Image forming device

Abstract

To provide an image forming apparatus that changes a setting temperature of a fixing temperature according to the type of a recording sheet, and to reduce image forming time in executing specific continuous image forming operation of forming images on a plurality of recording sheets including two or more types of recording sheets.SOLUTION: An image forming apparatus 500 comprises fixing means 25 that performs thermal fixing processing on a recording sheet carrying an image at a fixing temperature controlled to a setting temperature according to a type of the recording sheet. The image forming apparatus has control means 110 that executes specific continuous image forming operation of forming images on a plurality of recording sheets including two or more types of recording sheets S1, S2 different in the setting temperature from each other, in a specific image forming order in which a frequency at which the thermal fixing processing is continuously performed on the same type of recording sheets is higher than a normal image forming order in normal continuous image forming operation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] 2. Description of the Related Art Conventionally, there is known an image forming apparatus equipped with a fixing unit that performs a heat fixing process on a recording sheet bearing an image at a fixing temperature that is controlled to a set temperature according to the type of the recording sheet.

[0003] Patent Document 1 discloses an image forming apparatus that controls the feeding interval of paper (recording sheets) conveyed to the fixing nip portion to ensure sufficient time for the fixing roller and pressure roller to heat up when the temperature of the fixing roller or the like falls below a threshold in order to ensure fixability. Patent Document 1 also describes that the set temperature of the fixing temperature is changed when performing heat fixing processing on paper of different paper thicknesses (types). Summary of the Invention [Problem to be solved by the invention]

[0004] However, in an image forming device that changes the set temperature of the fixing temperature depending on the type of recording sheet, the image formation time may become long when performing a specific continuous image forming operation that forms images on multiple recording sheets containing a mixture of two or more types of recording sheets. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides an image forming apparatus including a fixing unit that performs a heat fixing process on a recording sheet carrying an image at a fixing temperature controlled to a set temperature according to the type of the recording sheet, and when a specific continuous image forming operation is performed to form images on a plurality of recording sheets including two or more types of recording sheets having different set temperatures, the specific image forming order in which the heat fixing process is performed on the same type of recording sheets in succession more frequently than in a normal image forming order during a normal continuous image forming operation is performed.continuous A control means for executing an image forming operation is provided. The specific continuous image forming operation includes a continuous double-sided image forming operation in which images are formed on both sides of a plurality of recording sheets containing a mixture of two or more types of recording sheets, and the normal continuous image forming operation includes a continuous double-sided image forming operation in which images are formed on both sides of a plurality of recording sheets by interleaf control, and the continuous double-sided image forming operation in the specific continuous image forming operation is an operation in which images are formed on a plurality of recording sheets that are alternately fed with two types of recording sheets. It is characterized by the following. [Effects of the Invention]

[0006] According to the present invention, in an image forming device that changes the set temperature of the fixing temperature depending on the type of recording sheet, it is possible to shorten the image formation time when performing a specific continuous image forming operation that forms images on multiple recording sheets that are a mixture of two or more types of recording sheets. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment. [Figure 2] 2 is a schematic diagram showing an example of a fixing device in the printer. [Figure 3] FIG. 2 is a block diagram showing the hardware configuration related to main control in the printer. [Figure 4] FIG. 10 is an explanatory diagram showing the normal image formation sequence when performing a specific continuous image forming operation (alternate double-sided printing operation) in which recording sheets are alternately fed from two paper feed cassettes and images are continuously formed on both sides of each recording sheet. [Figure 5] 5A and 5B are explanatory diagrams showing the printing sequence when the alternating double-sided printing operation described above is performed in the embodiment. [Figure 6] 10 is a flowchart illustrating an example of print order rearrangement control in an embodiment. [Figure 7] An explanatory diagram showing the normal printing order during single-sided printing operation, in which two recording sheets are fed from the first paper feed cassette and then one recording sheet is fed from the second paper feed cassette, and this is repeated twice to form images consecutively on one side of six recording sheets. [Figure 8] FIG. 4 is an explanatory diagram showing the printing sequence when the above-described single-sided printing operation is performed in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] An embodiment of an electrophotographic printer will be described below as an image forming apparatus to which the present invention is applied.

[0009] In this embodiment, an electrophotographic image forming apparatus will be described, but it may also be an image forming apparatus other than an electrophotographic image forming apparatus (such as an inkjet image forming apparatus or a stencil printing image forming apparatus). Also, the image forming apparatus of this embodiment is a single-drum monochrome image forming apparatus equipped with one photosensitive member, but is not limited to this, and may also be, for example, a tandem color image forming apparatus in which multiple photosensitive members are arranged along an intermediate transfer belt as an intermediate transfer member.

[0010] First, the basic configuration of the copying machine according to this embodiment will be described. FIG. 1 is a schematic diagram showing the configuration of a copying machine according to this embodiment. In the figure, a copying machine 500 includes a scanner 200, which is an image reading device, and a printer 100, which is the main body of an image forming device. The scanner 200 includes an automatic document feeder (ADF).

[0011] The printer 100 has a paper feed unit 102 that supplies recording sheets stored therein to a paper feed path 48, and an image creating unit 101 mounted thereon. The image creating unit 101 is provided with a drum-shaped photosensitive member 40 that serves as a latent image carrier. The image creating unit 101 also has an optical writing device 21 that serves as a latent image writing means.

[0012] The surface of the photoreceptor 40 is uniformly charged by the charging device 60, and then optically scanned by the optical writing device 21, which drives a light source based on image data. The light-irradiated portion of the surface of the photoreceptor 40, which is created by this optical scanning, has its potential attenuated and becomes an electrostatic latent image.

[0013] The electrostatic latent image formed on the surface of the photoconductor 40 is developed into a toner image by the developing device 61. Inside the developing device 61, a developer mixture of toner and magnetic carrier is stirred, causing the toner in the developer to become negatively charged by friction. A developing roller is disposed inside the developing device 61. A portion of the peripheral surface of the developing roller is exposed to the outside through an opening in the casing, facing the photoconductor 40. The developer drawn up by the developing roller is transported to a development zone facing the photoconductor 40 as the developing roller rotates. In the development zone, a development potential acts between the electrostatic latent image on the photoconductor 40 and the developing roller, to which a development bias is applied, causing the negatively charged toner to move from the roller side to the latent image side. This development potential causes the toner on the developing roller to separate from the magnetic carrier and transfer to the electrostatic latent image on the photoconductor 40. The electrostatic latent image on the photoconductor 40 is then developed with the toner into a toner image.

[0014] A transfer belt 10 is disposed below the photoreceptor 40 and presses the transfer belt 10 toward the photoreceptor 40. This forms a transfer nip where the photoreceptor 40 and the transfer belt 10 come into contact with each other. The transfer belt 10 is stretched over a transfer roller 11 to which a transfer bias is applied, and forms a transfer electric field between the transfer belt 10 and the electrostatic latent image on the photoreceptor 40.

[0015] When the printer 100 receives image data, it uses a driving means to move the transfer belt 10 endlessly counterclockwise in the figure. At the same time, it drives the photoreceptor 40 to rotate it as well. The toner image formed on the photoreceptor 40 is then transferred onto a recording sheet that is carried on the outer circumferential surface of the transfer belt 10 at the transfer nip and transported.

[0016] After passing through the transfer nip, residual toner that was not transferred adheres to the surface of the photoreceptor 40. This residual toner is removed from the surface of the photoreceptor 40 by a drum cleaning device 59 and then transported to a waste toner bottle. After cleaning, the surface of the photoreceptor 40 is uniformly charged again by a charging device 60.

[0017] Printer 100 separates and feeds out recording sheets one by one from one of a plurality of paper feed cassettes (four paper feed cassettes 44A, 44B, 44C, and 44D in the illustrated example) arranged in multiple stages in paper feed unit 102. The fed recording sheet is transported by a transport roller and enters paper feed path 48 of image forming unit 101. The recording sheet that enters paper feed path 48 of image forming unit 101 hits the registration nip of registration roller pair 49 and stops.

[0018] The pair of registration rollers 49 start rotating in synchronization with the timing at which the toner on the photoreceptor 40 reaches the transfer nip, and feeds the recording sheet toward the transfer nip. In the transfer nip, the toner image on the photoreceptor 40 is transferred onto the recording sheet by the action of the transfer electric field and nip pressure, forming an image.

[0019] The recording sheet that has passed through the transfer nip is sent to a fixing device 25 as a fixing means, where the image is fixed onto the surface of the recording sheet. Thereafter, the sheet is discharged outside the machine via a pair of discharge rollers 56, and then stacked on a paper discharge tray.

[0020] In a double-sided printing mode in which an image is formed on both sides of a recording sheet, a recording sheet with a toner image fixed only on the first side thereof passes through the fixing device 25 and is then sent to the lead-in path 28a of the re-feeding device 28, rather than to the discharge roller pair 56. The recording sheet sent to the lead-in path 28a is switched back by the switch-back section 28b of the re-feeding device 28, passes through the reversing feed path 28c, and is re-sent to the paper feed path 48 in an inverted state. The recording sheet is then sent from the paper feed path 48 to the transfer nip, where a toner image is transferred to the second side thereof, and is then discharged outside the apparatus via the fixing device 25 and the discharge roller pair 56.

[0021] FIG. 2 is a schematic diagram showing an example of the fixing device 25 in this embodiment. As shown in FIG. 2, the fixing device 25 includes a fixing roller 25a as a fixing member, a fixing heater 25d as a heating means provided on the fixing roller 25a, a pressure roller 25e as a pressure member, a pressure heater 25f as a heating means provided on the pressure roller 25e, a pressure roller fan 25g for cooling the pressure roller 25e, and a temperature sensor 25h as a temperature detection means.

[0022] The temperature of the fixing roller 25a is controlled by turning on and off the power to the fixing heater 25d. The heating means in the fixing device 25 is not limited to the heaters 25d and 25f, but may be other temperature-controllable heating means such as a lamp or an electromagnetic induction heating device (IH heater).

[0023] The pressure roller 25e is located below the fixing roller 25a and is rotatably pressed against the fixing roller 25a. The pressure roller 25e is provided with a pressure heater 25f inside. The temperature of the pressure roller 25e is controlled by turning on and off the power to the pressure heater 25f of the pressure roller 25e.

[0024] A fixing nip is formed by the pressure contact between the pressure roller 25e and the fixing roller 25a, and when the recording sheet S passes through this fixing nip, the toner image on the recording sheet S is fixed to the recording sheet S by heat and pressure.

[0025] FIG. 3 is a block diagram showing the hardware configuration related to the main control in the printer 100. As shown in FIG. The printer 100 includes a main body control unit 110 as a control means for controlling the entire printer. The operation and display unit 130, memory 111, image processing unit 112, paper feed unit 102, sheet transport unit 103 consisting of various transport rollers for transporting recording sheets S, image creation unit 101, and other components of the printer 100 operate in response to instructions from the main body control unit 110.

[0026] The main body control unit 110 realizes image forming operations through the cooperative operation of each component. The operation display unit 130 is a component that has both an operation input function for receiving operation instructions from the user and a display function for presenting information to the user. The image processing unit 112 is a component that processes input image data and performs image editing and arrangement as instructed. The memory 111 is a component that inputs and stores various data such as calculation data and image data.

[0027] The paper feed unit 102 is a component that feeds out a recording sheet from a specified one of the four paper feed cassettes 44A, 44B, 44C, and 44D. The image creation unit 101 is a component that executes the operation of forming an image on the recording sheet S, and is composed of components such as a toner image forming means that forms a toner image on the photosensitive member 40, a transfer belt 10, and a fixing device 25. The sheet transport unit 103 is a component that executes the operation of transporting the recording sheet fed from the paper feed unit 102 along a specified transport path, and is composed of components such as a pair of registration rollers 49 and a transport roller of the re-feed device 28.

[0028] Next, image formation order control, which is a feature of the present invention and controls the printing order (image formation order), will be described. FIG. 4 is an explanatory diagram showing the normal printing order when performing a specific continuous image forming operation (alternate double-sided printing operation) in which recording sheets S1 and S2 are alternately fed from two paper feed cassettes 44A and 44B and images are continuously formed on both sides of each recording sheet.

[0029] In this embodiment, interleaf control is performed in a normal continuous image formation operation in which recording sheets are fed from one of the paper feed cassettes and images are continuously formed on both sides of each recording sheet. Note that the interleaf control here refers to control that changes the printing order so that printing (transfer, fixation) is performed on one or more recording sheets from the (n+1)th sheet onward between printing (transfer, fixation) on the first side of the nth recording sheet and printing (transfer, fixation) on the second side of the nth recording sheet.

[0030] The example shown in FIG. 4 illustrates a printing sequence when performing interleaf control to perform alternating double-sided printing on four recording sheets. The printer 100 of this embodiment can accommodate up to three recording sheets in the re-feed device 28. Therefore, in the example shown in FIG. 4, first, (1) printing is performed on the first side of the first recording sheet S1 fed from the first paper feed cassette 44A, and the first recording sheet S1 is sent to the re-feed device 28. Next, (2) printing is performed on the first side of the second recording sheet S2 fed from the second paper feed cassette 44B, and the second recording sheet S2 is sent to the re-feed device 28. Also, (3) printing is performed on the first side of the third recording sheet S1 fed from the first paper feed cassette 44A, and the third recording sheet S1 is sent to the re-feed device 28.

[0031] After printing on the first side of this third recording sheet S1, (4) printing is performed on the second side of the first recording sheet S1 in the re-feeding device 28. Then, this first recording sheet S1 is discharged from the discharge roller pair 56 to the outside of the machine.

[0032] Next, (5) the first side of the fourth recording sheet S2 fed from the second paper feed cassette 44B is printed, and the fourth recording sheet S2 is sent to the re-feeding device 28. Thereafter, (6) the second side of the second recording sheet S2 in the re-feeding device 28 is printed, and the second recording sheet S2 is discharged from the discharge roller pair 56 to the outside of the apparatus. Also, (7) the second side of the third recording sheet S1 in the re-feeding device 28 is printed, and the third recording sheet S1 is discharged from the discharge roller pair 56 to the outside of the apparatus. After the second side of the third recording sheet S1 is printed, (8) the second side of the fourth recording sheet S2 in the re-feeding device 28 is printed, and the fourth recording sheet S2 is discharged from the discharge roller pair 56 to the outside of the apparatus.

[0033] If the recording sheets S1 and S2 stored in the two paper feed cassettes 44A and 44B are of different types with different set fixing temperatures, the set fixing temperature must be changed each time the type S1 or S2 of the recording sheets being sequentially conveyed to the fixing device 25 is switched (symbol W in FIG. 4). In this case, the recording sheet must be made to wait at the pair of registration rollers 49 to prevent it from being conveyed to the fixing device 25 before the fixing temperature reaches the new set temperature, resulting in a waiting time. In the normal printing order shown in FIG. 4, the type S1 or S2 of the recording sheets being conveyed to the fixing device 25 is switched five times, so the image formation time until the alternating double-sided printing operation for the four recording sheets is completed is extended by the time corresponding to the five waiting times.

[0034] FIG. 5 is an explanatory diagram showing the printing order (specific image formation order) when the above-described alternating double-sided printing operation is performed in this embodiment. In this embodiment, the printing sequence when performing the above-described alternating double-sided printing operation (specific image forming operation) is set so that the frequency with which the same type of recording sheet is subjected to the heat fixing process in succession (i.e., the frequency with which the same type of recording sheet is transported in succession to the fixing device 25) is higher than in the normal printing sequence shown in FIG.

[0035] According to this, when the above-mentioned alternating double-sided printing operation is performed, the number of times that the types S1 and S2 of the recording sheets conveyed to the fixing device 25 are switched is reduced compared to the normal printing order, and the number of waiting times can be reduced. Therefore, the image formation time until the alternating double-sided printing operation is completed can be reduced compared to the normal printing order.

[0036] Specifically, in this embodiment, when performing the above-mentioned alternating double-sided printing operation, the interleaf control that is performed in the normal printing order is turned off, so that the printing order is such that images are formed successively on each side of a recording sheet, and then images are formed successively on each side of the next recording sheet, in the order (image order of the print job) in which printing (transfer, fixing) is performed on both sides of the recording sheet.

[0037] In this case, as shown in FIG. 5, first, (1) printing is performed on the first side of the first recording sheet S1 fed from the first paper feed cassette 44A, and the first recording sheet S1 is fed to the re-feed device 28. Then, (2) printing is performed on the second side of the first recording sheet S1, and the first recording sheet S1 is discharged from the discharge roller pair 56 to the outside of the apparatus. Next, (3) printing is performed on the first side of the second recording sheet S2 fed from the second paper feed cassette 44B, and the second recording sheet S2 is fed to the re-feed device 28. Then, (4) printing is performed on the second side of the second recording sheet S2, and the second recording sheet S2 is discharged from the discharge roller pair 56 to the outside of the apparatus. The same process is then performed for the third recording sheet S1 and the fourth recording sheet S2.

[0038] According to the printing order (specific image formation order) of this embodiment shown in Fig. 5, the types S1 and S2 of the recording sheets to be conveyed to the fixing device 25 need only be switched three times. Therefore, while five waiting times would occur in the normal printing order shown in Fig. 4, the printing order of this embodiment shown in Fig. 5 only requires three waiting times, thereby reducing the image formation time.

[0039] In the case of a printing sequence in which printing (transferring and fixing) on ​​both sides of a recording sheet is performed in the order of successively forming images on each side of a recording sheet and then successively forming images on each side of the next recording sheet (image sequence of the print job) with interleaf control turned off as in this embodiment, the image formation time is longer than in the case of a normal printing sequence in which interleaf control is performed, since the effect of shortening the image formation time by interleaf control cannot be obtained. However, since the waiting time for changing the fixing temperature due to a change in the type of recording sheet conveyed to the fixing device 25 is usually sufficiently longer than the time that can be shortened by interleaf control, the effect of shortening the image formation time can be obtained by this embodiment.

[0040] Next, the length of the waiting time for changing the fixing temperature due to switching of the type of recording sheet conveyed to the fixing device 25 will be described. In the following explanation, an example will be given in which the recording sheets S1 stored in the first paper feed cassette 44A are thin paper with a low set temperature range for the fixing temperature, and the recording sheets SS stored in the second paper feed cassette 44B are thick paper with a high set temperature range for the fixing temperature. In this example, the set temperature corresponding to the thin paper recording sheet S1 is set at both the upper and lower limits of the set temperature range, while the set temperature corresponding to the thick paper recording sheet S2 is set at only the lower limit of the set temperature range. The lower limit of the set temperature range corresponding to the recording sheet S2 is higher than the upper limit of the set temperature range corresponding to the recording sheet S1.

[0041] The set temperature for each type of recording sheet is affected by the material, thickness, and process speed (rotation speed of the photosensitive drum) of the recording sheet. For example, the thicker the recording sheet, the more difficult it is for the recording sheet itself to heat up, and therefore a higher fixing temperature is required. Also, the faster the process speed, the shorter the time it takes for the recording sheet to pass through the fixing device 25, and therefore a shorter heating and fixing process is required, and therefore a higher fixing temperature is required.

[0042] In this example, at the point where the type of recording sheet conveyed to the fixing device 25 changes from thin paper S1 to thick paper S2, the fixing temperature of the recording sheet (thin paper) S1 after fixing falls within the set temperature range for the recording sheet S1. The fixing temperature at this time is lower than the lower limit of the set temperature range for the next recording sheet (thick paper) S2 conveyed after fixing the recording sheet (thin paper) S1. Therefore, the main body control unit 110 temporarily stops the image forming operation (thus causing a waiting time for the fixing temperature to rise) and controls the heating means, such as the fixing heater 25d, so that the detection result of the temperature sensor 25h of the fixing device 25 exceeds the lower limit of the set temperature range for the recording sheet (thick paper) S2 (the temperature that allows paper feeding). Then, when the detection result of the temperature sensor 25h of the fixing device 25 exceeds the lower limit of the set temperature range for the recording sheet (thick paper) S2 (the temperature that allows paper feeding), the image forming operation is resumed, and the recording sheet S2 is fed from the pair of registration rollers 49, the image is transferred onto the recording sheet, and the recording sheet S2 is conveyed to the fixing device 25.

[0043] Table 1 below summarizes the relationship between the fixing temperature rise wait time and the potentially related factors of room temperature, current fixing temperature, heater usage, and cooling fan operation status. As shown in Table 1, the lower the room temperature, the longer it takes for the fixing temperature to rise, resulting in a longer fixing temperature rise wait time. Furthermore, the greater the difference between the current fixing temperature and the paper feed allowable temperature (the lower limit of the set temperature range for recording sheets (cardboard) S2), the longer it takes to raise the fixing temperature to the paper feed allowable temperature, resulting in a longer fixing temperature rise wait time. Furthermore, the more heaters whose heater usage status is lit, the fewer heaters can be added to increase the fixing temperature, resulting in a longer fixing temperature rise wait time. Furthermore, when the cooling fan operation status is ON, it is possible to increase the fixing temperature rise rate by turning the cooling fan OFF. However, when the cooling fan operation status is OFF, this is not possible, resulting in a longer fixing temperature rise wait time.

[0044] [Table 1]

[0045] On the other hand, at the point where the type of recording sheet conveyed to the fixing device 25 switches from thick paper S2 to thin paper S1, the fixing temperature of the recording sheet (thick paper) S2 after fixing falls within the set temperature range for the recording sheet S2. The fixing temperature at this time is higher than the upper limit of the set temperature range for the next recording sheet (thin paper) S1 conveyed after fixing the recording sheet (thick paper) S2. Therefore, the main body control unit 110 temporarily stops the image forming operation (thus creating a waiting time for the fixing temperature to drop) and turns off heating means such as the fixing heater 25d so that the detection result of the temperature sensor 25h of the fixing device 25 falls below the upper limit of the set temperature range for the recording sheet (thin paper) S1 (the paper feed permitted temperature). Also, if necessary, the main body control unit 110 operates a cooling means such as a cooling fan provided in the printer 100. Then, when the detection result of the temperature sensor 25h of the fixing device 25 falls below the upper limit value (paper feeding permitted temperature) of the set temperature range of the recording sheet (thin paper) S1, the image formation operation is resumed, the recording sheet S2 is sent out from the registration roller pair 49, the image is transferred, and the sheet is transported to the fixing device 25.

[0046] Table 2 below summarizes the relationship between the fixing temperature drop wait time and the potentially related factors of room temperature, current fixing temperature, heater usage status, and cooling fan operation status. As shown in Table 2, the higher the room temperature, the longer it is difficult for the fixing temperature to drop, so the fixing temperature drop wait time tends to be longer. Furthermore, the greater the difference between the current fixing temperature and the paper feed allowable temperature (the lower limit of the set temperature range for recording sheets (cardboard) S2), the longer it takes to drop the fixing temperature to the paper feed allowable temperature, and the longer the fixing temperature drop wait time tends to be. Furthermore, the fewer heaters whose heater usage status is lit, the fewer heaters can be turned off to lower the fixing temperature, and the longer the fixing temperature drop wait time tends to be. Furthermore, when the cooling fan operation status is ON, the fixing temperature can be lowered by turning on the cooling fan, so the fixing temperature drop wait time tends to be longer.

[0047] [Table 2]

[0048] FIG. 6 is a flowchart showing an example of print order rearrangement control in this embodiment. When a print job is input, the main body control unit 110 first determines whether the print job is double-sided printing (S1). If it is determined that the print job is not double-sided printing (No in S1), the main body control unit 110 turns off interleaf control and starts image formation operation (printing) (S5).

[0049] On the other hand, if it is determined that double-sided printing is being performed (Yes in S1), the main body control unit 110 then determines whether alternating paper feed is being performed, in which recording sheets S1 and S2 are fed alternately from two paper feed cassettes 44A and 44B (S2). At this time, if it is determined that alternating paper feed is not being performed (No in S2), the type of recording sheet being conveyed to the fixing device 25 does not change frequently, so there is little waiting time for changing the fixing temperature due to a change in the type of recording sheet being conveyed to the fixing device 25. Therefore, the main body control unit 110 starts normal image formation operation (printing) with interleaf control turned on (S6).

[0050] If it is determined that the paper feed is alternate (Yes in S2), the main body control unit 110 then acquires the detection result of the temperature sensor 25h of the fixing device 25 and determines whether the difference between the current fixing temperature and the set temperature of each of the types S1 and S2 of recording sheets used in the print job is equal to or greater than a predetermined threshold (S3). In the above example, it determines whether any of the difference between the current fixing temperature and the upper limit of the set temperature range for recording sheet S1, the difference between the current fixing temperature and the lower limit of the set temperature range for recording sheet S1, and the difference between the current fixing temperature and the lower limit of the set temperature range for recording sheet S2 is equal to or greater than a predetermined threshold.

[0051] Since the fixing temperature before the print job is a relatively low temperature close to the fixing standby temperature, if the above differences are both equal to or less than a predetermined threshold, it can be seen that the types of recording sheets S1 and S2 used in the print job do not include any recording sheet types for which the set fixing temperature is set high. In this case, the temperature range over which the fixing temperature can be controlled in the print job remains a relatively low temperature range for all types of recording sheets, and even if the set fixing temperature is changed when the type of recording sheet being conveyed to the fixing device 25 is switched, almost no waiting time occurs.

[0052] Therefore, in this embodiment, if the difference between the current fixing temperature and the set temperatures of the recording sheet types S1 and S2 used in the print job is less than a predetermined threshold (No in S3), the main body control unit 110 starts normal image formation operation (printing) with interleaf control turned on (S6).

[0053] Next, if the difference between the current fixing temperature and the set temperatures for the recording sheet types S1 and S2 used in the print job is greater than or equal to a predetermined threshold (Yes in S3), the main body control unit 110 determines whether the image formation time (printing time) will be shortened by interleaf control (S4).

[0054] Specifically, for example, when the interleaf control is turned on for the print job, the main body control unit 110 acquires the number of times that switching between the types S1 and S2 of recording sheets conveyed to the fixing device 25 occurs (the number of times that waiting times occur), and calculates a predicted value for each waiting time. The predicted value for each waiting time can be calculated based on the room temperature, the current fixing temperature, the heater usage status, the cooling fan operation status, etc., in accordance with the trends summarized in Tables 1 and 2 above. Then, the total waiting time when the interleaf control is turned on is calculated from the acquired number of times that switching occurs and the predicted value for each waiting time.

[0055] Similarly, when the print job is executed with interleaf control turned off, the main body control unit 110 acquires the number of times that switching between the types S1 and S2 of recording sheets conveyed to the fixing device 25 occurs (the number of times that waiting times occur), and calculates a predicted value for each waiting time. The predicted value for each waiting time can be the value calculated earlier. Then, from the acquired number of times that switching occurs and the predicted value for each waiting time, the total waiting time when interleaf control is turned off is calculated.

[0056] The main body control unit 110 also calculates the time-saving effect of interleaf control for the print job (the difference between the printing time when interleaf control is turned off and the printing time when interleaf control is turned on). If the difference between the total waiting time when interleaf control is turned on and the total waiting time when interleaf control is turned off exceeds the time-saving effect of interleaf control, it can be predicted that the expected end time of image formation in the printing order (specific image formation order) with interleaf control turned off will be earlier than the expected end time of image formation in the normal printing order with interleaf control turned on. In this case, the main body control unit 110 determines that the image formation time (printing time) will not be shortened by interleaf control (No in S4), turns off interleaf control, and starts the image formation operation (printing) (S5).

[0057] Furthermore, if the difference between the total waiting time when interleaf control is turned on and the total waiting time when interleaf control is turned off is less than the time-saving effect of interleaf control, it can be predicted that the expected end time of image formation in a printing order (specific image formation order) with interleaf control turned off will be later than the expected end time of image formation in a normal printing order with interleaf control turned on. In this case, the main body control unit 110 determines that the image formation time (printing time) will be shortened by interleaf control (Yes in S4), and starts normal image formation operation (printing) with interleaf control turned on (S6).

[0058] In this embodiment, in order to shorten the image formation time with simple control in the alternating double-sided printing operation, the interleaf control is turned off to reduce the number of times the types S1 and S2 of recording sheets conveyed to the fixing device 25 are switched. However, if a printing order can be realized that can reduce the number of times of switching even more than the printing order when the interleaf control is turned off, that printing order may be adopted.

[0059] Furthermore, in this embodiment, a specific continuous image forming operation (alternate double-sided printing operation) in which recording sheets S1 and S2 are alternately fed from two paper feed cassettes 44A and 44B and images are continuously formed on both sides of each recording sheet has been described as an example, but the present invention is not limited to this. The present invention can be applied to any case in which images are continuously formed on multiple recording sheets that include a mixture of two or more types of recording sheets with different set temperatures. Therefore, the present invention is not limited to double-sided printing in which images are formed on both sides of each recording sheet, but can also be applied to single-sided printing in which images are formed on only one side of each recording sheet.

[0060] An example of single-sided printing in which an image is formed on only one side of each recording sheet will be described with reference to FIGS. 7 is an explanatory diagram showing the normal printing order during single-sided printing, in which two recording sheets S1 are fed from the first paper feed cassette 44A and then one recording sheet S2 is fed from the second paper feed cassette 44B, and this is repeated twice to form images consecutively on one side of six recording sheets. In the normal printing order shown in FIG. 7, printing (transfer and fixing) is performed in order starting with the first sheet, and the sheets are ejected directly to the outside of the printer in order starting with the first sheet. In this case, as shown in FIG. 7, the types of recording sheets S1 and S2 conveyed to the fixing device 25 are switched three times, so the image formation time until the single-sided printing operation for the six recording sheets is completed is extended by the time equivalent to the waiting time for three times.

[0061] FIG. 8 is an explanatory diagram showing the printing order (specific image formation order) when the above-described single-sided printing operation is performed in this embodiment. In this embodiment, as shown in Fig. 8, the first and second recording sheets S1 of the same type are printed (transferred and fixed) in order and then discharged to the outside of the machine. Following this, the third recording sheet S2 of a different type is printed and then discharged to the outside of the machine.

[0062] After printing on the third recording sheet S2, a sixth recording sheet S2 is fed from the second paper feed cassette 44B, and printing is performed on the sixth recording sheet S2, which is the same type as the third recording sheet S2. The sixth recording sheet S2 is then fed to the re-feeding device 28. At this time, the sixth recording sheet S2 is not switched back by the switchback section 28b of the re-feeding device 28, but is sent directly from the introduction path 28a to the reversing feed path 28c and kept waiting. As a result, when the sixth recording sheet S2 is re-fed from the reversing feed path 28c to the paper feed path 48, the recording sheet S2 is re-fed to the paper feed path 48 without being reversed.

[0063] After printing on the sixth recording sheet S2, the fourth and fifth recording sheets S1 are printed in order and then discharged outside the apparatus. Following the fifth recording sheet S1, the sixth recording sheet S2, which had been waiting in the re-feeding device 28, is re-sent from the reversing feeding path 28c to the paper feeding path 48, where it passes through the fixing device 25 without being printed on and is discharged outside the apparatus.

[0064] According to the printing order (specific image formation order) of this embodiment shown in Fig. 8, the types S1 and S2 of the recording sheets to be conveyed to the fixing device 25 only need to be switched twice. Therefore, while the normal printing order shown in Fig. 7 requires three waiting times, the printing order of this embodiment shown in Fig. 8 requires only two waiting times, thereby reducing the image formation time.

[0065] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is an image forming apparatus (e.g., printer 100, copier 500) equipped with a fixing means (e.g., fixing device 25) that performs a heat fixing process on a recording sheet S carrying an image at a fixing temperature that is controlled to a set temperature according to the type of the recording sheet, and when performing a specific continuous image forming operation (e.g., alternating double-sided printing operation) in which images are formed on multiple recording sheets that are a mixture of two or more types of recording sheets S1, S2 having different set temperatures, the image forming apparatus is characterized by having a control means (e.g., main body control unit 110) that performs the specific image forming operation in a specific image formation order (e.g., the order when interleaf control is turned off) in which the heat fixing process is performed on the same type of recording sheets in succession more frequently than in the normal image formation order during a normal continuous image forming operation (e.g., operation with interleaf control turned on). In an image forming apparatus that changes the set fixing temperature depending on the type of recording sheet, a specific continuous image forming operation may be performed to form images on multiple recording sheets containing two or more types of recording sheets, depending on the user's usage. Furthermore, such a specific continuous image forming operation may be performed to test the image forming apparatus. When performing such a specific continuous image forming operation, the type of recording sheets being sequentially conveyed to the fixing unit may be switched to a different type. In this case, the set fixing temperature must be changed and the new set fixing temperature must be set to the new set temperature before the new type of recording sheet is conveyed to the fixing unit. Therefore, a waiting time is generated to temporarily suspend the image forming operation to prevent the new type of recording sheet from being conveyed to the fixing unit before the new set fixing temperature is reached. This waiting time may occur each time the type of recording sheet being sequentially conveyed to the fixing unit is switched to a different type. Therefore, the more frequently the type of recording sheet being sequentially conveyed to the fixing unit is switched to a different type, the longer the image formation time. Therefore, in this embodiment, when the above-mentioned specific image forming operation is performed, continuous image formation is performed in a specific image forming order in which the heat fixing process is performed consecutively on the same type of recording sheets more frequently than in the normal image forming order. The normal image forming order here refers to the image forming order during a normal continuous image forming operation, such as when images are continuously formed on multiple recording sheets of the same type. According to this embodiment, when the above-mentioned specific image forming operation is performed, the number of times the type of recording sheets sequentially conveyed to the fixing unit is switched to a different type is reduced compared to the normal image forming order, thereby reducing the number of times the above-mentioned waiting time occurs. Therefore, the image formation time until image formation by the specific continuous image forming operation is completed can be shortened compared to when the specific continuous image forming operation is performed in the normal image forming order.

[0066] [Second mode] The second aspect is characterized in that, in the first aspect, the specific continuous image forming operation includes a continuous double-sided image forming operation that forms images on both sides of a plurality of recording sheets that are a mixture of two or more types of recording sheets, and the normal continuous image forming operation includes a continuous double-sided image forming operation that forms images on both sides of a plurality of recording sheets by interleaf control. According to this, when performing continuous double-sided image formation operations, the image formation time can be shortened by interleaf control in normal continuous image formation operations, and in specific continuous image formation operations, the number of waiting times can be reduced compared to when interleaf control is performed, thereby shortening the image formation time.

[0067] [Third aspect] The third aspect is the second aspect, characterized in that the specific image formation order is an image formation order in which image formation is performed successively on each side of a recording sheet, and then image formation is performed successively on each side of the next recording sheet. This allows a specific continuous image forming operation to be realized by a simple control of turning off the interleaf control.

[0068] [Fourth aspect] A fourth aspect is characterized in that, in the second or third aspect, the continuous double-sided image forming operation forms images on a plurality of recording sheets that are alternately fed with two types of recording sheets. According to this, in alternating printing in which images are formed on multiple recording sheets by alternately feeding two types of recording sheets, in certain continuous image formation operations, the number of waiting times can be reduced and the image formation time can be shortened compared to when interleaf control is implemented.

[0069] [Fifth mode] A fifth aspect is characterized in that, in any of the first to fourth aspects, when the control means executes the specific consecutive image forming operation, if the expected end time of image formation in the specific image forming order is later than the expected end time of image formation in the normal image forming order, the control means executes the specific image forming operation in the normal image forming order. In this case, by using a specific image formation order, it is possible to avoid a situation in which the image formation end time is delayed. [Explanation of symbols]

[0070] 10: Transfer belt 11: Transfer roller 21: Optical writing device 25: Fixing device 25a: Fuser roller 25d: Fixing heater 25e: Pressure roller 25f: Pressure heater 25g: Pressure roller fan 25h: Temperature sensor 28:Retransmission device 28a:Introduction path 28b: Switchback section 28c: Reverse feed path 40: Photoreceptor 44A to 44D: Paper cassette 48:Paper feed path 49: Registration roller pair 56: Discharge roller pair 59: Drum cleaning device 60: Charging device 61: Developing device 100: Printer 101: Imaging section 102:Paper feed section 103: Sheet transport unit 110: Main body control unit 111: Memory 112: Image processing unit 130: Operation display section 200: Scanner 500: Copier [Prior art documents] [Patent documents]

[0071] [Patent Document 1] Patent No. 5454254

Claims

1. An image forming apparatus including a fixing unit that performs a heat fixing process on a recording sheet carrying an image at a fixing temperature that is controlled to a set temperature according to the type of the recording sheet, a control means for executing a specific continuous image forming operation in which images are formed on a plurality of recording sheets including two or more types of recording sheets having different set temperatures, in a specific image forming order in which the frequency of successively performing the heat fixing process on the same type of recording sheets is higher than in a normal image forming order during a normal continuous image forming operation; the specific continuous image forming operation includes a continuous double-sided image forming operation for forming images on both sides of a plurality of recording sheets including two or more types of recording sheets mixed together, the normal continuous image forming operation includes a continuous double-sided image forming operation for forming images on both sides of a plurality of recording sheets by interleaf control, The image forming apparatus is characterized in that the continuous double-sided image forming operation in the specific continuous image forming operation is a operation in which images are formed on a plurality of recording sheets obtained by alternately feeding two types of recording sheets.

2. In the image forming apparatus according to claim 1, The image forming apparatus is characterized in that, when executing the specific consecutive image forming operation, if the expected image formation end time in the specific image forming order is later than the expected image formation end time in the normal image forming order, the control means executes the specific consecutive image forming operation in the normal image forming order.

3. An image forming apparatus having a fixing means for performing a heat fixing process on a recording sheet bearing an image at a fixing temperature controlled to a set temperature according to the type of the recording sheet, a control means for executing a specific continuous image forming operation in which images are formed on a plurality of recording sheets including two or more types of recording sheets having different set temperatures, in a specific image forming order in which the frequency of successively performing the heat fixing process on the same type of recording sheets is higher than in a normal image forming order during a normal continuous image forming operation; The image forming apparatus is characterized in that, when executing the specific consecutive image forming operation, if the expected image formation end time in the specific image forming order is later than the expected image formation end time in the normal image forming order, the control means executes the specific consecutive image forming operation in the normal image forming order.

4. 4. The image forming apparatus according to claim 3, the specific continuous image forming operation includes a continuous double-sided image forming operation for forming images on both sides of a plurality of recording sheets including two or more types of recording sheets mixed together, The image forming apparatus is characterized in that the normal continuous image forming operation includes a continuous double-sided image forming operation in which images are formed on both sides of a plurality of recording sheets by interleaf control.

5. 5. The image forming apparatus according to claim 1, 2 or 4, The image forming apparatus is characterized in that the specific image forming sequence is an image forming sequence in which image formation is performed successively on each side of a recording sheet, and then image formation is performed successively on each side of the next recording sheet.

6. 5. The image forming apparatus according to claim 4, The image forming apparatus is characterized in that the continuous double-sided image forming operation in the specific continuous image forming operation is a operation in which images are formed on a plurality of recording sheets obtained by alternately feeding two types of recording sheets.

Images