This article will discuss the causes and mechanism of corrugated cardboard warpage in detail, which is one of the basic principles of cardboard production. In addition, it is very helpful to arrange some operators to observe the whole process of papermaking process, so that they can better understand the characteristics of paper and solve the wraping problem of cardboard more reasonably.
Paper quality and characteristics
Paper making machines produce much wider paper that are rolled into one or more rolls. A separate slitting device is set to adjust the width of the paper, and then rewind the web along the specified paper core under controlled tension. The inner diameter of the paper core should be suitable for the clamp on the corrugated line.
The paper in the center of a paper machine is usually thicker than the paper on the edge. Similarly, the humidity in different positions of the paper machine is also different, which can be seen from the wet marks on the paper roll. Modern paper machines equipped with controllers can overcome these shortcomings, but these problems may still occur.
If protective measures are not taken, both ends of the paper roll may absorb moisture from the air during shipment. As far as cost reduction is concerned, it is quite attractive to buy several tons of paper at same time, but it may also cause risks such as many storage, so inspection is necessary protective measures to the storage. This is an independent issue outside the cardboard production, but the understanding of this issue can help operators decide whether to use a particular roll, leave it to adjust for a period of time, or dispose of the roll as waste. Corrugator Line Wet End employees should be provided with portable measuring instruments to help them check the roll twice before loading.
Paper is a natural product, so just as there are differences between different rolls of paper, there are also differences in paper in the same roll. The expansion or shrinkage of the original growth fiber kraft paper in the direction perpendicular to the paper texture is twice as large as that along the paper texture. Modern paper machines are specially designed to give random orientation to the fibers inside the paper, rather than straighten out the fibers along the running direction of the equipment.
If the paper expands due to the increase of humidity, move the paper back to the original humidity environment, and the paper will shrink to smaller size in the vertical and horizontal direction. This phenomenon is called "lag effect". If you repeat the same steps for the same paper again, the paper will further shrink as a whole, but the degree of shrinkage will decrease with each repetition. Paper with a high moisture content is more likely to absorb extra moisture than paper that was previously dried. If the paper is heated, the water absorption effect will be enhanced, similar to the expansion and shrinkage effects.
The size of lighter face paper changes more greatly under the effects. Tests conducted by our technician center(WEST RIVER) show that each 40 inch light face can shrink by 3 / 8 inch (9 mm / M). Therefore, if the face paper use heavy kraft paper and the inner paper use light paper, serious warpage may occur, which will bring trouble to the operation and processing of cardboard production. If 2 pounds / 1000 square feet of water is added to 65 pounds of paper (that is, 10 g / m2 of water is added to 300 g of face paper), the moisture content of the paper will increase by about 3%, but if the same weight of water is added to 26 pounds (i.e. 125g), the paper moisture will increase by about 8%, which aggravates the previous imbalance.
When the paper is stretched under the action of tension, the stretching range of light paper is larger. In addition, in recent years, with the increasing use of light-weight paper in the corrugated cardboard production process by carton enterprises, it is estimated that 40% of corrugated core paper and 10% of face paper in Europe will weigh less than 22 pounds (i.e. 100g / m2) in a few years. No matter how the equipment itself, its control system and precision should be improved, we should be aware that the operators must know what they are dealing with. Without training and experience in some areas, their chances of success are slim.
Note: when the paper is heated to a temperature below the equilibrium humidity, then in the subsequent rewetting process, the moisture content of the paper decreases and the size decreases. The test results show that all sheets of paper shrink uniformly. In the process of Corrugated processing, the paper can occur twice such a process.
Note: study the shrinkage ratio of different paper. According to the different weight of paper, the shrinkage can reach 9mm / m, and the forever shrinkage is 3mm / M ~ 4mm / m. The shrinkage of light kraft paper or recycled paper is higher than that of heavy-duty paper. This explains the problem in combining light and heavy tissue.
Warpage type
Compared with the causes of warpage, various types of warpage are more well known. It is necessary to define the type of warpage before a more extensive analysis of warpage problems and causes, because identifying warpage types is the first step in identifying the root cause of the problem.
All warping is caused by stress conflicts, mainly on the inner and outer layers of paper with different tile sizes (sometimes on corrugated core paper). Because the flat paperboard material has different tension or moisture content when it enters into the double-sided machine to bond together, resulting in the difference between the inner and outer face paper. With the effect of tension, the moisture inside the paperboard will eventually be distributed evenly, which will also cause warpage stress.
Forward warping (upward warping)
As the name suggests, forward warping is one of the most common types of warping. According to all the above paper characteristics, if the single side paperboard is wetter than the bottom paper when entering the duplexer, then when the excess moisture is transferred from the upper layer to the lower layer, the paperboard will bend upward as shown in the figure. This is because the upper paper shrinks and the lower paper expands.
