Image forming apparatus and image forming system

An image forming apparatus having a fixing unit fixing a developer image formed based on image data received onto a medium includes a target fixing temperature determination unit, a fixable temperature range computing unit, and a fixing temperature control unit. The target fixing temperature determination unit determines fixing temperature arranged based on the medium as target fixing temperature. The fixable temperature range computing unit computes a fixing temperature range fixable the developer image on the medium by using medium information of the medium while using the target fixing temperature as reference temperature. The fixing temperature control unit controls the fixing unit based on the fixable temperature range computed by the fixable temperature range computing unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus having a fixing temperature control function and an image forming system having the image forming apparatus.

2. Description of Related Art

Generally, a related art image forming apparatus employing an electrophotographic method includes a fixing unit fixing a developer image formed on a surface of an image carrier such as a photosensitive drum onto a recording medium. Such a fixing unit includes a heat roller having a heating element and a pressure roller disposed pressed against the heat roller. When the recording medium having the developer image transferred thereon passes between the heat roller and the pressure roller, the heat and pressure is applied, thereby fixing the developer image onto the recording medium.

The related art image forming apparatus having such a fixing unit usually includes a fixing temperature control function controlling fixing temperature by monitoring surface temperature of the heat roller. For example, Japanese Un-examined Patent Application Publication No. H05-46051 discloses a prior art image forming apparatus including a timer mechanism arranging a time between a time at which surface temperature of the heat roller reaches a prescribed temperature and a time at which outer circumference surface temperature of the pressure roller becomes uniformalized.

In the image forming apparatus having the fixing temperature control function described above, however, the fixing temperature to begin fixing operation is fixed so as to secure image quality of the image to be fixed onto the recording medium, and the fixing operation halts until the surface temperature of the heat roller reaches the prescribed fixing temperature. Consequently, for example, in a case of beginning printing operation immediately after the image forming apparatus is activated, the print operation does not begin until the heat roller reaches the prescribed fixing temperature, causing prolongation of a time period until completion of printing. Since the fixing temperature to begin the fixing operation is inflexible, appropriate fixing temperature corresponding to image data or the recording medium cannot be selected, causing difficulty in image quality enhancement.

The present invention provides an image forming apparatus capable of adjusting a time period until fixing temperature reaches an appropriate level to begin fixing operation according to image data or a recording medium. Moreover, the present invention provides the image forming apparatus capable of obtaining fixing stability and enhancing fixing quality by such an adjustment of the fixing temperature to the appropriate level to begin the fixing operation.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the invention, an image forming apparatus having a fixing unit fixing a developer image formed based on image data received onto a medium includes: a target fixing temperature determination unit determining fixing temperature arranged based on the medium as target fixing temperature; a fixable temperature range computing unit computing a fixing temperature range fixable the developer image on the medium by using medium information of the medium while using the target fixing temperature as reference temperature; and a fixing temperature control unit controlling the fixing unit based on the fixable temperature range computed by the fixable temperature range computing unit.

According to another aspect of the present invention, an image forming system includes an image processing apparatus processing image data and an image forming apparatus having a fixing unit fixing a developer image formed based on the image data received from the image processing apparatus onto a medium. The image forming apparatus includes: a communication unit communicating with the image processing apparatus; a target fixing temperature determination unit determining fixing temperature arranged based on the medium as target fixing temperature; a fixable temperature range computing unit computing a fixing temperature range fixable the developer image on the medium by using medium information of the medium while using the target fixing temperature as reference temperature; and a fixing temperature control unit controlling the fixing unit based on the fixable temperature range computed by the fixable temperature range computing unit.

Additional features and advantages of the present invention will be more fully apparent from the following detailed description of embodiments, the accompanying drawings and the associated claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, like reference numerals designate identical or corresponding parts throughout the several views.

First Embodiment

Referring toFIG. 1, an image forming apparatus101employing an electrophotographic method according to a first embodiment of the present invention is illustrated. The image forming apparatus101includes a fixing unit112fixing a developer image formed on a surface of an image carrier, for example, a photosensitive drum109, onto a recording medium. The image forming apparatus101of the present invention is now described in detail.

The image forming apparatus101includes: a sheet cassette102storing a sheet or sheets10serving as the recording medium or recording media; a feed roller103separately conveying the sheet10sheet by sheet from the sheet cassette102; a conveyance path104guiding conveyance of the sheet10fed from the feed roller103; a conveyance roller105conveying the sheet10to a conveyance belt106; the conveyance belt106conveying the sheet10in a course of image forming by an image forming unit108; a driven roller107aand a drive roller107btightly stretching the conveyance belt106; the image forming unit108forming the developer image with the electrophotographic method; a print head unit110forming an electrostatic latent image on a surface of the photosensitive drum109included in the image forming unit108with light; a transfer roller111transferring the developer image formed on the surface of the photosensitive drum109to the sheet10; the fixing unit112fixing the developer image transferred to the sheet10with application of heat and pressure; a conveyance path117guiding conveyance of the sheet10conveyed by the conveyance roller106; conveyance rollers118and119conveying the sheet10in a sheet ejection direction; an ejection roller120ejecting the sheet10; a stacker121stacking the sheet10ejected thereon; a control board122controlling the image forming apparatus101; a nonvolatile memory123storing a control program and the like of the image forming apparatus101; an operator panel124receiving a print setting provided by a user; and an interface connector125transmitting and receiving print data and the like.

The sheet cassette102serving as a box member includes an opening at an upper surface thereof, so that a plurality of sheets10on which the images to be formed are accumulated therein, and each of the plural sheets10is fed from the opening by the feed roller103.

The sheet cassette102includes the feed roller103disposed in such a manner as to contact the sheet10of the sheet cassette102. The feed roller103is made up of at least a roller pair and separates the plural sheets10sheet by sheet by rotation thereof rotated by power transmitted from a drive system (not shown), thereby supplying the sheet10to the conveyance path104.

The conveyance path104serving as a guide member guides the sheet10supplied from the sheet cassette10to the conveyance roller105. The conveyance roller105made up of a roller pair is disposed at an end of the conveyance path104. The conveyance roller105conveys the sheet10guided along the conveyance path104to the conveyance belt106.

The conveyance belt106serving as an endless belt member is disposed in such a manner as to contact the photosensitive drum109with certain pressure. The conveyance belt106is tightly stretched by the driven roller107aand the drive roller107bdisposed at each end thereof. The driven roller107aand the drive roller107bsupport the conveyance belt106with certain tension. The driven roller107aand the drive roller107bare rotated by power transmitted from a drive system (not shown), thereby operating the conveyance belt106.

The image forming unit108includes, for example, the photosensitive drum109, and forms the developer image corresponding to each color of black, yellow, magenta, and cyan by a developer image forming mechanism developing the electrostatic latent image formed on the photosensitive drum109. Such four colors of black, yellow, magenta, and cyan are abbreviated as K, Y, M, and C, respectively. Each of image forming units108K,108Y,108M, and108C is disposed above the conveyance belt106, and is disposed in sequence from an upper stream in a conveyance direction of the sheet10. Each of the image forming units108K,108Y,108M, and108C is substantially similar to one another except for the color. A description of the image forming units108K,108Y,108M, and108C is hereafter given by using the image forming unit108as representative of the image forming units108K,108Y,108M, and108C.

The photosensitive drum109can store an electrical charge on the surface thereof and serves as an image carrier forming the electrostatic latent image thereon by the light irradiated from the print head unit110(described later). The photosensitive drum109is a drum member rotatable around a central axis thereof, and is disposed in such a manner as to contact the conveyance belt106in a lowest portion of the image forming unit108.

The print head unit110includes a light-emitting element such as light-emitting diode (LED) and is disposed above the photosensitive drum109. The print head unit110allows the light-emitting element to emit the light based on the image data received, thereby exposing the electrostatic latent image on the surface of photosensitive drum109.

The transfer roller111is disposed in a position face to face with the photosensitive drum109through the conveyance belt106. When the sheet10passes between the photosensitive drum109and the conveyance belt106, high voltage is applied to the transfer roller111from a power source (not shown), thereby transferring the developer image formed by the image forming unit108to the sheet10.

The fixing unit112includes the heat roller113rotatable around a central axis thereof and the pressure roller116disposed in such a manner as to press against the heat roller113. The heat roller113includes a core metal, made of metal such as aluminum or iron, having a halogen lamp114serving as a heating member inside, and includes an elastic member, for example, silicone rubber, on a surface thereof. The surface of elastic member is coated by a coating layer or a tube having substantially the same function as the coating layer so as to secure separatability with the developer image transferred on the sheet10.

The temperature sensor115serving as a non-contact thermistor, for example, is disposed a certain space away from the surface of the heat roller113. The temperature sensor115measures surface temperature of the heat roller113. The halogen lamp114is turned on and off according to an execution result of a control program based on the temperature measured by the temperature sensor115, so that the surface temperature of the heat roller113is controlled.

The pressure roller116is disposed in such a manner as to contact the surface of the heat roller113with certain pressure applied by a tension spring (not shown). Consequently, the pressure roller116and the heat roller113contact each other with the certain pressure, thereby forming a nip portion in which the heat and pressure is applied to the sheet10.

The conveyance path117serving as a guide member guides the sheet10conveyed from the conveyance belt106to the ejection roller120. Each of the conveyance rollers118,119is made up of a roller pair and is disposed in a mid-stream of the conveyance path117. The conveyance rollers118,119convey the sheet10guided along the conveyance path117to the ejection roller120.

The ejection roller120made up of a roller pair is disposed at an end of the conveyance path117. The ejection roller120ejects the sheet10guided along the conveyance path117to the stacker121. The stacker121is disposed on a downstream side of the ejection roller120and stacks thereon the sheet10ejected from the ejection roller120.

The control board122serving as a board member includes a central processing unit (CPU, not shown) or the nonvolatile memory123thereon. The CPU included on the control board122executes the control program stored in the nonvolatile memory123, thereby controlling the image forming apparatus101.

The nonvolatile memory123includes a memory such as a flash memory, an ultra violet erasable programmable read only memory (UV-EPROM), or an electronically erasable and programmable read only memory (EEPROM), and stores the control program and the like of the image forming apparatus101.

The operator panel124includes a display mechanism, for example, a liquid crystal display (LCD), and an input mechanism, for example, a switch or switches. The operator panel124is disposed at an upper portion of the image forming apparatus101to facilitate input operation by the user. The operator panel124displays a printing progress state or an apparatus state of the image forming apparatus101and receives the print setting from the user.

The interface connector125serving as an interface connector connects with a local area network (LAN) or a universal serial bus (USB) cable.

In the printing operation of the above structure of the image forming apparatus101, the CPU on the control board122reads the control program stored in the nonvolatile memory123upon receiving the print data through the interface connector125after receiving the input of the print setting by the user through the operation panel124.

The CPU supplies operation instructions to the drive system (not shown) and the power source system (not shown) based on the control program read. Upon receiving the operation instructions, the drive system transmits drive power to each of the rollers, and the power source system begins initial operation to apply the high voltage.

The feed roller103begins to rotate by the drive power transmitted from the drive system, and separately conveys the plural sheets10sheet by sheet from the sheet cassette102. Each of the sheets10conveyed from the feed roller103is guided along the conveyance path104, and is conveyed to the conveyance roller105which conveys the sheet10to the conveyance belt106. The conveyance belt106is driven by rotation of the driven roller107aand the drive roller107b, and conveys the sheet10to a position in contact with the photosensitive drum109.

Herein, the print head unit110is driven based on the print data received and forms the electrostatic latent image on the photosensitive drum109. The image forming unit108develops the electrostatic latent image formed on the photosensitive drum109using the developer image forming mechanism. When the sheet10is conveyed between the photosensitive drum109and the conveyance belt106, the developer image is transferred on the sheet10by the transfer roller111applied with the high voltage by the power source system (not shown).

The sheet10having the developer image transferred thereon is conveyed to the fixing unit112by drive of the conveyance belt106. In the fixing unit112, the heat roller113is controlled in such manner as to have appropriate fixing temperature according to the execution result of the control program based on the temperature measured by the temperature sensor115. Upon reading the fixing unit112, the sheet10is applied with the heat and pressure, thereby fixing the developer image thereon.

The sheet10having the developer image fixed thereon is conveyed along the conveyance path117to the ejection roller120by rotation of the conveyance rollers118and119. The sheet10conveyed to the ejection roller120is ejected to the stacker121by rotation of the ejection roller120.

Referring toFIG. 2, a description of the control program executed by the CPU is given. The control program allows the print operation of the image forming apparatus101to be executable.

The control program includes: a transmitting-receiving unit201transmitting and receiving print data to and from a host computer and the like through the interface connector125; an image density information calculation unit202calculating image density information per prescribed region of the print data received; a target fixing temperature determination unit203determining fixing temperature arranged based on the sheet10as target fixing temperature; a fixable temperature range computing unit204computing a fixing temperature range in which the developer image can be fixed onto the sheet10by using the image density information while using the target fixing temperature as reference temperature; a fixing temperature control unit205controlling surface temperature of the heat roller113based on the fixable temperature range; a print startable judgment unit206judging whether or not to start the print operation based on the surface temperature of the heat roller113; a fixing temperature measurement unit207acquiring a measurement result of the surface temperature of the heat roller113measured by the temperature sensor115; a print control unit208controlling the print operation as a whole of the image forming apparatus101; a sheet feeding mechanism control unit209controlling feeding of the sheet10; and an image forming mechanism control unit210controlling image formation and a transfer process.

The transmitting-receiving unit201transmits and receives the print data to and from the host computer and the like through the interface connector125. Upon receiving the print data, the transmitting-receiving unit201notifies the image density information calculation unit202of reception of the print data.

The image density information calculation unit202, for example, partitions the print data to be formed on the sheet10into cells as illustrated inFIG. 3Aand determines a number of data dots in a cell unit of a prescribed region as illustrated inFIG. 3B. The image density information calculation unit202calculates the image density by dividing the number of data dots with respect to each cell determined by a number of all dots occupied in the cell. In a case where the image to be formed on the sheet10is a multi-color image, three colors such as cyan, magenta, and yellow are used for general color expression. Therefore, the image density information calculation unit202calculates the image density with respect to each color, and eventually calculates a sum of the image density of the three colors (hereafter referred to as added image density).

Since each maximum image density of the cyan, magenta, and yellow is one hundred (100) percent, the added image density of the three colors becomes three hundred (300) percents. However, each maximum image density of the three colors may be reduced to a value below one hundred (100) percent due to a problem of fixability, for example. Therefore, it must be noted that the added image density is not necessarily three hundred (300) percents.

The target fixing temperature determination unit203determines the fixing temperature arranged associated with thickness information of the sheet10as the target fixing temperature (hereafter referred to as “Tprint”). In the first embodiment, the fixing temperature serving as the reference temperature is arranged beforehand according to the thickness of the sheet10to be used for printing, so that a good printing result is obtained in a case where the printing operation is performed with the added image density of three hundred (300) percents. The target fixing temperature determination unit203determines the temperature of “Tprint” based on the thickness of the sheet10used for the printing. For example, where the thickness of the sheet10is one hundred twenty (120) μm, the target fixing temperature determination unit203determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius.

The fixable temperature range computation unit204computes the fixing temperature range in which the developer image can be fixed on the sheet10by using the added image density calculated by the image density information calculation unit202while using the temperature of “Tprint” determined by the target temperature determination unit203as the reference temperature. Herein, a portion having higher added image density uses a larger quantity of the developer, causing an increase in a heat amount to be removed by the developer. Therefore, the fixable temperature range computing unit204computes the fixable temperature range using the highest added image density among the added image density calculated by the image density information calculation unit202.

Particularly, the fixable temperature range computing unit204computes the fixable temperature range using a fixable temperature range offset table as illustrated inFIG. 4. For example, where the sheet10to be used for the printing has a thickness of one hundred twenty (120) μm, where the target fixing temperature determination unit203determines the temperature of“Tprint” to be one hundred eighty (180) degrees Celsius, and where the added image density calculated by the image density information calculation unit202is two hundred fifty (250) percents, the fixable temperature range computing unit204refers to the fixable temperature range offset table and computes upper limit temperature (hereafter referred to as “Tupper”) in the fixable temperature range to be one hundred eighty three (183) degrees Celsius (“Trpint” of 180° C.+an upper limit temperature offset value “Toff1” of 3° C.=183° C.) and lower limit temperature (hereafter referred to as “Tlower”) in the fixable temperature range to be one hundred seventy six (176) degrees Celsius (“Trpint” of 180° C.+a lower limit temperature offset value “Toff2” of −4° C.=176° C.).

The fixing temperature control unit205controls the surface temperature of the heat roller113by turning on and off the halogen lamp114based on the fixable temperature range computed by the fixable temperature range computing unit204.

The print starable judgment unit206judges whether or not to start the print operation based on the measurement result of the surface temperature of the heat roller113.

The fixing temperature measurement unit207acquires, based on an instruction of the fixing temperature control unit205or the print startable judgment unit206, the measurement result of the surface temperature of the heat roller113measured by the temperature sensor115. The acquired surface temperature of the heat roller113is notified to the fixing temperature control unit205or the print startable judgment unit206.

Where the print startable judgment unit206judges that the print operation is startable, the print control unit208controls the print operation of the image forming apparatus101as a whole.

The sheet feeding mechanism control unit209controls the feeding of the sheet10fed by the feed roller103and the like based on the control by the print control unit208.

The image forming mechanism control unit210controls the image formation provided by the image forming unit108and the transfer process provided by the transfer roller111and the like based on the control by the print control unit208.

Referring to a flowchart ofFIG. 5, an example procedure based the control program described above is illustrated.

When the print data are transmitted with respect to image forming apparatus101from the host computer, the transmitting-receiving unit201receives the print data through the interface connector125. The transmitting-receiving unit201notifies of reception of the print data with respect to the image density information calculation unit202(Yes in step S101). Upon receiving the notification, the image density information calculation unit202calculates the image density of the print data received (step S102). Herein, the image density information calculation unit202is assumed to calculate the added image density of the portion having the highest image density within a print page.

In step S103, the target fixing temperature determination unit203determines the temperature of “Tprint” based on the thickness of the sheet10.

Subsequently, the fixable temperature range computing unit204computes the temperature of “Tupper” and “Tlower” by using the added image density calculated by the image density information calculation unit202while using the temperature of “Tprint” determined by the target fixing temperature determination unit203as the reference temperature (step S104).

In step S105, the fixing temperature control unit205supplies the instruction with respect to the fixing temperature measurement unit207to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is below “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit205supplies the instruction to the power source system (not shown) to distribute the power to the halogen lamp114such that the surface temperature of the heat roller113is adjusted to the temperature of “Tprint.” Upon receiving the instruction from the fixing temperature control unit205, the power source system begins the power distribution to the halogen lamp114.

On the other hand, where the surface temperature of the heat roller113is above “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit205does not supply the power distribution instruction to the power source system (not shown).

After the surface temperature of the heat roller113reaches the temperature of “Tprint,” the fixing temperature control unit205controls the power source system (not shown) such that the surface temperature of the heat roller113remains at the temperature of “Tprint.”

Next, the print startable judgment unit206supplies the instruction with respect to the fixing temperature measurement unit207to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is within the fixable temperature range as a result of measurement thereof by the temperature sensor115(Yes in step S106), the print startable judgment unit206supplies a print start instruction to the print control unit208. Upon receiving the print start instruction, the print control unit208supplies print execution instructions to the sheet feeding mechanism control unit209and the image forming mechanism control unit210. Upon receiving the print execution instruction from the print control unit208, the sheet feeding mechanism control unit209allows the feed roller103and the like to start conveying the sheet10. The image forming unit108and the transfer roller111instructed by the image forming mechanism control unit210form the developer image on the sheet10conveyed.

The fixing unit112fixes the developer image on the sheet10. Subsequently, the sheet10having the developer image fixed thereon is guided along the conveyance path118and is conveyed to the ejection roller120. The ejection roller120ejects the sheet10on the stacker121(step S107).

Where the surface temperature of the heat roller113is outside the fixable temperature range as a result of measurement thereof by the temperature sensor115(No in step S106), the print startable judgment unit206does not supply the print start instruction to the print control unit208. The print startable judgment unit206is on standby until the surface temperature of the heat roller113reaches within the fixable temperature range. Where the surface temperature of the heat roller113reaches within the fixable temperature range, the print startable judgment unit206supplies the print start instruction to the print control unit208.

Now, an adjustment of a print startable timing made by the operation based on the control program is described in detail in comparison with a prior art apparatus.

A description of adjusting the print startable timing in a course of heating the fixing unit112is given below.

A time “t” consumed by the fixing unit112to be heated by turning on the halogen lamp114is expressed in Formula 1 as follows:
t=(T1−Troom)/h,Formula 1:
where “h” is a rate of heat temperature change, “Troom” is room temperature, and “T1” is print start temperature.

As illustrated inFIG. 6A, a time “ts1” consumed by the image forming apparatus101of the first embodiment until the printing operation is started is fifty eight (58) seconds according to Formula 1 above.
ts1=(170−25)/2.5=58 seconds,
where the print start temperature, that is, the lower limit temperature within the fixable temperature range “Tlower” is one hundred seventy (170) degrees Celsius, the rate of the heat temperature change “h” is two-point-five degrees Celsius per second (2.5° C./sec), and the room temperature “Troom” is twenty five (25) degrees Celsius. A double-headed arrow inFIG. 6Aindicates a print startable temperature range according to the present invention.

On the other hand, a time “ts1” consumed by the prior art apparatus until the printing operation is started is sixty two (62) seconds according to Formula 1.
ts1′=(180−25)/2.5=62 seconds,
where the print start temperature, that is, the target fixing temperature “Tprint” is one hundred eighty (180) degrees Celsius, the rate of the heat temperature change “h” is two-point-five degrees Celsius per second (2.5° C./sec), and the room temperature “Troom” is twenty five (25) degrees Celsius.

Therefore, according to the image forming apparatus101of the first embodiment, the printing operation can be started four (4) seconds (62 sec−58 sec) earlier than the prior art apparatus.

Next, a description of adjusting the print startable timing in a course of releasing the heat from the fixing unit112is given. In a case where the temperature of the fixing unit112once increases to a high level, for example, after successive printing, the fixing unit112accumulates the heat therein, causing an increase in difficulty of decreasing the temperature thereof. Consequently, a temperature change of the fixing unit112in a course of releasing the heat is moderate compared to a temperature change in a course of heating the fixing unit112.

A time “tt” consumed by the fixing unit112to release the heat is expressed in Formula 2 as follows:
tt=(T0−T1)/r,Formula 2:
where “r” is a rate of releasing temperature change, “T0” is temperature of the fixing unit112at the present time, and “T1” is print start temperature.

As illustrated inFIG. 6B, a time “ts2” consumed by the image forming apparatus101of the first embodiment until the printing operation is started is fifty (50) seconds according to Formula 2 above.
ts2=(200−190)/0.2=50 seconds,
where the print start temperature, that is, the upper limit temperature within the fixable temperature range “Tupper” is one hundred ninety (190) degrees Celsius, the rate of the releasing temperature change “r” is zero-point-two degrees Celsius per second (0.2° C./sec), and the temperature “T0” of the fixing unit112at the present time is twenty hundred (200) degrees Celsius. A double-headed arrow inFIG. 6Bindicates a print startable temperature range according to the present invention.

On the other hand, a time “ts2” consumed by the prior art apparatus until the printing operation is started is one hundred (100) seconds according to Formula 2.
ts2′=(200−180)/0.2=100 seconds,
where the print start temperature, that is, the target fixing temperature “Tprint” is one hundred eighty (180) degrees Celsius, the rate of the releasing temperature change “r” is zero-point-two degrees Celsius per second (0.2° C./sec), and the temperature “T0” of the fixing unit112at the present time is two hundred (200) degrees Celsius.

Therefore, according to the image forming apparatus101of the first embodiment, the printing operation can be started fifty (50) seconds earlier than the prior art apparatus.

In addition to the above advantage over the prior art apparatus, the fixing temperature range computing unit204according to the first embodiment refers to the fixable temperature range offset table as illustrated inFIG. 4and computes the fixable temperature range based on the added image density calculated by the image density information calculation unit202. Therefore, for example, where the sheet10to be used for the printing has the thickness of one hundred twenty (120) μm, where the target fixing temperature determination unit203determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, and where the added image density calculated by the image density information calculation unit202is two hundred fifty (250) percents, the fixable temperature range computing unit204computes the upper limit temperature “Tupper” in the fixable temperature range to be one hundred eighty three (183) degrees Celsius and the lower limit temperature “Tlower” in the fixable temperature range to be one hundred seventy six (176) degree Celsius (that is, the fixable temperature range is between 176° C. and 183° C.). Moreover, for example, where the sheet10to be used for the printing has the thickness of one hundred twenty (120) μm, where the target fixing temperature determination unit203determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, and where the added image density calculated by the image density information calculation unit202is fifty (50) percent, the fixable temperature range computing unit204computes the upper limit temperature “Tupper” in the fixable temperature range to be one hundred ninety (190) degrees Celsius and the lower limit temperature “Tlower” in the fixable temperature range to be one hundred sixty seven (167) degree Celsius (that is, the fixable temperature range is between 167° C. and 190° C.).

Therefore, the print startable timing in the course of heating the fixing unit112can be adjusted according to the added image density in the first embodiment as illustrated inFIG. 7A. For example, where the added image density is fifty (50) percent, the printing operation is started at a time “ts3” at which the temperature of the fixing unit112reaches the lower limit temperature “Tlower” of one hundred sixty seven (167) degrees Celsius in the fixable temperature range. Where the rate of the heat temperature change is two-point-five degrees Celsius per second (2.5° C./sec), and where the added image density is of two hundred fifty (250) percents, the printing operation can be executed three-point-six (3.6) seconds ((176−167)/2.5) earlier than a time “ts4” at which the fixing unit112reaches the lower limit temperature “Tlower” of one hundred seventy six (176) degrees Celsius in the fixable temperature range. Double-headed arrows on a left side and a right side inFIG. 7Aindicate the print startable temperature ranges in a case of the density of fifty (50) percent and two hundred fifty (250) percents, respectively.

Similarly, the print startable timing in the course of releasing the heat from the fixing unit112can be adjusted according to the added image density in the first embodiment as illustrated inFIG. 7B. For example, where the added image density is fifty (50) percent, the printing operation is started at a time “ts5” at which the temperature of the fixing unit112reaches the upper limit temperature “Tupper” of one hundred ninety (190) degrees Celsius in the fixable temperature range. Where the rate of the releasing temperature change is zero-point-two degrees Celsius per second (0.2° C./sec), and where the added image density of two hundred fifty (250) percents, the printing operation can be executed thirty five (35) seconds ((190−183)/0.2) earlier than a time “ts6” at which the fixing unit112reaches the upper limit temperature “Tupper” of one hundred eighty three (183) degrees Celsius in the fixable temperature range. Double-headed arrows on a left side and a right side inFIG. 7Bindicate the print startable temperature ranges in a case of the density of fifty (50) percent and two hundred fifty (250) percents, respectively.

The image density information calculation unit202described above calculates the added image density based on the number of data dots per prescribed region of the print data received. Alternatively, the image density information calculation unit202can calculate the added image density by measuring a layer thickness of each of cyan, magenta and yellow colors of the developer on the sheet10, for example, as illustrated inFIG. 8, and the fixable temperature range can be computed based on the added image density obtained.

Moreover, the fixing temperature range computing unit204described above refers to the fixable temperature range offset table as illustrated inFIG. 4and computes the upper limit temperature “Tupper” and the lower limit temperature “Tlower” in the fixable temperature range. Alternatively, the fixing temperature range computing unit204can arrange, for example, fixing temperature offset values in maximum added image density and minimum added image density beforehand, and can determine the temperature of “Tupper” and “Tlower” by computation based on such two offset values. In such a case, since each developer has different fixability, a correction is made by a fixability coefficient provided to each color, thereby computing the temperature of “Tupper” and “Tlower” more accurately.

The sheet feeding mechanism control unit209described above allows the conveyance of the sheet10to be started in a case where the temperature of the fixing unit112reaches within the fixable temperature range. Alternatively, the sheet feeding mechanism control unit209can allow the conveyance of the sheet10to be started before the temperature of the fixing unit112reaches the fixable temperature range by controlling a number of rotations of the feed roller103and the like.

According to the first embodiment described above, in a case where the added image density of the print data is low, the fixable temperature range is increased, thereby advancing a print startable timing. Therefore, a waiting time of a fixing temperature adjustment in the fixing unit112can be shortened. Particularly, in a case where the temperature of the fixing unit112once increases to a high level, for example, after successive printing, the fixing unit112accumulates the heat therein, causing an increase in difficulty of decreasing the temperature thereof. In such a particular situation, the waiting time of the fixing temperature adjustment can be shortened. Moreover, in a case where the added image density of the print data is high, the fixable temperature range can be reduced, thereby obtaining stable fixability and enhancing fixing quality.

Second Embodiment

Elements and print operation of an image forming apparatus2101according to a second embodiment is similar to those of the image forming apparatus101described above in the first embodiment. Like elements will be given the same reference numerals as above, and description thereof will be omitted. In the second embodiment, a control program executable of the print operation of the image forming apparatus2101is different from the first embodiment.

Referring toFIG. 9, the control program executed by a central processing unit (CPU) according to the second embodiment is illustrated.

The control program includes: a transmitting-receiving unit801transmitting and receiving print data to and from a host computer and the like through an interface connector125; an operator panel control unit802controlling an operator panel124; a medium information storage unit803storing thickness information of a sheet10, input by a user through the operator panel124, in a nonvolatile memory123; a target fixing temperature determination unit804determining fixing temperature arranged associated with the thickness information of the sheet10stored in the medium information storage unit803as target fixing temperature; a fixable temperature range computing unit805computing a fixing temperature range in which a developer image can be fixed onto the sheet10by using the thickness information of the sheet10while using the target fixing temperature as reference temperature; a fixing temperature control unit806controlling surface temperature of a heat roller113based on the fixable temperature range; a print startable judgment unit807judging whether or not to start the print operation based on the surface temperature of the heat roller113; a fixing temperature measurement unit808acquiring a measurement result of the surface temperature of the heat roller113measured by a temperature sensor115; a print control unit809controlling the print operation as a whole of the image forming apparatus2101; a sheet feeding mechanism control unit810controlling feeding of the sheet10; and an image forming mechanism control unit811controlling image formation and a transfer process.

The transmitting-receiving unit801transmits and receives print data to and from a host computer and the like through an interface connector125. Upon receiving the print data, the transmitting-receiving unit801notifies the target fixing temperature determination unit804of reception of the print data.

The operator panel control unit802controls the operator panel124receiving the thickness information of the sheet10input by the user.

The medium information storage unit803stores the thickness information of the sheet10input through the operator panel124in the nonvolatile memory123.

The target fixing temperature determination unit804reads the thickness information of the sheet10stored in the medium information storage unit803, and determines the fixing temperature arranged associated with the thickness information of the sheet10as target fixing temperature (hereafter referred to as “Tprint”). In a case where an image to be formed on the sheet10is a multi-color image, three colors such as cyan, magenta, and yellow are used for general color expression. Since each maximum image density of the cyan, magenta, and yellow is one hundred (100) percent, added image density of the three colors becomes three hundred (300) percents.

However, each maximum image density of the three colors may be reduced to a value below one hundred (100) percent due to a problem of fixability, for example. Therefore, it must be noted that the added image density is not necessarily three hundred (300) percents.

In the second embodiment, the fixing temperature serving as the reference temperature is arranged beforehand according to the thickness of the sheet10to be used for the printing, so that a good printing result is obtained in a case where the printing operation is performed with the added image density of three hundred (300) percents. The target fixing temperature determination unit804determines the temperature of “Tprint” based on the thickness of the sheet10used for the printing. For example, where the thickness of the sheet10is one hundred (100) μm, the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred seventy (170) degrees Celsius. Where the thickness of the sheet10is two hundred fifty (250) μm, the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius.

The fixable temperature range computation unit805computes the fixable temperature range in which the developer image can be fixed on the sheet10by using the thickness information of the sheet10while using the temperature of “Tprint” determined by the target temperature determination unit804as the reference temperature. Herein, the fixable temperature range includes upper limit temperature (hereafter referred to as “Tupper”) and lower limit temperature (hereafter referred to as “Tlower”).

Generally, a heat amount to be removed increases with an increase in the thickness of the sheet10to be used for the printing. Consequently, a temperature difference between the temperature of “Tprint” and “Tlower” may be arranged to be small while a temperature difference between the temperature of “Tprint” and “Tupper” can be arranged to be big so as to secure the fixability. Moreover, a heat amount to be removed decreases with a decrease in the thickness of the sheet10to be used for the printing. Consequently, the fixablity can be secured in a case where the temperature difference between the temperature of “Tprint” and “Tlower” is arranged to be big. However, in a case where the print data having high added image density are printed on the sheet10of a thin sheet, and in a case where the temperature of the heat roller113is high, the developer melted by the heat roller113sticks to the heat roller113, causing winding the sheet10around the heat roller113. Such a situation of winding the sheet10around the heat roller113may cause paper jam. Therefore, the temperature difference between the temperature of “Tprint” and “Tupper” needs to be small.

Based on such a condition, the fixable temperature range computing unit805computes the fixable temperature range. Particularly, the fixable temperature range computing unit805computes the fixable temperature range using a fixable temperature range offset table as illustrated inFIG. 10. For example, where the sheet10to be used for the printing has the thickness of one hundred (100) μm, and where the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred seventy (170) degrees Celsius, the fixable temperature range computing unit805refers to the fixable temperature range offset table and computes the upper limit temperature “Tupper” to be one hundred seventy five (175) degrees Celsius (“Trpint” of 170° C.+an upper limit temperature offset value “Toff1” of 5° C.=175° C.) and the lower limit temperature “Tlower” to be one hundred sixty (160) degree Celsius (“Trpint” of 170° C.+a lower limit temperature offset value “Toff2” of −10° C.=160° C.). Moreover, where the sheet10to be used for the printing has the thickness of two hundred fifty (250) μm, and where the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit805refers to the fixable temperature range offset table and computes the temperature of “Tupper” to be one hundred ninety two (192) degrees Celsius (“Trpint” of 180° C.+the upper limit temperature offset value “Toff1” of 12° C.=192° C.) and the temperature of “Tlower” to be one hundred seventy six (176) degree Celsius (“Trpint” of 180° C.+the lower limit temperature offset value “Toff2” of −4° C.=176° C.).

The fixing temperature control unit806controls the surface temperature of the heat roller113by turning on and off the halogen lamp114based on the fixable temperature range computed by the fixable temperature range computing unit805.

The print starable judgment unit807judges whether or not to start the print operation based on a measurement result of the surface temperature of the heat roller113.

The fixing temperature measurement unit808acquires, based on an instruction of the fixing temperature control unit806or the print startable judgment unit807, the measurement result of the surface temperature of the heat roller113measured by the temperature sensor115. The acquired surface temperature of the heat roller113is notified to the fixing temperature control unit806or the print startable judgment unit807.

Where the print startable judgment unit807judges that the print operation is startable, the print control unit809controls the print operation of the image forming apparatus2101as a whole.

The sheet feeding mechanism control unit810controls the feeding of the sheet10fed by the feed roller103and the like based on the control by the print control unit809.

The image forming mechanism control unit811controls the image formation provided by the image forming unit108and the transfer process provided by the transfer roller111and the like based on the control by the print control unit809.

Referring to a flowchart ofFIG. 11, an example procedure based the control program described above is illustrated. In the second embodiment, the fixing temperature serving as the reference temperature is arranged beforehand according to the thickness of the sheet10to be used for the printing, so that a good printing result is obtained in a case where the printing operation is performed with the added image density of three hundred (300) percents.

The thickness information of the sheet10is input by the user through the operation panel124. The operator panel control unit802notifies the medium information storage unit803of reception of the thickness information of the sheet10. The medium information storage unit803stores the thickness information of the sheet10in the nonvolatile memory123(step S201).

Subsequently, when the print data is transmitted with respect to the image forming apparatus2101from the host computer, the transmitting-receiving unit801receives the print data through the interface connector125. The transmitting-receiving unit801notifies of reception of the print data with respect to the target fixing temperature determination unit804(Yes in step S202).

Upon receiving the notification, the target fixing temperature determination unit804determines the temperature of “Tprint” based on the thickness information of the sheet10stored in the nonvolatile memory123(step S203).

Subsequently, the fixable temperature range computing unit805computes the temperature of “Tupper” and “Tlower” by using the thickness information of the sheet10while using the temperature of “Tprint” as the reference temperature (step S204).

In step S205, the fixing temperature control unit806supplies an instruction with respect to the fixing temperature measurement unit808to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is below “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit806supplies the instruction to a power source system (not shown) to distribute the power to the halogen lamp114such that the surface temperature of the heat roller113is adjusted to the temperature of “Tprint.” Upon receiving the instruction from the fixing temperature control unit806, the power source system begins the power distribution to the halogen lamp114.

On the other hand, where the surface temperature of the heat roller113is above “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit806does not supply the power distribution instruction to the power source system (not shown).

After the surface temperature of the heat roller113reaches the temperature of “Tprint,” the fixing temperature control unit806controls the power source system (not shown) such that the surface temperature of the heat roller113remains at the temperature of “Tprint.”

Next, the print startable judgment unit807supplies the instruction with respect to the fixing temperature measurement unit808to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is within the fixable temperature range as a result of measurement thereof by the temperature sensor115(Yes in step S206), the print startable judgment unit807supplies a print start instruction to the print control unit809. Upon receiving the print start instruction, the print control unit809supplies a print execution instructions to the sheet feeding mechanism control unit810and the image forming mechanism control unit811. Upon receiving the print execution instruction from the print control unit809, the sheet feeding mechanism control unit810allows the feed roller103and the like to start conveying the sheet10. The image forming unit108and the transfer roller111instructed by the image forming mechanism control unit811form the developer image on the sheet10conveyed.

The fixing unit112fixes the developer image on the sheet10. Subsequently, the sheet10having the developer image fixed thereon is guided along the conveyance path118and is conveyed to the ejection roller120. The ejection roller120ejects the sheet10on the stacker121(step S207).

Where the surface temperature of the heat roller113is outside the fixable temperature range as a result of measurement thereof by the temperature sensor115(No in step S206), the print startable judgment unit807does not supply the print start instruction to the print control unit809. The print startable judgment unit807is on standby until the surface temperature of the heat roller113reaches within the fixable temperature range. Where the surface temperature of the heat roller113reaches within the fixable temperature range, the print startable judgment unit807supplies the print start instruction to the print control unit809.

The fixable temperature range computing unit805according to the second embodiment refers to the fixable temperature range offset table as illustrated inFIG. 10and computes the fixable temperature range based on the thickness information of the sheet10. For example, where the sheet10to be used for the printing has the thickness of one hundred (100) μm, and where the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred seventy (170) degrees Celsius, the fixable temperature range computing unit805computes the temperature of “Tupper” to be one hundred seventy five (175) degrees Celsius and the temperature of “Tlower” to be one hundred sixty (160) degree Celsius (that is, the fixable temperature range is between 160° C. and 175° C.). Moreover, for example, where the sheet10to be used for the printing has the thickness of two hundred fifty (250) μm, and where the target fixing temperature determination unit804determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit805computes the temperature of “Tupper” to be one hundred ninety two (192) degrees Celsius and the temperature of “Tlower” to be one hundred seventy six (176) degree Celsius (that is, the fixable temperature range is between 176° C. and 192° C.).

Therefore, the print startable timing in the course of heating the fixing unit112can be adjusted according to the thickness of the sheet10in the second embodiment as illustrated inFIG. 12. For example, where the thickness of the sheet10is one hundred (100) μm, the printing operation is started at a time “ts7” at which the temperature of the fixing unit112reaches the lower limit temperature “Tlower” of one hundred sixty (160) degrees Celsius in the fixable temperature range. That is, where a rate of the heat temperature change is two-point-five degrees Celsius per second (2.5° C./sec), the printing operation can be started four (4) seconds ((170−160)/2.5) earlier than a time “ts7” at which the fixing unit112reaches the target temperature “Tprint” of one hundred seventy (170) degrees Celsius. Moreover, where the thickness of the sheet10is two hundred fifty (250) μm, the printing operation is started at a time “ts8” at which the temperature of the fixing unit112reaches the temperature of “Tlower” of one hundred seventy six (176) degrees Celsius. That is, where the rate of the heat temperature change is two-point-five degrees Celsius per second (2.5° C./sec), the printing operation can be started one-point-six (1.6) seconds ((180−176)/2.5) earlier than a time “ts8” at which the fixing unit112reaches the target temperature “Tprint” of one hundred eighty (180) degrees Celsius. Therefore, the thinner the sheet10in the course of heating, the more advantage the second embodiment can provide.

InFIG. 12, a dotted line led from the “Tupper” of 192° C., a solid line led from the “Tprint” of 180° C., a dotted line led from the “Tlower” of 176° C., a dotted line led from the “Tupper” of 175° C., a solid line led by the “Tprint” of 170° C., and a dotted line led by the “Tlower” of 160° C. indicate the fixable upper limit temperature in a case of the medium thickness of 250 μm, the target fixing temperature in a case of the medium thickness of 250 μm, the fixable lower limit temperature in a case of the medium thickness of 250 μm, the fixable upper limit temperature in a case of the medium thickness of 100 μm, the target fixing temperature in a case of the medium thickness of 100 μm, and the fixable lower limit temperature in a case of the medium thickness of 100 μm, respectively.

Similarly, the print startable timing in the course of releasing the heat from the fixing unit112can be adjusted according to the thickness of the sheet10in the second embodiment as illustrated inFIG. 13. For example, where the thickness of the sheet10is two hundred fifty (250) μm, the printing operation is started at a time “ts9” at which the temperature of the fixing unit112reaches the upper limit temperature “Tupper” of one hundred ninety two (192) degrees Celsius in the fixable temperature range. That is, where the rate of the releasing temperature change is zero-point-five degrees Celsius per second (0.5° C./sec), the printing operation can be started twenty four (24) seconds ((192−180)/0.5) earlier than a time “ts9” at which the fixing unit112reaches the target temperature “Tprint” of one hundred eighty (180) degrees Celsius. Moreover, where the thickness of the sheet10is one hundred (100) μm, the printing operation is started at a time “ts10” at which the temperature of the fixing unit112reaches the temperature of “Tupper” of one hundred seventy five (175) degrees Celsius. That is, where the rate of the releasing temperature change is zero-point-five degrees Celsius per second (0.5° C./sec), the printing operation can be started ten (10) seconds ((175−170)/0.5) earlier than a time “ts10” at which the fixing unit112reaches the target temperature “Tprint” of one hundred seventy (170) degrees Celsius. Therefore, the thicker the sheet10in the course of releasing the heat, the more advantage the second embodiment can provide.

InFIG. 13, a dotted line led from the “Tupper” of 192° C., a solid line led from the “Tprint” of 180° C., a dotted line led from the “Tlower” of 176° C., a dotted line led from the “Tupper” of 175° C., a solid line led by the “Tprint” of 170° C., and a dotted line led by the “Tlower” of 160° C. indicate the fixable upper limit temperature in a case of the medium thickness of 250 μm, the target fixing temperature in a case of the medium thickness of 250 μm, the fixable lower limit temperature in a case of the medium thickness of 250 μm, the fixable upper limit temperature in a case of the medium thickness of 100 μm, the target fixing temperature in a case of the medium thickness of 100 μm, and the fixable lower limit temperature in a case of the medium thickness of 100 μm, respectively.

The fixing temperature range computing unit805described above refers to the fixable temperature range offset table as illustrated inFIG. 10and computes the upper limit temperature “Tupper” and the lower limit temperature “Tlower.” Alternatively, the fixing temperature range computing unit805can arrange, for example, fixing temperature offset values in maximum thickness and minimum thickness of the sheet10beforehand, and can determine the temperature of “Tupper” and “Tlower” by computation based on such two offset values.

The sheet feeding mechanism control unit810described above allows the conveyance of the sheet10to be started in a case where the temperature of the fixing unit112reaches within the fixable temperature range. Alternatively, the sheet feeding mechanism control unit810can allow the conveyance of the sheet10to be started before the temperature of the fixing unit112reaches the fixable temperature range by controlling a number of rotations of the feed roller103and the like.

According to the second embodiment described above, a print startable timing can be advanced based on the thickness information of the sheet10. Therefore, a waiting time of a fixing temperature adjustment in the fixing unit112can be shortened. Particularly, in a case where the temperature of the fixing unit112once increases to a high level, for example, after successive printing, the fixing unit112accumulates the heat therein, causing an increase in difficulty of decreasing the temperature thereof. In such a situation, the waiting time of the fixing temperature adjustment can be shortened according to the second embodiment. Moreover, in a case where the sheet10is a thick sheet, the temperature difference between the temperature of “Tpirnt” and “Tlower” is arranged to be small, thereby obtaining stable fixability and enhancing fixing quality. Moreover, in a case where the sheet10is a thin sheet, the temperature difference between the temperature of “Tpirnt” and “Tupper” is arranged to be small, thereby reducing occurrences of the jam caused by winding the sheet10around the heat roller113.

Third Embodiment

Elements and print operation of an image forming apparatus3101according to a third embodiment is similar to those of the image forming apparatus101described above in the first embodiment. Like elements will be given the same reference numerals as above, and description thereof will be omitted. In the third embodiment, a control program executable of the print operation of the image forming apparatus3101is different from the first embodiment.

Referring toFIG. 14, the control program executed by a central processing unit (CPU) according to the third embodiment is illustrated.

The control program includes: a transmitting-receiving unit1201transmitting and receiving print data to and from a host computer and the like through an interface connector125; an operator panel control unit1202controlling an operator panel124; a medium information storage unit1203storing class information of a sheet10, input by a user through the operator panel124, in a nonvolatile memory123; a target fixing temperature determination unit1204determining fixing temperature arranged associated with the class information of the sheet10stored in the medium information storage unit1203as target fixing temperature; a fixable temperature range computing unit1205computing a fixable temperature range in which a developer image can be fixed onto the sheet10by using the class information of the sheet10while using the target fixing temperature as reference temperature; a fixing temperature control unit1206controlling surface temperature of a heat roller113based on the fixable temperature range; a print startable judgment unit1207judging whether or not to start the print operation based on the surface temperature of the heat roller113; a fixing temperature measurement unit1208acquiring a measurement result of the surface temperature of the heat roller113measured by a temperature sensor115; a print control unit1209controlling the print operation as a whole of the image forming apparatus3101; a sheet feeding mechanism control unit1210controlling feeding of the sheet10; and an image forming mechanism control unit1211controlling image formation and a transfer process.

The transmitting-receiving unit1201transmits and receives print data to and from a host computer and the like through an interface connector125. Upon receiving the print data, the transmitting-receiving unit1201notifies the target fixing temperature determination unit1204of reception of the print data.

The operator panel control unit1202controls the operator panel124receiving the class information of the sheet10input by the user.

The medium information storage unit1203stores the class information of the sheet10input through the operator panel124in the nonvolatile memory123.

The target fixing temperature determination unit1204reads the class information of the sheet10stored in the medium information storage unit1203, and determines the fixing temperature arranged associated with the class information of the sheet10as target fixing temperature (hereafter referred to as “Tprint”). In a case where an image to be formed on the sheet10is a multi-color image, three colors such as cyan, magenta, and yellow are used for general color expression. Since each maximum image density of the cyan, magenta, and yellow is one hundred (100) percent, added image density of the three colors becomes three hundred (300) percents.

However, each maximum image density of the three colors may be reduced to a value below one hundred (100) percent due to a problem of fixability, for example. Therefore, it must be noted that the added image density is not necessarily three hundred (300) percents.

In the third embodiment, the fixing temperature serving as the reference temperature is arranged beforehand according to the class of the sheet10to be used for the printing, so that a good printing result is obtained in a case where the printing operation is performed with the added image density of three hundred (300) percents. The target fixing temperature determination unit1204determines the temperature of “Tprint” based on the class of the sheet10used for the printing. For example, where the sheet10is a plain sheet, the target fixing temperature determination unit1204determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius.

The fixable temperature range computation unit1205computes the fixable temperature range in which the developer image can be fixed on the sheet10by using the class information of the sheet10while using the temperature of “Tprint” determined by the target temperature determination unit1204as the reference temperature. Herein, the fixable temperature range includes upper limit temperature (hereafter referred to as “Tupper”) and lower limit temperature (hereafter referred to as “Tlower”).

Generally, a heat amount to be removed varies depending on the class (e.g., the plain sheet, a glossy sheet, a label sheet, a postcard, an envelope, an OHP sheet) of the sheet10to be used for the printing. In this regard, a temperature offset value needs to be arranged beforehand to obtain a good fixability with respect to each class of the sheet10. Therefore, the fixable temperature range computing unit1205computes the fixable temperature range using a fixable temperature range offset table as illustrated inFIG. 15. For example, where the sheet10to be used for the printing operation is the plain sheet, and where the target fixing temperature determination unit1204determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit1205computes the temperature of “Tupper” to be one hundred ninety (190) degrees Celsius (“Trpint” of 180° C.+an upper limit temperature offset value “Toff1” of 10° C.=190° C.) and the temperature of “Tlower” to be one hundred sixty seven (167) degrees Celsius (“Trpint” of 180° C.+a lower limit temperature offset value “Toff2” of −13° C.=167° C.). For example, where the sheet10to be used for the printing operation is the glossy sheet, and where the target fixing temperature determination unit1204determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit1205refers to the fixable temperature range offset table and computes the temperature of “Tupper” to be one hundred eighty five (185) degrees Celsius (“Trpint” of 180° C.+the upper limit temperature offset value “Toff1” of 5° C.=1185° C.) and the temperature of “Tlower” to be one hundred seventy four (174) degrees Celsius (“Trpint” of 180° C.+the lower limit temperature offset value “Toff2” of −6° C.=174° C.).

The fixing temperature control unit1206controls the surface temperature of the heat roller113by turning on and off the halogen lamp114based on the fixable temperature range computed by the fixable temperature range computing unit1205.

The print starable judgment unit1207judges whether or not to start the print operation based on a measurement result of the surface temperature of the heat roller113.

The fixing temperature measurement unit1208acquires, based on an instruction of the fixing temperature control unit1206or the print startable judgment unit1207, the measurement result of the surface temperature of the heat roller113measured by the temperature sensor115. The acquired surface temperature of the heat roller113is notified to the fixing temperature control unit1206or the print startable judgment unit1207.

Where the print startable judgment unit1207judges that the print operation is startable, the print control unit1209controls the print operation of the image forming apparatus3101as a whole.

The sheet feeding mechanism control unit1210controls the feeding of the sheet10fed by the feed roller103and the like based on the control by the print control unit1209.

The image forming mechanism control unit1211controls the image formation provided by the image forming unit108and the transfer process provided by the transfer roller111and the like based on the control by the print control unit1209.

Referring to a flowchart ofFIG. 16, an example procedure based the control program described above is illustrated. In the third embodiment, the fixing temperature serving as the reference temperature is arranged beforehand according to the class of the sheet10to be used for the printing, so that a good printing result is obtained in a case where the printing operation is performed with the added image density of three hundred (300) percents.

The class information of the sheet10is input by the user through the operation panel124. The operator panel control unit1202notifies the medium information storage unit1203of reception of the class information of the sheet10. The medium information storage unit1203stores the class information of the sheet10in the nonvolatile memory123(step S301).

Subsequently, when the print data are transmitted with respect to the image forming apparatus3101from the host computer, the transmitting-receiving unit1201receives the print data through the interface connector125. The transmitting-receiving unit1201notifies of reception of the print data with respect to the target fixing temperature determination unit1204(Yes in step S302).

Upon receiving the notification, the target fixing temperature determination unit1204determines the temperature of “Tprint” based on the class information of the sheet10stored in the nonvolatile memory123(step S303).

Subsequently, the fixable temperature range computing unit1205computes the temperature of “Tupper” and “Tlower” by using the class information of the sheet10while using the temperature of“Tprint” as the reference temperature (step S304).

In step S305, the fixing temperature control unit1206supplies an instruction with respect to the fixing temperature measurement unit1208to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is below “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit1206supplies the instruction to a power source system (not shown) to distribute the power to the halogen lamp114such that the surface temperature of the heat roller113is adjusted to the temperature of “Tprint.” Upon receiving the instruction from the fixing temperature control unit1206, the power source system begins the power distribution to the halogen lamp114.

On the other hand, where the surface temperature of the heat roller113is above “Tprint” as a result of measurement thereof by the temperature sensor115, the fixing temperature control unit1206does not supply the power distribution instruction to the power source system (not shown).

After the surface temperature of the heat roller113reaches the temperature of “Tprint,” the fixing temperature control unit1206controls the power source system (not shown) such that the surface temperature of the heat roller113remains at the temperature of “Tprint.”

Next, the print startable judgment unit1207supplies the instruction with respect to the fixing temperature measurement unit1208to measure the surface temperature of the heat roller113by the temperature sensor115. Where the surface temperature of the heat roller113is within the fixable temperature range as a result of measurement thereof by the temperature sensor115(Yes in step S306), the print startable judgment unit1207supplies a print start instruction to the print control unit1209. Upon receiving the print start instruction, the print control unit1209supplies a print execution instructions to the sheet feeding mechanism control unit1210and the image forming mechanism control unit1211. Upon receiving the print execution instruction from the print control unit1209, the sheet feeding mechanism control unit1210allows the feed roller103and the like to start conveying the sheet10. The image forming unit108and the transfer roller111instructed by the image forming mechanism control unit1211form the developer image on the sheet10conveyed.

The fixing unit112fixes the developer image on the sheet10. The sheet10having the developer image fixed thereon is guided along the conveyance path118and is conveyed to the ejection roller120. The ejection roller120ejects the sheet10on the stacker121(step S307).

Where the surface temperature of the heat roller113is outside the fixable temperature range as a result of measurement thereof by the temperature sensor115(No in step S306), the print startable judgment unit1207does not supply the print start instruction to the print control unit1209. The print startable judgment unit1207is on standby until the surface temperature of the heat roller113reaches within the fixable temperature range. Where the surface temperature of the heat roller113reaches within the fixable temperature range, the print startable judgment unit1207supplies the print start instruction to the print control unit1209.

The fixable temperature range computing unit1205according to the third embodiment refers to the fixable temperature range offset table as illustrated inFIG. 15and computes the fixable temperature range based on the class information of the sheet10. For example, where the sheet10to be used for the printing operation is the plain sheet, and where the target fixing temperature determination unit1204determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit1205computes the temperature of “Tupper” to be one hundred ninety (190) degrees Celsius and the temperature of “Tlower” to be one hundred sixty seven (167) degree Celsius (that is, the fixable temperature range is between 167° C. and 190° C.). Moreover, for example, where the sheet10to be used for the printing operation is the glossy sheet, and where the target fixing temperature determination unit1204determines the temperature of “Tprint” to be one hundred eighty (180) degrees Celsius, the fixable temperature range computing unit1205computes the temperature of “Tupper” to be one hundred eighty five (185) degrees Celsius and the temperature of “Tlower” to be one hundred seventy four (174) degree Celsius (that is, the fixable temperature range is between 174° C. and 185° C.).

Therefore, the print startable timing in the course of heating the fixing unit112can be adjusted according to the class of the sheet10in the third embodiment as illustrated in FIG.17. For example, where the sheet10is the plain sheet, the printing operation is started at a time “ts11” at which the temperature of the fixing unit112reaches the lower limit temperature “Tlower” of one hundred sixty seven (167) degrees Celsius in the fixable temperature range. Where a rate of the heat temperature change is two-point-five degrees Celsius per second (2.5° C./sec), and where the sheet10is the glossy sheet, the printing operation can be executed two-point-eight (2.8) seconds ((174−167)/2.5) earlier than a time “ts12” at which the fixing unit112reaches the lower limit temperature “Tlower” of one hundred seventy four (174) degrees Celsius.

InFIG. 17, a dotted line led from the “Tupper” of 190° C., a dotted line led from the “Tupper” of 185° C., a solid line led from the “Tprint” of 180° C., a dotted line led from the “Tlower” of 174° C., and a dotted line led from the “Tlower” of 167° C. indicate the fixable upper limit temperature in a case of the plain sheet, the fixable upper limit temperature in a case of the glossy sheet, the fixable lower limit temperature in a case of the glossy sheet, and the fixable lower limit temperature in a case of the plain sheet, respectively.

Similarly, the print startable timing in the course of releasing the heat from the fixing unit112can be adjusted according to the class of the sheet10in the third embodiment as illustrated inFIG. 18. For example, where the sheet10is the plain sheet, the printing operation is started at a time “ts13” at which the temperature of the fixing unit112reaches the upper limit temperature “Tupper” of one hundred ninety (190) degrees Celsius in the fixable temperature range. Where the rate of the releasing temperature change is zero-point-two degrees Celsius per second (0.2° C./sec), and where the sheet10is the glossy sheet, the printing operation can be executed twenty five (25) seconds ((190−185)/0.2) earlier than a time “ts14” at which the fixing unit112reaches the upper limit temperature “Tupper” of one hundred eighty five (185) degrees Celsius.

InFIG. 18, a dotted line led from the “Tupper” of 190° C., a dotted line led from the “Tupper” of 185° C., a solid line led from the “Tprint” of 180° C., a dotted line led from the “Tlower” of 174° C., and a dotted line led from the “Tlower” of 167° C. indicate the fixable upper limit temperature in a case of the plain sheet, the fixable upper limit temperature in a case of the glossy sheet, the fixable lower limit temperature in a case of the glossy sheet, and the fixable lower limit temperature in a case of the plain sheet, respectively.

The sheet feeding mechanism control unit1210according to the third embodiment allows the conveyance of the sheet10to be started in a case where the temperature of the fixing unit112reaches within the fixable temperature range. Alternatively, the sheet feeding mechanism control unit1210can allow the conveyance of the sheet10to be started before the temperature of the fixing unit112reaches the fixable temperature range by controlling a number of rotations of the feed roller103and the like.

According to the third embodiment described above, a print startable timing can be advanced based on the class information of the sheet10. Therefore, a waiting time of a fixing temperature adjustment in the fixing unit112can be shortened. Particularly, in a case where the temperature of the fixing unit112once increases to a high level, for example, after successive printing, the fixing unit112accumulates the heat therein, causing an increase in difficulty of decreasing the temperature thereof. In such a situation, the waiting time of the fixing temperature adjustment can be shortened according to the third embodiment. Moreover, in a case where the sheet10is a special medium such as the glossy sheet and the label sheet, a fluctuation range of the fixing temperature can be reduced by narrowing the fixable temperature range, thereby enhancing the printing quality.

According to the first, second, and third embodiment, the image forming apparatuses101,2101, and3101capable of forming the multi-color image by the developer of plural colors are described above. However, an image forming apparatus capable of forming a monochrome image by the developer of a single color can control as similar to the above embodiments. The present invention can be applied to a facsimile machine, a multifunctional peripheral, a photocopier, and the like. In the embodiments described above, print setting including medium information input by the user is received through the operator panel124. Alternatively, the print setting including the medium information can be performed in the host computer.

The present invention has been described above with regard to particular embodiments, but the present invention is not limited thereto. As can be appreciated by those skilled in the art, numerous additional modifications and variation of the present invention are possible in light of the above-described teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.