Q&A about printing paper (117-127)

117. What is the stiffness of the paper?
Answer: The stiffness of the paper is the resistance of the paper to bending, ie rigidity or softness. Stiffness is related to the rheological properties of the paper, which is determined by the ability of the outer layer to stretch and the inner layer to withstand pressure when the paper is bent. The stiffness can measure the ability of the paper to support its own weight.

118. What factors affect paper stiffness?
A: The thickness of paper is the most important factor that affects stiffness, followed by tightness. If the tightness is kept constant, the stiffness is proportional to the third power of the thickness; if the ration is kept constant, the stiffness is proportional to the square of the thickness; if it is at a certain thickness, the stiffness is proportional to the tightness. The formula is expressed as follows:
T - sample thickness;
E - Young's modulus (ie, moment of inertia);
B - specimen width;
L - sample length.
Thus, the higher the elastic modulus, the higher the stiffness. Since the modulus is the stress strain per unit area, the deformation under a certain load is reduced and a higher stiffness can be obtained.
119. What factors influence the stiffness in papermaking?
Answer: The raw materials and the pulping method are different, and the stiffness of the copied paper is also different. The higher the pulping degree, the higher the paper-stiffness of the paper; the higher the hemicellulose content of the paper is, the higher the stiffness is; the paper stiffened by the chemical pulp is related to the stiffness of the individual fibers. And the stiffness of thicker paper is related to the strength of the bond between the fibers. When the basis weight of the paper is constant, in order to obtain the maximum stiffness, it is desirable to increase the bulk of the paper without reducing the bonding force between the fibers. The moisture content of the paper has a great influence on its stiffness. After the paper is wet, its stiffness will decrease linearly. Adding proper ingredients in the paper stock can also effectively increase the stiffness of the paper, and some can also reduce the stiffness degradation that occurs due to the increase in humidity.
120. What are the requirements for stiffness on printed paper?
A: Stiffness is a must-have feature for some printing papers. If the paper does not have a certain degree of stiffness, it will bring difficulties to the continuous paper and paper discharge in the printing process, and the printing speed cannot be improved, even to prints such as books. Use of poor performance. For sheetfed offset, the stiffness of paper is one of the most basic conditions.

121. What are the instruments for measuring the stiffness of paper and board?
A: There are many instruments used to measure the stiffness of paper and cardboard. There are Clark type, Taber type, Gurley type and horizontal stiffness tester and resonance stiffness tester. The Gurley type stiffness tester is commonly used. .

122. What is the surface strength of paper?
A: The surface strength of paper refers to the strength of the bond between the surface of fibers, glue, fillers or paint particles on the surface of the paper and between the coating and the paper base. It indicates the ability of the paper to resist the ink splitting force during the printing process. When the paper surface is separated from the inked plate or blanket, the separation force of the ink is greater than the binding force between the paper particles, the paper surface will produce a visible cracking phenomenon, discontinuous or continuous on the paper surface of the crack. The phenomenon of the inked area is called pulling, and therefore, the surface strength of the paper is often referred to as "pulling resistance."

123. What is the relationship between paper surface strength and water resistance?
A: The water resistance of paper refers to the ability of the paper to retain its original strength properties when it is subjected to water infiltration. Before the impact of water, the surface strength of the two papers is the same, but after being infiltrated with the same amount of water, the surface strength will change differently. Therefore, the analysis of the surface strength of the paper must take into account the strong water resistance weak.

124. What are the two types of papers?
Answer: The lapping of the paper is classified into dry lapping and wet lapping according to whether water is involved or not. Dry lapping has nothing to do with water, but because of the effect of the separation force of the ink on the surface of the paper, it has nothing to do with the water resistance of the paper, and dry picking may occur on both monochrome and multicolor machines. Wet naps take place with the participation of water and therefore relate to the water resistance of the paper. Wet naps are unique to the multicolor offset press. Under the wetting effect of the dampening solution, the binding force between the pigments between the non-coating paper fibers bonded by hydrogen bonding or the water-soluble glue as the adhesive will be significantly reduced, and dry picking will occur easily; Wet naps are related to the amount of water and the time the water stays on the paper.

125. What is the difference between powder dropping, hair loss and pulling?
Answer: Pulling is the peeling of the bound particles on the paper, and the falling-off is the shedding of loose particles on the paper. The pulling hair depends on the surface strength of the paper, and the falling hair depends on the cleanness of the paper surface. Although the performance of the paper is in two different aspects of the paper, the impact on the printing is the same. Therefore, in the actual printing, the surface of the blanket must be cleaned frequently to avoid the occurrence of printing failures.

126. What are the methods for measuring the surface strength of paper?
A: The common methods for measuring the surface strength of paper are the wax stick method and the IGT printability meter method.
1) The wax stick method (Dennison)
It refers to using 20 wax sticks with different labels and different adhesive power to stick the surface of the paper to determine the surface strength. During the measurement, one end of the wax stick was heated and melted, and then pressed vertically on the surface of the paper. After 15 minutes, the wax was used to indicate the surface strength of the paper. The higher the number, the higher the surface strength of the paper. The higher. This method is simple and can accurately distinguish papers with different surface strengths. However, due to the difference in structure between wax and printing ink, the adhesion, affinity and ink of the paper are not the same, and it is measured statically, and it cannot reflect the mechanical characteristics of the paper surface peeled off during the printing process. Therefore, it has only comparative significance.
2) IGT printability test method This method is based on the principle that the separation force is proportional to the separation speed when the fluid is separated between planes, ie, the faster the separation speed, the greater the separation force. The IGT printability tester measures the surface strength at different printing speeds with a certain viscosity under a certain pressure, and measures the continuous generation of filler points, coating spots, fluffing, and destruction of the paper surface on the paper surface. The distance at the time of foaming, tearing, and tearing is used to determine the maximum printing speed that does not produce a pulling phenomenon, that is, the critical speed for pulling. And use this speed to express the surface strength of paper, the unit is cm/s or m/s.

127. What is the optical performance of paper? What are the factors that affect the optical performance?
A: The optical properties of paper include whiteness (brightness), opacity, and gloss. These properties are determined by the reflection of visible light (including specular, diffuse, and diffuse reflections), transmission, and absorption on the paper. . The optical properties of paper and paperboard vary greatly due to their composition and processing. The main influencing factors are pulp type and whiteness, pulp processing methods and paper sheet forming methods, plastic materials contained in the paper, fillers, dyes, and surface coating and finishing methods.

- "Printing Technology - Equipment and Special Printing"