1. Field of the Invention
The present invention relates to an image forming apparatus provided with a recording head with a plurality of nozzle arrays, for forming an image on an image-formed material, and more particularly, relates to an image forming apparatus provided with at least one long recording head composed of a plurality of short recording heads.
2. Description of the Related Art
For example, the recording head of the ink-jet type image forming apparatus has a tendency to multiply or lengthen a recording device (ink-jet nozzle) in order to meet the requirements of high-speed image formation (image recording).
As such an image forming apparatus, a structure provided with a so-called line head for disposing (forming) a recording device (ink-jet nozzle) across one side (width direction) of an image-formed material (recording medium) is known.
An image forming apparatus provide with a line head can form an image all over this image-formed material by relatively moving the image-formed material in the direction orthogonal to the ink-jet nozzle array direction of a line head (sub scan direction) and then jetting ink on the image-formed material from the ink-jet nozzle. Since the image forming apparatus provide with a line head needs neither the movement of a carriage nor the intermittent conveyance of the image-formed material, it can rapidly and easily form an image.
However, the line head has disadvantages that its cost is high compared with a short recording head, that its quality yield is bad, that its reliability is low and the like.
An image forming apparatus utilizing the advantages in cost of a short recording head, in its quality yield, in its reliability and the like by disposing a plurality of short recording heads in each of which a plurality of ink-jet nozzles is arrayed in one direction (main scan direction) in order to solve these problems is also known.
However, in the line head composed of such short recording heads, if there is a phase difference in pitch between nozzle arrays (improper pitch), striped optical density unevenness, white-wiping and the like are sometimes formed.
Patent reference 1 (Japanese Patent Application Publication No. 2002-144542) discloses am image recording method capable of recording a high-quality image without color/optical density unevenness or the like in an image recording apparatus in which one recording head (line head) is obtained by disposing a plurality of short (recording) heads.
The image recording method disclosed by patent reference 1 is described below with reference to FIGS. 1A˜1C.
The recording head 10 shown in FIG. 1A comprises a plurality of short heads 12A and 12B. In this recording head 10, a plurality of adjacent ink-jet nozzles 11A of the short head 12A and a plurality of adjacent ink-jet nozzles 11B of the short head 12B are disposed in such a way that a part of them overlaps when viewed from the sub scan direction. This joint area (overlapped area) corresponds to ink-jet nozzles 11A-1˜11A-3 on the short head side 12A and ink-jet nozzles 11B-1˜11B-3 on the short head side 12B.
In image record by the recording head 10 with such a structure, for example, as shown in the area “a” of FIG. 1B, a high-optical density recording area occurs along the sub scan direction in the joint area of the ink-jet nozzles 11A-1˜11A-3 of the short head 12A and the ink-jet nozzles 11B-1˜11B-3 of the short head 12B, and a high-quality image cannot be recorded.
In such a case, a record control unit for controlling the recording head 10 to record an image, which is not shown in FIG. 1, determines the ink-jet nozzle 11A-2 of the short head 12A and the ink-jet nozzle 11B-2 of the short head 12B, which is shown by an one-dot chain line as one example, as a joint point and the use of the ink-jet nozzles 11A-1 and 11B-1 located further on the top end is stopped.
Since the space between the ink-jet nozzles 11A-2 and 11B-2, which are the joint point is narrower than a proper pitch, the o optical density in the joint point becomes higher than a proper value, as shown in the area “a” of FIG. 1A. In order to correct this, the record control unit stops the drive of one of the ink-jet nozzles 11 on every another line in the sub scan direction to record an image with proper optical density. In an example of an area “b” of FIG. 1B, the drive of the ink-jet nozzle 11B-2 of the short head 12B is stopped on every another line in the sub scan direction.
The image recording method of patent reference 1 can record a high-quality image without color/optical density unevenness and the like, by performing such control.
In this case, in an ink-jet recording head using a piezoelectric device (PZT), generally the amount of ink jetted from the ink-jet nozzle at the head end increases or decreases compared with that in an area other than the end, that is, a non-end area. In the case of an image recording apparatus with one recording head, even when there is a little change in optical density due to the change of the amount of ink jetted from a specific number of ink-jet nozzles on the end in such a phenomenon, a part whose optical density has changed becomes the end of an image recording area. Therefore, the optical density unevenness of a recorded image is not remarkable. However, in the case of an image recording apparatus with a line head obtained by adjacently disposing a plurality of recording heads, the joint part of adjacent recording heads becomes inner than the end of the image recording area. Therefore, when optical density unevenness occurs in this part, striped optical density unevenness, white-wiping and the like becomes remarkable in a recorded image.
Patent reference 2 (Japanese Patent Application Publication No. 2003-320647) discloses a method for visually reducing optical density unevenness due to the fluctuations of the jet amount of ink (ink jet volume) at the end of such a recording head.
The method of this patent reference 2 determines which an input image signal is, the end area signal or non-end area signal of a corresponding recording head. If it is determined that that it is the end area signal, an end area correction process is performed. If it is determined that that it is the non-end area signal, a non-end area correction process is performed.
In the end area correction process, the end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas. In the non-end area correction process, the non-end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas, and it is also corrected in such a way that a optical density value gradually decreases from the end toward the center. The method of patent reference 2 corrects the end and non-end areas and also reduces optical density unevenness.
However, although patent reference 1 discloses a method for improving the optical density unevenness of a nozzle array overlapped part between adjacent short heads, it does not disclose a method for improving unevenness due to a non-uniform recording characteristic between nozzles arrays. Therefore, only the method disclosed by patent reference 1 cannot improve unevenness due to this non-uniform recording characteristic.
Although patent reference 2 discloses a method for improving unevenness due to non-uniform ink jet volume from the ink-jet nozzles, the relative positions of short heads are adjusted and disposed in such a way that one ink-jet nozzle in the joint part of adjacent short heads can be matched with the other ink-jet nozzle in the joint part when viewed from the sub scan direction.
The recording devices (ink-jet nozzle) are formed at very fine intervals. For example, if its resolution is 300 dpi, the interval becomes 85 μm. In the method disclosed by patent reference 2, for example, a locating mechanism for locating a ink-jet nozzle of one short head and the ink-jet nozzle of the other adjacent short head at intervals of 85 μm with no error is needed, which incurs the cost-up of the apparatus.
The generation factor and degree of optical density unevenness due to the nozzle array overlapped part of this short head and those of optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles are different.
FIG. 2A shows the recording optical density characteristic of a recording head (short head) in which the ink-jet nozzle at the nozzle array end is higher than that at non-end.
FIG. 2B shows the recording optical density characteristic of a recording head (line head) in which the two short heads with the recording optical density characteristic shown in FIG. 2A are adjacently disposed in such a way that a part of their ink-jet nozzles are overlapped when viewed from the above-described sub scan direction.
Since as shown in FIG. 2A, the optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles is due to the little structural or manufacturing error in the jet amount of ink (ink jet volume) between recording heads, its optical density change value becomes small and gentle.
Since as shown in FIG. 2B, the optical density unevenness in the nozzle overlapped part of a recording head is due to a optical density change according to the phase difference in a ink-jet nozzle position between nozzle arrays of adjacent recording heads, its optical density change value becomes large and steep.
Therefore, in order to correct optical density unevenness due to these two factors, a sufficient correction effect cannot be obtained only by simply combining two methods (technologies) disclosed by patent references 1 and 2, and accordingly no high-quality image can be obtained.