Corrugated board warping analysis and control

In the production process of corrugated paper, defects in warpage of corrugated cardboard often occur, and when it is severe, the normal production of subsequent processes will be affected, and the chance of scrapping will be greatly increased, resulting in waste and increased production costs. For this reason, in the production process of corrugated paper, the occurrence of warpage of corrugated cardboard must be avoided as much as possible. This is an effective way to reduce consumption and reduce costs. This paper discusses and discusses the causes of the warpage of corrugated board and its peers.

Classification of Corrugated Cardboard Warpage

According to the different warping phenomenon of corrugated board (Fig. 1), it can be roughly divided into five types: vertical positive warping, longitudinal reverse warpage, transverse positive warpage, horizontal reverse warpage and compound warping.

1. Longitudinal positive warpage: As shown in (a), the warpage of the paperboard in the direction perpendicular to the corrugation, in which the paper is recessed inward, is called longitudinal positive warping. This warping phenomenon occurs more in the production of corrugated cardboard.

2. Longitudinal warpage: As shown in (b), the warpage of the paperboard outward in the direction perpendicular to the corrugation is called longitudinal warpage. This warping phenomenon is less frequent in corrugated board production.

3. Lateral positive warping: As shown in (c), the warpage of the paperboard in the direction of the corrugations, concave inwards of the paper, is called transverse positive warping. Such warpage often occurs in offset printing. Face paper in corrugated board.

4. Lateral warpage: As shown in (d), the warpage of the cardboard in the direction of the corrugations is called transverse warpage. This is also a frequent occurrence in the production of corrugated cardboard. Warp phenomenon.

5. Compound Warpage: There are two or more types of warpage on a piece of corrugated cardboard, which is called composite warping.

Several basic concepts

To facilitate the analysis and study of the causes and mechanisms of the warpage caused by corrugated board, several basic concepts must first be identified.

1. Vertical and horizontal definition of corrugated board: As shown in (Fig. 2), parallel to the direction of the corrugated groove on the corrugated board, namely the direction of the width of the roll, it is defined as the horizontal direction of the corrugated board; The vertical direction of the slot is defined as the longitudinal direction of the corrugated cardboard.

2. The external dimensions of paper and paperboard change with the change of moisture content: During the drying process of paper and paperboard (before it is completely dry), its external dimensions will shrink correspondingly with the decrease of moisture content. Under normal circumstances, the amount of contraction in the transverse direction is greater than the amount of contraction in the longitudinal direction, and the amount of shrinkage in the thickness direction is negligible.

3. The characteristics of corrugated board shrinkage during the drying process: Here is an example of a single corrugated three-layer corrugated cardboard. Single corrugated three-layer corrugated paperboard is made of single corrugated two-layer sheet and face paper after being glued by a coating machine and then compounded, bonded and dried. Corrugated board shrinks its outer dimensions in the drying process with the following characteristics:

(1) When the moisture content of the single-tile two-layer sheet is greater than the moisture content of the facial paper, the shrinkage of the single-sheet two-layer sheet is inevitably greater than the shrinkage of the face sheet. In this case, the two-layer paper is firmly bonded, so that the face paper It will be affected by the contraction force from the four sides to the center of the single tile. Since the longitudinal shrinkage of a single-wall two-layer sheet is compensated by the slight deformation of its corrugation waveform, warpage stress is less likely to occur in this direction, and the lateral shrinkage of a single-wall two-layer sheet cannot be compensated. Warpage stress is easily generated in this direction.

(2) When the moisture content of the facial tissue is greater than the moisture content of the single-layer two-layer sheet, the shrinkage of the facial tissue is inevitably greater than the shrinkage of the single-wall two-layer sheet. Similarly, the single-wall two-layer sheet will be subject to the face paper. The effect of the contraction force from the periphery to the center, at this time, warpage is liable to occur because the single-wall two-layer sheet has small longitudinal strength, and the transverse strength is large, so warpage deformation is not easily generated.

Causes of warpage in corrugated board

1. The influence of the difference in moisture content: Practice has proved that the difference in moisture content between the composite objects of corrugated board is the main cause of the warpage of corrugated board.

(1) If the moisture content of the single tile two-layer sheet is greater than the moisture content of the tissue paper, then in the process of bonding and drying, the shrinkage of the single-sheet two-layer sheet must be greater than the shrinkage of the sheet, which is then affected by the corrugated board. Restriction of the shrinkage characteristics, the shape of the cardboard will appear after the phenomenon of transverse warping. When the moisture content of a single tile is too large, there will be vertical and horizontal warpage simultaneously, that is, the phenomenon of composite warping.

(2) If the moisture content of the tissue paper is greater than the moisture content of the single layer two-layer sheet, then in the bonding and drying process, the shrinkage of the tissue must be greater than the shrinkage of the single layer two-layer sheet, also due to the corrugated board. Due to the restriction of shrinkage characteristics, the formed cardboard has a phenomenon of longitudinal warpage.

2. Influence of the amount of glue applied: In the production of corrugated board, the single-layer two-layer sheet sent by the single-sided machine must be evenly glued on the tip of the squeegee by a sizing machine, and then compounded with the face paper. At this time, if the amount of glue applied is too large, moisture in the adhesive will be rapidly absorbed and swelled by the tip and the top sheet of the single-sheet two-layer sheet, so that the moisture content on the side of the tissue will be much higher than that on the back side. The moisture content (assuming that the moisture content before gluing is the same), then the shrinkage of the tissue side during the drying process will be greater than the shrinkage of the liner side, so that the formed corrugated cardboard will appear laterally positive. The phenomenon of warpage, such warpage is particularly likely to occur in the corrugated cardboard coated offset paper.

3. The impact of production speed: After corrugated paperboard with compounded glue, it must also be heated through a drying tunnel to glue the paste, bond the paperboard, and then dry. However, in fact, the temperature distribution in the vertical direction of the drying tunnel is not uniform, and it is in an upwardly decreasing state. As shown in Figure 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 temperature of the drying tunnel is not reduced accordingly, then the moisture content of the surface of the corrugated board produced will be lower than the moisture content of the liner side, and the board will exhibit a phenomenon of longitudinal warpage. . In severe cases, it may even cause the cardboard to be brittle, open the plastic, and be scrapped.

Causes of corrugated board warping for many reasons, in addition to the above, the paper's water resistance differences, the production process is not suitable for the tension of the paper surface, the flyover on the single layer of two-story tile stacking too much, etc., but also will cause the corrugated cardboard Alice song. Whatever the reason, the main reason is ultimately due to the difference in moisture content between the composite objects that make up the corrugated board. Therefore, proper control of the moisture content of each composite object of corrugated cardboard is the key to controlling warpage.

Corrugated board warping control

Studies have shown that when the layers of paper that make up a corrugated board enter a drying tunnel and the difference in moisture content between them is greater than 2.5% to 3%, it is difficult to avoid warping of the board. This means that the difference in moisture content of 3% is the critical point for the warpage of corrugated cardboard. Therefore, in production, we should pay attention to control the moisture content between each layer of paper and single tile two layers, the difference between the moisture content between them is controlled within 3%.

1. Understand the moisture content of the base paper: When using the paper roll, operators of the corrugated paperboard production line should know their moisture content status in advance so that they can use the paper roll with the approximate moisture content as much as possible, which can effectively avoid the Finding time is too long to produce a lot of scraps.

2. Make full use of preheated cylinders: Preheating drying cylinders are installed in the corrugated board production line. When the moisture content of the roll paper is too different, it must be adjusted with a preheating cylinder so that the layers enter the drying tunnel. The difference in moisture content between paperboard and paper is less than 3%.

3. Correctly control the amount of glue applied: To produce high-quality corrugated board, it is also one of the keys to correctly grasp the amount of glue applied. If the amount of glue applied 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, the adhesive will not be firm, and it will be easy to debond and blisters.

Usually when the speed is faster, the amount of glue applied should be reduced accordingly; when the speed is slow, the amount of glue applied should be appropriately increased. When using paper with better water resistance, the amount of glue applied should be increased accordingly, and vice versa. Therefore, during the production process, the amount of glue applied by the gluing machine should be adjusted according to the change of production conditions. The principle of adjustment is: under the premise of ensuring the conformity of the adhesive strength, the amount of gluing should not be less.

4. Stacking correctly: Because the causes of warpage in corrugated board are various, it is difficult to completely control the occurrence of warpage due to pure* process adjustment. Therefore, it is required that the stacking process must be based on each corrugated board just produced. 10 pieces are piled up one after the other, piled up on top of each other, and then flattened with heavy objects until the temperature, humidity and indoor temperature and humidity of the corrugated cardboard are basically the same. This can effectively avoid the warping phenomenon of corrugated board.

Although the cause of the warpage of corrugated board is complex and there are many factors that affect it, as long as it is taken seriously and carefully handled, the warpage of corrugated board can still be controlled.

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