Dry offset and offset printing of plastic printing

Dry relief offest

In the case of color printing, the thickness of the ink layer is only 2 to 3 μm. If the ink is directly transferred from the printing plate to the plastic surface, it is difficult to be uniform, so the dry offset printing method is used. Dry offset printing is a combination of embossing and offset printing. It is transferred from the relief to the blanket and transferred to the surface of the substrate. Since the letterpress does not require water, the name of dry offset printing comes from this. The advantage is that the ink transferred from the printing plate to the blanket can be uniformly transferred to the plastic surface due to the elasticity of the rubber regardless of the irregularities of the printed plastic.

The difference between dry offset printing and offset printing lies in that the offset printing plate is a flat surface, which uses a photosensitive material to make a lipophilic graphic, the remaining blank part is hydrophilic, and then the principle of oil-water repulsion is used so that the ink is only stained on the ink. The graphic part of the edition; and the dry offset printing plate is a relief printing plate, the blank part is concave, does not need to use the fountain solution, can cause the ink to stick only in the protruding graphic part.

Printed plastic dry offset printing plate is generally a resin plate with a metal substrate or a plastic substrate. After the exposure, the resin of the graphic portion is cured, and the uncured blank portion of the resin is removed by development, and finally the protruding graphic is left. The ink roller sticks ink on the letterpress of the letterpress, and then it is transferred to the blanket to form a color picture, and then it is rolled through the blanket and the printing object, and the ink is delivered once. Because the printing plate is only in contact with the rubber, its resistance to printing is quite high, reaching 100 to 2 million impressions.

There are two kinds of drying methods in the printing process: infrared drying and ultraviolet drying.

Infrared drying is to evaporate and dry the solvent in the solvent ink, and the remaining binder and pigment adhere to the plastic surface. Since the temperature of the printed plastic is also increased by heating and drying the volatile solvent, it should be noted that the temperature of the drying tunnel cannot exceed the heat-resistant temperature of the plastic and cause plastic deformation.

Purple meal drying is the use of UV ink printing, after a certain wavelength of ultraviolet irradiation to initiate polymerization and curing, the entire ink layer for the network cross-linked structure, ink layer, chemical resistance, abrasion resistance is better than solvent-based ink . Due to its advantages of instant drying, low temperature curing, and solvent-free emission, it has been widely used at home and abroad in recent years.

Many plastic products are printed circumferentially. In order to fully irradiate the ultraviolet light, a short tunnel device is installed in the subsequent part of the printing press. There is a conveyor belt inside, which conveys the printed plastic parts and rotates them horizontally. Irradiated on all ink film surfaces and instantly cured.

Plastic products and sheet-like printing materials are different. Many plastic products need to be repackaged and packaged one by one after printing. If improper solvent inks are used, residual solvent exists after the ink film is formed, and environmental requirements are not met. The use of UV ink instant curing, there is no environmental problem; in addition to the use of UV ink, printability is particularly good, due to the absence of solvent, the ink fountain, uniform ink roller on the viscosity change is minimal, onboard stability, network will not expand The printing and graphic arts are clear. In the UV drying process, the energy consumption is only 1/5 of the infrared drying, and the printing capacity is as high as 18,000 to 2,000 times per hour, which greatly shortens the work cycle and improves work efficiency.

In order to improve the adhesion to the ink, the surface of the plastic product must be subjected to a pre-treatment at the time of printing. In particular, a non-polar method having a low surface tension is a flame-retardant treatment. The flame contains excited O, NO-, OH-, and NH+4 to form a very thin oxide layer on the surface of the plastic to increase its critical surface tension. The treatment is very good, typically up to 48-60 dyne per cm. Flame treatment is applicable to various shapes of plastic products, and the circumferential plastic products can be rotated to allow each side to be processed. The main variables affecting the flame treatment effect are the air/gas ratio, the distance between the treated surface and the blue top of the visible flame, the speed at which the plastic product passes through the flame, and so on.

This kind of printing machine is generally an automatic multicolor machine, the layout is satellite type, the structure is compact, and the production efficiency is high. A few years ago, such machines also relied on imports, but with localization in recent years, the prospects of dry offset printing are promising.