Abstract:

Objective Because of the different surface properties of printing materials, the spreading and penetration of ink on the substrate are different. Fabrics are composed of regular fiber structure, which makes its interface have a directional diffusion path. The spreading and penetration behavior of ink on fabrics would causes the diffusion area of ink to be much larger than the theoretical area, known as the dot gain phenomenon, which leads to blurred edges of printing images, changes in color scales, and seriously affects the image restoration effect. Therefore, it is particularly important to reduce the excessive diffusion of ink on fabric and improve the quality of image printing.

Method In order to solve the problem of poor image restoration caused by excessive diffusion of ink on the fabric, this research proposed a method of cluster dot printing, which uses image processing software to gather and print the originally scattered printing points according to certain rules, which could effectively reduce the excessive diffusion of ink and achieve the effect of high-precision image reproduction.

Result Because of the difference in the material and structure of the printing medium, the diffusion of ink is different (Fig. 3). The perimenter of a single ink drop on the cotton fabric is 2.26 times that on the copper proofing paper, and the area is increased by 36% (Tab. 1). The ink expansion rate on cotton fabric was significantly higher than that on copper proofing paper, which results in that the printing method applicable to the paper may not be applicable to the cotton fabric. Different printing methods were adopted to print on cotton fabric. The area expansion rate of scattered printing points on cotton fabric was as high as 40%, while the area expansion rate of clustered printing ellipse points on cotton fabric was only 16%, and the area expansion rate is reduced by 24%. Therefore, the clustered printing mode was shown to be effective in reducing the diffusion of ink on cotton fabric (Fig. 5). The change of printing mode could improve and reduce the ink diffusion and thus the printing effect. Lines are the units that constitute patterns. When printing lines of 0.5 mm, the difference in line thickness formed by clustered printing is less than 10%, far less than the 30% of scattered printing, greatly improving the uniformity of the print line, and making the print spacing closer to the ideal spacing (Fig. 6). Clustered printing could better restore the color scale of the image. Compared with scattered printing, the reflection rate of light was reduced by 9%. Compared with the points used for clustered printing, the light absorption rate of dispersion printing was significantly enhanced. This indirectly suggests that the point expansion of scattered printing may lead to the contact between adjacent points, which increases the actual ink area, thus increasing the light absorption (Fig. 8). The use of clustered printing would better restore the image and improve the printing effect. The final image of scattered printing would be much darker than the original image with some noise, which affect the printing quality. Compared with scattered points, the pattern printed in clustered printing mode would be brighter and reduce noise. This is because polymerization printing can effectively reduce the diffusion of ink on the fabric and improve the printing quality (Fig. 9).

Conclusion From the perspective of ink diffusion on the substrate, the diffusion of a single ink drop on the cotton fabric is greater than that on the copper proofing paper, which results in that the printing method applicable to the paper may not be applicable to the cotton fabric. Compared with scattered printing, the clustered printing technology gathers randomly printed dots to form dots of different shapes according to certain rules, which can effectively reduce the diffusion of ink on cotton fabrics, reduce the dot gain of ink drops, and thus improve the resolution of image repruduction. When printing lines, clustered printing can effectively avoid the uneven thickness of lines, and make the actual line width closer to the theoretical width. When printing images with different grayscales, the clustered printing can effectively avoid the grayscale darkening of the image caused by dot enlargement, and reduce the noise in the printing process, so as to improve the printing quality.

Key words: ink jet printing, clustered printing, area expansion rate, dot transfer characteristic, image restoration, ink jet printing for cotton fabric