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 paperboards. On the one hand, the labor intensity of workers is very high, and on the other hand the speed of the production line 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 rejection rate, the inability to use automated 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 DEVELOPMENT) put forward the number of ketones expressed warpage (WARP), and to illustrate the warping of corrugated cardboard in the actual use of Permissible degree. If 1/4 in (about 6 mm) of warp is present at a width of 24 in (approximately 600 mm as a unit), the degree of warpage is defined as 1/4, and this number is taken 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 warping factor of the 1/4 warpage of the 24 in width board is also 1/4. Then, with respect to wide paper sheets, the warpage of a unit of 24 inches wide is expressed as a formula that is inversely proportional to the unit square, that is:
Warp degree = warping factor * (unit squared) 2
(WARP)=WARP FACTOR*(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 Tensile warpage such as warping of warp in the longitudinal and transverse directions. Lateral warping is divided into upward warping and downward S warping.
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, moisture, heat, and nature and type of tension paper.
1, the nature and type of paper:
In terms of the nature of the paper, the paper absorbs water and swells, drying shrinks. The wet paper has the property of absorbing large water. The size of the change in size is more than twice as large in the horizontal direction than 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 kind of paper may be of different grades, and warpage tends to occur when the two grades of corrugated paperboards with great differences are registered in the A-level and the C-level.
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 to dry, this situation the impact of warpage Great. It also causes uneven gluing on machinery and operation, resulting in inconsistent moisture in the cardboard. Japan’s survey of 16 plants showed that the amount of single-sided corrugators is at least 2.4 g/m2, a maximum of 8.79 g/m2, and the amount of paper machine attachment is at least 2.8 g/m2. 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.
With respect to the heating problem, when a double-sided machine is used, single-faced corrugations absorb moisture and heat at the single-face corrugating machine, and also absorb water from the glue on the paste-gluer machine. Therefore, the single-faced corrugated cardboard tends to be horizontally stretched. On the other hand, paperboards that are in direct contact with the flatbed dryer on a double sided machine tend to shrink when dried.
In the process of production of corrugated cardboard, the tension of the paper from the paper-feeding frame to the double-machine is inconsistent. This inconsistency contributes to the inconsistency of lateral shrinkage of the paperboard.
Fifth, the main causes of various warpage and countermeasures 1, upward warpage, downward warpage As mentioned earlier, single-sided corrugated cardboard in a single-faced corrugating machine and paste and paste at the office, horizontal elongation, and Noodle paper (face paper) is in an over-dried state due to direct contact with the preheater and the double-sided plate dryer, and is fed into the corrugated cardboard after entering the cooling section of the double-sided machine. The corrugated cardboard is exposed to the air. In order to achieve moisture balance, moisture is released from the side of the single-faced corrugated sheet with a large moisture content, and at the same time shrinkage occurs; while the dried noodle sheet absorbs moisture and elongates, a warp 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. The warpage is caused by the unbalanced water content of the two-sided liner paper of the corrugated cardboard. Regarding the influence of the raw paper moisture used, 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 moisture and warpage of raw paper.
When the liner paper using a single-faced corrugated cardboard has 3% more paper than the face paper of a corrugated cardboard, 92% of the roll paper is warped. When the surface paper moisture of the corrugated cardboard is 3% larger than that of the single corrugated cardboard, 75% becomes a warp.
Therefore, when selecting the base paper, the moisture fluctuation of the noodle paper (noodle paper of single-sided 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 the single-faced corrugator and the double-sided machine as consistent as possible.
(4) Minimize the amount of glue on the corrugated board and then at the paste machine.
The above four items are all designed to solve the problem of cardboard upturning, so as to reduce the excessive water setting when the single-faced 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) Reducing the face paper and preheating the wrap angle on the machine.
(2) Properly stretch the base paper.
(3) Reduce the heating temperature of the dual engine or increase the speed.
In addition to this, we must also pay attention to:
(1) Single-faced corrugated cardboard with uneven corrugations.
(2) The viscosity and concentration of the gum are appropriate.
(3) The single-sided corrugated board entering the double-sided machine cannot be fanned.
(4) The pressure of the press 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 paperboard and the solution to upward warpage have been introduced, and the solution to the warpage of the lower warp has not been difficult to find from the above description.
2. S-warped warpage and warpage are simply warped, S- warpage and diagonal warpage described later, is a trace of trouble. When this kind of warpage occurs, it can be piled up by flipping 180 degrees in groups of 10-20 sheets 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) Large amount of water is left on the roll of the liner.
(3) The heating temperature of the double-sided machine is not uniform.
The upper spot on the ordinate indicates the percentage of moisture in the paper, and the lower half indicates 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 driving side changes from 0.2 to 1.0, which confirms the wet spot on the web banner to the web. The warpage of the cardboard part 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 show the relationship between them. That is, the relationship between the width of the wet spot and the average moisture content of the base paper and the percentage of the excess moisture when the product of the percentage of excess moisture reaches 40 or more can reach 1 or more, and unacceptable warping occurs (for example, the width of the wet spot is 10 in When this part 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. Warpage in the length direction The warpage of adult corrugated cardboard has no relation with the base paper. This is due to the tension adjustment in the production process of corrugated cardboard. 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 produced by the combination of longitudinal and lateral warping. The reason for this is that the single-sided corrugated paperboard on the bridge is overloaded, the tension on one-sided corrugated board is too large (tension is the cause of longitudinal warpage), and the other is upturned, downturned, or warped (as described above. It is due to the fact that the difference in the moisture content of the corrugated board has destroyed the balance of telescopicity). 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 the single-faced corrugated cardboard or tissue paper; the tension of the paper can be adjusted by adjusting the brake pad tension, the angle of the preheating machine can be adjusted, and the angle of the pretreatment machine can be adjusted; Means control the tension of the board; 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 through the analysis of the verification results of cardboard warpage by foreign institutions, we have a comprehensive understanding of warping, 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.)