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The cardboard bulges into depressions in a larger area, which is called warping.

The formation of cardboard warpage is more:
There is “positive” warpage, also known as “upward warpage”, which means that the cardboard bulges toward the side of the tissue paper.
The opposite is “reverse” warpage.
One side is convex and the other side is concave, which is “S-shaped warping”. The warping is developed by taking the diagonal of the cardboard as the axis to be “twisted warping”, also known as “corrugated warping”.
The warping axis is parallel to the corrugated direction, which is called “length direction” warping. Except for forward warpage, other types of warpage are rare.

Use warped cardboard to make cartons, the surface of the box is not good, and the shape cannot be square, which affects the appearance. Due to the uneven surface of the box, it is easy to lose stability when subjected to force, which reduces the compressive resistance of the carton. The warped cardboard is not easy to accumulate, and the workmanship is not good, and it cannot enter the printing slotting machine smoothly, which seriously affects the printing effect and the accuracy of slotting.

The fundamental reason for the warpage of the cardboard is the imbalance of the paper quality: the different degrees of expansion and contraction of the various components of the cardboard are caused.

The water content of the paper is different, and the grain is different in the vertical and horizontal directions, and the degree of shrinkage is also different.

When the backing paper (face and inner) on both sides of the cardboard is different in expansion and contraction, it is easy to cause the cardboard to warp. Compared with thin and small paper, the use of thick and heavy paper has a relatively stable degree of expansion and less deformation. Therefore, when the thickness or weight of the face paper and the inner paper are different, the cardboard is likely to warp. Therefore, when matching paper in carton design or production, the grammage and quality of the tissue should be equal or close.

Due to the different fiber directions of paper, when heated, its transverse shrinkage is twice as large as its longitudinal shrinkage. Therefore, when mixing paper in production, the fiber direction of the surface and the inner paper should generally be the same to reduce the warpage of the cardboard.

On the corrugated board machine, the paper cannot shrink due to traction along the length of the cardboard, but the lateral shrinkage of the paper cannot be controlled. This is another major cause of “positive warpage”.

In production, the shrinkage and deformation of the paper can be controlled by adjusting the heat to reduce the warpage of the cardboard.

The cause of “frontal warpage” is generally that the lining paper and the single corrugation are too moist, and the surface paper is too dry. Therefore, the drying degree of the lining paper and single corrugation should be increased to reduce the preheating of the opposite surface paper. For example, increase the preheating area of ​​the corrugated paper and lining paper on the single-facer, lower the temperature of the hot plate on the cladding machine, reduce the number of gravity rollers, and appropriately increase the vehicle speed to reduce heat transfer. When parking, you should lift the cardboard away from the hot plate, or spray the paper on the face, switch to a high-concentration and high-viscosity adhesive, and reduce the amount of glue.

The reason for “reverse side warping” is just the opposite of the above, because the single-sided corrugation is too dry and the tissue paper is too wet. Therefore, the opposite method should be adopted to control.

S-type warpage usually means that the edges of the paper are too wet and the base paper shrinks severely. Can increase the preheating time of the paper. It may also be the reason why the material of the upper and lower cardboard is too different.

The causes of distortion and warping are: the tension of the single-sided corrugated through the bridge is too large; the quality of the base paper is not good; the temperature distribution of the hot plate is uneven, and the moisture of the base paper is uneven.