In the packaging trade of cigarette pack printing production, paper curling will cause serious problems for post-press processing and use. For example, after the printed sheet is curled, the positioning error during the subsequent cross-cutting, hot stamping, and die-cutting processing will increase, and the paper will not be smooth, which will cause the machine to stop the paper and seriously affect the efficiency and quality of the subsequent processing; After the finished cigarette pack curls, it will cause difficulties for the paper-gluer and automatic packaging machine. Generally, low basis weight paper (such as coated paper) is more likely to curl than high basis weight paper (whiteboard). How to ensure the flatness of the paper for cigarette pack printing in the processing process and storage and use is a problem that we must take seriously.
1. The relationship between paper stiffness and curl
The stiffness of the same amount of paper depends on the thickness and length of the pulp fibers used in papermaking. Chinese printing companies are strong. The stiffness of thick and long fiber paper must be better than that of short and fine fiber paper. In addition, the stiffness also depends on the papermaking method and paper structure. For example, the stiffness of the three-layer paper of the "sandwich" structure is stronger than that of the "whole" structure. The curl caused by the forced winding of paper by the external force of online publishing is difficult to recover smoothly during the paper stretching process. Especially when winding with a small core, the paper closer to the core curls more strongly.
The characteristic of paper curl caused by winding is that the paper curls in the fiber direction, which provides a good identification method for judging whether the product is such a curl.
2. The relationship between the moisture content of the paper and the curl
Generally speaking, the standard moisture content of paper just off the paper machine is 7% Â± 2% education, but the moisture content of the paper during storage and printing process will change, which is also a major cause of paper curl, 90 % Of the paper curl is due to this. The moisture of paper is mainly stored between paper fibers. Therefore, the increase in moisture and the loss will cause the distance between the fibers of the paper to increase or shrink, which is mainly reflected in the change in the lateral size of the paper. When the moisture content of the paper changes uniformly, Heidelberg will only cause the lateral size of the paper to change. This is a problem that needs to be avoided in printing registration, but this does not cause the paper to curl. The curling is caused by the uneven changes in the moisture content of the paper.
There are roughly two cases of unbalanced changes in paper moisture content. The edges of the paper and the center of the paper are unevenly absorbed or lost. For example, stacked sheets of paper or roll paper, if there is no wrapping film and other isolated packaging, the edge of the paper is easy to absorb moisture and the "lotus edge" phenomenon occurs, or the paper edge loses moisture and the "tight edge" phenomenon occurs. For example, the printing surface of the paper blocks the capillary pores on the surface of the paper due to the printing ink, and the non-printing surface is generally not treated, so the water capacity of the two sides of the paper is very different. If the paper is exposed to a temperature and humidity environment that is incompatible with its moisture content for a long time, it will inevitably cause the paper to curl horizontally: when the paper absorbs moisture from the external environment, the lateral expansion between the fibers of the non-printed paper is greater than that of the printed surface. , The paper will curl to the printing surface, crystal engraving; when moisture is lost, the lateral shrinkage between the fibers of the non-printing surface paper is greater than the printing surface, the paper will curl to the non-printing surface, food packaging design. Another example is the use of aluminum-plated transfer paper in the packaging of commercial products to account for a large proportion. Since the printing surface of this type of packaging is composed of aluminum coating and adhesive layer, which hinders the loss of moisture on the printing surface, it is easy to curl the paper during use: If you use Mann in an air-conditioned environment with low humidity Roland, it is easy to produce the phenomenon of curling to the non-printing surface; when used in an environment with high humidity, it is easy to produce the phenomenon of curling to the printing surface. Therefore, the horizontal curl of the paper is an inevitable result of the uneven change of the water content in the paper. This provides a good method for determining the cause of the paper's lateral curl.
The paper production and use locations are in different regions, different seasons, or in an air-conditioned constant temperature and humidity environment and a non-air-conditioned and non-air-conditioned environment with different temperature and humidity, and they will inevitably encounter paper curling problems.
Published in the wet composite production process (composite surface coating water-based composite adhesive, back surface coating waterproof adhesive and drying) commonly used in composite paper and transfer paper, if the water content of the adhesive, composite speed and drying link, etc. Improper control leads to large differences in the gain and loss of moisture on both sides of the composite paper and the transfer paper, which will inevitably produce curling in an undefined direction.
There may be some other causes of curling in paper produced from composite materials. For example, when the paper and plastic are compounded, the plastic film is stretched and deformed due to the excessively high compounding temperature or the traction tension of the plastic film is too large. After the composite is cured, cutting the large sheet or peeling the plastic film will also cause the paper to curl.
To solve the curling caused by winding, in addition to selecting a large-diameter paper core web offset press as much as possible when ordering, a humidification device and a rewind device can also be used for correction on the printing machine. Because humidification can make the water molecules penetrate into the paper fiber molecules, poster printing increases the distance between the fiber molecules. The loose fibers quickly reduce the stiffness, so that the original curled fiber becomes flat under the action of external force. However, because the humidification device on the printing machine is difficult to control, plus the pre-press process when humidifying the coated printing paper during high-speed operation, moisture cannot quickly penetrate into the fiber gaps inside the paper, so it is difficult to control the paper smoothing effect In addition, cost is also an important factor hindering its application. At present, this method has not been adopted by the industry. It is to make the reverse side of the paper curl pass a paper guide roller with a diameter of less than 50mm and a large wrap angle. The mechanical force is used to reverse the curled fiber to transform KBA to achieve the purpose of flattening the paper. At the same time, the rewind device can automatically adjust the wrap angle according to the position of the paper in the roll diameter, that is, adjust the force of the reverse bend according to the degree of paper bending, to avoid overcorrection.
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