The printing quality can't improve, have you done a good job in these aspects?

Starting with the first digital printing of label products, up to now, corrugated boxes, flexible packaging, folding cartons and other printed products can be completed using digital printing equipment. The proportion of digital printing in the field of packaging printing is gradually rising. At the same time, the quality of digital printing has become a topic of discussion. This article introduces the core technology of digital printing-the key technology that affects printing quality.

High-quality color image reproduction requires sufficient dynamic range, and precise control of image tone, neutral gray balance, graininess, color gamut range, detail and sharpness, which requires the need to improve the recording resolution of inkjet printheads. The fundamental solution to this problem lies in reducing the recording dot size of inkjet printing. After the ink droplets are ejected onto the surface of the printing material such as paper, the remaining kinetic energy impacting on the paper will be dissipated in the form of expansion on the surface of the printing material and penetration into the printing material. Only after the expansion and penetration process is completed, the ink droplets are converted into records point. It can be seen from this process that the volume of ejected ink droplets has a causal relationship with the size of the recording dot on the surface of the printing material, so reducing the size of the recording dot means reducing the size of the ejected ink droplet. At present, the ink droplet size of the nozzles used for fine image printing has been reduced to 3.5 picoliters (pl, 1 picoliter = 10-12 liters), and the ink droplet size of the nozzles used for outdoor printing and packaging printing is between 9 and 50 pl, such as The HP ScitexFB7600 product manual marked the ink drop as 42pl.

Recording resolution and gray scale

The recording resolution refers to the number of dots that the inkjet device can print within a unit distance of the printing material. Generally speaking, the higher the recording resolution, the smaller and denser the recording point of the ink droplet conversion, and the finer the printing effect. However, the above conclusions are only applicable to binary inkjet technology, that is, printing can only be completed by ejecting or not ejecting ink droplets, and most word nozzles now support multi-grayscale printing based on the principle of density modulation. Multi-grayscale printing refers to a printing technique that changes the density of recording dots through the change of ink droplet size under the same recording resolution, thereby producing different tones. The research results show that the visual effective resolution is approximately equal to the product of the recording resolution and the square root of the gray level of the recording point. For example, a combination of 360dpi recording resolution and 16 gray levels can produce a visual effect equivalent to 1400dpi binary printing. Many commercial inkjet devices on the market support multi-grayscale printing, such as HP Scitex 11000 through HP Scitex high dynamic range printing technology, can eject ink droplets of three sizes of 15pl, 30pl, and 45pl, and control the size of the ink droplets dynamically , So that each recording point can form a density of up to 16 gray levels, to achieve a smooth transition color printing effect, fine expression of image details.

Matching of substrate and ink

The surface characteristics and porous nature of the paper determine the degree of ink expansion and penetration in the paper fibers. After the ink spreads and absorbs in the paper fibers, the edge of the recording point will become blurred, limiting the edge definition of the recording point, and the adjacent recording points are very likely to squeeze together, causing the resolution of the recording point to actually decrease. The ink penetrates too much into the paper, resulting in the ink layer formed on the surface of the final paper being too thin, which in turn affects the optical density and tone performance of the printed result. In addition, the expansion and penetration of ink will also lengthen the drying time. In order to speed up the drying process, some equipment uses forced hot air flow to deal with the surface of the paper evaporation drying technology.

Faced with the problem of inkjet printing ink matching with paper, three solutions have emerged:

One is to make an article on ink, such as HP Scitex series equipment, Agfa Jeti series equipment, Konica Minolta KM-1 all use UV ink, use UV ink to quickly dry to solve this problem, and then Xerox Triver 2400 uses high-convergence phase change ink. The waxy ink melts and ejects ink droplets. The ink droplets reach the surface of the paper and become gelatinous after cold curing, thus effectively preventing the ink from penetrating on the surface of the paper.

Another solution is to pre-treat the paper before printing and use water-based inks for printing, such as inkjet printing equipment from manufacturers such as Kodak, HP, and Fujifilm.

The third option is to use the transfer method (that is, the ink droplets are first sprayed onto the transfer blanket, and the ink is evaporated to form an ink film, and then transferred to the surface of the paper). The devices that use this solution are Landa S10 and Canon Voyager.

There are many types of printing materials for packaging and printing. In addition to paper, plastic, metal, glass and various composite materials are also relatively common. These materials generally have the characteristics of non-absorptive to ink. Inkjet printing can use UV ink to solve the problem of ink drying, but then it is the problem of the degree of fixation of the dried ink on the surface of the printing material. At present, the means to solve this problem is mainly to pre-treat the surface of the printing material before printing. In addition, when UV inkjet printing is performed on curved surfaces such as cans and bottles, or often with deformed leather, the flexibility of the dried ink layer is high, which is a challenge for UV inkjet printing.

The author is from Shanghai Publishing and Printing College Zheng Liang