Discussion on Several Problems in Fabric Digital Printing(I)

I. Overview

Six years ago, Zhejiang Honghua Computer Technology Co., Ltd. has started producing six-color digital printing machines for reactive dyes for fabrics. Shanghai Hualun Printing and Dyeing Company used it to produce hundreds of patterns. Process using cotton cloth training drift mercerization → rolling alkali (Soda 2% + urea 10% + alginate + anti-stain salt S appropriate amount) → drying → reactive dye ink inkjet printing → Steam (102 °C, 8 minutes) → washing → Soaping, printing effect and color fastness basically meet the requirements.

At that time, using water-soluble dyes as colorants, the color fastness of printed fabrics could easily meet the requirements of the index. Cotton, polyester, silk, wool, nylon, acrylic can use the appropriate dye for ink. However, due to color density, dye uptake rate, fixation rate mismatch and other reasons, the hue and shade of the color mixture are difficult to completely match, which is common in active, dispersed, and acid dye inkjet printing. The problems that existed at the time of paint printing were: dark color, low light color, poor printability, and easy clogging of nozzles.

In the past five years, digital textile printing technology has made great progress. However, with scale production, several technologies still need to be further improved. We will summarize, analyze and discuss the following issues from the information we have obtained for your reference and correction.

Second, on the digital printing means

Digital printing includes 'read' and 'write' two contents: the first 'read and write' is a digital video camera or scanner that reads the optical radiation signal, decomposes it into XYZ tristimulus values, and modulates it into a digital write RAM memory. The second 'read-write' is when the computer reads the digital information in RAM and copies it through the printer. Both reading and writing have a factor of error: the former modulates light into a digital image. Because of the difference in incident light and illuminance, the captured image can have various differences in hue and brightness; the latter reduces the digital pigment to hue. Due to the non-standard 'triple primary colors' and the concentration mismatch, the hue difference occurs.

In theory, the red, yellow and blue ternary color light can be mixed to obtain a 'white' color. Red, yellow and blue ternary color pigments can be mixed to obtain a 'black' color. The colors of the light belong to the additive mixture. Red, yellow, and blue, plus white light can spell various shades of light and lightness, as seen on a television screen. The mixture of colors is a subtraction mixture, with red, yellow, blue plus black, you can call out a variety of colors. The wavelengths of the three primary colors of CIE are: red 700.0; green 546.1; blue 435.8. If the reflected light of the three colors of red, green and blue exactly meets the wavelength requirements of the above three primary colors, various colors can be called out in different proportions. However, pigments with such pure shades do not actually exist, but there are only a few variegated colors. In this way, subtractive blending uses complementary colors to subtract each other, which inevitably brings in the third color component, further reducing the vividness of the color. In this way, it is impossible to use red, yellow, and blue primaries for pigment mixing, and magenta (red violet), yellow, and cyan (blue-green), or red, green, and blue for ternary colors to increase Green and red purple vividness. Because human vision is more sensitive to the bright colors of these two colors.

Inkjet printers have three colors (magenta, yellow, cyan), four colors (three colors plus black), six colors (four colors plus light blue and light magenta), and seven colors (six colors plus light black), etc. [6] . The 6-8 color inks used for textile reactive dye printing have green, red, blue and orange colors in addition to the four basic colors. The basic colors of ink companies are not the same. Regardless of the color of the reactive dye used, due to the inherent color impure, the second read-write copy of the image, the hue and the original can not be completely consistent.

Printing methods: needle, inkjet, laser and other printers. Digital fabric printing, currently using inkjet printing technology. There are three types of nozzles for ink jet printing, and the nozzle diameters are mostly in the range of 10-100 μm, and they have all been used for fabric printing.

The nozzle diameter of the printing machine is 50 μm. **The oscillator divides the flow to produce 62,500 drops of ink per second; advancement speeds up to 18-20m/s*** print resolution up to 1440dpi.

'Continuous' is equivalent to the pressure of 2 atmospheric pressure water gun, continuous injection, using the inductor to change the direction of the droplets, or shoot to the fabric or shoot to the recovery tank, the advantage is that the printing speed is fast, the disadvantage is the fineness is not good; 'bubble (BubbleJet-BJ) or thermal spray: like a steam gun, drops are dropped into the gun as needed, and the thermal element of the nozzle warms up to 350°C instantaneously. The low-boiling components in the ink quickly evaporate, generating explosive pressure and spraying the droplets. Out, the advantage is high definition, the disadvantage is that the inkjet direction is scattered, the nozzle is easily blocked; piezoelectric type: like a water cartridge, the piezoelectric ceramic deformation with pulse current, rapid extrusion of the ink in the gun, some models are also You can control the pressure drop to change the size of ink droplets, and the printing accuracy is higher. The advantage is that no heating is needed during ink jet, so no chemical change occurs in the ink and the nozzle is not easy to clog. The disadvantage is that the cost of the nozzle is high. Previously the most widely used office desktop inkjet printers, Canon and Hewlett-Packard (HP) used bubbles, and EPSON (Electronics) used piezoelectric. Now Canon has also added a piezoelectric type.

In terms of print speed, the continuous firing cycle is 62,500 drops/second and the printing speed is 300 meters/minute. The recording speed is 2000 symbols/second, and 48,000 newspapers can be printed per hour, which is more than 10 times more efficient than other printing methods. The other two types of current high-end inkjet printers can eject 2 microliters of ultra-fine ink droplets (diameter less than 1/10 of the hairline). The frequency is 5000 times per second, can be regarded as: Seiko secret agents.

Office printing paper, generally used matt printing paper; photo printing, with a thick coating of high-gloss photographic paper; high glossy paper, strong ink absorption, ink dot diffusion is small, the printed picture is sharp and sharp, dark part of the density, crystal clear translucent Very realistic. The sharpness and richness of fabric ink-jet printing are still difficult to compare with the print quality of paper, especially photos and pictorials.

In recent years, the development of multi-headed, multi-mouthed, large ink cartridge ink jet printers has increased the production capacity from 0.1 m/min to 2 m/min. Piezoelectric nozzles have accounted for 80% of the market [5]. Paint ink-jet printing is on the rise. There are several units that produce pigment inks. However, from the actual situation, the richness, sharpness, and color fastness of the ink-jet paints have yet to be further improved. In general, there are three key technologies for the development of paint ink-jet printing: 1. Select suitable pigments and adhesives to improve color density, lightness, and color fastness, respectively; 2. Quickly absorb ink droplets, reduce diffusion, and improve printing. Clarity; 3, to solve the nozzle blockage (now a fabric inkjet printer, most of them have thousands of nozzles), is an important prerequisite for the industrialization of digital printing.

Three, on the pigment

Ink-jet printing ink droplets fall on the receptor (paper or cloth) and are arranged in a matrix without overlapping each other.

Various mixed colors of ink jet printing are the combination of the basic color point arrangement and the result of visual synthesis. The perception of images is the image obtained through psychological comparison. The color of the face in the photo is exactly the same as that of a real person, and the inconsistency is good. However, if the fabric printing has a significant color difference as it is, customers will not accept it. In the point that it must be the same, the inkjet printing of fabrics is more demanding than the photo.

(I) Test conditions

5 million digital cameras;

CanonBJC255SP bubble three-color printer;

AcerScan-320P scanner with 600dpi;

Printing paper: matt printing paper, newsprint, crepe paper;

Fabric effect tester.

Note: There are three kinds of printing inks used in the printer: 1 original monochrome black ink; 2 original three-color printing ink; 3 commercially available bulk three-color ink. The bulk three-color ink is used as a print test, and the original three-color ink cartridge is used to print the final 'scripts' to correctly reflect the various color variations that occur in the test. (Note: The paper printing set can be used in black and white drafts.)

(b) Operation method

The film is taken with a digital camera; the film is printed on the film by the printer; the scanner scans the printed image into a 'remake' film; the film is continuously enlarged several times in a computer word file (the image is elongated (vertical or landscape) After 10 times cut out 9/10, then stretch 10 times, then cut out 9/10 again. Such a repetition. One-time enlargement up to 2x106 times, if you want to further enlarge, there will be a virtual image, unable to print)

(c) Test results:

1. The negatives taken by the digital camera and the photos printed by the printer are actually different from the colors of the real faces.

2, the original film and remake film, take the chin part of the local, repeatedly zoomed in, get Image-1-1, Image-1-2, Image-2-1, Image-2-2 four images (see Figure 3 ),

3, as can be seen from Figure 3, Image-2 printed with bulk ink is much redder than the original film Image-1, and the chin part is magnified by 103-106 times, and it can be seen that Image-2-2 also has a dim color. With but few, this shows that the basic color of the ternary is too light, and the picture taken is reddish.

4, three yuan basic color printing, it is impossible to increase the cyan ink jet ink to correct the hue. The only thing that is feasible is to increase the concentration of the cyan ink, and if the concentration cannot be increased, the cyan pigment with high coloring power can be used instead. Experiments show that the color density matching of the ternary basic color is a key requirement.

5. Bulk ink is formulated with dye. The reason why the cyan dye is very light is due to the solubility of the dye.

6. If the paint is used as inkjet ink, it can not be limited by the dye solubility and fixation rate. In addition, pigments with higher coloring power can be selected among various dyes.

(to be continued)

Reprinted from: Graphic Arts