At present, only a small part of the more than one hundred unit-type flexo presses introduced by GINA are printing self-adhesive materials, and most of the products printed by flexo presses are thin and similar. The most typical ones are in three categories. The first category is cigarette skins. The paper used is mostly single-sided coated paper of about 70 g / m2 to 90 g / m2. The second category is beer labels, mostly single-sided coated paper or waterproof coated paper of about 75 g / m2, and high-grade beer labels are vacuum-plated plain paper or cloth paper. The third type is tipping paper printing, which uses tipping paper base of about 30 g / m2. Almost all users who have printed thin paper report that they will encounter a series of problems. These problems can be summarized in the following words: "wrinkle", "shrink", "warp" and "static". Below, we will discuss the causes and solutions of these faults in detail.
One of the faults: "Wrinkle"
When the flexo printing machine "walks" thin paper, the easiest and first problem is that the paper produces "wrinkles" along the direction of the paper. The wider the machine (ie, the wider the paper), the thinner the paper The easier it is to wrinkle. When this fault occurs, the first reason you should think of is that the tension is too large. The general situation is that a certain local tension of the machine is too large, then wrinkles are generated in this area and affect the back part. For example, sometimes the unwinding tension is set too high, wrinkles are generated in the unwinding part, and the wrinkling becomes more serious as it goes to the back, or even breaks the paper. Some users originally ordered the flexo printing machine as a thick paper type. Once used to print thin paper, it is easy to wrinkle. At this time, wrinkling generally occurs in the middle and rear of the flexo printing machine. Sometimes, this problem can be solved by reducing the tension setting of each part, but the most fundamental solution is to purchase a set of rollers suitable for printing thin paper, including the impression cylinder and the paper feed and paper output rollers.
Another reason for wrinkling of thin paper is that the deflection angle of the rectification device is too large, so that the tension of the paper at the rectification device is tight on one side and loose on the other side, so it is easy to produce wrinkles. If it is an unwinding part, there will be "bark" wrinkles on the paper feed roller, of course, on the loose side of the paper. The solution to this problem is to allow the correction to "straighten up" (press the Servo-Center key), slow down the paper for a while, adjust the appropriate lateral position of the paper and stabilize it, stop, and then adjust the electric eye of the correction device to make it correct The position of the paper edge to be monitored is aligned at this time, and the deviation correction system is switched to the "automatic" state at this time. In this way, the deflection angle of the correcting device during the "automatic" working process is minimized, and the possibility of wrinkles is minimized. At this time, the horizontal position of the paper roll must be marked, and the position must be aligned when replacing the paper roll.
Sometimes when printing on thin paper, after the original paper roll is removed from the package, the paper near the surface of the outer package will absorb moisture in the air and wrinkle. When the wrinkled paper on the outer layer enters the machine, it will definitely cause problems. Of course, this situation is only temporary, as long as this part of the paper is passed or simply cut off and reconnected at a lower speed, normal production can be started. As for how to avoid the above-mentioned problems, we suggest that during the transportation and storage process, the paper roll packaging should be strictly guaranteed to be removed before the machine is printed, to minimize the time the paper roll absorbs moisture in the air.
Other causes of wrinkling of thin paper include: foreign material (such as large dry ink sticks) or dents on the surface of the impression roller guide roller; the parallelism of individual rollers is too poor, etc. In addition to adjusting the tension and checking the machine, you can also wrap two layers of electrical tape on the inside of the paper guide roller surface about 15 mm away from the two paper edges if necessary to prevent the paper from moving laterally inward. . A paper guide roller with a spiral line can also be used to stretch the thin paper outward. But when the stiffness of the thin paper is too small and the tension is relatively large, it will wrinkle due to the spiral thread, so it is necessary to analyze the specific problems.
Fault two: "Shrink"
In order to improve production efficiency, the current flexographic printing presses are very focused on improving the drying capacity. Generally, natural wind, hot air (high, medium, and low crotch) and infrared (infinitely adjustable and non-adjustable) are used. Some drying methods or their combination, some machines are also equipped with two-stage ultraviolet curing system. In this way, people will find a phenomenon in the proofing or production, that is, a set of films made from the same set of films will have a lateral size deviation, 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, so for thin paper, if the water-based ink is printed on a large area, the "printing wet" on the surface of the paper will expand and widen. The larger the printing area, the anilox roller line. The lower the number, the more obvious the widening phenomenon. When drying after printing, the paper will be narrowed due to "shrinkage". The larger the amount of heating and the more water volatilization, the narrower the width of the paper will be. The heating and drying systems of most flexo printing machines (including the UV curing system) have a minimum speed starting working point, such as 15 m / min. For safety reasons, below this speed value, the infrared and ultraviolet drying devices are not turned on, and will only turn on when it reaches or exceeds this value. It is conceivable that if our startup speed is only slightly higher than this speed value, the amount of paper heating is large, and the shrinkage will be obvious. The farther away the unit is, the greater the lateral overprint error. Therefore, in the printing production process, the lifting speed should be increased as much as possible, and unnecessary hot air or infrared drying should be reduced as far as possible in the production, so as to avoid the occurrence of lateral overprinting errors. We used to print tipping paper on a 20-inch wide flexo press, and only after increasing the speed to more than 80 meters / minute, did we ensure that no lateral overprinting errors would occur.
Of course, there is one exception. When we printed the beer label using cloth-textured vacuum aluminum-plated paper, there was a large deviation in the horizontal printing. After careful inspection and testing, the factor of width change caused by the loss of moisture due to drying was ruled out, and it was finally found that even if the machine runs empty The change in paper width may be caused by the change in paper tension on the machine. [next]
Trouble three: "Curl"
"Warping" here refers to the warpage of the paper in the direction perpendicular to the direction of the paper. At this time, looking at the paper along the direction of the paper, you will see that the sides of the paper or the finished product are upward. Since the running speed of modern packaging machinery is getting faster and faster, the quality of products is bound to be higher and higher. Once the printed products are warped, it will bring some trouble to the packaging line operation. Therefore, whether it is a tobacco factory or a beer factory, the flatness requirements for printed products are very strict.
To explain in a simple sentence, the occurrence of warpage is caused by the difference in the tensile force (stress) of the front and back sides of the thin paper, and most of them are caused by the changes in the humidity of the paper surface. Thin paper has a certain thickness even if it is thin. If one side (or both sides) of the paper is damp (humidified) and the tension on the front and back sides is inconsistent, and the stiffness of the paper along the fiber direction is generally better than the transverse direction, then the two are common Under the action, the paper will inevitably warp. Thick paper is less prone to warping because of its better overall stiffness.
For the warpage of paper and products, we can analyze in four stages and find solutions gradually.
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-coated 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, the amount of water applied to the front of the paper will hinder some moisture from entering the paper from the front, and the back will be more damp, which will cause the paper to warp slightly. .
2. Start printing water-based ink and water-based varnish, but do not apply ultraviolet varnish. The startup speed is appropriately higher. The printed semi-finished products are cut into small sheets and placed on a table to observe the results. There are two cases here. If you print a large-area solid-field version and start it slowly, warpage will most likely occur. On the contrary, if the printing area is small and the startup speed is fast, warpage is less likely to occur. This is because the front of the paper is printed with water-based ink or water-based varnish, and the surface of the paper is initially dampened and deformed, and there is a temporary "arching" phenomenon, which is the opposite of warpage. However, after drying, especially when the amount of heating is large, the wetted part will "shrink" after the water loss, that is, the surface of the paper has tension, causing warpage.
3. Separately apply a layer of UV varnish and observe whether the paper warps after the UV varnish is applied. If the formula of varnish is not suitable, it will often produce additional tightening stress during curing, causing warpage. In this case, contact the UV varnish supplier as soon as possible to improve the performance of the UV varnish, or increase the line number of the anilox roller of the UV varnish unit.
4. The printed product is flat, but after the printed product is placed in the packaging plant of the tobacco factory or brewery for a period of time, the product warps. The relative humidity of the cigarette factory's roll-up package workshop is generally around 65%, and the humidity of the beer factory's labeling workshop is not lower than this value. The vast majority of flexo products are placed in this environment with only one option, namely humidification on the single (back) side. Because the front side is 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 "moisture".
For the warpage problems that occur in the above four stages, we can only adjust the ink reflective oil in the second and third stages, and increase the startup speed as much as possible to reduce the amount of heating. How to make an article on the back of the paper to make the paper have double waterproof function? In practice, we came up with a clever way to make a small modification to the AQUAFLEX flexo printing machine, that is, add 2 paper guide rollers above the first unit's print head wall plate, only change The flexo printing unit can be changed to a gravure backcoating unit in one pass. Use the anilox roller as a gravure cylinder to print the full plate, and find a seamless rubber dyeing sleeve or a flexographic printing plate cylinder with the full plate as the gravure printing cylinder, adjust the plate cylinder The clutch pressure action and the anilox roller supply deinking action ensure that the back of the paper is inked when starting up, and the back of the paper is a certain distance away from the anilox roller when it is stopped to prevent the paper from breaking due to wetting. The liquid in the ink tray is used for back coating, and some users simply use a mixture of diluent and water. Shaanxi New Century Co., Ltd. can provide a better liquid formula, that is, use polyacrylic acid and pull powder to boil and add a certain proportion of water, the price is very cheap. The thickness of the back coating can be changed according to the number of lines of the anilox roller or the ratio of the back coating liquid. The fundamental purpose of backcoating is to increase the "tightening" stress on the back of the paper, offset the stress on the front due to ink and varnish, and to prevent the mixing process on the back of the paper. Practice has proved that this method is not only effective, but also economical and practical, and it has been popularized and applied on many Yajiafa flexo printing machines.
The fourth fault: "static"
"Static electricity" refers to the static electricity generated during high-speed startup production. Once the static electricity of tissue paper products is generated, it will cause some trouble for the orders that need to be collected on the paper collection table when cut into single sheets. After the paper is cut or die-cut, the finished product tends to go down against the bottom roller, sometimes falls, and sometimes just happens to be caught between the bottom roller and the front end of the take-up table. When it falls, only a small amount of waste is caused, which does not hinder the overall situation. If the paper is jammed, it needs to be shut down to clean up, which is even more wasteful.
There are two main reasons for the above problems. One is the poor stiffness of the tissue itself, and the other is static electricity. There are also two solutions, one is to install an effective anti-static device in front of the sheet-cutting and die-cutting units, and the other is to install a fixed thin steel sheet on the front of the take-up table that is against the surface of the paper roller and ensures a tight seam. , The purpose is to force "shovel" the product that is going down to make it enter the normal paper collection stage.
Sometimes, the tissue paper product will fly upward on the take-up table together with the sheet-cutting and die-cutting rollers. The main reason is that the varnish is not cured enough and is a little sticky. At this time, it is generally necessary to improve the curing performance of the varnish. Frequently replace the sponge lining on the sheet-cutting and die-cutting rolls, add a small amount of talcum powder if necessary, and add less and more frequently. The most fundamental thing is to ensure that the varnish is completely cured and not sticky.
When printing thin paper on a flexo machine, it is easy to break the paper. This problem is more serious than when the paper is jammed. There are two main reasons: firstly, the tension of the paper is set too high, and secondly there are defects such as tears at the edges of the paper. In most cases, the paper breakage is caused by the combination of two reasons. Once a tear occurs at the edge of the paper, it is easy to break the paper at the rectifier, and then at the middle and rear of the machine. It is easy to break the paper due to the increasing tension of the paper. In the process of paper roll transportation and loading and unloading, try to avoid damage to the edge of the paper. Once it is found that the edge of the paper is damaged, it must be handled in time and then printed. For example, if there are only one or two paper edge breaks, you can use adhesive tape to attach it. For a multi-layer break, use a utility knife to draw a long transition fillet in the longitudinal direction to avoid breakage due to stress concentration. It is best to remove all layers of paper that are very damaged.
Talking about the problems of sheet cutting, die cutting and paper collection, here are two registration problems of sheet cutting and die cutting. It should be said that the tension control of the unit-type flexo printing machine when printing thin paper is easier than printing cardboard . However, because the sheet-cutting station is far away from the printing station of the sheet-cutting line and the paper is relatively long, the registration error of the sheet-cutting is relatively larger than the registering error between colors. According to our experience, if the front overprint is good and the deviation between sheet and die cutting is large, it is mostly because the rear tension is not adjusted properly. The reasons for the overprint error in the front and the registration error in the back sheet and die cut are too large, the reason is more complicated. The first may be the uneven tension of the paper, and the second is the adjustment of the tension of the front and rear of the machine. The general principle is to first ensure that the paper is evenly stretched, then adjust the printed overprint, and finally the registration of sheet cutting and die cutting. The vast majority of current flexo printing machines are equipped with a computer monitoring system. What you see on the screen are the overprinted crosshairs of each group that have been magnified many times. The value information is used to guide the adjustment of tension and ensure the registration and registration.
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