Trouble printing and elimination of thin paper (2)

Explained in a simple sentence, the occurrence of warpage is caused by the difference in tension (stress) between the front and the back of the tissue, and the vast majority is due to the temperature change on the surface of the paper. The thin paper is thin and has a certain thickness. If the single side (or both sides) of the paper is wet (humidified) and the tension between the front and back sides is inconsistent, and the stiffness of the paper along the fiber direction is generally better than that of the transverse direction, the joint action of the two is common. Next, the paper will inevitably warp. Thick paper is less prone to warpage because of its overall stiffness.
For the warpage of paper and products, we can analyze it in four stages and gradually find a solution.
1. Observe the situation of the paper in the unwinding part, that is, without printing, just pull the paper out of the paper roll for a period of time and compare it. The general single-copper paper is slightly warped at this time because the moisture of the base paper roll is generally only about 5%, and the balance in the workshop with a relative humidity of about 50% can increase the moisture to about 7%. In the process of moisture absorption, due to how much paper is applied on the front side of the paper, it will prevent some moisture from entering the paper from the front side, and the back side is relatively more damp, causing the paper to warp slightly. Of course, the frontal coating will also have certain restrictions on the changes of the paper fibers. .
2, start printing water-based ink and water-based gloss, but not on the UV oil. The boot speed is appropriately higher, and the printed semi-finished products are cut into small leaflets and placed on the table to observe the results. There are two cases here. If a large area is printed on the ground and the boot speed is slow, warpage will occur. On the contrary, warpage is less likely to occur if the printing area is smaller and the starting speed is faster. This is due to the fact that the surface of the paper is printed with water-based ink or water-based gloss, and the surface of the paper has been initially deformed by dampness and has a short "arching" phenomenon, contrary to warpage. However, after drying, especially when the amount of heating is large, the moisture-saturated part of the surface of the paper after water loss will occur a phenomenon of “shrinkage”, that is, there is tension on the surface of the paper. Causes warping.
3, a separate layer of UV Varnish, observed after the UV Varnish after the paper is warped. If the formula of the varnish is not suitable, it will tend to generate additional shrinkage stress during the curing process, causing warpage. At this time, please contact the supplier of UV varnish as soon as possible to improve the performance of the UV varnish, or increase the number of anilox roller lines of the UV varnishing unit.
4. The printed product is flat, but after the printed product is placed in the tobacco factory or brewery packaging plant for a period of time, the product warps. The relative humidity of the cigarette factory's coiling and packing workshop is generally about 65%, and the humidity of the labeling workshop of the brewery is not lower than this value. The vast majority of flexographic products placed in this environment have only one option, namely, single ( Back) humidification. Because the front surface has been covered with ink and varnish, it is difficult for moisture to enter the paper from the front, and the back surface is warped due to “wet”.
For the warpage problems occurring in the above four stages, we can only adjust the ink and varnish in the second stage and the third stage, and increase the starting speed as much as possible to reduce the heating amount. How to make a fuss on the back of the paper to make the paper double waterproof? In practice, we came up with a clever way to make a small modification to AQUAFLEX's flexo press, which is to add 2 paper guide rollers above the print head wall plate of the first unit. With a paper path, the flexo unit can be changed to a gravure back coat unit.
The anilox roller is used as a gravure cylinder for printing the full-plate version, and at the same time, a seamless rubber dyeing sleeve or a full-flexible plate printing cylinder is used as a gravure printing cylinder to adjust the plate cylinder. The clutch pressure action and the anilox roller are used for deinking to ensure that the back of the paper is inked when starting the printer. The back of the paper feeding machine leaves the anilox roller at a certain distance to prevent the paper from breaking due to wetness. The liquid in the ink tray is used for back coating, and some users simply use a mixture of a diluting agent and water. Shaanxi New Century Company can provide a good liquid formula, that is, using polyvinyl alcohol and open powder plus water to add a certain proportion of water, the price is very the back of the whole family can change the thickness of the anilox roller according to need Or change the ratio of back coating liquid. The fundamental purpose of the back coating is to increase the “stretching” stress on the back of the paper, offsetting the front-side stress due to ink, varnish, and preventing the back side from humidifying. Practice has proved that this method is not only effective, but also economical and practical. It has been popularized and applied on many AJA flexographic printing machines for printing paper.
Failure of the fourth: "static"
"Electrostatic" refers to the static electricity generated during high-speed start-up production. Once static electricity is generated in thin paper products, the order for the receipt of paper on the delivery table for cutting into single sheets will cause a practical trouble. After the paper is cut or die-cut, the finished product often goes down against the bottom roller, sometimes dropping off, and sometimes just between the bottom roller and the front end of the delivery table. Falling out will only result in a small amount of waste, which will not affect the overall situation. If the paper is stuck, it will need to be shut down and the waste will be greater.
There are two main reasons for the above problems. First, the stiffness of the thin paper itself is poor, and the second is static electricity. There are two ways to solve this problem. One is to install an effective static elimination device in front of the cutting and die-cutting unit, and the other is to install a fixed thin steel sheet on the front surface of the delivery table to ensure a tight seam. . The purpose is to forcibly pull down the products that are going down and put them into the normal delivery stage.
Sometimes, thin paper products will fly upward along with the slicing and die-cutting rollers on the delivery table. The main reason is that the lacquer is not cured thoroughly and is a little sticky. In this case, it is generally necessary to improve the curing properties of the lacquer. When changing the sponge pack on the cutting and die cutting rolls frequently, add a small amount of talcum powder if necessary. The most fundamental thing is to ensure that the varnish is completely cured and not sticky.
Speaking of cutting, die-cutting, and paper-receiving issues, here are two sentence-cutting and die-cutting registration issues. It should be said that the tension control of the united flexographic printing machine when printing thin paper is easier than printing paper jamming. . However, due to the long distance between the sheet cutting station and the printing station of the tangent line, the paper sheet is relatively long, so the registration error of the sheet cutting is relatively larger than that of the printing sheet between the colors. According to our experience, if the front overprint is better and the cut is larger and the die cut is larger, it is mostly because the back tension is not adjusted properly. When the front overprinting error and the back slicing and die-cutting registration errors are both too large, the reason is more complicated. The first possibility is that the tension of the paper is not uniform, followed by the adjustment of the front and rear tension of the machine. The general principle is to ensure that the paper is pulled evenly, and then adjust the printing overprint, and finally the cut and die-cut sets. The vast majority of the current flexo presses are equipped with a computer monitoring system, which is seen on the screen. Zoom in many times over the set of overprint reticle, if long-term careful observation of the Fluor screen, can be read on the top of a lot of valuable information, used to guide the adjustment tension, guarantee overprinting and registration. With regard to this issue, we will discuss with you in the future "Printing Technology."