Patent ID: 12228870

Below, a color printer1according to one embodiment of the present disclosure will be described in detail while referring to the accompanying drawings. In the following description, directions will be referenced from the perspective of a user operating the color printer1. Specifically, when usingFIG.1as an example, the right side of the color printer1inFIG.1, which is the near side from the perspective of the user, will be called the “front”; the left side of the color printer1inFIG.1, which is the far side from the perspective of the user, will be called the “rear”; the near side of the drawing inFIG.1will be called the “left”; and the far side of the drawing will be called the “right”. The up-down directions inFIG.1will be called “up” and “down”.

The color printer1is configured to form images on both sides of a sheet S, such as a recording paper or transparency. As illustrated inFIG.1, the color printer1primarily includes a main casing2and, disposed inside the main casing2, a sheet-feeding unit3, an image-forming unit4, and a conveying unit9.

The main casing2primarily has an upper cover21, a discharge tray22, and a discharge opening23. The upper cover21is configured to open and close by pivoting up and down around a rotational axis (not illustrated) provided at the rear of the main casing2. The discharge tray22supports sheets S on which images have been formed. After image formation, the sheets S are discharged through the discharge opening23into the discharge tray22.

The sheet-feeding unit3is disposed in the bottom section of the main casing2. The sheet-feeding unit3is primarily provided with a sheet tray31that accommodates sheets S, a sheet-pressing plate32, a pickup roller33, a separating roller34, a conveying roller36, and a registration roller37. The sheet-pressing plate32of the sheet-feeding unit3presses the sheets S in the sheet tray31toward the pickup roller33, and the pickup roller33feeds sheets S in the sheet tray31toward the image-forming unit4. The separating roller34separates the sheets S fed from the sheet tray31into single sheets that the conveying roller36and registration roller37supply one at a time to the image-forming unit4.

The image-forming unit4is configured to form images by transferring toner images onto sheets S. The image-forming unit4is primarily provided with four exposure heads5, four process units6, a transfer unit7, and a fixing unit8.

The exposure heads5are disposed above to face corresponding photosensitive drums63. Each exposure head5is provided with light-emitting elements of an LED array (not illustrated) arrayed in the left-right direction along the bottom edge of the exposure head5. The exposure head5exposes the top surface of the corresponding photosensitive drum63by flashing the light-emitting elements on the basis of image data. The exposure heads5are retained on the upper cover21via retaining members (not illustrated). When the upper cover21is opened, the exposure heads5move with the upper cover21and separate from the photosensitive drums63.

The process units6are arranged in a series in the front-rear direction in the area between the discharge tray22and the sheet tray31. The process units6can be mounted in and removed from the main casing2while the upper cover21is open. Each process unit6is configured of a drum cartridge61, and a developing cartridge62that is detachably mounted on the drum cartridge61. Each drum cartridge61has a photosensitive drum63, a charger64, and the like. Each developing cartridge62has a developing roller65and, while not designated with reference numerals in the drawing, a supply roller, a thickness-regulating blade, a toner-accommodating section that holds toner for use as a developer, and the like.

The four process units6are designated by reference numerals6Y,6M,6C, and6K in order from front-to-rear that accommodate toner in the corresponding colors yellow, magenta, cyan, and black. In this specification and in the drawings, the letters Y, M, C, and K for the corresponding colors yellow, magenta, cyan, and black will be appended to reference numerals of the developing cartridges62, photosensitive drums63, and the like when identifying components associated with specific colors of toner.

The transfer unit7is provided in the main casing2between the sheet tray31and the process units6. The transfer unit7is primarily provided with a drive roller71, a follow roller72, an endless conveying belt73, and four transfer rollers74. The conveying belt73is stretched around the drive roller71and follow roller72such that the outer surface of the conveying belt73opposes the four photosensitive drums63aligned in the front-rear direction. The transfer rollers74are arranged to oppose corresponding photosensitive drums63from inside the loop formed by the conveying belt73such that the conveying belt73is pinched between the transfer rollers74and the corresponding photosensitive drums63.

The fixing unit8is disposed to the rear of the process units6and the transfer unit7. The fixing unit8primarily includes a heating roller81, and a pressure roller82that faces and presses against the heating roller81. A heater (not illustrated) is disposed inside the heating roller81. The heater is a halogen lamp, for example, that emits light and generates heat when energized, heating the heating roller81with the radiant heat.

In the image-forming unit4, the chargers64uniformly charge the surfaces of the rotating photosensitive drums63, and the corresponding exposure heads5expose the charged surfaces to form electrostatic latent images on the photosensitive drums63based on image data. In the meantime, the supply rollers supply toner from the toner-accommodating sections to the corresponding developing rollers65, and the toner carried on the developing rollers65is regulated to a uniform thickness while passing between the developing rollers65and the thickness-regulating blades as the developing rollers65rotate.

The toner carried on each developing roller65is supplied onto the corresponding photosensitive drum63, developing the electrostatic latent image on the photosensitive drum63into a visible toner image. Subsequently, the toner image on each photosensitive drum63is transferred onto a sheet S supplied from the sheet-feeding unit3as the sheet S is conveyed between the photosensitive drum63and the conveying belt73. The toner images transferred onto the sheet S are thermally fixed to the sheet S as the sheet S is conveyed between the heating roller81and pressure roller82, thereby forming an image on the sheet S. After the toner image is thermally fixed to the sheet S, a conveying roller83conveys the sheet S out of the fixing unit8and onto a conveying path91.

The conveying unit9is configured to convey sheets S exiting the image-forming unit4either outside the main casing2or back toward the image-forming unit4. The conveying unit9primarily includes the conveying path91, a conveying roller92, a discharge roller93, a reconveying path94, and a plurality of reconveying rollers95provided along the reconveying path94.

The conveying path91extends upward from a point near the conveying roller83, and then curves forward and extends toward the discharge opening23. The reconveying path94extends downward from a point near the rear side of the conveying roller83, curves and extends forward along the bottom of the sheet tray31, and then curves and extends upward toward the conveying roller36.

The conveying roller92and discharge roller93are configured to be rotatable in forward and reverse rotations. Specifically, when rotating forward as indicated by solid arrows, the conveying roller92and discharge roller93convey the sheet S from the image-forming unit4toward the discharge opening23. When rotating in reverse as indicated by dashed arrows, the conveying roller92and discharge roller93convey a sheet S that has exited from the image-forming unit4back toward the image-forming unit4.

When an image is being formed on only one side of a sheet S, the conveying roller83of the conveying unit9conveys the sheet S out of the image-forming unit4, and the forward-rotating conveying roller92and discharge roller93convey the sheet S toward the discharge opening23and discharge the sheet S through the discharge opening23onto the discharge tray22.

On the other hand, if images are being formed on both sides of the sheet S, the conveying roller83of the conveying unit9conveys the sheet S out of the image-forming unit4, the forward-rotating conveying roller92and discharge roller93convey the sheet S toward the discharge opening23and are temporarily halted before the trailing edge of the sheet S passes the conveying roller92. Subsequently, the conveying roller92and discharge roller93are rotated in reverse to guide the sheet S, which has an image formed on one side, onto the reconveying path94. Once on the reconveying path94(see the dashed line), the reconveying rollers95, conveying roller36, and registration roller37supply the sheet S back to the image-forming unit4.

After the sheet S has been resupplied to the image-forming unit4, the image-forming unit4forms an image on the other side of the sheet S, and the conveying roller83conveys the sheet S out of the image-forming unit4. Subsequently, the forward-rotating conveying roller92and discharge roller93of the conveying unit9convey the sheet S toward the discharge opening23and discharge the sheet S through the discharge opening23onto the discharge tray22.

The color printer1is configured to form images in a monochrome mode and a color mode. In the monochrome mode, only the process unit6K is used to form a black image on the sheet S. In the color mode, all process units6Y,6M,6C, and6K are used to form a multicolor image on the sheet S.

When the color printer1operates in the color mode, all developing rollers65Y,65M,65C, and65K are in contact with the corresponding photosensitive drums63Y,63M,63C, and63K as illustrated inFIG.2A. This state will be called a first contact state. In other words, while the color printer1operates in the color mode, the developing rollers65Y,65M,65C, and65K are placed in the first contact state. When the color printer1operates in the monochrome mode, on the other hand, only the developing roller65K contacts the corresponding photosensitive drum63K while the developing rollers65Y,65M, and65C are separated from the corresponding photosensitive drums63Y,63M, and63C, as illustrated inFIG.2B. This state will be called a second contact state. In other words, while the color printer1operates in the monochrome mode, the developing rollers65Y,65M,65C, and65K are placed in the second contact state.

Further, when the color printer1is conveying a sheet S back toward the image-forming unit4or when image formation is complete, all developing rollers65Y,65M,65C, and65K are separated from the corresponding photosensitive drums63Y,63M,63C, and63K, as illustrated inFIG.2C. This state will be called a separated state. In other words, while the color printer1is conveying a sheet S back toward the image-forming unit4or after image formation is complete, the developing rollers65Y,65M,65C, and65K are placed in the separated state.

Next, the configuration of drive mechanisms in the color printer1will be described. As illustrated inFIG.3, the color printer1is provided with a first motor110, a second motor210, a switchback drive mechanism120, a first developing drive mechanism130, a fixing drive mechanism140, a conveying drive mechanism150, a reconveying drive mechanism170, a photosensitive member drive mechanism220, a belt drive mechanism230, a second developing drive mechanism240, and a separation mechanism300.

The first motor110functions to apply a drive force to the conveying roller92, discharge roller93, and the like. The first motor110is configured to switch the rotating direction of an output shaft (not illustrated) supplying the drive force. By switching the rotating direction of the output shaft, the first motor110can rotate the conveying roller92and discharge roller93in forward or reverse directions.

The second motor210functions to apply a drive force to: the photosensitive drums63Y,63M,63C, and63K; the developing rollers65Y,65M,65C, and65K (developing cartridges62Y,62M,62C, and62K) that supply toner in the corresponding colors yellow, magenta, cyan, and black to the respective photosensitive drums63Y,63M,63C, and63K; the conveying belt73; and the like. When the second motor210is driven, the drive shaft (not illustrated) of the second motor210that supplies the drive force always rotates in the same direction.

The switchback drive mechanism120functions to transmit the drive force of the first motor110to the conveying roller92and discharge roller93. The switchback drive mechanism120is configured of a plurality of gears (not illustrated). The switchback drive mechanism120is configured to rotate the conveying roller92and discharge roller93forward when the output shaft of the first motor110is rotated in one direction (hereinafter referred to as “forward rotation”) and to rotate the conveying roller92and discharge roller93in reverse when the output shaft of the first motor110is rotated in another direction opposite the one direction (hereinafter referred to as “reverse rotation”). Note that since the configuration and layout of gears employed in the present disclosure are widely known, detailed illustrations and descriptions of the drive mechanisms have been omitted from this specification.

The first developing drive mechanism130functions to transmit the drive force of the second motor210to the developing roller65K (the developing cartridge62K). The first developing drive mechanism130is primarily configured of a plurality of gears (not illustrated). The first developing drive mechanism130is further provided with a developing clutch139. The developing clutch139is an electromagnetic clutch having a known configuration. The developing clutch139is configured to switch between a connected state enabling a drive force to be transmitted from the second motor210to the developing roller65K, and a disconnected state preventing the drive force from being transmitted from the second motor210to the developing roller65K.

When the developing rollers65are in the first contact state illustrated inFIG.2Aor the second contact state illustrated inFIG.2B, the developing clutch139is in its connected state for transmitting the drive force of the second motor210to the developing roller65K. When the developing rollers65are in the separated state illustrated inFIG.2C, the developing clutch139is in its disconnected state for preventing the drive force of the second motor210from being transmitted to the developing roller65K. Note that when the developing rollers65are being switched (the developing rollers65are moving) from the separated state to the first contact state or second contact state, the developing clutch139preferably begins transmitting the drive force from the second motor210to the developing roller65K while the developing roller65K is still in a separated state, i.e., before the developing roller65K contacts the photosensitive drum63K, so that the rotation of the developing roller65K is stable when the developing roller65K contacts the photosensitive drum63K.

The fixing drive mechanism140functions to transmit the drive force of the first motor110to the heating roller81. The fixing drive mechanism140is configured of a plurality of gears (not illustrated). The fixing drive mechanism140is configured to rotate the heating roller81clockwise inFIG.3when the output shaft of the first motor110is in forward rotation and not to transmit the drive force to the heating roller81when the output shaft of the first motor110is in reverse rotation. A temperature sensor ST for detecting the temperature of the fixing unit8is provided near the fixing unit8. The temperature sensor ST detects the temperature of the heating roller81. Output from the temperature sensor ST is inputted into a control device10. The control device10can acquire detected temperatures from the temperature sensor ST at any timing.

The conveying drive mechanism150functions to transmit the drive force of the first motor110to the pickup roller33, registration roller37, and the like (the sheet-feeding unit3). The conveying drive mechanism150is primarily configured of a plurality of gears (not illustrated). The conveying drive mechanism150is configured to rotate the pickup roller33and the like in only one direction, regardless the rotating direction of the output shaft in the first motor110. The conveying drive mechanism150is also provided with a feeding clutch166. The feeding clutch166is an electromagnetic clutch having a known configuration. The feeding clutch166is configured to switch between a connected state that enables a drive force to be transmitted from the first motor110to the pickup roller33, and a disconnected state that prevents a drive force from being transmitted from the first motor110to the pickup roller33.

The reconveying drive mechanism170functions to transmit the drive force of the first motor110to the reconveying rollers95. The reconveying drive mechanism170is primarily configured of a plurality of gears (not illustrated). The reconveying drive mechanism170is configured to rotate the reconveying rollers95clockwise inFIG.3, regardless the rotating direction of the output shaft in the first motor110.

The photosensitive member drive mechanism220functions to transmit the drive force of the second motor210to the photosensitive drums63Y,63M,63C, and63K. The photosensitive member drive mechanism220is configured of a plurality of gears (not illustrated). The photosensitive member drive mechanism220is configured to rotate the photosensitive drums63Y,63M,63C, and63K clockwise inFIG.3when the second motor210is driven and to halt driving of the photosensitive drums63Y,63M,63C, and63K when the drive of the second motor210is halted.

The belt drive mechanism230functions to transmit the drive force of the second motor210to the drive roller71(the transfer unit7). The belt drive mechanism230is configured of a plurality of gears (not illustrated). The belt drive mechanism230is configured to rotate the drive roller71counterclockwise inFIG.3when the second motor210is driven and to halt driving of the drive roller71when the drive of the second motor210is halted.

The second developing drive mechanism240functions to transmit the drive force of the second motor210to the developing rollers65Y,65M, and65C (the developing cartridges62Y,62M, and62C). The second developing drive mechanism240is primarily configured of a plurality of gears (not illustrated). The second developing drive mechanism240is configured to transmit the drive force of the second motor210to the developing rollers65Y,65M, and65C when the developing rollers65are in the first contact state illustrated inFIG.2Aand not to transmit the drive force of the second motor210to the developing rollers65Y,65M, and65C when the developing rollers65are in the second contact state illustrated inFIG.2Bor the separated state illustrated inFIG.2C. Note that when the developing rollers65switch from the second contact state or separated state to the first contact state, the second developing drive mechanism240preferably transmits the drive force from the second motor210to the developing rollers65Y,65M, and65C while the developing rollers65Y,65M, and65C are still in the separated state, i.e., before the developing rollers65Y,65M, and65C contact the photosensitive drums63Y,63M, and63C, so that the rotation of the developing rollers65Y,65M, and65C is stable when the developing rollers65Y,65M, and65C contact the photosensitive drums63Y,63M, and63C.

As illustrated inFIGS.2A through2C, the separation mechanism300can switch the developing rollers65between the first contact state illustrated inFIG.2A, the second contact state illustrated inFIG.2B, and the separated state illustrated inFIG.2C. The separation mechanism300is primarily provided with a separation cam310, and a separation drive mechanism330.

The separation cam310is a translation cam elongated in the front-rear direction and provided along the side of the process units6. The separation cam310is supported so as to be movable in the front-rear direction relative to the main casing2. Formed in the top surface of the separation cam310is a single recess312that can engage with a rotational shaft (not designated with a reference numeral) of the developing roller65K; three recesses313capable of engaging with respective rotational shafts of the developing rollers65Y,65M, and65C; and sloped surfaces314that slope upward from front to rear, extending from the rear ends on the bottom surfaces of the respective recesses312and313to the top surface of the separation cam310. The recess312is formed longer in the front-rear direction than the recesses313. The separation cam310has a gear rack311provided in the bottom surface of the front end portion thereof.

The separation drive mechanism330functions to transmit the drive force of the first motor110to the separation cam310. As illustrated inFIG.3, the separation drive mechanism330is configured of a plurality of gears that includes a separation cam drive gear335, and a separation clutch339. The gear rack311of the separation cam310is meshingly engaged with some of gear teeth provided on the formed on the outer circumferential portion of the separation cam drive gear335. The separation clutch339is an electromagnetic clutch having a known configuration. The separation clutch339is configured to switch between a connected state that enables the drive force of the first motor110to be transmitted to the separation cam drive gear335, and a disconnected state that prevents the drive force from being transmitted from the first motor110to the separation cam drive gear335. When the separation clutch339is in the connected state, the separation drive mechanism330is configured to rotate the separation cam drive gear335counterclockwise inFIGS.2A through2Cwhen the output shaft of the first motor110is in forward rotation, and to rotate the separation cam drive gear335clockwise inFIGS.2A through2Cwhen the output shaft of the first motor110is in reverse rotation.

If the first motor110is rotated in reverse from the first contact state illustrated inFIG.2A, in which all developing rollers65contact the corresponding photosensitive drums63, the separation cam drive gear335rotates clockwise in the drawing and the separation cam310moves forward. At this time, the rotational shafts of the developing rollers65Y,65M, and65C are first among the four developing rollers65engaged in the recesses312and313to move upward along the corresponding sloped surfaces314so that the developing rollers65Y,65M, and65C separate from the corresponding photosensitive drums63Y,63M, and63C, as illustrated inFIG.2B.

If the separation cam310is halted at this stage, the developing rollers65remain in the second contact state, in which only the developing roller65K contacts the photosensitive drum63K. If the separation cam drive gear335is rotated farther clockwise from the state illustrated inFIG.2B, the separation cam310moves farther forward. At this time, the rotational shaft of the developing roller65K moves upward along the corresponding sloped surface314, and the developing roller65K separates from the photosensitive drum63K, as illustrated inFIG.2C. As a result, the developing rollers65are in the separated state, i.e., all developing rollers65are separated from the corresponding photosensitive drums63.

If the first motor110is rotated forward from the separated state illustrated inFIG.2C, the separation cam drive gear335rotates counterclockwise in the drawings and the separation cam310moves rearward. At this time, the rotational shaft of the developing roller65K is first among the four developing rollers65supported on the top surface of the separation cam310to move downward and become engaged in the corresponding recess312, bringing the developing roller65K into contact with the photosensitive drum63K.

If the separation cam310is halted at this stage, the developing rollers65are in the second contact state. If the separation cam drive gear335continues to rotate counterclockwise from the state illustrated inFIG.2B, the separation cam310moves farther rearward. At this time, the rotational shafts of the developing rollers65Y,65M, and65C move downward and engage in the recesses313so that the developing rollers65Y,65M, and65C contact the corresponding photosensitive drums63Y,63M, and63C, as illustrated inFIG.2A. As a result, the developing rollers65are in the first contact state.

Although the separation cam310is configured to be capable of engaging with the rotational shafts of the developing rollers65in the present embodiment, the separation cam310may be configured to engage with any portions of the developing cartridges62provided that the separation cam310can switch the developing rollers65between a state in contact with the photosensitive drums63and a state separated from the photosensitive drums63.

Next, configurations for controlling the drive functions of the color printer1will be described. As illustrated inFIG.1, the color printer1is provided with the control device10, a first sheet sensor11, and a second sheet sensor12.

The first sheet sensor11and second sheet sensor12are provided for detecting sheets S being conveyed inside the main casing2. As one example, each of the first sheet sensor11and second sheet sensor12is configured of an actuator that pivots when contacted by a sheet S, and an optical sensor that detects pivoting of the actuator. The first sheet sensor11is disposed between the registration roller37and the conveying belt73along the conveying path of the sheet S. The second sheet sensor12is disposed between the fixing unit8and the conveying roller83along the conveying path of the sheet S. In the following description, the first sheet sensor11and second sheet sensor12are in an ON state when detecting a sheet S and an OFF state when not detecting a sheet S.

The control device10is configured to control driving of the developing rollers65(the developing cartridges62), conveying roller92, discharge roller93, and the like by controlling the first motor110, second motor210, developing clutch139, and the like. The control device10is disposed at a suitable position within the main casing2. As illustrated inFIG.3, the control device10is provided with: a CPU10a, which is an arithmetic unit; a memory10b; an input/output interface (not illustrated); and the like.

The control device10executes control by performing various arithmetic processes based on programs and the like stored in the memory10b, and detection results acquired from the sheet sensors11and12and the temperature sensor ST (seeFIG.3). The memory10bincludes ROM, RAM, flash memory, and the like. The control device10may be provided with an application-specific integrated circuit (ASIC) or may be configured of integrated circuits constituting a portion of an ASIC.

Next, a control process executed by the color printer1having the above configuration will be described in detail with reference toFIGS.4through7.FIG.4illustrates steps in a printing process executed by the control device10. The control device10begins this printing process upon receiving a print command from an external PC (not illustrated), for example. Hereinafter, each procedure in the following processes will be indicated by a step number using “S” as an abbreviation for “step.”

In S10ofFIG.4, the control device10first determines whether the current printing mode of the color printer1is the color mode. If the current printing mode is the color mode (S10: YES), the control device10executes a color printing process in S12and subsequently ends the printing process. However, if the current printing mode is not the color mode (S10: NO), i.e., when the current printing mode is the monochrome mode, the control device10executes a monochrome printing process in S14, and subsequently ends the printing process ofFIG.4.

FIG.5illustrates steps in the color printing process. In S20at the beginning ofFIG.5, the control device10turns on (energizes) the heater in the heating roller81. Subsequently, in S22the control device10waits until a detected temperature T of the temperature sensor ST has reached a temperature T1(S22: NO). Once the detected temperature T is greater than or equal to the temperature T1(S22: YES), in S24the control device10turns on (begins driving) the first motor110. Here, the temperature T1is a temperature determined in advance to be the temperature at which driving of the first motor110is to be started. The temperature T1is stored in the memory10b, for example.

Next, in S26the control device10waits until the time elapsed after starting to drive the first motor110has reached a time TM1(S26: NO). Once the elapsed time has reached the time TM1(S26: YES), in S28the control device10turns on (begin driving) the second motor210. Here, the time TM1is a time determined in advance to be the wait time for which the control device10must wait after beginning to drive the first motor110before starting to drive the second motor210. The time TM1is also stored in the memory10b, for example. This elapsed time is measured by a timer, for example. The timer may be implemented by a function of the control device10when the control device10is equipped with a timer function or may be configured of a software timer in the memory10bwhen the control device10is not equipped with a timer function. WhileFIG.5does not show the processes of starting, stopping, and resetting the timer and the like, the control device10starts, stops, and resets the timer at prescribed timings when determining the elapsed time. Specifically, the control device10resets and starts the timer at the same time the control device10turns on the first motor110.

Thus, the time TM1is used in the present embodiment for determining the timing at which driving of the second motor210is started, but a temperature Tp of the fixing unit8may be used instead.

In S30the control device10waits until the elapsed time after starting to drive the second motor210has reached a time TM2(S30: NO). Once the elapsed time has reached the time TM2(S30: YES), the control device10advances to S32. Here, the time TM2is a time determined in advance to be the wait time that the control device10must wait after starting to drive the second motor210until the second motor210has become stable. The time TM2is stored in the memory10b, for example.

In S32the control device10waits until the detected temperature T of the temperature sensor ST is greater than or equal to a temperature T2(S32: NO). Once the detected temperature T is greater than or equal to the temperature T2(S32: YES), in S34the control device10instructs the separation drive mechanism330to switch the developing rollers65from the separated state inFIG.2Cto the first contact state inFIG.2A. Here, the temperature T2is a temperature determined in advance to be the temperature at which the developing rollers65are allowed to be pressed against the photosensitive drums63. The temperature T2is stored in the memory10b, for example.

Next, in S36the control device10waits until the elapsed time after issuing an instruction to press the developing rollers65against the photosensitive drums63has reached a time TM3(S36: NO). Once the elapsed time has reached the time TM3(S36: YES), in S38the control device10turns on (switches to the connected state) the feeding clutch166. In S40the control device10executes printing and subsequently ends the color printing process. Here, the time TM3is a time determined in advance to be the wait time that the control device10must wait after instructing the separation drive mechanism330to press the developing rollers65against the photosensitive drums63before turning on the feeding clutch166. The time TM3is stored in the memory10b, for example.

FIG.7Ais a timing chart illustrating control timings for various components in the color printer1during the color mode. As illustrated inFIG.7A, the heater is turned on at a timing t0, and the control device10begins driving the first motor110at a timing t1at which the detected temperature T of the fixing unit8reaches the temperature T1. At a timing t2, which occurs a time TM1after the timing t1, the control device10begins driving the second motor210. At a timing t3, when the detected temperature T of the fixing unit8reaches the temperature T2after a time TM2has elapsed from the timing t2, the control device10begins pressing the developing rollers65against the photosensitive drums63. At a timing t4, a time TM3after the timing t3, the control device10turns the feeding clutch166on. InFIG.7A, the temperature T3denotes the temperature of the fixing unit8during printing (the fixing temperature). In other words, the control device10sets the target temperature of the detected temperature T for printing to the temperature T3and controls the heater so that the detected temperature T is at the temperature T3during printing.

FIG.6illustrates steps in the monochrome printing process. Since the monochrome printing process is similar to the color printing process described above inFIG.5with only some modifications, the same reference numerals inFIG.5are used inFIG.6, and descriptions of these processes are omitted as appropriate.

In S50ofFIG.6, the control device10determines whether the time elapsed after starting to drive the first motor110has reached a time TM1′. That is, while the control device10determines in S26ofFIG.5whether the elapsed time after starting to drive the first motor110has reached a time TM1, in S50ofFIG.6the control device10determines whether this elapsed time has reached the time TM1′. Here, the relationship between time TM1′ and time TM1is TM1′>TM1. In other words, the timing at which driving of the second motor210is started in the monochrome mode is later than that in the color mode. This is because the second motor210has the function of driving the photosensitive drums63. Accordingly, the photosensitive drums63begin rotating when the control device10begins driving the second motor210.

Since the life of the photosensitive drums63is shortened as the rotating time of the photosensitive drums63accumulates, the life of the photosensitive drums63can be prolonged by suppressing their rotating time. Accordingly, the start timing for driving the second motor210can be delayed from the timing used for the color mode because only the photosensitive drum63K among the four photosensitive drums63Y,63M,63C, and63K is used for printing in the monochrome mode and the photosensitive drum63K is positioned farthest downstream in the conveying direction of the sheet S. Therefore, the control device10is configured to delay the timing at which driving of the second motor210is started.

As described for the process in S26, the temperature Tp of the fixing unit8may be used in place of the time TM1for determining the start timing for driving the second motor210in S50. However, when the temperature of the fixing unit8is used in place of the time TM1′, the control device10preferably uses a temperature Tp′ that is higher than the temperature Tp because this higher temperature will delay the timing at which driving of the second motor210is started, thereby prolonging the life of the photosensitive drums63.

In the color printing process described above, the control device10instructs the separation drive mechanism330to press the developing rollers65against the photosensitive drums63when the detected temperature T of the temperature sensor ST is greater than or equal to the temperature T2. However, in the monochrome printing process, the control device10controls the developing roller65K to be pressed against the photosensitive drum63K in S54once the control device10determines in S52that a time TM4has elapsed after the timing of the rise of the detected temperature above T2. Here, the time TM4is set in advance as the wait time that the control device10must wait after detecting that the temperature T has reached or exceeded the temperature T2before the developing roller65K can be pressed against the photosensitive drum63K. The time TM4is stored in the memory10b, for example.

The time TM4is determined by considering the time required for the sheet S to be conveyed from the photosensitive drum63Y to the photosensitive drum63K. Accordingly, a toner image on the photosensitive drum63K is ready to be transferred onto the sheet S once the sheet S has reached the photosensitive drum63K. Unlike the process described above in S34for pressing all developing rollers65against their corresponding photosensitive drums63, in the process of S54the control device10instructs the separation drive mechanism330to switch the developing rollers65from the separated state illustrated inFIG.2Cto the second contact state illustrated inFIG.2B. In the monochrome printing process, the control device10may be configured to press the developing roller65K against the photosensitive drum63K when the first sheet sensor11has detected the sheet S, rather than when the time TM4has elapsed after the timing of the detected temperature T exceeding the temperature T2.

FIG.7Bis a timing chart illustrating control timings for various components in the color printer1during the monochrome mode. As illustrated inFIG.7B, the heater is turned on at a timing t0, and the control device10begins driving the first motor110at a timing t2at which the detected temperature T of the fixing unit8reaches the temperature T1. At a timing t21, which occurs a time TM1′ after the timing t1, the control device10begins driving the second motor210. The detected temperature T of the fixing unit8reaches the temperature T2at a timing t3, which occurs a time TM2following the timing t21. At a timing t4, which occurs a further time TM3after the timing t3, the control device10turns on the feeding clutch166. At a timing t5, which occurs a time TM4after the timing t3, the control device10begins pressing the developing roller65K against the photosensitive drum63K.

Since the time TM4is longer than the time TM3and both times TM3and TM4are measured from the same starting point of the timing t3, the control device10begins pressing the developing roller65K against the photosensitive drum63K later than the timing at which the sheet S is fed from the sheet tray31. The time TM3has the same length in bothFIGS.7Aand7B, and measurement of the time TM3begins at the same timing in both processes. However, the time TM3inFIG.7Ais defined as the wait time that the control device10must wait after instructing the developing rollers65to be pressed against the photosensitive drums63before the control device10turns on the feeding clutch166(see the description of S36), while the time TM3inFIG.7Bmust be defined as the wait time that the control device10must wait after the detected temperature T of the temperature sensor ST reaches or exceeds the temperature T2before the control device10can turn on the feeding clutch166.

As described above, the color printer1in the present embodiment is provided with the sheet tray31that accommodates sheets S; the pickup roller33that feeds sheets S from the sheet tray31; the conveying belt73that is used when transferring toner images onto sheets S; the photosensitive drum63Y that contacts the conveying belt73; the photosensitive drum63K that contacts the conveying belt73downstream from the photosensitive drum63Y in the circulating direction of the conveying belt73; the developing roller65Y that is used when forming multicolor images on sheets S and is configured to be switchable (movable) between a contact state for contacting the photosensitive drum63Y and a separated state for separating from the photosensitive drum63Y; the developing roller65K that is used when forming monochrome images on sheets S and is configured to be switchable between a contact state for contacting the photosensitive drum63K and a separated state for separating from the photosensitive drum63K; the fixing unit8that fixes toner images formed on the sheets S; and the control device10.

The control device10raises the temperature of the fixing unit8up to the fixing temperature T3by causing the heating roller81to generate heat. At the same time, the control device causes the photosensitive drum63Y and photosensitive drum63K to rotate, and causes the conveying belt73to circularly move. When forming a multicolor image on a sheet S, the control device10places the developing rollers65Y and65K in contact with the corresponding photosensitive drums63Y and65K once the temperature of the fixing unit8has reached the temperature T2, which is lower than the temperature T3, and feeds a sheet S from the sheet tray31onto the conveying belt73when the time TM3has elapsed after the temperature of the fixing unit8reaching the temperature T2. When forming a monochrome image on a sheet S, the control device10controls the developing roller65K to contact the photosensitive drum63K when the time TM4has elapsed after the temperature of the fixing unit8reaching the temperature T2and feeds a sheet S from the sheet tray31to the conveying belt73when the time TM3has elapsed after the temperature of the fixing unit8reaching the temperature T2.

In this way, the color printer1of the present embodiment is configured to feed a sheet S from the sheet tray31to the conveying belt73once the time TM3has elapsed after the temperature of the fixing unit8reaching the temperature T2, regardless of whether a multicolor image or a monochrome image is being formed on the sheet S. However, the color printer1controls the developing roller65Y to contact the photosensitive drum63Y once the temperature of the fixing unit8has reached the temperature T2when forming a multicolor image on a sheet S, while the control device10controls the developing roller65K to contact the photosensitive drum63K once the time TM4has elapsed after the temperature of the fixing unit8reaching the temperature T2when forming a monochrome image on a sheet S.

This method enables the timing at which the developing rollers65Y and65K contact the corresponding photosensitive drums63Y and63K to be set while accounting for the difference in timings at which images are formed on photosensitive drums63during multicolor image formation and monochrome image formation. On the other hand, the timing at which sheets S are fed from the sheet tray31is uniformly set to the timing at which the time TM3has elapsed after the temperature of the fixing unit8reaching the temperature T2, regardless of whether a multicolor image or a monochrome image is being formed. This enables the color printer1to set suitable timings at which the developing rollers65contact the photosensitive drums63and at which the fixing unit8is ready (the timing at which the temperature of the fixing unit8has reached the temperature T3and the sheet S has arrived at the fixing unit8).

Note that in the present embodiment, the toner image is an example of the claimed developer image. The multicolor image is an example of the claimed multicolor image, and the monochrome image is an example of the claimed single-color image. The conveying belt73is an example of the claimed belt. The photosensitive drum63Y is an example of the claimed first photosensitive drum. The photosensitive drum63Y is positioned farthest upstream in the direction in which the conveying belt73moves, and is used when multicolor images are formed. The photosensitive drum63K is an example of the claimed second photosensitive member. The photosensitive drum63K is positioned downstream of the photosensitive drum63Y in the direction in which the conveying belt73moves, and is used when monochrome images are formed. The developing roller65Y is an example of the claimed first developing roller. The developing roller65Y is positioned farthest upstream in the direction in which the conveying belt73moves, and is used when multicolor images are formed. The developing roller65K is an example of the claimed second developing roller. The developing roller65K is positioned downstream of the developing roller65Y in the direction in which the conveying belt73moves, and is used when monochrome images are formed. In this case, the toner in yellow is an example of the claimed first-color toner, and the toner in black is an example of the claimed second-color toner. The photosensitive drum63M and developing roller65M and the photosensitive drum63C and developing roller65C are also examples of the claimed first photosensitive drum and first developing roller. In this case, the toner in magenta or cyan is an example of the claimed first-color toner. The transfer unit7is an example of the claimed transfer unit, and the conveying belt73is an example of the claimed belt. The fixing unit8is an example of the claimed fixing device. The separation mechanism300is an example of the claimed separation mechanism. The control device10is an example of the claimed controller. The temperature T3is an example of the claimed fixing temperature. The temperature T2is an example of the claimed first temperature. The time TM3is an example of the claimed first period of time. The time TM4is an example of the claimed second period of time.

Further, the conveying belt73is used for conveying sheets S so that toner images can be transferred directly from the photosensitive drums63Y and63K onto the sheets S.

This enables the color printer1to set the timing at which the developing rollers65contact the photosensitive drums63while accounting differences in the conveying distance from the pickup position of the sheet S to the position at which the initial image is transferred onto the sheet S between color image formation and monochrome image formation.

The time TM4is set longer than the time TM3.

Accordingly, when forming a monochrome image on a sheet S, the developing roller65K is made to contact the photosensitive drum63K after the conveyance of the sheet S has been started, thereby reducing unnecessary rotation of the developing roller65K.

Further, the control device10begins rotating the photosensitive drum63Y and photosensitive drum63K and circularly moving the conveying belt73once the temperature of the fixing unit8has reached the temperature Tp, which is lower than the temperature T2. The temperature Tp is an example of the claimed second temperature.

Thus, the control device10can begin rotating each of the photosensitive drum63Y and photosensitive drum63K before the developing roller65Y and developing roller65K have been made to contact the respective photosensitive drum63Y and photosensitive drum63K.

The color printer1is also provided with the first motor110for driving the pickup roller33; and the feeding clutch166that can switch between a connected state in which the drive force of the first motor110can be transmitted to the pickup roller33, and a disconnected state in which the drive force of the first motor110cannot be transmitted to the pickup roller33. The control device10activates the first motor110when the temperature of the fixing unit8has reached the temperature T1, which is lower than the temperature T3, and switches the feeding clutch166to the connected state while the first motor110is driving the pickup roller33. The temperature T1is an example of the claimed third temperature.

This allows the first motor110to be in a standby state when the pickup roller33is driven, so that the conveying time of the sheet S to the conveying belt73after pickup is appropriate. That is, the first motor110can be rotating at the target speed when the control device10switches the feeding clutch166to the connected state.

The color printer1is further provided with the second motor210for driving the developing roller65Y and developing roller65K. The control device10does not drive the developing roller65Y and developing roller65K while the developing roller65Y and developing roller65K are in the separated state but begins driving the developing roller65Y and developing roller65K when the developing roller65Y and developing roller65K are moving from the separated state toward the contact state.

Thus, the developing roller65Y and developing roller65K are not rotated unnecessarily in the separated state but are rotating when they reach the contact state. Hence, the developing roller65Y and developing roller65K can be quickly prepared to form toner images on the photosensitive drum63Y and photosensitive drum63K.

While the invention has been described in conjunction with various example structures outlined above and illustrated in the figures, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the example embodiments of the disclosure, as set forth above, are intended to be illustrative of the invention, and not limiting the invention. Various changes may be made without departing from the spirit and scope of the disclosure. Therefore, the disclosure is intended to embrace all known or later developed alternatives, modifications, variations, improvements, and/or substantial equivalents. Some specific examples of potential alternatives, modifications, or variations in the described invention are provided below:(1) In the embodiment described above, the color printer1that executes color printing using four colors of toner (Y, M, C, and K) is used as an example, but the number of colors can be more or less than four.(2) In the embodiment described above, the color printer1capable of forming images on both sides of a sheet S is used as an example, but the present invention can also be applied to printers capable of forming images on only one side of a sheet S.(3) In the embodiment described above, the stand-alone color printer1is used as an example, but the present invention may be applied to copiers, multifunction peripherals, or the like as well.(4) The color printer1of the present embodiment described above is provided with exposure heads5for exposing the photosensitive drums63, but exposure in the color printer1may be performed using laser scanners instead.(5) The transfer unit7in the present embodiment is provided with the conveying belt73for conveying sheets S so that toner images can be transferred directly from the photosensitive drums63onto the sheets S. However, the transfer unit7may be provided with an intermediate transfer belt instead. In this case, toner images are initially transferred from the photosensitive drums63onto the intermediate transfer belt, and the intermediate transfer belt subsequently transfers those toner images onto the sheets S.