Corrugated board warping factors and solutions

1. Overview Warpage of corrugated board is a very common and troublesome phenomenon in the production process of corrugated board.
For example, we can see that at some paperboard mills' cardboard output lines, it is necessary to manually reverse the stacked cardboard. On the one hand, the labor intensity of workers is very high. On the other hand, the speed of production lines is affected. In the latter equipment, for example, in the paper feeding operation of the fully automatic printing slot machine, we often see that in order to make the paper feed smoothly, the worker presses the paper board down firmly before the paper feeding table. In fact, the warpage of cardboard will bring many unfavorable factors to the enterprise, such as the increased labor intensity of workers, the increase of reject rate, the inability to use automatic machines normally, and the impact on productivity. In order to make everybody attach importance to this issue as much as possible and draw lessons from the experience of foreign companies in this area, it is very necessary to draw a solution.
Second, the definition of warpage At present, there is no clear definition of cardboard warping in the United States, in the United States CID (CORRUGATED INDUSTRY DEVELPMENT) proposed numerical warpage (WARP), and to illustrate the warping of corrugated cardboard in the actual use of Permissible degree. If a warp of 1/4 inch (approximately 6 mm) in width 24 in (approximately 600 mm as a unit) is defined as warpage equal to 1/4 (FIG. 1), this number is used as the allowable limit of warpage. At the same time, the formula (1) is defined. From the formula, we have obtained that the paper board warpage coefficient of the paper board with a warpage of 24 in width is also 1/4. Then, with regard to wide paper sheets, the warpage of 24 in is used as a unit, and a formula inversely proportional to the square of this unit is published, that is: Warpage = Warp coefficient * (unit squared) 2 (1) (WARP ) = WARP FACTOR î–‚ (UNIT) 2
For example, the permissible value of the warp degree of a paper sheet having a 1/4 WF in 48 inches wide in the United States is 1 mm in the above formula.
The allowable value of warpage in Japan is the same as that in the United States. For example, the allowable warpage limit is 9/6in (14.1mm) for a width of 36in, and the allowable warpage limit is 1in (25.4mm) for 48in (1200mm).
Third, warping types Corrugated board warping types can be cited as follows:
(1) Longitudinal warpage (2) Lateral warpage (3) Longitudinal and transverse warp warp twisted warp and the like. Lateral warping is divided into upward warping and downward S warping. as shown in picture 2:
Fourth, the main cause of warping The cause of warping is very complex, summed up is due to the fact that noodle paper (ie, corrugated paper, paper and paper) in the production due to a variety of reasons caused by their expansion and contraction differences.
The causes of warpage are: 1. The nature and type of paper 2. The nature and type of moisture, heat, and tension paper.
(1) The nature and type of paper:
In terms of the nature of the paper, the paper absorbs moisture and swells and shrinks when dried. The wet paper has the property of absorbing large water. The size change is more than twice as large in the horizontal direction as in the vertical direction.
The type of paper, such as the degree of stretching of kraft paper and tea board paper, varies depending on the composition of the fibers and the manufacturing method. The same type of paper may be of different grades. If corrugated paperboards with very different grades, such as A grades and C grades, are combined, the warp tends to occur.
(2) moisture, heat, tension in the corrugated board processing process, single-sided corrugating machine and the paste machine at the glue, the glue contains a lot of water, and then heated for drying, this situation has a great impact on warpage Big. In the machinery and operation will also cause uneven glue, resulting in inconsistent moisture content of the cardboard. Japan's survey of 16 plants showed that the amount of single-sided corrugator attachment was at least 2.4 g/m2, a maximum of 8.79 g/m2, and the amount of paper machine attachment was at least 2.8 g/m2, with a maximum of 18.6. g/m2. In addition, with respect to the horizontal attachment fluctuations of corrugated cardboard, the survey results also confirmed that the single-faced corrugating machine is 0.5-3.5 g/m2, and the paper-bonding machine is 0.5-14.7 g/m2. Explain the uneven moisture content.
Regarding the heating problem, the single-faced corrugated board has absorbed moisture and heat at the single-faced corrugating machine when passing the double-sided machine, and also absorbs moisture from the glue at the pasteurizing machine. Therefore, the single-faced corrugated cardboard has the tendency of horizontal elongation. On the one hand, paperboard that is in direct contact with the flatbed dryer on a double-sided machine tends to shrink when dried.
In the process of producing corrugated cardboard, the tension of the paper from the paper-feeding frame to the double-sided machine is inconsistent. This inconsistency contributes to the inconsistency of lateral shrinkage of the paperboard. V. Main Causes and Countermeasures for Various Warpages Upward warpage, downward warpage As mentioned earlier, single-sided corrugated cardboard is glued on single-faced corrugating machines and gluing machines, with large lateral elongation, while the liner (paper) is directly connected to the preheater and The flat-bed dryer of the double-sided machine is brought into an over-dried state and enters the cooling section of the double-sided machine and is then sent out as a corrugated cardboard. The corrugated cardboard is exposed to the air. In order to achieve a moisture balance, moisture is discharged from the single-faced corrugated side of the large moisture, and at the same time, the shrinkage occurs; while the dried noodle paper absorbs moisture and elongates, so that a warping phenomenon occurs. If the above situation is reversed (actually, the bonding speed is too high as compared with the drying capability of the flat-plate dryer), the warping occurs.
Here again, the paper moisture problem is repeated, and the warpage is caused by the moisture imbalance of the two-sided liner paper of the corrugated cardboard. Regarding the influence of the used raw paper moisture, CID conducted a large-scale experiment, ie, the raw paper of different moisture was used as the paper of single-sided corrugated cardboard and the paper of corrugated cardboard to investigate the relationship between the moisture and warpage of the raw paper.
When the liner paper using a single-faced corrugated cardboard had 3% more paper than the surface paper of the corrugated cardboard, 92% of the paper was warped.
When paper with corrugated cardboard has 3% more moisture than a single corrugated cardboard, 75% becomes warped.
Therefore, when selecting the base paper, the moisture fluctuation of the noodle paper (noodle paper of one-side machine and noodle paper of corrugated cardboard) must be kept within 3%. As a solution to warpage, besides the moisture of the base paper, note also on the corrugated board machine:
(1) Increase the wrap angle between the preheater and the preprocessor at the single-faced corrugator.
(2) Minimize the amount of glue on a single-faced corrugator.
(3) Reduce the amount of single-sided corrugated cardboard stacked on the bridge and keep the speed of single-faced corrugator and double-sided machine as consistent as possible.
(4) Minimize the amount of glue on corrugated cardboard at the paper machine. The above four items are all designed to solve the problem of cardboard upturning, so as to reduce as much as possible when the single-sided corrugated cardboard enters the double-sided machine. At the same time, in order to increase the water content of corrugated cardboard as much as possible, attention should be paid to:
(1) Reduce the wrap angle on the warming machine.
(2) Properly stretch the base paper.
(3) Reduce the heating temperature of the double-sided machine, or increase the speed.
In addition to this, we must also pay attention to:
(1) Single-faced corrugated cardboard without uneven corrugations.
(2) The viscosity and concentration of the gum are appropriate.
(3) The single-sided corrugated cardboard entering the double-sided machine cannot be fanned.
(4) The pressure of the pressure roller on the double-sided machine is moderate, and the lateral pressure is even.
In the above, the cause of upward warping and downward warpage of the board and the solution to upward warpage have been introduced. The solution to the warping of the lower warp is not difficult to find from the above description, and it is not listed here.
2. S-warped warpage and warpage are simple warping, S warping and diagonal warping described later, is a very troublesome warping. When this kind of warpage occurs, it can be piled up by flipping 180 degrees in a group of 10-20 pieces of cardboard at the production line. At the same time, when such warpage is serious, in addition to the above-mentioned method of accumulation, a heavy object must be used to flatten the warpage. Otherwise, it will cause obstacles in the subsequent box making process.
The cause of S-warping is:
(1) The horizontal wet stripe on the hang-up roll.
(2) The moisture on the edge of the liner roll is large.
(3) The heating temperature of the double-sided machine is not uniform.
According to the United States CID experiment, the above (1), (2) is the main cause of S-warping.
The upper half of the ordinate represents the percentage of paper moisture, and the lower half represents the warpage of the paperboard produced. The abscissa indicates the web width width (inches). If the wet spot is on the operation side of the corrugating machine, a warp of warpage of 0.8 occurs on the operation side. If the web is used in reverse, the operating side decreases by 0.3 from the warpage before flipping to 0.8; but the drive side changes from 0.2 to 1.0, which confirms the wet spot on the web banner to the corrugated cardboard. Partial warping is influential.
The relationship between the width of the wet spot in the direction of the noodle base paper banner and the degree of excess moisture is related to the warpage. CID uses the following figure to express the relationship between them. That is, the relationship between the width of the wet spot and the average moisture of the base paper is such that when the product of the percentage of excess moisture reaches 40 or more, the warp degree can reach 1 or more, and unacceptable warping occurs (for example, the width of the wet spot is 10 in. When a part of water contains 4% of the average moisture content, uncontrollable warpage can occur.
The main key to overcome the S-type warping from the above three items is that the moisture content of the base paper should be even, and then the control temperature of the double-sided machine should be even.
3. Longitudinal warpage of the warped corrugated board is irrelevant to the original paper, which is caused by the tension adjustment in the corrugated board production process. That is, the tension of the corrugated cardboard facing paper sheet and the three-layer preheating machine is too tight and the tension difference between the corrugated cardboard and the single-faced corrugated cardboard is too large. At the same time, the tension on the corrugated cardboard is too tight, which is one of the reasons for the length error in the paper cutter. The solution is to reduce the difference in tension between single-sided corrugated cardboard and corrugated cardboard on paper.
4. Diagonal Warpage Diagonal Warpage is the warp created by the combination of longitudinal and lateral warpage. The reason for this is that the single-sided corrugated board on the bridge is overloaded, the tension on the single-faced corrugated board is too large (tension is the cause of the longitudinal warpage), and the upturned, downward warped, or S-shaped warp is observed. It is due to the fact that the difference in the moisture in the corrugated cardboard breaks the balance of the telescoping. Therefore, in order to solve the diagonal warpage, it is necessary to comprehensively consider the above-mentioned various situations, and eliminate the above-mentioned unfavorable conditions one by one during operation, and observe while operating. For example, the water content of the cardboard can be balanced by spraying water on a single-faced corrugated cardboard or facial paper; the tension of the paper can be adjusted by adjusting the brake pad tension, the wrap angle of the preheater can be adjusted, and the angle of the pretreatment machine can be adjusted. The tension of the board is controlled; the speed of the corrugated board production line can be controlled to operate under stable conditions, and the amount of accumulation on the bridge frame can be reasonably reduced without overloading.
After analyzing the warpage factors of corrugated cardboard, especially after analyzing the verification results of cardboard warpage by foreign institutions, we have a comprehensive understanding of warp and warping types and solutions of paperboard. It is believed that it will play an active guiding role in improving the quality of production board. (Zheng Zhengpei, Carton Machinery Branch of Shanghai Electric Group Printing & Packaging Co., Ltd.)