1. Field of the Invention
The present invention relates to an inkjet printhead. More particularly, the present invention relates to a piezoelectric inkjet printhead that can reduce a volume of a pressure chamber to increase a number of channels per inch (CPI).
2. Description of the Related Art
In general, inkjet printheads are devices for printing a predetermined color image by ejecting a small volume of droplet of printing ink at a desired position on a print medium, such as a sheet of paper or a fabric. Inkjet printheads are largely categorized into two types, depending on the ink ejection mechanism used. A first type is a thermal inkjet printhead, in which a heat source is employed to form and expand bubbles in ink, causing ink droplets to be ejected. A second type is a piezoelectric inkjet printhead, in which a piezoelectric element is deformed to exert pressure on ink, causing ink droplets to be ejected.
A conventional piezoelectric inkjet printhead is illustrated in FIGS. 1 and 2. Referring to FIGS. 1 and 2, a manifold 13, a plurality of restrictors 12 and a plurality of ink chambers 11, which, in combination, constitute ink channels, are formed on a channel plate 10. A plurality of nozzles 22, corresponding to the plurality of ink chambers 11, are formed on a nozzle plate 20. A piezoelectric actuator 30 is disposed on the channel plate 10. The manifold 13 is a path through which ink introduced from an ink reservoir (not shown) is supplied to the plurality of ink chambers 11. The restrictors 12 are flow paths through which ink is introduced from the manifold 13 to the plurality of ink chambers 11. The plurality of ink chambers 11, in which ink to be ejected is contained, are typically arranged on one or both sides of the manifold 13. The driving of the piezoelectric actuator 30 causes a change in the volume of a corresponding ink chamber 11, thereby producing a pressure change in the ink chamber 11 which results in ink ejection, or ink introduction. To this end, portions of the channel plate 10 form upper walls of the ink chambers 11 and act as vibration plates 14 that are deformed by the piezoelectric actuator 30.
In the operation of a conventional piezoelectric inkjet printhead constructed as described above, as the vibration plate 14 is deformed in a downward direction by the driving of the piezoelectric actuator 30, the volume of the ink chamber 11 is reduced and an internal pressure of the ink chamber 11 is accordingly changed (i.e., the pressure in the ink chamber 11 is increased), causing ink contained in the ink chamber 11 to be outwardly ejected through the nozzle 22. Subsequently, as the vibration plate 14 returns to its original state by the driving of the piezoelectric actuator 30, the volume of the ink chamber 11 is increased and an internal pressure of the ink chamber 11 is accordingly changed (i.e., the pressure in the ink chamber 11 is reduced), causing ink to introduced to the ink chamber 11 from the manifold 13 via the restrictor 12.
When an image is printed using the conventional piezoelectric inkjet printhead having structure described above, the resolution of the image is greatly affected by the number of nozzles per inch. The number of channels per inch (CPI) generally indicates the number of nozzles per inch, and the number of dots per inch (DPI) is generally a measure of the resolution of the image.
In the conventional piezoelectric inkjet printhead illustrated in FIGS. 1 and 2, the volume of ink droplets ejected through the nozzle 22 is greatly affected by the extent of displacement of the vibration plate 14. That is, the greater the displacement of the vibration plate 14, the greater the volume of the ink droplets, and, conversely, the less the displacement of the vibration plate 14, the less the volume of the ink droplets. The displacement of the vibration plate 14 is dependent on the area of the vibration plate 14, and the area of the vibration plate 14 is dependent on the volume of the ink chamber 11. In the conventional inkjet printhead, if the vibration plate 14 is deformed by the driving of the piezoelectric actuator 30, ink is ejected through the nozzle 22. However, the ink also flows back toward the manifold 13 via the restrictor 12. Thus, due to this backflow, the displacement of the vibration plate 14 must be great enough to accommodate both the volume of the ink droplet ejected from the nozzle 22 and the volume of the ink that flows back toward the manifold 13. Accordingly, to eject ink droplets of uniform volume, the displacement of the vibration plate 14 should take into account the amount of ink backflow. Thus, the area of the vibration plate 14 and the volume of the ink chamber 11 should be such that the change in volume of the ink chamber 11, caused by the piezoelectric actuator 30 displacing the vibration plate 14, is greater than or equal to the volume of the ink droplet plus the volume of the ink backflow.
The number of CPI of the piezoelectric inkjet printhead is generally in inverse proportion to a distance DN between adjacent nozzles 22. Thus, to increase the number of CPI of the printhead, the distance DN between the adjacent nozzles 22 should be reduced. However, the conventional piezoelectric inkjet printhead having the structure described above has limitations in reducing the distance DN between the adjacent nozzles 22, for the previously mentioned reasons. That is, reducing the distance DN generally necessitates reducing the area occupied by the ink chambers 11, which in turn may require a reduction in the size of the vibration plate 14 and a concomitant reduction in the effective displacement of the vibration plate 14. Thus, increasing the CPI of the printhead may be at odds with maintaining a desired volume of ink displacement.
In another aspect of the conventional inkjet printhead, the conventional inkjet printhead typically prints an image on a sheet of paper by reciprocating in a direction orthogonal to a feed direction of the sheet, i.e., by reciprocating in a width direction of the sheet. That is, during printing, the typical inkjet printhead moves back and forth across the paper in order to print images thereon. Accordingly, the conventional inkjet printhead has a slow printing speed, due to the need to move the printhead across the width of the sheet before advancing the sheet.
Inkjet printheads having the same length as the width of a sheet of paper have recently been developed in an attempt to increase printing speed. Such printheads may have a plurality of nozzles that are arrayed across the width of the sheet of paper. This may permit the printing of an image on the sheet at high speed, without reciprocation in the width direction of the sheet. An inkjet printhead having this structure is generally known as a page-wide inkjet printhead.
However, in order to print an image with sufficiently high resolution without any reciprocation in a width direction of a printing sheet of paper, the number of CPI of the printhead needs to be equal to the number of DPI of an image. However, it is not straightforward to increase the CPI to match the desired DPI of an image. Since the conventional piezoelectric inkjet printhead has structural limitations in increasing the number of CPI for the reasons described above, it is difficult to have the same number of CPI as the number of DPI of the image.
Accordingly, to satisfy the recent demands for an image with higher resolution, continuous efforts are needed to increase the number of CPI of a printhead.