Do you know the defects of flexo preprinting?
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The increase in flexo dot has always been the biggest reason for the quality of peer-to-peer flexo printing, because there are two pressures in the flexo process, one is the ink pressure on the anilox roller, and the other is the plate pressure. The printing pressure of the printing roller. Since the hardness of the flexographic plate used for pre-printing is generally about 70° Shore, the plate is deformed by hardness due to the mechanical error caused by the tangent of each roller, and the resulting dot gain is almost inevitable. The focus of early flexographic quality control was on the captain's control of the two pressures, so the print quality was closely related to the individual factors of the operator. The skill and quality of the first-line operator determines the quality of each print, so the quality of the flexo is floating, and the dot gain is unstable. Because the flexo dot is enlarged, the number of plate lines used for flexo printing has never been more than 133lpi before, and the minimum dot size is 2%. Compared with the 175lpi of gravure and offset printing, it is obviously insufficient at the highlight level.
Industry blame on the quality of flexo products
The number of printing plates is too low, and the level of the offset printing process is too large;
The highlight requires a gradual change to zero without a hard mouth, and the dark tone requires the layer to be opened without being leveled. At present, most of the flexo printing is difficult to achieve;
Offset four-color overprint can achieve a variety of colors, flexo multi-spot color and two overprints, making printing cost too high;
The version fee is much higher than offset printing.
Number of plates
The number of plate lines is the first problem. There are several aspects to the use of high-line plates in flexo printing, one of which is the plate. The boundary of the plate to zero, that is, the minimum dot diameter is related to whether the naked eye can see the hard mouth. The 1% dot diameter of 150 lpi is 19 μm, and the gradient mesh is distinguishable from 1% to 0%. The 1% dot diameter of 175lpi is 16μm, and the gradient to 0% of the critical line is not easy to distinguish. A dot diameter of 15 to 16 μm forms a quality indicator. Therefore, if the pattern design needs to be graded to 0%, a 150 lpi plate must be used with a hybrid mesh, and the density of the FM network is used to mask the excessive dot diameter. When using a 175lpi plate, it can be directly attenuated by the characteristics of the AM network. Offset printing has been doing this for a long time, and this process is quite mature. The technology that the flexo plate reaches the minimum dot diameter is the first to drive the Kodak NX plate because the square laser aperture that generates the dots is 10 μm. Esko HD technology can also be achieved, although the circular laser used is larger in diameter, but the characteristics of the plate material in UV curing can also make the dot diameter smaller by the corrosion of the plate washing solvent. The label industry uses more HD outlets, and it has proven to be very effective through actual combat.
Another important issue with high-line plates is the choice of anilox rolls. When I was in the first line of flexographic printing, I used the argument of the ratio of the number of printing lines and the number of anilox rolls in the industry at the time. It was proposed to calculate the opening diameter of the anilox roll in accordance with the minimum dot diameter ≥ To choose an anilox roller. The original intention of this method is to avoid accidentally planting into the cell cavity due to the small diameter of the dot. The ink stained around the dot will stick to the printing plate, especially adhered to the non-inking area between the two dots, resulting in blockage (or Called the net), resulting in quality failure. However, this will inevitably require the number of anilox rolls, and the anilox rolls with a high number of small inks are inevitable. The same anilox roller, the price of high line and low line is very different. At the same time, the small amount of ink of the anilox roller also brings the defect of insufficient color density. For this reason, the flexo printing process often repeats the overlapping of two identical colors to overcome the deficiencies. I have used the method of selecting the surface tension of the printing plate to match the surface tension of the ink to solve this problem. I hope that even if the dot is mistakenly planted into the cell, the ink will only be on the top of the dot that needs to be transferred, and the non-inking zone will not. Contaminated with unwanted ink. Or even if it is contaminated, the printed area of the printing plate should be self-cleaning where it should not be inked, without stopping the plate. The author defines this technology as the self-cleaning theory of the flexographic version, which has been verified in the actual machine test of the flexo printing machine.
Dot shape
The second problem is that the choice of the flexo dot increase curve, in other words, the choice of the shape of the dot problem. This problem involves the objective needs of the market for rubber to soft, we often need to require flexo according to the quality characteristics of offset printing. Like it, this is critical. In fact, as long as the two methods can be used to achieve the square circle, there is no difficulty in theory, but in the details of the 50% dot, that is, near the expansion threshold, it is very particular. It is necessary to avoid the jump failure caused by the increase of the flexographic dot, which is another meaning of "ink fill in", which is also the level near the critical point that must be avoided.
Gray balance
The third question is how the flexo controls the gray balance during the printing process. If it can be achieved, flexo printing can also use the basic color stacking process to achieve various spot colors like offset printing. It can be imagined that when flexo printing is no longer printed with 6-color, 8-color or even 10-color machines, how much equipment cost, material consumption, etc. can be reduced? But for a long time, the gray balance of flexo printing only exists in the color separation range of the plate making company, which is extremely difficult to achieve on the printing machine. Because of each printing, the ink pressure and printing pressure of flexo printing will change, and the dot gain will change. The original designed C, M, Y, and K color ratios will definitely be out of shape.
So, can you achieve gray balance? It is the only way to control the human factors to the lowest level by using the characteristics of the machine equipment like offset printing. Fortunately, this kind of equipment has already been made abroad, and it can also be produced in China. This is the roller pillow structure of the flexographic printing machine. The author introduced this technique in an article reviewing the new technology of flexo labeling a few years ago. The foreign introduction is petal, but its core technology is the pitch control of the gear. I have summarized it into a roller pillow structure, similar to the shoulder iron of an offset press, but it is different.
Flexo controls the gray balance in the printing process, and an important part is to control the viscosity of the ink. This problem is not difficult for label printing companies using UV inks, but it is difficult for companies that use solvent-based inks or water-based inks for a long time. For a long time, the customary thinking of the flexo world, liquid inks should use low viscosity, otherwise there will be problems in the printing process. In fact, the determination of ink viscosity is determined by the characteristics of the inking mechanism. If the flexo press inking mechanism can accommodate higher viscosity inks, such as flexo UV inks, why bother to lower the ink viscosity?
The development, detection and correction of the flexographic reduction curve will also be an indispensable task. It is necessary to use the reduction curve of the printing plate to make up for the shortcomings in actual printing.
Combining the above three elements, it is not difficult to achieve the gray balance in the printing process. Based on this, we will minimize the personal factors of the operator in the printing process, so that the quality of the single-machine multi-class printing is consistent, thus effectively reducing the cost of flexo printing.
Plate cost
The fourth question is how to reduce the flexo printing fee. For a long time, the photosensitive resin flexo has been monopolized by large foreign companies, and the high price of the flexographic version used in China has been criticized by the industry. This big problem that has plagued China's flexo world has been loosened in recent years. The birth of the domestic flexo and the smooth promotion, at least in the field of thick edition, the price of the flexo has been significantly flattened. However, in the field of thin plates, especially in the field of laser flexography, the price is still very high. Comparing the obvious price difference between the flexographic version and the two types of thick and thin products, if we print the same quality products with a lower-priced thicker version, we don’t have to rely on the price reduction of the plate supplier, but we must control the fate. Is it in your own hands? In the field of flexo preprinting, the low-cost flexo plate is used to achieve the improvement of the original printing quality, which is worthy of our exploration.