If you have any needs pls contact me-
Whatsapp number of Ivy: +86 18933516049 (My Wechat +86 18933510459)
Email me: 01@songhongpaper.com
Currently, among the over one hundred unit-level flexographic printing machines imported into the country, only a small fraction are utilized for printing self-adhesive materials. The majority are employed in the production of thin paper-based products. Nearly all users engaged in thin paper printing have reported encountering a series of operational challenges. These issues can be summarized as follows: "wrinkling," "shrinking," "curling," "static electricity," and "paper breakage." This discussion will systematically examine the root causes of these problems and propose corresponding solutions.
Wrinkling
During the operation of flexographic printing machines with thin paper, the most frequent and initial issue encountered is the formation of wrinkles along the direction of web travel. The likelihood of wrinkling increases with the width of the printing machine (i.e., wider web) and decreases in paper thickness-indicating that wider and thinner papers are more susceptible to this defect.
1. Cause of Fault: Excessive Tension
In general, elevated tension in any given section of the machine may lead to localized wrinkling, which subsequently affects downstream processes. For instance, if the unwind tension is set too high, wrinkles may originate at the unwind station and progressively worsen throughout the print path, potentially leading to web breaks. Additionally, some users initially procured flexographic printing machines designed for heavier substrates; when these machines are repurposed for thin paper printing without appropriate adjustments, wrinkling commonly occurs in the middle to final sections of the press.
Solution: This issue can be effectively mitigated by reducing the tension settings across individual machine sections, ensuring optimal web handling and minimizing deformation.
02. Fault cause: The deflection angle of the correction device is too large.
The tension condition of the paper at the correction device is that one side is tight while the other side is loose, which makes it prone to creasing. For the unwinding part, the "tree bark" creasing phenomenon can be observed on the paper feed tension roller, of course, on the side where the paper is looser.
Solution - To correct the deviation "reposition yourself" (press the Servo-Center key). Slowly move the paper for a while, adjust the appropriate horizontal position of the paper and stabilize it. Then stop the machine, adjust the optical sensor of the deviation correction device to align it with the current paper edge position, and then switch the deviation correction system to the "automatic" mode for operation. This way, the deviation angle of the deviation correction device during the "automatic" operation can be minimized, and the possibility of wrinkles can be reduced to the lowest. At this time, a clear mark should be made on the horizontal position of the paper roll. When changing to the next paper roll, the position must be aligned correctly.
03. Fault causes: There are foreign objects (such as large dry ink lumps) or dents on the surface of the imprinting drum-type paper guide roller; the parallelism of some rollers is out of tolerance, etc.
Solution - In addition to adjusting the tension and inspecting the machine, if necessary, two layers of electrical tape can also be wound on the inner side of the paper guide roller surface, about 15 millimeters away from the two paper edges, to prevent the paper from moving inward laterally. Another option is to use a paper guide roller with spiral lines, allowing the thin paper to extend outward. However, when the stiffness of the thin paper is too low and the tension is relatively too high, the spiral lines may cause wrinkling instead. Therefore, specific analysis is required for each situation.
Furthermore, when printing on thin paper, after removing the packaging from the original paper roll, the paper near the outer packaging surface will absorb moisture from the air and develop wrinkles. When the wrinkled outer layer of paper enters the machine, problems are bound to occur.
Of course, this situation is only temporary. As long as this part of the paper is moved at a slower speed or simply cut off and reconnected, normal production can be resumed. As for how to avoid the above problems, we suggest that during transportation and storage, the paper rolls should be kept in perfect packaging until they are about to be put into the printing machine, and the time for the paper rolls to absorb moisture from the air should be minimized as much as possible.
"Shrink"
Cause of failure: To enhance production efficiency, modern flexographic printing machines place great emphasis on improving the drying capacity. Usually, a combination of natural air, hot air (divided into high, medium, and low three types), infrared rays (including adjustable and non-adjustable types), and other drying methods or their combinations are employed. Some machines are even equipped with two-stage ultraviolet curing systems. Thus, during proofing or production, a phenomenon is observed that the lateral dimensions of a set of plates made from the same film can vary, with errors as small as less than 1 millimeter and up to 2-3 millimeters when the error is large. In practice, we have found that if the film or printing plate is in perfect condition, the lateral dimension deviation is caused by the opposite effects of "wetting during printing" and "drying".
As we all know, about 50% of the components in water-based ink are water. For thin paper, if large areas of water-based ink are printed, the surface of the paper will "get wet" and expand, becoming wider. The larger the printing area and the lower the number of wire rollers, the more obvious the widening phenomenon will be. After the printing is completed, when the paper is dried, it will shrink and narrow. The greater the amount of heating and the more water evaporates, the more severely the paper width will narrow.
The heating and drying systems of most flexographic printing machines (including UV curing systems) have a minimum speed threshold for operation, such as 15 meters per minute. For safety reasons, when the speed is lower than this value, neither the infrared nor the ultraviolet drying devices will be activated. They will only be turned on when the speed reaches or exceeds this value. It is easy to imagine that if our startup speed is only slightly higher than this value, the heating of the paper will be significant and the shrinkage will be obvious. The farther apart the units are, the greater the error in crosswise registration.
Solution: During the printing production process, the conveying speed of the machine should be increased as much as possible, and unnecessary hot air or infrared drying should be minimized during production to avoid lateral registration errors. We once printed water-soluble paper on a 20-inch wide flexographic printing machine. Only after increasing the speed to above 80 meters per minute did we ensure that there were no lateral registration errors.
Of course, there was one exception. When we used the fabric-coated vacuum aluminum-coated paper to print beer labels, there was a significant deviation in the horizontal printing. After careful inspection and testing, we ruled out the factor of width change caused by water loss during drying. Eventually, we discovered that even when the machine was running without paper, the width of the paper would still change. The reason might be due to the change in the tension of the paper on the machine.
"Curl" (Curl
Here, "warp" refers to the warping of the paper in the direction perpendicular to the paper feeding direction. When viewed along the paper feeding direction, one can see that the two sides of the paper or the finished product are upwardly warped. Due to the increasingly fast operation speed of modern packaging machinery, the requirements for product quality are also becoming higher and higher. Once the printed products warp, it will cause certain troubles for the packaging production line operation.
Cause of the fault: In a nutshell, the warping occurs due to the difference in tension (stress) between the front and back sides of the thin paper, and the majority of the cases are caused by changes in the humidity of the paper surface. Even the thinnest paper has a certain thickness. If one side (or both sides) of the paper gets wet (humidified) and causes inconsistent tension on the front and back sides, and the stiffness along the fiber direction of the paper is generally better than that in the transverse direction, then under the combined effect of these two factors, the paper will inevitably warp. Thick paper is less likely to warp because its overall stiffness is better.
Solution: For the warping of paper and products, we can analyze it in four stages.
Observe the condition of the paper at the unwinding part, that is, without going through printing, simply pull a section of the paper from the paper roll and place it for a period of time for comparison. Generally, the single copper paper will slightly warp at this time. This is because the moisture content of the original paper roll is usually only about 5%, and in a workshop with a relative humidity of about 50%, reaching equilibrium can increase the moisture content to about 7%. During the process of absorbing moisture, since the coating on the front side will somewhat prevent some moisture from entering the paper from the front, while the back side gets more moisture, causing the paper to slightly warp. Of course, the coating on the front side will also have a certain restriction on the deformation of the paper fibers.
2. Start printing water-based ink and water-based varnish, but do not apply ultraviolet varnish. The starting speed should be appropriately higher. Cut the semi-finished products after printing into small sheets and place them on a table to observe the results. There are two situations here. If large areas are printed with solid ink and water-based varnish, and the starting speed is slow, warping is likely to occur. On the contrary, if the printing area is smaller and the starting speed is faster, warping is less likely to happen. This is because the front side of the paper, after being printed with water-based ink or water-based varnish, is initially deformed due to moisture absorption, showing a brief "arching" phenomenon, which is exactly the opposite of warping. However, after drying, especially when the heating is strong, the dehydrated part will shrink after losing water, causing a "shrinking" phenomenon on the surface of the paper, that is, there is tension on the surface, which leads to warping.
3. Apply a single layer of UV varnish and observe whether the paper warps after the UV varnish is applied. If the varnish formula is inappropriate, it often causes additional tension during the curing process, leading to warping. At this point, contact the UV varnish supplier as soon as possible to improve the performance of the UV varnish or increase the number of lines on the mesh roller of the UV varnishing unit.
4. The printed products are flat, but after being placed in the packaging workshop of a cigarette factory or a beer factory for a period of time, they start to warp. The relative humidity in the cigarette rolling and packaging workshop is generally around 65%, and the humidity in the labeling workshop of the beer factory is no lower than this value. For the majority of flexographic products placed in such environments, there is only one option: single (back) surface humidification. Because the front side is covered with ink and varnish, it is difficult for moisture to enter the paper from the front, while the back side warps due to "moisture absorption".
Regarding the distortions that occurred in the above four stages, we could only adjust the ink reflective oil in the second and third stages and try to increase the startup speed to reduce the heating volume. How to make use of the back side of the paper to achieve double waterproofing function? In practice, we came up with a clever solution. We made a minor modification to the AQUAFLEX flexographic printing machine, adding two paper guiding rollers above the printing head wall plate of the first unit. By simply changing the paper feeding route, the flexographic unit could be transformed into an intaglio back coating unit. The screen roller is regarded as an intaglio cylinder that prints a full plate solid area, while a seamless rubber dyeing sleeve or a flexographic printing plate cylinder filled with a solid area is used as the intaglio impression cylinder. Adjust the clutch pressure action of the printing plate cylinder and the ink supply and removal action of the screen roller to ensure that the paper's back side is inked when starting up and that the back side of the paper is at a certain distance from the screen roller when stopping, to prevent the paper from breaking due to moisture. The liquid in the ink tray is used for back coating. Some users simply use a mixture of diluent and water. The thickness of the back coating can be changed by adjusting the number of lines of the screen roller or the ratio of the back coating liquid. The fundamental purpose of back coating is to increase the "tightening" stress on the back side of the paper, counteract the stress on the front side caused by ink and varnish, and prevent the mixing process on the back side of the paper. Practice has proved that this method not only has good results but is also economical and practical, and has been widely promoted and applied in many AQUAFLEX flexographic printing machines for printing thin paper.
"Static electricity"
"Static electricity" refers to the static electricity generated during the rapid production process. Once static electricity occurs in thin paper products, it can cause certain problems when cutting them into single sheets and requiring the use of a receiving table to collect the paper. After the paper is cut into sheets or die-cut, the finished products often slide down closely against the bottom roller. Sometimes they fall off, and sometimes they get stuck between the bottom roller and the front end of the receiving table. When they fall off, it only results in a small amount of waste, which does not affect the overall situation. However, if the paper gets stuck, the machine needs to be stopped for cleaning, resulting in greater waste.
01. Cause of failure: The thin paper has poor tensile strength.
Solution - Install an effective static electricity dissipation device before the die-cutting and trimming units.
02. Fault cause: Static electricity
Solution - Install a fixed thin steel plate at the front end of the paper receiving table, which should be pressed against the surface of the paper roller and ensure a perfect fit. The purpose is to forcibly "scrape" the downward-moving products up, allowing them to enter the normal paper receiving stage.
03. Fault cause: The paper sheet would fly up along with the folding cutter and the die-cutting roller on the receiving table. The main reason is that the varnish did not cure thoroughly and it was a bit sticky.
Solution - Generally, it is necessary to improve the curing performance of the varnish. Also, the sponge lining on the cutting and die-cutting rollers can be replaced frequently. If necessary, a small amount of talcum powder can be added, but do so sparingly and frequently. The most fundamental solution is to ensure that the varnish is fully cured without becoming sticky.
"Cutting Paper"
Fault cause: When flexographic printing machine prints thin paper, paper breakage sometimes occurs. This problem is more serious than when printing thick paper. The main reasons are as follows: Firstly, the tension setting of the paper is too high. Secondly, there are defects such as tears at the edge of the paper. In most cases, the paper breakage is caused by the combined effect of these two reasons. Once a tear occurs at the edge of the paper, paper breakage is likely to happen at the deviation device. Then, in the middle and rear part of the machine, due to the increasing tension of the paper, paper breakage is prone to occur.
Solution: During the transportation and handling of the paper rolls, efforts should be made to prevent the paper edges from getting damaged. If any damage to the paper edges is detected, it should be dealt with promptly before starting the printing process. For instance, if there are only a few broken edges, they can be taped over with adhesive tape. For a broken edge that is part of multiple layers, a longer transitional arc can be made using a utility knife along the longitudinal direction to avoid cracking due to stress concentration. For those severely damaged outer layers of paper, it is best to remove all of them.
The most ideal pH range for ink is 8.5 to 9.2.
2. The thickness of the steel scraper of the inkjet printing machine is generally set at 0.15mm.
3. The pressure for flexographic printing usually ranges from 10 N/cm² to 50 N/cm.
4. Generally, the number of lines on the screen roller is 4 to 5 times that of the printing screen lines.
5. The laser engraving mesh roller technology enables the engraving line density to reach a high precision of 1000 lines per inch.
6. When the pH value is lower than 8.2, the viscosity of water-based ink will increase and the drying process will accelerate.
7. The pH value range of some special water-based inks is different from that of ordinary water-based inks. The pH value of gold and silver inks is 7.5 to 8.0.
8. The usage of the general pH stabilizer is to add 1-2% of the total ink volume every 30-40 minutes and stir evenly, or to add the stabilizer to the circulating ink pump.
In inkjet printing, the addition amount of defoamer is generally no more than 0.5%.
When adding surfactants to the ink, the maximum addition ratio should not exceed 2%.
