1. Field of the Invention
The present invention relates to a liquid ejecting head that accurately ejects ink droplets, and an ink jet printing apparatus.
2. Description of the Related Art
In recent years, serial scan type ink jet printing apparatuses (liquid ejecting apparatuses) have been increasing rapidly, in which a print head, as a liquid ejecting head, ejects ink droplets (liquid droplets) during movement with respect to a print medium to print on the print medium. These ink jet printing apparatuses have the advantages of being easily miniaturized and capable of printing color images relatively easily. In addition to the serial scan type ink jet printing apparatus, which prints images by moving the print head, full line type ink jet printing apparatuses are available. This type of printing apparatus uses a long print head extending all along a print area of the print medium in a width direction to print images without the need to move the print head.
Attempts have been made to reduce the size of ejected ink droplets in order to improve the quality of printed images. Furthermore, the frequency in use of the ink jet printing apparatuses has recently been increasing. Thus, attempts have been made to improve durability performance.
One method for reducing the size of liquid droplets ejected from the liquid ejecting head (including the print head) is to reduce the size of ejection ports through which liquid droplets are ejected. However, the reduced size of each of the ejection ports increases the flow resistance to the liquid at the ejection port. This may prevent desired liquid ejection performance and ejection efficiency from being achieved. That is, the thickness of an orifice plate in which the ejection ports are formed increases relative to the reduced opening area of the ejection port. For example, Japanese Patent Laid-Open Nos. 2004-042652 and 2004-042651 propose a configuration for reducing the flow resistance to ink (liquid) at the ejection port while maintaining the strength of the print head.
In the proposed print heads, the ejection port is formed by a first ejection port portion located closer to an opening portion of the ejection port and a second ejection port portion communicating with the first ejection port portion. In these print heads, the flow resistance to the ink can be reduced by reducing the thickness only of the opening portion (first ejection port portion) of the ejection port in the orifice plate. Thus, a decrease in the strength of the orifice plate can be inhibited. Furthermore, when bubble energy resulting from bubble of the ink is utilized to eject the ink, bubbles generated in the ink are efficiently grown from the second ejection port portion toward the first ejection port portion. This improves ink ejection performance and efficiency.
Furthermore, various attempts have been made to improve the durability performance of the print head. The improved durability performance of the print head enables an increase in the number of possible droplet ejections during the working life of the print head.
According to an ink ejecting method utilizing an ink film boiling phenomenon, an electrothermal converting element generates heat to cause the ink to bubble so that the resulting bubble energy is utilized to eject the ink through the ejection port. When the electrothermal converting element generates heat, a thermochemical reaction occurs between the surface of a protective film covering the electrothermal converting element and the ink. As a result, the protective film may be oxidized or dissolved. Furthermore, a possible impact force caused by cavitation during a debubbling process may scrape or damage the protective film. The degraded function of the protective film may cause the ink to be in appropriately ejected or result in inappropriate printing. To deal with this, attempts have been made to improve the durability performance by improving the protective film and the shape of nozzles.
A nozzle shape for decreasing the possible impact force resulting from cavitation is described in Japanese Patent Laid-Open Nos. 2002-321369 and 2002-248769. In this nozzle shape, the center line of an ink channel is displaced from the center line of the electrothermal converting element. Thus, the position where bubbles generated by the electrothermal converting element are defoamed can be fluctuated and shifted. Furthermore, by changing the flow of the ink during the defoaming, the defoaming can be prevented from occurring over the electric heating element. Additionally, by displacing the center line of the electrothermal converting element from the center line of a bubbling chamber, the amount of displacement of the center of the ejection port from the center of the electrothermal converting element can be increased by the amount of displacement of the center of the ejection port from the center of the bubbling chamber and can be minimized. Thus, the ink is prevented from collecting in the vicinity of the ejection port, allowing the ink to be accurately ejected even with the defoaming position moved. Thus, controlling the ink flow in association with the defoaming allows bubbles to be biased toward the side of the electrothermal converting element. This enables a reduction in the possible impact force exerted on the electrothermal converting element by cavitation during the defoaming. The durability of the print head can thus be improved.
However, the reduced amount of ejected ink (liquid) relatively makes the effects of the nozzle structure on ink droplets (droplets) more significant. For example, if the center of the bubbling chamber (energy acting chamber) with the electrothermal converting element provided therein is displaced from the center of the ejection port to make the bubbling chamber asymmetric, the ejected ink droplets are more significantly affected. Specifically, when the bubble energy resulting from bubbling of the ink in the bubbling chamber is utilized to eject the ink and the bubbling chamber is then refilled with ink through an ink channel, a plane is assumed which divides the bubbling chamber into two parts. The plane is almost perpendicular to an element board on which the electrothermal converting element is formed and is almost parallel to the ink channel. The plane passes through the center of the electrothermal converting element. If the plane is used to divide the asymmetric bubbling chamber into two parts, one of the parts has a larger first area and the other has a smaller second area. If such an asymmetric bubbling chamber is refilled with ink through the ink channel, the ink flows from the first area to the second area.
A force in the direction of the flow acts on the ink refilled into the bubbling chamber. Thus, a trailing end (trailing portion) of the ink droplet resulting from ejection of the ink in the bubbling chamber is likely to bend in a direction from the first area toward the second area. As a result, the trailing portion of the ink droplet is torn away and separated into fine satellites. The satellites may impact the print medium at an inaccurate position (impacting accuracy) or become small floating mists.
The small mist, separated from the trailing portion of the ink droplet, may float between the print head and the print medium and adhere to the print head or the print medium. If the mist adheres to a peripheral part of the ejection port in the print head, the mist may hinder movement of the ejected ink and reduce the accuracy of the ink droplet impact.
The reduced impact accuracy of the ink droplets may cause ink dots to be formed at unexpected positions on the print medium. This may degrade the quality of a printed image. Furthermore, if the ink mist adheres to the print head and an unspecified part in the printing apparatus, the printing apparatus may malfunction. Additionally, the ink adhering to the printing apparatus may stain the print medium to degrade the print quality of the printed medium.