Thin paper printing 5 major printing failures
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Almost all users who have printed thin papers have encountered a series of problems that can be summarized in the following words: “wrinkle”, “shrink”, “warp”, “static” and “breaking”. Below, we will discuss in detail the causes and solutions of these faults.
Wrinkle
When the flexo press is "walking" thin paper, the easiest and first problem is that the paper produces "wrinkles" along the paper feed direction on the machine. The wider the machine (ie the wider the paper), the thinner the paper. The easier it is to wrinkle.
Cause of failure: excessive tension
In general, if the local tension of the machine is too large, wrinkles will occur in the area and affect the latter part. For example, sometimes the unwinding tension is set too large, and wrinkles are generated in the unwinding portion, and the wrinkles become more serious and even break the paper. Some users originally ordered the flexo press as a thick paper type. Once used to print thin paper, it is easy to wrinkle, and the wrinkles usually occur in the middle and back of the flexo press.
Solution - This problem can be solved by lowering the tension setting of each part, but the most fundamental solution is to purchase a set of rollers suitable for printing thin paper, including impression cylinders and paper feed and paper output rolls.
Cause of failure: The deflection angle of the correcting device is too large
The tension condition of the paper at the correcting device is that one side is tight and the other side is loose, so wrinkles are easily generated. If the unwinding part is found, a "bark" wrinkle is found on the paper-feeding roller, of course, on the loose side of the paper.
Solution - Let the correction be "swing yourself" (press the Servo-Center button), take a piece of paper and adjust the horizontal position of the appropriate paper and stabilize it, then stop, then adjust the electric eye of the correcting device to make it meet the current needs. The position of the monitored paper edge is aligned, and the correction system is switched to the "automatic" state. By doing so, the angle of deflection of the correcting device during "automatic" operation is minimized, minimizing the possibility of wrinkling. At this time, the horizontal position of the paper roll should be made an obvious mark. When changing the next roll of paper, the position must be aligned.
Cause of failure: The surface of the impression cylinder type paper guide roller has foreign matter (such as a large dry ink block) or dents; the parallelism of individual rollers is excessively poor.
Solution - In addition to adjusting the tension and checking the machine, if necessary, two layers of electrical tape can be wrapped around the inside of the paper guide roller at a distance of about 15 mm from the sides of the paper to prevent lateral movement of the paper. It is also possible to use a spiral paper guide roller to extend the tissue outward. However, when the stiffness of the tissue is too small and the tension is relatively large, it will wrinkle due to the spiral, so the specific problem is analyzed.
In addition, when printing on tissue paper, after the original paper roll is removed from the package, the paper near the surface of the outer package absorbs the moisture in the air and wrinkles, and when the outer wrinkled paper enters the machine, it is sure to cause problems.
Of course, this situation is only temporary. If you take this part of the paper at a lower speed or simply cut it off and reconnect it, you can start production normally. As for how to avoid the above problems, we suggest that during the transportation and storage process, it is necessary to strictly ensure that the paper roll is packaged and ready to be printed on the machine before removing the package, so as to minimize the time for the paper roll to absorb moisture in the air.
Shrink
The reason for the failure: In order to improve production efficiency, the current flexographic printing machine pays great attention to improving the drying ability. Generally, natural wind, hot air (high, medium and low) and infrared (infinitely adjustable and non-adjustable) are used. ) A variety of drying methods or combinations of them, some machines are also equipped with a two-stage UV curing system. In this way, in the proofing or production, people will find a phenomenon that the horizontal dimension of the set of the same set of film will be biased, the error is less than 1 mm, and the error is 2-3 mm. In practice, we have found that if there is no problem with the film or the printing plate, the deviation of the lateral dimension is caused by the two opposite processes of "printing wet" and drying.
As we all know, about 50% of the water-based ink is water. For thin paper, if the water-based ink is printed on a large area, the surface of the paper will be expanded and widened. The larger the printing area, the anilox roller line. The lower the number, the more obvious the widening phenomenon. When the paper is dried after printing, the paper is narrowed by "shrinkage". The larger the amount of heating and the more the water volatilizes, the more narrow the paper width becomes.
Most flexo press heating and drying systems (including UV curing systems) have a minimum speed starting point, such as 15 m/min. For safety reasons, below this speed, the IR and UV drying units are not turned on and will be turned on at or above this value. As you can imagine, if our boot speed is only slightly higher than this speed, the paper will be heated a lot and the shrinkage will be obvious. The farther apart the unit, the greater the lateral overprint error.
Solution: In the printing production process, the speed of the lift should be increased as much as possible, and unnecessary hot air or infrared drying should be minimized in the production to avoid lateral overprinting errors. We used to print tipping paper on a 20-inch wide flexo press to increase the speed to 80 m/min or more before ensuring no lateral overprinting errors.
Of course, there is an exception. When we used the printed vacuum aluminized paper to print the beer label, the horizontal printing showed a large deviation. After careful inspection and testing, the factors that caused the width change due to the loss of moisture due to drying were eliminated, and it was finally found that even if the machine was empty, the paper would occur. The paper width varies, possibly due to changes in the paper pull on the machine.
"Curl"
The "warping" mentioned here refers to the warpage of the paper in the direction perpendicular to the paper feeding direction. When you look at the paper feeding direction, you will see the paper or the finished product curl up. As the speed of modern packaging machinery is getting faster and faster, the requirements for product quality are bound to become higher and higher. Once the printing products are warped, it will bring certain troubles to the packaging line operation.
The cause of the malfunction: explained in a simple sentence, the occurrence of warpage is caused by the difference in tensile force (stress) between the front and back of the thin paper, and most of it is caused by the humidity change of the paper surface. The thin paper is also thin and has a certain thickness. If the one side (or both sides) of the paper is damp (wet), the tension between the front and the back is inconsistent, and the stiffness of the paper in the fiber direction is generally better than the horizontal direction. Under the action, the paper will inevitably warp. Thick paper is less prone to warping because its overall stiffness is better.
Solution: For the warpage of paper and products, we can analyze in 4 stages.
1. Observe the situation of the paper in the unwinding section, that is, without printing, just pull the paper out of the paper roll and place it for a while to compare. The general single copper paper is slightly warped at this time, because the moisture of the original paper roll is generally only about 5%, and the balance is increased to about 7% in a workshop with a relative humidity of about 50%. In the process of moisture absorption, because the front side of the paper is coated, it will prevent some water from entering the paper from the front side, while the back side is relatively moist, which makes the paper slightly warped. Of course, the front coating will also restrict the deformation of the paper fiber. .
2. Start printing water-based ink and water-based varnish, but do not apply UV varnish. The boot speed is appropriately higher, and the printed semi-finished products are cut into small sheets, which are placed on the table to observe the results. There are two cases here. If the solid version is printed on a large area and the boot is slow, most of the warpage will occur. Conversely, if the printing area is small and the booting speed is faster, warping is less likely to occur. This is because the front side of the paper is printed with water-based ink or water-based varnish. At the beginning, the surface of the paper is deformed by moisture, and there is a short "arching" phenomenon, which is just the opposite of warpage. However, after drying, especially when the amount of heating is large, the surface of the damp will be "shrinked" after the water is lost, that is, the surface of the paper has a tension, causing warpage.
3. Apply a layer of UV varnish alone to observe whether the paper is warped after the UV varnish is finished. If the formulation of the varnish is not suitable, it will often generate additional shrinkage stress during the curing process, causing warpage. At this time, contact the UV varnish supplier as soon as possible to improve the performance of the UV varnish or increase the number of anilox rolls in the UV glazing unit.
4. The printed product is flat, but the product is warped after being placed in the tobacco factory or brewery packaging workshop for a period of time. The relative humidity of the tobacco factory's coiling workshop is generally around 65%, and the humidity of the brewery labeling workshop is not lower than this value. The vast majority of flexo products are placed in this environment with only one option, namely single (back) surface humidification. Since the front side has been covered with ink, varnish, etc., it is difficult for moisture to enter the paper from the front side, and the back side is warped due to "wet".
For the warpage problem occurred in the above four stages, we can only adjust the ink reflective oil in the second and third stages, 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 the flexo press, ie above the print head wall of the first unit By adding two paper guide rollers, you can change the flexo unit to a gravure back coating unit by simply changing the paper path. Use the anilox roller as a gravure cylinder that prints the full version of the field, and at the same time find a seamless rubber dyeing sleeve or a flexographic plate cylinder printed on the ground as a gravure cylinder to adjust the plate cylinder. The clutch press action and the anilox roller supply and deinking action ensure that the back side of the paper is inked when the machine is turned on. When the machine is stopped, the back side of the paper leaves a certain distance from the anilox roller to prevent the paper from being broken due to wetting. The liquid in the ink tray is used for back coating, and some users simply use a mixture of a thinner and water. The thickness of the back coating can be changed as needed to change the number of lines of the anilox roll or to change the ratio of the back coating liquid. The fundamental purpose of the back coating is to increase the "shrinkage" stress on the back side of the paper, offset the stress on the front side due to ink, varnish, and prevent the mixing process on the back side of the paper. Practice has proved that this method is not only effective, but also economical and practical. It has been promoted and applied on many Ajapa flexo printing machines.
"electrostatic"
"Static" refers to the static electricity generated during high-speed start-up production. Once the thin paper products generate static electricity, it will cause some trouble for the order that is cut into a single sheet and needs to be received on the delivery table. After the paper has been cut or die-cut, the finished product tends to go down against the bottom roller, sometimes falling down, and sometimes just between the bottom roller and the front end of the delivery tray. When it falls, it only causes a small amount of waste, which does not hinder the overall situation. If the paper is stuck, it needs to be shut down for cleaning and waste.
The cause of the malfunction: the stiffness of the tissue itself is poor
Solution - install an effective static elimination device in front of the cutting and die cutting unit.
Cause of failure: static electricity
Solution - Install a fixed thin steel sheet on the front end of the delivery table that is placed against the surface of the paper roll to ensure a tight seam. The purpose is to forcibly "shovel" the downwardly moving product into a normal delivery. stage.
The cause of the malfunction: On the delivery table, it will fly up together with the slitting and die-cutting rolls. Mainly the glazing oil is not cured enough, a little sticky.
Solution - generally need to improve the curing performance of varnish, you can also often replace the sponge lining on the slitting and die-cutting rolls, if necessary, add a small amount of talc powder, add less, and add. The most fundamental thing is to ensure that the varnish is completely cured and not sticky.
"broken paper"
The cause of the malfunction: When the flexo printing machine prints thin paper, it is easy to break the paper. This problem is more serious than when the paper jam is taken. There are two main reasons: firstly, the pulling force of the paper is set too large, followed by defects such as breakage at the edge of the paper. In most cases, the two causes cause a paper break. Once the edge of the paper breaks, the paper breaks easily at the corrector, and then at the middle and rear of the machine, the paper breaks easily due to the increasing tension of the paper.
Solution: During the transportation and loading and unloading process of the paper roll, it is necessary to avoid damage to the paper edge. Once it is found that the paper edge is damaged, it should be processed in time, and then start printing. For example, if only one or two paper edges are broken, it can be pasted with adhesive tape. For a multi-layered break, a long transitional fillet can be drawn along the longitudinal direction with a utility knife to avoid breakage due to stress concentration. It is best to remove all the layers of paper that are very damaged.
1. The most ideal range for ink pH is 8.5-9.2.
2. The thickness of the inkjet printer steel scraper is generally 0.15mm.
3. The flexographic printing pressure is generally between 10 N/cm2 and 50 N/cm.
4. Generally, the number of anilox rolls is 4-5 times the number of screens added to the print.
5. Laser engraving anilox technology enables the number of engraving lines to reach a high precision of 1000 lines/in.
6. When the pH is lower than 8.2, the viscosity of the water-based ink will increase and the drying will increase.
7. The pH range of some special water-based inks is different from ordinary water-based inks. The pH of gold and silver inks is 7.5~8.0.
8. The general pH stabilizer is used to add 1~2% of the total ink amount every 30~40 minutes to stir evenly, or add the stabilizer to the circulating ink pump.
9. In ink printing, the amount of defoamer added is generally not more than 0.5%.
10. When adding surfactant to the ink, the addition ratio should not exceed 2%.