Image forming apparatus that changes nip pressure based on sheet characteristics

An image forming apparatus includes a plate, a feeding unit that feeds a recording material placed on the plate, an image forming unit that form an image on the fed recording material, a conveying unit to convey a recording material pinched by a nip pressure from a rotating member nip portion, a switching unit to switch the nip pressure, a detecting unit, and a control unit. The detecting unit detects property information regarding the fed recording material. In response to an instruction to form the image on the recording material and before detection of property information completes, the control unit controls to switch the nip pressure based on information regarding image forming on the recording material, and after detection of the property information completes, the control unit controls to switch the nip pressure based on the information regarding the image forming on the recording material and the detected property information.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates to an image forming apparatus including a curl correction mechanism configured to correct a curl on a recording material and a fixing unit configured to fix an image onto a recording material.

Description of the Related Art

Conventionally, some image forming apparatuses such as a copier and a printer may have a curl correction mechanism configured to correct a curl on a recording material formed when passing through a fixing unit. The curl correction mechanism has a nip portion formed by members such as rollers or a belt and applies pressure to a curled recording material at the nip portion while conveying the recording material.

Japanese Patent Laid-Open No. 6-258906 discloses an arrangement configured to adjust nip pressure to be generated by a curl correction mechanism and to be applied to a curled recording material based on a printing ratio of an image formed on the recording material. In order to form a full-color image on a recording material, the printing ration of all colors (Y, M, C, K) are calculated, and the adjustment of nip pressure is then started. This, however, may take long time until completion of the nip pressure adjustment. Conveyance of a recording material to the curl correction mechanism before the nip pressure adjustment completes can be prevented by setting to delay feeding of a recording material from a feeding port of a cassette, for example. This may result in a longer first print output time (FPOT) that is a time period from a time when an image forming instruction is given to a time when the first recording material on which an image is formed is discharged to outside of the main body of the image forming apparatus.

According to Japanese Patent Laid-Open No. 6-258906, a way to address this issue is that the printing ration of three colors (Y, M, C) are calculated, and the printing ratio of the remaining one color (K) is estimated from information regarding the printing ration of the three colors. Thus, the nip pressure adjustment can be started before the calculation of the printing ration of all colors completes. Thus, the nip pressure adjustment can complete earlier than the configuration which calculates the printing ration of all colors, and the recording material can be fed earlier. Therefore, an increase of the FPOT can be prevented.

A curl formed on a recording material may vary in accordance with not only the printing ration but also the type of a recording material (plain paper, thick paper, or thin paper) and the ambient temperature and humidity. Among them, the type of a recording material can automatically be discriminated by detecting property information such as a surface condition, a thickness, and a basis weight of the recording material. Such property information can be obtained by a paper type detecting sensor such as an optical sensor or an ultrasonic sensor provided in a conveying path. The paper type detecting sensor may be provided at each of feeding ports of a plurality of cassettes, which however increases the cost. Therefore, in many cases, a paper type detecting sensor may be provided at a position following a position where conveying paths from the feeding ports join.

A recording material conveying path having a reduced length for reduction of the size of an image forming apparatus may reduce the space between the paper type detecting sensor and the curl correction mechanism. In this configuration, after detection by the paper type detecting sensor completes, the nip pressure adjustment by the curl correction mechanism may start. In this case, when the nip pressure adjustment based on a result of detection of property information regarding a recording material takes time, the recording material may reach the curl correction mechanism before the nip pressure adjustment completes. In order to prevent this, the recording material may need to reach the curl correction mechanism in delayed timing by, for example, reducing the conveying speed of the recording material or temporarily stopping the recording material in the conveying path. This also results in an increased FPOT.

It is also configured such that the nip pressure can be changed not only in the curl correction mechanism but also in the fixing unit configured to fix an image to a recording material. Changing the nip pressure in the fixing unit based on the type of a recording material and an ambient temperature and humidity may improve quality of an image fixed onto the recording material. However, also in the fixing unit, like the curl correction mechanism, when the nip pressure adjustment starts after completion of detection by the paper type detecting sensor, the recording material may reach the fixing unit before the nip pressure adjustment completes.

SUMMARY OF THE INVENTION

The disclosed image forming apparatus works towards preventing a first print output time (FPOT) from increasing in a configuration which changes the nip pressure in a conveying unit configured to convey a recording material such as a curl correction mechanism and a fixing unit based on a detection result provided by a paper type detecting sensor provided in a conveying path.

According to an aspect of the present invention, an image forming apparatus includes a plate configured to receive a recording material placed on the plate, a feeding unit configured to feed a recording material placed on the plate, an image forming unit configured to form an image on the recording material fed by the feeding unit, a conveying unit including a first rotating member and a second rotating member, wherein the conveying unit is configured to convey the recording material on which the image has been formed by the image forming unit and is pinched by a nip pressure from a nip portion formed by the first rotating member and the second rotating member, a switching unit configured to switch the nip pressure in at least three levels, a detecting unit provided between the feeding unit and the conveying unit and configured to detect property information regarding the recording material fed by the feeding unit, and a control unit configured to control the switching unit, wherein, in response to an instruction to form the image on the recording material and before detection of property information regarding the recording material by the detecting unit completes, the control unit is configured to control the switching unit to switch the nip pressure based on information regarding image forming on the recording material, and wherein, in response to the instruction to form the image on the recording material and after detection of the property information regarding the recording material by the detecting unit completes, the control unit is configured to control the switching unit to switch the nip pressure based on the information regarding the image forming on the recording material and the property information detected by the detecting unit.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

General Configuration of Image Forming Apparatus

With reference toFIG. 1A, a general configuration of an image forming apparatus according to a first embodiment will be described. According to this embodiment, the image forming apparatus may be a laser beam printer, for example.

A laser beam printer100(hereinafter, called a printer100) has process cartridges5Y,5M,5C, and5K. The process cartridges5Y,5M,5C, and5K are detachably attached to a main body of the printer100. The four process cartridges5Y,5M,5C, and5K have an identical structure but are different in that they apply toners (developing agents) of different colors, that is, yellow (Y), magenta (M), cyan (C), and black (K) to form an image. References Y, M, C, and K are omitted in the following descriptions except for descriptions regarding a member corresponding to a specific color.

Each of the process cartridges5has a toner container23, a photoconductive drum1, a charging roller2, a developing roller3, a cleaning blade4, and a waste toner container24. Exposure devices7are disposed below the process cartridges5and are configured to expose the photoconductive drums1based on an image signal.

The photoconductive drums1are evenly charged to a predetermined polarity and potential by a corresponding charging roller2while the photoconductive drums1are rotating. The photoconductive drums1undergo image exposure by the exposure devices7so that electrostatic latent images corresponding to target polychrome images can be formed.

The exposure devices7applied according to this embodiment are polygon scanners having laser diodes and bring laser beams modulated based on image information onto the photoconductive drums1to form electrostatic latent images. The electrostatic latent images formed on the photoconductive drums1are developed by developing rollers3. Toners of the colors are adhered to the electrostatic latent images on the photoconductive drums1through the developing rollers3to develop toner images. The toners in the toner containers23are negatively charged nonmagnetic single-component toners, and electrostatic latent images are developed by a contact developing method with the nonmagnetic single-component toners.

An intermediate transfer belt unit includes an intermediate transfer belt8(image bearing member), a driving roller9, and an opposing roller for secondary-transfer10. Primary transfer rollers6are placed inside the intermediate transfer belt8by facing the photoconductive drums1and are configured to receive a primary transfer bias of positive polarity from a primary transfer bias power supply, not illustrated. A motor, not illustrated, rotates the driving roller9so that the intermediate transfer belt8rotates, which is followed by rotation of the opposing roller for secondary-transfer10. The photoconductive drums1rotate in directions indicated by arrows illustrated nearby inFIG. 1A, and the intermediate transfer belt8rotates in a direction indicated by an arrow illustrated nearby inFIG. 1A. A primary transfer bias with a positive polarity is applied to the primary transfer rollers6. Thus, toner images on the photoconductive drums1undergo primary transfer to the intermediate transfer belt8in order from the toner image on the photoconductive drum1Y. After that, the stacked toner images of four colors are conveyed to a secondary transfer roller11.

The cleaning blades4for the photoconductive drums1are in pressure contact with the photoconductive drums1to remove residual toners left on surfaces of the photoconductive drum1without being transferred to the intermediate transfer belt8and other residual substances on the photoconductive drums1. A partial visible image may remain on the intermediate transfer belt8without being transferred to paper P at the position of the secondary transfer roller11. Such a visible image left on the belt is removed by a cleaning operation using a cleaning blade21and is collected to a waste toner container22.

A feeding/conveying device12has a feeding roller14(feeding unit) configured to feed paper P from a cassette13(plate) storing paper P (recording materials) and a conveyance roller pair15configured to convey paper P fed thereto. Paper P conveyed from the feeding/conveying device12is conveyed to a registration roller pair16along a conveyance guide40and is conveyed to the secondary transfer roller11by the registration roller pair16. A bias of a positive polarity is applied to the secondary transfer roller11so that the toner images of four colors on the intermediate transfer belt8are transferred to the conveyed paper P (hereinafter, called secondary transfer).

A paper type detecting sensor54is placed on a downstream side of the registration roller pair16in the conveying direction for paper P and on an upstream side of the secondary transfer roller11. The paper type detecting sensor54is a sensor configured to detect property information of paper P conveyed thereto. When property information regarding paper P is to be detected by the paper type detecting sensor54according to this embodiment, the conveyance of the paper P is stopped. Particularly, a feeding motor, not illustrated, is stopped from a time when the leading edge of paper P is detected by the registration sensor16S to a time when the leading edge of the paper P certainly reaches the position of the paper type detecting sensor54. The paper type detecting sensor54will be described below in detail.

The paper P after toner images are transferred thereto is conveyed to a fixing device17. The fixing device17is a film heating fixing unit including a fixing film18containing a heater30and a temperature sensor31configured to measure a temperature of the heater30and a pressurizing roller19for pressure contact with the fixing film18. A fixing/discharge sensor17S is configured to detect reach of paper P to the fixing device17. The fixing device17is configured to heat and press paper P to fix toner images transferred to the paper P to the paper P. When toner images are fixed to the paper P, the paper P may curl. In the configuration according to this embodiment, paper P may curl in a direction of winding around the pressurizing roller19. The paper P to which toner images are fixed is conveyed by a decurling roller pair25in a curl correction mechanism29and is discharged by a discharge roller pair20to outside of the printer100. The curl correction mechanism29is a mechanism configured to correct a curl formed on paper P and can correct the curl by applying force to the paper P in the opposite direction of the direction that the paper P is winding around the pressurizing roller19. The curl correction mechanism29will be described below in detail.

When paper P having passed through the fixing device17is not discharged to undergone printing on a second surface of the paper P, the paper P having passed through the fixing device17is conveyed toward a reverse point201. A double-sided flapper61can switch the conveying direction for paper P between a discharging direction and an inverting direction. After paper P passes through the reverse point201, the paper P is conveyed by a reverse roller pair50in a direction for discharging paper to outside. When the trailing edge of paper P passes through the reverse point201and the reverse roller pair50nips the paper P, the reverse roller pair50is stopped once. The reverse roller pair50is then rotated in the opposite direction of the direction of conveyance up to this point so that the paper P can be conveyed toward a double-sided conveying path.

Within the double-sided conveying path, paper P is conveyed by a first double-sided conveying roller pair51, a second double-sided conveying roller pair52, and a third double-sided conveying roller pair53. At a junction200, the double-sided conveying path joins a conveyance guide40arranged between the conveyance roller pair15and the registration roller pair16. The reversed paper P is conveyed again to the secondary transfer roller11by the registration roller pair16. Toner images of four colors on the intermediate transfer belt8are transferred to the second surface of the paper P. The fixing device17fixes the toner images transferred to the second surface. The double-sided flapper61is switched to the discharging direction so that the double-sided printed paper P is discharged to outside of the printer100.

The printer100has an environmental sensor60configured to detect an ambient temperature and humidity of the printer100.

Discrimination of Paper Type

FIG. 1Billustrates a detail configuration of the paper type detecting sensor54according to this embodiment. The paper type detecting sensor54has an LED55a, an LED55b, a phototransistor56a, and a phototransistor56b.

Light emitted from the LED55ais irradiated to a surface of paper P on the conveyance guide40through a slit57a. The conveyance guide40has a window for irradiating light to a back surface of paper P. The reflected light from the paper P is gathered through slits57band57cand is received by the phototransistors56aand56b. The phototransistor56areceives diffused reflected light of light emitted from the LED55a, and the phototransistor56breceives specular reflected light thereof. A value based on the amount of diffused reflected light received by the phototransistor56ais output to the discrimination control unit113, and a value based on the amount of specular reflected light received by the phototransistor56bis output to the discrimination control unit113. Thus, a CPU104obtains a gloss (x=specular reflection output/diffused reflection output). In other words, a surface property of the paper P can be detected as property information regarding the paper P. Light emitted from the LED55bis irradiated to a back surface of the paper P through a condensing guide57dconfigured to collect light. The transmitted light through the paper P is received by a phototransistor56athrough a slit57b. The phototransistor56areceives transmitted light of light emitted from the LED55b. A value based on the amount of transmitted light received by the phototransistor56ais output to the discrimination control unit113. Then, the CPU104obtains a transmittance (y=output from the phototransistor56a). In other words, the thickness of the paper P can be detected as property information regarding the paper P.

The printer100discriminates the paper type based on the gloss and transmittance of the paper P obtained by detection of the paper P performed by the paper type detecting sensor54. According to this embodiment, referring toFIG. 1C, five paper types (plain paper, thin paper, thick paper, gloss paper, and smooth paper) may be discriminated. With reference toFIG. 1C, a case will first be described in which the gloss x is lower than a threshold value A. If the transmittance y is lower than a threshold value B, the printer100discriminates thick paper as the paper type of the paper P. If the transmittance y is higher than the threshold value B and is lower than a threshold value C, the printer100discriminates plain paper as the paper type of the paper P. If the transmittance y is higher than the threshold value C, the printer100discriminates thin paper as the paper type of the paper P. Next, a case will be described in which the gloss x is higher than the threshold value A. If the transmittance y is lower than a threshold value D, the printer100discriminates gloss paper as the paper type of the paper P. If the transmittance y is higher than the threshold value D, the printer100discriminates smooth paper as the paper type of the paper P. Thus, by detection of paper P performed by the paper type detecting sensor54, the paper type of the paper P can be discriminated. The threshold values A to D may be stored in a non-volatile memory114illustrated inFIG. 2.

According to this embodiment, the LED55ais arranged to irradiate LED light to a surface of paper P diagonally at a predetermined angle as illustrated inFIG. 1B. The LED55bis arranged to irradiate LED light to the paper P from a front position of the phototransistor56aas illustrated inFIG. 1B.

General Control Over Image Forming Apparatus

FIG. 2is a control block diagram according to this embodiment. The printer100has a controller102and an engine control unit101. The controller102is connected to a host computer103over a network or via a printer cable, for example, and gives a print instruction to the engine control unit101based on settings in the host computer103. The engine control unit101includes a circuit having the CPU104and a ROM and a RAM, not illustrated, and is configured to execute a program for controlling devices within the printer100.

The CPU104is connected to an image forming control unit110configured to control a charging bias, for example, and a conveyance control unit111configured to drive motors for rotating pairs of rollers on a conveying path and may instruct them to form an image on paper P and convey the paper P. The CPU104is further connected to a switching control unit112, a discrimination control unit113, and a non-volatile memory114. The switching control unit112is configured to switch the nip pressure of the decurling roller pair25. The discrimination control unit113is configured to discriminate the paper type of paper P based on a detection result from the paper type detecting sensor54. The CPU104is further connected to a temperature sensor31, an environmental sensor60, a registration sensor16S, and a fixing/discharge sensor17S and can obtain results detected by the sensors.

When the conveyance control unit111drives the curl correction motor73, the decurling roller pair25rotates along the conveying direction of paper P. Driving the conveyance control unit111drives a double-sided motor70rotate the first double-sided conveying roller pair51, the second double-sided conveying roller pair52, and the third double-sided conveying roller pair53. When the switching control unit112drives a switching motor74, a pressure control cam28, which will be described below, rotates so that the nip pressure of the decurling roller pair25is changed.

The discrimination control unit113discriminates a paper type of the paper P detected by the paper type detecting sensor54with reference to a paper type discrimination table as illustrated inFIG. 1Cbased on the gloss and transmittance of paper P. The gloss of the paper P is detected based on an output value with light emitted from the LED55a, and the transmittance of the paper P is detected based on an output value with light emitted from the LED55b. Therefore, the gloss and transmittance of the paper P are detected not simultaneously but sequentially such as detecting the gloss first and next detecting the transmittance.

Next, control processing to be performed for executing a printing operation will be described. First, the controller102transmits a print reservation command to the engine control unit101based on a print instruction given from the host computer103. The engine control unit101starts a preparation operation for performing a printing operation in order of reception of print reservation commands from the controller102and waits for a print start command transmitted from the controller102. When the preparation operation completes and the printer100gets ready for printing, the controller102transmits a print start command to the engine control unit101. In response to the print start command, the engine control unit101outputs a /TOP signal being reference timing for output of a video signal to the controller102and starts a printing operation based on the print reservation command.

The print reservation command may designate information regarding a paper type, a paper feeding port, and the size of paper P. When the print reservation command designates a specific paper type such as plain paper or thick paper, the engine control unit101determines image forming conditions such as a target temperature for a corresponding fixing temperature adjustment and the nip pressure of the decurling roller pair25and performs a printing operation under the conditions. When the print reservation command does not designate a specific paper type but designates automatic discrimination mode, the discrimination control unit113automatically discriminates the paper type of the paper P, and the engine control unit101performs a printing operation under the image forming conditions based on the discriminated paper type. In the automatic discrimination mode, image forming conditions are not determined until the paper type discrimination completes. Therefore, the engine control unit101is to output the /TOP signal after the paper type discrimination completes.

Curl Correction Mechanism

FIG. 3Aillustrates a configuration of the curl correction mechanism29. The decurling roller pair25includes a curl correction roller25a(first rotating member) and a curl correction opposed roller25b(second rotating member). The curl correction roller25ais made of silicone rubber foam having an Asker C hardness of about 30, and the curl correction opposed roller25bis made of iron. The curl correction roller25ahaving a lower hardness presses the curl correction opposed roller25bhaving a higher hardness to form a nip portion along an outer periphery of the curl correction opposed roller25b. The paper P is pinched by the nip portion to be conveyed so that a curl formed on the paper P by the fixing film18and pressurizing roller19can be corrected toward the opposite direction. In other words, the paper P is nipped along the outer periphery of the curl correction opposed roller25bso that a curl formed in a direction of winding around the pressurizing roller19can be corrected. When the switching control unit112drives the switching motor74, the pressure control cam28rotates in a direction indicated by an arrow inFIG. 3A. The rotation stop position of the pressure control cam28may be changed to switch the pressure that the curl correction roller25aapplies to the curl correction opposed roller25bthrough a pressure lever27and a pressurizing spring26.

FIG. 3Billustrates three states with different nip pressures of the decurling roller pair25. In a low pressure state29a, the decurling roller pair25has a lowest nip pressure. A medium pressure state29bmay be obtained by rotating the pressure control cam28by 72° in clockwise from the low pressure state29a. Here, the nip pressure of the decurling roller pair25is higher than that in the low pressure state29a. A high pressure state29cmay be obtained by rotating the pressure control cam28by 110° in clockwise from the medium pressure state29b. Here, the nip pressure of the decurling roller pair25is higher than that in the medium pressure state29b. When the pressure control cam28in the high pressure state29crotates by 178° in clockwise, the pressure state29ais obtained. These three curl correction states (states of the nip pressure of the decurling roller pair25) can be changed only in one direction along the direction indicated by the arrows illustrated inFIG. 3B.

When a leading edge of paper P enters to the decurling roller pair25while the curl correction state (nip pressure) is changing, there is a possibility that a paper jam (paper jam) will occur. This is caused by instable directions of movement of the entering paper P after passing through the decurling roller pair25due to the instable nip pressure of the decurling roller pair25and thus variations of the conveying path of the leading edge of the paper P. Accordingly, the operation for switching the curl correction state is paused before the paper P enters to the nip according to this embodiment. Here, a predetermined margin time occurs before the switching control unit112stops the switching motor74and the switching of the curl correction state is actually stopped. According to this embodiment, the switching of the curl correction state is discontinued when the leading edge of paper P reaches a first position upstream of the decurling roller pair25. The first position in consideration of such a margin time may be a position 10 mm upstream of the decurling roller pair25, for example.

The paper P can be conveyed in a stable manner after the switching of the curl correction state is discontinued and the leading edge of the paper P reaches a second position downstream of the decurling roller pair25. The second position for stable conveyance of paper P may be a position 50 mm downstream of the decurling roller pair25, for example. According to this embodiment, during a period from a time when the leading edge of paper P reaches a position 10 mm upstream of the decurling roller pair25to a time when it reaches a position 50 mm downstream of the decurling roller pair25, the switching of the curl correction state is temporarily discontinued.

Curl Correction Control

Next, curl correction control will be described. According to this embodiment, curl correction states are determined from the following four viewpoints. In order to switch the curl correction state, the switching motor74is driven, and the pressure control cam28is rotated by a desired phase angle from the number of driving steps of the switching motor74. Here, the switching motor74may be a stepping motor.

A first viewpoint may be an amount of moisture (hereinafter, absolute moisture content) in ambient air of the printer100. When the ambient temperature or humidity of the printer100is low and the absolute moisture content is low, paper P contains a low amount of moisture, which means a small curl may be formed on the paper P. When the absolute moisture content is high on the other hand, paper P contains a higher amount of moisture, which means that a large curl may be formed on the paper P. Accordingly, the nip pressure for correcting the curl is to be switched based on the size of the formed cur depending on the absolute moisture content. The absolute moisture content can be detected by the environmental sensor60.

A second viewpoint may be a temperature of the fixing device17when a printing operation starts (hereinafter, a fixing temperature). Here, the term “fixing temperature” refers to a temperature of the heater30before the temperature increases with a printing operation. In a case where a printing operation starts at a high fixing temperature after a short period of time passed from the last printing operation, a fixing operation is started with a small temperature difference between the fixing film18and the pressurizing roller19. Therefore, a small curl is formed on paper P. This state is called a Hot state. On the other hand, in a case where a printing operation starts at a fixing temperature reduced to a substantially normal temperature after a long period of time passed from the last printing operation, a fixing operation is performed with a large temperature difference between the fixing film18and the pressurizing roller19. Therefore, a large curl is formed on paper P. This state is called a Cold state. Accordingly, the nip pressure for correcting a curl is to be changed based on the size of the formed curl depending on the temperature of the fixing device17. A Hot state or a Cold state may be discriminated by the CPU104by comparing the temperature of the heater30detected by the temperature sensor31and a threshold temperature prestored in the non-volatile memory114. In other words, when the detected temperature of the heater30is higher than the threshold temperature, the CPU104discriminates a Hot state. If the detected temperature of the heater30is lower than the threshold temperature, the CPU104discriminates a Cold state.

A third viewpoint is printing side information indicating simplex print or duplex print. When duplex print processing is performed, the degree of a curl formed during a printing operation performed on the first side is reduced during a printing operation performed on the second side. Thus, the degree of the curl formed on paper P is smaller than the resulting degree of a curl formed in simplex print processing. This is because a curl is formed during the second side printing in the opposite direction of the direction of the curl formed during the first side printing so that the curl formed during the first side printing can be cancelled. Accordingly, the nip pressure for correcting a curl is to be changed based on the size of the formed curl depending on the type of printing, simplex print and duplex print, to be executed. The printing side information is contained in a print reservation command transmitted from the controller102.

A fourth viewpoint is a paper type of paper P on which printing is to be performed. Comparing thin paper having low stiffness and thick paper having high stiffness, higher nip pressure may be needed to correct a curl on the thick paper than that for correcting a curl on the thin paper where the degrees of curls on the thick paper and the thin paper are substantially equal. Correcting a curl on thin paper with high nip pressure may possibly result in formation of a curl in the opposite direction of the direction in which the original curl has been formed. In a case where paper type information is contained in a print reservation command transmitted from the controller102, the discrimination control unit113may discriminate a paper type based on property information detected by the paper type detecting sensor54.

Thus, according to this embodiment, a curl correction state is determined based on information regarding those four viewpoints by using a nip pressure selection table on Table 1. Referring to Table 1, when an absolute moisture content is equal to or higher than 20 [g/m3], the absolute moisture content is handled as “High”. When an absolute moisture content is equal to or higher than 5 [g/m3] and lower than 20 [g/m3], the absolute moisture content is handled as “Medium”. When an absolute moisture content is lower than 5 [g/m3], the absolute moisture content is handled as “Low”. Further referring to Table 1, when a fixing temperature upon print start is equal to or higher than 70 [° C.], the fixing temperature is handled as “Hot”. When a fixing temperature upon print start is lower than 70 [° C.], the fixing temperature is handled as “Cold”. For example, in a state that an absolute moisture content is “High”, fixing temperature upon print start is “Hot”, when simplex print is to be performed on plain paper, the nip pressure of the decurling roller pair25is switched to the high pressure state29c.

In a case where a user gives a print instruction by designating a paper type in advance, the CPU104upon print start can obtain the paper type information. Thus, a curl correction state can be determined with reference to the nip pressure selection table as illustrated on Table 1.

On the other hand, in a case where a print instruction is given in an automatic discrimination mode for discriminating a paper type by using the paper type detecting sensor54, the CPU104cannot obtain paper type information until the detection operation performed by the paper type detecting sensor54completes.

In a case where a curl correction state is determined upon completion of the detection operation performed by the paper type detecting sensor54and the switching of the curl correction state is then started, the leading edge of paper P may possibly reach the decurling roller pair25before the switching completes. This is caused when the time period for switching the curl correction state is longer than the time period for conveying the paper P from the paper type detecting sensor54to the decurling roller pair25. Table 2 illustrates switching times from one curl correction state to another curl correction state illustrated inFIG. 3B. According to this embodiment, the time for conveying paper P from the paper type detecting sensor54to the position 10 mm upstream of the decurling roller pair25is equal to 1250 [ms]. Referring to Table 2, switching the curl correction state from the low pressure state29ato the high pressure state29ctakes 1432 [ms], which is longer than 1250 [ms] for conveying the paper P.

According to this embodiment, it is controlled to determine a provisional curl correction state before a detection operation performed by the paper type detecting sensor54completes in the automatic discrimination mode and then start switching of the curl correction state. This can increase the possibility that the switching can be performed before the leading edge of paper P reaches the decurling roller pair25even when it is determined based on obtained paper type information to perform switching of the curl correction state.

Provisional Curl Correction State

With reference toFIG. 4and Table 2, a method for determining a provisional curl correction state will be described.FIG. 4is a flowchart for determining a first curl correction state based on printing side information being information excluding paper type ((information excluding property information), a fixing temperature upon print start, and an absolute moisture content). The term “first curl correction state” refers to a provisional curl correction state to be determined based on information excluding a paper type. A control based on the flowchart inFIG. 4may be executed by the CPU104based on a program stored in the non-volatile memory114, for example.

First, the CPU104obtains printing side information designated by the controller102(S100) and discriminates simplex print or duplex print to be executed. Next, the CPU104obtains a fixing temperature upon print start from the temperature sensor31and discriminates “Hot” or “Cold” (S101). Next, the CPU104obtains an absolute moisture content from the environmental sensor60and discriminates “High”, “Medium”, or “Low” (S102).

The CPU104then narrows candidates for the curl correction state for switching the nip pressure of the decurling roller pair25based on the information excluding a paper type (printing side information, the fixing temperature upon print start, and the absolute moisture content) and the nip pressure selection table (S103). Hereinafter, candidate states are represented by (S1, S2, . . . SN). For example, in a case where simplex print is to be executed under conditions including “High” as an absolute moisture content and “Cold” as a fixing temperature upon print start, candidates for the curl correction state include the low pressure state29aand the medium pressure state29bwithin the intersection of the “Cold” column and “High” row in Table 1.

If it is determined that there is one candidate for the curl correction state (S104), the CPU104determines the candidate curl correction state S1as the first curl correction state (S105). On the other hand, if it is determined that there are two or three curl correction state candidates (S104), the CPU104obtains a maximum switching time Ti for each of the curl correction state candidates (S1, S2, . . . SN) (S106). Here, the term “maximum switching time Ti” refers to a maximum value of a time period for switching from a curl correction state candidate Si to another curl correction state candidate Sj. The CPU104determines a curl correction state Si for minimizing the maximum switching time Ti as the first curl correction state (S107).

For example, in a case where the low pressure state29aand the medium pressure state29bwithin the intersection of the “Cold” column and “High” row on Table 1 are curl correction state candidates, the switching time from the low pressure state29ato the medium pressure state29bis equal to 995 [ms], and the switching time from the medium pressure state29bto the low pressure state29ais equal to 1561 [ms]. Here, defining that i=1 is the low pressure state29aandi=2 is the medium pressure state29b, there are only two curl correction state candidates. Therefore, the maximum switching time T1=995 [ms], and the maximum switching time T2=1561 [ms]. Because the curl correction state for minimizing the maximum switching time Ti is the low pressure state29awith i=1, the CPU104determines the low pressure state29aas the first curl correction state.

Curl Correction Operation

With reference toFIG. 5, a method according to this embodiment will be described which determines a curl correction state in response to a print instruction from the controller102. A flowchart inFIG. 5is to be executed on a first piece of paper P to be printed. In a case where the second and subsequent pieces of paper P have the same paper type as that of the first piece of paper P in a continuous printing operation, the nip pressure determined for the first piece of paper P is used for executing a curl correction operation. A control based on the flowchart inFIG. 5may be executed by the CPU104based on a program stored in the non-volatile memory114.

First, the CPU104determines which of a print instruction in the automatic discrimination mode from the controller102or a print instruction designating a paper type has been received (S200). If it is determined that a print instruction in the automatic discrimination mode has been received, the CPU104determines the first curl correction state as a provisional curl correction state according to the method described with reference toFIG. 4(S201). The CPU104then starts a control for switching the nip pressure of the decurling roller pair25to the determined first curl correction state (S202). In parallel with this processing paper P is fed from the cassette13, and the paper type detecting sensor54starts the detection operation on the paper P. Here, the CPU104switches the nip pressure of the decurling roller pair25to the first curl correction state before the paper type detecting sensor54at least completes the detection operation.

Next, the CPU104determines whether the paper type detecting sensor54completes the detection operation or not (S203). If it is determined that the detection operation has completed, the CPU104discriminates the paper type based on the detected property information and determines a second curl correction state based on the information including the paper type (S204). The term “second curl correction state” refers to a final curl correction state to be determined based on four kinds of information of printing side information, a fixing temperature upon print start, an absolute moisture content, and paper type information. The CPU104determines whether the first curl correction state and the second curl correction state are different or not (S205). If it is determined that the first curl correction state and the second curl correction state are different, the CPU104starts a control for switching the nip pressure of the decurling roller pair25to the determined second curl correction state (S206). On the other hand, if the first curl correction state and the second curl correction state are not different, that is, if the switching of the nip pressure is not necessary, the processing advances without performing the operation in S206.

When the paper type detecting sensor54detects the property information regarding paper P, the registration roller pair16is stopped, and the paper P is therefore stopped. When the registration roller pair16is driven again, the paper P is conveyed again toward the secondary transfer roller11in synchronization with toner images on the intermediate transfer belt8. According to this embodiment, in order to prevent the reach of the leading edge of paper P to the decurling roller pair25before completion of the switching of the nip pressure of the decurling roller pair25, the CPU104executes the following control flow.

In S207, the CPU104determines whether the leading edge of paper P has reached the position 10 mm upstream of the decurling roller pair25or not based on the time when the fixing/discharge sensor17S detects the leading edge of the paper P. If it is determined that the leading edge has reached the position, the CPU104checks whether the switching of the nip pressure of the decurling roller pair25has completed (S208). If it is determined that the switching has not completed, the CPU104temporarily stops the switching of the nip pressure of the decurling roller pair25(S209). The CPU104determines whether the leading edge of the paper P has reached the position 50 mm downstream of the decurling roller pair25or not based on the time when the fixing/discharge sensor17S detects the leading edge of the paper P (S210). If it is determined that the leading edge has reached the position, the CPU104resumes the switching of the nip pressure of the decurling roller pair25(S211). The switching operation performed on the nip pressure is temporarily stopped so that the leading edge of the paper P can be prevented from entering to the nip of the decurling roller pair25while the nip pressure is being switched and that the paper P can be conveyed in a stable manner.

If it is determined in S200that a print instruction designating a paper type has been received, the CPU104determines the second curl correction state based on the printing side information, the fixing temperature upon print start, and the absolute moisture content as well as the already obtained paper type information (S212). The CPU104starts a control for switching the nip pressure of the decurling roller pair25to the determined second curl correction state (S213).

Next, specific operations of this embodiment will be described with reference to the timing charts inFIGS. 6 and 7.FIG. 6illustrates a case where the switching of the nip pressure of the decurling roller pair25can be performed before the leading edge of the paper P reaches the decurling roller pair25.FIG. 7illustrates a case where the switching of the nip pressure of the decurling roller pair25cannot be performed before the leading edge of the paper P reaches the decurling roller pair25.

FIG. 6illustrates a timing chart in a case where simplex print in the automatic discrimination mode is instructed when the absolute moisture content is “High”, the fixing temperature upon print start is “Cold”, and plain paper is stored in the cassette13. Referring toFIG. 6, the initial curl correction state (initial state) is the high pressure state29c. In this case, the low pressure state29ais the first curl correction state, as described above. For printing on plain paper, the medium pressure state29bis determined as the second curl correction state.

The engine control unit101in response to a print start command from the controller102(T500), the switching control unit112determines the low pressure state29aas the first curl correction state and starts switching control over the nip pressure of the decurling roller pair25(T501). After the switching control starts, the discrimination control unit113discriminates plain paper as the paper type of the paper P based on a detection result from the paper type detecting sensor54. The switching control unit112determines the medium pressure state29bas the second curl correction state from the information including a paper type (T503) and starts the switching control over the nip pressure of the decurling roller pair25(T504). The engine control unit101obtains a time (T506) when the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25from the time (T505) when the fixing/discharge sensor17S detects the leading edge of the paper P. Because the switching of the nip pressure of the decurling roller pair25ends at T506, the printing operation is continued.

FIG. 7illustrates a timing chart in a case where simplex print in the automatic discrimination mode is instructed when the absolute moisture content is “High”, the fixing temperature upon print start is “Hot”, and plain paper is stored in the cassette13. Referring toFIG. 7, the initial curl correction state (initial state) is the high pressure state29c. In this case, the first curl state is the low pressure state29a, and the second curl state is the high pressure state29c.

The engine control unit101in response to a print start command from the controller102(T600), the switching control unit112determines the low pressure state29aas the first curl correction state and starts switching control over the nip pressure of the decurling roller pair25(T601). After the switching control starts, the discrimination control unit113discriminates plain paper as the paper type of the paper P based on a detection result from the paper type detecting sensor54. The switching control unit112determines the high pressure state29cas the second curl correction state from the information including a paper type (T603) and starts the switching control over the nip pressure of the decurling roller pair25(T604). The engine control unit101obtains a time (T606) when the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25from the time (T605) when the fixing/discharge sensor17S detects the leading edge of the paper P. Because the switching of the nip pressure of the decurling roller pair25has not ended at T606, the switching control unit112stops the switching motor74. The switching control unit112drives the switching motor74again and resumes the switching at a time (T607) when the leading edge of the paper P reaches a position 50 mm downstream of the decurling roller pair25.

According to this embodiment, for printing in the automatic discrimination mode, the nip pressure of the decurling roller pair25is switched to the first curl correction state determined from information excluding a paper type before a detection operation performed by the paper type detecting sensor54completes. After the detection operation performed by the paper type detecting sensor54completes, the nip pressure of the decurling roller pair25is switched to the second curl correction state determined based on the information including a paper type. Thus, an FPOT substantially equal to that of a mode in which a user designates a paper type in advance can also be realized in the automatic discrimination mode. In other words, a longer FPOT in the automatic discrimination mode can be prevented.

Further according to this embodiment, the switching of the nip pressure of the decurling roller pair25is temporarily stopped when the switching of the nip pressure of the decurling roller pair25is not performed before the leading edge of paper P reaches the nip of the decurling roller pair25even though the control as described above is executed. When the leading edge of the paper P reaches a position 50 mm downstream of the decurling roller pair25, the switching of the nip pressure is resumed. The interruption of the switching of the nip pressure while a predetermined amount from the leading edge of paper P is passing through the nip can stabilize the orientation of the paper P while being conveyed, which can avoid a paper jam due to variations of the conveying path of the leading edge of the paper P.

Second Embodiment

According to the first embodiment, in a case where the switching of the nip pressure of the decurling roller pair25is not completed when the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25, the switching is temporarily stopped. The switching is resumed when the leading edge of the paper P reaches a position 50 mm downstream of the decurling roller pair25so that the paper P can be prevented from entering to the nip of the decurling roller pair25while the nip pressure is being switched. However, there is a possibility that a curl of the paper P may not be sufficiently corrected because appropriate pressure is not applied to the paper P during a period from a time when the paper P reaches the decurling roller pair25to the time when the switching completes.

According to a second embodiment, in a case where there is a possibility that the switching from the first curl correction state to the second curl correction state may not be completed when the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25, the timing for image forming may be delayed. Principal parts of the descriptions are the same as those of the first embodiment, and differences from the first embodiment will be described below.

With reference toFIG. 8, a method according to this embodiment will be described which determines a curl correction state in response to a print instruction received from the controller102. A flowchart illustrated inFIG. 8is to be executed on a first piece of paper P to be printed. A control based on the flowchart inFIG. 8is executed by the CPU104based on a program stored in the non-volatile memory114, for example.

First, the CPU104in response to a print instruction from the controller102, the image forming control unit110inhibits start of an image forming operation (S300). The term “image forming operation” refers to an operation for forming a toner image on the photoconductive drum1or the intermediate transfer belt8, and the operation does not include a paper feeding operation for feeding paper P from the cassette13. Because the processing in S200to S206is the same as the processing in the flowchart inFIG. 5, any repetitive descriptions will be omitted.

If it is determined in S205that the first curl correction state and the second curl correction state are different, the CPU104starts a control for switching the nip pressure of the decurling roller pair25to the second curl correction state in S206. The CPU104then calculates a time T1to2for switching from the first curl correction state to the second curl correction state. The CPU104reads out a conveyance time Td stored in the non-volatile memory114, for example. The term “conveyance time Td” refers to a time period from a time when the detection operation performed by the paper type detecting sensor54completes and the registration roller pair16is driven again to convey paper P to a time when the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25. The CPU104determines whether the conveyance time Td is longer than the calculated switching time T1to2or not (S301).

If the CPU104determines in S301that the conveyance time Td is shorter than the switching time T1to2, it is estimated that the leading edge of the paper P may reach the decurling roller pair25before the switching of the nip pressure of the decurling roller pair25completes. According to this embodiment, the timing for forming toner images on the photoconductive drums1and the intermediate transfer belt8is delayed. More specifically, the CPU104calculates an image-forming waiting time Tw (S302). The image-forming waiting time Tw may be calculated by using Expression (1).
Image-forming waiting timeTw=Td−T1to2  (1)

This embodiment assumes that the conveyance time Td is equal to 1250 [ms]. After waiting for passage of the image-forming waiting time Tw (S303), the CPU104permits the image forming control unit110to start an image forming operation inhibited in S300(S304). Thus, forming toner images on the photoconductive drums1and the intermediate transfer belt8is started. The registration roller pair16is then driven again, and the paper P stopped at the position of the paper type detecting sensor54is conveyed again in synchronization with the toner images formed on the intermediate transfer belt8.

If it is determined in S301that the conveyance time Td is longer than the switching time T1to2, the CPU104permits the image forming control unit110to start an image forming operation (S304).

If it is not determined in S205that the first curl correction state and the second curl correction state are different, that is, if the switching of the nip pressure is not necessary, the CPU104permits the image forming control unit110to start an image forming operation (S305).

If it is determined in S200that a print instruction designating a paper type is received, the nip pressure of the decurling roller pair25is changed without detecting a property state of the paper P by the paper type detecting sensor54. Accordingly, the CPU104permits the image forming control unit110to start an image forming operation without waiting for passage of an image-forming waiting time (S306). Then, the second curl correction state is determined (S212), and a control starts for switching the nip pressure of the decurling roller pair25to the second curl correction state (S213).

Next, specific operations according to this embodiment will be described with reference to a timing chart inFIG. 9.FIG. 9illustrates a timing chart in a case where simplex print in the automatic discrimination mode is instructed when the absolute moisture content is “High”, the fixing temperature upon print start is “Hot”, and the cassette13stores plain paper. Referring toFIG. 9, the initial curl correction state is the high pressure state29c. The first curl correction state is the low pressure state29a, and the second curl correction state is the high pressure state29c.

The engine control unit101in response to a print start command from the controller102(T700), the switching control unit112determines the low pressure state29aas the first curl correction state and starts switching control over the nip pressure of the decurling roller pair25(T701). After the switching control starts, the discrimination control unit113discriminates plain paper as the paper type of the paper P based on a detection result from the paper type detecting sensor54. The switching control unit112determines the high pressure state29cas the second curl correction state from the information including a paper type (T703) and starts the switching control over the nip pressure of the decurling roller pair25(T704). If the switching time T1to2from the first curl correction state to the second curl correction state is longer than the conveyance time Td, the engine control unit101permits to start an image forming operation after waiting for the image-forming waiting time Tw (T704). This can delay the timing for forming toner images on the photoconductive drums1and the intermediate transfer belt8and thus can delay the timing for conveying the paper P again. By delaying the timing for conveying the paper P, the switching of the nip pressure of the decurling roller pair25completes at a time (T705) when the paper P reaches a position 10 mm upstream of the decurling roller pair25.

Thus, according to this embodiment, an increase of the FPOT in the automatic discrimination mode can be prevented. Furthermore, according to this embodiment, even under the control as described above, image forming timing is delayed in a case where the switching of the nip pressure of the decurling roller pair25does not complete before the leading edge of the paper P reaches the decurling roller pair25. Thus, a curl correction operation can be executed with a proper nip pressure across the entire paper P. This can reduce towards eliminating the possibility that a curl on a paper P cannot be sufficiently corrected in the first embodiment.

Third Embodiment

According to the first embodiment, before a detection operation performed by the paper type detecting sensor54completes, the nip pressure of the decurling roller pair25is switched to the first curl correction state. After the detection operation performed by the paper type detecting sensor54completes, the nip pressure is switched to the second curl correction state. In consideration of durability of the decurling roller pair25, the number of times of switching of the nip pressure may be reduced. According to this embodiment, the number of times of switching of the nip pressure of the decurling roller pair25is reduced. Principal parts of the descriptions are the same as those of the first embodiment, and differences from the first embodiment will be described below.

First, a case will be described in which simplex print in the automatic discrimination mode is instructed when the absolute moisture content is “High”, the fixing temperature upon print start is “Cold”, and the cassette13stores thick paper. The initial curl correction state is the medium pressure state29b. In this case, according to the first embodiment, candidates for a curl correction state are narrowed to the low pressure state29aand the medium pressure state29bbased on information excluding a paper type, and the CPU104determines the low pressure state29aas a first curl correction state. After the paper type is discriminated as thick paper, the medium pressure state29bis determined as a second curl correction state. In other words, the curl correction state is switched in order of the medium pressure state29b→the low pressure state29a→the medium pressure state29b.

Here, the number of times of switching can be reduced with a longer conveyance time Td from completion of the detection operation performed by the paper type detecting sensor54to reach of the leading edge of the paper P to a position 10 mm upstream of the decurling roller pair25. A case will be examined in which the printing in the embodiment above is performed when the conveyance time Td is equal to 1600 [ms], for example. The time period for switching from the medium pressure state29bbeing an initial curl correction state to the low pressure state29ais equal to 1561 [ms] with reference to Table 2 and is shorter than the conveyance time Td. Thus, even without switching the nip pressure of the decurling roller pair25to the low pressure state29abefore the detection operation performed by the paper type detecting sensor54completes, the switching of the nip pressure completes before the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25. In other words, even when the switching of the nip pressure of the decurling roller pair25starts after the detection operation performed by the paper type detecting sensor54completes, the switching of the nip pressure completes before the leading edge of the paper P reaches a position 10 mm upstream of the decurling roller pair25. Therefore, in this case, the medium pressure state29bis kept without provisionally switching the nip pressure to the first curl correction state (low pressure state29a).

According to this embodiment, when the times for switching from an initial curl correction state to curl correction state candidates are all shorter than the conveyance time Td, the provisional switching to the first curl correction state is not executed. After the detection operation performed by the paper type detecting sensor54completes, switching to the second curl correction state is only executed. According to this embodiment, the printer100has conveying speeds for four types of paper P, and the printer100determines a conveying speed based on the paper type and paper size of the paper P and conveys the paper P at the determined conveying speed. Table 3 illustrates conveyance times Td corresponding to the conveying speeds.

With reference toFIG. 10, a method according to this embodiment will be described which determines a curl correction state when a print instruction is received from the controller102. The processing on the flowchart inFIG. 10is to be executed on a first piece of paper P to be printed. A control based on the flowchart inFIG. 10is executed by the CPU104based on a program stored in the non-volatile memory114, for example.

Because the processing in S200and S201is the same as the processing in the flowchart inFIG. 4, any repetitive description will be omitted. After the first curl correction state is determined in S201, the CPU104obtains a maximum value Tmax of switching times from the curl correction state Sb before switching to curl correction state candidates Sj to be reduced in S201(S400). The curl correction mechanism29has a position detecting sensor, not illustrated, and the CPU104can detect the curl correction state Sb before switching or the initial curl correction state. The CPU104reduces the conveying speeds based on the paper size of the paper P, which is notified in the print reservation command and obtains a minimum conveyance time Tmin (S401). For example, for long paper having a length of 431.9 mm in the conveying direction, the conveying speeds are reduced to one of a ¾ speed, a ½ speed, and a ⅓ speed. It may be configured such that the paper size can automatically be detected based on the position of a rear end regulating plate provided in the cassette13.

The CPU104determines whether the maximum switching time Tmax is longer than the obtained minimum conveyance time Tmin or not (S402). If it is determined that the maximum switching time Tmax is longer than the minimum conveyance time Tmin, the CPU104starts a control for switching the nip pressure of the decurling roller pair25to the determined first curl correction state (S202). If it is determined that the maximum switching time Tmax is shorter than the minimum conveyance time Tmin, the CPU104does not perform the control for switching the nip pressure of the decurling roller pair25to the first curl correction state. After the paper type detecting sensor54completes the detection operation (S203), the CPU104starts a control for switching the nip pressure of the decurling roller pair25to the second curl correction state (S204). Because the processing in S205to S213is the same as the processing in the flowchart inFIG. 4according to the first embodiment, any repetitive description will be omitted.

Next, specific operations according to this embodiment will be described with reference to a timing chart inFIG. 11.FIG. 11illustrates a timing chart in a case where simplex print in the automatic discrimination mode is instructed when the absolute moisture content is “High”, the fixing temperature upon print start is “Cold”, and the cassette13stores thick paper. Referring toFIG. 11, the initial curl correction state is the medium pressure state29b. The conveying speed depending on the paper size is set to a ¾ speed.

When the engine control unit101receives a print start command from the controller102(T800), the switching control unit112narrows curl correction state candidates to the low pressure state29aand the medium pressure state29band determines the low pressure state29aas a first curl correction state (T801). Here, the switching time Tmax (=1561 [ms]) from the medium pressure state29bbeing a curl correction state before switching to the low pressure state29ais shorter than the minimum conveyance time Tmin (=1575 [ms]). Accordingly, the control is not executed for switching the nip pressure of the decurling roller pair25to the first curl correction state. The discrimination control unit113then discriminates thick paper as the paper type of paper P based on a detection result from the paper type detecting sensor54. The switching control unit112determinates the medium pressure state29bas a second curl correction state based on information including the paper type (T803). Here, because the curl correction state is the medium pressure state29b, the switching control ends, and the printing operation continues.

Thus, according to this embodiment, an increase of the FPOT in the automatic discrimination mode can be prevented. Furthermore, according to this embodiment, in a case where the switching times from a curl correction state before switching to curl correction state candidates are all shorter than the conveyance time Td, switching to a provisional first curl correction state is not executed. The switching to the final second curl correction state is only executed so that the number of times of switching of the curl correction state can be reduced and that the lifetime of the decurling roller pair25can be increased.

Fourth Embodiment

According to the first and second embodiments, the nip pressure of the decurling roller pair25is provisionally switched to a first curl correction state before the paper type detecting sensor54completes a detection operation. After the paper type detecting sensor54completes the detection operation, the nip pressure is switched to a final second curl correction state. Even when the nip pressure is switched to the first curl correction state in advance, the switching to the second curl correction state may not complete before the leading edge of the paper P reaches the decurling roller pair25, as described above. In this case, the first curl correction state may be determined again by using history of past paper type detection results. This is because, in a case where one paper feeding port (cassette) is selected in continuously executed printing operation, there is a high possibility that paper P of the same paper type is fed from the paper feeding port.

FIG. 12is a flowchart illustrating the control.FIG. 12is fundamentally the same as the flowchart illustrated inFIG. 5according to the first embodiment and is different from the flowchart inFIG. 5in that processing in S500and S501. The non-volatile memory114illustrated inFIG. 2stores information indicating a paper type of paper P fed from the feeding/conveying device12in the last print operation.

After the first curl correction state is determined in S201, the CPU104obtains a maximum value Ti of times for switching from the first curl correction state to another curl correction state candidates. The CPU104then reads out the conveyance time Td stored in the non-volatile memory114, for example. The CPU104determines whether the conveyance time Td is longer than the maximum switching time Ti (S500). If it is determined that the conveyance time Td is shorter than the maximum switching time Ti, it is determined that paper of the same paper type as the paper type lastly fed from feeding/conveying device12is to be conveyed. Then, the CPU104determines again the first curl correction state based on the information indicating the lastly fed paper type (S501). The information indicating the paper type of lastly fed paper is stored in the non-volatile memory114. The CPU104then starts a control for switching the nip pressure of the decurling roller pair25to the determined first curl correction state (S202).

Thus, if the paper type detected by the paper type detecting sensor54is the same as the paper type of the lastly fed paper, the first curl correction state determined based on the paper type of the lastly fed paper is identical to the final second curl correction state. This works towards eliminating necessity for the operation for switching the nip pressure of the decurling roller pair25and can prevent inconvenience in which the switching to the second curl correction state cannot be completed before the leading edge of the paper P reaches the decurling roller pair25.

Variation Examples

Having described that, according to the first to third embodiments, the paper type detecting sensor54is configured to detect reflected light and transmitted light. However, embodiments are not limited thereto. For example, a paper type detecting sensor54may be applied which has an ultrasonic wave detecting unit58and a surface property detection unit59as illustrated inFIG. 13. In the ultrasonic wave detecting unit58, ultrasonic waves are transmitted from a transmitting unit58a, ultrasonic waves attenuated through paper P are received by a receiving unit58b. The CPU104detects a basis weight of the paper P as property information based on the amplitude value of the ultrasonic waves received by the receiving unit58b. The surface property detection unit59includes an irradiating unit59a, a focusing unit59b, and an image capturing unit59c. The irradiating unit59ais configured to irradiate light to paper P, and the focusing unit59bis configured to focus light reflected by a surface of the paper P. The image capturing unit59cis a light receiving unit configured to receive light focused by the focusing unit59band captures a surface image of the paper P from the received light. The CPU104then detects surface a property (unevenness) of the paper P as property information based on the surface image captured by the image capturing unit59c. The discrimination control unit113discriminates a paper type based on the detected basis weight and surface property.

According to the first to third embodiments, the nip pressure of the decurling roller pair25included in the curl correction mechanism29is switched. However, the embodiments are not limited thereto.

For example, the disclosed information is also applicable to the fixing device17configured to fix toner images to paper P. The fixability of toner images to paper P may depend on the nip pressure in fixing processing and may also depend on environmental conditions and the paper type of paper P. Therefore, a fixing nip pressure for optimum fixability may be set in consideration of a paper type, for example. Accordingly, the nip pressure to be applied in fixing processing may be changed based on a paper type detection result from the paper type detecting sensor54, like the aforementioned embodiments.

In other words, it may be configured such that the nip pressure between the fixing film18and the pressurizing roller19included in the fixing device17illustrated inFIG. 1can be switched, and the present embodiment may be applied to the switching of the nip pressure.FIG. 14illustrates a configuration of the fixing device17which can switch the nip pressure between the fixing film18and the pressurizing roller19. The fundamental configuration is the same as that of the curl correction mechanism29illustrated inFIG. 3. Referring toFIG. 14, when the switching motor74is driven, the pressure control cam28rotates in a direction indicated by arrow illustrated inFIG. 14. The pressure control cam28switches the pressure applied from the pressurizing roller19to the fixing film18through the pressure lever27and the pressurizing spring26.

It is assumed here that the switching control over the pressure applied from the pressurizing roller19to the fixing film18is equivalent to the switching control over the curl correction state in the curl correction operation as described above. This allows fixing toner images to paper P with a fixing nip pressure suitable for the paper type without increasing an FPOT in that case.

Having described that, according to the first to third embodiments, the nip pressure can be switched in three levels, embodiments are not limited thereto. The nip pressure may only be required to be switched in at least two levels.

Having described laser beam printers according to the aforementioned embodiments as examples, the examples are not limited thereto. An embodiment is also applicable to a printer or a copier of other printing systems such as an ink-jet printer.

This application claims the benefit of Japanese Patent Application No. 2016-233352 filed Nov. 30, 2016, which is hereby incorporated by reference herein in its entirety.