Dirty printing is a common fault in color printing. The occurrence of such failures will affect the printing quality at least, and will cause large-scale scrapping of products, which will cause great losses to enterprises. There are many reasons for the stickiness of printed products, such as thick printing ink layer, the back of the paper is too smooth, improper ink adjustment, uncomfortable printing pressure, etc. The stickiness is not only caused by the characteristics of the ink, but also related to the environmental temperature and humidity. Only if the technology is carefully controlled can the printing quality of the product be guaranteed.
1. Correctly understand the relationship between printing pressure and ink layer thickness
Appropriate printing pressure is an important condition for achieving uniform transfer of the ink layer on the layout. Insufficient or uneven layout pressure will affect the uniform transfer of the ink layer. When the pressure of the layout is insufficient or uneven during printing, the ink color of the printed product will bloom, and the color thickness of the entire layout is also inconsistent. Increasing the amount of ink can indeed make up for the defects caused by the poor contact between the plate ink layer and the substrate caused by the lack of printing pressure, but it is also easy to cause the printed matter to become dirty. Therefore, the printing pressure should be adjusted first, and then the amount of ink in the layout should be adjusted, so as to obtain a good printing effect.
2. Handle the relationship between the printability of the paper and the thickness of the ink layer
Paper with poor surface smoothness absorbs ink quickly and absorbs a large amount of ink. When printing this kind of paper, if the printing pressure is insufficient or uneven, the bottom is easy to bloom after printing, if the amount of ink is increased, the ink layer is thicker, and if the blot drying speed is slow, it is easy to appear sticky. For paper with poor surface smoothness, the printing pressure should be increased accordingly; for paper with better surface smoothness such as coated paper and glass cardboard, the ink layer should be as thin as possible to avoid sticking. From the perspective of anti-sticking, printing a large area of ​​the plate should use "dark ink thin printing", that is, the hue of the ink is adjusted to be darker, and the ink layer is correspondingly thinner. Avoid sticking.
3. Grasp the relationship between plate characteristics and printing ink layer thickness
The quality of the printing plate, the thickness of the printing ink layer and the printing pressure have a certain relationship. If the smoothness of the printing surface is poor, the ink layer and the pressure required for its printing are large; otherwise, it is small. If using zinc plate printing, due to its poor ink affinity and poor surface gloss, only by increasing the printing pressure and the thickness of the ink layer can the full effect of the printing ink layer be achieved, but it is easy to cause stickiness, so it is not suitable to use the zinc plate Print color-printed products with more than 8 format. The photosensitive resin plate has good surface gloss, soft texture, flatness and ink affinity. The printing ink layer is thinned accordingly, and it can still achieve a good printing effect, and it is not easy to appear sticky. Therefore, when printing large-area images, photosensitive resin plates should be used.
4. Avoid sticky back surface caused by improper printing ink layer thickness
To determine whether the printing ink layer is moderate, the main basis is that the printing plate is evenly pressed, the printing plate is evenly exposed to the bottom, and the printing is not false, and the color of the printed product matches the sample. If the printing pressure is uneven or insufficient, the gloss of the printed paper is poor, the color of the printed original is dark, and the color of the ink is adjusted lightly, it is easy to cause the judgment of the thickness of the printing ink layer to be inaccurate. The back of the printed product will be sticky due to the thick ink layer.
5. Avoid the sticky and dirty printing caused by improper operation
(1) Improper adjustment of the ink roller, the amount of ink is too large or uneven
When the ink roller head and shaft hole have a large gap or are eccentric due to wear; when the colloid elasticity of the ink roller becomes poor, the ink roller makes uneven contact with the plate surface during the rolling and brushing of the ink along the plate surface, which is likely to cause local plate brushing Unevenness, thin ink layer. In addition, if the ink roller is adjusted too high or too high, it is also easy to make the layout ink layer coated insufficiently or unevenly. At this time, if the ink volume is increased blindly, it will make the local layout ink layer thick and cause sticking. Therefore, when printing a large-area plate product, the ink roller device requires high precision, the rubber roller has a good roundness, the rubber roller is soft and hard, the ink roller and the ink roller are in uniform contact, and the height adjustment should be accurate to reduce the stickiness of the printed product Opportunity.
(2) Improper receipt
When printing and receiving paper, the angle and height of the paper should be adjusted accurately according to the size of the printed product and the degree of softness of the paper. Otherwise, the edge of the printed sheet will easily touch the imprint of the previous printed sheet, which will damage the ink on the surface of the printed product. In addition, the printed sheets should be handled lightly to prevent sticking due to the movement and friction of the upper and lower printed sheets. Depending on the ink-receiving condition of the printing plate, the amount of air-drying paper should be mastered as appropriate to prevent heavy pressure from sticking.
(3) The bottom of the printing plate is not real, and the pressure is uneven
The plywood material is not strong enough, and the compression deformation coefficient is large. If this material is used as a plate support, the printing pressure and ink layer thickness must be increased, so it is easy to make the printed matter sticky. Therefore, it is better not to use plywood bottom brackets when printing on the ground. Magnetic plate or other metal bottom brackets should be used to make the printed ink layer thin and not spend, which is conducive to anti-sticking.
(4) The paper is too acidic, the climate is humid, and the printed matter is not easy to dry
When printing with offset paper or some domestic coated paper, the ink layer is not easy to dry due to the thick printing ink layer and the acidity of the paper, and it is also prone to sticking. At this time, dry oil should be properly added to the ink to prevent the printed matter from sticking.
(5) Knocking on paper creases causes friction on printed products
When printing coated paper, due to the smooth surface of the paper, if the paper creases are struck, it will easily rub against the imprint and cause sticking. Therefore, knocking on paper should be controlled according to the layout structure and paper characteristics. For thicker coated paper, try not to knock, just use the method of upper or lower kneading, and rub the paper straight and firm.
In summary, there are many reasons for the stickiness of color prints in printing. As long as we are technically well-controlled, not only can we get better anti-sticking effect, but also can improve the quality of printing and production efficiency. .
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