1. Field
The present application relates to a printing apparatus, a printing method, an image processing apparatus, a storage medium, and a print control apparatus.
2. Description of Related Art
Printing apparatuses have been known which form an image on a printing medium by repeating a sequence of scanning operation for printing (hereinafter referred to as print scanning operation) and sub-scanning operation for conveying the printing medium. For the print scanning operation, a printing head ejects ink through a plurality of ejection openings arranged therein while scanning the printing medium. The print scanning operation of such a printing apparatus is generally performed by a multipass method in which a plurality of print scanning operations are performed on a unit region of a printing medium.
It has been known that printing apparatuses of this type have a problem of beading, which is a phenomenon of ink droplets connected by ejecting ink droplets close to each other on a printing medium in print scanning operation. Beading may lead to a nonuniform image and degraded image quality. In order to reduce the occurrence of beading, US 2007/0109604 discloses that ink droplets are applied in a dispersed manner to a printing medium to avoid beading of landed ink droplets in a stage of image formation. This technique is achieved by using mask patterns including printable pixels in an arrangement calculated for reducing the repulsive potential between the printable pixels.
In recent years, ink jet printing has been increasingly used to produce printed mutter for various applications, and various types of ink and printing media have been being used accordingly. For forming highly bright images, a combination of an ink and a printing medium has been known. In this combination, an ink containing a pigment is fixed to the surface of a printing medium less permeable to the ink (hereinafter referred to as less permeable printing medium).
The present inventors have however found that printing operation using an ink containing a pigment and a less permeable printing medium may cause marked beading and thus fail to provide satisfactory image quality. In addition, the present inventors have found the reason of this disadvantage. More specifically, the reason is that when such an ink is fixed to the surface of a less permeable printing medium, the ink is kept in a liquid state on the printing medium for a longer time than the conventional case where ink is fixed to a printing medium by permeation of the ink.
This disadvantage will now be described in detail. FIG. 1 schematically shows a droplet 51 of a first ink containing a pigment on a layer 50 of a second ink containing a pigment, formed by ejecting the second and the first ink in that order onto the same region on a less permeable printing medium 3, in section taken along a plane perpendicular to the surface of the printing medium 3.
When an ink containing a pigment and a less permeable printing medium are used for printing, ink droplets are fixed to the surface of the printing medium without permeating into the printing medium, thus forming an ink layer to cover the surface of the printing medium, as shown in FIG. 1. When another ink is additionally ejected to a region where an ink has been applied, therefore, the ink ejected later is applied to a layer made of the previously ejected ink, but not on the surface of the printing medium.
In the following description, the first and the second ink have substantially the same surface tension in liquid phase, but the wettability of the second ink on a layer made of the first ink is higher than the wettability of the first ink on a layer made of the second ink. Wettability is an index of how easily a droplet spreads over a solid surface and wets it. The wettability of a liquid can be estimated by the contact angle of the liquid, which is the angle between a solid surface and the tangent to a droplet of the liquid at the contact point with the solid surface. As a droplet spreads more easily over a solid surface and wets it, the contact angle decreases.
The contact angle of an ink varies depending on the surface tension γL of the ink in liquid phase and the critical surface tension of a solid surface to which the ink is applied. Critical surface tension is one of the indices of the surface tension of a solid. As the surface tension γL of an ink in liquid phase increases, or as the critical surface tension of a solid surface to which the ink is applied decreases, the contact angle of the ink increases. The surface tension γL of an ink depends on the chemical properties of the ink, and the critical surface tension of a solid surface depends on the chemical properties of the solid surface, for example, an ink layer.
Even though the surface tensions γL in liquid phase of two inks are the same, the ink layers made of each of the inks on a printing medium have different critical surface tensions. In the present embodiment, the critical surface tension of the layer of the first ink on a printing medium is larger than the critical surface tension of the layer of the second ink on the printing medium.
When the second ink and the first ink are applied in that order to a printing medium, as shown in FIG. 1, the first ink is applied to an ink layer of the second ink having a lower critical surface tension, and the contact angle of the first ink is relatively large. Consequently, force F acting on the ink droplet in a direction parallel to the surface of the printing medium, or in the direction in which the ink droplet spreads over the surface and wet it, decreases.
FIG. 2 shows the surface of a printing medium onto which a plurality of droplets of the second ink and the first ink have been ejected in that order as in the case of FIG. 1, wherein the droplets of each ink are applied close to each other by the same print scanning operation.
When the first ink, applied after the second ink, has come in contact with the ink layer 50 of the second ink, as shown in FIG. 2, force F acting on the ink droplets in the direction in which the droplets spread over the surface and wets it is small, and accordingly, the droplets tend strongly to aggregate together. This causes marked beading, and consequently, the final image formed on the printing medium exhibits markedly increased graininess.
Furthermore, the ink droplets 51 cover only a small area of the ink layer 50. Accordingly, the final image is undesirably defined by a mixture of an area whose uppermost layer is made of the ink droplets 51 and an area whose uppermost layer is made of the ink layer 50. Thus, the uniformity of the resulting image is also degraded.
In particular, if an ink containing a fluorocarbon surfactant is first applied, the fluorocarbon surfactant reduces the critical surface tension of the layer of this ink, thus causing marked beading.
Therefore, if an ink having a relatively low wettability to an ink layer is fixed to the ink layer, the quality of the resulting image in, for example, graininess and uniformity is likely to be degraded. Such degradation resulting from beading becomes particularly significant when an image with a tone level from halftone to high tone is printed, that is, when ink droplets are applied close to each other at high density in one print scanning operation.