Drop size
Label products are the first to use digital printing. 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 and printing is gradually increasing. At the same time, the quality of digital printing has become a topic of discussion. This article introduces the core technology of digital printing – a key technology that affects print quality.
High-quality color image reproduction requires sufficient dynamic range and precise control of image tonality, neutral gray balance, graininess, gamut range, detail and sharpness, which requires an increase in the recording resolution of the inkjet printhead. The fundamental solution to this problem is to reduce the size of the recording dots for inkjet printing. After the ink droplets are ejected onto the surface of the printing material such as paper, the residual kinetic energy impinging 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 when the expansion and infiltration process is finished, the ink droplets are converted into records. point. As can be seen from this process, there is a causal relationship between the volume of the ejected ink droplets and the size of the recording dots on the surface of the printing material, and thus reducing the size of the recording dots means reducing the size of the ejected ink droplets. At present, the size of the ink droplets for the fine image printing has been reduced to 3.5 picolitres (pl, 1 picolitre = 10-12 liters), and the size of the ink droplets for outdoor inkjet printing and packaging printing is between 9 and 50 pl, such as The HP Scitex FB7600 product manual has an ink drop of 42 pl.
Record resolution and gray level
Recording resolution refers to the number of dots that an inkjet device can print within a unit distance of the substrate. In general, the higher the recording resolution, the smaller the denser the recording point of the ink drop conversion, and the finer the printed effect. However, the above conclusions only apply to binary inkjet technology, that is, printing can only be done by nozzle ejection or no ink droplet ejection, and most word nozzles now support multi-gradation printing based on the density modulation principle. Multi-gradation printing refers to a printing technique that changes the density of recording dots by changing the size of the ink droplets at the same recording resolution, thereby producing different color tones. The 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, the recording resolution of 360 dpi combined with 16 gray levels can produce a visual effect equivalent to 1400 dpi binary printing. Many of the commercially available inkjet devices on the market support multi-gradation printing. For example, the HPScitex 11000 can eject ink droplets of 15 pl, 30 pl, and 45 pl through HP Scitex high dynamic range printing technology to dynamically control the droplet size. , so that each recording point can form a density of up to 16 gray levels, achieving a smooth transition color printing effect, and finely expressing image details.
Substrate and ink matching
The surface characteristics and porosity of the paper determine the extent to which the ink expands and penetrates into the paper fibers. The expansion and absorption of the ink in the paper fibers will blur the edges of the recording dots, limiting the sharpness of the edges of the recorded dots, and the adjacent recording dots are likely to be crowded together, resulting in a de facto reduction in the resolution of the recording dots. The ink penetrates too much into the interior of the paper, causing the ink layer formed on the surface of the final paper to be too thin, which in turn affects the optical density and tone performance of the printed result. In addition, the expansion and penetration of the ink also prolongs the drying time. In order to speed up the drying process, some equipment uses an evaporative drying technique that uses forced hot air flow to treat the surface of the paper.
In the face of the problem of matching inkjet printing ink to paper, three solutions have emerged:
One is to make a fuss about ink, such as HP Scitex series equipment, Agfa's Jeti series equipment, Konica Minolta KM-1 use UV ink, use UV ink to dry quickly to solve this problem, and then like Xerox Riverr The 2400 uses high-melting phase-change ink. After the waxy ink is melted, the ink droplets are ejected. When the ink droplets reach the surface of the paper, they are gelled after being solidified by cooling, thus effectively preventing the ink from penetrating on the surface of the paper.
Another solution is to pre-process the paper before printing, using water-based inks, such as Kodak, Hewlett-Packard, Fujifilm and other manufacturers of inkjet printing equipment.
The third option is to use the transfer method (that is, the ink droplets are sprayed onto the transfer blanket first, and the ink is evaporated to form the ink film and then transferred to the paper surface). The equipment using this scheme is Landa S10, Canon Voyager.
There are many kinds of printing materials for packaging and printing. In addition to paper, plastics, metals, glass and various composite materials are also common. These materials generally have the property of being non-absorbent to ink, and inkjet printing can use UV ink to solve the problem of ink drying, but it is followed by 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-press the surface of the printing material. In addition, when UV inkjet printing is performed on a surface such as a can, a bottle, or the like, or a surface in which a deformed leather is often used, there is a high demand for flexibility of the ink layer after drying, which is a challenge for UV inkjet printing.
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