Q&A about printing paper (89-97)

89. What are the two types of smoothness of paper?

A: The smoothness of the paper under the pressure of printing is called the smoothness of printing, and the smoothness of the surface of the paper in the free state is called the apparent smoothness. Apparent smoothness depends on the texture of the appearance of the paper, printing smoothness is a comprehensive manifestation of the apparent smoothness of the paper and the surface compressibility, has a direct impact on the quality of printed matter, is one of the most important printability indicators of paper .

90. What is the effect of print smoothness on print quality?

A: Print smoothness is a prerequisite for obtaining prints faithful to originals on paper. It determines the degree of contact between the surface of the paper and the surface of the plate or blanket inking at the moment of imprinting, which is an important factor affecting whether ink transfer is comprehensive and whether the graphic is clear.

(1) The smoothness of printing affects the ink requirement of the paper, that is, a certain printing density, the amount of ink required for the paper or the layout. The rougher the paper, to reach a certain density, the thicker the ink film needed on the plate, this will increase the print product inhomogeneity and print-through, affect the dot quality and increase the possibility of mechanical dot gain.

(2) Printing smoothness affects the uniformity of the paper's ink. When paper with poor smoothness is printed in the field, the density of printing is not uniform; the quality of screen printing products and dot quality is poor, and there are serious dot losses. Especially, small dots in the bright tuning part are more likely to be lost, affecting the reproduction of the highlight layer. .

(3) Printing smoothness also affects the glossiness of printed products. High print smoothness is conducive to forming a uniform and smooth ink film on the paper surface, thereby improving the glossiness of printed products.

91. How to measure the smoothness of paper?

A: There are many ways to detect the smoothness of paper, such as microphotographing, stylus method, optical method, printing test method, capacitance method, optical contact method, and air leakage method. One of the more practical and widely used air leakage methods.
Date: June 2, 2005 92. How does the softness and compressibility of paper affect printing?

A: The softness and compressibility of paper in printing affect the degree of contact between the blanket and the surface of the paper. The softer the paper, the easier it is to be compressed under the effect of printing pressure, and the better the paper surface is completely in contact with the blanket. If printing on strong paper, because the paper itself has a certain degree of uneven thickness and uneven surface, the contact between the blanket and the paper surface is light and heavy, so the printed images are also different shades, even the loss of dots . If printed on soft, elastic, compressible paper, these properties can make up for the inconsistencies in paper thickness and uneven surface, promote good contact between the paper and the layout, and make the ink transferred to the paper delicate , accurately reproduce the layout of the text.

93. What is the degree of paper dust?

A: Dust is the dark, yellow, dark yellow and brownish impurities and dust that can be seen on the surface of the paper sheet with the naked eye and are significantly different from the paper surface color. Some are caused by the fiber material itself and are also caused by external factors.

Dust degree refers to the amount of dust with a certain area or a certain size range observed on a sample of a certain area, expressed as the number of dust on a unit area, or the equivalent area of ​​dust on paper or cardboard per unit area ( Mm2) to indicate.

94. The degree of paper dust on the print?

Answer: Dust-laden papers, especially those with black dust, form blemishes on printed graphics and are prone to errors such as punctuation marks and numbers. Even if it does not cause mistakes or become a waste product, more dust will inconvenience reading.

95. What is the retractility and dimensional stability of the paper?

A: The rate of expansion and contraction is the effect of water vapor in the air, that is, the change in relative humidity, which causes the size of the paper to shrink or elongate. When the paper is impregnated with water or over-dried, it will also undergo dimensional changes. The property of paper that expands or contracts due to changes in humidity or moisture is called dimensional stability. The smaller the expansion rate, the better the dimensional stability; the larger the expansion rate, the worse the dimensional stability.


96. What is the impact of stretch rate and dimensional stability on printed paper?

A: When the paper has a good dimensional stability, there will be no significant expansion and deformation during printing. For printed paper, the rate of retraction is one of the most important quality indicators because color prints often undergo overprinting. Once printed, the paper is stretched once. In offset printing, if the expansion and contraction rate is large, it will cause serious overprint or image distortion problems. Therefore, the printing paper requires a minimum retractility and better dimensional stability.

97. What are the main reasons for paper stretching?

A: The reasons for the stretching of paper are essentially two things: First, the individual fibers shrink or swell; second, the fibers are tight or loose. The paper's stretching characteristics are closely related to the way the paper is formed and the amount of traction that the paper is subjected to during the manufacturing process. Generally, the paper's horizontal stretching rate is greater than that of the vertical direction.

The factors affecting the paper retractility are the degree of beating and the tightness of the paper. The degree of tension of the paper with high degree of beating is high and the rate of retraction is also large, because the pulping can increase the tightness of the paper and reduce the gap that can accommodate the expansion of the fibers; And, after the pulping, the fiber binding force is enhanced, making it difficult for the fibers to slip and reduce the expansion and contraction of the fibers. Slurry types and fillers can also affect the paper's shrinkage rate. In general, adding fillers and sizing can also reduce the paper's stretch rate.

In addition, after the paper has been wetted and dried for the second time, the stretch rate produced by the paper is smaller than that caused by the first wetting and drying. If wetting is repeated and drying is repeated, the amount of stretching generated by each cycle will gradually decrease.

- "Printing Technology - Equipment and Special Printing"