Feeding process and feed failure

When printing with flatbed paper, the first process is also a critical one. The paper feeding process is a process of feeding sheets, which are stacked on the paper feed table, to the printing mechanism without fail. In the paper feeding process, the following problems are mainly caused by the performance and quality of the paper.

Alice and curl. During the printing of the flatbed paper, it is occasionally found that the edges of the paper placed on the paper-feeding table are curled or curled up by themselves, and as a result, the paper-feeding device cannot feed the paper one by one into the printing mechanism. Even if the paper can be fed, it does not allow the tooth decay and registration device to work properly, resulting in a significant decrease in print quality.

There are two main types of paper curling and curling. One is white curl, and the other is curl after printing.

White curl refers to the curl that occurs without the paper before printing. The curling phenomenon of white paper has both habit curl and stretch curl. Habitual curling is caused by the inertia of the paper when it is rolled into a roll before it is cut into flat paper. The axis of the curl is always at right angles to the longitudinal direction of the paper. The core of a paper roll or paper roll is most likely to buckle when it is cut into flat paper.

Stretch curling is caused by the difference in the telescoping of the front and back sides of the paper. Paper that expands and contracts due to changes in moisture content will not curl if both sides of the paper are stretched the same. There is a difference in the curling of the paper on both sides.

If the paper changes from a moisture content to another moisture content, it will inevitably expand and contract. Although the change in moisture content is the same, the expansion and contraction rates on the front and back sides of the paper are different. When there is a difference in the expansion and contraction of the front and back surfaces, the direction of curling always curls toward the side where the elongation is small, or curls toward the side where the shrinkage is high. Obviously, the greater the gap between the two sides, the more severe the degree of curling.

Unprinted white paper is placed on the paper table after being disassembled. If the moisture content of the air is lower than the equilibrium moisture of the paper (that is, the paper is wet and the air is dry), the moisture in the paper is volatilized into the air. After the water evaporates, the fibers in the paper shrink due to the reduction of moisture. Because the shrinkage of the fibers is always severe in the transverse thickness, and the shrinkage in the longitudinal direction is small, the reverse side of the paper has a large degree of contraction due to the longitudinally arranged fibers, and the front side of the paper shrinks due to the relative decrease in the fibers arranged longitudinally. small. Therefore, when the paper is dried, it is always curled to the reverse side. Because one or a stack of paper is placed in dry air, the four corners dry fast and have low curl resistance, so the curl is always more severe at the four corners. For single-sided coated paper, the ratio of stretch of the non-coated surface (reverse surface) is significantly greater than that of the coated surface (front surface), so when the single-coated paper is dried, it curls toward the non-coated surface. Most of the white paper curling that occurs in the printing factory is curled due to paper drying.

The method of preventing and resolving the curl of white paper is to perform humidity conditioning, or origami or crepe paper. Humidity treatment is to adapt the paper storage environment and the humidity of the use environment to the moisture content of the paper itself. Origami, crepe paper is the curled paper is curled in the opposite direction of the curl, or use the rubber hammer and wood stick in the curling direction of the opposite side of the paper, through the fold or enthalpy to reduce the curl of the paper.

Observing the changes in the placement process of a flat single-sided coated paper, it is found that if the coated side of a single-side coated paper is piled up in a high-humidity environment, a few sheets of paper are placed on top of the crucible. It will absorb moisture from the air, and the moisture will penetrate to the non-coating layer on the back, so it begins to curl up from the four corners after absorbing moisture. This created a gap with the next sheet of paper, allowing the intrusion of moist air and subsequent curling of the next sheet of paper. Repeatedly, the curl develops in turn.

On the other hand, if the coated side of the single-sided coated paper is stacked face down in a high-humidity environment, although the top few sheets absorb moisture quickly, the paper curls toward the coated surface, but curls. The phenomenon will not develop to the bottom of the paper basket, and it will not even develop. This is because in this case, the resistance of the curl is much greater than the resistance of the corner curl.

Therefore, when stacking the paper baskets, analyze the relationship between the moisture content of the paper and the humidity of the storage environment, or make the paper face up, or make the paper face up, always make the paper curled in the downward position. Under normal circumstances, the paper is often dehumidified in the storage environment, so it should be placed face up. The same is true for non-coated paper.

Of course, not all paper curls. If the expansion and contraction rates of the front and back surfaces of the paper are close to each other or the moisture and storage of the paper and the humidity of the printing environment are appropriate, curling does not occur. In addition, the degree of beating, the degree of tightness, the amount of filling, the degree of sizing, etc. of the paper are also related to the curling property. Therefore, the disadvantages of solving the curl of the paper are, of course, not all the paper curls. If the expansion and contraction rates of the front and back surfaces of the paper are close to each other or the moisture and storage of the paper and the humidity of the printing environment are appropriate, curling does not occur. In addition, the degree of beating, the degree of tightness, the amount of filling, the degree of sizing, etc. of the paper are also related to its curling properties. Therefore, the problems in solving the problem of paper curling must also be emphasized in the paper making process.

Curl after printing is different from curling of white paper. It occurs when printing has entered the delivery table, and it is first curled to the non-printing surface, and then it is turned to curl toward the printing surface.

The reason for curling after printing is not that there is a difference in the stretching rate of the paper itself, but that there is a difference in the water content on both sides of the paper. That is, the printing surface of the paper, especially the non-image portion, absorbs the wetting water.

After the wetting water adheres to the printed surface, the fibers of the surface layer begin to elongate, which tends to curl toward the non-printed surface. However, in the printing press, because the paper is subject to various restrictions, it can still maintain its flat state. However, after the paper arrives at the delivery table, the tendency to curl appears.

Normally, if one side of the paper is coated with water, this side will expand and curl on the opposite side. However, when the paper begins to dry, the curling direction will gradually reverse, and finally it will bend toward the side that has been wiped. This is because the paper undergoes wetting elongation, and if it is dried to its original dryness, it shrinks smaller than the original size. In the single-sided printing, a part of the moisture attached to the printing surface quickly evaporates, another part penetrates quickly into the interior of the paper layer, and the unprinted surface can also absorb a print superposed thereon. The longitudinal direction of the tissue paper is curved upwards toward the printing surface in the axial direction.

This kind of curling is necessary for the operation of the other side of the printing, it is bound to affect the next printing. If it is a print that only prints one side, it also cuts the ribbon after printing. Processing such as folding and binding brings many difficulties.

In order to prevent and reduce post-impression curling, efforts should be made to reduce the amount of infiltrating water, and the printing plant should also be provided with suitable temperature, humidity and good storage conditions.

Source: Paper Easy