Study on the properties and printability of coated white paper (2)

4.4 Surface Strength The surface strength of coated white paperboard refers to the degree of bonding between fibers, fibers and fillers, and rubber. The stronger the binding force, the higher the paper surface strength, and vice versa. In the printing, the surface strength of the paper surface is often measured by the degree of hair loss, powder removal, and peeling. During the printing process, only about half of the ink on the plate or the blanket is transferred to the paper each time the stamp is imprinted. That is, the ink layer is separated during printing, part of the ink is transferred to the paper, and part of the ink remains. On the surface of the plate or blanket. The viscosity of the ink is a force that prevents the separation of the ink layer, so that the separation sometimes does not occur on the ink film, but occurs at the weakest point where the paperboard fibers, fillers, and glue are bonded. As the viscosity of the printing ink increases with the increase of the printing speed, the surface of the paper printed on the high-speed printing machine (8000 sheets/hour or more) is subject to a large pulling force. If the bonding force between the surface structure of the cardboard is not sufficient to resist the tension generated when the ink film is separated, fluffing, dusting, and even peeling of a small piece of paper (called peeling) may occur during printing.

In the reproduction of prints, printing is done in a very short time (instantaneous), and there is a complex mechanical interaction between paper and ink. As the speed of printing presses increases, the effect of peel tension increases. To make the ink transfer to the surface of the paper correctly without destroying the paper surface structure, the surface strength (bonding force) of the paper must satisfy the following formula:

T≥W+Q+N
Bonding force between T-fiber and fillers W-ink adhesion to paper Q-peel tension N-print plate attractiveness to ink

The fibers on the surface of the paper are individually pulled off and called as lint-free fibers. If these fibers are stuck on the surface of the printing plate or blanket, the next few sheets will have defects. When severe hair loss occurs, the paper hair will move against the direction of ink transfer, mix with the ink, and fill in gaps between the printing plate's Internet point or lines to cause a paste plate. Mild stencils affect the quality of the printed matter and cause the sharpness of the graphic and text to decline. Severe stencils can cause the plate to be scrapped and the printing job cannot be carried out. During the printing process, due to the poor surface strength of the paperboard, sometimes a small area of ​​surface fiber stripping occurs on the surface of the paperboard. After the peeling, the shredded fragments stick to the printing plate or blanket, and after several times of embossing, they may adhere firmly. On a printing plate or blanket, this part of the printing will be higher than the rest of the printing plate or blanket, the printed text will appear in the corresponding part of a dot, and the surrounding is not a print. White ring of ink - ring white spot. This will have a direct impact on the quality of the product, and even if it is severe, it will make production impossible.

From the above analysis, we know that the stripping, decontamination, and lint removal during printing are basically determined by the surface strength of the coated white paperboard, but the T value for a particular coated white paperboard is hard to increase in the printing plant. Therefore, only the right side of the above relation is used to adapt the T value on the left side of the relation. Below we discuss some of the items on the right side of the relationship. The change of W value can be started from two aspects: reducing the viscosity of the ink or reducing the thickness of the ink film under possible conditions; the change of the Q value depends on the speed of the printing press and the radius of the impression cylinder, and the speed of the printing press. The faster, the greater the peel tension, and vice versa, the smaller, if the speed is certain, the smaller the radius of the impression roller, the greater the peel tension, of course, the larger the radius, the smaller the peel tension; the change in the N value is mainly for the lithography In terms of printing, it refers to whether the blanket has been excessively viscous, such as sulfur loss and stickiness. At this time, the value of N must be large. Therefore, certain measures must be taken to reduce the value of N. We mentioned earlier that it is difficult to increase the T value when printing, but because we do not properly handle it will cause the T value to decrease. This requires our printers to maintain the original T value of the paper as far as possible in the actual work, while also trying to reduce the influence of external factors on the paper surface, ensure that the paper reaches the balance of force in the printing process, and make the printing work smoothly. get on.

The surface strength of coated white paperboard is mainly determined by the raw materials and processes of papermaking. For example, the surface strength of coated white paperboard made from straw fibers such as wheat straw pulp and eucalyptus pulp is generally low, and it is made from wood pulp and cotton pulp. The coated white paperboard has a higher surface strength. Paper surface strength is also related to the selection of fillers and compounds, the degree of calendering and other factors.

4.5 Smoothness Smoothness indicates the smoothness and smoothness of the coated surface. The application of the white paperboard coating layer is to improve the smoothness of the paperboard surface, and the smoothness that cannot be obtained by the application of the calender can be obtained by the coating. The printing effect depends to a large extent on the smoothness of the paperboard surface. The clarity of the text printed on the uneven paperboard surface is affected because the smoothness is related to the ink transfer and the smoothness is determined. The tightness of the board in contact with the printing plate. The thickness of the ink film on the printing plate is generally 3 to 10 μm. About half of the ink is transferred to the cardboard when it is imprinted with the cardboard. If the depth of the pit on the cardboard exceeds the thickness of the ink film, the plate is imprinted during printing. Corresponding to this pit can not be in contact with the paper. If the site is a field, a white point will be produced; if it is a text, it will result in the phenomenon of a broken pen and less painting.

4.6 Absorption performance The absorption properties of coated white paperboard include oil absorption and water absorption. Oil absorption and water absorption are two completely different properties of paper. Of these two properties, we are more concerned with oil absorption because the vast majority of printing inks are oil-based materials. The printing process requires that the paperboard has a considerable absorption capacity, so that the paperboard can absorb the ink as soon as possible, the paperboard absorbs the link material in the ink during printing, and the pigment particles are left on the paperboard surface to quickly dry. In general, the pores between cardboard fibers are larger than the diameter of the pigment particles. During printing, the ink is pressed into the pores of the paperboard. Under the effect of the fiber capillary, the binder is absorbed into the small fiber gaps smaller than the diameter of the pigment particles. Paper fibers are interwoven into an infinite number of capillaries. The amount of ink sucked can be discussed by the size of the gap. After the ink fills the gap, the capillary in the cardboard absorbs the binder in the ink. If the gap between the fibers is small, the capillary action of the fibers is damaged and the ink absorption performance is poor. If the pores are too large, the capillary not only absorbs the binder but also absorbs the pigment, resulting in the phenomenon of print-through.

The oil-absorbing property of the coated white paperboard is closely related to the printability. Low oil absorption, the ink will not be quickly absorbed by the paper after it is transferred to the paper, so that the ink that has not dried will be transferred to the back of the other paper covered above, causing it to become dirty. If the oil absorption is too high, it will infiltrate excessively, and the cardboard surface will lose its luster. The binder material will infiltrate into the paper fibers. The rapid penetration may cause the powder to pulverize, leaving the pigments on the surface unprotected. After the ink film layer printed on the surface of the paper is dried, the phenomenon that the pigment encounters a slight friction and becomes powdery and falls off occurs. After printing, the pigment on the surface of the paper is separated from a white line by the fingernail. When the carton is in transit, its printed surface may be scratched sometimes. The main reason is that the carton of the ink is excessively absorbed by the carton, so there is no film with enough fixed pigment left. The root cause of this effect lies in both paper and ink: In terms of ink, because the viscosity of the vehicle is too low, the ink dries too slowly; in paper, due to too low water, the ink in the coating layer absorbs too quickly; The reasons for both paper and ink are that the ink vehicle penetrates too much into the paper. The measures taken in printing are to add a varnish capable of forming a hard dry film in the ink, and it is also possible to consider adding more desiccants for accelerating the drying of the ink. On the paperboard, the composition of some coating layers can be changed slightly to slow the absorption of the ink.

In summary, the printability of paperboard has a great influence on the quality of packaging products. We can improve the print quality of paperboard by improving the printability of paperboard. In the actual printing, the overall appraisal of the printability of the paperboard, but also through the actual printing or analog printing methods, the paperboard glossiness, the quality of the ink transfer, the phenomenon of lint removal, etc. to evaluate the cardboard Printing performance.

Author: Department of Paper Engineering, Printing Engineering College Shaanxi University of Science Chen Chang