In recent years, particularly in Europe, America and other countries, ultra-thin and super-miniature corrugated cardboard has quietly occupied more and more corrugated cardboard markets, because it also has good physical properties and printing performance of corrugated cardboard and cardboard. Compared with the traditional thick cardboard, it has the characteristics of good strength, strong cushioning force, good elasticity, saving material, light weight, and good printing effect. Compared with traditional corrugated cardboard, micro-corrugated cardboard has the advantages of small size, high rigidity, compact structure, strong and flat, light and thin materials, good compressive performance, and the ability to directly print with an offset press, which has changed in the past only in the flexo version. The printing process that directly prints on the press and can only be printed after the offset press first, simplifies the process, shortens the production cycle, and improves the production efficiency.
The commonly used miniature corrugated cardboard has F-type (0.75mm), G-type (0.5mm), N-type (0.46mm), O-type (0.3mm) and so on, are three-layer composition of instant paper, crucible paper and bottom paper . At the same time, micro-corrugated cardboard also has the following characteristics: (1) high strength, can enhance product protection, increase the strength of cardboard than 40%; (2) light weight, 40% lighter than thick cardboard, lighter than cover corrugated cardboard 20 %; (3) smooth surface, exquisite designs, colorful colors, and strong visual effects.
1. Offset principle and its requirements for corrugated board printing
Currently used in corrugated board printing are flexographic printing, gravure printing, and recoating of first offset printing paper. Among them, flexo printing is the most commonly used, but the printing accuracy is not very high, generally only a few low-grade rough cartons can be printed, and gravure printing and offset printing are preprinting processes, that is, first printing a good pasta and then proceeding Bonding, although it can obtain better quality, the process is complicated and the cost is high. Therefore, offset printing directly on corrugated cardboard is a new process pursued by the packaging printing industry. The current KBA Rapida Model 105 and Manroland 700, 900 can both be offset directly on the miniature corrugated board, and the printing quality can reach a very high level.
Offset printing is the use of the natural law of oil and water immiscibility, in the printing plate almost on the same plane, so that the graphics part only adsorbs ink, the blank part only absorbs water, and transfer the graphic ink to the substrate through the blanket. . Due to its high image restoration and color reproduction capabilities, offset printing is currently the most widely used and most mature printing method. At present, it accounts for more than 50% of all printing in China and mainly prints on paper. In recent years, with the rapid development of packaging and printing, flexographic printing, gravure printing, and screen printing have made tremendous progress, and the development momentum and proportion of offset printing have declined.
At present, since the offset printing technology has been made directly on the miniature corrugated paper, a breakthrough has been made, which will make the offset printing process return to life. In the field of traditional fiber cardboard boxes, such as alcohol, small appliances, shoes, hardware tools, microelectronics, computer software, counter sales, fast food and so on, offset printing has already begun because of its better printing quality. Traditional thick corrugated cardboard competes for the market.
However, the use of fountain solution in offset printing (the fountain solution is mainly to keep the blank part of the printing plate clean and the main component of the fountain solution is water), which makes the offset printing process become very complicated. The mixture of ink and water will produce ink emulsification, which will cause the paper board to absorb water and cause deformation and strength, which will affect the color, viscosity, and drying performance of the ink. Therefore, the control of water and ink balance becomes critical. If the amount of water is too large, the ink will be over-emulsified, the drying will be slow, and the color will become light. In particular, the corrugated board will absorb more moisture and the compressive strength and surface strength will decrease, and even the printing pressure will not withstand the collapse. The increase in the size of the cardboard causes inconsistency in the deformation of the facial tissue and the inner paper, which degrades the printability. Therefore, the direct offset water control of the corrugated cardboard is more strictly controlled than that of the normal paper printing. However, if the amount of ink is too large, defects such as dot gains, hierarchical levels, and dirtiness will occur. Therefore, the ink balance must be controlled during the printing process, especially the amount of water.
Direct offset printing on corrugated board, in pressure control, requires lighter than ordinary paper. Since the middle corrugated medium of corrugated cardboard is empty, if the pressure is too great, it will cause failures such as dot enlargement and hierarchical merger, and at the same time, it will produce the phenomenon of â€œsqueeze boardâ€, and in severe cases it will cause crushing. Therefore, the pressure control must be more precise.
Due to the special structure and special requirements of corrugated board, this type of offset press generally adopts a special blanket with good compressibility and a certain hardness, so that the different compression resistance of the corrugated cardboard surface can be compensated by the compression deformation of the blanket. Performance and deformability to improve print quality.
2. Impact of offset printing on corrugated board printing
(1) Effect on the strength of corrugated board
The great pressure of offset printing will reduce the compressive strength of the corrugated board; the use of fountain solution will cause the surface strength and compressive strength of the board to decrease due to water absorption.
(2) "Panboard" phenomenon
The washboard phenomenon is the most common quality problem in corrugated board printing. This phenomenon may occur if the pressure and ink amount are not well controlled during printing.
(3) Imbalance of ink and wash
The greatest impact on quality in offset printing is ink-water balance, especially when the water volume is too large to have a greater impact on micro-corrugated cardboard.
(4) Fountain solution acidity
Too strong acid will make drying slower and corrode the printing plate; too weak acidity will not form an effective hydrophilic protective layer on the blank part of the printing plate.
(5) Blanket performance
The properties of the blanket include surface properties and compression deformation properties. The surface properties are the assurance of ink absorption and transfer of ink, and the compression deformation properties are the basis for obtaining high quality images on corrugated cardboard.
Direct offset printing micro-corrugated cardboard is a brand-new printing process that achieves a very high level of printing effect and printing quality. It is basically comparable to the printing quality of paper. It will become a high-grade corrugated cardboard box and a carton. The preferred cardboard for packaging can bring huge economic benefits to packaging and printing companies, and is one of the main directions for the development of offset printing in the future. This is a re-ignited light for the development of offset printing.
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