Cause Analysis and Control of Warpage of Corrugated Cardboard
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In the production process of corrugated paper, the defects of corrugated cardboard warping often occur. In severe cases, the normal production of subsequent processes will be affected, and the chance of scrapping will be greatly increased, resulting in waste and increasing production costs. For this reason, in the production process of corrugated paper, it is necessary to avoid the occurrence of warpage of corrugated cardboard, which is an effective way to reduce consumption and reduce costs. This paper discusses the reasons for the warpage of corrugated board and its control.
Classification of corrugated board warpage
According to the different warping phenomenon of corrugated cardboard, it can be roughly divided into five types: longitudinal positive warping, longitudinal anti-warping, horizontal warping, lateral anti-warping and compound warping.
1. Longitudinal warpage: The warpage phenomenon in which the paperboard is recessed inward in the direction perpendicular to the corrugated sheet is called longitudinal warpage. This warpage occurs more in corrugated board production.
2. Longitudinal anti-warping: The warping phenomenon of the paperboard in the direction perpendicular to the corrugated, the outward convexity of the facial paper is called longitudinal anti-warping. This warpage occurs less frequently in corrugated board production.
3. Horizontal warpage: The warping phenomenon of the cardboard in the corrugated direction and the concave inward of the paper is called the lateral warping. This warping often occurs in the corrugated board of the offset printing paper.
4. Lateral anti-warping: The warping phenomenon of the cardboard in the direction of corrugation, the outward warping of the facial paper is called lateral anti-warping, which is also a kind of warping phenomenon often produced in the corrugated board production process.
5. Composite warping: There are two or more types of warping on a corrugated board, which is called compound warping.
Several basic concepts
In order to facilitate the analysis and study of the causes and mechanisms of warpage of corrugated cardboard, several basic concepts must be clarified first.
1. Definition of longitudinal and transverse direction of corrugated board: parallel to the direction of the corrugated groove on the corrugated board, ie the direction of the web of the paper roll, which is defined as the transverse direction of the corrugated board; the direction perpendicular to the corrugated groove is defined as corrugated board Portrait.
2. The outer dimensions of paper and paperboard vary with the moisture content: during the drying process (before drying), the outer dimensions of the paper and paperboard will shrink as the moisture content decreases. Normally, the amount of lateral shrinkage is greater than the amount of shrinkage in the longitudinal direction, and the amount of shrinkage in the thickness direction is ignored here.
3. Characteristics of shrinkage of corrugated cardboard during drying: Here, a single corrugated three-layer corrugated cardboard is taken as an example. The single corrugated three-layer corrugated board is made by laminating, bonding and drying the single corrugated two-layer sheet and the face paper through the glue coating machine. The shrinkage of corrugated cardboard during its drying process has the following characteristics:
(1) When the moisture content of the single-wall two-layer sheet is greater than the moisture content of the paper, the shrinkage of the single-wall two-layer sheet is inevitably larger than the shrinkage of the paper, and the two-layer paper is firmly bonded, so the paper It will be subjected to the contraction force from the circumference to the center from the single-wall two-layer sheet. Since the longitudinal shrinkage of the single-wafer two-layer sheet is compensated by the slight deformation of the corrugated waveform, warping stress is not easily generated in this direction, and the lateral shrinkage of the single-wafer two-layer sheet cannot be compensated, so Warping stress is easily generated in this direction.
(2) When the moisture content of the facial paper is greater than the moisture content of the single-wall two-layer sheet, the shrinkage of the facial paper is inevitably larger than the shrinkage of the single-wafer two-layer sheet. Similarly, the single-wafer two-layer sheet will be subjected to the paper from the facial tissue. The effect of the contraction force from the periphery to the center, at this time, since the longitudinal strength of the single-walled two-layer sheet is small, warpage is likely to occur, and when the lateral strength is large, warping deformation is less likely to occur.
Causes of warpage of corrugated cardboard
1. Influence of moisture content difference: Practice has proved that the difference in moisture content between the composite objects of corrugated board is the main reason for the warpage of corrugated board.
(1) If the moisture content of the single-wall two-layer sheet is greater than the moisture content of the paper, then in the bonding and drying process, the shrinkage of the single-wall two-layer sheet must be greater than the shrinkage of the paper, and then the corrugated cardboard Constrained by the shrinkage characteristics, the formed cardboard has a lateral anti-warping phenomenon. When the moisture content of the single-wall two-layer sheet is too large, there will be a vertical and horizontal anti-warpage at the same time, that is, a composite warp phenomenon occurs.
(2) If the moisture content of the facial paper is greater than the moisture content of the single-wall two-layer sheet, then the shrinkage of the facial tissue during the bonding and drying process must be greater than the shrinkage of the single-wafer two-layer sheet, also due to the corrugated cardboard. Constrained by the characteristics of the shrinkage amount, the longitudinally warped phenomenon occurs in the formed cardboard.
2. Influence of the amount of glue applied: When producing corrugated board, the single-wall two-layer piece sent by the single-sided machine must be evenly coated on the tip of the coat by the glue applicator, and then compounded with the face paper. At this time, if the amount of glue applied is too large, the moisture in the paste will be quickly absorbed and swelled by the tip of the single-layer two-layer sheet and the face paper, so that the moisture content on the side of the paper will be much higher than that on the side of the paper. The moisture content (assuming the water content before coating is consistent), then the shrinkage on the side of the paper will be greater than the shrinkage on the side of the paper during the drying process, so the corrugated cardboard after molding will appear laterally positive. The phenomenon of warpage, which is particularly prone to occur in corrugated cardboard with offset printing paper.
3. Influence of production speed: Corrugated cardboard after being glued and compounded must also be heated by drying tunnel to make the paste gelatinized, the cardboard is bonded, and then dried. In fact, the temperature distribution in the vertical direction of the drying tunnel is not uniform and is in a state of decreasing upward.
Therefore, there is also a gradient in the drying speed of the cross-section of the corrugated board during the drying process. If the production speed is too slow and the drying tunnel temperature is not reduced accordingly, the moisture content of the corrugated board produced on the paper side will be lower than that on the paper side, and the longitudinal warpage of the cardboard will occur. . In severe cases, it may even cause the board to become brittle, open and scrapped.
There are many reasons for the warpage of corrugated cardboard. In addition to the above, the difference in paper water resistance, the unsuitable tension of the paper during the production process, and the excessive accumulation of the single-layer two-layer sheet on the flyover will also cause the corrugated cardboard to be warped. song. But for whatever reason, the most important thing is that the moisture content between the composite objects that make up the corrugated board is different. Therefore, proper control of the moisture content of each composite object of corrugated paperboard is the key to controlling warpage.
Corrugated board warping control
Studies have shown that when the layers of paper that make up the corrugated board enter the drying tunnel, the moisture content between them is greater than 2.5% to 3%, it is difficult to avoid the warpage of the cardboard. This means that the difference in 3% moisture content is the critical point at which corrugated cardboard produces warpage. Therefore, in production, care should be taken to control the moisture content between each layer of paper and the single-wall two-layer sheet, and to control the difference in moisture content between them within 3%.
1. Understand the moisture content of the base paper: When using the paper roll, the operators of the corrugated board production line should know their moisture content in advance, so as to use the paper rolls with similar water content as close as possible, which can effectively avoid the cause. Exploring the adjustment time is too long to produce a large amount of waste.
2. Make full use of the preheating cylinder: Preheating dryer is installed in the corrugated board production line. When the moisture content difference of the paper is too large, it must be adjusted by the preheating cylinder to make the layers enter the drying tunnel. The difference in moisture content between paperboard and paper is less than 3%.
3. Correctly master the amount of glue: To produce high-quality corrugated board, it is also one of the keys to correctly master the amount of glue applied. If the amount of glue is too large, the paperboard is soft, the strength is poor, and it will warp. If the amount of glue is too small, the adhesion is not strong, and it is easy to degumming and foaming.
Generally, when the vehicle speed is faster, the amount of glue applied should be correspondingly reduced; when the vehicle speed is slow, the amount of glue applied should be appropriately increased. When using paper with better water resistance, the amount of glue should be increased accordingly, and vice versa. Therefore, in the production process, the amount of glue applied to the glue applicator should be adjusted according to the change of production conditions. The principle of adjustment is: under the premise of ensuring the bond strength is qualified, the amount of glue should be less and less.
4. Correct stacking: Because corrugated cardboard has many reasons for warpage, it is often difficult to completely control the occurrence of warpage by simply adjusting the process. Therefore, it is required that the stacking process must be made of corrugated cardboard just produced. 10 pieces of a stack, stacked on the front and back, and then flattened with heavy objects until the temperature and humidity of the corrugated board and the indoor temperature and humidity are basically the same. This can effectively avoid the occurrence of warpage of corrugated cardboard.
Although the cause of warpage of corrugated cardboard is complicated and has many influencing factors, as long as it is taken seriously and carefully manipulated, the warpage of corrugated cardboard can be controlled.