Corrugated board flexo printing pressure control

Flexo printing has made great progress in recent years, especially in packaging and printing, showing its unique advantages. In corrugated board printing, although there are offset printing, screen printing, etc., but due to flexographic printing plate is a flexible plastic material, it has the advantage of other printing methods can not be compared, according to statistics, foreign in corrugated cardboard In printing, flexo printing accounted for more than 95% of the market share. In China, more than 70% of corrugated board use flexographic printing.

However, it is precisely because the flexographic printing plate is a flexible plastic material, which makes it difficult to avoid a defect, which is the printing is easy to make dot gain and text expansion paste and so on. One of the main factors causing such failures is the improper control of printing pressure, which is true for line and text printing, and the requirement for printing pressure is higher for screen printing. Only by properly controlling the printing pressure can a beautiful, high-quality product be printed. In printing, if the pressure is too heavy, it will cause dot expansion and text expansion and paste, 30% to 40% dots have hollow spots, and there will be serious hard-mouth phenomenon when printing; the pressure of printing plate and anilox roller is too high. Large, paper-based writing (especially fine-text writing) is vague, and the margins of printed artwork are not clear. In addition, in multi-color printing, if the former color printing pressure is too high and the color ink viscosity is too high, and the drying is too slow, it will result in poor overprinting; but if the printing pressure is insufficient, because the surface of the corrugated cardboard is not as good as other papers Smooth and flat, uneven, it will appear bad ink transfer effect. Therefore, in the corrugated flexo printing, the reasonable control of the pressure is an important aspect affecting the printing quality. We know that in the flexo printing process, there are mainly three pressure points (actually three embossing belts, namely ink fountain roller and anilox roller, anilox roller and plate roller, plate roller and embossing roller), These three pressure points are all very important in terms of operation technology. In the printing process, great care must be taken. It is not possible to rely solely on experience, but should be controlled under controlled conditions. It is impossible to make the three pressure points out of control, or even print under the wrong operating conditions. If so, the quality of the printed crystal will not be able to talk about. So, how to correctly control the printing status of these three pressure points? Generally speaking, the following three aspects can be considered.

Contact pressure between ink fountain roller and anilox roller

The main effect of the pressure between the ink fountain roller and the anilox roller is to control the amount of ink to be printed and evenly transfer the ink. Relative to the pressure of the anilox roller and the plate roller, the plate roller and the embossing roller, the pressure of the ink roller surface may be slightly larger because the ink fountain roller is used to squeeze out the ink on the surface of the anilox roller. If the pressure between the two rollers is large, the amount of ink on the anilox roller is small; otherwise, the amount of ink is more. However, if the pressure of the two rollers is too large (ie, the gap is small), the amount of ink on the anilox roller is relatively reduced. Although the dots can be printed clearly, the characters and lines can be printed clearly, but the ink volume and gloss are lacking. In addition, due to the pressure between the two rollers is too large, the anilox roller and the ink fountain roller are bent at both ends, the printer gear beats, and the teeth are broken. If the pressure between the two rollers is too small (ie, the gap is too large), once the critical point of the web height is reached, the anilox roller will lose the effect of ink transfer, resulting in uneven ink-eating on the printing plate of the printing plate, and the ink volume during printing is not easy. The control eventually resulted in overprinting of the product's printed film, resulting in smeared ink smudges, poor clarity, riddled dots, line spreads, and false hairs. The control of the printing pressure is related to the size of the printing area. Generally, the printing area is larger and the pressure can be lighter to increase the amount of ink, thereby increasing the density and vividness and brightness of the field. Generally smaller or more delicate printing area, the pressure will be slightly larger to reduce the amount of ink to improve the clarity of the print.

This pressure must be horizontally tangent to both ends of the anilox roller, and there must be no deviation, otherwise it will affect the clarity and the printing effect of the printed crystal. Specifically, the pressure between the two rollers can be relatively large when printing cable products, small text, and lines; large-size and field products can be relatively small. The general control method is: Using a 0.10mm thick gauge, plug in the contact point. Need to plug in a few more points, and it should be stuffed in tight conditions. Kraft paper can also be cut into 60-80mm long strips, placed on the contact point, according to the length of the anilox roller, evenly put a long 4-6, short put three. Adjusting the ink fountain roller to the kraft paper can be pulled under relatively tight conditions. If it is easy to pull it up or it is easy to insert the thick gauge into it, there is a gap between the two, which is abnormal. The ink fountain roller and the anilox roller must work under contact. In some companies, the number of anilox rollers used in printing is high, and the amount of ink is insufficient when printing color blocks and large letters. Therefore, the operator often increases the ink supply by increasing the gap between the ink fountain roller and the anilox roller. . However, if there is a gap between the two, the effect of quantitative ink supply by the anilox roller is lost. Therefore, the use of anilox roller must have a "quantitative" concept of ink supply, can not use the gap to adjust the amount of ink, there is a gap in the control of the ink is not good, the actual print incontestably, there will be corduroy-like streak; cable version will produce Chromatic aberration. If the ink fountain roller and the anilox roller are under normal contact, the ink on the roller wall of the anilox roller still cannot be squeezed out, which means the viscosity of the ink is too high. The viscosity of the ink should be adjusted. Between the ink roller and the anilox roller, The pressure is useless.

Contact pressure between anilox roller and plate roller

This pressure is mainly to evenly transfer the ink on the anilox roller to the printing surface of the printing plate. It has a great relationship to the resolution of printed dots. In general, the contact pressure is light, that is, the anilox roller is in contact with the plate, and the plate is deformed to meet the light pressure requirement. If the pressure is too high, the pressure of the anilox roller on the printing plate will be large, which will not only affect the service life of the printing plate, but also the image dot after the printing will increase, and the word or line version will become thicker or double-image, making the image tone higher. The level of darkening deepens, low-level network points and other failures. The products printed in this way have lost tone levels, poor image clarity, poor tone reduction, and unbalanced three colors. At the same time, due to over-printing pressure, the printing plate's printing rate is reduced, and the printing is easy to pile up. Dirty points and other faults. In addition, because the pressure is too great, the anilox roller touches the side of the printing plate at the time of printing, and the printing plate is compressed after being deformed, so that the front of the printing pattern and the text has a hard mouth and serious burrs. At present, quality issues in this area can be observed on many prints. However, the pressure is too small to print on the plate. Therefore, in actual operation, it is necessary to have a flexible grasp depending on the actual situation. If the accuracy of the machine is poor, the pressure may be appropriately increased; otherwise, the pressure may be reduced. For example, the plate thickness is 3.94mm, the lining thickness is 3.05mm, the double-sided adhesive is 0.1lmm, the hanging polyester is 0.10mm, and the total thickness is 7.20mm. According to the requirement of lighter pressure, it is better that the two rollers are in tangential contact, that is, the distance between the anilox roller and the plate roller is kept at 7.20mm. However, due to the fact that the anilox roller, the plate, and the roller all have positional errors, The tangential state is impossible. After testing the anilox roll misalignment and radial jump force 0.02-0.04mm, the plate roller jump force 0.01-0.02mm, the plate thickness error is ±0.02mm, the maximum error is 0,08mm With a printing pressure of 0.02mm, the total amount is 0.1mm. As long as the anilox roller is fully accessible to the printing plate, the ink in the anilox roller can be transferred to the printing plate, and the requirements for ink transfer are achieved. Therefore, with a feeler gauge and thickness gauge of 6 mm, the distance between the anilox roller and the plate roller is controlled to be 7.10 mm, which can meet the requirement of light pressure. In the ordinary printing process, the linear velocity of the surface of the printing plate roller should be less than the linear velocity of the surface of the anilox roller, and the ink transfer effect is good. As the pressure between the two increases, the amount of ink transfer is small. The best printing pressure is to adjust the pressure between the anilox roller and the plate roller to make them uniform in size, and make the ink layer of the anilox roller touch the round surface of the plate surface horizontally. This must be done with long-term practice.

The pressure between the plate roller and the platen roller

The pressure between the plate roller and the embossing roller is mainly to ensure that the ink layer obtained on the plate is correctly transferred to the printing material. This is the last key of flexographic printing. It is also directly related to the quality of the printing product. . In general, the contact pressure between the plate roller and the platen roller is also light, but in the actual printing process, since the corrugated cardboard is not as smooth as the ordinary paper, it is difficult to do this in printing. However, it is also necessary to strictly control the pressure and try to do light pressure. The pressure between the two rollers is too large, the printed dots will be spread out, the middle color is light, surrounded by a deep circle, affecting the image level, so that the dark tone level is blurred, while the text, line version is spread blotting bilateral; pressure is too light, then Imprinting materials on the printing materials. The best pressure control is to make the pressure at both ends of the platen roller and the plate roller the same, and the two are in round-cut horizontal contact. When the printing part of the printing plate is in contact with the printing material, the dots do not substantially expand, the images are all clearly printed, the writing and the lines are clear, and the printing is not double-printed. The distance between the plate roller and the platen roller is the thickness of the plate plus the thickness of the corrugated board. The total thickness of the plate can be controlled, but the thickness of the corrugated board is difficult to control. The printing quality of corrugated cardboard is closely related to the thickness error and flatness of corrugated cardboard itself. Among the printed materials, corrugated paper is the worst, not only the thickness of a piece of cardboard is very different, and the thickness and corrugation level error of the same paperboard is also relatively large. When printing, it is required to meet the thickness requirements of all corrugated cardboard and achieve good The quality control is very difficult.

As for the control of the gap between the plate roller and the embossing roller, according to practical experience, it is necessary to achieve the following requirements: the total thickness of the gap two plates + the thickness of the corrugated board—the amount of error in the thickness of the corrugated board (usually 0.5 mm)—roller The amount of misalignment with the plate (usually 0.1mm). The feeler and gauge are used to control the gap between the plate roller and the embossing roller. Within this control range, it is already possible to ensure that the inked portion of the corrugated paper is completely printed out. However, due to the thickness error of corrugated paper, when printing thick corrugated cardboard, the large amount of compression of the printing plate is not conducive to improving the printing quality. Therefore, to ensure the printing quality, the quality of the corrugated cardboard should be improved as much as possible to reduce the thickness error. If you do not control the operation, the pressure will be too serious hard-mouthed, on the ground inconsistent, ink ink part of the line is shallow, non-ink-edge hard mouth and ink is darker. The excessive pressure on the printed cable version will cause the network to enlarge the paste, and 30%-40% of the outlets have hollow spots. In addition, in the printing process, according to the specific circumstances of the printing, it must be adjusted at any time to keep it in an optimal and stable state.

To ensure the printing quality of corrugated paper, it is necessary to reasonably control the pressures of the above three aspects. During the trial printing and printing of the school, if there is a phenomenon that partial printing is not possible, generally do not take measures to increase the pressure, because although this can temporarily solve the problem of partial printing, it will make most of the rest of the imprinting. Deformation is not worth the candle. The correct approach should be to use the pad version of the method to solve, that is, in accordance with the actual situation in the pad after the film pad method, because the corrugated board printing is generally printed on the plate attached to the film, so use pad method Still more convenient. In addition, the printing plate lining technique can also be used. For example, today's ideal air cushion type lining technology, practice has proved that it can effectively reduce the printing surface pressure, control the dot expansion, to avoid deformation of the printing plate, a good compensation effect for the oversized plate roller, for low quality Corrugated board printing also has good printability, and compared to thicker printing plates, the use of air cushions can also reduce the cost of nearly 1/3, so it is a worthy approach to promote.

(author / Qi Cheng)

<China Packaging News>