Image forming apparatus and control method

In an image forming apparatus, upon receipt of a printing instruction, a controller starts rotation of a heating member, starts rotation of a photoconductor drum when a standby time has elapsed from a time of the start of the rotation of the heating member, starts conveyance of a sheet after the start of the rotation of the photoconductor drum, and forms a toner image on the sheet after the start of the conveyance of the sheet. The controller is configured to be operated in one of selectable modes including a multicolor mode in which a toner image is formed using both of toner of a first color and a second color and a monochrome mode in which a toner image is formed using toner of the second color only. The standby time adopted in the monochrome mode is longer than the standby time adopted in the multicolor mode.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from Japanese Patent Application No. 2021-086030 filed on May 21, 2021, Japanese Patent Application No. 2021-200661 filed on Dec. 10, 2021, and Japanese Patent Application No. 2022-070648 filed on Apr. 22, 2022, the disclosures of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates to an image forming apparatus with a controller for controlling rotations of a photoconductor drum and a heating member, and a method of control implemented therein.

BACKGROUND ART

An image forming apparatus may include a controller capable of executing a preliminary (pre-process) rotation process of causing a photoconductor drum to make a predetermined number of (or more) rotations while causing a charger to operate, for a period of time after receipt of a printing instruction before start of feed of a first sheet, so as to stabilize a surface potential of the photoconductor drum. In this technical scheme, typically, a fuser motor is activated to start rotation of a heating roller (heating member) at a time when the temperature of a fuser assembly has reached a fuser motor rotation permissible temperature after receiving a printing instruction, and thereafter, a process motor is activated to start the preliminary rotation process (to start rotation of the photoconductor drum) at a time when the temperature of the fuser assembly has reached a process motor rotation permissible temperature. After activating the process motor, a laser diode for exposure of the photoconductor drum is caused to emit light, and after the output of the laser diode is stabilized, a sheet feed is started, so that a sheet is fed after the temperature of the fuser assembly has reached an image quality assurance temperature.

SUMMARY

Determination of sheet feed timing based on activation of the process motor depending upon the temperature of the fuser assembly would be subject to variation by the influence of the ambient temperature or the temperatures of the heating roller or the like at the time of receipt of the printing instruction. On the other hand, the preliminary rotation process is not affected by the influence of temperatures and performed for a fixed period as predetermined.

For adequate printing, the preliminary rotation process should be completed after receipt of the printing instruction before the leading edge of the first sheet arrives at the photoconductor drum. On this account, the start of the preliminary rotation process may be set at an earlier time with allowance made for the variation of the sheet feed timing to ensure that the preliminary rotation process is completed before the leading edge of the first sheet arrives at the photoconductor drum. However, setting such an earlier time for starting the preliminary rotation process would result in unnecessary rotation of the photoconductor drum between the time of completion of the preliminary rotation process and the arrival of the leading edge of the first sheet at the photoconductor drum.

There is a need to obviate unnecessary rotation the photoconductor drum would make after receipt of the printing instruction before the leading edge of the first sheet arrives at the photoconductor drum.

In one aspect, an image forming apparatus comprising a sheet feed mechanism, a conveyor belt, a first photoconductor drum, a second photoconductor drum, a heating member, and a controller is disclosed herein. The sheet feed mechanism is provided to feed a sheet. The conveyor belt is provided to convey the sheet fed by the sheet feed mechanism in a direction of conveyance of the sheet. The first photoconductor drum is provided to form a toner image of a first color on the sheet conveyed by the conveyor belt. The second photoconductor drum is provided to form a toner image of a second color on the sheet conveyed by the conveyor belt. The second photoconductor drum is located downstream of the first photoconductor drum in the direction of conveyance of the sheet. The heating member is configured to heat a sheet, the heating member being rotatable. The controller is configured to be operated in one of selectable modes including a multicolor mode in which a toner image is formed using both of toner of the first color and toner of the second color and a monochrome mode in which a toner image is formed using toner of the second color only. The controller is configured to exercise, upon receipt of a printing instruction, control which comprises: starting rotation of the heating member; starting rotation of the first and second photoconductor drums when a first standby time has elapsed from a time of the starting of the rotation of the heating member if the controller is operated in the monochrome mode, or when a second standby time longer than the first standby time has elapsed from the time of the starting of the rotation of the heating member if the controller is operated in the multicolor mode; causing the sheet feed mechanism to feed a sheet after the starting of the rotation of the first and second photoconductor drums; and forming a toner image on the sheet after causing the sheet feed mechanism to feed the sheet. The standby time adopted when the controller is operated in the multicolor mode is longer than the standby time adopted when the controller is operated in the monochrome mode.

In another aspect, an image forming apparatus comprising a photoconductor drum, a heating member, and a controller is disclosed herein. The photoconductor drum is provided to form a toner image on a sheet. The heating member is configured to heat a sheet, and comprises a heating roller that is rotatable. The controller is configured to execute, upon receipt of a printing instruction, processes comprising a heating process, a pre-fixing rotation process, a pre-process rotation process, and a sheet feed process. The heating process is a process of heating the heating roller. The pre-fixing rotation process is a process of starting rotation of the heating roller and causing the heating roller to rotate for a first execution period TMp from a time of the starting of the rotation of the heating roller. The pre-process rotation process is a process of starting rotation of the photoconductor drum when a standby time TMw has elapsed from the time of the starting of the rotation of the heating roller and causing the photoconductor drum to rotate for a second execution period TMd from a time of starting of rotation of the photoconductor drum. The second execution period TMd is shorter than the first execution period TMp. The sheet feed process is a process of starting feed of a sheet if both of a first condition and a second condition are satisfied where the first condition is satisfied if the pre-fixing rotation process has been completed, and the second condition is satisfied if a temperature of the heating roller has become equal to or higher than a predetermined temperature. The standby time TMw satisfies an equation:
TMw=(TMp+TMf)−TMd
where TMf is a time period from a time of the starting of the feed of the sheet until a leading edge of the sheet arrives at the photoconductor drum.

In still another aspect, a method of control implemented in an image forming apparatus including a sheet feed mechanism provided to feed a sheet, a conveyor belt provided to convey the sheet fed by the sheet feed mechanism in a direction of conveyance of the sheet, a first photoconductor drum provided to form a toner image of a first color on the sheet conveyed by the conveyor belt, a second photoconductor drum provided to form a toner image of a second color on the sheet conveyed by the conveyor belt, and a heating member configured to heat a sheet is disclosed herein. The second photoconductor drum is located downstream of the first photoconductor drum in the direction of conveyance of the sheet. The heating member is rotatable. The control is exercised in one of selectable modes including a multicolor mode in which a toner image is formed using both of toner of the first color and toner of the second color and a monochrome mode in which a toner image is formed using toner of the second color only. The method to be executed upon receipt of a printing instruction comprises: starting rotation of the heating member; starting rotation of the first and second photoconductor drums when a first standby time has elapsed from a time of the starting of the rotation of the heating member if the controller is operated in the monochrome mode, or when a second standby time longer than the first standby time has elapsed from the time of the starting of the rotation of the heating member if the controller is operated in the multicolor mode; causing the sheet feed mechanism to feed a sheet after the starting of the rotation of the first and second photoconductor drums; and forming a toner image on the sheet after causing the sheet feed mechanism to feed the sheet.

DESCRIPTION OF EMBODIMENTS

As shown inFIG.1, a color printer1as an example of an image forming apparatus includes a housing2, a sheet feeder unit3, an image forming unit4, a fixing device8, and a conveyor unit9. The housing2includes a sheet output tray21provided in an upper surface thereof.

The sheet feeder unit3is provided in a lower space inside the housing2, and includes a sheet feed tray31configured as a receptacle to hold and serve sheets S, and a sheet feed mechanism32configured to feed a sheet S held in the sheet feed tray31, to the image forming unit4. The sheet feed mechanism32includes a pickup roller33, a separation roller34, a separation pad35, and a registration roller36.

In the sheet feeder unit3, sheets S in the sheet feed tray31are fed by the pickup roller33toward the separation roller34, and separated one from the others between the separation roller34and the separation pad35. The registration roller36aligns the edge of the sheet S and conveys the sheet S toward the image forming unit4.

The image forming unit4includes an exposure device5, four process units6, and a transfer unit7.

The exposure device5is provided in an upper space inside the housing2, and includes a light source, a polygon mirror and other components which are not illustrated. The exposure device5is configured to scan a surface of each photoconductor drum61with a light beam, to thereby expose the surface of the photoconductor drum61to the light beam.

Each of the four process units6includes a photoconductor drum61, a charger62, a development roller63, a drum cleaner64, and a static eliminator lamp65. The drum cleaner64is configured to clean the surface of the corresponding photoconductor drum61, and the static eliminator lamp65is configured to irradiate the surface of the corresponding photoconductor drum51with light to thereby eliminate static electricity. Each of the four process units6contains, in its inside space, toner as an example of a developer.

The process units6containing toner of respective colors of yellow, magenta, cyan, and black, as designated by reference characters6Y,6M,6C, and6K, respectively, are arranged in this sequence in a direction of conveyance of a sheet S. In the following description and associated drawings, when the photoconductor drums51, the development rollers61or other members are specified for respective colors of toner, signs of Y, M, C, and K are appended to reference numerals of the respective members for yellow, magenta, cyan, and black.

As shown inFIG.2, the development roller63is movable between a contact position in which the development roller63is in contact with the photoconductor drum61and a separate position in which the development roller63is separate from the photoconductor drum61. The color printer1further includes a contact/separation mechanism300and a controller100. The contact/separation mechanism300is a mechanism that causes the development roller63to move between the contact position and the separate position. The controller100includes a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), etc., and is configured to execute various processes, in response to receipt of a printing instruction, or the like, according to programs prepared in advance.

The controller100is configured to be operated in one of selectable modes including a multicolor mode in which a toner image is formed using toner of a plurality of colors, a monochrome mode in which a toner image is formed using toner of a single color, and an all-separate mode in which all the development rollers63are separated from the corresponding photoconductor drums61. To switch the modes, the controller100causes the contact/separation mechanism300to operate to move one or more of the development rollers63. In the multicolor mode, all the development rollers63are kept in contact with the corresponding photoconductor drums61. In the monochrome mode, only the development roller63K for black is kept in contact with the corresponding photoconductor drum61K, and the other three development rollers63Y,63M,63C for yellow, magenta, and cyan are kept out of contact with the corresponding photoconductor drums61Y,61M,61C. In the all-separate mode, all the development rollers63are kept out of contact with the corresponding photoconductor drums61. The all-separate mode is adopted, for example, when the photoconductor drums61are subject to cleaning.

In the following description, “first” will be prefixed to the legends of members to be used in the multicolor mode and not to be used in the monochrome mode, which are specified with signs of Y, M and C appended to their reference numerals, and “second” will be prefixed to the legends of members to be used in both of the multicolor mode and the monochrome mode, which are specified with a sign of K appended to their reference numerals, as the case may be. For example, the photoconductor drums61Y,61M and61C may be expressed as the first photoconductor drums61Y,61M and61C. Similarly, the photoconductor drums61K may be expressed as the second photoconductor drum61K.

As shown inFIG.1, the transfer unit7includes a drive roller71, a follower roller72, a conveyor belt73, and four transfer rollers74. The conveyor belt73is an endless belt, and stretched between and looped over the drive roller71and the follower roller72. Inside the conveyor belt73, the transfer rollers74are located in positions corresponding to the photoconductor drums61. The conveyor belt73is nipped between the transfer rollers74and corresponding photoconductor drums61.

The charger62is a member that charges a surface of the corresponding photoconductor drum61. In other words, for example, the first charger62Y charges a surface of the first photoconductor drum61Y. The second charger62K charges a surface of the second photoconductor drum61K. The exposure device5exposes the charged surface of each photoconductor drum61to light, and an electrostatic latent image as formulated according to image data is formed on the surface of the photoconductor drum61.

The development roller63is a member that supplies toner to an electrostatic latent image formed on the corresponding photoconductor drum61. Thus, a toner image is formed on the photoconductor drum61. Thereafter, a sheet S is conveyed to an interface between the photoconductor drum61and the corresponding transfer roller74, and the toner image on the photoconductor drum61is transferred to the sheet S.

The fixing device8is a device configured to thermally fix a toner image on a sheet S. The fixing device8includes a heating roller81configured to heat a sheet S, and a pressure roller82configured to nip a sheet S in combination with the heating roller81. The heating roller81has a shape of a circular cylinder, and contains a heater H in its cylindrical inside space. The pressure roller82is pressed against the heating roller81, and configured to rotate by following the rotational motion of the rotating heating roller81.

The conveyor unit9is, principally, configured to convey a sheet S outputted from the fixing device8toward the outside of the housing2. The conveyor unit9mainly includes a conveyor roller91, and an output roller92. A sheet S outputted from the fixing device8is conveyed by the conveyor roller91to the output roller92, and ejected by the output roller92onto the sheet output tray21.

As shown inFIG.3, the color printer1further includes a main motor M1as an example of a first motor, a process motor M2as an example of a second motor, a sheet feed clutch C1, a development clutch C2, a contact/separation clutch C3, charging bias supply circuits V, and a temperature sensor83.

The main motor M1is a motor that supplies driving force to the sheet feed mechanism32, the heating roller81, the development roller63K (i.e., second development roller63K for black color, for use in the monochrome mode and the multicolor mode), and the contact/separation mechanism30. The main motor M1is connected via gearing (not shown) and the sheet feed clutch C1to the pickup roller33. The sheet feed clutch C1is a clutch capable of switching between an engagement state in which a driving force of the main motor M1is transmitted to the pickup roller33and a disengagement state in which transmission of the driving force from the main motor M1to the pickup roller33is interrupted.

The main motor M1is connected via gearing (not shown) to the heating roller81. The main motor M1is connected via gearing (not shown) and the development clutch C2to the development roller63K. The development clutch C2is a clutch capable of switching between an engagement state in which the driving force of the main motor M1is transmitted to the development roller63K and a disengagement state in which transmission of the driving force from the main motor M1to the development roller63K is interrupted.

The main motor M1is connected via gearing (not shown) and the contact/separation clutch C3to the contact/separation mechanism300. The contact/separation clutch C3is a clutch capable of switching between an engagement state in which the driving force of the main motor M1is transmitted to the contact/separation mechanism300and a disengagement state in which transmission of the driving force from the main motor M1to the contact/separation mechanism300is interrupted.

The process motor M2is a motor that supplies driving force to the development rollers63Y,63M and63C (i.e., first development rollers for yellow, magenta and cyan, for use in the multicolor mode), the four photoconductor drums61, and the drive roller71. The process motor M2is connected via gearing (not shown) to the three development rollers63Y,63M and63C, the four photoconductor drums61, and the drive roller71.

The charging bias supply circuit V is a circuit that provides a charging bias to the charger62. Four charging bias supply circuits V are provided one for each of the four chargers62. For example, the first charging bias supply circuit VY for yellow color, (i.e., one of the three first charging bias supply circuits V) is provided for the first charger62Y, and the second charging bias supply circuit VK is provided for the second charger62K.

The temperature sensor83is a sensor configured to detect a temperature of the heating roller81. The temperature detected by the temperature sensor83is provided to the controller100.

The controller100is configured to execute, after receipt of instruction, several processes which include a heating process, a pre-fixing rotation process, a pre-process rotation process and a sheet feed process. The heating process is a process of heating the heating roller81. More specifically, in the heating process, the heating roller81is heated by the heater H to increase the temperature of the heating roller81to a fixing temperature To (see alsoFIG.6). To start the heating process, the controller100activates the heater H before activating the main motor M1. The pre-fixing rotation process is a process of rotating the heating roller81and the pressure roller82before starting an image forming process for making the conditions of the heating roller81and the pressure roller82suitable for the fixing process. To be more specific, the pre-fixing rotation process is a process of starting rotation of the heating roller81and causing the heating roller81to rotate for a first execution period TMp from a time of the starting of the rotation of the heating roller81. To start the pre-fixing rotation process, the controller100activates the main motor M1before activating the process motor M2.

The controller100is configured to change the first execution period TMp according to an ambient temperature, and a type of a sheet S. Specifically, the controller100sets the first execution period TMp on the basis of a first table stored in a memory such as RAM. The first table contains information of several parameters including the ambient temperatures and the types of a sheet S, each combination of which is associated with the first execution period TMp, showing a relationship between each combination of the parameters and the first execution period TMp.

The lower the ambient temperature, the longer the first execution period TMp the controller100sets. The thicker the thickness of the sheet S and the larger the area of the sheet S, the longer the first execution period TMp the controller100sets.

The pre-process rotation process is a process of rotating the photoconductor drums61and the like before starting the image forming process for making the conditions of the photoconductor drums61or the like suitable for the image forming process. To be more specific, the pre-process rotation process is a process of starting rotation of the photoconductor drums61and causing the photoconductor drums61to rotate for a second execution period TMd from a time of starting the rotation of the photoconductor drums61. In the pre-process rotation process, the photoconductor drums61are caused to rotate while the chargers62are being operated. To start the pre-process rotation process, the controller100activates the process motor M2.

As shown inFIG.6, the second execution period TMd includes a first period TM1, a second period TM2, a third period TM3, a fourth period TM4, and a fifth period TM5. The first period TM1is a time period elapsed from a time of starting of rotation of the process motor M2until a lock signal for the process motor M2turns on. The second period TM2is a time period elapsed from a time of turning-on of the lock signal for the process motor M2until rotation of the photoconductor drum stabilizes. The third period TM3is a time period elapsed from a time of starting of supply of the charging bias to the charger62by the charging bias supply circuit V until the supply of the charging bias by the charging bias supply circuit V stabilizes. The fourth period TM4is a time period elapsed from a time of completion of stabilization of the charging bias until the photoconductor drum61makes N rotations where N is a natural number indicative of the number of full rotations. The fifth period TM5is a time period elapsed from a time at which the development roller has been pressed into contact with the photoconductor drum until the development roller63makes M rotations where M is a natural number indicative of the number of full rotations. Each of the number of rotations N, M may be set at one or any number more than one. Each of the first period TM1, second period TM2, third period TM3, fourth period TM4and fifth period TM5is predetermined as a fixed value determined by means of experiment, simulation, etc. Therefore, the second execution period TMd takes on a fixed value.

The sheet feed process is a process of starting feed of a sheet S if both of a first condition and a second condition are satisfied where the first condition is satisfied if the pre-fixing process has been completed, and the second condition is satisfied if a temperature of the heating roller81has become equal to or higher than a predetermined temperature. In the following description, the predetermined temperature will also be called a pickup permissible temperature Tp.

The controller100is configured to start the pre-process rotation process when a standby time TMw has elapsed from the time of starting of the pre-fixing rotation process.

The standby time TMw is a value that satisfies the following inequality:
A−α≤TMw≤A
where A=(TMp+TMf)−TMd, TMf is a feed time period from a time of the starting of the feed of a sheet S until a leading edge of the sheet S arrives at the photoconductor drum61, and α is a time period expended for the photoconductor drum61to make a half turn.

Specifically, the controller100sets the standby time TMw on the basis of a second table stored in a memory such as RAM. The second table contains a list of items of information on the first execution periods TMp, the feed time periods TMf and the second execution periods TMp, each combination of which is associated with the standby time TMw, showing a relationship between each combination of the periods and the standby time TMw.

Herein, α takes on a fixed value. It is to be understood that α may be changed where appropriate as long as it is not longer than a time period expended for the photoconductor drum61to make a half turn. For example, α may be a time period expended for the photoconductor drum61to rotate a one-third turn, a one-quarter turn, a one-fifth turn, or through any angle less than 180 degrees. α may be set at 0. That is, A may be equal to TMw (A=TMw). In the present embodiment, the standby time TMw is set with α being 0, to satisfy the equation, TMw=(TMp+TMf)−TMd.

The feed time period TMf takes on a value determined by a distance for a sheet S to be conveyed from the pickup roller33to the photoconductor drum61(conveyance distance), and a speed at which the sheet S is to be conveyed (conveyance speed).

The conveyance distance from the pickup roller33to one photoconductor drum61located in a position upstreammost in the direction of conveyance of a sheet S among photoconductor drum(s)61used for the image forming process varies with the mode selected, i.e., the conveyance distance in the multicolor mode and the conveyance distance in the monochrome mode are different from each other. To be more specific, the conveyance distance in the multicolor mode is a distance from the pickup roller33to the first photoconductor drum61Y; the conveyance distance in the monochrome mode is a distance from the pickup roller33to the second photoconductor drum61K. Accordingly, the feed time period TMf is set at a value greater in the monochrome mode than in the multicolor mode.

The controller100sets the feed time period TMf on the basis of a third table stored in a memory such as RAM. The third table contains information on the feed time periods TMf associated with the modes (the multicolor mode and the monochrome mode), showing a relationship between each of the modes and the feed time period TMf.

Next, a detailed description will be given of an operation of the controller100with reference to the flowchart shown inFIG.4. In the initial state before execution of the process ofFIG.4, the clutches C1, C2and C3are disengaged, and the four development rollers63are separate from the corresponding photoconductor drums61.

As shown inFIG.4, the controller100first determines whether or not a printing instruction has been received. (S1). If it is determined in step S1that no printing instruction has been received (No), then the controller100brings the process to an end.

If it is determined in step S1that a printing instruction has been received (Yes), then the controller100sets a first execution period TMp, a second execution period TMd, and a feed time period TMf (S2). Specifically, in step S2, the controller100sets the first execution period TMp based on the first table. The controller100retrieves a fixed value corresponding to the second execution period TMd from the memory and sets the retrieved fixed value as the second execution period TMd. The controller100sets a feed time period TMf based on the third table.

After step S2, the controller100sets a standby time TMw based on the second table (S3). Specifically, in step3, the controller100sets the standby time TMw which satisfies the equation:
TMw=(TMp+TMf)−TMd.

After step S3, the controller100turns on the heater H (S4), and determines whether or not the temperature T detected by the temperature sensor83is equal to or higher than a main motor rotation start temperature Tm (S5). If it is determined in step S5that T is not equal to or higher than Tm (No), then the controller100repeats the process of step S5.

If it is determined in step S5that T≥Tm (Yes), then the controller100starts causing the main motor M1to operate and starts causing the heating roller81to rotate, to thereby start the pre-fixing rotation process (S6). After step S6, the controller100determines whether or not the standby time TMw has elapsed from the time of the starting of the pre-fixing rotation process (S7).

If it is determined in step S7that the standby time TMw has not elapsed yet (No), then the controller100repeats the process of step S7. If it is determined in step S7that the standby time TMw has elapsed (Yes), then the controller100starts the pre-process rotation process shown inFIG.5(S8).

As shown inFIG.5, in the pre-process rotation process, the controller100first starts causing the process motor M2to operate, and causes the development rollers63Y,63M and63C and the photoconductor drums61to rotate (S31). In step S31, additionally, the controller100turns on the development clutch C2to cause the development roller63K to rotate as well.

After step S31, the controller100determines whether or not the rotation of the process motor M2has stabilized (S32). Herein, after the process motor M2is caused to operate, the rotational speed of the process motor M2increases, and when the rotational speed of the process motor M2has reached a predetermined rotational speed, the process motor M2outputs a process motor lock signal to the controller100. Upon receipt of the process motor lock signal, the controller100determines that the process motor M2has stabilized.

It is understood that the time period expended from the starting of the rotation of the process motor M2to the outputting of the process motor lock signal is an approximately fixed period, more specifically, equivalent to the first period TM1described above, irrespective of the ambient temperature, or the like. Therefore, the controller100may be configured to determine, in step S32, whether or not the first period TM1has elapsed from the time of the starting of the rotation of the process motor M2to thereby determine whether or not the rotation of the process motor M2has stabilized.

If it is determined in step S32that the rotation of the process motor M2has not stabilized yet (No), then the controller100repeats the process of step S32. If it is determined in step S32that the rotation of the process motor M2has stabilized (Yes), then the controller100determines whether or not the rotations of the photoconductor drums61have stabilized (S33). To be more specific, the controller100determines, in step S33, whether or not the second period TM2has elapsed from a time of completion of stabilization of the rotation of the process motor M2to thereby determine whether or not the rotations of the photoconductor drums61have stabilized.

If it is determined in step S33that the rotations of the photoconductor drums61have not stabilized yet (No), then the controller100repeats the process of step S33. If it is determined in step S33that the rotations of the photoconductor drums61have stabilized (Yes), then the controller100turns on the static eliminator lamps65, and starts supplying charging biases from the charging bias supply circuits V to the chargers62to turn on the chargers62(S34).

After step S34, the controller100turns on the contact/separation clutch C3at a predetermined point in time, to start bringing the development rollers63into contact with the corresponding photoconductor drums61(S35). To be more specific, the contact/separation clutch C3is turned on at such a point in time that the development rollers63being pressed in contact with the photoconductor drums61finish making M rotations at a time t8that is, as shown inFIG.6, a time just after a lapse of a time period (TM3+TM4) from a time t5at which the chargers62have been turned on. In other words, the turning-on of the contact/separation clutch C3is timed to complete causing the development rollers63to be pressed in contact with the photoconductor drums61at a time t7that is a time just after a lapse of a time period (TM3+TM4−TM6) from the time t5.

At a time t9that is a time just after a lapse of a time period (TM3+TM4+TM5) from the time t5at which the chargers62have been turned on, the photoconductor drums61charged by the chargers62with stable charging biases have made N or more rotations, and the development rollers63being pressed against the photoconductor drums61have made M rotations. Accordingly, the photoconductor drums61and the development rollers63come to exhibit good conditions suitable for the image forming process; therefore, the pre-process rotation process is completed at the time t9. After the pre-process rotation process is completed (i.e., after the photoconductor drums61and the development rollers63come to exhibit good conditions suitable for image forming process), the controller100may not necessarily bring the pre-process rotation process to an end; rather, after a lapse of time expended for completion of the pre-process rotation process, the controller100continues causing the photoconductor drums61and the development rollers63to rotate and proceeds to a printing process (image forming process) of forming a toner image on a sheet S.

Referring back toFIG.4, after starting the pre-process rotation process in step S8, the controller100determines whether or not both of a first condition and a second condition are satisfied where the first condition is satisfied if the first execution period TMp has elapsed from a time of starting of the main motor M1and the second condition is satisfied if a temperature T detected by the temperature sensor83is equal to or higher than the pickup permissible temperature Tp (S9). If it is determined in step S9that at least one of the first condition and the second condition is not satisfied (No), then the controller100repeats the process of step S9. If it is determined in step S9that the both of the first condition and the second condition are satisfied (Yes), then the controller100turns on the sheet feed clutch C1to start feeding a first sheet S (S10).

After step S10, the controller100causes the image forming unit4to perform an image forming process (S11) to print sheets S of which the number is specified in the printing instruction received in step S1, and brings this process ofFIG.4to an end. At the end of this process, the controller100restores the states of the clutches C1, C2and C3and the contact/separation mechanism300to their initial states.

Next, a detailed description will be given of examples of an operation of the controller100. In one example shown inFIG.6, the detected temperature T reaches the pickup permissible temperature Tp during the first execution period TMp for execution of the pre-fixing rotation process.

As shown inFIG.6, the controller100, upon receiving the printing instruction (at a time t1), turns on the heater H (not shown). Then, the temperature T of the heating roller81increases at a specific rate. When the temperature T of the heating roller81reaches the main motor rotation start temperature Tm (at a time t2), the controller100turns on the main motor M1to start the pre-fixing rotation process. When the pre-fixing rotation process is started, the heating roller81and the pressure roller82are caused to rotate, and the rotating pressure roller82absorbs heat from the heating roller81. Accordingly, the temperature T of the heating roller81increases at a rate lower than the aforementioned specific rate.

In the example ofFIG.6, the temperature T of the heating roller81reaches the pickup permissible temperature Tp at a time t21preceding a time t11of completion of the pre-fixing rotation process. If the temperature T of the heating roller81reaches the pickup permissible temperature Tp, the second condition is satisfied; however, at the time t21, the first condition has not been satisfied yet. Therefore, the controller100does not start feeding of a sheet S at this time t21. When the temperature T reaches the fixing temperature To, the controller100controls the heater H so as to keep the temperature T at the fixing temperature To.

When the standby time TMw has elapsed from the time t2of starting the pre-fixing rotation process (i.e., at a time t3), the controller100turns on the process motor M2to start the pre-process rotation process. Accordingly, the photoconductor drums61are caused to start rotating. At the time t3, as described above, the controller100does not only turn on the process motor M2, but also turns on the development clutch C2to cause the development rollers63to rotate.

When the first period TM1has elapsed from the time t3, the rotation of the process motor M2stabilizes (at a time t4). When the second period TM2has elapsed from the time t4, the rotations of the photoconductor drums61stabilize (at a time t5).

When a time period (TM1+TM2) has elapsed from the time t3(i.e., at the time t5), the controller100turns on the static eliminator lamps65, and turns on the chargers62. When the third period TM3has elapsed from the time t5, the charging biases supplied to the chargers62stabilize (at a time t6).

At a predetermined time after the time t6, the controller100turns on the contact/separation clutch C3(at a time t7). When the first execution period TMp has elapsed from the time t2of starting the pre-fixing rotation process (i.e., at a time t11), the first condition is satisfied; therefore, the controller100turns on the sheet feed clutch C1to start feeding a sheet S.

When the fourth period TM4has elapsed (i.e., the photoconductor drums61have made N rotations) from the time t6at which the charging biases have stabilized (i.e., at a time t8), the development rollers63brought into contact with the corresponding photoconductor drums61at the time t7have made M rotations while being pressed against the corresponding photoconductor drums61. When the fifth period TM5has elapsed (i.e., the development rollers63pressed against the corresponding photoconductor drums61have made M rotations) from the time t8(i.e., at a time t9), the photoconductor drums61and the development rollers63come to exhibit good conditions suitable for the image forming process, and the pre-process rotation process comes to an end. In short, the pre-process rotation process is completed after a lapse of a time period (TMw+TMd) from the time t2of the starting of the pre-fixing rotation process.

On the other hand, the leading edge of a sheet S started being fed at the time t11arrives at the photoconductor drum61(at the time t9) after a lapse of the feed time period TMf from the time t11. In other words, if the detected temperature T reaches the pickup permissible temperature Tp during the first execution period TMp, the leading edge of the sheet S arrives at the photoconductor drum61(at the time t9) after a lapse of a time period (TMp+TMf) from the time t2of the starting of the pre-fixing rotation process.

The standby time TMw is set at a time period which satisfies the equation TMw=(TMp+TMf)−TMd, as described above. Therefore, the time period (TMw+TMd) elapsed from the time t2of starting the pre-fixing rotation process until completion of the pre-process rotation process is equal to the time period (TMp+TMf) elapsed from the time t2of starting the pre-fixing rotation process until arrival of the leading edge of a sheet S at the photoconductor drum61. Accordingly, the pre-process rotation process is determined to get completed at a time when the leading edge of the sheet S arrives at the photoconductor drum61, and thus, unnecessary rotation the photoconductor drum61would make after receipt of the printing instruction before the leading edge of the sheet S arrives at the photoconductor61can be reduced.

In another example shown inFIG.7, the detected temperature T reaches the pickup permissible temperature Tp after completion of the pre-fixing rotation process. The example ofFIG.7differs from the example ofFIG.6only in the timing of starting the feed of a sheet S, and the following process to be executed in the example ofFIG.7is substantially the same as that ofFIG.6.

In the example ofFIG.7, the temperature T of the heating roller81reaches a pickup permissible temperature Tp (at a time t22) when a time period (TMp+TMf+TMa) which is longer than the first execution period TMp has elapsed from the time t2. Therefore, at the time t22, the controller100turns on the sheet feed clutch C1, and start feeding a sheet S. In this instance, the photoconductor drum61rotates unnecessarily during a time period (TMa+TMf) from the time t9of completion of the pre-process rotation process (the end of the second execution period TMd) to a time t23at which the leading edge of a sheet S arrives at the photoconductor drum61. However, in the example ofFIG.7as well, the photoconductor drums61are not caused to rotate during the standby time TMw; therefore, unnecessary rotation of the photoconductor drums61can be reduced in comparison, for example, with a configuration in which the pre-fixing rotation process and the pre-process rotation process are caused to start at the same time.

In the present embodiment as described above, the following advantageous effects can be achieved.

If the temperature T of the heating roller81reaches the pickup permissible temperature Tp before completion of the pre-fixing rotation process, the pre-process rotation process is completed at a time when the leading edge of a sheet S arrives at the photoconductor drum61; thus, unnecessary rotation of the photoconductor drums61can be reduced.

The above-described embodiment may be modified and implemented in various forms as will be described below.

In the above-described embodiment, the color printer1is given as an example of an image forming apparatus; however, the image forming apparatus may be a monochrome printer, or other apparatus such as a copier, a multifunction peripheral, etc. In an alternative embodiment, such as a monochrome printer, without any means for rendering a development roller movable between the contact position and the separate position, the second execution period TMd may not include the fifth period TM5(time period necessary for the development roller to make M rotations).

Parameters such as the first execution period TMp may not be set based on the tables as described above, but may be, for example, computed using predetermined formulae.

The elements described in the above embodiment and its modified examples may be implemented selectively and in combination.