reversing rollers  41  temporarily into the switchback section  43 , while being guided by the flipper  42 . After that, the print sheet PA being guided by the flipper  42  is returned from the switchback section  43 , to re-feed to the printer  3  using associated reversing rollers  41 . 
     At the printer  3 , the print sheet PA is transferred by the belt transfer section  16  with the non-printed side of print sheet PA facing the inkjet heads  27 , while having images printed on the non-printed side by inkjet heads  27 . After that, the both-side printed print sheet PA is dried in the sheet dryer  4 , and transferred to the sheet discharger  5 . 
     Then, at a step S 9 , the controller  7  operates to determine whether or not the current print job is complete, on bases including the number of frames of image data to be printed and the number of printed sheets. If the printing is determined as being complete (Yes at the step S 9 ), then the control flow goes to a step S 16 , where the controller  7  enters a designated process. 
     At the step S 16 , the controller  7  operates to stop the circulation pump  23 . This stops circulation of aqueous ink  50  in the ink circulation system  17 . 
     Next, at a step S 17 , the controller  7  operates to store in the HDD  54  a completion time PTE and print time PTA of the current print job, for use in a subsequent print job. 
     The printing process then goes to an end. 
     On the other hand, if the print time PT is determined as being the threshold Th n  or more (Yes at the step S 7 ), then the control flow goes to a step S 10 , where the controller  7  operates to drive the degassing pump  24   a , to degas aqueous ink  50  circulating in the ink circulation system  17 . Here, the length of print time PT being the threshold Th n  or more refers to an extent of status of aqueous ink  50  to be developed within the long print time PT, having much dissolved gases forming bubbles, with high probabilities of failures in discharge of aqueous ink  50 . 
     Next, at a step S 11 , the controller  7  operates to execute a turned-down image printing process. The turned-down image printing process refers to a process of printing images at a print speed turned down from, or slower than, a print speed in the normal image printing process. The turned-down image printing process affords to control dissipation of heat at the inkjet head  27 , suppressing formation of bubbles by dissolved gases in aqueous ink  50 . It is noted that the turned-down image printing process is similar in control action to the normal image printing process, excepting the print speed. 
     Next, at a step S 12 , the controller  7  operates to determine whether or not the time of degassing VT has elapsed a threshold ThV. If the degassing time VT is determined as having elapsed the threshold ThV (Yes at the step S 12 ), then the control flow goes to a step S 13 , where the controller  7  operates to accelerate associated implements including feed rollers  12 , register rollers  15 , and transfer rollers  32 , to start the normal image printing process at a normal print speed faster than the print speed in the turned-down image printing process. Here, the length of degassing time PT exceeding the threshold ThV refers to an extent of status of aqueous ink  50  degassed to remove much dissolved gasses therein, with low probabilities of failure in discharge of aqueous ink  50  to be achieved even in execution of the normal image printing process at the normal print speed. 
     Next, at a step S 14 , the controller  7  operates to check if the number of printed sheets has attained a preset sheet number, to determine whether or not the current print job is complete. If the print job is determined as being complete (Yes at the step S 14 ), then the control flow goes to a step S 15 , where the controller  7  operates to stop the degassing pump  24   a . This stops degassing aqueous ink  50 . 
     In due course, the controller  7  operates to execute processes at the steps S 16  and S 17 , so the printing process goes to an end. 
     (Performances of Duplex Printing Machine) 
     Description is now made of performances of the duplex printing machine  1 . 
     According to the duplex printing machine  1  described, there is a controller  7  adapted for comparison between a print time PT and a threshold Th n  to determine a degassing of aqueous ink  50  to be made or not. In other words, the controller  7  is adapted to predict a necessity of degassing to operate for driving a degassing pump  24   a  of a degasser  24 . This affords to eliminate delays in initiation of a degassing that might have caused failures in discharge of aqueous ink  50  in a printing. Unlike techniques in the past, it affords to eliminate also issues in following an initiation of a printing to detect an amount of gases dissolved in aqueous ink  50 , that might have lead to a delayed start of ink, resulting in a frequent failure in discharge of ink. Accordingly, there is an enhanced quality achieved in printed images, allowing for a suppressed occurrence of a forced shutdown in a printing. 
     According to the duplex printing machine  1 , there is a controller  7  adapted for comparison between a print time PT and a threshold Th n  to determine a degassing of aqueous ink  50  to be unnecessary, to operate for a printing without driving a degassing pump  24   a . The duplex printing machine  1  thus allows for a reduced power consumption at the degassing pump  24   a , with an implemented energy saving. 
     According to the duplex printing machine  1 , there is a controller  7  adapted to have a threshold Th n  of print time PT associated with an ink temperature T giving significant influences on failures of ink discharge, to thereby determine a degassing to be made or not. The duplex printing machine  1  according to the first embodiment thus affords, even in short print times PT, to eliminate failures in discharge of aqueous ink  50  due to increased heat dissipation at an inkjet head  27  with high ink temperatures T. Even in long print times PT on the contrary it affords to print images without driving a degassing pump  24   a , subject to controlled heat dissipation at the inkjet head  27  with moderate ink temperatures T. The first embodiment thus permits implementing an energy saving duplex printing machine  1 . 
     According to the duplex printing machine  1 , there is a controller  7  adapted to work for a necessary degassing, to select a turned-down image printing process slower in print speed than a normal image printing process, with reduced tendencies for dissolved gases in ink to form bubbles. This permits the duplex printing machine  1  to start printing images without waiting a completion of degassing, thus allowing for a saved time before completion of the printing. 
     According to the duplex printing machine  1 , there is a controller  7  adapted to work for a short interval of time between a previous clock of printing and a current clock of printing, to add a previous print time PT to a current print time PTA to set the sum as a print time PT, for use in determination of a degassing to be made or not. The duplex printing machine  1  is thus permitted to work, even in succession of different print jobs to be executed within a short while, to determine a necessity of degassing with a favorable precision, allowing for a suppressed failure in discharge of aqueous ink  50 . 
     Second Embodiment 
     Description is now made of a second embodiment according to a modification in printing process of the first embodiment.  FIG. 6  is an explanatory flowchart of a printing process according to the second embodiment.  FIG. 7  is an example of threshold table listing print time thresholds associated with numbers of vibration cycles of piezoelectric elements. With respect to the embodiment described, like elements are designated by like reference signs, omitting redundancy. Likewise, with respect to the embodiment described, like control steps are designated by like step numbers, omitting redundancy. 
     In the printing process according to the second embodiment, as shown in  FIG. 6 , there is a step S 21  following a step S 1 , where the controller  7  operates for calculation to determine a vibration frequency f on bases including a set of image data of an input print job. As used herein, the vibration frequency f refers to the number of vibration cycles of the piezoelectric element set per unit time. The greater the vibration frequency f, the greater the heat dissipation of piezoelectric elements in the inkjet head  27  with an enhanced bubbling in aqueous ink  50 . According to the second embodiment, in the HDD  54 , there is stored as shown in  FIG. 7  a threshold table Tb 2  listing a number of domain intervals of vibration frequency f of piezoelectric elements Δf n  (n=1, 2, . . . ) (=f 1 ˜f 2 , f 2 ˜f 3 , f 3 ˜f 4 , . . . ) and a number of thresholds of print time Th n  (n=1, 2, . . . ) related thereto in a one-to-one corresponding manner. 
     After that, the controller  7  operates to execute designated processes at steps S 2  to S 5 , like the first embodiment. 
     Next, at a step S 22 , the controller  7  operates to extract, from the threshold table Tb 2  stored in the HDD  54 , a threshold Th n  of print time corresponding to a vibration frequency interval Δf n  covering a vibration frequency f of piezoelectric elements calculated from an image data set, and set up the same. In the example of threshold table Tb 2  shown in  FIG. 7  according to the second embodiment, there is a threshold The of print time to be set for frequencies f belonging to a vibration frequency interval Δf 2 =f 2 ˜f 3 . 
     After that, the controller  7  operates to execute designated processes at steps S 7  et seq., like the first embodiment. 
     According to the second embodiment described, there is a controller  7  adapted to have a threshold Th n  of print time PT associated with a vibration frequency f giving significant influences on failures of ink discharge, to thereby determine a degassing to be made or not. The duplex printing machine  1  according to the second embodiment thus affords, even in short print times PT, to eliminate failures in discharge of aqueous ink  50  due to increased heat dissipation at an inkjet head  27  with high frequencies f. 
     Even in long print times PT on the contrary it affords to print images without driving a degassing pump  24   a , subject to controlled heat dissipation at the inkjet head  27  with moderate frequencies f. The second embodiment thus permits implementing an energy saving duplex printing machine  1 . 
     Third Embodiment 
     Description is now made of a third embodiment according to a modification in printing process of the embodiments described.  FIG. 8  is an explanatory flowchart of a printing process according to the third embodiment.  FIG. 9  is an example of threshold table listing print time thresholds associated with combinations of numbers of ink temperature intervals and numbers of vibration cycles of piezoelectric elements. According to the third embodiment, in the HDD  54 , there is stored as shown in  FIG. 9  a threshold table Tb 3  listing a number of domain sections of ink temperature ΔT n  (n=1, 2, . . . ) (=T 1 ˜T 2 , T 2 ˜T 3 , T 3 ˜T 4 , . . . ) combined with a number of domain intervals of vibration frequency f of piezoelectric elements Δf n  (n=1, 2, . . . ) (=f 1 ˜f 2 , f 2 ˜f 3 , f 3 ˜f 4 , . . . ), and a number of thresholds of print time Th n  (n=1, 2, . . . ) related to the combinations in a one-to-one corresponding manner. With respect to the embodiments described, like elements are designated by like reference signs, omitting redundancy. Likewise, with respect to the embodiments described, like control steps are designated by like step numbers, omitting redundancy. 
     In the printing process according to the third embodiment, as shown in  FIG. 8 , the controller  7  operates to execute designated processes at steps S 1 , S 21 , and S 2  to S 5 , like the second embodiment. 
     Next, at a step S 23 , the controller  7  operates to extract, from the threshold table Tb 3  stored in the HDD  54 , a threshold Th n  of print time one-to-one corresponding to a combination of an ink temperature interval ΔT n  covering an ink temperature T input from the temperature sensor  29  and a vibration frequency interval Δf n  covering a vibration frequency f of piezoelectric elements calculated from an image data set, and set up the same. In the example of threshold table Tb 3  shown in  FIG. 9  according to the third embodiment, there is a threshold Th 2  of print time to be set for a combination of any ink temperature T belonging to an ink temperature interval ΔT 2 =T 2 ˜T 3  and any vibration frequency f belonging to a vibration frequency interval Δf 2 =f 2 ˜f 3 . 
     After that, the controller  7  operates to execute designated processes at steps S 7  et seq., like the first embodiment. 
     According to the third embodiment described, there is a controller  7  adapted to have a threshold Th n  of print time PT associated with combination of an ink temperature and a vibration frequency f calculated from an image data set of a print job, to thereby determine a degassing to be made or not. The third second embodiment thus affords to enjoy effects of both the first embodiment and the second embodiment. 
     Although the present invention has been described by use of embodiments, the present invention should not be construed as being restrictive to the embodiments described. The scope of the present invention should be defined within the scope of appended claims as well as a scope equivalent to the scope of claims. There may be modifications in part of the embodiments described, as follows. 
     The embodiments described may have constituent elements thereof altered or changed as necessary in shape, value, material, or the like. The embodiments described may be combined as necessary. 
     In the embodiments described, the present invention has been applied to an aqueous ink-addressing inkjet type duplex printing machine. The present invention may well be applied to, among others, a solvent system ink-addressing inkjet type duplex printing machine and any of inkjet type printing machines of a one-side printing system or else. 
     In the embodiments described, the present invention has been applied to a printing machine provided with an ink circulation system. The present invention may well be applied to a printing machine adapted to supply ink to an inkjet head without circulating ink. 
     In the embodiments described, there has been use of a print time PT calculated from an image data set of a print job, for checking if it does or does not exceed a threshold Th n  (n=1, 2, . . . ), to determine a necessity of driving a degassing pump. There may be use of any applicable method else to determine the necessity of driving a degassing pump. For instance, there may be use of a vibration frequency f calculated from an image data set of a print job, for checking if it does or does not exceed a threshold, to determine a necessity of driving a degassing pump. Or else, there may be use of a drive frequency fD of a belt transfer section  16 , for checking if it does or does not exceed a threshold, to determine a necessity of driving a degassing pump. Here, the drive frequency fD refers to the number of lines to be printed per unit time in a process of printing line by line along a transfer direction of a print sheet. The drive frequency fD may be determined by calculation from a vibration frequency f calculated on bases including an image data set of a print job. For instance, there may be use of a drive frequency fD calculated on bases including a total vibration cycle number N of a piezoelectric element set, an ink discharge rate or speed VI, a total number of lines to be scanned at an inkjet head, and a print time. 
     There may be a controller  7  configured to determine a necessity of degassing every ink color. Preferably, this controller  7  should determine a necessity of degassing on bases including an ink temperature of a respective color of ink, a vibration frequency f or a total vibration cycle number N of a piezoelectric element set of a respective color of ink, a print ratio of a respective color of ink in an image data set, and the number of ink droplets per pixel of a respective color of ink. 
     In the embodiments described, there has been a threshold Th n  of print time PT extracted from a threshold table Tb 1 , Tb 2 , or Tb 3 . There may be a controller  7  configured to calculate in advance a threshold of print time before an initiation of printing, on bases including an ink temperature T, a total vibration cycle number N, or a vibration frequency f. 
     The present application claims the benefit of priority under 35 U.S.C. §119 to Japanese Patent Application No. 2009-222918, filed on Sep. 28, 2009, and Japanese Patent Application No. 2010-212553, filed on Sep. 22, 2010, the entire content of which are incorporated herein by reference.