Flexo Printing Quality Analysis and Resolution (1)

Since the 1990s, under the impetus of the Professional Committee of the China-India Printing Association, the flexible printing industry in China has made great progress in the flexo printing process. The quality of China's flexographic products has been increasing. However, from the analysis of information fed back by flexographic printers, there are still some unsolved problems in theory and practice in the flexo printing process, and product quality often fluctuates. This article intends to analyze the quality issues that are common in flexo printing. The corresponding solution.

one. Block version

Blocking plate is one of the common diseases of flexo printing. Many products of domestic and foreign counterparts have this problem. The so-called block edition, that is, the problem of dirty printing of small outlets in the printing of level printing. During the printing process, when the dots that are not connected to each other on the printing plate are printed on the paper or film for some reason, the ink of the circular dots overflows. The two dots and sometimes the dots are irregularly connected together. The plate is dirty. When printing is stopped, it can be found that the printing plate has poor transfer of ink. On the same printing plate, the ink on the solid part is completely transferred, the printing plate is clean, and the cable is covered with a lot of excess ink. When the ink is serious, it is usually wiped clean with a rag, and it is necessary to clean it carefully with a soft brush. There are two types of blockages, one is 30% or less, especially 5% or less, that is, we often say that the highlights, there is a blockage range of 45% -55%, due to the flexible version of the tone The expansion is generally around 20%. Therefore, the 50% critical point of offset printing is generally about 70% on flexo printing. In this way, 45%-55% of the area on the printing plate is the part where the edges of the two circular screen points are relatively close. This part of the blocked version is a blockage in another sense. On plain webs that are relatively easy to print, little attention is paid to such unpredictable problems. The foreign counterparts in the flexo printing industry call this phenomenon the ink fill in, that is, the ink blocks the depression between two adjacent dots on the printing plate, also called bridging, while the domestic counterparts are caged and dirty. There is no garbage in the accumulated ink, and there is no dust. The solution can be completely solved with the solvent. It is just the ink. The ink that appears in the part where the ink should not appear is doing the trick. The reasons for the block version are as follows:

1. Mismatches between the number of plate lines and the number of anilox roller lines

The key point of flexo printing, unlike other printing methods, is an anilox roller. The anilox roller is the metering roller for ink transfer. What kind of anilox roller will give you what kind of ink. The main parameters of the anilox roller are: line number, ink storage volume and the angle of the cell. The number of lines of the anilox roller refers to the number of cells per unit length in the axial direction of the anilox roller, and the number of screens is also the number of lines. The minimum unit of the composition image is the dots, and the number of dots per inch is the number of lines of the printing plate. In the flexo printing process, the matching of the number of printing plate lines and the number of anilox roller lines is a very important issue. Our previous point of view is that the ratio of the number of printing plate lines to the number of anilox roller lines is a common parameter: the number of anilox roller lines should be 4-7 times the number of printing plate lines. Since the area of ​​the dot and the area of ​​the cell are not completely contained in the position, there will be a few dots of ink in the dot of the dot at the edge of the dot. This excess ink cannot be completely transferred from the anilox roller's cell site to the printing plate's dot, like the corresponding ink. Instead, it is transferred in an arc shape to the mesh point according to its viscosity and surface tension. The good or bad transfer constitutes the hidden danger of blocking the plate. Some later problems prompted us to correct our own views. The first batch of CDI printing plates produced in China, when a test version made by a foreign expert of Shanghai Xinhua Company was printed at Tiger Packaging (Shanghai) Co., Ltd., we used an 800 L/inch anilox roller. When analyzing the data on the test strip, we found a strange phenomenon. For 133 L/inch plate test strips, the size of the 5%, 3%, 2%, and 1% of the dots at each site is: 5% dot area> 3% dot area> 2% dot area, but 1 % dot is not the case, under the observation with a 75 times magnifying glass, 1% dot area is obviously a lot more than 3% or 5% dot area. In the 150 L/inch test strip area, we also observe that 3% of the Internet sites follow a large-to-small change, but in the 2% and I% regions, irregular or large or small The outlets are also chaotic in shape, and they have long and random corners. These phenomena show vividly the fact that these irregular points are when the fine dots are in contact with the cells on the anilox roller. Because the diameter of the dot is smaller than the diameter of the opening of the cell, the dot plugs of the printing plate are Into the network point, when the printing plate on the network and the network point to leave, not only its surface, but also around it also covered with ink, transferred to the printing substrate, the above weird phenomenon. Therefore, our view on the above issues is also amended to: When matching the number of anilox roller lines with the number of plate lines, we cannot rely only on a qualitative analysis of 4-7 times, and we should further propose the minimum dot of the plate. The diameter of ≥ anilox cell opening diameter of such a quantitative analysis. For example, the number of plate lines is 100L/inch and 150 L/inch, respectively, according to the traditional qualitative algorithm 4-7 times, 100 L/inch plate with 400-700 L/inch anilox roller, 150 L/inch plate with 600-1050 L/inch anilox roller. According to the quantitative calculation, the minimum dot of 100 L/inch printing plate is 2%, and its dot diameter can be obtained as 4l u. According to the above formula, the anilox roller should be equipped with a mesh opening of 600u/inch for 38u, if the minimum mesh is not 2% but 5%, the diameter is 64u, as long as the mesh opening is 400u/inch of 58u. can. Similarly, if the minimum network point of 150 L/inch is 2%, the diameter of its dot can be obtained as 27 u, and a 900 L/inch anilox roller with a cell opening of 25 u should be provided. If the minimum dot size on a 150 L/inch plate is 5% and the dot diameter is 43 u, a 550 L/inch anilox roller with a 42 u mesh opening is required. From this point of view, quantitative calculations are reasonable in terms of conclusions and are economically cost-effective.

2. Abnormal printing pressure

The printing pressure mentioned in the flexo printing process includes two contents. One is the pressure of the anilox roller on the plate roller. We call it the ink pressure, and the other is the pressure of the plate roller on the platen roller. Call it printing pressure. This pressure is due to the compression of the printing plate by the gap between the two rollers. The main causes of pressure anomalies are mechanical precision errors and unreasonable adjustments.

The mechanical accuracy error mainly refers to the radial runout error of each roller, which is also called the concentricity error in the industry. Foreign countries call it TIR. The radial runout error of each roller should be controlled within 10-20 u. Whether it is a satellite flexo press or a unit flexo press, this is a key figure.

The unreasonable adjustment amount mainly refers to that the gap between the rollers is not adjusted properly, and in particular, the adjustment of the gap between the anilox roller and the plate roller is the main reason for the abnormal ink pressure. If the inking pressure is too high, the printing plate internet point is under pressure and deformation, and the ink on the printing dot is too much, it will inevitably overflow to the surrounding area. At this time, as long as the printing pressure is not appropriate, the blocking plate is likely to occur.

The unevenness of the printing plate, that is, its thickness error caused by the high-speed operation, is one of the causes of the abnormal printing pressure. According to the data provided by the material supplier, such as the 1.4mm thin plate of DuPont Celli, the maximum error is 13u. The double-sided tape we use, whether it is 0.38mm or 0.50mm, often causes partial unevenness in coating unevenness due to the glue. If the error position of the printing plate misregisters with the error position of the double-sided tape, the unevenness of the printing plate will cause intermittent squeezing when the printing roller rotates at a high speed.

3. Improper fit of the ink to the surface tension of the printing plate

The surface wetting tension of paper and film can be measured. The ink surface also has wet tension and can be measured. All kinds of inks, due to different resins, different formulations, the surface tension is different. The greater the difference between the surface tension of the ink and the wetting tension of the printed substrate surface, the better the printing suitability. In flexographic short ink printing, the ink is first transferred to the plate and then transferred to the printing substrate. In this way, there is a problem with the tension of the ink and the printing plate. For inks and printing plates, since the difference in tension is small, the ink can be easily transferred from the printing plate to the printing substrate without leaving or leaving very little on the printing plate if the surface tension of the ink and the printing plate are different. If it is not very small, the ink on the printing plate will not be completely transferred, and there will be ink filling the depressed part between the two dots. This is the blockage.
Based on the above reasons, the corresponding solution is:

(1) Select the number of anilox roller lines according to the principle of the minimum dot diameter of the printing plate ≥ the opening diameter of the anilox roller dot opening, and do not need to be confined to the traditional 4-7 times.

(2) Targetedly resolve abnormal printing pressure.

a. Machines that exceed the standard for mechanical accuracy error shall be repaired and brought up to standard.

b. Control the ink pressure, ink pressure should not be too large. Ink dots with excessive ink pressure, a round of ink is uneven, half shallow and half deep, or there is a trace of ink overflow around the dot, and dots with excessive printing pressure are deformed by round dots. Become an ellipse. When the pressure on the ink is adjusted, it should be light and heavy, and the pressure should be gradually added. It cannot be overweight and then returned. At this time, the printing plate is full of ink, including the depressed area of ​​the printing plate. This has not yet imprinted has caused a hidden version of the hidden dangers. After adding the ink pressure and gradually adding the printing pressure, after the observation printing pressure is normal, all the printing plates are printed with ink, and the dots are not deformed. At this time, the pressure on the ink is retreated slightly to observe the printed products. Speed ​​up again. If the ink pressure drops too much, a hollow dot will appear on the printed product. At this time, the ink saturation will be different, but at this time, the phenomenon of blocking will be greatly improved.

c. Adjusting the unevenness of the printing plate and the double-sided tape is to adopt a pad printing plate instead of increasing the printing pressure, which is an important concept in operation. Commonly used leveling measures are that the ground version can be taped below, and the thickness of a single-sided tape is generally around 50-60 u. The cable version can be coated with shellac, with a layer thickness of approximately 4 u.

d. Use low-density double-sided tape to buffer the abnormal deformation of the dots. The double-sided adhesive has good elasticity. Although it is easy to appear pinholes in the actual printing, it is the best choice when printing the cable. The dots are not easily deformed and have good recovery.
e. From the order in which the plates are pressed, it is necessary to select the pattern to be gradually pressed from the part where the number of lines is less to a greater number, that is, from the part where the density is low to the part where the density is high at the part where the printing plate protrudes. Gradually pressured. The change in pressure at this time is gradual rather than sudden. If the opposite is true, the probability of blockage increases dramatically.

f Pay attention to the vertical uninterrupted layout. Due to the discontinuity in typesetting, there will always be high and low discrepancies in the raised portions of the printing plate in the longitudinal direction. At this time, the impact of pressure will be regular. If equal-width beading is applied on both sides of the plate, the bead will cover the entire platen roller so that the impact of pressure will be reduced. Or stagger typesetting, in the entire perimeter to make it not completely low position, then the roller will run more smoothly, the impact will be smaller.

(3) A reasonable solution to the ink and plate tension coordination.

a Select the appropriate ink and look for ink that is about the same as the surface tension of the plate. For example, UV inks, because the resin is different from the formula, the probability of this problem is not very high.

b. Select the plate parameters to make the plate surface harder. This can select the plate material, or you can choose to change the UVC and UVA parameters during the platemaking process. When the printing plate is being washed after being stopped for any reason during printing on the machine, do not rush to start the machine. Use compressed air to dry the plate and do not start it immediately without drying. Because the printing plate dry or not has a great relationship with its surface tension.

c. Adjust the surface parameters of the printing plate with a special solvent to make it close to the oil