The performance and function of common paper for packaging and printing

Third, cast coated paper
Cast coated paper, also known as high-gloss coated paper, commonly known as glass cardboard, is a single-sided high-gloss coating paper. Its main purpose is to print trademarks, packaging, postcards, greeting cards, invitations, etc. for high-end goods and exquisite handicrafts.

The characteristics of cast-coated paper are: It has extremely high glossiness and smoothness, and can print very fine network lines, with strong reproducibility of dots, tone, color and gloss, clear images and strong three-dimensional effect. It does not have super calendering, it has a higher gloss than the normal super-calendered coated paper. Moreover, it has a lower degree of tightness and high bulkiness than coated paper, and also has good elasticity and good ink repellency. Can not afford to lose powder and other advantages.

The raw material of the cast coated paper is basically the same as the coated paper, and its production method is: after the raw paper is coated, the coated layer is in a semi-wet state and has plasticity, so that the coated layer is closely attached to the high-precision ground, On the sticky chrome-plated cast cylinder surface, as the cylinder surface rotates, the paint layer is heated and dried to form a film, the plasticity of the paint film disappears, and the paint film is automatically peeled off from the cylinder surface to obtain a cast coated paper. The reason why the paper surface of the cast coated paper is smooth as a mirror is that when the paint layer is tightly attached to the cylinder surface of the cast coating cylinder under high pressure, the pigment particles are suppressed from being heated and shrink and dry, and the result is along the cylinder surface. The shape of the surface is oriented and the reflection matching the cylinder surface is obtained. Although the paper surface has a high gloss, it is rich in pores and has a large ink absorption. This is an obvious feature of cast-coated paper. The cast coated paper requires the paper surface to be flat and has no impact on the use of curling. The color of each batch of paper must not be significantly different. The paper edge trimming should be neat and clean, with no folds, breakages, marks, and visible streaks and the effect of the applied coating hemp pit.

The basis weight of the cast coated paper is 100 to 300 g/m2, which is a flat sheet paper, and the specifications are 787×1092 mm, 850×1168 mm, and 880×1230 mm. The technical standards for cast coated paper are shown in Table 3. At present, cast-coated paper is also mainly imported.

Fourth, the printing performance of packaging printing paper
The printing performance of paper is not a definite concept because: First, various printing methods have different requirements for paper; second, even if a completely consistent printing method is used, it is not possible to use only one index to indicate the printing of paper. performance. It is generally believed that when printing in this way or that way, the printing performance of the paper is expressed collectively on the basis of those indicators that ensure the quality of the print. Regardless of the type of printing used, paper uniformity, ink acceptance, whiteness, opacity, smoothness, gloss, surface micro-geometry, softness (compressibility), surface pull and strength, and other properties The quality indicators have an impact on the printing performance of the paper. The elasticity of paper also has an effect. When printing with a rotary press, the paper's elongation index is more important than offset printing. When printing on a certain paper, the minimum deformation of the paper or the dimensional stability during wetting and subsequent drying is also an important requirement.

The surface structure of the paper and its microscopic geometry are important factors in determining the printing performance of the paper. The paper surface is rough and bumpy. Using this type of paper for printing will cause the printed product to be strewn with pens, the dots are empty, and the level is reduced, and the ink absorption capability is also faster, resulting in blotting, dullness and other unfavorable consequences. The limit of the micro-unevenness (roughness) of various printing papers determines the thickness of the printed image and the hardness of the printing plate. For literary works and illustrations with strokes or coarse mesh images, the micro-unevenness can also be larger when printing with elastic plates, and can be slightly smaller when printing with hard plates. For fast printing of the illustrated version, the paper used should have a fairly high level of flatness and smoothness, and should also have a microporous surface structure. From this point of view, it is of particular importance to have a large number of highly dispersed inorganic fillers inside the fiber structure, or to apply a thin layer of strong inorganic coating on the paper machine on the paper surface.

All the points of the printing plate are in complete contact with the paper surface, which is a necessary condition for obtaining good printing results. The more uneven the paper surface, the harder it is to guarantee this condition. However, if the paper is softer, the more it can be compressed under the pressure of the printing plate, the more it can ensure that the paper surface is in complete contact with the printing plate, even if the paper surface is rough. In this case, the allowable range of microscopic unevenness on the paper surface can be slightly relaxed. The softness of the paper is particularly important in letterpress and gravure printing, while the importance in engraving lithography is slightly less.

The use of ground wood pulp in the stock improves the paper's many printing properties. The use of groundwood fibers helps to reduce the paper's transparency, increase the porosity, improve the ink's ability to absorb ink, softness and elasticity, and also helps to obtain better-textured paper. However, the use of groundwood pulp in paper stock is limited, otherwise it will reduce the paper's whiteness, smoothness, gloss, lightfastness, and major mechanical strength indicators.

The good effect of the inorganic filler on the printing properties of the paper is manifested in the improvement of the paper's opacity and whiteness and the reduction of the wetting linear deformation of the paper. The most promising filler is precipitated calcium carbonate. Calcium carbonate can impart high whiteness, opacity, and lightfastness to paper, and at the same time give good gloss and smoothness to calendered paper. The addition of calcium carbonate helps to improve the printability of the paper, the resulting print is clear, and the ink does not penetrate the back of the paper. However, calcium carbonate cannot be used as a filler in acidic media because it decomposes CO2, creating foam, reducing paper formation and filler efficiency, and also making paper sizing difficult.

In many cases, in order to obtain high-quality prints, coated paper, that is, a paper coated with an inorganic suspension or a water-based paste, is used for printing with different printing methods. The purpose of the printing paper coating is to make the paper surface even and dense, so as to ensure that the printing ink is uniformly absorbed, and a higher screen mesh can be used in printing.

The paper's ability to absorb ink and the quality of printed products are extremely important and must be given sufficient attention. The ink absorption capacity of paper refers to the degree of absorption of the binder in the ink by the paper. The ink-absorbing ability of the paper and the depth of penetration of the ink into the sheet, when other conditions are the same, are determined by the structure of the paper, and in particular by a series of factors determined by its capillary adsorption properties and ink properties. Since the surface of the fiber is not absolutely smooth, it has a lot of holes. These holes can absorb a certain amount of linking material. The more holes there are, the greater the absorption is, and therefore the ink has a strong absorption ability.

When printing with the same method, the paper's porosity and the viscosity of the ink are among the most important factors. Paper with high porosity has air content in the range of 75%, and the tightest air content in paper is about 13%. On tight, low-porosity paper, printing with sticky ink may cause ink stickiness, ie, The wet ink printed on the paper is transferred to the back of the paper placed on it.

The strong penetration of the ink into the paper (for example, when paper with a high porosity and a low-viscosity ink) may cause printing defects. This is manifested by the fact that the printing ink penetrates the paper sheet and is printed on the opposite side of the paper sheet, or because most of the ink's pigment does not remain on the surface of the paper, but penetrates into the paper's interior, it will get dull grey prints. .

In order to improve the quality of printed products, we must adjust the nature of the ink and the amount of dry oil added and discharged according to the ink absorption capacity of the paper.

(Author: Luo Guanglin, Xi'an University of Technology)

Packaging Printing and Processing