In the production process of corrugated paper, the defects of corrugated board warpage often occur, which will affect the normal production of subsequent processes in serious cases, greatly increasing the probability of scrapping, thereby causing waste and increasing production costs. Therefore, during the production process of corrugated paper, warpage of corrugated board must be avoided as much as possible, which is an effective way to reduce consumption and reduce costs. This article discusses the cause analysis and control of corrugated cardboard warpage with colleagues.
Classification of corrugated cardboard warpage
According to the difference of corrugated cardboard warpage (Figure 1), it can be roughly divided into five types: vertical forward warpage, vertical reverse warpage, horizontal positive warpage, horizontal reverse warpage and composite warpage.
1. Longitudinal positive warping: as shown in (figure a), the paperboard is warped in the direction perpendicular to the corrugation, and the tissue is recessed inward, which is called longitudinal positive warping. This phenomenon of warpage occurs more frequently in the production of corrugated board.
2. Longitudinal reverse warpage: as shown in (figure b), the paperboard warps in the direction perpendicular to the corrugation, and the paper is convex outward, which is called longitudinal reverse warpage. This kind of warpage phenomenon occurs less frequently in the production of corrugated board.
3. Transverse positive warpage: as shown in (Figure c), the warpage of the cardboard along the corrugated direction and the inward concave of the paper is called lateral positive warpage. This kind of warpage phenomenon often occurs in mounting offset printing Corrugated cardboard.
4. Horizontal warpage: as shown in (Figure d), the warpage phenomenon of cardboard in the corrugated direction and the outward convex surface of the paper is called lateral reverse warpage. This is also a common occurrence in the production process of corrugated cardboard. Kind of warping.
5. Compound warping: If there is more than two types of warping on a corrugated board, it is called compound warping. [next]
Several basic concepts
In order to facilitate the analysis and study of the causes and mechanisms of corrugated cardboard warpage, several basic concepts must be clarified first.
1. Vertical and horizontal definition of corrugated cardboard: as shown in (Figure 2), parallel to the direction of the corrugated groove on the corrugated cardboard, that is, the direction of the width of the paper roll, it is defined as the horizontal direction of the corrugated cardboard; The vertical direction of the groove is defined as the longitudinal direction of the corrugated cardboard.
2. The external dimensions of paper and paperboard change with the change of their moisture content: during the drying process (before reaching absolute dryness), the external dimensions of paper and paperboard will shrink accordingly as the moisture content decreases. Normally, the amount of shrinkage in the transverse direction is greater than that in the longitudinal direction, and the amount of shrinkage in the thickness direction is ignored here.
3. The characteristics of shrinkage of corrugated cardboard during the drying process: here we use single corrugated three-layer corrugated cardboard as an example. The single-corrugated three-layer corrugated cardboard is made by single-corrugated two-layer sheet and facial paper after being glued by a glue coating machine, and then compounded, bonded and dried. The shrinkage of the corrugated cardboard in the drying process has the following characteristics:
(1) When the moisture content of the single-layer two-layer sheet is greater than the moisture content of the facial paper, the shrinkage of the single-tile two-layer sheet must be greater than the shrinkage of the facial paper. At this time, because the two-layer paper has been firmly bonded, the facial paper It will be affected by the contraction force from the surrounding to the center of the single-layer two-layer sheet. Since the longitudinal shrinkage of a single tile two-layer sheet will be compensated by the slight deformation of its corrugated waveform, warping stress is unlikely to occur in this direction, but the lateral shrinkage of a single tile two-layer sheet cannot be compensated, so in Warping stress is easily generated in this direction.
(2) When the moisture content of the facial tissue is greater than that of the single-layer two-layer sheet, the shrinkage of the facial tissue must be greater than the shrinkage of the single-tile two-layer sheet. Similarly, the single-tile two-layer sheet will be affected by the The shrinkage force from the four sides to the center, at this time, because the longitudinal strength of the single tile two-layer sheet is small, it is easy to produce warpage, and the lateral strength is large, it is not easy to produce warpage deformation.
Causes of warpage of corrugated cardboard
1. Effect of moisture content difference: Practice has proved that the difference in moisture content between the composite objects of corrugated cardboard is the main reason for the warpage of corrugated cardboard.
(1) If the moisture content of the single-layer two-layer sheet is greater than the moisture content of the facial paper, then during the bonding and drying process, the shrinkage of the single-tile two-layer sheet must be greater than the shrinkage of the facial paper. Restricted by the shrinkage characteristics, the paperboard after molding has a phenomenon of lateral warpage. When the water content of the single tile two-layer sheet is too large, there will be vertical and horizontal reverse warpage at the same time, that is, the phenomenon of composite warpage.
(2) If the moisture content of the facial paper is greater than that of the single-layer two-layer sheet, then during the bonding and drying process, the shrinkage of the facial paper must be greater than the shrinkage of the single-layer two-layer sheet. Restricted by the characteristics of shrinkage, the paperboard after molding has a positive longitudinal warping phenomenon.
2. The effect of the amount of glue: When producing corrugated cardboard, the single-layer two-layer sheet sent by the single-sided machine must be evenly glued on the corrugated tip by the glue machine, and then compounded with the facial 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 corrugated tip of the single tile two-layer sheet and the facial paper, so that the moisture content on the side of the facial paper will be much higher than the side of the inner paper Moisture content (assuming the same moisture content before coating), then the shrinkage on the side of the paper side will be greater than the shrinkage on the side of the paper side during the drying process, so the corrugated cardboard after molding will appear horizontally positive The phenomenon of warpage, which is particularly likely to occur in corrugated cardboard mounted with offset paper.
3. The effect of production speed: Corrugated paperboard after compounding with glue must also be heated through a drying tunnel to make the glue paste, the paperboard is bonded, and then dried. In fact, the temperature distribution in the vertical direction of the drying tunnel is not uniform, showing a state of decreasing downwards. As shown in Figure 3.
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Therefore, during the drying process of corrugated cardboard, there is also a gradient in the drying speed of the cross section. If the production vehicle speed is too slow, and the temperature of the drying tunnel has not been reduced accordingly, the moisture content of the corrugated cardboard produced will be lower than the moisture content of the paper side of the corrugated board, and the board will be warped longitudinally. . In severe cases, it may even cause the cardboard to become brittle and unglued and scrapped.
There are many reasons for the warpage of corrugated cardboard. In addition to the above, the difference in water resistance of paper, the unsuitable tension of the facial paper in the production process, and the excessive accumulation of single-layer two-layer sheets on the flyover can also cause the warpage of corrugated cardboard. song. But no matter what the reason, in the final analysis, the most important thing is caused by the different moisture content of the composite objects that make up the corrugated cardboard. Therefore, properly controlling the moisture content of each composite object of corrugated board is the key to controlling warpage.
Corrugated board warpage control
Studies have shown that when the layers of paper that make up corrugated cardboard enter the drying tunnel, when the difference in 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 of 3% moisture content is the critical point of corrugated board warpage. Therefore, in production, we should pay attention to control the moisture content between each layer of paper and the single-layer two-layer sheet, and control the difference in moisture content between them within 3%.
1. Understand the moisture content of the base paper: When using paper rolls, the operators of the corrugated board production line should understand their moisture content in advance, so as to use paper rolls with similar moisture content as much as possible, so as to effectively avoid the It takes a long time to adjust and produce a lot of waste film.
2. Make full use of the preheating cylinder: preheating drying cylinders are installed in the corrugated cardboard production line. When the moisture content of the roll paper is too large, it must be adjusted by the preheating cylinder to make each layer enter the drying tunnel The difference in moisture content between cardboard and paper is less than 3%.
3. Correctly control the amount of glue: To produce high-quality corrugated cardboard, correct control of the amount of glue is also one of the keys. If the amount of glue is too large, the cardboard will be soft, and the strength will be poor, and it will warp; if the amount of glue is too small, it will not adhere well, and it is easy to degumm and foam.
Generally, when the vehicle speed is faster, the amount of glue should be reduced accordingly; when the vehicle speed is slower, the amount of glue should be appropriately increased. When using paper with better water resistance, the amount of glue should be increased accordingly, otherwise it should be reduced accordingly. Therefore, in the production process, the amount of glue applied by the glue applicator should be adjusted according to changes in production conditions. The principle of adjustment is: on the premise of ensuring the bonding strength is qualified, the amount of glue should be less or not more.
4. Correct stacking: because the corrugated cardboard is warped for many reasons, it is often simple * process adjustment is difficult to completely control the occurrence of warping, so the stacking process must require the corrugated cardboard just produced per Stack 10 pieces in a stack, stacked in front and back, and then flatten it with a heavy object until the temperature and humidity of the corrugated cardboard are basically the same as the room temperature and humidity. This can effectively avoid the occurrence of corrugated board warpage.
Although the reasons for the warpage of corrugated cardboard are complex and there are many influencing factors, as long as they are taken seriously and operated carefully, the warpage of corrugated cardboard can still be controlled.
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