Five factors inaccurate overprinting
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The overprinting accuracy of letterpress prints is one of the important factors to measure the quality of embossed products. However, due to improper operation and process measures, there are many faults such as unstable positioning and misregistration in the printing process, which are harmful and must be taken seriously. There are many reasons for the inaccurate printing of embossed products:
1. Improper ink adjustment, too high ink viscosity, unstable position of the printed sheet front gauge
Practice has shown that maintaining the proper viscosity of the ink is an important condition for ensuring the quality of the printed product. The ink lacking a certain viscosity cannot be printed with an ink layer which is firmly colored, full and bright. However, there is no uniform standard for ink viscosity, and the viscosity of the ink can be adjusted empirically based on the characteristics of the printed material (substrate) and the structure of the printing plate. If the viscosity is adjusted too low, the rubber roller will slip, resulting in uneven ink transfer and ink distribution, and a "oily" shape will appear on the printed surface. If the viscosity of the ink is too large, the printed sheet in the embossing is easily pulled by the ink and slips with the drum, causing a malfunction in which the predetermined position is not accurate. For example, when the DT4OO four-open automatic platform printing machine prints a large-area solid version, if the ink is too viscous, it will have a strong pulling effect on the printed sheets on the drum, so that the sheets slide outward from the teeth, the front gauge The error can reach about 3mm, and the adverse consequences are very serious. In order to prevent this kind of failure, when printing a letterpress product with a large amount of ink absorption, according to the viscosity of the ink, an appropriate amount of detackifier or varnish may be added as appropriate to reduce the adhesion of the ink to the printed sheet to ensure the printing precision. .
2, the adjustment of printing pressure, the impact on printing quality
Printing pressure is an important factor in achieving uniform transfer of the print. The ideal printing ink layer is derived from a moderately balanced printing pressure. In general, when the surface of the substrate is rough and the printing surface is large, the printing pressure should be increased accordingly. However, in actual operation, the printing pressure cannot be blindly increased to satisfy the visual effect of the printed ink layer. The printing pressure is too heavy, which not only accelerates the wear of the machine and the printing plate, but also easily causes overprinting misalignment. This is because when the printing pressure is too large, on the one hand, the printed sheet will be deformed, causing the printing plate to appear enlarged, causing the overprinting to be inaccurate; on the other hand, the printing pressure is too large, and the printing on the cylinder will be oppressed when the printing is performed. The pulling force causes the printed sheet to slip outward from the gripper, which will make the pre-pressed position unstable and appear small. In order to avoid such adverse consequences, the pressure adjustment of the letterpress printing should be gradually and lightly pressed in the case where the amount of ink supplied to the layout is normal, so that the appropriate printing pressure can be accurately adjusted to ensure the printing quality.
3, the printing plate bottom is not solid, the flatness and the compressive strength difference cause the printing to be inaccurate.
The hardness and precision of the plate support (sticker bottom) used for letterpress printing greatly affects the printing accuracy of the product. If the traditional gluing plate is used as the plate for large-format solid letterpress printing, the compression deformation is easy due to the poor flatness, fastness and compression resistance of the plywood. In the printing process, it is bound to be uneven due to uneven pressure or empty bottom. Large printing pressure, which is easy to cause distortion of the printed surface, making the printed surface larger than the actual version. In addition, since the wooden bottom is easy to bow, the printing of the cylinder in the embossing will inevitably cause slippage due to slight vibration or movement of the printing plate, resulting in misregistration of the product. Therefore, in the printing plate with large printing width and pressure, it is better to use a magnetic plate or an aluminum plate to ensure the accuracy of product overprinting.
4, the good lining process technology to prevent the deformation of the printed product caused by overprinting
The lining is a bridge to realize the transfer of the layout, which has a certain influence on the printing precision. Due to differences in the structure of the equipment, the requirements for the thickness of the lining are also different. Only by strictly controlling the thickness of the lining of the machine according to the specifications of the printing machine can the printing layout be kept out of shape. If the thickness of the lining is not enough, the longitudinal dimension of the printed layout is often smaller than the corresponding printing plate length; if the thickness of the lining exceeds the specified parameters of the equipment, the printed layout is larger than the actual printing surface, which is another factor affecting the printing precision.
In addition, if the lining package is not tight enough, there are phenomena such as looseness, bulging, and large-area excavation of the outer lining, or only the side of the nip part is pasted on the padding paper, and one example of the tipping position is not pasted, letting it float This will also cause the printing plate to be greatly deformed, resulting in printing misalignment. In order to ensure the accuracy of the letterpress printing, it is preferable to use a neutral and rigid packing material, and to wrap the packing material tightly.
5. The offset of the roller bite causes the position of the printed sheet to be inaccurate.
When the solid version is printed, if the pressure of the roller is insufficient, the printed sheet in the embossing can easily slip outward from the gnawed teeth due to the degree of reverse peeling resistance of the printing sheet. For example, the DT400 four-open automatic platform printing machine is prone to the above-mentioned drawbacks when printing a large area of the field letterpress product. The main reason for the research is that the pressure of the spring steel piece of the original machine drum is not enough, so that the bite does not bite the paper edge. The author has added steel sheets to the drum teeth to achieve the seizure of the sheets, preventing the sheets from slipping and moving when the embossing is performed, and the printing accuracy is better ensured. In addition, the machine has another drawback that affects the positioning of the sheet, which deserves attention. That is, after the printed sheet enters the drum to bite the teeth, there is a slight backlash at the end of the printed sheet at the moment of closing the crepe paper, which causes a printing skew phenomenon. The main reason is that ink or agglomerated paper dust accumulates in the teeth corresponding to the skewed side. When the teeth are closed, the accumulated dust in the teeth just hits the edge of the paper, causing the front position to be inaccurate. As long as the dust in the teeth is cleaned up, the fault can be eliminated.
In summary, the good operation and process technology can not only effectively guarantee the printing quality of the letterpress printing, improve the printing precision of the product, but also improve the production efficiency of the enterprise.