Patent Publication Number: US-8983360-B2

Title: Method and system for printing having a web jam detecting unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2009-265695 filed in Japan on Nov. 20, 2009. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing system that includes a first printer that forms an image on continuous-form printing paper and a second printer that receives the continuous-form printing paper fed from the first printer and forms an image on the continuous-form printing paper, and to a printing method. 
     2. Description of the Related Art 
     Conventionally, web printing systems (printing systems) have been proposed and brought into practical use as printing system that uses two printers to form images on both sides of elongated print paper (web) that has standard-compliant feed holes. An example of such a web printing system is disclosed in Japanese Patent No. 3680989. 
       FIG. 9  is a schematic perspective view illustrating basic configuration of a conventional web printing system. Referring to  FIG. 9 , the web printing system includes two printers P 1  and P 2  that are arranged in series and that form images on a web by electrophotography. The first printer P 1  disposed upstream performs printing on a first surface (front side) of a web W. The web W is delivered from the first printer to a turnover device where the web W is turned upside down and then sent into the second printer P 2  disposed downstream of the first printer. The second printer P 2  performs printing on a second surface (back side) of the web W. The web printing system configured as described above includes a control device  20  that controls operations of the printer P 1  and the printer P 2 . 
       FIG. 10  is an exemplary schematic diagram illustrating a configuration of the printer P 1 , P 2  illustrated in  FIG. 9 . In  FIG. 10 , a printing process section of a web printing unit that employs electrophotographic method includes a photosensitive member  3 . The photosensitive member  3  is configured to transfer and form an electrostatic latent image of a print image to the web W while rotating in a direction the web W is conveyed for printing. Various units (not illustrated) including an electrostatic charging unit, an exposure unit, a developing unit, and a cleaning unit, which are used in forming the electrostatic latent image on the surface of the photosensitive member  3 , are provided in the periphery of the photosensitive member  3 . 
     In both lateral-side portions of the web W, standard-compliant feed holes (which engage with the conveying unit  1  and  5 , and will be described later), for conveying the web W, are provided continually at regular intervals along the web conveying direction. The web conveying unit  1  is disposed upstream of a transfer device  4  in the web conveying direction, and the web conveying unit  5  is disposed downstream of the transfer device  4  in the web conveying direction. The web conveying units  1  and  5  convey the web W through a transfer region while applying a certain tension on the web W. A web-jam detecting sensor  2  is attached to the web conveying unit  1 . The web-jam detecting sensor  2  is configured to detect a jam of the web W. 
     The web conveying units  1  and  5  convey the web W to a thermal fixing device. The thermal fixing device includes a pre-heating plate  6 , a heating roller  7 , and a pressure roller  8 . The pre-heating plate  6  is a heating plate that preheats the web W so that fixation is performed reliably by the heating roller  7  and the pressure roller  8 . A puller roller  9  conveys the web W having undergone fixation to discharge the web W to the outside of the printer through a shooter  10 . A buffer mechanism  15  is provided to adjust slack in the web W. The buffer mechanism  15  is configured to be pivotable about its pivot center, which is an end of the buffer mechanism  15  on an upstream side relative to the feed direction of the web W. 
       FIG. 11  is an exemplary schematic diagram illustrating a configuration of a conventional web conveying unit. Referring  FIG. 11 , the web conveying unit  1  includes tractor pins  11 , which are to be inserted into the standard-compliant feed holes H for conveying the web W, tractor doors  12  having a mechanism that freely opens and closes the tractor doors  12 , and web-jam detectors  2   a  and  2   b  movable to open and close in synchronization with the tractor doors  12 . The above described elements are arranged in a pair. The web-jam detecting sensor  2  may be, for instance, a web-jam sensor of a transmission type; more specifically, the web-jam detecting sensor  2  may be a light-sensitive detector that includes a pair of the web-jam detector  2   a  that emits sensor light  13  and the web-jam detector  2   b  that receives the sensor light  13 . 
     In  FIG. 11 , only portions of the tractor doors  12 , which are portions near the web-jam detectors  2   a  and  2   b , are illustrated. 
       FIG. 12  is a schematic cross-sectional view illustrating how the web-jam detectors  2   a  and  2   b  operate. When the web W is jammed at the web conveying unit  1  as indicated by a dotted line in  FIG. 12  and causes the sensor light  13  to be shielded by the web W, the web-jam detector  2   b  does not receive the sensor light  13  any more. The web-jam detector  2   b  detects a jam of the web W in this manner. 
     The web-jam detectors  2   a  and  2   b  are configured to open and close in synchronization with the tractor doors  12 . Accordingly, the web-jam detector  2   b  detects a web jam even when the web-jam detector  2   b  is prevented from receiving the sensor light  13  because one of the tractor doors  12  is open or because the tractor door  12  is lifted up by a jam of the web W. 
     In the web printing system configured as described above, the web-jam detecting sensor  2  detects a jam of the web W irrespective of operations of the first printer P 1  and the second printer P 2 . 
     However, such an unfavorable situation as described below may occur in the conventional technique described above. 
     For instance, when printing operation pauses due to a trouble or the like or when printing operation is halted by an operator because of other circumstance, the web W is left as being loaded in the first printer P 1  and the second printer P 2  in a state where no printing is performed. Under such circumstances, the web W pausing in the thermal fixing device of the first printer P 1  continues to be heated by the pre-heating plate  6  and the heating roller  7 . Thus, as time passes, the moisture in the web W evaporates, and the web W partially and greatly shrinks and deforms due to heat. 
     When printing is resumed from such a state as described above, the web W that has the portion greatly deformed by heat shrinkage (hereinafter, “thermally shrunk portion”) is conveyed to the turnover device to be turned upside down and then fed into the second printer P 2 . When the thermally shrunk portion passes through the web conveying unit  1  of the second printer P 2 , the tractor door  12  may be lifted up due to the shape of the thermally shrunk portion, disadvantageously causing the web-jam detecting sensor  2  to detect a web jam in some cases. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, a printing system includes: a first printer that forms an image on continuous-form printing paper; and a second printer that receives the continuous-form printing paper fed from the first printer and forms an image on the continuous-form printing paper, and the first printer includes: a pre-heating unit that preliminarily heats the continuous-form printing paper prior to fixation; and a fixing unit that fixes a not-yet-fixed image formed on the continuous-form printing paper onto the continuous-form printing paper, and the second printer includes: a jam detecting unit that detects a jam of the continuous-form printing paper at an upstream position where the second printer forms the image on the continuous-form printing paper; and a control unit that controls the jam detecting unit such that the jam detecting unit does not perform the jam detection for a period of time, over which a deformed portion of the continuous-form printing paper that has been at rest at the pre-heating unit of the first printer passes through the detection position of the jam detecting unit. 
     According to another aspect of the present invention, a printing method to be performed in a printing system that includes a first printer that forms an image on continuous-form printing paper and a second printer that receives the continuous-form printing paper fed from the first printer and forms an image on the continuous-form printing paper, the first printer including: a pre-heating unit that preliminarily heats the continuous-form printing paper prior to fixation; and a fixing unit that performs the fixation by fixing a not-yet-fixed image onto the continuous-form printing paper, the printing method including: performing jam detection by detecting a jam of the continuous-form printing paper in the second printer at an upstream position where the second printer forms the image on the continuous-form printing paper; and performing control in the second printer such that the jam detection is not performed for a period of time, over which a deformed portion of the continuous-form printing paper that has been at rest at the pre-heating unit of the first printer passes through the detection position of the jam detecting unit. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of relevant elements of a signal processing system related to web-jam detection performed in printers P 1  and P 2  according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating, in a developed manner, elements located on a feed path of a web W between a transfer point of the printer P 1  and a transfer point of the printer P 2 ; 
         FIG. 3  is an explanatory diagram illustrating an example buffer-amount table; 
         FIG. 4  is an explanatory diagram of calculation of a buffer amount; 
         FIG. 5  is another explanatory diagram of calculation of a buffer amount; 
         FIG. 6  is a flowchart illustrating an example control procedure for web-jam detection performed by a main control unit  22  according to the embodiment; 
         FIG. 7  is a flowchart illustrating an example procedure related to setting of a web-jam-error masking period to be referred to at Step S 102  in the flowchart of  FIG. 6 ; 
         FIG. 8  is a block diagram illustrating another example of relevant elements of the signal processing system related to web-jam detection; 
         FIG. 9  is a schematic perspective view illustrating basic configuration of a conventional web printing system that includes printers P 1  and P 2 ; 
         FIG. 10  is a schematic configuration diagram illustrating an example configuration of the printer P 1 , P 2  illustrated in  FIG. 9 ; 
         FIG. 11  is a schematic configuration diagram illustrating an example configuration of a conventional web conveying unit; and 
         FIG. 12  is a schematic cross-sectional view illustrating how web-jam detectors  2   a  and  2   b  operate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     Basic configuration of a printing system according to an embodiment of the present invention is similar to the configuration of the conventional system illustrated in  FIGS. 9 to 12 . Accordingly, elements that are same as those in  FIGS. 9 to 12  are denoted by like reference numerals and symbols. 
       FIG. 1  is a block diagram illustrating an example of relevant elements of a signal processing system related to web-jam detection performed in the printers P 1  and P 2  illustrated in  FIG. 2 . Each of the printers P 1 , P 2  includes a web-jam detecting unit  30  that detects a web jam by using the web-jam detecting sensor  2 , which is a integral portion of the tractor doors  12  (see  FIGS. 11 and 12 ) of the web conveying unit  1 . 
     Referring to  FIG. 1 , the web-jam detecting unit  30  includes the web-jam detecting sensor  2 , a web-jam-information receiving unit  21 , and a main control unit  22 . The web-jam detecting sensor  2  may be a web-jam sensor of a transmission type similar to the sensor described above with reference to  FIG. 12 ; more specifically, the web-jam detecting sensor  2  may be a light-sensitive detector that includes a pair of the web-jam detectors  2   a  that emits the sensor light  13  and the web-jam detector  2   b  that receives the sensor light  13 . By providing the web-jam detecting unit  30  with the web-jam detecting sensor  2 , false detection caused by undulation of a web can be prevented. 
     The web-jam-information receiving unit  21  determines whether a web jam has occurred during printing based on a detection signal fed from the web-jam detecting sensor  2 . Upon web jam detection, the web-jam-information receiving unit  21  transmits web-jam information to the main control unit  22 . 
     Upon receiving the web-jam information, the main control unit  22  recognizes that a web jam has occurred, stops printing, and transmits a notification of the web jam detection to the control device  20 . Meanwhile, the main control unit  22  includes a timer  23  serving as a timepiece. 
     As described above, the web printing system includes the control device  20  (see  FIG. 9 ) that controls printing performed by the printer P 1  and the printer P 2 . The control device  20  may be provided on either one of the printers P 1  and P 2 , or, alternatively, independent from the printers P 1  and P 2 . Each of the main control units  22  of the printer P 1  and P 2  is coupled to the control device  20  via a communication channel  25 . Although the communication channel  25  is schematically indicated by a solid line in  FIG. 1 , the communication channel  25  may be realized by an appropriate communication medium, such as wired communication, wireless communication, or optical communication. 
     The control device  20  is configured to communicate with each of the main control units of the printers P 1  and P 2  and read information from each other. The control device  20  includes a touch panel  24  for displaying information for an operator and receiving information entered by an operator. 
     When the web printing system configured as described above is in a paused state, the web W continues to be heated by the pre-heating plate  6  and the heating roller  7  that partially greatly deform the web W. Hence, a thermally shrunk portion is formed on the web W. 
     Conveyance of the web W is resumed as the printing is resumed. During a period where the thermally shrunk portion formed on the web W by the printer P 1  passes through the web conveying unit of the printer P 2 , the thermally shrunk portion may lift up the tractor door  12 , which may cause the web-jam detecting sensor  2  to make false detection of a web jam. 
     To this end, in the present embodiment, false detection of a web jam resulting from detection of the thermally shrunk portion by the web-jam detecting sensor  2  is prevented from by not allowing the printer P 2  to perform the web-jam detection operation for a period, over which the thermally shrunk portion passes through a detection position of the web-jam detecting sensor  2  of the printer P 2 . 
       FIG. 2  is a schematic diagram illustrating, in a developed manner, elements located on a feed path of the web W between a transfer point of the printer P 1  and a transfer point of the printer P 2 . 
     Referring to  FIG. 2 , an operator inputs and sets L, which is the length between the transfer point (position of the transfer device  4 ) of the printer P 1  and the transfer point of the printer P 2  of the web W loaded on a web feed path, to the control device  20  through the touch panel  24  in advance. 
     The length of L is a fixed value having been set so as to allow the printer P 2  to print an image of an original or the like on a back side a page which corresponds to the front side on which the printer P 1  has printed an image of an original or the like. The buffer mechanism  15  serving as an adjusting unit that allows slack in the web W and adjusts the slack to thereby keep the length L constant is disposed at a portion indicated with the caption “INTERMEDIATE FEED PATH” in  FIG. 2 . In  FIG. 2 , the web W is illustrated as being tensioned tight and extending linearly; however, the web W has slack at each of the buffer mechanism  15  and the intermediate feed path. 
     When resuming printing, the control device  20  transmits the length L between the transfer point of the printer P 1  and the transfer point of the printer P 2  of the web W loaded on the web feed path to the main control unit  22  of the printer P 2  via the communication channel  25 . 
     In the main control unit  22  of the printer P 2 , a distance X from the transfer point of the printer P 1  to a leading end of the shooter  10  of the printer P 1 , a distance Y from the web-jam detecting sensor  2  of the printer P 2  to the transfer point of the printer P 2 , and a distance K from a nip between the heating roller  7  and the pressure roller  8  of the printer P 1  to the leading end of the shooter  10  of the printer P 1  are stored in advance. The main control unit  22  calculates a timing (error mask timing) T, which is a period of time that elapses before the thermally shrunk portion reaches the web-jam detecting sensor  2  of the printer P 2 , from Equation (I):
 
 T=S/Z =( L−X−Y+K )/ Z   (I)
 
     where S is the distance from the nip between the heating roller  7  and the pressure roller  8  of the printer P 1  to the web-jam detecting sensor  2  of the printer P 2 , and Z is print speed of the printers. 
     When the distance from the transfer point of the printer P 1  to the nip between the heating roller  7  and the pressure roller  8  of the printer P 1  is assigned to X in Equation (I), the need of taking the distance K into account is eliminated. 
     Meanwhile, the buffer mechanism  15  is provided to absorb the difference between a conveyance speed of the web W by the web conveying unit  5  and a conveyance speed of the web W by the heating roller  7  and the pressure roller  8 . The buffer mechanism  15  keeps the tension placed on the web W constant by changing a pivot angle of the buffer mechanism  15 . Accordingly, the distance X from the transfer point to the leading end of the shooter  10  varies because the amount of the slack in the web W on the feed path varies depending on operation of the buffer mechanism  15 . The amount of the slack in the web W on the feed path is referred to as a buffer amount (adjustment amount), depending on which the distance X varies. 
     In contrast, the distance L is a fixed value as described above. This causes a necessity for making a change to the error mask timing T obtained from Equation (I) depending on the buffer amount. Hence, the distance X is desirably set to different value by displacing with the buffer amount. 
     The pivot angle of the buffer mechanism  15  is classified into a plurality of levels (e.g., five levels) and assumed as buffer positions. The buffer mechanism  15  of the printer P 1  includes a position sensor unit, such as an angle sensor or an encoder, which measures the pivot angle thereof. A transmitting unit of the printer P 1  transmits the pivot angle of the buffer mechanism  15  obtained by the position sensor unit to the control device  20 . 
     The control device  20  receives the pivot angle from the printer P 1  and transmits it to the printer P 2 . The main control unit  22  of the printer P 2  receives the pivot angle from the control device  20  and determines a buffer amount associated with a buffer position of a pivot-angle level, into which the pivot angle is classified. The buffer amount is determined with reference to a reference distance X that is taken in a neutral state where the buffer mechanism  15  is pivoted by a neutral pivot angle, which is a median of pivot angles. 
     The printer P 2  stores a buffer-amount table, which corresponds to an adjustment-amount table, which contains buffer amounts associated with pivot angles of the buffer mechanism  15  of the printer P 1  with reference to a reference buffer position in a storage medium, such as memory. The reference buffer position is a buffer position in the neutral state where the buffer mechanism  15  is pivoted by the neutral pivot angle. 
       FIG. 3  is an explanatory diagram illustrating an example buffer-amount table. As illustrated in  FIG. 3 , buffer positions and buffer amounts associated with each other are stored in the buffer-amount table. The buffer position is a position that corresponds to a pivot angle detected by an angle sensor, an encoder, or the like. The buffer amount is a value, which varies depending on the distance X, and varies relative to the reference buffer amount obtained in the neutral state. 
     In  FIG. 3 , the lower limit position is the position of a minimum pivot angle in a range where the buffer mechanism  15  can pivot as illustrated in  FIG. 4 . The upper limit position is a position where the buffer mechanism  15  is pivoted by a maximum pivot angle. 
     When printing pauses, the main control unit  22  obtains the buffer amount that corresponds to a pivot angle of the buffer mechanism  15  in this paused state by referring to the buffer-length table stored in the memory. The main control unit  22  performs addition or subtraction of the buffer amount to or from the reference distance X, thereby obtaining a corrected value of the distance X. The main control unit  22  sets this corrected value as a value to be assigned to X in Equation (I). 
     The main control unit  22  of the printer P 2  calculates the error mask timing T from Equation (I) whereas the main control unit  22  of the printer P 1  obtains information about the distance X for use in solving Equation (I). Therefore it is necessary to transfer the information about the distance X obtained by the main control unit  22  of the printer P 1  to the main control unit  22  of the printer P 2 . This transfer may be preferably performed by the control device  20 . 
     Assuming that the length of the thermally shrunk portion is approximately equal to Q, which is the distance from the pre-heating plate  6  to the heating roller  7  in the thermal fixing device, the main control unit  22  calculates M, which is a period of time for use in determining a web-jam-error masking period from Equation (II).
 
 M=Q/Z   (II)
 
     The main control unit  22  of the printer P 2  is thus capable of recognizing, respectively, the error mask timing T, which is the time from the resumption of printing to the thermally shrunk portion reaches the web-jam detecting sensor  2 ; and the web-jam-error masking period (period M), for which error detected in the web-jam detection is to be masked, and which is the period of time, over which the thermally shrunk portion passes through the web-jam detecting sensor  2  (period of time required for passage). Accordingly, the main control unit  22  is thus capable of setting a period of time, in which web-jam detection control is not performed, by causing the timer  23  to count (measure) elapsed time from start of printing, which is notified by the control device  20 . 
     In the example described above, the value of the distance X for use in calculating the timing T from Equation (I) has been determined based on the pivot angle of the buffer mechanism  15  in the printing-paused state; however, the timing T may alternatively be obtained by using a fixed value as the distance X. In this alternative configuration, a distance from the transfer point to the leading end of the shooter  10  in a state where the buffer amount of the buffer mechanism  15  is at its maximum is desirably used as the distance X in Equation (I). This allows the web-jam-error masking period to start prior to arrival of a leading end of the thermally shrunk portion at the web-jam detecting sensor  2  based on the time T irrespective of the pivot angle of the buffer mechanism  15 . 
     In addition, the period M is desirably set to such a value that allows the web-jam-error masking period is set within a period the thermally shrunk portion passes through the web-jam detecting sensor  2  irrespective of buffer amount of the buffer mechanism  15 . For instance, it is preferable to assign a sum of the distance of the difference between the minimum value and the maximum value of the buffer amount and the distance from the pre-heating plate  6  to the heating roller  7  to the distance Q in Equation (II). 
     Using fixed values as values of the timing T and the period M allows the main control unit  22  of the printer P 2  to perform calculation by using the distances X, K, and Q calculated based on information about lengths of the feed paths and sizes of the units and the like of the printer P 2  itself as the distances X, K, and Q of the printer P 1  in Equations (I) and (II) because the printer P 1  and the printer P 2  are identical to each other in structure. 
     For a situation where the printer P 1  and the printer P 2  have different structures, for instance, a following configuration may be employed. The main control unit  22  of the printer P 1  transmits the distances X, K, and Q of the printer P 1  itself to the control device  20 , and the control device  20  transmits the distances X, K, and Q to the main control unit  22  of the printer P 2 . This configuration allows the main control unit  22  of the printer P 2  to obtain information about the distances X, K, and Q for use in solving Equations (I) and (II). 
     In the present embodiment, the buffer amount is obtained by accessing the buffer-amount table, in which the buffer positions and the buffer amounts are associated with each other, stored in the memory or the like in advance; however, a method for obtaining a buffer amount is not limited thereto. For instance, a configuration to be described below for dynamically calculating a buffer amount, which causes the distance X displace, based on the buffer position (pivot angle of the buffer mechanism  15 ) may be alternatively employed. 
       FIGS. 4 and 5  are explanatory diagrams for calculating a buffer amount.  FIG. 4  is a diagram of a situation where the buffer mechanism  15  is at the lower limit position. As illustrated in  FIG. 4 , in the situation where the buffer mechanism  15  is at the lower limit position, the main control unit  22  calculates x1, which is the distance from a distal end, which is on the opposite side from the pivot center, of the buffer mechanism  15  to the pre-heating plate  6 , from Equation (III):
 
 x 1 =B−A   (III)
 
     where A is the length of the buffer mechanism  15 , and B is the distance from the pivot center of the buffer mechanism  15  to the pre-heating plate  6 . 
     The transmitting unit of the printer P 1  transmits: A which is the length of the buffer mechanism  15 ; B which is the distance from the pivot center of the buffer mechanism  15  to the pre-heating plate  6 ; and θ which is the pivot angle of the buffer mechanism  15  detected by the position sensor unit of the printer P 1 , to the control device  20 . The control device  20  transfers A, B, and θ to the printer P 2 . The main control unit  22  of the printer P 2  receives A, B, and θ from the control device  20 . 
       FIG. 5  is a diagram illustrating a situation where the buffer mechanism  15  has pivoted. 
     In this situation, the main control unit  22  calculates x2, which is the distance from the distal end of the buffer mechanism  15  to the pre-heating plate  6 , from Equation (IV).
 
 x 2=√( A   2   +B   2 −2 AB  cos θ)  (IV)
 
     The main control unit  22  uses Equations (III) and (IV), and the buffer mechanism  15  obtains the buffer amount Δx, which is a displacement of the web conveyance distance from the lower limit position using Equation (V).
 
Δ x=x 2 −x 1  (V)
 
     The main control unit  22  adds the buffer amount Δx obtained from Equation (V) to the distance X. 
     The method described above allows dynamic calculation of a buffer amount to thereby adjust the distance X. 
     A web-jam detection control procedure performed by the printing system configured described above according to the present embodiment is described below.  FIG. 6  is a flowchart illustrating an example control procedure for the web-jam detection performed by the main control unit  22  according to the present embodiment. 
     Referring to  FIG. 6 ; the main control unit  22  monitors whether the printer is performing printing at Step S 101 . If the printer is performing printing (YES at Step S 101 ), the main control unit  22  monitors whether the printer is in the web-jam-error masking period at Step S 102 . If the printer is in the web-jam-error masking period (YES at Step S 102 ); the main control unit  22  does not perform the web-jam detection. If the printer is not in the web-jam-error masking period (NO at Step S 102 ), the main control unit  22  monitors the web-jam detection at Step S 103 . 
     If the main control unit  22  detects a web jam (YES at Step S 103 ), the main control unit  22  performs web-jam-error processing that includes transmitting a notification of occurrence of the web jam to the control device  20  and causing printing (web conveyance) to pause at Step S 104 . 
     In the procedure illustrated in  FIG. 6 , the web-jam detection is not performed during the web-jam-error masking period at Step S 102 . Alternatively, a method of preventing false detection of a web jam by, in lieu of setting the error masking period, increasing a required number of recheck times to determine that a web jam has occurred for the period corresponding to the web-jam-error masking period to thereby decrease sensitivity to web-jam error detection may be employed. 
       FIG. 7  is a flowchart illustrating an example procedure related to setting of the web-jam-error masking period to be referred to at Step S 102  in the flowchart of  FIG. 6 . 
     Referring to  FIG. 7 ; the main control unit  22  monitors whether printing has been started at Step S 201 . If the printing has been started (YES at Step S 201 ), the main control unit  22  causes the timer  23  to start counting time at Step S 202 . 
     The main control unit  22  monitors whether the error mask timing T calculated in advance (see Equation (I)) has elapsed at Step S 203 . If the time T has elapsed (YES at Step S 203 ), it is indicated that a thermally shrunk portion has reached the web-jam detecting sensor  2 . Accordingly, the main control unit  22  sets the printer to be the web-jam-error masking period at Step S 204 . Subsequently, the main control unit  22  monitors whether the period M, which is the web-jam-error masking period calculated in advance, has elapsed at Step S 205 . If the web-jam-error masking period M has elapsed (YES at Step S 205 ), the main control unit  22  cancels the web-jam-error masking period setting (Step S 206 ). 
     In the present embodiment, the web-jam-error masking period is determined by causing the timer  23  to count the period M calculated in advance based on the error mask timing T. However, the web-jam-error masking period may be determined by other method, example of which includes a method of defining the web-jam-error masking period by counting the number of printed pages. 
       FIG. 8  is a block diagram illustrating another example of relevant elements of the signal processing system related to the web-jam detection. In  FIG. 8 , elements identical or corresponding to those illustrated in  FIG. 1  are denoted by like reference numeral and descriptions thereabout are omitted. 
     Referring to  FIG. 8 , when printing is resumed, the main control unit  22  calculates the timing T which is a time thermally shrunk portion reaches the printer P 2 . In a situation where after the printing is resumed, printing pauses before the thermally shrunk portion reaches the printer P 2 , another thermally shrunk portion is formed by the pre-heating plate  6  and the heating roller  7  of the printer P 1  during a period, over which the printing pauses. Accordingly, at second resumption of the printing, or, put another way, when the printing is resumed for the second time, the web W has two thermally shrunk portions on the web feed path between the printer P 1  and the printer P 2 . 
     In this case, the main control unit  22  calculates the error mask timing T 1  of the thermally shrunk portion formed at first from Equation (VI):
 
 T 1 =T−Tp   (VI)
 
     where Tp is a period of time (in other words, print duration) that has elapsed from resumption of the printing until the printing pauses again. 
     If calculated value of the time T 1  is equal to or greater than one, the main control unit  22  stores the value in a storage device  26 . 
     The main control unit  22  counts, when printing is resumed for the second time, each of the error mask timing T (see Equation (I)) and the error mask timing T 1  related to arrival of the thermally shrunk portion at the printer P 2 . When the time has elapsed, the main control unit  22  sets the printer to enter a web-jam-error masking period of the period M to thereby mask the web-jam detection for each of the thermally shrunk portions. This allows masking of an error resulting from detection of a web jam to be started at each of points in time where the thermally shrunk portions individually reach the web-jam detecting sensor  2 , thereby allowing appropriate web-jam detection. 
     In the example described above, it is assumed that there are two thermally shrunk portions on the web. However, the main control unit  22  is adaptable to a situation where there are three or more thermally shrunk portions by calculating, for each of the thermally shrunk portions, time that is to elapse before the thermally shrunk portion reaches the web-jam detecting sensor  2  of the printer P 2  by performing similar calculation to that according to Equation (VI) and storing results of the calculation in the storage device  26 . 
     In a case when the printing pauses in the middle of a web-jam-error masking period of the web-jam detecting sensor  2 . The main control unit  22  stops a process that corresponds to the web-jam-error masking period. When resuming the printing, it is possible to deal with the above described situation by resuming the operations that correspond to the paused web-jam-error masking period. 
     According to the embodiment described above, the web printing system free from false detection of a web jam in the printer P 2  even when printing is resumed from a paused state can be provided. Prevention against false detection of a web jam that may be caused by a thermally shrunk portion that has been at rest at the pre-heating plate  6  of the printer P 1  may be achieved. Prevention against false detection that may be caused due to a plurality of thermally shrunk portions may also be achieved. 
     In the embodiment discussed above, the printer P 1  forms an image on the first surface of the web W while the printer P 2  forms an image on the second surface of the web W; however, the present invention is similarly applicable to a configuration where the printer P 2  additionally forms an image of the first surface of the web W as well. 
     In the embodiment described above, the standard-compliant feed holes H are created in both lateral-side portions of the web W so that the web W is conveyed by the web conveying units  1  and  5 ; however, the present invention is similarly applicable to a configuration where the holes H are not created in both lateral-side portions of the web W. 
     In the embodiment described above, the printer P 1  and the printer P 2  are identical to each other in structure; however, the present invention is similarly applicable to a configuration where the printer P 1  and the printer P 2  have different structures. 
     The embodiments and modifications described above may be combined appropriately without departing from the scope of the present invention. In the embodiments described above, printing paper is used as the web; however, the present invention is similarly applicable to a configuration where a thermally deformable print medium, such as film, is used as the web. 
     The present invention is applicable to any printing system that includes a first printer that forms an image on elongated print medium by performing image forming process that involves thermal fixing and a second printer that receives the print medium with the image printed thereon fed from the first printer and forms an image on the print medium that is conveyed at the same conveyance speed as that in the first printer irrespective of image forming process and structure of the second printer. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.