Precautions for flexo printing machines

Precautions for flexo printing machines

The flexographic printing machine operation is mainly to the following points:

①Many of the manuscripts of flexographic printing are transferred from offset printing. There is a big difference between the two printing processes. Therefore, it is not possible to copy the offset printing process. It must be based on the characteristics of the flexographic printing process itself.

②The adjustment of printing pressure, printing speed and tension of each part should be appropriate. Because the china flexo printing machine is composed of a layer of harder polyester plate base and highly elastic photosensitive material, its most obvious feature is its elasticity. In the printing process, a certain printing pressure needs to be applied. Although light pressure printing is used in flexographic printing, slight changes in printing pressure can also cause changes in color and gradation replication, leading to the expansion of image dots on the printed matter. Moreover, if the printing pressure is high, the dots will be enlarged seriously.

③The printing speed should be determined according to the specific production situation. When the two colour flexo printing machine is started, start at a low speed, adjust the position of the printing plate cylinder of each color group, and gradually increase the machine speed after reaching the register, and should be compatible with the drying performance of the ink. Also note that the machine speed cannot be too fast to prevent ink splashing, causing unnecessary waste and even affecting production.

④ Pay attention to the adjustment and control of the tension. If the tension is improper, it will cause problems such as inaccurate overprinting, skewed feeding, and uneven winding.

⑤ During the printing process, check the viscosity, PH value and drying performance of the ink at any time. For the control of flexographic printing ink, mainly these three aspects. In the production process, due to the volatilization of water and solvents, the performance of the ink becomes unstable, the viscosity increases, and the pH value decreases, which affects the printability. Therefore, in the printing process, at regular intervals, an appropriate amount of PH stabilizer or corresponding solvent (to be combined with the specific printing speed and ink drying conditions) should be added to adjust the pH and viscosity of the ink. At the same time, since the flexographic printing equipment is short-station printing, the stroke between color and color is relatively short. If the solvent in the ink is not completely dried, it will cause poor color stacking or adhesion.

Troubleshooting of Flexographic Printing

1. Printing quality problems of flexographic dots

In flexo printing machine, screen printing is often used to express the color and tone of the product. In the printing production process, if the production process is not properly controlled, quality problems such as dot loss and dot deformation and expansion are prone to occur. For example, small dots in high-gloss parts of the layout are easy to lose, and dark-tone dots are prone to expand, paste and become solid. Due to the expansion and deformation of the dots, the layout of the printed product lacks a sense of hierarchy, and the quality of printing and copying is significantly reduced. The printing plate is soft and elastic, which is the main reason why the flexographic printing dots are prone to quality problems. Process analysis shows that in the flexographic printing process, under the action of pressure, the graphic size of the printing plate becomes larger, so that the dot area also becomes larger, and the small dots are easily lost due to abrasion or uneven pressure. On the other hand, since flexographic printing uses low-viscosity ink, when the contact pressure between the surface of the printing plate and the printed product is slightly larger, the ink on the printing plate will spread outward, and the middle of the expanded dots will have a thin ink layer and edges. The ink layer is thick, forming similar hollow dots. Observing with a magnifying glass, the edges of the dots can be seen to be blurred. This is the basic feature of flexographic printing dot increase.

The production process also shows that the higher the number of dot lines of the flexographic plate, the greater the dot gain; the thicker the ink particles used, the larger the dot expansion; the larger the ink delivery or the ink is too thin, the larger the dot expansion will be. Larger; the greater the printing pressure and the pressure of the anilox roller, the larger the dot expansion; the softer the substrate, rubber roller, pad or flexographic plate, the larger the dot expansion; the worse the accuracy of the flexo printing machine china is, the larger the dot expansion Bigger. In addition, the thickness of the printing plate is inconsistent, and it is easy to enlarge or lose the printing dots. Therefore, to prevent printing quality problems from flexographic plates, we must not only pay attention to the process technology and operation technology, but also choose high-quality and high-precision plates. Printing quality.

2.the problem of deformation of the layout size

It is an unavoidable effect of flexographic printing that the layout specifications are distorted. This is due to the particular nature of a flexographic printer as a high-speed rotary machine, combined with having a smaller radius for the printing plate cylinder resulting in greater bending distortion of the printing plate when pasted onto its surface. As a result, size and registration changes of the printed images and text become more pronounced. To address these issues, plate makers often reduce the layout size per a special formula, and compress gradation levels accordingly. Additionally, if the web material being printed is especially soft, stretching and shrinking along with tension and pressure during printing can further challenge accuracy. Notably, there will be convexity for both graphics and text elements on a flexographic plate with concave blank areas; additionally deformation increases when print speed is slower and pressure higher or when machine precision is low. To prevent such issues from hindering process design, efforts should be made to ensure smooth plate application to the cylinder while preventing re-sticking deformation, use appropriate pressure that isn't too strong, maintain steady roll material tension as it's conveyed, keep constant print speed neither too fast nor slow as well as ensure regular maintenance/lubrication of all machinery

3. Process technical measures to improve the quality of flexographic printing

Make appropriate adjustments to the network cable version

The production process displays that the use of low-line flexographic printing can acquire bright and distinct printing quality with strong contrast. However, when the number of screen lines is too low, the substrate material's surface gloss is not ideal and the fine gradations within the print layout may be easily lost, resulting in poor clarity. In addition to this, as compared to other printing processes, flexographic printing has a larger dot enlargement rate and hence, lesser dot reproduction quality. Mostly, small dots below 2% are hard to replicate. Thus, adjusting level of the printing plate suitably according to equipment and raw material characteristics is essential to make flexographic printing suitable for gradation reproduction range in order to obtain better print quality effect.

Determine the number of screen lines according to the characteristics of the printed material

Under normal circumstances, 80-100 lines per inch is suitable for plate making when using rougher paper and a higher rotation speed for the printing machine. For New Year pictures, monthly calendars, trademarks, pictorials, postcards, desk calendars, product advertisements, book covers and more on offset paper, plastic film and whiteboard paper as well as white cardboard, 100-133 lines per inch are recommended. Printing fine pictures, product advertisements, book covers and illustrations on coated paper or painted newspapers requires 150～175 lines per inch for an optimal result. The best outcome will be achieved with 175～200 lines per inch when printing high-end products such as those on high-grade coated paper and glass card paper such as packaging and decoration prints.

Choose the right anilox roll

The anilox roller plays an essential role in the ink application to the plate, having a major effect on the quality of the finished product. The number of screens determines how much ink is deposited on the page; a higher number resulting in less ink transfer. A high screen count allows for a thin and even layer of ink, which meets any requirements for detailed and layered prints, particularly those needing a glossy finish, while also keeping an even and consistent flow of ink.

When printing screen products, there is a risk of paste and dot expansion if the amount of ink applied is too much. This is because the dot area of the screen plate is small, and that a low-line number anilox roller will mean each cell covers more than part of the plate's dot area. Therefore, this can cause the edges of dots on the plate to cause inking which then results in paste and dot enlargement. To ensure screen dot definition is up to standard it is best to use a high-screen line anilox roller for ink transfer.

Check the quality of the printing process

The printing quality of the resin plate is impacted directly. The film and the resin plate should be in uniform and close contact when printing to prevent dot loss and expansion. Exposure time should be regulated depending on the graphic structure of the plate. When washing, do so cautiously to avoid harm to dots and fine lines. To allow for greater endurance during printing, drying temperature should be controlled between 120°C and 130°C in order for the plate to reach an optimum hardness.

Adjust the appropriate printing pressure

Excessive printing pressure is the main reason for the expansion of flexographic printing outlets. The printing pressure is large, the layout is easy to expand and deform the dots due to excessive extrusion, and the ink at the edges of the dots is also easily transferred to the printed sheet, causing paste, ghosting or dot expansion.

The production situation shows that using a smaller printing pressure is an important measure to improve the quality of dot printing. Therefore, the printing pressure should be adjusted reasonably, and the pressure should be adjusted as light as possible. The actual pressure control generally controls the compression of the printing plate within 0.1mm. Generally, the pressure of printing fine mesh products is relatively small, and the pressure is greater when printing solid products. The hardness of the flexographic plate is low, and it is more uniform when in contact with the substrate, and the printed ink color is also more uniform, but the dots increase greatly, and the level of image reproducibility is poor. Therefore, when printing smooth surface substrates and screen products, the hardness of the printing plate should be slightly higher, and when printing substrates with rough surface and solid prints, the hardness of the printing plate should be lower to improve the printing quality of the product.

Adjust the ink supply pressure

If the ink supply pressure is too large, on the one hand, it is easy to cause the printing plate dots to wear and deform, on the other hand, the ink on the ink roller is easy to form a squeeze phenomenon on the plate surface, which makes the ink layer on the plate surface uneven or causes paste defects. If the pressure of the ink roller is too light, it is easy to cause uneven and insufficient ink reception on the layout, and make the dot printing unclear. Therefore, it is necessary to adjust the contact pressure between the ink roller and the plate surface of the plate cylinder.

Choose fine ink for printing

If you use poorly fine ink to print dots, then, due to the coarse pigment particles in the ink, the printed product layout is prone to paste and dot hairy defects, and the printing endurance of the printing plate will also decrease. Therefore, printing mesh products should choose ink with good fineness. The ink density with good fineness can make the dots of printed products clear and full.

Adjust the viscosity and fluidity of the ink

The ink is highly viscous, and it is easy to cause uneven ink transfer, dot fuzzing, splattering and paste printing. The viscosity of the ink is small, it is easy to emulsify the ink, and make the page dirty and other undesirable situations. There is a close relationship between the fluidity of ink and its viscosity. An ink with a higher viscosity has a higher consistency and a lower fluidity. On the contrary, the liquidity is greater. If the fluidity of the ink is too small, it is easy to cause uneven and uneven coating, resulting in poor reproduction of the printed product layout. Excessively fluid inks tend to cause insufficient dot printing and poor clarity. Therefore, when the viscosity and fluidity of the ink are not suitable, auxiliary materials can be used to adjust.

The factors that cause flexographic printing quality problems are diverse, sometimes caused by a defect factor, and sometimes caused by the coexistence of several undesirable reasons. A correct understanding of the root causes of printing quality failures, and by controlling printing operations and process technology, not only can prevent and avoid the opportunities for various printing failures, but also effectively improve production efficiency and product printing quality.

These china flexo printing machine manufacturers offer a range of flexo printing machines suitable for various applications, such as label printing, packaging printing, and more. It's important to do your research and compare the features, specifications, and prices of different machines before making a decision on which one to purchase.