Relationship between Paper Smoothness and Printing and Its Measuring Instruments (I)

The quality of paper not only marks the development level of the paper industry, but also affects the quality of printed products. As the main printing material for printing, paper plays a key role in the quality of printed products. The parameters of the paper are mainly quantitative, size, smoothness, gloss, whiteness, ink absorption and so on. Among the many performance indicators of paper, smoothness is an important parameter for paper evaluation, and whether the smoothness index meets the requirements is one of the standards for measuring paper quality.

Smoothness refers to the time required for a certain amount of air to escape from the atmosphere between the sample surface and the annular plate surface at a given contact state and a certain pressure difference, expressed in seconds (s). This definition is mainly based on the Buick type. The measuring principle of the smoothness meter is defined. The measuring principle is: placing paper and cardboard on a glass plate, applying a specific pressure to create a semi-vacuum, drawing in air and passing air through the contact surface, and measuring the time required for the degree of vacuum to change within a specified range. In layman's terms, smoothness refers to the degree of smoothness or roughness of paper and paperboard (including macro smoothness and micro smoothness), which is actually the degree of unevenness of the surface of paper and paperboard.

The form of smoothness

Smoothness is one of the most important quality indicators for printing paper. It is related to fiber ratio, filler, fine fiber, uniformity and calendering. The smoothness is closely linked to the surface profile of the paper. In the printing, the smoothness of the surface generally refers to roughness, flatness, and compressibility. The smoothness of the paper affects the writing process, the quality of the package, and even the quality of the print.

Paper surface smoothness can be expressed in four forms, as shown in Figure 1:

Figure 1 The form of smoothness

(1) The surface macro and micro smoothness are all good;

(2) The macroscopic smoothness is not good, that is, there is a large area of ​​uneven thickness or undulations, but the micro smoothness is very good;

(3) The macroscopic smoothness is good, but the micro smoothness is poor, that is, there are small pits on the paper surface that affect the printing effect;

(4) Both macroscopic and microscopic smoothness are not good.

For the first case, of course, there will be a good printing effect during the printing process. For the second case, as long as a compressible pad or a certain printing pressure is added to the printing, good contact with the plate can be obtained, and the printing effect is also good. In the third case, it is not easy to achieve good contact between the paper and the layout. Even if the printing pressure is increased, the contact area can only be increased, but it is difficult to obtain complete contact. Because the paper is a porous material, its compressibility is limited, so this microscopic roughness is very important for the printing effect, and it is difficult to achieve a satisfactory printing effect in the actual printing process. For the fourth case, it is more difficult to achieve the contact between paper and printing plate, and it is generally impossible to achieve a satisfactory printing effect.

The relationship between smoothness and print quality

Due to printing pressure, ink thickness, printing methods and other reasons, the paper smoothness will have a non-negligible impact on the quality of the print; and the paper smoothness affects the printing register, the number of screen lines, the printing speed, and the printing pressure. And other printing parameters. When the surface of the paper is impressed, it can contact the printing plate or the blanket with the largest contact area. The ink transfer can be completed more evenly and completely, and the graphic image can be clearly and fully reproduced on the paper. In order to obtain a satisfactory copying effect, paper with a poor smoothness will affect the number of screen printing lines, dot reproduction, increase, ink amount, and transfer and color rendering of printed products.