Image forming apparatus

Abstract

To appropriately perform suppressing of discharge of small liquid droplets so as not to deteriorate image quality as much as possible.SOLUTION: In a borderless process, on a suppression region including the outside of a sheet P, either a process of reliably suppressing use of medium ink droplets and small ink droplets or a suppression relaxation process of relaxing degree of suppression of the use of the medium ink droplets and the small ink droplets in the suppression region is performed. Switching between these processes is performed based on at least one of the total number of used sheets P and the total usage time of a printer 100.SELECTED DRAWING: Figure 7

Description

Technical Field

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

Background Art

[0002] Conventionally, there has been an image forming apparatus that discharges droplets such as ink droplets on both the inside and outside of a medium. The printer according to Patent Document 1 is an example of this, and as borderless printing, ink droplets are also discharged outside the medium.

[0003] In such an image forming apparatus, there may be a mist in which minute droplets float inside the apparatus. When the mist occurs repeatedly, there is a high possibility of adverse effects due to the inside of the apparatus being soiled with droplets.

[0004] The mist is likely to occur when relatively small droplets are discharged outside the medium. Therefore, Patent Document 1 describes that by suppressing the discharge of small droplets to the outside of the medium, it is difficult to generate mist. When the discharge of small droplets is suppressed, the image quality formed on the medium may deteriorate.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The occurrence situation of adverse effects due to mist also varies depending on the usage mode of the apparatus. That is, depending on the usage mode of the apparatus, there may be a case where even if some mist occurs, the adverse effects due to it are relatively unlikely to occur. Therefore, suppressing the discharge of small droplets regardless of the usage mode of the apparatus as in Patent Document 1 leads to unnecessarily deteriorating the image quality.

[0007] An object of the present invention is to provide an image forming apparatus that appropriately suppresses the ejection of small droplets without significantly degrading the image quality.

Means for Solving the Problems

[0008] The image forming apparatus of the present invention includes a droplet ejection head capable of ejecting small droplets and large droplets respectively, a medium supply unit that supplies a medium to the droplet ejection head, and a control unit. The control unit forms an image indicated by image data on the medium by selectively ejecting the small droplets and the large droplets toward both the inside and the outside of the medium supplied by the medium supply unit, and controls the droplet ejection head and the medium supply unit to execute an image forming process. The image forming process includes a normal droplet determination process for determining which of the small droplets and the large droplets to use based on the image data, a suppression droplet determination process for determining which of the small droplets and the large droplets to use based on the image data so as to suppress the use of the small droplets compared to the normal droplet determination process, a normal ejection process for ejecting the droplets determined by the normal droplet determination process from the droplet ejection head, and first and second suppression ejection processes for ejecting the droplets determined by the suppression droplet determination process from the droplet ejection head. The control unit executes the normal ejection process for a normal region that is an area inside the medium excluding the end portion of the medium, and for a suppression region consisting of the end portion of the medium and the outside of the medium, when at least either the number of media used in the past in the image forming process or the usage period of the apparatus does not satisfy a predetermined condition, the first suppression ejection process is executed, and when at least either the number of media or the usage period satisfies the predetermined condition, a suppression relaxation process is executed. The suppression relaxation process includes at least either the second suppression ejection process and the normal ejection process. The second suppression ejection process is a process of ejecting the droplets determined by the suppression droplet determination process from the droplet ejection head so that the degree of suppressing the use of the small droplets in the suppression region is lower than that of the first suppression ejection process.

Effects of the Invention

[0009] For the normal area inside the end of the medium, normal ejection processing is performed. For the suppression area including the outside of the medium, in certain cases, first suppression ejection processing is performed. The first suppression ejection processing is a process that suppresses the use of small droplets compared to the normal ejection processing. As a result, the image quality of the image formed on the medium may deteriorate.

[0010] On the other hand, when at least one of the number of media used in the past and the usage period of the apparatus satisfies a predetermined condition, suppression relaxation processing is performed. The suppression relaxation processing includes at least one of second suppression ejection processing and normal ejection processing. The degree of suppressing the use of small droplets in the second suppression ejection processing is smaller than that in the first suppression ejection processing. The normal ejection processing does not suppress the use of small droplets. Therefore, when the suppression relaxation processing is performed, the image quality of the image formed on the medium does not deteriorate or the degree of deterioration is smaller than that in the first suppression ejection processing.

[0011] The number of media used in the past and the usage period of the apparatus are indicators of the degree of soiling inside the apparatus due to the generation of mist. When the soiling inside the apparatus has advanced, the first suppression ejection processing is necessary, but this deteriorates the quality of the image formed on the medium. On the other hand, when the soiling inside the apparatus has not advanced so much, it is appropriate to perform suppression relaxation processing to maintain the image quality. Therefore, by switching between the first suppression ejection processing and the suppression relaxation processing based on the number of media used in the past and the usage period of the apparatus, the processing for suppressing the use of small droplets is ensured while preventing unnecessary deterioration of the image quality.

Brief Description of the Drawings

[0012]

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BEST MODE FOR CARRYING OUT THE INVENTION

[0013] Hereinafter, the printer 100 according to a preferred embodiment of the present invention will be described with reference to the drawings. In the following description, the vertical direction is defined based on the state in which the printer 100 is installed for use (the state shown in FIG. 1), the longitudinal direction of the housing 100a is defined as the left - right direction in a plan view, and the short - side direction of the housing 100a is defined as the front - rear direction.

[0014] The printer 100 (corresponding to the "image forming apparatus" of the present invention) is an apparatus that forms an image on a sheet P. As shown in FIG. 1, the printer 100 mainly includes a housing 100a, a conveyance mechanism 2, a carriage 4, a head 5, a moving mechanism 6, a paper discharge tray 7, a cartridge mounting portion 8, and a control portion 9.

[0015] The conveyance mechanism 2 (the "medium supply unit" of the present invention) supplies the sheet P accommodated in the lower part of the housing 100a to the head 5 above it. The conveyance mechanism 2 has a pair of conveyance rollers 23, a pair of conveyance rollers 24, and a guide portion 25. The guide portion 25 guides the sheet P conveyed upward by rollers disposed in the lower part of the housing 100a forward.

[0016] The pairs of conveyance rollers 23 and 24 are arranged at intervals in the front - rear direction. The pair of conveyance rollers 23 is composed of a driving roller that rotates by the drive of a conveyance motor 23a (see FIG. 3) and a driven roller that rotates along with the driving roller. The pair of conveyance rollers 24 is composed of a driving roller that rotates by the drive of a conveyance motor 24a (see FIG. 3) and a driven roller that rotates along with the driving roller. The sheet P guided forward by the guide portion 25 is further conveyed forward by the pairs of conveyance rollers 23 and 24.

[0017] The head 5 (corresponding to the "ink droplet ejection head" of the present invention) includes a plurality of nozzles 51 formed on the lower surface and a driver IC 52 (see FIG. 3). When the driver IC 52 is driven under the control of the control unit 9, ink is ejected from the nozzles 51. The ink reaches an area on the paper P at the image recording position facing the lower surface of the head 5. While the paper P is being conveyed by the conveyance mechanism 2, the ink from the head 5 adheres to the paper P, thereby forming an image on the paper P.

[0018] The driver IC 52 can eject ink droplets from the nozzles 51 while changing the size of the ink droplets. In the present embodiment, small ink droplets, medium ink droplets, and large ink droplets having different droplet sizes are selectively used. The sizes of these droplets are in the relationship of small ink droplets < medium ink droplets < large ink droplets. Note that the small ink droplets and the medium ink droplets correspond to the "small droplets" of the present invention. Also, the large ink droplets correspond to the "large droplets" of the present invention.

[0019] The head 5 is mounted on the carriage 4. The carriage 4 is movable by a moving mechanism 6. The moving mechanism 6 has two guide rails 61, 62 and a carriage motor 63 (see FIG. 3). The two guide rails 61, 62 are arranged at intervals in the front-rear direction and each extends in the left-right direction. The carriage 4 is arranged so as to straddle the two guide rails 61, 62. The carriage 4 is connected to the carriage motor 63 via a belt or the like. When the carriage motor 63 is driven under the control of the control unit 9, the carriage 4 moves in the left-right direction along the guide rails 61, 62. Hereinafter, the left-right direction in which the carriage 4 moves may be referred to as the main scanning direction, and the front-rear direction in which the paper P is conveyed may be referred to as the sub-scanning direction, respectively.

[0020] A carriage encoder 11 is installed on the carriage 4. The carriage encoder 11 (corresponding to the "position detection unit" of the present invention) can detect the position of the carriage 4 in the main scanning direction by being used together with the scale 12. The scale 12 extends in the left-right direction slightly in front of the guide rail 62. The scale 12 is formed with a transmission part that transmits light and a blocking part that blocks light alternately along the left-right direction. The carriage encoder 11 has a light emitting part and a light receiving part arranged so as to sandwich the scale 12. The state where the light from the light emitting part passes through the transmission part of the scale 12 is detected by the light receiving part receiving the transmitted light. On the other hand, the state where the light from the light emitting part is blocked by the blocking part of the scale 12 is detected by the light receiving part not receiving the light from the light emitting part. When the carriage 4 moves to one of the left and right, the carriage encoder 11 alternately repeats detecting the former state and the latter state. Therefore, based on the detection result of the carriage encoder 11, the position of the carriage 4 can be detected by counting the switching of the above states. The detection result of the carriage encoder 11 is output to the control unit 9.

[0021] A platen 55 is arranged below the head 5 (see FIGS. 1 and 2). The platen 55 has a rectangular parallelepiped main body 55a and a plurality of ribs 55b protruding upward from the upper surface of the main body 55a. The upper surface of the rib 55b contacts the paper P and supports the paper P. The rib 55b is formed in the region α of FIG. 1 which is inside the paper P in the left-right direction.

[0022] The paper P on which an image is formed by the head 5 is discharged to the paper discharge tray 7 by the conveyance mechanism 2. The paper discharge tray 7 is in front of the head 5 in the housing 100a and is arranged at the upper part of the housing 100a.

[0023] The cartridge mounting portion 8 is installed at the right front end of the housing 100a. The cartridge mounting portion 8 can detachably mount four ink cartridges 8a that store black, yellow, cyan, and magenta inks respectively. Ink is supplied from the ink cartridge 8a mounted on the cartridge mounting portion 8 to the head 5 via a tube (not shown) or the like.

[0024] The control unit 9 controls the entire printer 100. As shown in FIG. 3, a conveyance motors 23a and 24a, a driver IC 52, a carriage motor 63, a carriage encoder 11, etc. are electrically connected to the control unit 9.

[0025] As shown in FIG. 3, the control unit 9 includes a CPU (Central Processing Unit) 91, a ROM (Read Only Memory) 92, a RAM (Random Access Memory) 93, an ASIC (Application Specific Integrated Circuit) 94, etc. Programs executed by the CPU 91 and the ASIC 94, various fixed data, etc. are stored in the ROM 92. Further, the ROM 92 includes a writable area composed of an EEPROM (Electrically Erasable Programmable Read-Only Memory) or the like. History information representing the usage mode of the printer 100 is written in this area (see the following description). Image data indicating an image to be formed on the paper P and data necessary during program execution are temporarily stored in the RAM 93. The image data is acquired from an external device such as a PC or a USB (Universal Serial Bus) memory. The ASIC 94 rewrites the data in the RAM 93. In addition, the control unit 9 has a hardware clock that holds the current time.

[0026] Note that the control unit 9 may be configured such that only the CPU 91 performs various processes, or only the ASIC 94 performs various processes, or the CPU 91 and the ASIC 94 cooperate to perform various processes. Also, the control unit 9 may be configured such that one CPU 91 performs processing alone, or a plurality of CPU 91s perform processing in a shared manner. Further, the control unit 9 may be configured such that one ASIC 94 performs processing alone, or a plurality of ASIC 94s perform processing in a shared manner. Hereinafter, various processes performed by the control unit 9 will be described.

[0027] The processes performed by the control unit 9 according to the present embodiment include an image forming process, a history information recording process, and a pre-scan process. Among these, the image forming process is a process of forming an image on the sheet P by controlling the operations of the conveyance mechanism 2 and the head 5 based on image data. In the image forming process, based on the position of the carriage 4 indicated by the detection result of the carriage encoder 11, the driving of the conveyance motors 23a and 24a and the carriage motor 63 is controlled, so that the sheet P is sent to the head 5 and ink is ejected from the head 5 at appropriate positions and timing.

[0028] The image forming process according to the present embodiment includes a process with a border and a process without a border. As shown in FIG. 4, in the process with a border, the ink from the head 5 is ejected only toward the inside of the sheet P. In the process without a border, the ink from the head 5 is ejected across the outer edge of the sheet P toward both the inside and both sides thereof.

[0029] In the image formation process, the size of the ink droplets ejected from the head 5 is selected based on the image data. The image data includes data indicating pixel values for each pixel forming the image. FIG. 5 is a graph of the threshold values used when determining the size of the ink droplets in the printing with a border. The graph of FIG. 5 represents the relationship between the threshold value and the size of the ink droplets when the pixel value has a value range of 0 to 1023. In FIG. 5, the threshold value (hereinafter referred to as the "none / small threshold value") that separates non-ejection of ink and small ink droplets is 256, the threshold value (hereinafter referred to as the "small / medium threshold value") that separates small ink droplets and medium ink droplets is 512, and the threshold value (hereinafter referred to as the "medium / large threshold value") that separates medium ink droplets and large ink droplets is 768. Specifically, when the pixel value of a certain pixel is 256 or less, no ink is ejected for that pixel. When a certain pixel value is greater than 256 and 512 or less, small ink droplets are used for that pixel. When a certain pixel value is greater than 512 and 768 or less, medium ink droplets are used for that pixel. When a certain pixel value is greater than 768, large ink droplets are used for that pixel.

[0030] Furthermore, when determining the size of the ink droplets, error diffusion processing is performed to handle the error between the density of the pixels formed on the paper P by the actually ejected ink droplets and the density indicated by the original pixel values. Specifically, when the size of the ink droplets is determined for a certain pixel, the error between the error reference value corresponding to the ink droplet of that size and the pixel value is passed on to the pixel values of its adjacent pixels as follows. FIG. 6 shows an example of error diffusion processing when the threshold values shown in FIG. 5 are set and the error reference values for large ink droplets, medium ink droplets, and small ink droplets are set to 800, 500, and 300, respectively. As shown in FIG. 6, when the pixel values of adjacent pixels 1 to 3 are all 500, the ink droplets in pixel 1 are determined to be small ink droplets in light of FIG. 5. At this time, the error reference value for small ink droplets is 300, while the pixel value of pixel 1 is 500, so the difference 500 - 300 = 200 is passed on to the adjacent pixel 2. Therefore, when determining the size of the ink droplets in pixel 2, the determination is made based on the value 700 obtained by adding the carry-over error of 200 and the original pixel value of 500. With respect to the pixel value of 700, the size of the ink droplets is determined to be medium ink droplets in light of FIG. 5. At this time, the error reference value for medium ink droplets is 500, while the pixel value of pixel 2 is 700, so the difference 700 - 500 = 200 is passed on to the adjacent pixel 3. Similarly, the ink droplets in pixel 3 are determined to be medium ink droplets in light of FIG. 5 based on the pixel value of 700 obtained by taking into account the original pixel value and the carry-over error. It is assumed that error diffusion processing is also performed when determining the size of the ink droplets in the following.

[0031] The history information recording process is a process of writing history information to the ROM 92. The history information includes information indicating the total number of sheets of paper P used and information indicating the total usage time of the printer 100. The total number of sheets used is counted for each of the framed process and the frameless process performed in the past in the image forming process. Each time the control unit 9 executes the image forming process, the history information is updated so that the number of sheets of paper P used in that process is separately added to the total number of sheets of paper P used for the framed process and the frameless process. Also, when the power of the printer 100 is first turned on, the control unit 9 obtains the current time as the start date and time of use of the printer 100 by referring to the hardware clock. Then, the control unit 9 records the obtained start date and time of use in the ROM 92 as history information indicating the total usage time. The subsequent total usage time of the printer 100 can be obtained as the time from the start date and time of use indicated by the history information to the current time.

[0032] The pre-scan process is a process of checking whether the detection result of the carriage encoder 11 accurately indicates the position of the carriage 4. The pre-scan process is performed by moving the carriage 4 along a predetermined movement path by controlling the carriage motor 63 prior to the execution of the image forming process. When the carriage 4 reaches a predetermined position at the completion of the pre-scan process, if the detection result of the carriage encoder 11 indicates this, the detection result is determined to be accurate. On the other hand, when the position of the carriage 4 at the completion of the pre-scan process is different from the above-mentioned predetermined position, the detection result of the carriage encoder 11 is determined to be inaccurate.

[0033] One of the reasons why the carriage encoder 11 shows inaccurate detection results as described above is due to mist floating inside the housing 100a of the printer 100. Mist is a phenomenon in which the ink ejected from the head 5 disperses during flight and becomes minute ink droplets, floating inside the housing 100a of the printer 100. When mist occurs repeatedly, the carriage encoder 11 and the scale 12 get soiled with ink, increasing the risk of inability to perform accurate detection with these components. That is, the inaccurate detection result of the carriage encoder 11 by the pre-scan process corresponds to the serious soiling of the carriage encoder 11 and the scale 12 due to mist. Note that the function of the control unit 9 to determine whether the detection result of the carriage encoder 11 is accurate by the pre-scan process can detect the degree of soiling due to mist of the carriage encoder 11 and the like, and thus corresponds to the function of the "soiling detection unit" of the present invention.

[0034] The main cause of mist generation lies in the above borderless process. In the borderless process, the ink droplets ejected inside the sheet P fly through the gap g1 in FIG. 2 and reach the sheet P. On the other hand, the ink droplets ejected outside the sheet P fly through the gap g2 in FIG. 2 and reach the upper surface of the main body 55a of the platen 55. That is, the ink droplets ejected outside the sheet P fly a longer distance compared to the ink droplets ejected inside the sheet P. As a result, the latter ink droplets are more likely to disperse during flight and become mist compared to the former ink droplets. That is, the ink droplets ejected outside the sheet P are likely to generate mist. Also, mist occurs more frequently when small and medium ink droplets are used compared to when large ink droplets are used.

[0035] Therefore, in the borderless process, the ink ejection range is divided into a suppression area and a normal area. As shown in FIG. 7, the suppression area is an area consisting of the edge of the paper P and the outside of the paper P. The normal area is the inside of the paper P, excluding the edge of the paper P. In the normal area, when determining the size of the ink droplets, the same threshold as in the printing with a border is used (corresponding to the "normal droplet determination process" of the present invention). That is, for example, the threshold shown in FIG. 5 is used. Then, the ink droplets of the size determined thereby are ejected from the head 5 to the normal area of the paper P (corresponding to the "normal ejection process" of the present invention). On the other hand, in the suppression area, when determining the size of the ink droplets, in a specific case (described later), the use of medium and small ink droplets is suppressed compared to the normal area (corresponding to the "suppression droplet determination process" of the present invention). Then, the ink droplets of the size determined in this way are ejected from the head 5 to the suppression area of the paper P (corresponding to the "first and second suppression ejection processes" of the present invention).

[0036] Specifically, in the borderless process, when determining the size of the ink droplets, for example, the thresholds shown in the graphs G1 and G2 of FIG. 7 are used. The graph G1 shows the threshold used for each pixel at each position in the main scanning direction. The graph G2 shows the threshold used for each pixel at each position in the sub-scanning direction. In either graph, a constant threshold is used for the pixels within the normal area. Also, in the suppression area, the threshold decreases linearly as the distance from the normal area increases, and the threshold becomes zero in the area outside the paper P. In this way, the relationship between the position in the main scanning direction and the threshold and the relationship between the position in the sub-scanning direction and the threshold correspond to each other. Therefore, hereinafter, mainly the relationship between the position in the main scanning direction and the threshold will be described, and the description of the relationship between the position in the sub-scanning direction and the threshold will be omitted.

[0037] Graph G1 is applicable at any of the no / small threshold, small / medium threshold, and medium / large threshold. For example, the graph in Fig. 8(a) is applicable to the medium / large threshold, the graph in Fig. 8(b) is applicable to the small / medium threshold, and the graph in Fig. 8(c) is applicable to the no / small threshold respectively. In Fig. 8(a), the threshold at the position x1 in the normal region is 768. The thresholds at the positions x2, x3, and x4 in the suppression region are 600, 450, and 100 respectively. Note that x2, x3, and x4 are positions farther from the normal region in that order. In Fig. 8(b), the threshold at the position x1 in the normal region is 512. The thresholds at the positions x2, x3, and x4 in the suppression region are 400, 300, and 100 respectively. In Fig. 8(c), the threshold at the position x1 in the normal region is 256. The thresholds at the positions x2, x3, and x4 in the suppression region are 200, 150, and 100 respectively.

[0038] Fig. 9 is a graph showing the relationship between the thresholds in Figs. 8(a) to 8(c) and the size of the ink droplets at each of the positions x1, x2, x3, and x4. Compared with the graph at the position x1, the graphs at x2, x3, and x4 show that the use of both medium ink droplets and small ink droplets is suppressed. Also, the degree of suppression is greater at positions farther from x1. Thus, in borderless printing, the generation of mist caused by the ink discharged outside the paper P is suppressed by suppressing the use of medium ink droplets and small ink droplets, which are likely to cause the generation of mist. Note that in Figs. 8(a) to 8(c), the fact that the minimum value is 100 at any of the no / small threshold, small / medium threshold, and medium / large threshold corresponds to the fact that neither medium ink droplets nor small ink droplets are used, and the threshold for separating ink non-discharge and large ink droplets is 100, as shown by the graph at the position x4 in Fig. 9.

[0039] Incidentally, the occurrence status of the adverse effects caused by mist also varies depending on the usage mode of the printer 100. For example, when not much time has passed since the start of using the printer 100, or when the total number of sheets of paper P used for image formation is small, even if mist is generated due to the use of the printer 100, the dirt caused by the mist may not have accumulated in the housing 100a to the extent that adverse effects are caused. That is, depending on the usage mode of the printer 100, even if some mist is generated, adverse effects may be relatively unlikely to occur.

[0040] On the other hand, in the borderless process, if the use of medium ink droplets and small ink droplets is suppressed for the suppression area, the image quality of the image formed on the paper P may deteriorate. Regardless of the usage mode of the printer 100, if the use of medium ink droplets and small ink droplets is similarly suppressed for the suppression area, even though the risk of adverse effects caused by mist is low, it may unnecessarily reduce the image quality.

[0041] Therefore, in the present embodiment, when a predetermined condition regarding the usage mode of the printer 100 is satisfied, the control unit 9 executes a suppression relaxation process in which the suppression of the use of medium ink droplets and small ink droplets for the suppression area is relaxed. The predetermined condition is a condition indicating that the degree of dirt in the housing 100a due to the generation of mist in the housing 100a is low. The suppression relaxation process includes at least one of a process of reducing the degree of suppression of the use of medium ink droplets and small ink droplets in the suppression area and a process of not suppressing the use of medium ink droplets and small ink droplets themselves.

[0042] As an example, the medium / large threshold value in Fig. 10(a) is the same threshold value as in Fig. 8(a) and is used when the above-mentioned predetermined conditions are not satisfied. That is, it is used when the risk of adverse effects caused by mist is relatively high or when there is a relatively high risk of a sharp increase in that risk. Similarly, when the above-mentioned predetermined conditions are not satisfied, the small / medium threshold value in Fig. 8(b) and the no / small threshold value in Fig. 8(c) are used. Fig. 10(b) is a graph showing the relationship between the threshold value of the pixel value used when determining the size of the ink droplet at the positions of x1 and x5 and the size of the ink droplet in this case. Note that x5 indicates the outermost position of the suppression region. The size of the ink droplet is determined using the large threshold values in Fig. 8(a) (Fig. 10(a)), Fig. 8(b), and Fig. 8(c) (corresponding to the "suppression droplet determination process" of the present invention), and the ink droplet of the determined size is ejected from the head 5 to the suppression region (corresponding to the "first suppression ejection process" of the present invention). Thereby, in the suppression region, the use of medium ink droplets and small ink droplets is surely suppressed.

[0043] On the other hand, when the above-mentioned predetermined conditions are satisfied, a process using the relaxation threshold value A or B is performed for the entire suppression region. Both the relaxation threshold values A and B include the no / small threshold value, the small / medium threshold value, and the medium / large threshold value set so that the degree of suppressing the use of medium ink droplets and small ink droplets is smaller than when using the threshold values in Figs. 8(a) to 8(c). Further, the relaxation threshold value B is set so that the degree of suppressing the use of medium ink droplets and small ink droplets is smaller than the relaxation threshold value A.

[0044] As a specific example, FIG. 11(a) shows the relationship between the medium / large threshold value at the relaxation threshold value A and the position in the main scanning direction. FIG. 12(a) shows the relationship between the medium / large threshold value at the relaxation threshold value B and the position in the main scanning direction. Both the relaxation threshold values A and B show threshold values larger than the threshold value in FIG. 10(a) at each of the positions x2, x3, and x4 in the suppression region. Also, the medium / large threshold value in the suppression region at the relaxation threshold value A linearly decreases as it moves away from the normal region, whereas the medium / large threshold value in the suppression region at the relaxation threshold value B is a constant threshold value of the same magnitude as the threshold value in the normal region. In other words, the relaxation threshold value B does not suppress the use of medium ink droplets and small ink droplets in the suppression region. The process of discharging ink droplets of the size determined using the relaxation threshold value B from the head 5 to the suppression region corresponds to the "normal discharge process" of the present invention. Similarly, for the small / medium threshold value and the none / small threshold value at the relaxation threshold value A, threshold values larger than the threshold values in FIGS. 8(b) and 8(c) are set at each of the positions x2, x3, and x4 in the suppression region. Also, for the small / medium threshold value and the none / small threshold value at the relaxation threshold value B, the threshold value in the suppression region is set to a constant threshold value of the same magnitude as the threshold value in the normal region. FIG. 11(b) is a graph showing the relationship between the relaxation threshold value A and the size of the ink droplets at the positions of x1 and x5. FIG. 12(b) is a graph showing the relationship between the relaxation threshold value B and the size of the ink droplets at the positions of x1 and x5.

[0045] The above-mentioned predetermined conditions are determined based on the history information recorded in the ROM 92. As a specific example, the control unit 9 divides the total number of printed sheets of the borderless printing indicated by the history information by the total usage time [year] of the printer 100 indicated by the history information. Thereby, the number of sheets of the paper P used per year in the borderless printing is calculated. When the number of sheets of the paper P used per year is relatively large, it is presumed that the borderless process is performed at a high frequency, and thus the dirt inside the housing 100a due to mist rapidly progresses. On the other hand, when the number of sheets of the paper P used per year is relatively small, it is presumed that the frequency of the borderless process is low, and thus the dirt inside the housing 100a due to mist progresses slowly. Therefore, when the number of sheets of the paper P used per year calculated in this way does not exceed the reference value [sheets / year], the control unit 9 determines that the above-mentioned predetermined conditions are satisfied. On the other hand, when the number of sheets of the paper P used per year is equal to or more than the reference value, the control unit 9 determines that the above-mentioned predetermined conditions are not satisfied. The reference value is obtained, for example, by dividing the upper limit number of sheets of the paper P used for borderless printing by the durability time of the printer 100 so that the adverse effect due to mist reaches the limit. Note that the reference value may be set by other methods.

[0046] The relaxation thresholds A and B may be selectively used based on the comparison between the number of sheets of the paper P used per year and a plurality of reference values. Specifically, the first reference value is set to the value obtained by dividing the upper limit number of sheets by the durability time as described above, and the second reference value is set to half of the first reference value. Then, when the number of sheets of the paper P used per year does not exceed the first reference value and exceeds the second reference value, the relaxation threshold A is used. Also, when the number of sheets of the paper P used per year does not exceed the second reference value, the relaxation threshold B is used.

[0047] In addition, when it is determined that the detection result of the carriage encoder 11 is inaccurate due to the pre-scan process, the thresholds in FIGS. 8(a) to 8(c) are used regardless of the number of sheets of paper P used per unit time. The fact that the detection result of the carriage encoder 11 is inaccurate indicates that the contamination of the carriage encoder 11 or the scale 12 is serious as an adverse effect of mist. Whether the position detection of the carriage 4 by the carriage encoder 11 is accurate has a significant impact on the continued use of the printer 100. For this reason, as described above, when the detection result of the carriage encoder 11 is inaccurate, it is possible to reliably suppress the generation of mist by reliably suppressing the use of medium ink droplets and small ink droplets using the thresholds in FIGS. 8(a) to 8(c).

[0048] The processing flow by the control unit 9 for selecting a threshold based on the number of sheets of paper P used per unit time in borderless printing will be described with reference to FIG. 13. First, the control unit 9 performs a pre-scan (S1) and determines whether the detection result of the carriage encoder 11 is accurate (S2). If it is determined that the detection result of the carriage encoder 11 is inaccurate (S2, No), the control unit 9 selects the thresholds in FIGS. 8(a) to 8(c) (S8). Then, the series of processes ends. If it is determined that the detection result of the carriage encoder 11 is accurate (S2, Yes), the control unit 9 calculates the number of sheets of paper P used per unit time in borderless printing by dividing the total number of borderless printed sheets indicated by the history information by the total usage time of the printer 100 indicated by the history information (S3). Next, the control unit 9 determines whether the number of sheets of paper P used per unit time calculated in S3 is less than or equal to the second reference value (S4). When it is determined that the number of sheets of paper P used per unit time is less than or equal to the second reference value (S4, Yes), the control unit 9 selects the relaxed threshold B in FIG. 11 (S5). Then, the series of processes ends.

[0049] In S4, when it is determined that the number of sheets of paper P used per unit time calculated in S3 is not less than or equal to the second reference value (S4, No), the control unit 9 determines whether the number of sheets of paper P used per unit time calculated in S3 is less than or equal to the first reference value (S6). When it is determined that the number of sheets of paper P used per unit time is less than or equal to the first reference value (S6, Yes), the control unit 9 selects the relaxation threshold A (S7). Then, the series of processes ends.

[0050] In S6, when it is determined that the number of sheets of paper P used per unit time calculated in S3 is not less than or equal to the first reference value (S6, No), the control unit 9 selects the thresholds in FIGS. 8(a) to 8(c) (S8). Then, the series of processes ends.

[0051] According to the present embodiment described above, for the suppression region of the paper P, when a predetermined condition is not satisfied, a process of ejecting ink droplets having a size determined using the thresholds in FIGS. 8(a) to 8(c) from the head 5 is performed. This process is a process that surely suppresses the use of medium ink droplets and small ink droplets.

[0052] On the other hand, when a predetermined condition is satisfied, a suppression relaxation process is performed. The suppression relaxation process includes at least either a process of reducing the degree of suppression of the use of medium ink droplets and small ink droplets or a process of not suppressing the use of medium ink droplets and small ink droplets as compared with the case of using the thresholds in FIGS. 8(a) to 8(c). Therefore, when the suppression relaxation process is performed, the image quality of the image formed on the paper P does not deteriorate, or the degree of deterioration of the image quality is smaller than that in the case where the degree of suppression of the use of medium ink droplets and small ink droplets is not reduced.

[0053] Here, the above-mentioned predetermined condition is that the number of sheets of paper P used per unit time does not exceed a reference value. The number of sheets of paper P used per unit time is obtained by dividing the total number of sheets of paper P indicated by the history information by the total usage time of the printer 100 indicated by the history information. This serves as an indicator of the extent to which the inside of the housing 100a has become dirty due to the generation of mist within the printer 100. When the dirt inside the printer 100 has progressed, it is necessary to surely suppress the use of medium ink droplets and small ink droplets in the suppression area, but this reduces the quality of the image formed on the paper P. On the other hand, when the dirt inside the printer 100 has not progressed so much, it is appropriate to perform a suppression relaxation process to maintain the image quality. Therefore, based on the number of sheets of paper P used per unit time, the process of surely suppressing the use of medium ink droplets and small ink droplets and the suppression relaxation process are switched, so that the quality of the image is not unnecessarily degraded, and the process of suppressing the use of medium ink droplets and small ink droplets is ensured.

[0054] Also, according to the present embodiment, the above-mentioned predetermined condition is that the number of sheets of paper P used per unit time does not exceed a reference value. The number of sheets of paper P used per unit time is calculated using both the total number of sheets of paper P used and the total usage time of the printer 100.

[0055] In contrast, as the above-mentioned predetermined condition, a condition that either one of the total number of sheets used and the total usage period does not exceed a reference value may be used. This is because the total number of sheets used and the total usage period each independently serve as an indicator of the degree of accumulation of dirt due to mist.

[0056] However, when either the total number of sheets used or the total usage period does not exceed a reference value as a predetermined condition, the predetermined condition will be satisfied unless the dirt inside the housing 100a due to mist reaches a considerable level. As a result, the processing using the threshold values in FIGS. 8(a) to 8(c) is not performed until the dirt inside the housing 100a due to mist becomes serious. Therefore, for example, when a usage mode is such that the dirt inside the housing 100a due to mist progresses rapidly, since the suppression relaxation processing is initially performed, the dirt inside the housing 100a will progress in a short period, and as a result, the usable period of the printer 100 may be unexpectedly shortened. Thus, as in this embodiment, by using the number of sheets of the paper P used per unit time as a reference, when there is a possibility that the dirt will progress in a short period, the processing of surely suppressing the use of medium ink droplets and small ink droplets using the threshold values in FIGS. 8(a) to 8(c) is executed, and thereby, it is possible to suppress the usable period of the printer 100 from being unexpectedly shortened.

[0057] Also, in this embodiment, the total number of sheets of the paper P is the number of sheets related to the borderless processing. As described above, the generation of mist is mainly caused by the borderless processing. Therefore, by using the number of sheets used for the borderless processing as a reference, it is possible to appropriately switch between the processing of surely suppressing the use of medium ink droplets and small ink droplets and the suppression relaxation processing.

[0058] Also, in the suppression relaxation processing of this embodiment, ink droplets having a size determined using the relaxation threshold A or B are ejected from the head 5 to the entire suppression region. Therefore, compared with the ejection of ink droplets based on the threshold values in FIGS. 8(a) to 8(c), the image quality is improved in the entire suppression region.

[0059] Note that in the suppression relaxation processing, the ejection of ink droplets using the relaxation threshold A or B may be performed not on the entire suppression region but only on a part of it. For example, the same ink droplets as those in the normal region may be ejected onto a part of the suppression region, and the ejection of ink droplets using the relaxation threshold A or B may be performed on the remaining part of the suppression region.

[0060] In the above-described embodiment, when the number of sheets of paper P used per unit time does not exceed the second reference value, the relaxation threshold B is used. When the number of sheets of paper P used per unit time exceeds the second reference value but does not exceed the first reference value, the relaxation threshold A is used. The relaxation threshold B suppresses the use of medium ink droplets and small ink droplets to a lesser extent than the relaxation threshold A. That is, the degree of suppressing the use of medium ink droplets and small ink droplets is in accordance with the number of sheets of paper P used per unit time. As a result, the degree of suppressing the use of medium ink droplets and small ink droplets is appropriately adjusted according to the progress of soiling due to the generation of mist.

[0061] <Other Modifications> As described above, the embodiments of the present invention have been described with reference to the drawings. However, the specific configuration should not be considered to be limited to these embodiments. The scope of the present invention is shown by the claims, rather than the description of the above embodiments, and further includes all modifications within the meaning and scope equivalent to the claims.

[0062] For example, in the above-described embodiment, in the suppression relaxation process, whether to use the relaxation threshold A or the relaxation threshold B according to the number of sheets of paper P used per unit time is switched, that is, whether to suppress the use of medium ink droplets and small ink droplets is switched. Instead of this, it may be switched whether to use the relaxation threshold A or the relaxation threshold C according to the number of sheets of paper P used per unit time. The relaxation threshold C is set such that, like the relaxation threshold A, the use of medium ink droplets and small ink droplets is suppressed in the suppression region, but the degree of suppressing the use of medium ink droplets and small ink droplets is smaller than the relaxation threshold A.

[0063] Also, in the above-described embodiment, the total usage time of the printer 100 is acquired based on the start date of use. Instead of this, the total usage time of the printer 100 may be acquired based on the start date of manufacture. In this case, in the manufacturing process of the printer 100, the start date of manufacture will be recorded in the ROM 92. Also, the total length of the period during which the power of the printer 100 is on may be used as the total usage time of the printer 100.

[0064] Also, in the above-described embodiment, the total number of sheets of the paper P used is the integrated value of the number of sheets of the paper P related to the borderless process. Instead of this, without distinguishing between the bordered process and the borderless process, the integrated value of the number of sheets of the paper P used in all image formation processes may be used as the total number of sheets used.

[0065] Also, in the above-described embodiment, it is assumed that the small ink droplets and the medium ink droplets correspond to the "small droplets" of the present invention, and the large ink droplets correspond to the "large droplets" of the present invention. Instead of this, on the assumption that the small ink droplets correspond to the "small droplets" of the present invention and the large ink droplets and the medium ink droplets correspond to the "large droplets" of the present invention, the present invention may be applied. In this case, the use of small ink droplets will be suppressed in the suppression area. Also, there may be two types or four or more types of ink droplets. In this case, the present invention may be applied by dividing a plurality of types of ink droplets into two groups of small and large ones, with the former corresponding to the "small droplets" of the present invention and the latter corresponding to the "large droplets" of the present invention.

[0066] Also, in the above-described embodiment, dirt caused by mist on the carriage encoder 11 or the like is detected by the pre-scan process, and when the dirt is serious (when the detection result of the carriage encoder 11 is inaccurate), the thresholds in FIGS. 8(a) to 8(c) are used. Instead of or in addition to this, a detection unit (corresponding to the "dirt detection unit" of the present invention) for detecting dirt caused by mist on other parts inside the housing 100a may be installed, and the thresholds in FIGS. 8(a) to 8(c) may be used based on the detection result of the detection unit. This is because when mist is repeatedly generated by repeating the borderless process, mist may adhere to various locations inside the housing 100a, and dirt may accumulate due to this.

Explanation of Signs

[0067] 2 Conveying mechanism 5 Head 6 Moving mechanism 9 Control unit 11 Carriage encoder 12 Scale 100 Printer

Claims

1. A droplet ejection head capable of ejecting small droplets and large droplets respectively, A medium supply unit that supplies a medium to the droplet ejection head, And a control unit, The control unit, By selectively ejecting the small droplets and the large droplets toward both the inside and the outside of the medium supplied by the medium supply unit, the droplet ejection head and the medium supply unit are controlled to form an image indicated by image data on the medium, and an image forming process is executed, The image forming process, A normal droplet determination process for determining which of the small droplets and the large droplets to use based on the image data, A suppression droplet determination process for determining which of the small droplets and the large droplets to use based on the image data so as to suppress the use of the small droplets compared to the normal droplet determination process, A normal ejection process for ejecting the droplets determined by the normal droplet determination process from the droplet ejection head, And first and second suppression ejection processes for ejecting the droplets determined by the suppression droplet determination process from the droplet ejection head, The control unit, Executes the normal ejection process for a normal area that is an area inside the medium excluding the end portion of the medium, For a suppression area consisting of the end portion of the medium and the outside of the medium, when at least one of the number of media used in the past and the usage period of the apparatus in the image forming process does not satisfy a predetermined condition, the first suppression ejection process is executed, and when at least one of the number of media and the usage period satisfies the predetermined condition, a suppression relaxation process is executed, The suppression relaxation process includes at least one of the second suppression ejection process and the normal ejection process, The second suppression ejection process is a process of ejecting the droplets determined by the suppression droplet determination process from the droplet ejection head so that the degree of suppressing the use of the small droplets in the suppression area is lower than that of the first suppression ejection process. An image forming apparatus characterized by this.

2. The predetermined condition is that at least one of the number of media and the usage period does not exceed a reference value. The image forming apparatus according to claim 1, characterized by this.

3. The predetermined condition is that the number of media used per unit time calculated using both the number of media and the usage period does not exceed a reference value. The image forming apparatus according to claim 1, characterized by this.

4. The control unit, When the image forming process is the first image forming process, a second image forming process for controlling the droplet ejection head and the medium supply unit so as to form an image indicated by the image data on the medium by selectively ejecting the small droplets and the large droplets only toward the inside of the medium supplied by the medium supply unit is selectively executable with the first image forming process, The image forming apparatus according to claim 1, wherein the number of the media is the number of the media used in the first image forming process.

5. The suppression relaxation process is a process of performing at least one of the normal ejection process and the second suppression ejection process on a part of the suppression area and performing the first suppression ejection process on the remaining part of the suppression area. The image forming apparatus according to claim 1, characterized in that

6. The suppression relaxation process is a process of performing the normal ejection process or the second suppression ejection process on all of the suppression area. The image forming apparatus according to claim 1, characterized in that

7. In the suppression relaxation process, the control unit reduces the degree of suppressing the use of the small droplets according to the number of media used per unit time calculated using both the number of the media and the usage period. The image forming apparatus according to claim 1, characterized in that

8. a part where the droplets adhere due to repetition of the image forming process, further comprising a stain detection unit that detects stains caused by adhesion of the droplets to the part, The control unit executes the first suppression ejection process on the suppression area based on the detection result of the stain detection unit regardless of whether the predetermined condition is satisfied. The image forming apparatus according to claim 1, characterized in that

9. a moving mechanism for moving the droplet ejection head, further comprising a position detection unit that optically detects the position of the droplet ejection head, The image forming apparatus according to claim 8, wherein the part is the position detection unit.

Images