Why does the board warp in the production of corrugated paper?

Why does the board warp in the production of corrugated paper? In the production process of corrugating paper, the defects of corrugating paper board warping often occur, which will affect the normal production of subsequent processes in serious cases, greatly increasing the scrap probability, resulting in waste and increasing the production cost Therefore, in the production process of corrugated paper, we must try to avoid the occurrence of warpage of corrugated board, which is an effective way to reduce consumption and cost In this paper, the cause analysis and control of corrugated board warpage are discussed together with peers Classification of corrugated board warping: according to the different warping phenomena of corrugated board (Fig 1), it can be roughly divided into five types: longitudinal warping, longitudinal warping, transverse warping, transverse warping and composite warping (Fig 1) 1 Longitudinal positive warpage: as shown in (Fig a), the warpage of the paperboard in the direction perpendicular to the corrugation, which is called longitudinal positive warpage This kind of warping phenomenon occurs frequently in the production of corrugated board 2 Longitudinal anti warping: as shown in (Figure b), the paper board warps outward along the direction perpendicular to the corrugating, which is called longitudinal anti warping This kind of warping phenomenon seldom occurs in the production of corrugated board 3 Transverse positive warpage: as shown in (Figure C), the warpage of the paperboard along the corrugating direction and the face paper concave inward is called transverse positive warpage, which often occurs in the corrugated board mounted with offset paper 4 Transverse warpage: as shown in (Figure d), the warpage phenomenon of the paper board protruding outward along the corrugating direction is called transverse warpage, which is also a common warpage phenomenon in the production process of corrugated paper board 5 Composite warpage: if there are more than two types of warpage on a corrugated board at the same time, it is called composite warpage Several basic concepts: in order to analyze and study the cause and mechanism of corrugated board warpage, several basic concepts must be defined first 1 Definition of longitudinal and transverse direction of corrugated board: as shown in Figure 2, the direction parallel to the corrugated groove on the corrugated board, that is, the door width direction of the paper roll, is defined as the transverse direction of the corrugated board; the direction perpendicular to the corrugated groove is defined as the longitudinal direction of the corrugated board (Fig 2) 2 The overall dimensions of paper and paperboard change with the change of moisture content: during the drying process of paper and paperboard (before absolute drying), the overall dimensions will shrink with the decrease of moisture content Generally, the horizontal shrinkage is larger than the longitudinal shrinkage, and the shrinkage in the thickness direction is ignored here 3 The characteristics of corrugated board shrinkage in the drying process: take single corrugated three-layer corrugated board as an example Single corrugating three-layer corrugated board is made of single corrugating two-layer sheet and surface paper after gluing by gluing machine and then compounding, bonding and drying The shrinkage of corrugated board in the drying process has the following characteristics: (1) when the moisture content of single tile and two-layer sheet is greater than that of the paper, the shrinkage of single tile and two-layer sheet must be greater than that of the paper At this time, because the two-layer paper has been firmly bonded, the paper will be affected by the shrinkage force from the four sides to the center from the single tile and two-layer sheet Because the longitudinal shrinkage of the single tile and two-layer sheet will be compensated by the micro deformation of its corrugating waveform, it is not easy to generate warping stress in this direction, while the transverse shrinkage of the single tile and two-layer sheet cannot be compensated, so it is easy to generate warping stress in this direction (2) When the moisture content of the face paper is greater than that of the single tile and two-layer sheet, the shrinkage of the face paper must be greater than that of the single tile and two-layer sheet Similarly, the single tile and two-layer sheet will be affected by the shrinkage force from the four sides to the center of the face paper At this time, because the longitudinal strength of the single tile and two-layer sheet is small, it is easy to generate warpage, while the transverse strength is large, it is not easy to generate warpage The causes of corrugated board warpage: 1 The influence of water content difference: practice has proved that the main cause of corrugated board warpage is the difference of water content between composite objects (1) If the moisture content of the single tile and two-layer sheet is greater than that of the face paper, the shrinkage of the single tile and two-layer sheet must be greater than that of the face paper in the process of bonding and drying At this time, due to the shrinkage characteristics of the corrugated board, the formed paperboard will appear the phenomenon of transverse reverse warpage When the water content of single tile and two-layer is too large, there will be vertical and horizontal reverse warpage at the same time, that is, composite warpage (2) If the moisture content of the face paper is greater than that of the single tile and two-layer sheet, the shrinkage of the face paper must be greater than that of the single tile and two-layer sheet in the process of bonding and drying Also, due to the characteristics of the shrinkage of the corrugated board, the formed board will appear the phenomenon of vertical warping 2 Influence of gluing quantity: in the production of corrugated board, the single tile and two-layer sheets sent by the single side machine must be uniformly glued on the edge of the corrugated board by the gluing machine, and then compounded with the surface paper At this time, if the amount of glue is too large, the water in the paste will be quickly absorbed and moistened by the edge of the single tile lamination and the face paper, so that the water content of the face paper side will be much higher than that of the inside paper side (assuming that the water content is the same before glue application) In the process of drying, the shrinkage of the side of the face paper will be greater than the shrinkage of the side of the inside paper, so the corrugated board after forming will appear the phenomenon of transverse positive warpage, which is particularly easy to occur in the corrugated board mounted with offset paper 3 Influence of production speed: the corrugated board after gluing and compounding must also be heated through the drying channel to make the paste paste paste, the board is bonded, and then dried In fact, the temperature distribution in the vertical direction of the drying channel is uneven, showing an upward decreasing state As shown in Figure 3 (Fig 3) therefore, there is also a gradient of drying speed on the cross section of corrugated board during the drying process If the production speed is too slow, and the drying channel temperature is not reduced correspondingly, then the moisture content of the surface paper side of the corrugated board produced will be lower than the moisture content of the inside paper side, and the paper board will appear the phenomenon of vertical warping In serious cases, it may even cause the cardboard to be brittle and glue opened, and then it will be scrapped There are many reasons for the warpage of corrugated board In addition to the above, the difference of water resistance of paper, the unsuitable tension of surface paper in the production process, and the excessive accumulation of single tile and two-layer sheets on the overpass will also cause the warpage of corrugated board However, no matter what the reason is, in the final analysis, Zui is mainly caused by the different moisture content between the composite objects that make up the corrugated board Therefore, the key to control warpage is to properly control the moisture content of each composite object of corrugated board Warpage control of corrugated board: some studies have shown that when the moisture content difference between the layers of paper that make up corrugated board is greater than 2.5% ~ 3%, it is difficult to avoid the warpage of the board This means that the difference of 3% moisture content is the critical point of warpage Therefore, in the production, we should pay attention to the control of the moisture content between each layer of paper and single tile, and the difference between them should be controlled within 3% 1 Know 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 status in advance, so as to use the paper roll with similar moisture content as much as possible, so as to effectively avoid producing a large number of waste sheets due to the long adjustment time 2 make full use of preheating cylinder: preheating drying cylinder is set in the corrugated board production line When the moisture content difference of the roll is too large, preheating cylinder must be used to adjust so that the moisture content difference of each layer of board and paper entering the drying channel is less than 3% 3 Mastering the gluing quantity correctly: to produce corrugated board, mastering the gluing quantity correctly is also one of the keys If the amount of glue is too large, the paperboard will become soft, weak and warped; if the amount of glue is too small, the paperboard will not bond firmly, and it is easy to degumme and blister Generally, when the speed is fast, the amount of glue should be reduced correspondingly; when the speed is slow, the amount of glue should be increased appropriately When the paper with better water resistance is used, the amount of gelatinization should be increased correspondingly, and vice versa Therefore, in the production process, the gluing amount of the gluing machine should be adjusted according to the change of the production situation The adjustment principle is: on the premise of ensuring the bonding strength is qualified, the gluing amount should be less rather than more 4 Correct stacking: because there are many reasons for the warpage of corrugated board, it is often difficult to completely control the warpage by simply * process adjustment Therefore, it should be required that the newly produced corrugated board should be stacked every 10 Stack one by one, stack up in front and back, and then flatten it with heavy objects until the temperature and humidity of corrugated board are basically the same as that of indoor In this way, the warpage of corrugated board can be effectively avoided Although the causes of corrugated board warpage are complex and there are many influencing factors, as long as we take it seriously and operate it carefully, the warpage of corrugated board can be controlled.