[China Packaging Network News] In the crater processing process of the box, the warpage of the board is often one of the important issues plagued the paper processing industry. In addition to cardboard warpage will cause die-cutting position, reduce efficiency, but also affect the squareness after the box, weakening the carton compressive strength of the whole box. Due to the complexity of the factors that cause warpage and the environmental temperature and humidity, many packaging companies are plagued with difficulties. Here, Xiao Bian offers you some practical methods:
Definition of warp board warpage
Generally speaking, we can call warping as long as the cardboard is away from the straight state. However, as the warpage of the quality category, slight warping is not a product defect, and warping is only possible when the degree of warping has an effect on the production, appearance and performance of the carton. In order to better analyze the warping problem of color box, we can define it by introducing the arch modulus. After measurement, we found that for the coated paperboard, when the arch modulus exceeds 0.05, it will easily cause die-cutting and affect the cutting efficiency. For boxed boxes, when one of them has a buckling coefficient of more than 0.03, the appearance of the box and the pressure resistance of the box are significantly affected. Therefore, we can call it a warpage defect when the coated cardboard and boxed box curvature values exceed 0.05 and 0.03, respectively.
Attachment: Calculation formula of warping factor:
Analysis of the Causes of Board Warping in the Pit
In the box crater process, the cause of cardboard warpage defects is caused by the fact that, in addition to a few cases caused by the warp of the craters and the warpage of the corrugated cardboard itself, the majority of the causes are caused by the water absorbed by the cardboard inside the glue. The change in the size of the color surface and the corrugated board, the inconsistency in the dimensional change has led to the occurrence of warpage after the pit.
In order to understand the change in the size of the cardboard after the pit and the law of warpage, we have found some laws by detailed analysis of the experiments:
1) Colored paper and corrugated cardboard will exhibit elongation after absorbing moisture; and the more water absorbed, the greater the extent of elongation. When the ratio of expansion and contraction of colored paper and corrugated cardboard is inconsistent, warp will occur in the cardboard after the pit. If the ratio of the stretching of the confetti is greater than the expansion and contraction ratio of the corrugated cardboard, the warpage of the cardboard after the crater is represented by the upward bend, and vice versa.
2) When the cardboard absorbs all of the absorbed moisture and returns to the original moisture content, the color paper will be slightly smaller than the original size, but the corrugated cardboard is slightly larger than the original size.
3) If the cardboard is placed in a dry environment after the pit, its size will shrink more than before the pit. The drier the weather, the greater the contraction. Moreover, the shrinkage of colored paper is significantly higher than the shrinkage of corrugated board. This is why, in the dry season, the reason why color boxes tend to bend upwards after being stacked in warehouses for a period of time is obvious.
4) The stretch ratio in the horizontal direction of the cardboard is 10 times or more that of the vertical direction. That is to say, if the length of colored paper or corrugated cardboard is horizontal stripes and the width is vertical stripes, the size of its change will be larger, and vice versa.
Improve methods
After finding out the law of cardboard warping in the pit, we can find some ways to improve it. It has been proved through practice that the warpage of the cardboard can be fundamentally improved as long as the glue with a higher solid content is selected and the correct paper grain direction is maintained, supplemented by the correct stacking method.
1) Increase the solid content of glue
According to the above experimental results, the amplitude of the elongation of colored paper and corrugated cardboard after absorbing moisture increases with the increase of absorbed moisture, so if the cardboard absorbs less water at the time of crater, the cardboard will have less elongation. We can use a higher solid content of stumbling glue to improve the board warpage defects in pits. The higher the solid content, the lower the water content of the glue, and the glue with a higher solid content can reduce the amount of glue per unit area of the board, thus ensuring that the water absorbed by the cardboard per unit area is greatly reduced, thereby improving the warpage. . From the results obtained from multiple experiments, the warpage of the board can be significantly improved when the solid content of the glue is kept above 24%.
Table 1: Effect of Solid Content of Glue on Cardboard Warpage
2) Ensure correct paper grain direction
In order to ensure that the board does not exhibit warping defects during the crater process, the factory must keep the vertical ridges parallel to the long sides when ordering the flat coated white hoe or flattening the roll (Figure 2). Since the stretch ratio of the horizontal stripes in the horizontal direction is more than 10 times that of the vertical stripes, the size change will be smaller. Moreover, because the short side is in the horizontal direction and the long side is in the vertical direction, the absolute value of the web shrinkage is much smaller than that of the horizontal print, which can reduce the warpage.
Experiments show that a 270g/m2 3S coated paperboard, the long side direction of the paperboard is parallel to the vertical direction, and when the water content of the paperboard is increased from 11.6% to 16.6%, the horizontal stripe elongation of the paperboard is found. It is 21 times that of the vertical stripes, and the horizontal stripes are stretched by 0.2 mm in the direction of the 2.7 mm vertical stripes. (See Table 2)
Table 2: Cardboard Size Changes in Correct Paper Orientation
If the vertical direction of the cardboard is parallel to the short side, according to the elongation of Table 1, the horizontal direction will be stretched by 4.2mm and the vertical direction will be extended by 0.1mm. The effect of the paper grain direction on the size change of the board is evident. (See Table 3)
Table 3: Changes in board size in the wrong paper grain direction
3) The bubbly paper with low water absorbency and the paperboard with high water absorbability have obvious bonding effect
Practice has shown that if the back of confetti is low in water absorbency, and the water absorption of the linoleum paper with which it is attached is high, most of the water in the glue is absorbed by the wavy paper. The cardboard is clearly flat. However, it is advisable to avoid such operations in wet weather and it is easy for customers to make complaints due to the softness of cardboard.
We have done an extreme experiment and put a hundred sheets of confetti on the back of the lacquer before boring. As a result, the cardboard that was pulled out was extremely straight. Therefore, the packaging plant should grasp the surface water absorbency of the colored paper and the tile before it is used.
4) The correct stacking method
In addition, in the process of squeezing pits, the paperboards that have just completed sag pits are stacked and stacked in a positive and negative manner, which can also improve the warpage of the cardboard to some extent. For paperboards that have been rammed and warped, it is more feasible and effective to apply a certain amount of pressure on them. After a certain period of time, the warped paperboard will be significantly improved.
