The working environment is the basic condition for achieving the ideal printing effect. The working environment problems mainly include three aspects: temperature, humidity and cleanliness. If the working environment of the offset printing workshop is unreasonable, it may bring some practical difficulties and process failures to the production of offset printing.
1 The impact of cleanliness of the operating environment
The production workshop should first be clean and dust-free. Dust is the most culprit that causes excessive pinhole leakage of printed products. When printing, if there is a lot of dust in the workshop, it is easy to adsorb to the surface of the substrate and the printing plate, and even mixed into the ink, it will block the mesh during printing, so that normal printing cannot be performed.
2 The effect of temperature and humidity on the substrate
The water content of the paper changes with the change of environmental temperature and humidity, and the paper's water absorption and water loss ability is related to its own nature, structure and raw materials. Generally speaking, the higher the density of the paper, the longer the fiber and the higher the sizing degree, the lower the water content. vice versa. The water content of the paper produced by the paper mill is usually 5% to 7%. During the transportation, storage and printing of paper, as the temperature and humidity of the environment change, its water content will increase or decrease. This change in water content Offset printing is often harmful and unhelpful.
1) The paper absorbs water. The paper absorbs water and causes the density around it to decrease, the fiber gap increases, and the paper deforms to show a "lotus edge", and the four sides undulate and warp. The difficulty of this deformed paper for offset printing is: For the machine with a hem type front gauge, it is difficult to enter the front gauge due to the undulation of the paper gripper. The operator must increase the pressure of the pressed paper to achieve the purpose. The consequence is that the side gauge does not pull the paper in place, the overprint is not accurate, and even the paper will cross the front gauge and enter the printing unit directly, causing the plate to be broken, the blanket and other failures; for the machine with the front swing type, the convex part of the paper enters the front When it is regular, it will collide with the paper tongue and pad of the front gauge, which will cause the paper to not be in place and affect the accuracy of overprinting. At the same time, the paper is slightly uneven, which also makes Feida difficult to separate the paper normally.
2) Paper loss of water The loss of paper will cause shrinkage and form "tight edges", that is, concave (or convex) in the middle, and the four sides are raised (or down). When printing this paper, it is difficult to separate the paper. The second is that when it enters the impression cylinder, the stress release is uneven, and it is easy to wrinkle, which makes the product scrapped. Therefore, before printing, the paper should be piled up in the printing workshop to allow it to adapt to the workshop environment, so that its temperature and humidity can be balanced with the workshop environment; or install the temperature and humidity adjustment equipment in the paper library to facilitate the paper and printing workshop The temperature and humidity of the environment are consistent. In the printing process, strictly control the moisture of the layout, and neatly stack the semi-finished products.
3 The influence of temperature and humidity on ink
The viscosity and fluidity of the ink change with temperature. The higher the temperature, the lower the viscosity and the greater the fluidity; the lower the humidity, the faster the evaporation and drying of the ink, and the higher the humidity, the slower the evaporation and drying of the ink. This characteristic of the ink has a great relationship with the production process and the quality of the print. The performance is: the appropriateness of the viscosity directly affects the transfer of the ink, the firmness of the imprint, the amount of ink penetration and the gloss of the printed product. If the fluidity is too small, the ink will not spread smoothly and unevenly, resulting in unevenness of the ink color on the same printing surface; if the fluidity is too large, the imprint will not be transferred accurately, the graphic level is not clear, the ink color is not full, and the color Not bright, reducing the print quality. There is also a close relationship between the viscosity and fluidity of the ink. While increasing the viscosity of the ink, it will reduce its fluidity and vice versa. It can be seen that, according to temperature changes, correct adjustment of the viscosity and fluidity of the ink is important for printing. In the cold winter, the temperature is too low to easily cause the following two process failures.
1) When printing under the condition of 8 ～ 10 ℃, because the fluidity of the ink is too small, the viscosity of the ink is greatly reduced due to the addition of ink adjustment oil, the ink forms floating ink on the printing plate and text, and is transferred to the water roller, water After the roller turns for one week, the ink is inked on the blank of the printing plate. As a result, the ink roller and the water roller simultaneously supply ink to the printing plate, forming a special ghost image. The ghost image is exactly the circumference of the water roller from the actual mark.
2) The temperature is too low, the ink viscosity is too large, it is difficult to adjust the ink viscosity and fluidity to make it have good printability. At this time, if you add various auxiliary agents to reduce the viscosity of the ink, it is necessary to increase its fluidity, which will lead to failure of the printing process. Because the temperature is too low, the internal cohesion of the ink increases, the activity of the oil molecules weakens, and the dispersion force decreases, which makes the ink not easy to break the wire. During printing, the imprint gripping force is not enough to overcome the combined force of the adhesion and peeling force of the blanket to the paper. , As a result, the paper was stuck on the blanket, and it was impossible to produce normally.
The above is a brief summary of the author through years of actual production. It is recommended that qualified manufacturers improve the working environment of the workshop as much as possible. If a specific link is not paid attention to, it will affect the printing effect. Therefore, to achieve high-quality printing results, we must pay attention to every small link in the process.
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