Paper absorbency and its effect on print quality
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The ink absorption of paper refers to the absorption capacity of paper by ink. It is related to the absorption of low viscosity components in the ink by the capillary pores of the paper and the penetration of certain components in the ink into the paper. It is the quality of the printed matter. One of the important factors.
Paper absorption process
The ink absorption of the paper is not only related to the looseness of the paper and the state of the capillary, but also related to the surface properties of the paper fiber, the content of the filler, the pigment, the rubber, the composition and characteristics of the ink, the printing method, the printing pressure and the like. In actual printing, the absorption of ink by paper can be divided into two stages.
The first stage is the printing press imprinting moment, which relies on the printing pressure to press part of the ink transferred to the surface of the paper into the larger pores of the paper, that is, the whole ink (including the pigment in the ink) enters the paper pores. This process is generally called Pressure infiltration phase. At this stage, the absorption of ink by the paper mainly depends on factors such as the printing pressure, the structure of the paper, and the viscosity of the ink. If the printing pressure is large or the structure of the paper is loose, the paper will absorb the ink more strongly. In general, paper with loose printed structure, such as newsprint, letterpress paper, etc., should have a lower printing pressure and a lower ink viscosity. For printing paper with tight structure, such as coated paper, the printing pressure can be appropriately larger, and the ink viscosity can be slightly higher.
The second stage is to leave the embossed area from the paper until the ink is completely dry. This stage mainly relies on the capillary action of the paper to absorb the ink, which is called the free osmosis stage. At this stage, the binder separates from the ink as a whole and enters the interior of the paper at a relatively slow rate through the rough surface of the small pores and paper fibers. Therefore, this process is actually a process in which the binder migrates from the ink to the paper pores, because the binder is separated from the ink as a whole, and the conjunctival properties remaining on the paper ink film will be changed. The fixation and drying of the ink are also in the process. carry out. At this stage, the rate at which the paper absorbs the ink determines the gloss of the print and whether offset, chalking, etc. occur. When the ink is transferred to the paper, the low molecular weight binder (solvent) will begin to penetrate into the paper layer as time passes, so that the solvent content in the ink layer on the paper surface will decrease, and the ink viscosity will increase. The ink layer is condensed.
The ink absorbency of the paper depends on the number of paper capillaries and the size of the capillary diameter. Paper is a porous material. There are many gaps between the fibers and fibers inside the paper, between the fibers and the filler, and between the pigment particles. These gaps are equivalent to many capillary tubes. Under the action of these capillaries, the binder in the ink can be absorbed, and the thicker the diameter of the capillary, the faster the absorption of the ink. Therefore, the pore structure of the paper determines its absorption of the ink.
The looser the paper, the larger the pores and the stronger the ink absorption. For example, newsprint is relatively loose and has strong ink absorption. Therefore, the ink transferred to the newsprint is fixed fast, which is conducive to the improvement of printing speed. Coated paper is not as loose as offset paper, and the pores are not as large as offset paper. However, the fixation of ink during printing is not necessarily slower than offset paper. There are two main reasons. First, the amount of ink supplied by coated paper is small; Second, there are numerous small and small capillary tubes between the pigment particles attached to the surface of the coated paper. The absorption capacity of many fine capillary tubes is much larger than that of a few large capillary tubes. In addition, the coated paper has a strong selective selectivity for the binder, allowing the low molecular component having strong fluidity in the binder to penetrate into the paper, and the polymer component in the binder remains in the ink layer, so that the ink layer can be improved. Firmness and gloss.