Why is the cardboard warped during corrugated paper production?

Analysis and Control of the Causes of Warpage of Corrugated Cardboard

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 warping phenomenon:

According to the different warping phenomenon of corrugated cardboard ( Fig. 1) , it can be roughly divided into five types: longitudinal positive warping, longitudinal anti-warping, lateral warping, lateral anti-warping and composite warping.

                                (figure 1)

1. Longitudinal positive warpage: As shown in ( Fig. a) , the warpage of the cardboard in the direction perpendicular to the corrugated inward, the inward concave of the facial paper is called longitudinal warping. This warpage occurs more in corrugated board production.

2. Longitudinal anti-warping: As shown in ( Fig. b) , the warping phenomenon of the cardboard protruding outward in the direction perpendicular to the corrugated sheet is called longitudinal anti-warping. This warpage occurs less frequently in corrugated board production.

3. Horizontal warping: As shown in ( Fig. c) , the warping phenomenon of the cardboard in the corrugated direction and the inward concave of the facial paper is called the lateral warping. This warping often occurs in the offset printing. The corrugated board of the paper.

4. Lateral anti-warping: As shown in ( Fig. d) , the warpage of the cardboard in the direction of corrugation and the outward convexity of the facial paper is called lateral anti-warping, which is also a common occurrence in the production process of corrugated cardboard. Kind of warping phenomenon.

5. Composite warpage: There are two or more types of warping on a corrugated board, which is called compound warpage.

A few 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: as shown in ( Fig. 2) , parallel to the direction of the corrugated groove on the corrugated board, that is, the direction of the web of the paper roll, which is defined as the transverse direction of the corrugated board; The vertical direction of the groove is defined as the longitudinal direction of the corrugated cardboard.

                            (figure 2)

2. The outer dimensions of paper and paperboard vary with the moisture content: paper and paperboard shrink in size as the moisture content decreases during the drying process ( before drying ) . 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

Features:

(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 periphery 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, the 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-layer 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 face paper. 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 water 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 during the bonding and drying process, the shrinkage of the single-wall two-layer sheet must be greater than the shrinkage of the paper, which is affected by the corrugated board. 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 simultaneously, that is, a composite warp phenomenon occurs.

(2) If the moisture content of the paper is greater than the moisture content of the single-layer two-layer sheet, then the shrinkage of the paper during the bonding and drying process must be greater than the shrinkage of the single-wall two-layer sheet, also due to the corrugated board. Constrained by the characteristics of the shrinkage amount, the formed cardboard is longitudinally warped.

2. Influence of the amount of glue applied: When producing corrugated board, the single-layer 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 combined 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 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 the 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 the 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. As shown in ( Figure 3) .

                                (image 3)

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 main reason 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 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: cause warpage because the corrugated cardboard are many, often difficult to completely adjust the simple process control * warpage phenomenon, therefore, a step must be newly required stacking of corrugated board produced per 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.

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