Printing wrinkles and exclusion

Paper wrinkling during printing is a common failure during offset printing. The occurrence of paper wrinkling during printing inevitably affects the quality of the product, resulting in defective products and waste products. In severe cases, printing production cannot be performed normally. Paper creases during printing, which are caused by various factors, caused by the unevenness of the paper, and improper adjustment of printing equipment. The key to solving paper wrinkling is to do a good job of preprinting and proper adjustment of printing equipment. 1. Wrinkled paper caused by unevenness of paper is a porous multiphase material consisting of plant fibers, fillers, rubber, pigment, and water. Among them, plant fiber is the most basic ingredient of paper. It has the characteristics of water swelling and water loss shrinkage, which makes the paper sensitive to changes in the temperature and humidity of the environment. Sharp changes in the climatic environment often cause changes in the paper phase, so that the paper changes the normal paper phase, flatness decreases, the paper can not be transported smoothly on the paper feed plate during printing, can not be correctly positioned, resulting in overprinting is not allowed, the paper from Wrinkles and other failures. The wrinkling phenomenon caused by the paper unevenness generally has the following situations. 1. Paper wrinkles caused by the edge of the paper Wrinkled sheet paper (or several sheets of paper suspended from one another, with air between the paper layers) can change in the humidity within a short period of time Maintain balance with atmospheric humidity. In the paper to the atmospheric humidity is in the process of equilibrium, the paper will have the corresponding elongation or contraction, because the elongation or shrinkage is uniform as a whole, so it does not affect the flatness of the paper. However, the paper is different in the case of stacking. A pile of flat, low water content paper is placed in a room with a relatively high relative humidity. At this time, the humidity in the room will penetrate from the top of the pile and the sides of the stack to the inside of the pile. However, this penetration can only reach a certain depth, and then the speed of its penetration quickly slows down. As a result, the water absorption of the pile of paper stops only a few centimeters deeper at the top and side. After this phenomenon of local moisture absorption, the paper can no longer maintain its flatness. Because the internal moisture of the paper stack does not change, deformation does not occur, but the sides of the paper stack are stretched after absorbing moisture, so the paper will inevitably bend around and form a wavy shape. For the paper on the top of the stack, because it can absorb moisture uniformly from above and maintain its flatness, the undulations caused by the lower part of the paper must have an effect on the upper part of the paper, so that the paper on the top of the stack is also wavy. Another kind of wavy paper can be caused by "dew point". In the winter or early spring and autumn season, the paper that was unpacked and unpacked was often very flat, but soon after one or two hours, it was found to be wavy, similar to the situation where dry paper had just been unpacked during the rainy season. This is because cold paper enters the workshop where the temperature is higher. If the temperature difference reaches the “dew point” value, a large amount of water droplets will condense on the sides of the stack, and the water droplets are quickly absorbed by the edges of the paper. Paper wavy. During printing, the paper that is fluffy in the short side direction of the paper is pressed against the middle portion of the paper dragging edge by a large printing pressure. Although the paper itself has a certain elasticity, some bow wrinkles can be eliminated. However, if it exceeds a certain limit, it will not be able to cope with wrinkling. See (Figure 1) Figure 1 As this type of wrinkling is squeezing from the left and right sides of the paper toward the middle, two bows are often formed on the trailing edge of the paper. Since the arrangement of plant fibers in paper has directionality, the radial expansion and contraction of plant fibers is much larger than the lengthwise expansion and contraction. Therefore, the short side of longitudinal paper and the long side of transverse paper are larger and smaller than other sides and wavy bends. It also means that the wrinkling caused by corrugated paper is known to cause wrinkling when compared to horizontal paper. 2. Paper wrinkles caused by the tight edges of the paper A stack of flat paper with a relatively high moisture content is placed in a room with low relative humidity. At this time, part of the moisture on the side of the paper stack is distributed to the surrounding space, and the paper The inside of the pile does not change, resulting in the shrinkage of the edge of the paper, and the interior is loose and wavy, forming a tight edge of the paper. The slackening of the middle part of the paper during printing is carried forward with the squeezing of the paper. If such unwinding cannot be completely absorbed, bow wrinkles (Figure 2) will occur. This bow wrinkle generally does not reach the edge of the paper, mainly occurs in the center of the paper is slightly biased toward the mouth. Figure 2. Paper wrinkles caused by curling of paper corners Due to poor transport of paper, horizontal paper or excessive moisture during printing tends to make the paper corners buckle (warp up). Paper corner buckles tend to produce small bow-shaped bow wrinkles. Under the paper angle, on the JZ108 and J2203 type machines, the swinging teeth of the middle delivery paper speed up to feed paper after nipping the paper, and the air resistance makes the paper angle go down. When the teeth are given to the impression cylinder, the impression cylinder clenches. It is possible that the paper bite at the corners of the paper is too small. After covering the cylinder of the impression cylinder, the corners of the paper will swell, and after embossing, there will be a small crooked bow. When the paper corners are upturned and the paper-feeding teeth are accelerated to feed paper, the air resistance will cause the paper corners in the paper-feeding tooth gap to float upward. As a result, the embossing cylinder will not bite into the paper corners and will be imprinted. Small beak oblique bow wrinkle, severe corner defects will occur see (Figure 3). Figure 32. Wrinkling Caused by Improperly Adjusted Equipment Wrinkling caused by improper adjustment of printing equipment is generally caused by the mechanism controlling the transfer of paper. There are usually several situations. 1. Feeding Paper Wrinkles Due to Paper Misadjustment Caused by improper paper transfer For the swing-type paper feed mechanism, the distance between the swinging feed paper feed and the feed station and the impression cylinder should be kept at a certain distance, usually paper 3 times the thickness. If the distance is too small, the positioned paper will be touched, resulting in inaccurate overprinting. When the spacing is too large, the paper forms an excessively large wavy edge when the paper is handed over, causing misprints or tearing at the edge of the mouth, and even wrinkles in the paper. In the same way, when the hem-type transfer mechanism conveys the paper, the bite line of the transfer teeth should also be on the same straight line in the same plane, so that the two bite lines of the bite pinching axis and the pinching axis are at the minimum spacing ( The limits are on the respective planes and parallel to each other (Figure 4). Figure 4 1 - Teeth 2 - Teeth 3 - Paper (To Be Continued) (Author: Jiangsu Press and Publication School) Editor of the Responsibility/ Li Danmei