Image forming device that reduces consumption of toner used for non-printing purpose

When carrying out monochrome printing, an engine unit of a printer calculates an amount of toner consumed as lubricant by a color photoreceptor according to the number of sheets to be printed (S63). When carrying out color printing, the engine unit calculates an amount of toner remaining on the color photoreceptor after the printing based on an amount of toner necessary for printing calculated from a printing rate and an amount of actually transferred toner that is calculated from the transfer rate (S65). The toner remaining on the color photoreceptor is used as lubricant, and the engine unit calculates an amount of toner by subtracting the residual toner amount from the amount of toner as lubricant calculated from the number of sheets for monochrome printing (S67), and supplies the calculated amount of toner as lubricant to the color photoreceptor when carrying out monochrome printing.

This application is based on Japanese Patent Application No. 2009-070122 filed with the Japan Patent Office on Mar. 23, 2009, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image forming devices, and in particular to a tandem image forming device that forms an image by fusing toner to a print medium.

2. Description of the Related Art

Some image forming devices provided with a developing device for each color and capable of carrying out tandem color printing include a drive mechanism that does not separately control driving of a photoreceptor in the developing device for monochrome printing and driving of a photoreceptor in the developing device for color printing. When carrying out monochrome printing using an image forming device having such a configuration, a photoreceptor for color printing rotates along with a photoreceptor for monochrome printing.

Each photoreceptor is in contact with a cleaner blade for removing residual toner that remains after transfer. When carrying out monochrome printing using the image forming device as described above, a toner for printing is not supplied to the photoreceptor for color printing. Accordingly, when carrying out monochrome printing, the photoreceptor for color printing rotates in a state in which the photoreceptor and the cleaner blade are brought into contact with the toner for printing therebetween. This can lead to deterioration of the photoreceptor for color printing due to friction occurring between the photoreceptor and the cleaner blade.

Therefore, Japanese Laid-Open Patent Publication No, 2005-099215 (hereinafter referred to as Document 1), for example, discloses a technique for supplying the photoreceptor for color printing with a predetermined amount of toner as lubricant between the photoreceptor and the cleaner blade, in addition to toner used for printing purpose when carrying out monochrome printing using an image forming device with such a configuration.

However, as the image forming device disclosed in Document 1 supplies the toner as lubricant in addition to the toner for printing, this poses a problem of increasing consumption of toner used for non-printing purpose.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problems, and an object of the present invention is to provide an image forming device provided with a developing device for each color and capable of carrying out tandem color printing, with which consumption of toner used for non-printing purpose can be reduced.

In order to achieve the above object, according to one aspect of the present invention, an image forming device includes: a first photoreceptor for forming monochrome picture-image that corresponds to black toner; a second photoreceptor for forming color picture-image that corresponds to color toner; cleaner blades that are respectively in contact with the first and second photoreceptors; developing devices corresponding to respective colors of the toner; a drive mechanism for causing the first and second photoreceptors to rotate; a picture-image forming unit for forming an image by transferring a toner image developed on the first and second photoreceptors to an image carrier; a supplying unit for supplying the toner to the first and second photoreceptors; and a controller. The drive mechanism causes the second photoreceptor to rotate when forming a monochrome picture-image using the picture-image forming unit. The controller causes the supplying unit to supply a predetermined amount of toner as lubricant to the second photoreceptor at a timing determined based on an amount of toner remaining on the second photoreceptor after the transfer to the image carrier.

Preferably, the controller includes: a first counter for integrating a first toner amount obtained from the amount of operation of the picture-image forming unit for forming a monochrome image; and a second counter for integrating a second toner amount obtained from the amount of operation of the picture-image forming unit for forming a color image, the second toner amount being the amount of toner remaining on the second photoreceptor, wherein at the timing at which a value obtained by subtracting a value of the second counter from a value of the first counter reaches a threshold value, the controller causes the supplying unit to supply the toner of an amount corresponding to the threshold value as lubricant to the second photoreceptor.

More preferably, the controller calculates the first toner amount by multiplying the amount of operation of the picture-image forming unit for forming a monochrome image by a unit amount of toner, and integrates the first toner amount using the first counter.

Preferably, the controller calculates the second toner amount based on the amount of operation of the picture-image forming unit for forming a color image, a printing rate of the color image, and a transfer rate from the second photoreceptor to the image carrier, and integrates the second toner amount using the second counter, the second toner amount being the amount of toner remaining on the second photoreceptor after the transfer to the image carrier.

More preferably, the controller calculates the second toner amount using the transfer rate corresponding to an environmental condition within the image forming device, the second toner amount being the amount of toner remaining on the second photoreceptor after the transfer to the image carrier.

Preferably, in the image forming device, the second photoreceptor includes a plurality of color photoreceptors respectively corresponding to the plurality of colors of the toner. The second counter integrates each of the colors of the toner, the controller calculates the second toner amount for each color of the toner and integrates the second toner amount using the second counter for the corresponding color, the second toner amount being the amount of toner remaining on the second photoreceptor after the transfer to the image carrier, and the controller causes the supplying unit to supply the toner of the predetermined amount to one of the color photoreceptors at the timing at which the threshold value is reached, the one of the color photoreceptors having the value obtained by subtracting corresponding one of the values of the second counter from the value of the first counter reaches the threshold value.

Alternatively, it is preferable that, in the image forming device, the second photoreceptor includes a plurality of color photoreceptors respectively corresponding to the plurality of colors of the toner. The second counter integrates each of the colors of the toner, the controller calculates the second toner amount for each color of the toner and integrates the second toner amount using the second counter for the corresponding color, the second toner amount being the amount of toner remaining on the second photoreceptor after the transfer to the image carrier, and the controller causes the supplying unit to supply the toner of the predetermined amount to all of the color photoreceptors at the timing at which the value obtained by subtracting at least one of the values integrated by the second counter for the respective colors of the toner from the value of the first counter reaches the threshold value.

Preferably, the controller sets the value obtained by subtracting the value of the second counter from the value of the first counter to 0 when the value of the second counter is greater than the value of the first counter.

Preferably, the controller causes the supplying unit to supply the predetermined amount of toner in a period between one image forming operation and a subsequent image forming operation by the picture-image forming unit during which an image forming operation is not carried out, wherein the controller sets the period by extending an interval between the image forming operations.

Alternatively, it is preferable that the controller causes the supplying unit to supply the predetermined amount of toner in a period during which an image forming operation by the picture-image forming unit is not carried out, wherein the controller sets the period by temporarily stopping the image forming operation.

According to the present invention; it is possible to reduce consumption of toner used for non-printing purpose with an image forming device provided with a developing device for each color and capable of carrying out tandem color printing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described below with reference to the drawings. In the following description, like components and components are denoted by like reference numerals. Names and functions of thereof are also the same.

While an example in which an image forming device is a tandem color printer (hereinafter referred to as a printer) is described in the following description, the image forming device is not limited to a printer. The image forming device may be a copying machine, a facsimile, or an MFP (Multi Function Peripheral) that is a device having these functions integrated therein.

Referring toFIG. 1, a printer1according to the present embodiment is provided with an image formation unit and a carrying unit of paper as a print medium.

The image formation unit includes a belt2as an intermediate transfer member provided substantially at a center within printer1and suspended internally with a plurality of rollers. Along belt2, a cartridge28athat corresponds to Yellow (Y), a cartridge28bthat corresponds to Magenta (M), a cartridge28cthat corresponds to Cyan (C), and a cartridge28dthat corresponds to Black (K) are disposed. These components are representatively referred to as cartridges28.

Cartridges28include, as a toner picture-image forming mechanism that forms a toner image using an electrostatic recording method, photoreceptors3a,3b,3c, and3d, charging units5a,5b,5c, and5d, exposing units6a,6b,6c, and6d, developing units4a,4b,4c, and4d, and cleaner blades9a,9b, and9c, and9d, respectively. These components are representatively referred to as photoreceptors3, charging units5, exposing units6, developing units4, and cleaner blades9, respectively.

Each of developing units4includes a developing roller and a supplying roller that are not shown in the drawing. Further, each developing unit4includes a space for filling toner that is not shown in the drawing. When developing unit4is operated, the supplying roller is rotated and the toner within this space is supplied to the developing roller. The developing roller is disposed at a position that corresponds to corresponding one of photoreceptors3, and carries the supplied toner to a position that faces toward photoreceptor3. As a surface of photoreceptor3is exposed, the toner on the developing roller moves to an exposed portion of photoreceptor3at the position that faces toward photoreceptor3. As a result, a toner image is formed on photoreceptor3.

Charging units5evenly charge the surfaces of photoreceptors3. Each of exposing units6exposes an image pattern of the corresponding color on photoreceptor3, thereby forming a latent image. Each developing unit4supplies the toner to photoreceptor3and develops a toner image on photoreceptor3. Each photoreceptor3transfers the toner image formed on belt2.

Cleaner blades9are brought in contact with photoreceptors3and take residual toner off from photoreceptors3. This mechanism will be described later.

The image formation unit further includes an intermediate transfer belt cleaner7, a waste toner box15, toner bottles25a,25b,25c, and25d, and a secondary transfer roller11.

Intermediate transfer belt cleaner7is disposed around belt2, and separates residual toner on belt2from belt2.

Waste toner box15is used to contain the separated residual toner. Toner bottles25a,25b,25c, and25drespectively include agitating blades26a,26b,26c, and26d, and operate the respective agitating blades to supply toner to corresponding cartridges28.

Secondary transfer roller11forms a pair with a roller that internally suspends belt2with belt2interposed therebetween, and transfers the toner image that has been transferred to belt2on a paper sheet that has been carried.

A sensor31is disposed around the image formation unit, and measures an environment around the image formation unit. The measured environment is an environment relating to an image forming function, and at least one condition of the environment such as humidity, temperature, and an atmospheric pressure is measured. Sensor31may be provided around each cartridge of the corresponding toner color.

The carrying unit includes a paper feeding roller8, a carrying roller30, timing roller10, secondary transfer roller11as described above, a pair of fuser rollers12aand12b(representatively referred to as fuser rollers12), a paper discharge roller13, double-side path carrying rollers14aand14b, and a paper sheet detecting sensor27.

Paper feeding roller8feeds a paper sheet from a container16as a cassette for containing paper sheets that is disposed at a lower portion within printer1. Carrying roller30carries the paper sheet that has been fed by paper feeding roller8. Timing roller10temporarily stops the paper sheet that is carried by carrying roller30. Fuser rollers12are disposed with the paper sheet that is carried interposed therebetween, and fuses the toner image transferred onto the paper sheet by heating. Paper discharge roller13discharges the paper sheet that has been fused or carries the paper sheet to a double-side carrying path29. Double-side path carrying rollers14aand14bcarries the fused paper sheet carried by paper discharge roller13to timing roller10through double-side carrying path29. Paper sheet detecting sensor27is disposed at a position in the carrying path at which the paper sheet comes immediately after passing through secondary transfer roller11, and senses that a frontal end of the paper sheet and/or a rear end of the paper sheet pass through this position.

As shown inFIG. 2A, photoreceptors3a,3b,3c, and3d, secondary transfer roller11, and timing roller10are connected to a main motor18. Main motor18drives these rollers to rotate in a clockwise direction as shown by an arrow in the figure.

Out of developing units4included in cartridge28, developing units4a,4b, and4cfor color printing are connected to a developing motor20, and developing unit4dfor monochrome printing is connected to a developing motor21. Developing motor20drives developing units4a,4b, and4cfor color printing, and developing motor21drives developing unit4dfor monochrome printing.

Alternatively, as shown inFIG. 2B, developing motor20is not included in printer1, and all of developing units4included in cartridge28may be connected to developing motor21. In this case, developing motor21drives only developing unit4dfor monochrome printing by rotating in a forward direction and drives all of developing units4a,4b,4c, and4dby rotating in a reverse direction.

Because of the configuration as shown inFIG. 2Aor2B, all of photoreceptors3a,3b,3c, and3dthat are connected in series to main motor18are driven to rotate by main motor18. Accordingly, even when carrying out monochrome printing, photoreceptors3a,3b, and3cfor color printing are also driven to rotate along with photoreceptor3dfor monochrome printing. Developing unit4dcan be driven to rotate separately from developing units4a,4b, and4c, either by being connected to developing motor21that is different from a motor to which developing units4a,4b, and4cfor color printing are connected (FIG. 2A), or by developing motor21rotating in the reverse direction (FIG. 2B).

Toner bottles25aand25bare connected to a toner supplying motor23, and toner bottles25cand25dare connected to a toner supplying motor24. Agitating blades26a,26b,26c, and26dare operated by rotary drive of the motor. Fuser rollers12are connected to a fuse motor19, and rotated by rotary drive of fuse motor19. Double-side path carrying rollers14aand14bare connected to double-side path carrying motor22, and rotated by rotary drive of double-side path carrying motor22.

As shown inFIG. 3, photoreceptor3rotates by main motor18driving in a state in which photoreceptor3is in contact with cleaner blade9. With this configuration, the residual toner on the surface of photoreceptor3is taken off by cleaner blade9. The residual toner that has been taken off is discharged to a path for collecting waste toner that is not shown in the drawing, and forms a toner pool at a portion90at which cleaner blade9and photoreceptor3are in contact with each other, which is partially shown by a dotted line in the figure, or more specifically, at an edge portion of cleaner blade9that is in contact with photoreceptor3. As the toner that forms the toner pool is present between cleaner blade9and photoreceptor3, the toner serves as lubricant, and it is possible to suppress friction between cleaner blade9and photoreceptor3due to the rotation of photoreceptor3.

Printer1further includes an engine unit100that controls an overall operation and a controller200that carries out image processing according to a control signal from engine unit100. Referring toFIG. 4, engine unit100includes a control unit101including a CPU (Central Processing Unit), a nonvolatile memory102that is attached to engine unit100, a unit communication unit103for communicating with a nonvolatile memory that is attached to cartridge28and the like, a load communication unit104for communicating with various loads such as main motor18, and a sensor communication unit105for communicating with sensor31. Controller200includes a control unit201including a CPU and a nonvolatile memory202that is attached to controller200.

Memory102stores a program that is executed by control unit101, and memory202stores a program that is executed by control unit201. Control units101and201control printer1as a whole by reading necessary programs from memories102and202and causing the CPUs to execute the programs.

The memory that is attached to cartridge28and the like stores information of consumables, and unit communication unit103reads the information of consumables from the memory according to a control signal of control unit101and passes the information to control unit101. Further, load communication unit104transmits a control signal generated by control unit101to a load that is to be controlled by the generated signal.

Referring further toFIG. 4, control unit201of controller200includes a calculation unit211. While calculation unit211is a function that is realized mainly by the CPU of control unit201executing the program, calculation unit211can be implemented as a different configuration other than the CPU.

Calculation unit211calculates a printing rate from image data to be printed. The printing rate refers to a ratio of an image area to a printing area in a paper sheet. Specifically, the number of dots that configure an image to be printed is divided by the number of dots that corresponds to a printing area of a printing paper sheet.

Calculation unit211extracts the image area (the number of dots that form the image) from the image data to be printed. Further, calculation unit211stores in advance the printing area (the number of dots for the printing area) for each size of paper. Calculation unit211calculates the printing rate by dividing the number of dots that form the extracted image by the number of dots corresponding to the printing area that is stored for a specified size of the paper sheet. Preferably, calculation unit211extracts the image area for each toner color from the image data to be printed, and calculates the printing rate for each toner color. Calculation unit211outputs the calculated printing rate to engine unit100along with a print instruction,

Control unit101of engine unit100includes, as shown inFIG. 5A, an input unit110for inputting an environmental condition, a counting unit111for counting an amount of rotation, a storage unit112, a first calculation unit113for calculating an amount of residual toner, a second calculation unit114for calculating an amount of toner as lubricant, and a supply control unit115. While these are functions that are realized mainly by the CPU of control unit101executing the program described above, these functions can be implemented as a different configuration other than the CPU.

Input unit110accepts an input, from sensor communication unit105that communicates with sensor31, of an environment value such as humidity and temperature received from the sensor. The accepted environment value is inputted to first calculation unit113,

Counting unit111includes a counter111a. Counting unit111counts an amount of rotation of the developing roller such as timing roller10and secondary transfer roller11, and integrates the amount of rotation using counter111a. The amount of rotation of the developing roller is also considered as a driving amount for image formation of developing unit4. Preferably, counting unit111includes counter111afor each color, and counts the amount of rotation of the developing roller for each color. Counting unit111at least includes counter111afor monochrome printing and counter111afor color printing, and counts the amount of rotation of the developing roller for each print mode.

As shown inFIG. 5B, storage unit112stores transfer efficiency that corresponds to the environmental condition. Specifically, while photoreceptor3and belt2are fixed, a positional relation of these components can change depending on the environment due to characteristics of materials. With the change of the position, there is a case where the toner image on photoreceptor3cannot be completely transferred to belt2and remains on photoreceptor3. The toner remaining on photoreceptor3after the transfer is referred to as “remaining toner” or “residual toner”. As the positional relation between photoreceptor3and belt2changes according to the environment, the transfer efficiency changes according to the environment. In other words, an amount of residual toner also changes according to the environment,

WhileFIG. 5Bshows the transfer efficiency that is associated with the humidity as a condition of the environment that most affects the transfer efficiency, temperature or an atmospheric pressure that can be measured by sensor31may also be taken as an environmental condition that affects the transfer efficiency, or two or more of humidity, temperature, and the atmospheric pressure may be combined as the environmental condition.

First calculation unit113calculates the amount of residual toner on photoreceptors3a,3b, and3cfor color printing for each page of color printing. The amount of residual toner for each page can be obtained by subtracting an amount of toner that is actually transferred from photoreceptor3to belt2from an amount of toner consumed to print the page. The amount of toner consumed to print a single page can be obtained by multiplying an amount of toner consumed when forming an image without spaces on a printing paper sheet by a printing rate of this page. First calculation unit113stores the amount of toner consumed when forming an image without spaces on a paper sheet as a reference consumption, and multiplies this amount by the printing rate entered by controller200, thereby calculating the amount of toner consumed to print the page.

The amount of toner that is actually used in the transfer out of the amount of toner consumed to print the page can be obtained by multiplying the amount of toner consumed to print the page by the transfer efficiency. As the transfer efficiency is affected by the environment as described above, first calculation unit113calculates the amount of toner used for the transfer by reading the transfer efficiency when carrying out color printing out of the transfer efficiency of the environmental conditions stored in storage unit112taking an environment value when carrying out color printing that is entered by input unit110as the environmental condition. First calculation unit113includes a counter113a, and integrates the calculated amount of residual toner using counter113a.

Preferably, first calculation unit113includes counter113a for each toner color. In this case, first calculation unit113calculates the amount of residual toner for each toner color by calculating the amount of toner consumed to print the page using the printing rate calculated by calculation unit211for each toner color, and integrates the amount of residual toner for each toner color using corresponding counter113a.

Second calculation unit114stores in advance a unit amount of toner necessary as lubricant for photoreceptors3a,3b, and3cfor color printing for a single sheet of monochrome printing which will be described later. Second calculation unit114calculates, based on the amount of rotation of the developing roller in monochrome printing and the unit amount, the amount of toner necessary as lubricant, that is, the amount of toner consumed as lubricant by photoreceptors3a,3b, and3cfor color printing that is in operation when carrying out monochrome printing. Second calculation unit114includes a counter114a, and integrates the calculated amount of toner necessary as lubricant using counter114a. Similarly to first calculation unit113, second calculation unit114can include counter114afor each toner color.

As described above, when carrying out printing, belt2and photoreceptor3are brought into contact with each other. As described above, photoreceptors3a,3b,3c, and3dare driven to rotate by main motor18, both when carrying out color printing and when carrying out monochrome printing. When carrying out color printing, developing units4a,4b, and4cfor color printing are driven by developing motor20, and developing unit4dfor monochrome printing is driven by developing motor21, thereby forming toner images on photoreceptors3a,3b,3c, and3d. When carrying out monochrome printing, developing unit4dfor monochrome printing is driven by developing motor21, thereby forming a toner image only on photoreceptor3dfor monochrome printing. Driving developing units4a,4b, and4cfor color printing that are not used when carrying out monochrome printing causes the toner within cartridges28a,28b, and28cfor color printing to be scraped and frictioned with developing blades or the like that are not shown in the drawing, and deteriorated. Therefore, developing units4a,4b, and4cfor color printing are not driven. On the other hand, the toner within cartridges28a,28b, and28cfor color printing is not supplied to photoreceptors3a,3b, and3cfor color printing that are not used when carrying out monochrome printing. Therefore, the toner pools are not formed at contact portions90respectively between photoreceptors3a,3b, and3cand cleaner blades9a,9b, and9c. As a result, photoreceptors3a,3b, and3crotate directly in contact with cleaner blades9a,9b, and9cwithout the toner interposed therebetween.

Supply control unit115carries out a supply operation, at a predetermined timing, of supplying toner that is not used for printing to photoreceptors3a,3b, and3c, in order to suppress frictional loads between photoreceptors3a,3b, and3cand cleaner blades9a,9b, and9cwhen carrying out monochrome printing and to prevent the cleaner blades from being scraped or a rotary torque from increasing. The supplied toner forms the toner pool at contact portion90, and serves as lubricant.

Supply control unit115calculates an amount obtained by subtracting the amount of residual toner on photoreceptors3a,3b, and3cfor color printing calculated by first calculation unit113from the amount of toner necessary as lubricant calculated by second calculation unit114as an integrated toner amount as an amount of toner to be supplied as lubricant. Supply control unit115calculates the integrated toner amount by subtracting a counting value of counter113afrom a counting value of counter114a.

Supply control unit115stores in advance the amount of toner to be supplied as lubricant in a single supply operation as a threshold value. Supply control unit115compares the amount of toner stored as the threshold value and the calculated integrated toner amount. When the integrated toner amount reaches the amount of toner to be supplied, supply control unit115determines that it is the timing at which the toner as lubricant is to be supplied. When supplying the toner as lubricant, supply control unit115outputs a control signal to a necessary operational mechanism that is not shown in the drawing, and causes the mechanism to interrupt the printing operation and to supply the toner of the above described amount that is stored as the threshold value during a non-image-forming period. As used herein, the “non-image-forming period” may be a period during which the printing operation is temporarily stopped, or may be a period between image forming operations that is provided by extending an interval between the image forming operations longer than usual.

It was confirmed by the experimentation by the inventor that it is adequate to supply the toner as lubricant of about 40 mg to each of photoreceptors3a,3b, and3cfor color printing for every 100 sheets in monochrome printing. Therefore, the unit amount for the amount of toner necessary as lubricant per sheet for monochrome printing stored in second calculation unit114can be, for example, 0.4 mg. It should be noted that it is possible to increase the amount of toner held as the toner pool depending on a shape of contact portion90between cleaner blade9and photoreceptor3.

Examples of supplying the toner as lubricant includes, as described above, supplying toner of 40 mg every time when 100 sheets of monochrome printing is carried out. It is possible to supply the toner more stably by supplying toner of 40 mg every time when printing 100 sheets of paper than by supplying toner of 0.4 mg every time when printing a single sheet of paper. Further, it is possible to reduce the number of times of driving cartridges28a,28b, and28cto supply the toner. Accordingly, supply control unit115stores the amount of toner necessary as lubricant for a predetermined number of printing sheets (here, 40 mg for every 100 sheets) as the threshold value.

Preferably, supply control unit115controls the supply by calculating the integrated toner amount for each toner color by subtracting the counting value of counter113afor each toner color from the counting value of counter114a, and determining whether or not it is the timing to supply the toner as lubricant for each toner color. Supply control unit115resets counter114aand counter113awhen the supply operation has been carried out.

An operational flow of printer1is described referring toFIG. 6. An operation shown in a flow chart ofFIG. 6is an operation started by the print instruction issued by an operation unit of printer1that is not shown in the drawing, or other devices connected to printer1that are not shown in the drawing, and implemented by control unit101of engine unit100and control unit201of controller200reading necessary programs respectively from memories102and202and causing the respective CPUs to execute the programs to control the components shown inFIGS. 4 and 5A.

Referring toFIG. 6, after calculation unit211calculates the printing rate from the image data to be printed in step S10, controller200outputs the printing rate along with the print instruction to engine unit100in step S20.

The printing operation is carried out in step S30by being controlled by control unit101of engine unit100. When the printing operation starts, counting unit111counts the amount of rotation of the developing roller at a predetermined interval, for example, every time when a single sheet is printed in step S40.

Next, in step S60, first calculation unit113and second calculation unit114carry out an operation shown inFIG. 7, and calculate the amount of toner necessary as lubricant. Specifically, referring toFIG. 7, if a printing mode is the monochrome mode (YES in step S61), in step S63, second calculation unit114adds the unit amount of the amount of toner necessary as lubricant for photoreceptors3a,3b, and3cfor color printing to counter114a, as shown inFIG. 8, for every single page of monochrome printing (step S631). In an example ofFIG. 8, the addition is made by 0.4 mg each time.

If the printing mode is the color mode (NO in step S61), in step S65, first calculation unit113carries out an operation shown inFIG. 9and calculates the amount of residual toner. Specifically, referring toFIG. 9, in step S651, the amount of toner to be consumed for a single page of color printing is calculated by multiplying the amount of toner to be consumed when forming the image without spaces in a printing paper sheet by the printing rate of this page. First calculation unit113reads the transfer efficiency that corresponds to the environmental condition that has been entered by storage unit112in step S653, and, in step S655, calculates the amount of residual toner by subtracting the amount of toner that is actually used for the transfer from the amount of toner to be consumed for a single page of color printing that has been calculated in step S651. In step S655, the amount of residual toner is calculated based on, for example, the following equation.
Residual Toner Amount=Amount of Toner to be Consumed for Single Page of Color Printing×(1-Transfer Efficiency).

In step S653, first calculation unit113adds the amount of residual toner calculated in step S655to counter113a.

It should be noted that, in step S65, the amount of residual toner may be calculated for each toner color. In this case, the amount of residual toner calculated for the corresponding toner color is added to counter113acorresponding to the toner color.

When either the calculation of the amount of toner necessary as lubricant in step S63, or the calculation of the amount of residual toner in step S65is carried out, supply control unit115calculates the integrated toner amount as the amount of toner to be supplied as lubricant in step S67, as shown inFIG. 10, based on the counting value of counter114aas the integrated toner amount of the toner necessary as lubricant and the counting value of counter113a as the integrated amount of the amount of residual toner. Specifically, referring toFIG. 10, in step S671, supply control unit115calculates the integrated toner amount by subtracting the counting value of counter113a from the counting value of counter114a. As the integrated toner amount of the toner to be supplied as lubricant does not become a negative value, if the calculation result in step S671is negative (YES in step S673), the integrated toner amount is set to be 0 (step S675). Preferably, the calculation of the integrated toner amount shown inFIG. 10is carried out for each toner color.

After the amount of toner to be supplied as lubricant is calculated through the above process (step S60), as shown inFIG. 11, a process of comparing the integrated toner amount that has been calculated in step S60with the threshold value is carried out, and it is determined whether or not the operation of supplying the toner as lubricant is necessary. Specifically, referring toFIG. 11, the integrated toner amount that has been calculated in step S60is compared with 40 mg which is the amount of toner to be supplied as lubricant in the single supply operation stored as the threshold value, and if it is determined that the calculated integrated toner amount reaches the threshold value of 40 mg (YES in step S701), it is determined to be the timing to supply the toner as lubricant to each of photoreceptors3a,3b, and3cfor color printing, and a request for the operation is set in step S703. When the integrated toner amount is calculated for each toner color, the operation in step S11is carried out for each toner color and the timing for supplying the toner as lubricant is determined for each toner color. Further, preferably, the request for the operation for each color may be set by determining the toner as lubricant is to be supplied to all of photoreceptors3a,3b, and3cfor color printing when the integrated toner amount is calculated for each toner color and when it is determined to be the timing for supplying the toner as lubricant for one of the toner colors in the operation in step S11.

As a result of the above determination, when the request for the toner supply operation is set (YES in step S70), supply control unit115carries out an operation as shown inFIG. 12in step S80, and causes the necessary supply operation to be carried out. Specifically, referring toFIG. 12, when the request for the toner supply operation is set based on the above determination (YES in step S801), supply control unit115interrupts the printing operation that is being carried out in step S803, and outputs a control signal for carrying out the operation of supplying the toner of the amount that is stored as the threshold value (40 mg in the above example) as lubricant to each of photoreceptors3a,3b, and3cfor color printing to the necessary load during the non-image-forming period. As a result, the toner of the above amount is supplied as lubricant to each of photoreceptors3a,3b, and3cfor color printing. Then, in steps S805and S807, supply control unit115resets counter114aand counter113a. Preferably, supply control unit115supplies the toner as lubricant to photoreceptor3that corresponds to the toner color for which the request for the toner supply operation is set.

As the above operations are carried out in printer1, the toner supplied to photoreceptors3a,3b, and3cfor color printing as lubricant when carrying out monochrome printing does not overlap with the toner remaining on photoreceptors3a,3b, and3cfor color printing when carrying out color printing, and thereby reducing the consumption of the toner used for non-printing purpose.