Patent Grant
8948682
1. Field of the Invention
The present invention relates to an image forming apparatus, and specifically relates to an image forming apparatus that prevents image quality deterioration caused by condensation of water vapor generated from sheets when fixing a toner image thereto.
2. Description of the Related Art
Conventionally, in electrographic image forming apparatuses such as copiers, printers and facsimile machines, when forming an image on a sheet, first, a toner image formed in an image forming unit is transferred to a sheet. Subsequently, the toner image is fixed to the sheet by means of heating and pressurization using a fixing unit, thereby forming an image on the sheet.
Example of such image forming apparatus include an image forming apparatus including a re-transport unit for reversing a sheet with a toner image fixed to one side thereof and transporting the sheet to an image forming unit again. In two-side printing for forming an image on two sides of a sheet, a sheet with a toner image fixed to one side thereof is reversed by the re-transport unit, and is transported again to the image forming unit, thereby forming an image on the back side of the sheet.
Examples of the fixing unit include a thermal pressure fixing-type fixing unit including a fixing roller and a pressure roller, the fixing unit applying heat and pressure to a sheet simultaneously by means of the fixing roller and the pressure roller, thereby fixing a toner image to the sheet. In the case of such fixing unit, when fixing a toner image to a sheet, a considerable amount of heat is applied from the fixing roller, which heats the sheet, to the sheet. Accordingly, in the toner image fixing, moisture contained in the sheet evaporates, generating water vapor.
When water vapor is generated, if the temperature of the image forming apparatus body subsequently becomes relatively low, water vapor may condense in a sheet transport path. Upon occurrence of condensation, when a sheet passes through the sheet transport path, droplets adhere to the sheet. In order to prevent condensation, conventional image forming apparatuses include a fixing unit with an enhanced air tightness to absorb water vapor within the fixing unit. Another example of a conventional image forming apparatus is configured to discharge generated water vapor to the outside of the apparatus via a louver provided at an upper portion of the image forming apparatus (see Japanese Patent Application Laid-Open No. H08-254938).
In conventional image forming apparatuses, with an increase in speed in recent years, an amount of heat transferred from the fixing roller to a sheet is also increased, resulting in an increase in an amount of generated water vapor itself. The conventional configuration that enhances the air tightness of the fixing unit, thereby absorbing water vapor within the fixing unit has a limit on the water vapor absorption, causing difficulty in water vapor collection.
When water vapor is insufficiently discharged and collected, if the temperature of the image forming apparatus body becomes relatively low, for example, in two-side printing, water vapor condenses on a guide member that guides a reversed sheet. In recent years, for downsizing and enhanced two-side printing productivity of the image forming apparatuses, a switchback roller pair provided in a re-transport unit to reverse and transport a sheet is arranged in the vicinity of the fixing unit in some cases. In this case, water vapor condenses also on the surface of the switchback roller pair.
Upon occurrence of condensation, for example, when two-side printing is performed in a condition in which the image forming apparatus body is not sufficiently warmed up, like in the case of a cold start, water vapor condensing on the guide member and the switchback roller pair adheres to a sheet, which is to be reversed and transported, in the form of droplets. When droplets adhere to a sheet, the electric resistance value of the part of the sheet surface to which the droplets adhere is lowered compared to the surrounding part of the sheet surface to which no droplets adhere, which may cause image dropouts during transfer of a toner image in a transfer unit.
The present invention has been made in view of the above-mentioned existing circumstances, and provides an image forming apparatus capable of preventing image quality deterioration caused by condensation in two-side image formation.
An image forming apparatus according to the present invention includes: a sheet feed unit that feeds a sheet to an image forming unit forming a toner image on the sheet; a fixing unit that fixes the toner image to the sheet; a re-transport unit that reverses the sheet with the toner image and re-transports the sheet to the image forming unit when forming an image on both sides of the sheet; and a control unit that controls a sheet feed operation of the sheet feed unit, wherein, when starting duplex image formation for forming an image on both sides of a sheet after an one-side image formation for forming an image on one side of a sheet, as the number of the sheet with the image formed on the one side thereof in the one-side image formation is greater, the control unit retards a timing to cause the sheet feed unit to start the sheet feed operation.
Also, an image forming apparatus according to the present invention includes: a sheet feed unit that feeds a sheet to an image forming unit forming a toner image on the sheet; a fixing unit that fixes the toner image to the sheet; a re-transport unit that reverses the sheet with the toner image and re-transports the sheet to the image forming unit; and a control unit that controls a sheet transport operation of the re-transport unit, wherein, when performing duplex image formation for forming an image on both sides of a sheet after an one-side image formation for forming an image on one side of a sheet, the control unit controls the re-transport unit so as to temporarily halt the sheet on the both sides of which the toner image is to be formed in the re-transport unit or to decrease a sheet transport speed.
The present invention enables prevention of image quality deterioration caused by condensation in two-side image formation.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Embodiments of the present invention will be described in detail below with reference to the drawings.
The image forming unit 1B includes scanner units (3YM and 3CK), and four process cartridges 2 (2Y, 2M, 2C and 2K) for forming toner images of four colors: yellow (Y), magenta (M), cyan (C) and black (Bk). The image forming unit 1B also includes an intermediate transfer unit 4 arranged above the process cartridges 2. Each process cartridge 2 includes a photosensitive drum, which is an image bearing member (not illustrated) for forming a toner image.
The intermediate transfer unit 4 includes an intermediate transfer belt 4a wrapping around a drive roller 5, a tension roller 5a and a driven roller 5b. The intermediate transfer unit 4 includes primary transfer rollers 4b provided inside the intermediate transfer belt 4a, which are brought into contact with the intermediate transfer belt 4a at positions facing the photosensitive drums. The intermediate transfer belt 4a is constructed of a film-like member and is arranged so as to be in contact with the respective photosensitive drums, and rotates in the direction of the arrow (counterclockwise) by means of the drive roller 5 that is driven by a drive unit (not illustrated).
A positive transfer bias is applied to the intermediate transfer belt 4a by means of the primary transfer rollers 4b. Negatively-charged toner images for respective colors on the photosensitive drums are sequentially transferred to the intermediate transfer belt 4a in a superimposed manner. Consequently, a full color image is formed on the intermediate transfer belt. At a position facing the drive roller 5 in the intermediate transfer unit 4, a secondary transfer roller 9, which is included in a secondary transfer unit 30 that transfers the full color image formed on the intermediate transfer belt to a sheet S, is provided. The sheet transport unit 1C transports a sheet to the secondary transfer unit 30.
The fixing unit 11 is arranged above the secondary transfer roller 9, and a delivery roller pair 12 and a switching member 13 are arranged above the fixing unit 11. The delivery roller pair 12, which is a delivery unit, delivers a sheet with an image fixed thereto to the outside of the apparatus. A re-transport unit 1D includes a switchback roller pair 14 as sheet reverse and transport rollers (reverse and transport unit) capable of normal and reverse rotation. The switchback roller pair 14 is arranged above the fixing unit. The switching member 13, which is swingably supported, selectively guides a sheet with an image fixed thereto by the fixing unit 11 to one of the delivery roller pair 12 and the switchback roller pair 14. A sheet reversed by the re-transport unit 1D is transported by the sheet transport unit 1C to the secondary transfer unit 30 again.
In
An image forming operation of the full color laser beam printer 1 configured as described above will be described. Upon start of an image forming operation, first, the scanner unit 3 irradiates laser light (not illustrated) based on image information from, such as a personal computer (not illustrated) to sequentially expose the surfaces of the photosensitive drums, which are uniformly charged to have a predetermined polarity and potential, thereby forming an electrostatic latent image on each of the photosensitive drums. Subsequently, the electrostatic latent images are developed using toner and visualized.
For example, first, laser light from the scanner unit 3YM according to an image signal for a yellow color component is applied to the photosensitive drum in the process cartridge 2Y for yellow, thereby forming an electrostatic latent image for yellow on the photosensitive drum. The electrostatic latent image for yellow is developed using yellow toner from a developer unit and thereby visualized as a yellow toner image. Subsequently, the yellow toner image reaches a primary transfer unit where the photosensitive drum and the intermediate transfer belt 4a are in contact with each other, with rotation of the photosensitive drum. The toner image on the photosensitive drum is transferred to the intermediate transfer belt 4a by means of the primary transfer bias applied to the primary transfer roller 4b.
Next, a part of the intermediate transfer belt 4a, which bears the yellow toner image, is moved. Before this movement, a magenta toner image is formed on the photosensitive drum of the process cartridge 2M for magenta with a method similar to the above. The magenta toner image is transferred to the intermediate transfer belt 4a on the yellow toner image. Similarly, as the intermediate transfer belt 4a moves, a cyan toner image and a black toner image are transferred so as to superimpose the yellow and magenta toner images in the primary transfer unit. Consequently, a full color toner image is formed on the intermediate transfer belt.
In parallel to the toner image forming operation, the feed cassette unit 5 sends out sheets S set therein by means of a pickup roller 6. Subsequently, the sheets S are separated one by one by means of a feed/retard roller 7 and transported to a registration roller pair 8. At that moment, the registration roller pair 8 halts, and a sheet S is brought into contact with a nip portion of the registration roller pair 8 in a halt state to form a loop of the sheet S, thereby correcting skewing of the sheet S. In the present embodiment, the feed cassette unit 5 and the registration roller pair 8 are included in a sheet feed unit that feeds a sheet S to the image forming unit 1B.
After the correction of the skewing of the sheet S, the registration roller pair 8 is driven at a timing for aligning the position of the full color toner image on the intermediate transfer belt and the position of the sheet S in the secondary transfer unit 30. The sheet S is transported to the secondary transfer unit 30. In the secondary transfer unit 30, the full color toner image is transferred to the sheet at a time by means of a secondary transfer bias applied to the secondary transfer roller 9. The waste toner, which was not secondary transferred to the sheet S, is collected by a cleaning unit 10, which is provided on the intermediate transfer belt.
The sheet S with the full color toner image transferred thereto is transported to the fixing unit 11. In the fixing unit 11, toner of the respective colors are fused and mixed upon receipt of heat and pressure, and fixed to the sheet S as a full color image.
In the case of one-side printing for forming an image only on one side of a sheet (one-side image formation mode), the sheet S with an image fixed thereto is delivered to an delivery tray 17 by means of an delivery roller pair 12 provided on the downstream side in a sheet transport direction of the fixing unit 11. In the case of one-side printing, the sheet with an image fixed thereto by the fixing unit 11 is delivered to the outside of the apparatus without passing through the re-transport unit 1D.
An operation for the case of two-side printing (duplex image formation mode) for forming an image on both sides of a sheet S will be described. As illustrated in
After the sheet S is transported by a certain amount and the trailing edge of the sheet S passes by the switching member 13, the switchback roller pair 14 is halted temporarily, and the switching member 13 is rotated in the direction of arrow A. Subsequently, the switchback roller pair 14 is reversely rotated, thereby the sheet S proceeding onto the re-transport path R1 as illustrated in
When fixing the toner image in the fixing unit 11, moisture contained in the sheet S evaporates owing to heating by the fixing unit 11 and turns into water vapor. As illustrated in
Since the amount of the water vapor W is proportional to the number of sheets to be subjected to one-side printing, as the number of sheets subjected to one-side printing increases, the amount of condensation on the switchback roller pair 14, etc., increases. The number of sheets subjected to one-side printing increases, the printer body 1A is warmed up more. When the printer body 1A is warmed up, the water vapor condensation evaporates. In other words, even though the water vapor temporarily condenses, the condensation on the switchback roller pair 14, etc., disappears as the printer body 1A is warmed up.
In the present embodiment, when forming an image on the back side of a sheet after one-side printing in a state in which the water vapor W condenses on the switchback roller pair 14, etc., the sheets are made to wait according to the number of sheets in the one-side printing, and then, two-side printing is started. More specifically, a timing for starting a sheet feed operation by a sheet feed unit 5A including the feed cassette unit 5 and the registration roller pair 8 illustrated in the later-described
In the present embodiment, a sheet feed operation refers to an operation for sending out one sheet from a stack of sheets using the pickup roller 6 and the feed/retard roller 7 of the feed cassette unit 5. In the present embodiment, a timing for sending out one sheet from a stack of sheets using the pickup roller 6 and the feed/retard roller 7 of the feed cassette unit 5 is changed according to the number of sheets in one-side printing.
The following mode for changing a timing for a sheet feed operation to feed a sheet to the image forming unit 1B by means of the sheet feed unit 5A may also be employed. A sheet sent out of the feed cassette unit 5 is halted temporarily before the image forming unit 1B, for example, at the registration roller pair 8. Then, the timing for sending out the sheet toward the image forming unit 1B using the registration roller pair 8 is changed according to the number of sheets in one-side printing.
A sequence, according to an amount of generated water vapor W, for performing an image forming operation for two-side printing after the end of the image forming operation for one-side printing in the present embodiment will be described with reference to
When the printer body 1A accepts a print job, a control unit 50, which controls, e.g., an image forming operation of the image forming unit 1B and driving of the feed cassette unit 5, which is illustrated in the later-described
If the accepted print job is a two-side print job (Yes in S100), the control unit 50 determines whether or not the counter value C is less than or equal to a threshold value C0, which is a pre-set criterion for starting a two-side print job, that is, C≦C0 (S102). Then, if the counter value C is less than or equal to the threshold value C0 (Yes in S102), a two-side print job is carried out. Subsequently, after the end of the two-side print job, the control unit 50 resets the counter value C, that is, sets the counter value C to 0 (S103), and the printing is terminated.
If the counter value C exceeds the threshold value C0 (No in S102), this means that water vapor W condenses on the switchback roller pair 14, etc. Accordingly, the control unit 50 deducts a value ΔC2, which is a condensation evaporation amount per unit time, from the counter value C without carrying out the two-side print job. Consequently, the counter value C has a value of C−ΔC2 (S104).
In the present embodiment, simultaneously with power-on of the printer body 1A, the control unit 50 starts a timer 52, which illustrated in the later-described
The sequence illustrated in
When one-side printing is performed, the water vapor generation amount ΔC1 per unit sheet count is added to the counter value C, which is set to 0, for printing on every sheet. The condensation evaporation amount ΔC2 per unit time is deducted from the counter value C according to the passage of time immediately after the printing. When one-side printing is consecutively performed for a plurality of sheets, the relationship between the counter value C and the threshold value C0 is C>C0 as illustrated in
Even though a two-side print job is accepted in this state, the control unit 50 does not start the two-side print job until the counter value C is less than or equal to the threshold value C0, which is the criterion for starting a two-side print job, that is, the relationship is C≦C0. In other words, when C>C0, even though a two-side print job is accepted, the control unit 50 does not start two-side printing until predetermined wait time for becoming the relationship C<C0 has passed. More specifically, the control unit 50 does not start a sheet feed operation by means of the feed cassette unit 5. In a state in which water vapor condenses, predetermined wait time is provided to retard a timing for the feed cassette unit 5 to start feeding a sheet, compared to that in the case of the preceding one-side print job.
In one-side image formation, a sheet feed operation is started at predetermined time intervals (at every predetermined timing). In one-side image formation, an interval between sheets is a predetermined interval. When two-side image formation for forming an image on both sides of a sheet is started in a state in which water vapor condenses after the end of one-side image formation, an operation will be performed as described below. In other words, after the end of the last sheet feed operation for the one-side image formation, a sheet feed operation is started after the passage of predetermined wait time so as to be later than the predetermined timing. The predetermined wait time is optimum time according to the number of sheets in the last one-side print job.
When the number of sheets in one-side print job is greater than or equal to N0, wait time T increases according to the number of sheets in the one-side print job. In other words, when the number of sheets in the one-side print job is N (N0≦N<N1), wait time T is time according to the number of sheets in the one-side print job. Meanwhile, as the number of sheets in a one-side print job increases exceeding N0, the condensation amount also increases. When the number of sheets in a one-side print job is greater than or equal to N1, the water vapor condensation amount ΔC1 and the amount ΔC2 of water vapor discharged to the outside of the apparatus and evaporated condensation reach a state of equilibrium, and therefore, it is expected that the condensation amount becomes constant. In other words, if the number of sheets in one-side print job is greater than or equal to N1, the wait time can be made to be constantly T1. In the present embodiment, for a one-side print job for a number of sheets greater than or equal to a certain number, fixed wait time T1 is provided regardless of the number of sheets in the print job.
For two-side printing, in a state in which water vapor condenses, the wait time T is provided to retard a timing for starting feeding a sheet compared to that in the preceding one-side print job. Thereby, droplets can be prevented from adhering on the surface of the sheet S.
As described above, in the present embodiment, in two-side printing, as the number of sheets with a toner image formed on one side thereof in the one-side print job increases, the timing for starting a sheet feed operation is retarded compared to the timing for forming a toner image on one side of a sheet. In other words, when starting a two-side print job after the end of a one-side print job, if the number of sheets with an image formed on one side thereof in the one-side print job is greater than or equal to a predetermined number, a sheet feed operation is started at a later timing after the end of the last sheet feed operation in the one-side print job. As the number of sheets in the one-side print job increases, an interval between the last sheet in the one-side print job and a first sheet in the two-side print job is increased.
Consequently, in two-side printing after one-side printing, two-side printing can be started after passage of optimum wait time according to the number of sheets in the one-side printing from the state in which water vapor W condenses on the switchback roller pair 14 and the switching member 13. As a result, droplets can be prevented from adhering to the surface of the sheet S, and occurrence of image dropouts due to a decrease in resistance of the sheet surface caused by the droplet adherence can be prevented during toner image transfer in the secondary transfer unit 30.
A second embodiment of the present invention will be described.
In the present embodiment, optimum wait time is set according to the environment in which the image forming apparatus body is installed. In the present embodiment, in order to detect the conditions of the environment in which the image forming apparatus body is installed, a temperature sensor 53 and a humidity sensor 54, which is illustrated in
Low temperature/low humidity, ordinary temperature/low humidity and high temperature/low humidity: C0=C0L
Low temperature/ordinary humidity, ordinary temperature/ordinary humidity and high temperature/ordinary humidity: C0=C0N
Low temperature/high humidity, ordinary temperature/high humidity and high temperature/high humidity: C0=C0H
A sequence, according to an amount of generated water vapor W, when performing an image forming operation for two-side printing after the end of an image forming operation for one-side printing in the present embodiment will be described with reference to
If the accepted print job is a two-side print job (Yes in S300), the control unit 50 determines the environment in which the image forming apparatus body is installed based on temperature information from the temperature sensor 53 and humidity information from the humidity sensor 54 (S302). Based on the determination, a threshold value C0 is set according to the environment from among the three threshold values C0 (S303, S304 and S305).
The control unit 50 determines whether or not the counter value C is less than or equal to a threshold value C0, which is a pre-set two-side print job start criterion, that is C≦C0 (S306). If the counter value C is less than or equal to the threshold value C0 (Yes in S306), a two-side print job is carried out. After the end of the two-side print job, the control unit 50 resets the counter value C, that is, sets the counter value C to 0 (S307), and the printing is terminated.
If the counter value C exceeds the threshold value C0, this means that water vapor W condenses on a switchback roller pair 14, etc. Accordingly, the control unit 50 deducts a condensation evaporation amount ΔC2 per unit time from the counter value C without performing the two-side print job. Accordingly the counter value C has a value of C−ΔC2 (S308).
Low temperature/low humidity, ordinary temperature/low humidity and high temperature/low humidity: N0=N0L
Low temperature/ordinary humidity, ordinary temperature/ordinary humidity and high temperature/ordinary humidity: N0=N0N
Low temperature/high humidity, ordinary temperature/high humidity and high temperature/high humidity: N0=N0F
In the present embodiment, the relationship between the sheet counts N0 in the respective environments is N0L>N0N>N0H. In the sequence, as an environment is a higher temperature/higher humidity, wait time occurs even in a one-side print job for a smaller sheet count. This is because a sheet S before printing generally holds an amount of moisture according to the environment in which the image forming apparatus body is installed and thus, as an environment is a higher humidity, a large amount of water vapor W is generated. In a space around the switchback roller pair 14 and the switching member 13 immediately after the end of one-side printing, the effect of the temperature of the fixing unit 11 is dominant, and the effect of the temperature of the environment in which the image forming apparatus body is installed is small. In a high temperature/high humidity environment, an amount of water vapor W generated from a sheet S is large, and therefore, the natural phenomenon that water vapor W adhering to the switchback roller pair 14 and the switching member 13 is hardly dried can be expected with high accuracy.
In the present embodiment, when the number of sheets in a one-side print job exceeds a certain number, the amount of water vapor condensation and the amount of water vapor discharged to the outside of the apparatus and condensation evaporation reach a state of equilibrium, and therefore, it is expected that the condensation amount becomes constant. Thus, for a one-side print job for a number of sheets that is greater than or equal to a certain sheet count, fixed wait time is set according to the respective environments in which the image forming apparatus body is installed as follows, not depending on the number of sheets in the print job.
Low temperature/low humidity, ordinary temperature/low humidity and high temperature/low humidity: T1=T1L
Low temperature/ordinary humidity, ordinary temperature/ordinary humidity and high temperature/ordinary humidity: T1=T1N
Low temperature/high humidity, ordinary temperature/high humidity and high temperature/high humidity: T1=T1H
In the present embodiment, the relationship between the wait times for the respective environments is T1H>T1N>T1L. This is because in a high temperature/high humidity environment, the amount of water vapor W generated from sheets S is large, requiring more wait time.
In the present embodiment, when two-side printing is performed in a state in which water vapor condenses, wait time T is provided and the timing for starting feeding sheets is retarded according to the environment in which the printer body 1 is installed compared to the timing in the preceding one-side print job. Consequently, in two-side printing after one-side printing, two-side printing can be started after passage of optimum wait time according to the number of sheet in the one-side printing and the environment in which the printer body 1 is installed, from the state in which water vapor W condenses on the switchback roller pair 14 and the switching member 13. As a result, image dropouts can be prevented from occurring due to a decrease in resistance of the surface of a sheet S caused by droplets adhering to the surface.
With digitalization and a multi-functionalization in recent years, in a case where, for example, an image reading unit is arranged at an upper portion of the printer body, the above-described water vapor discharge via a louver often becomes difficult because the image reading unit lies in the way. In the case of an apparatus of an inner-delivery type in which a sheet with an image formed thereon is delivered between the printer body and the image reading unit, it is effective to retard the timing for starting two-side printing like in the first and second embodiments in order to avoid the problem of droplets in two-side printing.
Also, in the case of an apparatus of an inner-delivery type in which a relay transport unit that transports a sheet with an image formed thereon to a sheet processing unit provided in a side portion of the printer body is provided in an in-body space, water vapor discharge via a louver becomes difficult due to the existence of the relay transport unit. Accordingly, the control to retard the timing for starting two-side printing like in the first and second embodiments in order to avoid the problem of droplets in two-side printing is effective also for a configuration including a relay transport unit.
A third embodiment of the present invention will be described.
In two-side printing, a sheet S with an image formed on one side thereof is reversed and transported by the switchback roller pair 14. In this time, droplets X adhering to the nip portion of the switchback roller pair and water vapor condensed on a lower surface of the reverse guide 23 adhere to the sheet S. The sheet S is transported in the direction indicated by arrow B illustrated in
In the present embodiment, as illustrated in
Subsequently, the sheet S is halted for a predetermined period of time, and the transport of the sheet S is resumed after the adhered droplets X are evaporated such that the adhered droplets X do not affect transfer to the back side of the sheet S. The sheet S is transported through a registration roller pair 8 to an image forming unit 1B and printing is performed on the back side. When the sheet S is transported to the image forming unit 1B, the droplets X adhering to the sheet S have been evaporated, enabling prevention of image defects caused by the droplets X.
As described above, in the present embodiment, when performing two-side printing, a sheet S is temporarily halted in the vicinity of the fixing unit 11 to evaporate droplets adhering to the sheet S. As a result, image droplets can be prevented from occurring as a result of a decrease in resistance of a sheet surface owing to droplets adhering to the sheet surface. For example, when the reverse guide 23 has a high temperature, a similar effect can also be provided by decreasing the sheet transport speed to transport the sheet at a low speed, without halting the sheet S in the vicinity of the fixing unit 11.
A fourth embodiment of the present invention will be described. In the present embodiment, as in the above-described third embodiment, in two-side printing, when a trailing edge of a sheet S comes to the vicinity of a fixing unit, the transport of the sheet S is halted to evaporate droplets adhering to the sheet S. The halt time is changed according to the number of sheets in immediately-preceding one-side printing.
For example, when two-side printing is performed after one-side consecutive printing on a small number of sheets, water vapor adhering to the switchback roller pair and the reverse guide 23 does not converge to form droplets. Accordingly, where the number of sheets in one-side consecutive printing is small, no image defects are caused even though the sheet S is not halted for a certain period of time in the vicinity of the fixing unit 11.
In the present embodiment, when a trailing edge of a sheet S with droplets X adhering thereto is positioned in the vicinity of the fixing unit 11, the above-described control unit 50, as illustrated in
In the present embodiment, as illustrated in
As described above, in the present embodiment, in two-side printing, sheet transport operation of the re-transport unit 1D is controlled so as to temporarily halt a sheet S in the vicinity of the fixing unit 11 for a period of time according to the number of sheets subjected to one-side printing to evaporate droplets adhering to the sheet S. Consequently, the droplets X adhering to the sheet S can be evaporated such that the droplets X do not affect transfer of a toner image to the back side of the sheet S, and thus, image defects caused by the droplets X can be prevented. Even with the configuration as described above, when two-side printing is consecutively performed after one-side printing, droplets X adhere to a first sheet S. Accordingly, second and onward sheets S are consecutively transported without being halted in the vicinity of the fixing unit 11.
A fifth embodiment of the present invention will be described.
In
In the present embodiment, the vents 11b and 24a form a communication portion that provides communication between the re-transport path R1 and the fixing unit 11. As a result of forming the vents 11b in the upper cover 11a and forming the vents 24a in the lower reverse guide 24, respectively, when a sheet S passes between the reverse guide 23 and the lower reverse guide 24, warm air from the fixing unit 11 is blown against the sheet S. Consequently, droplets adhering to the sheet S are partially evaporated.
In the present embodiment, also, the sheet S is temporarily halted in the vicinity of the fixing unit 11 for a period of time according to the number of sheets subjected to one-side printing, and then transport of the sheet S is resumed. When the sheet S is halted in the vicinity of the fixing unit 11, droplets adhering to the sheet have been partially evaporated, and thus, the halt time for the sheet can be shortened.
As described above, in the present embodiment, in two-side printing, droplets adhering to a sheet S are partially evaporated by warm air from the fixing unit 11. Subsequently, the sheet is temporarily halted in the vicinity of the fixing unit 11 to evaporate the droplets adhering to the sheet S. Consequently, the droplets X adhering to the sheet S can be evaporated such that the droplets X do not affect transfer of a toner image to the back side of the sheet S. Accordingly, image defects caused by the droplets X can be prevented. Even with the configuration as described above, when consecutive two-side printing is performed after one-side printing, droplets X adhere to a first sheet S. Therefore, second and onward sheets S are transported consecutively without being halted in the vicinity of the fixing unit 11.
A sixth embodiment of the present invention will be described.
The image forming unit 100A includes a process cartridge 109 including, e.g., a photosensitive drum 109a, a charge roller, a developer sleeve and a toner container (which are not illustrated), and a laser scanner 111 that exposes a surface of the photosensitive drum 109a to form an electrostatic latent image on the photosensitive drum. The sheet feed unit 100B includes a feed cassette 102 that stacks sheets S therein, and a pickup roller 103 that feeds the sheets S in the feed cassette one by one. The transfer unit 100C includes the photosensitive drum 109a, and a transfer roller 110 that is brought into pressure-contact with the photosensitive drum 109a to form a transfer nip and transfers a toner image on the photosensitive drum 109a to the sheet when a sheet passes by the transfer nip.
An image forming operation in the monochrome printer 100 will be described. Upon input of image information from an external information apparatus such as a personal computer, laser light is emitted from the laser scanner 111 based on the input information. The laser light is irradiated to the photosensitive drum 109a that rotates clockwise. Consequently, an electrostatic latent image is formed on the photosensitive drum 109a uniformly charged at a predetermined polarity and potential by the charge roller (not illustrated). With rotation of the developer sleeve (not illustrated), adequately-charged toner is supplied onto the photosensitive drum 109a and adheres to the electrostatic latent image. Consequently, the electrostatic latent image is developed and visualized as a toner image.
In parallel to the toner image formation operation, sheets S stacked in the feed cassette 102 are sequentially sent out by a pickup roller 103 one by one, stating from an uppermost one. Subsequently, the sheets S are separated one by one by a retard roller pair 104, and then transported to a registration roller pair 105 whose rotation has been halted. A forward edge of a sheet S that has reached the registration roller pair 105 hits a nip of a registration roller pair 105. Subsequently, the sheet S is transported until the sheet S forms a predetermined loop and is subjected to skewing correction.
The sheet S subjected to skewing correction by the registration roller 105 is transported to the transfer unit 100C by the registration roller pair 105 in line with a timing for an image to be drawn out, while the size of the sheet S is determined by sheet size detecting unit 121. A toner image formed on a photosensitive drum in the transfer unit 100C is transferred to a predetermined position of the sheet by a transfer roller 110.
The sheet S to which the toner image has been transferred is transported to the fixing unit 114. In the fixing unit 114, the unfixed toner image is heated and pressurized to be fixed to the sheet surface. After the fixing of the toner image, in the case of one-side printing, the sheet S is transported from the fixing unit 114 by a triple roller pair 125 and a switchback roller pair 118. Subsequently, the sheet is delivered onto a delivery tray 117 by a delivery roller 116, with the printed side down.
The monochrome printer 100 can form an image on both sides of a sheet. When performing two-side printing on a sheet S, as illustrated in
The sheet S transported into the re-feed transport path 119, as illustrated in
When one-side printing is consecutively performed in the monochrome printer 100 with the above-described configuration, a large amount of water vapor adheres to a reverse guide 123 and condenses there, thereby forming droplets. After one-side consecutive printing, if two-side printing is performed before the droplets are evaporated, the droplets adhering to the reverse guide 123 adhere to the sheet S that has been transported there. A part of a sheet S to which droplets particularly often adhere is a part around an forward edge of the sheet S which is reversed and transported in the direction of arrow D by the switchback roller pair 118 in two-side printing.
The reversed and transported sheet S is transported to the re-feed transport path 119. Regardless of the sheet size, when the forward edge of the sheet S with droplets adhering thereto comes to a position in a vicinity L of the fixing 114, as illustrated in
An image forming apparatus having a configuration that is different from those in the above-described first to fifth embodiments also can prevent image defects caused by droplets, by temporarily halting a sheet in the vicinity of the fixing unit 114 in two-side printing, thereby evaporating droplets adhering to the sheet S. A similar effect can also be provided by transporting a sheet S at a low speed in the vicinity L of the fixing 114.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2009-251391, filed Oct. 30, 2009, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-251391 | Oct 2009 | JP | national |
This is a divisional of U.S. patent application Ser. No. 12/905,198, filed Oct. 15, 2010, pending.
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| Number | Date | Country | |
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| 20130195532 A1 | Aug 2013 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12905198 | Oct 2010 | US |
| Child | 13826026 | US |
