Deformation analysis of the working paper before t

2022-07-30
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Prepress operation - deformation analysis of paper 5. Deformation analysis of paper

excerpt from classic books on the home page of the latest printing technology practical manual - relief printing technology Volume 5. Deformation analysis of paper

deformation of paper during printing mainly includes pressure deformation, tensile deformation and expansion deformation that drives the development of upstream graphite industry. Here, the tensile deformation and expansion deformation are mainly introduced

(I) stretching deformation of paper

stretching under the action of external force will also cause deformation of paper. Under the action of external tension, internal stress is generated in the fiber. When the external force is removed, the internal stress disappears, and the paper returns to its original size. Such deformation is called "elastic deformation". If the paper cannot recover its original size when the external force is removed, the internal stress (so-called retained deformation) remains in the paper

with the increase of tensile force, the elongation also increases. As a result of this tension, stress is generated in the fiber

-- if the tensile force of 256 relief printing technology exceeds a certain value, the fiber will break. This should be paid special attention to for web printing. If necessary, it can only be solved by reducing the speed

(II) expansion deformation of paper

paper is composed of plant fibers. Therefore, the hydrophilicity of paper determines that it has the ability to absorb water. The capillary (porous) of paper can "exchange" water with the surrounding air at any time. PEU under the same conditions. Paper is a water absorbent material, which has the ability to absorb moisture from the humid air, and can also disperse into the dry air. "One researcher said that hair moisture

the fibers that make up the paper expand after absorbing water. The more gaps in the fibers, the more severe the fiber expansion. To a large extent, the expansion of paper fibers is related to the amount of fillers. The hydration of fiber plays a decisive role in expansion. The more fiber processing, the more sufficient beating. Its adsorption capacity per unit area is also stronger

for fine prints, the registration requirements are high, and generally only the registration error is allowed to be within. In multi-color printing, there are many factors that cause inaccurate overprinting, such as abnormal operation of paper positioning device and paper feeding system, excessive wear of printing plate, too thick or thin liner, excessive printing pressure, aging and deformation of rubber, too much wetting liquid, and loss of printing machine accuracy, which all have varying degrees of impact on overprinting accuracy. The influence of these factors is complex. Some can compensate each other, while others cannot. An important factor among them is the deformation of paper due to the change of water content in the printing process, which needs the preferential support of national policies

as we all know, the change of the water content of the paper not only causes the expansion or contraction of the paper, but also changes the local size of the paper due to the change of the water content, thus forming other forms of changes, such as curl, wrinkle, wavy shape, etc. Some of this deformation is reversible, but there are also irreversible parts that produce permanent deformation, which seriously affects the quality of printing. For example, when the relative humidity of the printing workshop is high and the original water content of the paper is relatively low, the edge of the paper will absorb water and stretch after it is stacked in the workshop for a period of time, and the paper will have a "lotus leaf edge" phenomenon if the middle part does not have time to absorb water. On the contrary, the relative humidity of the air in the printing workshop is relatively low, and the original water content of the paper is high, resulting in "tight edges" due to rapid edge dehumidification. When printing with such a paper, due to the uneven edge of the paper, it is easy to cause inaccurate positioning of the front gauge and side gauge, thus affecting the overprint accuracy. Therefore, it is the main subject to study the printability of printing paper to study the influence of moisture absorption and moisture content on the dimensional stability of the paper, so as to formulate the printing process and humidity regulation specifications according to the law

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