JPS60153669A - Ink-jet recorder - Google Patents

Abstract

PURPOSE:To allow high-quality variable density pictures by drying forcibly ink drops printed on recording paper and then printing an ink drop at the same point. CONSTITUTION:Ink drops printed on a recording paper 7 are forcibly dried by an ink drying means. After completion of the drying, the following ink drop 6 is printed at the same point. With this forcible drying, an ink solvent on the recording paper disapperars, only a dyestuff or a pigment and trace quantity of a high-boiling point solvent remain, and there is no problem for double printing. Density is controlled by the number of printed ink drops. The maximum number of double printing is decided by that of gradation required by a variable density picture. Thus high-quality various density pictures can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inkjet recording apparatus for displaying grayscale images and grayscale characters.

An inkjet head of a well-known drop-on-demand type inkjet recording device will be explained. FIG. 1 is a sectional view showing an example of the configuration. One end of the injection block 1 is
It is open to the atmosphere through an orifice 2 for ejecting ink droplets, and the other end communicates with an ink supply hole 3. Ink is supplied to the ejection chamber 1 through the ink supply hole 3. The flexible upper plate forming part of the wall of the injection chamber 1 includes:
A piezoelectric element 5 is attached to form a small mora. By applying an electric pulse to the piezoelectric element 5, the upper plate 4 injects and bends in the direction of reducing the volume of the chamber 1, generating a pressure wave inside, and this pressure wave causes the georifice 2. During recording, ink droplets 6 are ejected toward the recording paper 7.

Conventionally, when displaying pseudo-gradation using the inkjet or jet head, an area gradation method such as a dither method or a density bang method has been used. This is a method of expressing gradation by controlling the area covered by ink per unit area υ on the recording paper surface. More specifically, one pixel is n
This is a method of expressing (nxm+x) gradations from 0 to nXm@ by expressing it as a dot matrix of Xm and depending on how many of the nXm dots are turned on. For example, when the dot matrix is 2×2, five gradations shown in FIG. 2 can be expressed, and the pixel resolution is V2, which is the dot resolution. In the figure, 8 indicates a black dot on the recording paper.

If the dot matrix is 3.times.3, for example, 10 gradations shown in FIG. 3 can be expressed, but the pixel resolution will be v3. Similarly, if the dot matrix is 4 x 4, 17 gradations can be expressed, but the pixel resolution is l/
In the case of 4 to 5×5, 26 gradations can be expressed, but the pixel resolution becomes 115.

As described above, there is a trade-off between the number of gradations that can be expressed and the pixel resolution, and there is a problem in that if a sufficient number of gradations is attempted, the image becomes rougher. What is one way to satisfy both?

The goal is to increase the dot resolution.

However, in actual printers, there is a limit to the dot resolution. In the case of a drop-on-dead type inkjet recording device, increasing dot resolution requires making the ink droplet diameter smaller, but this introduces a smaller nozzle diameter and increases the risk of nozzle clogging.

Also, the dot positioning accuracy becomes relatively strict.
As for the problem, 10 to 12 rounds/fight is considered to be the practical limit. Therefore, in printing grayscale images using conventional on-demand inkjet recording devices, it has been difficult to achieve both a sufficient number of gradations and pixel resolution.

In addition to the area gradation method, there is a density gradation method as a density expression method. This expresses shading by controlling the pixel density itself, and it is possible to achieve high multi-pixel resolution as one dot corresponds to one pixel, but the recording method itself can control the pixel density. It is possible only for some recording methods such as thermal recording method and sublimation thermal transfer recording method. Overprinting in the wire dot recording method can also be considered a type of density gradation method, but in conventional inkjet recording devices, overprinting causes excessive moisture on the recording paper, causing problems such as ink flow, strike-through, and deflection of the recording paper. It was impractical.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inkjet recording apparatus that enables a density gradation method to produce finer and higher quality gradation images.

The inkjet recording device of the present invention is an inkjet recording device that reduces the volume of the ejection blockage by applying electric pulses to a piezoelectric element and causes ink droplets to fly from the ejection chamber to the recording medium. After the ink droplets ejected are forced to dry, the next ink droplet is ejected at the same point, and the density is controlled by the number of ink droplets ejected at the same point.

According to the present invention, the density is controlled by repeatedly ejecting ink droplets onto the recording paper a plurality of times.

If the maximum number of overlapping operations is n times, it is possible to display gradations of L:=n+1 (1). In order to avoid problems such as ink flowing out and bleed-through due to overprinting, ink droplets on the recording paper are forcibly dried by an ink drying means. After the drying is completed, the next ink droplet is applied. Due to forced drying, the ink solvent on the recording paper is blown away, leaving only the dye (or pigment) and a small amount of high-boiling point solvent, so there is no problem in the next overprinting.

As the ink, a normal aqueous ink can be used, but from the viewpoint of drying properties, it is better to use less high boiling point solvent. The maximum number of overlapping operations n is determined by the number of gradations required for the grayscale image.

The recording density on the recording paper increases as the dye density increases, but eventually becomes saturated.

The saturation value is determined by the type of dye and recording paper, but approximately □
It is about 1.5 for coated paper and about 1.2 for uncoated paper.

The ink density used in this apparatus is determined by overprinting n times, and the dye density is determined so that a recording density approximately equal to this saturation density value can be obtained.

A hot air heater or an infrared heater can be used as the ink drying means. Another method is to embed a heating element in the platen itself. Another method is to place a line-type thermal head on the back side of the recording paper. In this case, there is almost no need to control the thermal head, and it is sufficient to leave it energized during printing.

FIG. 4 shows an example of ink dye density versus recording density characteristics using water-based water-based dye ink and uncoated paper. Since it is uncoated paper, the saturation recording density (logarithm of the ratio of the amount of incident light to the amount of reflected light)
is approximately 1.2.

Table 1 shows experimental values of recording density when inks with different dye densities were overprinted.

Table 1 However, in this case, no wire or forced drying means was used, and the experiment was conducted in such a state that the next dot was placed and mixed together while the previous dot was still dry. As a result, it was found that the recording density of the dots did not increase but rather decreased as the dots were overshot. This is because the moisture on the Mori recording paper is too high, causing a large amount of the ink to penetrate into the fibers, and as a result, a large amount of dye is absorbed into the fiber layer.

Next, examples of the present invention will be described. An experiment similar to that shown in Table 1 was conducted using a forced drying method in which the next dot was implanted after the previous dot had completely dried.

This is shown in Table 2. As a result, a clear increase in density was obtained by overlapping ink droplets.

From the results shown in Table 2, it can be seen that in order to increase the recording density by overlapping ink dots, it is necessary to sufficiently dry the ink on the recording paper before depositing the next ink drop. Forced drying means are essential to obtain a sufficient recording speed.

FIG. 5 shows the increase in recording density due to multiple overprinting of the same dye density. As shown in this figure, the recording density increases with the number of overlapping strikes, and the increasing characteristic is
It can be seen that the ink dye once vs. recording density characteristic is in line with the figure. Therefore, it can be seen that finer recording density characteristics can be obtained by using ink with a lower dye concentration.

As explained above, according to the present invention, by forcibly drying the ink using the ink drying means, it is possible to print multiple dots overlapping each other and display shading, and it is possible to display shading at one point as in the density gradation method. Therefore, sufficient pixel resolution can be achieved even with a dot resolution that is not too high, and the dot positioning accuracy is also low, and the scanning mechanism can be constructed at low cost.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an inkjet head of a known inkjet recording device, and FIGS. 2 and 3 are 2×2 and 3×3 pixel maps of a conventional inkjet recording device, respectively.
A display diagram showing an example of IJ Fuchs gradation expression, FIG. 4 is a characteristic diagram showing the dye density vs. recording density characteristic, and FIG. 5 is a characteristic diagram showing the number of overlapping dots vs. recording density in an embodiment of the present invention. . In the figure, 1...Ejection chamber, 2...Orifice, 3...Ink supply hole, 4...
・Flexible upper plate, 5...Piezoelectric element, 6.
...Ink drop, 7...Recording paper, 1 figure% 2 figure 4 figure cow 5 figure 7 [H technique]

Claims (1)

[Claims] In an inkjet recording device that reduces the volume of a jet blockage by applying an electric pulse to a piezoelectric element and causes ink droplets to fly from the ejection chamber to a recording medium, the ink droplets shot onto the recording medium are forced to dry. After that, the next ink droplet is ejected onto the same point, and the density is controlled by the number of ink droplets ejected onto the same point.