Image forming apparatus and control method thereof

An image forming apparatus of the invention conveys paper fed by a paper feeding mechanism piece by piece to a paper discharging unit through a paper conveying path. An image forming unit is arranged in a midway portion of this paper conveying path and executes an image forming process for printing an image based on image data on paper being conveyed. A sensor that detects a type of the paper conveyed is arranged on an upstream side with respect to the image forming unit on the paper conveying path. Image forming process conditions for the image forming unit are set in accordance with a detection result of the sensor. When the paper fed is paper not detected by the sensor, the image forming process is not started. After paper type detection by the sensor, the paper is directly discharged. When the paper fed is paper of the same type as paper detected by the sensor, the image forming process is started without performing paper type detection. Moreover, paper being conveyed is conveyed to the image forming unit, in which the image forming process is started, in synchronization with a printing operation of the image forming unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image forming apparatuses that can print characters and figures on various kinds of paper such as a printer and a copying machine.

2. Description of the Related Art

In general, in this type of image forming apparatuses, printing on various kinds of paper having different thicknesses and the like is possible. However, a difference in a paper type affects transferability, fixability, and the like in printing. Therefore, for example, in an image forming apparatus of an electrophotographic system, improvement in an image quality is realized by, for example, changing a quantity of toner for transferring an image to paper, transfer conditions, and fixing conditions according to paper thickness.

Conventionally, an operator visually checks and manually designates paper type information such as thickness. However, for example, because this work is extremely complicated, it is proposed to automatically detect a paper type using a transmission optical sensor or the like.

For example, in JP-A-2004-277057, paper feeding mechanism that feeds paper piece by piece is provided for each paper feeding cassette that stacks and holds paper and, on an outgoing side of the paper feeding mechanism, a paper type sensor that detects type (thickness, etc.) of paper fed by this paper feeding mechanism is provided. In preparation for printing to be performed later, a leading end of one piece of paper at the top is fed from the paper feeding cassette by a predetermined quantity by the paper feeding mechanism at timing prior to an original paper feeding command and a type of the paper is detected by the paper type sensor.

In this way, it is possible to acquire paper type information at the timing prior to the original paper feeding command. This makes it possible to perform, for an image forming process of an image forming unit, proper setting using the detected paper type information before the original paper feeding command for printing is received.

In this type of image forming apparatuses, in general, plural paper feeding cassettes as paper supplying sources are provided. Pieces of paper of types different from one another are often provided in the plural paper feeding cassettes, respectively, to make it possible to select a desired paper type from these paper feeding cassettes and perform printing. In this case, pieces of paper fed from the respective paper feeding cassettes by paper feeding mechanisms are separately sent to a common paper conveying path leading to the image forming unit, and sent to the image forming unit and printed, respectively.

A case in which the conventional technique described above is applied to such image forming apparatuses will be examined. For example, it is assumed that paper is stored in a paper feeding cassette and the paper feeding cassette is set in an image forming apparatus main body. In this case, to detect a paper type in advance in preparation for printing after this, as described above, a leading end of one piece of paper is fed from the paper feeding cassette by a predetermined quantity by the paper feeding mechanism and a type of the paper is detected by the paper type sensor. Thereafter, if an original paper feeding command for the paper, the type of which is detected, is continuously issued, no problem occurs because it is possible to send the paper fed halfway to the image forming unit as it is. However, if an original paper feeding command for paper stored in another paper feeding cassette is issued, it is likely that the paper, the type of which is detected, hinders feeding and conveyance of the paper according to the original paper feeding command.

In short, in the image forming apparatus having the plural paper feeding cassettes, it is not definitely decided to which one of the paper feeding cassettes (paper types) an original paper feeding command is given. The paper feeding cassette is changed according to a print request. Therefore, as described above, it often occurs that, after a paper type of a paper feeding cassette set anew is detected, an original paper feeding command is given to another paper feeding cassette.

In this case, since the paper, the type of which is detected, stays in a state in which the leading end thereof is fed by the predetermined quantity, it is likely that the paper hinders movement of paper fed from another paper feeding cassette to a common conveying path. To prevent the paper from hindering the movement of another piece of paper, it is necessary to provide the paper type sensor in an extremely narrow portion near the paper feeding mechanism and arrange and set the paper type sensor such that the leading end of the paper in the type detected state does not project over the conveying path of another piece of paper. Therefore, the paper type sensor is subjected to restriction in terms of arrangement. Even if the paper type sensor is arranged in this way, when a quantity of paper feeding by the paper feeding mechanism is unstable, the leading end may project to the common conveying path and interfere with the movement of another piece of paper. When double feeding occurs, remaining paper may project to the common conveying path.

To surely prevent such interference, paper once fed to the sensor portion only has to be returned into the paper feeding cassette after the detection by the sensor.

However, to return the paper partially fed from the paper feeding cassette into the paper feeding cassette after the paper type detection, a complicated mechanism is required, which causes a failure. It is not preferable to return the paper once fed because a burden on the paper is heavy and a jam is caused.

Providing the paper type sensor near the paper feeding mechanism leads to misdetection of a paper type as well. The paper feeding mechanism takes out uppermost paper stacked and held in the paper feeding cassette with a pickup roller and separates and feeds the paper piece by piece with a paper feeding roller arranged on a front surface side of the paper and a separation roller arranged on a rear surface side of the paper. However, when a separating action is not sufficient, two pieces of paper may be delivered together right behind the paper feeding roller and the separation roller. Even if the two pieces of paper are delivered together, in most cases, only one piece of paper is delivered to the conveying path by an action of the separation roller. However, when the paper type sensor is provided near the separation and paper feeding mechanisms, the paper type sensor detects these two pieces of paper. Thus, for example, in the case of a sensor that detects paper thickness, misdetection is caused.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an image forming apparatus that can perform printing under optimum conditions according to a paper type and without deteriorating performance because the image forming apparatus performs paper feeding for detecting a paper type prior to original paper feeding and, after the detection of the paper type, sends paper in a state in which the paper does not hinder conveyance of another piece of paper and prepares for the original paper feeding after that.

According to an aspect of the invention, an image forming apparatus includes: a paper feeding mechanism that feeds paper piece by piece; a paper conveying path that conveys, when paper is fed by this paper feeding mechanism, the paper to a paper discharging unit; an image forming unit that is arranged between the paper feeding mechanism and the paper discharging unit on this paper conveying path and executes an image forming process for printing an image based on image data on the paper being conveyed by the paper conveying path; a sensor that is arranged on an upstream side with respect to the image forming unit on the paper conveying path and detects a type of paper conveyed by this paper conveying path; a condition setting unit that sets conditions for an image forming process of the image forming unit in accordance with a value detected by this sensor; an image-forming-process control unit that does not start the image forming process when paper fed to the paper conveying path by the paper feeding mechanism is paper not detected by the sensor and starts the image forming process without awaiting paper type detection when paper fed to the paper conveying path is detected by the sensor; and a paper-conveying-path control unit that causes, when paper fed to the paper conveying path is paper not detected by the sensor, the paper conveying path to discharge the paper through the image forming unit, in which the image forming process is not started, and causes, when paper fed to the paper conveying path is paper of a type same as paper detected by the sensor, the paper conveying path to convey the paper to the image forming unit, in which the image forming process is started, in synchronization with a printing operation of the image forming unit.

According to another aspect of the invention, an image forming apparatus includes: a paper feeding mechanism that feeds paper piece by piece; a main paper conveying path that conveys, when paper is fed by this paper feeding mechanism, the paper to a paper discharging unit; an image forming unit that is arranged between the paper feeding mechanism and the paper discharging unit on this main paper conveying path and executes an image forming process for printing an image based on image data on the paper being conveyed by the main paper conveying path; a sensor that is arranged on an upstream side with respect to the image forming unit on the main paper conveying path and detects a type of paper conveyed by this main paper conveying path; a sub paper conveying path that is divided from a downstream side with respect to the image forming unit on the main paper conveying path, joins with the upstream side with respect to the image forming unit on the main paper conveying path, and, in operation, switches back paper conveyed from the image forming unit to the paper discharging unit and joins the paper to the upstream side of the image forming unit; a condition setting unit that sets conditions for an image forming process of the image forming unit in accordance with a value detected by this sensor; an image-forming-process control unit that does not start the image forming process when paper fed to the main paper conveying path by the paper feeding mechanism is paper not detected by the sensor and starts the image forming process according to the paper feeding to the main paper conveying path when paper fed to the paper conveying path is paper of a type same as paper detected by the sensor; and a paper-conveying-path control unit that conveys, when paper fed to the main paper conveying path is paper not detected by the sensor, the paper to the sub paper conveying path, starts the image forming process, and causes the sub paper conveying path to convey the paper conveyed through the sub conveying path to the image forming unit, in which the image forming process is started, in synchronization with a printing operation of the image forming unit and causes, when paper fed to the paper main conveying path is paper of a type same as paper detected by the sensor, the main paper conveying path to convey the paper to the image forming unit, in which the image forming process is started, in synchronization with a printing operation of the image forming unit.

According to still another aspect of the invention, a control method of an image forming apparatus includes: a step of feeding paper piece by piece to a main paper conveying path that conveys fed paper to a paper discharging unit; a step of detecting a type of the paper fed to the main paper conveying path with a sensor; a step of setting, in accordance with a detection result of the sensor, image forming process conditions for an image forming unit that executes an image forming process for forming an image based on image data and printing the image on paper being conveyed by the paper conveying path; a step of not starting the image forming process when paper fed to the paper main conveying path is paper, a type of which is not detected by the sensor, passing the paper through the image forming unit, in which this image forming process is not started, actuating a sub paper conveying path that is divided from an exit side of this image forming unit and joins with an upstream side with respect to the image forming unit of the main paper conveying unit, switching back paper conveyed from the image forming unit to the paper discharging unit, joining the paper to the upstream side of the image forming unit, starting the image forming process, and conveying the paper to the image forming unit in synchronization with printing timing of the image forming unit; and a step of starting the image forming process without performing paper type detection when paper fed to the main paper conveying path is paper of a type same as type detected by the sensor and causing the main paper conveying path to convey the paper to the image forming unit in synchronization with a printing operation of the image forming unit.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be hereinafter explained in detail with the accompanying drawings as examples.

FIG. 1shows a structure of an image forming apparatus according to a first embodiment of the invention.FIG. 2shows control unit of the image forming apparatus as functional blocks. InFIG. 1, reference numeral1denotes an apparatus main body. A transparent original stand (a glass plate)2for placing an original is provided in an upper part of the apparatus main body1. A cover3is provided on this original stand2to be freely opened and closed. A carriage4is provided on a lower surface side of the original stand2. An exposure lamp5is provided in the carriage4. The carriage4is capable of reciprocatingly moving along the lower surface of the original stand2. This carriage4reciprocatingly moves while lighting the exposure lamp5to expose an original on the original stand2. A reflected light image of the original placed on the original stand2is obtained by this exposure. The reflected light image is projected on a CCD (Charge Coupled Device)10by reflecting mirrors6,7, and8and a lens block for magnification9. The CCD10outputs an image signal corresponding to the reflected light image of the original.

A scan unit (203inFIG. 2) that optically scans an image of the original placed on the original stand2is constituted by the carriage4, the exposure lamp5, the reflecting mirrors6,7, and8, the lens block for magnification9, and the CCD10.

Reference numeral220denotes an image forming unit, which is constituted by a print engine (213inFIG. 2) constituted by an exposing unit11and the like, photoconductive drums21,22,23, and24, a transfer belt25, a drive roller26, and a process unit (214inFIG. 2) constituted by a transfer-roller driving unit and the like. This image forming unit220forms an image based on image data (the image signal outputted from the CCD10) and executes an image forming process for printing the image on paper being conveyed. The image signal outputted from the CCD10is supplied to the exposing unit11first after being appropriately processed. The exposing unit11emits a laser beam B1corresponding to an image signal of a yellow color, a laser beam B2corresponding to an image signal of a magenta color, a laser beam B3corresponding to an image signal of a cyan color, and a laser beam B4corresponding to an image signal of a black color to a photoconductive drum21for the yellow color, a photoconductive drum22for the magenta color, a photoconductive drum23for the cyan color, and a photoconductive drum24for the black color, respectively.

The photoconductive drums21,22,23, and24are arrayed substantially in the horizontal direction at fixed intervals. The transfer belt25of an endless shape is provided above these photoconductive drums21,22,23, and24. The transfer belt25is laid over the drive roller26, guide rollers27,28, and29, and a driven roller30. The transfer belt25is subjected to power from the drive roller26and rotates to travel in a counterclockwise direction. The guide roller27is provided to move up and down freely. The guide roller27is subjected to rotation of a cam31and moved to the transfer belt25side to displace the transfer belt25to the photoconductive drums21,22,23, and24side.

Primary transfer rollers41,42,43, and44are provided to be movable up and down in positions opposed to the photoconductive drums21,22,23, and24across the transfer belt25. The primary transfer rollers41,42,43, and44are moved (lowered) to the transfer belt25side to bring the transfer belt25into contact with the photoconductive drums21,22,23, and24and transfer visible images on the photoconductive drums21,22,23, and24to the transfer belt25.

Although not shown in the figure, a cleaner, a charge removing lamp, a charging unit, and a developing unit are displayed one after another around the photoconductive drum21(and the other photoconductive drums22,23, and24as well). The cleaner has a cleaning blade, which comes into contact with the surface of the photoconductive drum21, and scrapes off a developing material remaining on the surface of the photoconductive drum21with the cleaning blade. The charge removing lamp removes electric charges remaining on the surface of the photoconductive drum21. The charging unit applies a high voltage to the photoconductive drum21to charge the surface of the photoconductive drum21with electrostatic charges. The laser beam B1emitted from the exposing unit11is irradiated on the surface of the photoconductive drum21subjected to the charging. An electrostatic latent image is formed on the surface of the photoconductive drum21by this irradiation. The developing unit supplies a developing material (a toner) of the yellow color to the surface of the photoconductive drum21to visualize the electrostatic latent image on the surface of the photoconductive drum21.

Plural paper feeding cassettes50as paper supplying sources are provided below the exposing unit11. A large number of pieces of paper P of types different from one another are stored in these paper feeding cassettes50in a stacked state. Paper feeding mechanisms221that feed the paper in the paper feeding cassettes50piece by piece from the top are provided in exit portions (on the right side in the figure) of these paper feeding cassettes50, respectively. The paper P is taken out piece by piece from any one of the paper feeding cassettes50by this paper feeding mechanism221. This paper feeding mechanism221for taking out paper is constituted by a pickup roller51, a paper feeding roller52a, and a separation roller52b. The paper feeding mechanism221separates the paper P taken out from the paper feeding cassette50and feeds the paper to a paper conveying path53piece by piece.

The paper conveying path53extends to a paper discharge port54above through the driven roller30of the image forming unit220. The paper discharge port54faces a paper discharging unit55leading to an outer peripheral surface of the main body1. Conveying rollers56are provided near the paper feeding mechanisms221, respectively, on a start end side of the paper conveying path53. When paper is fed to this paper conveying path53by any one of the paper feeding mechanisms221, the paper conveying path53conveys the paper fed to the paper discharging unit55.

A secondary transfer roller57is provided, across the transfer belt25, in a position opposed to the driven roller30in a midway portion of the paper conveying path53. Registration rollers58are provided in a position before the driven roller30and the secondary transfer roller57in a conveying direction. The registration rollers58deliver the paper P into a space between the transfer belt25and the secondary transfer roller57at timing synchronizing with a transfer operation by the transfer belt25and the secondary transfer roller57. The secondary transfer roller57transfers, while nipping the paper P delivered from the registration rollers58between the secondary transfer roller57and the transfer belt25on the driven roller30, a visible image transferred on the transfer belt25to the paper P and prints the visible image on the paper P. In other words, the registration rollers58convey the paper P to the image forming unit220having the transfer belt25and the secondary transfer roller57in synchronization with a printing operation of the image forming unit220.

A heat roller59for heat fixing and a press-contact roller60in contact with this heat roller59are provided in a position of the paper conveying path53further on a downstream side than the secondary transfer roller57. The image transferred to the paper P is fixed by the heat roller59and the press-contact roller60. A paper discharging roller61is provided at an end of the paper conveying path53.

Reference numeral222denotes an automatic duplex unit (hereinafter referred to as ADU). The ADU222has a paper conveying path62that is divided from a downstream side with respect to the image forming unit220on the paper conveying path53(the end of the conveying path53) and joins with an upstream side with respect to the image forming unit220on the paper conveying path53(an upstream side position of the registration rollers58). This paper conveying path62reverses the front and the back of the paper P for duplex printing. The paper conveying path62is provided with the conveying rollers63,64, and65and, in operation, switches back paper conveyed from the image forming unit220to the paper discharging unit55and joins the paper to the upstream side of the image forming unit220. In this way, the paper P returned to the upstream side of the image forming unit220by the paper conveying path62joins with the paper conveying path53. The paper P is delivered to the image forming unit220by the registration rollers58in synchronization with a printing operation of the image forming unit220. Therefore, the visible image on the transfer belt25is transferred to the rear surface of the paper P and printed.

The paper conveying path53that conveys the paper fed by the paper feeding mechanism221to the paper discharging unit55is set as a main paper conveying path and the paper conveying path62for reversing the front and the back of the paper is set as a sub paper conveying path. This sub paper conveying path62comes into the operation state described above when the duplex printing is designated by a not-shown control panel or the like provided in the apparatus main body1.

Reference numeral223denotes a paper type sensor, which is arranged on the upstream side with respect to the image forming unit220on the main paper conveying path53(a position further on the upstream side than the registration rollers58) and detects a type of paper conveyed by the paper conveying path53.FIG. 3is schematically shows the main paper conveying path53section inFIG. 1extracted and enlarged. InFIG. 3, a positional relation among the paper type sensor223, the registration rollers58, and joining points on the main paper conveying path53is clearly shown. As this paper type sensor223, a publicly known sensor that discriminates a paper type by detecting the thickness and luminous transmittance of paper only has to be used.

In an example inFIG. 1, the two paper feeding cassettes50are provided as paper feeding sources. However, it goes without saying that the number of paper feeding cassettes50may be three or more. Besides, although not shown in the figure, a manual paper feeding mechanism (hereinafter referred to as SFB) and a large capacity paper feeder (hereinafter referred to as LCF) capable of stacking and feeding several thousand pieces of paper are used. Paper feeding paths from the SFB and LCF also join with the main paper conveying path53. The paper type sensor223is arranged further on the downstream side than joining points of the paper feeding paths from the paper feeding sources SFB and LCF. Therefore, it is possible to detect, with one paper type sensor223, all types of paper fed from the respective paper feeding sources and conveyed on the main paper conveying path53.

A cleaner36is provided across the transfer belt25in a position opposed to the driver roller26of the image forming unit220. This cleaner36has a cleaning blade36a, which comes into contact with the transfer belt25, and scrapes off a developing material remaining on the transfer belt25with the cleaning blade36a. Hooks71,72,73, and74are provided near the primary transfer rollers41,42,43, and44. These hooks71,72,73, and74can engage with shafts of the primary transfer rollers41,42,43, and44and lift the shafts while rotating and move the primary transfer rollers41,42,43, and44in an upward direction in the figure. It is possible to set a full color mode, a fully separated mode, and a monochrome mode according to which ones (one) of the primary transfer rollers41,42,43, and44are lifted.

A control circuit for the apparatus main body1will be explained usingFIG. 2. Reference numeral200denotes a main controller. A control panel controller201, a scan controller202, and a print controller210are connected to the main controller200. The main controller200collectively controls the control panel controller201, the scan controller202, and the print controller210.

The scan unit203is connected to the scan controller202. As described above, the scan unit203is constituted by the carriage4, the exposure lamp5, the reflecting mirrors6,7, and8, the lens block for magnification9, the CCD10, and the like and optically scans an image of an original placed on the original stand2.

The print controller210is connected to the paper type sensor223and is connected to a ROM211for control program storage, a RAM212for data storage, the print engine213, the process unit214, and a conveying unit215, respectively. As described above, the print engine213is constituted by the exposing unit11and the like. As described above, the process unit214is constituted by the photoconductive drums21,22,23, and24, the transfer belt25, the drive roller26, the transfer-roller driving unit, and the like. The conveying unit215is constituted by conveying mechanisms, driving circuits for the conveying mechanisms, and the like in the paper conveying paths53and62.

As shown inFIG. 2, the print controller210has a condition setting unit210a, a process control unit for image formation210b, and a paper-conveying-path control unit210cas functions.

The condition setting unit210aoptimally sets conditions for the image forming unit220(the process unit214inFIG. 2is equivalent to the image forming unit220) in accordance with a detection result of the paper type sensor223.

The process control unit for image formation210bcontrols start of the image forming process in the image forming unit220according to whether a type of paper fed from the paper feeding cassette50to the main paper conveying path53by the paper feeding mechanism221is detected by the paper type sensor223. When a paper type is not detected yet, the process control unit for image formation210bdoes not start the image forming process. On the other hand, when a paper type is already detected (for example, when remaining pieces of paper are fed from the same cassette), the process control unit for image formation210bcan start the image forming process without awaiting paper type detection.

When the paper P is fed to the paper conveying path53by the paper feeding mechanism221, the paper-conveying-path control unit210ccontrols the paper conveying path53to convey this paper to the paper discharging unit55. The paper-conveying-path control unit210ccontrols the registration rollers58to convey paper, which is conveyed by the paper conveying path53, to the image forming unit220, in which the image forming process is started, in synchronization with a printing operation of the image forming unit220.

In the constitution described above, for example, when one of the paper feeding cassettes50stores one new paper type and is set in the apparatus main body1, the paper type of the paper P stored in this paper feeding cassette50is not detected by the paper type sensor223yet. In this state, when a paper feeding command is issued to perform printing on the paper in the paper feeding cassette50, in which this undetected paper type is stored, the paper feeding mechanism221corresponding to this paper feeding cassette50feeds one piece of the paper P in the paper feeding cassette50to the main paper conveying path53.

The paper-conveying-path control unit210ccauses the conveying roller56to rotate and convey the paper P fed to the paper discharging unit55. In the middle of this conveyance, a paper type of the paper P is detected by the paper type sensor223.

When a paper type of the paper fed to the main paper conveying path53is not detected yet as described above, the process control unit210bdoes not start the image forming process in the image forming unit220. At this point, since the main paper conveying path53continues to convey the paper fed, the paper passes through the image forming unit22, in which the image process is not started, and is directly discharged to the paper discharging unit55.

When a paper feeding command is issued for paper stored in the paper feeding paper cassette50same as the paper, the paper type of which is detected as described above, the process control unit210bdetermines that the paper fed this time is paper of the same type as the paper, the paper type of which is already detected by the paper type sensor223. At this point, since conditions for the image forming unit220are optimally set according to the paper type already detected, the process control unit210bstarts the image forming process without awaiting paper type detection. In this case, the paper-conveying-path control unit210ccontrols the registration roller58to deliver the paper to the image forming unit200in synchronization with a printing operation of the image forming unit220. At this point, since the image forming process is already started, printing based on image data is promptly performed in the image forming unit220.

As described above, in the image forming apparatus according to the first embodiment of the invention, in the case in which a paper type is not detected yet, when first piece of paper is fed to the main paper conveying path53, the paper is preliminarily passed to the paper discharging unit55while a paper type thereof is detected in the middle of the preliminary paper passage. In other words, a paper type is detected by the paper type sensor223in a process of the preliminary paper passage. Therefore, at a stage when second and subsequent pieces of paper are fed to the main paper conveying path53, since the image forming process is optimally set and the image forming process is started according to this paper feeding, it is possible to promptly perform printing under optimum conditions and realize high performance.

Since the paper preliminarily passed for paper type detection is discharged, unlike in the past, paper, a paper type of which is already detected, does not stop in the middle of paper feeding because of an instruction for feeding of paper of another paper type and hinder feeding and conveyance of another piece of paper and it is unnecessary to once return the paper to a feeding source to prevent this hindrance. Therefore, a structure is simplified and it is possible to prevent occurrence of a paper jam and the like and perform smooth printing.

Since the paper preliminarily passed for paper type detection and discharged is not printed yet, it is possible to use the paper again as paper for printing and no waste occurs.

An image forming apparatus according to a second embodiment of the invention will be explained. An image detecting apparatus according to the second embodiment returns paper preliminarily passed for paper type detection to the upstream side of the image forming unit220using the automatic duplex unit222without discharging the paper and continuously executes printing.

Therefore, the paper-conveying-path control unit210cof the print controller210is configured to, when paper fed to the main paper conveying path53by the paper feeding mechanism221is paper, a paper type of which is not detected by the sensor223, actuate the sub paper conveying path62of the automatic duplex unit222. The process control unit210bis configured to, when paper fed to the main paper conveying path53by the paper feeding mechanism221is paper not detected by the sensor223, not start the image forming process but start the image forming process when the sub paper conveying path62is actuated as described above.

In the constitution described above, in the case in which one of the paper feeding cassettes50stores a new paper type and is set in the apparatus main body1, when a paper feeding command for printing is issued for the paper P stored in this paper feeding cassette50, the paper feeding mechanism221feeds paper from the paper feeding cassette50to the main paper conveying path53. When one piece of paper is fed to the main paper conveying path53in this way, the paper-conveying-path control unit210ccauses the conveying roller56to rotate and convey the paper P fed to the paper discharging unit55. A paper type of the paper P is detected by the paper type sensor223in the middle of this conveyance.

The condition setting unit210aoptimally sets, in accordance with a detection result of the paper type sensor223, conditions for the image forming process of the image forming unit220to match a paper type detected. On the other hand, since a paper type of the paper fed to the main paper conveying path53is not detected yet, the process control unit210bdoes not start the image forming process in the image forming unit220. However, since the main paper conveying path53is continuing to convey the paper fed, the paper passes through the image forming unit220in which the image process is not started.

At this point, since the paper-conveyance controlling means210cactuates the sub paper conveying path62of the duplex automatic unit223, the sub paper conveying path62catches the paper conveyed from an exit of the image forming unit220to the paper discharging unit55and switches back the paper. Therefore, paper preliminarily passed is joined to the upstream side of the image forming unit220while the front and the back thereof are reversed. The process control unit210bhas started the image forming process because the sub paper conveying path62is actuated. The paper-conveyance controlling means210ccontrols the registration roller58to convey the paper returned to the upstream side of the image forming unit220as described above to the image forming unit220, in which the image forming process is started, in synchronization with print timing of the image forming process. Therefore, predetermined printing is applied to the paper preliminarily passed by the image forming unit220.

As described above, when a paper feeding command for printing is given to the paper feeding cassette50, if a paper type of the paper feeding cassette is not detected yet, a first piece of paper is preliminarily passed and a paper quality of the paper is detected, and, then, the paper is returned to the upstream side of the image forming unit220by the sub paper conveying unit62without being discharged. Thus, it is possible to perform printing with the image forming process optimally set according to the paper quality detected.

As in the first embodiment, concerning second and subsequent pieces of paper, the image forming process is started without awaiting paper type detection and printing based on image data is promptly performed in the image forming unit220under optimum conditions.

In this second embodiment, as in the first embodiment, unlike in the past, paper preliminarily passed does not stop in the middle of paper feeding and hinder feeding and conveyance of another piece of paper and it is unnecessary to once return the suspended paper to a feeding source to prevent this hindrance. Since the existing duplex automatic unit222is used, it is possible to perform printing on the paper preliminarily passed without providing a new mechanism.