The corrugated cardboard produced by the corrugated cardboard production line is prone to warping. The warped corrugated cardboard brings many difficulties to the production of the next process, such as printing and slotting. This article mainly explains the causes and principles of warping in the production process of corrugated cardboard. Through detailed analysis, personnel related to corrugated line operations can understand the comprehensive solutions for transverse warping, longitudinal warping and S-shaped warping.
Five factors that affect the warping of corrugated cardboard!
1. The correctness and proficiency of personnel operating techniques:
In the process of cardboard production, the most important factor is people first. If people's mental state is not good and their mentality is not good, then the cardboard cannot be adjusted; secondly, the lack of knowledge. Therefore, they do not know how to make adjustments; finally, there is a problem of execution. For example, everyone knows the cause of cardboard warping, but no one comes forward to coordinate and execute it, so problems will also arise. Therefore, only one person cannot adjust the cardboard warpage, and the cardboard warpage requires the coordination of the entire production line.
2. Improve the function of machinery and equipment
(1) Downtime
The longer the stop time of the corrugated production line, the greater the degree of cardboard warpage. If the corrugating line is shut down frequently, the probability of cardboard warping will increase significantly. The reason is that once the equipment stops, the cardboard will stay on the drying tunnel, and the longer the downtime, the more warped the cardboard will be.
When the cardboard stays on the drying tunnel, the bottom cardboard dries quickly, while the upper cardboard dries slowly, and the moisture of the lower cardboard is easily transferred to the uppermost cardboard. Due to the evaporation of water from bottom to top, the moisture of the upper cardboard is getting higher and higher, and the moisture of the bottom cardboard is getting less and less. When the moisture balance is restored, the cardboard naturally bends upwards. Therefore, it is particularly important to control the number of abnormal downtime and abnormal downtime.
(2) Production speed
Production speed will directly affect the evaporation of paperboard moisture. Generally speaking, when the vehicle speed is 150 m/min, the moisture will be reduced by 2%; if the vehicle speed is 45 m/min, the moisture will be reduced by 4%. It can be concluded that the slower the vehicle speed, the more moisture will transfer from bottom to top, and the greater the probability of cardboard warping.
(3) Preheat cylinder
a. Running speed: The speed will affect the amount of heat transferred from the preheating cylinder to the paper.
b. Coated area: the more the coating is, the more heat is transferred and the more water evaporates; the less the coating is, the less heat is transferred and the less water evaporates.
c. Tension of the base paper: The larger the wrap angle of the preheating cylinder, the tighter the wrap and the greater the friction. Conversely, the smaller the wrap angle, the looser the wrap.
d. Base paper contact surface: It has a certain relationship with the contact surface (liner paper or surface paper). As some people say, if the base paper is replaced, the cardboard will be flat, which is caused by the different tension of the inner paper.
Of course, in the process of adjusting the above points, the balance roller also needs to be adjusted synchronously.
2. Lateral downward warping
The principle of lateral downward warping is the opposite of lateral upturning, because the moisture of the face paper is too high, which can increase the moisture of the inner paper, and increase the amount of glue on the single-sided machine, or increase the amount of paper accumulation in the flyover, because the amount of paper accumulation of the flyover increases the moisture is not easy to disperse.
In addition, we can reduce the preheating of the backing paper (inner paper) to make the bottom paper a little more moisture, we can also spray water on the backing paper, we can also reduce the speed of the tile wire, that is, increase the speed of the hot plate, these are the opposite measures to lateral upturning.
3. Longitudinal bend
The longitudinal bending is mainly caused by the pulling force, and we mainly adjust the angle of the preheating cylinder package angle and the brake of the base paper holder. If the brake pulls down means that the pull of the face paper is too large, the pull of the face paper is too large, then the wrapping angle is adjusted a little smaller, the brake is relaxed, if it is bent upward, it means that the pull of the inner paper is large, that is, the angle of the triple preheating cylinder and the angle of the position of the large belt below increase, thereby improving the tension of the liner paper.
4. Equipment control
In terms of equipment, there must be a certain automatic control method, such as the parallelism of corrugated rolls, pressure rollers, paste rolls, and pressure needs to be set, otherwise in the case of difficult loading, your adjustment will be meaningless. The preheating cylinder and temperature must be kept uniform, and only when the temperature is uniform can the increase and decrease of the wrapping angle have a real change.
In this process, there is also a device for spraying water. We know that the position of the hot plate, the position of the face paper, and the amount of paper accumulation on the flyover are all realized by automatic control, so we need to grasp the control value. Of course, it is also necessary to pay attention to the air permeability of the conveyor belt, usually the better the air permeability, the lower the error. All in all, the points mentioned above must have corresponding control values and magnitude.

3. Control of moisture uniformity of base paper
Every 1% change in the moisture content of the base paper will change the size of the corrugated board by 0.06% to 0.10%. Because the moisture content of the cardboard can change by 15% to 20% when it passes through the corrugated cardboard production line, it may cause a relative change in the size of the cardboard by 0.9% to 2%. If the moisture error reaches 3%, it is enough to cause the cardboard to warp.
The moisture content of the same paper roll also needs to be controlled within 3%, otherwise it will cause different degrees of cardboard warping. Whether it is different rolls of paper or the same roll of paper, if the moisture on both sides and the middle of the cardboard changes, it will cause different degrees of warping. Therefore, careful control of the moisture content in the base paper is essential for flat board production.
Moisture content measurements at normal speeds showed that the moisture removal was somewhat lower in the "loose" regions of the web than in the "tight" regions. The fluff of the paper roll is actually a change in moisture, which has two effects: one is that it will affect the tension, that is, the pulling force; the other is that it will affect the moisture of the cardboard, which itself is caused by moisture.
Another very important factor is the flyover. When the overpass is running normally with accumulated paper, the cardboard produced is straight; when the overpass is empty, the track of the cardboard warpage deviates in the reverse direction; These are caused by how much moisture is emitted.
In addition, the single tile section and the double tile section should be carried out at the same speed. The continuous production of the corrugated cardboard production line not only needs to be connected, but also maintains a constant production speed to minimize the problem of cardboard warpage.
4. Stability and reliability of heating and gas supply
The heating and air supply needs to be stable, continuous and uniform. If the air supply is unstable and the vehicle speed is stable, it will cause the heat of the cardboard to be inconsistent and warp.
By the way, let’s mention the amount of paste, we just need to remember that more glue means more water. Because the water-powder ratio is 3:1, the water that goes up will evaporate. If you put more glue on a certain part, it means that there is more moisture in this part. Due to the different shrinkage when the water is balanced, the cardboard is warped.
There are three factors affecting the state of gluing: the gap between the gluing roller and the gluing roller will directly affect the amount of gluing; the gap between the gluing roller and the corrugated roller will directly affect the amount of gluing; amount of paste.
5. The natural climate of the production environment
In the storage of cardboard, there are also factors that affect the unevenness of the cardboard, such as the crookedness of the cardboard stack, and the exposed cardboard is warped due to the difference in distribution area and force. In addition, the outdoor temperature and humidity will also cause the cardboard to warp. If the humidity is high, it will easily warp.

Equipment, paper selection, control during operation and post-processing are all factors that affect the warping of cardboard, and solving the bending of paperboard is controlled for these factors. In terms of base paper, the moisture content is generally (8±1)% best, and the moisture spread of the entire width is uniform (the deviation is controlled within 1%), otherwise it is easy to form "S" shape or diagonally curved cardboard. In terms of paper distribution, the grammage of face paper and lining paper cannot be too different, generally should not differ by more than 50 grams, and raw paper with high moisture content and low moisture content should not be produced together.
1. Lateral upturning
The main reason for the lateral upturn of the cardboard is uneven moisture, and the cardboard is bent towards the side with high moisture, that is, the moisture of the inner paper is high. So how to reduce the moisture of the inner paper?
The first method is to reduce the amount of paste on the single-sided machine, and at the same time increase the amount of paste in the place of the paste machine. Because the two correspond to each other, increase here, decrease there.
The second method is to increase the moisture of the facial paper, spray water on the facial paper to increase its humidity, and also reduce the wrapping angle of the cardboard on the preheating cylinder, which can also improve the moisture of the facial paper.
The third method is to reduce the temperature of the hot plate, so that the moisture of the cardboard below is not easy to transfer upward, and at the same time can increase the speed of the car, and also solve the problem of the lateral upward tilting of the cardboard.
 

Based on the above several situations, the summary is as follows:
Causes and solutions of transverse S-type warping and diagonal warping
a. The moisture of the base paper is uneven, and the paper force is uneven. b. The base paper holder arm bracket is unbalanced, resulting in the inconsistent heating degree of the paper in the preheating cylinder wrap corner c. The installation level of the paper splicer.
Solution: Calibrate base holder arm level or splicer level, and add balance rollers to the most suitable.

Causes and solutions of lateral upturning
The temperature of the face paper is too high, and the temperature difference between the paper and the backing paper is too large, resulting in the shrinkage force of the backing paper after cooling is greater than that of the face paper.
Solution: a. Increase the wrapping angle of the backing paper and shrink the backing paper in advance. b. Increase the amount of facial paper glue. c. Minimize low-speed operation. d. Appropriately reduce the steam pressure of the hot plate. e. Reduce the corners of the facial paper. f. Appropriate paper spray.

Causes and solutions of lateral warpedia
The temperature of the backing paper is too high, and the temperature difference between the paper and the face paper is too large, resulting in the shrinkage force of the face paper after cooling is greater than the shrinkage force of the backing paper. (The principle and solution are the opposite of upturning)
Solution: a. Increase the corner of the tissue wrap and shrink the paper in advance. b. Increase the amount of liner glue. c. Minimize high-speed operation. d. Appropriately increase the steam pressure of the hot plate. e. Reduce the corners of the backing paper. f. Proper bottom spray.

Causes and solutions of longitudinal warpage
The tension of the facial paper and single tile in the paste machine is inconsistent.
Solution: Adjust the brake tension on the face paper or flyover.

Summery:
After the cardboard appears to be bent, how many measures are available?
First, the surface of the warped cardboard is wiped with water and then forward and reverse pressure is performed.
Second, the warped cardboard storage environment is humidified.
The third is counterpressure after steam injection. The newly produced cardboard has a high temperature, high humidity, and moisture is dispersed from the middle to the surroundings, at this time it is necessary to stack the front and back in time, and press it with a wooden board, push it to a ventilated, easy heat dissipation place to store for more than 4-8 hours, so that the cardboard is relatively flat after cooling.

What aspects do I need to pay attention to when stacking goods?
First, the cargo driver unloads the cardboard to be stacked flat on the pallet, and is not allowed to exceed the pallet. In principle, stacking crookedly can easily cause the cardboard to bend, leading to customer complaints.
Second, if the customer's storage conditions are in a ventilated place, the cardboard needs to be wrapped to prevent a large amount of moisture loss and cause bending and warping.