Precautions for Label Printing with a Flexo Press

Selection of color sequence

At present, the graphic part of the label printed with flexographic printing is mainly based on the field, lines, text, etc. Most of the printing materials used are paper, aluminum foil, plastic film, stickers, etc. When water-soluble or solvent-based inks are used to print absorbent printing materials, after infrared hot air drying, the materials will tend to pull up, which will affect the accuracy of overprinting. Therefore, care must be taken when considering the printing color sequence to avoid such problems. Color plates containing large areas should be printed as far as possible in the cell behind, because the substrate is infrared-dried after absorbing a large amount of ink, and the soaring change of the field cell is naturally larger than that of other cells. The effect of overprinting in the printing process is also relatively large. If such units are printed after text and lines, more or less will bring certain positive impact on overprinting. If it is necessary to place the field part in the front color group, it can be consciously made a little larger in the up-down direction to make up for the losses caused by the soaring of the printing materials and make overprinting more accurate.

Platemaking considerations

1. Trapping

When making a flexo, a certain amount of trapping is beneficial to print registration, provided the trap size is appropriate. If the trap is set too small or trapping is not done, it will cause misregistration in the printing (dark trapping); if the trap is set too large, a large outline will appear on the edge of the print, affecting the effect. Visual effects, thus affecting the overall effect of the label.

2. Layout Design

Since flexo printing is basically based on web printing, especially the application of seamless sleeve technology, the paper is greatly saved, but some flexographic printers cannot achieve on-line die cutting, or need to undergo subsequent processing (cutting, etc.) In order to produce finished products, it is necessary to fully consider leaving sufficient margin for subsequent processing when designing layouts. We should not consider the paper-saving rate unilaterally and ignore the qualification rate of subsequent processing.

Selection of raw and auxiliary materials

When using flexographic printing labels, the choice of raw and auxiliary materials will directly affect the quality of printed products, so its choice should not be limited to unit price, but should be the first element of the quality of printed products, and the synthesis of the product The cost is aggregated and analyzed for selection. The major raw and auxiliary materials that need to be considered are mainly paper and ink.

1. Paper

In general, the label pattern is mainly on the ground, which puts certain requirements on the smoothness, whiteness and glossiness of the paper. If you want to get a satisfactory label printing effect in the field, the smoothness of the paper must be high to ensure that the printed product has a uniform density value, so that the surface of the printed product will not cause hair loss. Because the paper with high glossiness and whiteness can enhance the contrast between the ink and the paper, and make the picture more clear, the flexographic ink shows a more vivid effect. Therefore, when choosing the flexo label paper, The two indicators of gloss and whiteness are particularly important. The paper's opacity has more influence on the quality of flexo labels than the above two indicators. Therefore, the whiteness and glossiness of the paper should be selected under the condition that the paper opacity is consistent, and the general whiteness should be Above 85%, the gloss should be higher than 55%.

In addition, since flexo printing generally uses web printing, the tensile strength of the paper is an important selection indicator. Only good tensile strength can ensure that the paper stays steady and continuous throughout the printing process, and the label maintains good overprint accuracy. In addition, the surface strength of the paper is also an important selection indicator, only the paper has a good surface strength The printing in the field is full, and there will be no occurrence of prints or uneven colors on the printed surface.

2. Ink

Since the label is given a vivid color, the proportion of the pigment in the ink will be increased to some extent, but this will have a certain impact on printing. The proportion of pigment is too high (above 35%), the viscosity of the ink will increase, and the thixotropy of the ink will become larger. In the printing process, the transfer of ink will be affected. At this time, the ink must be stirred as necessary. And as far as possible, place the ink in a suitable temperature (20°C).

When selecting the ink, the particle size of the ink and the drying performance of the ink should also be considered. Especially when there are large areas of color patches on the label, and there are text, lines or halftone images, it is necessary to consider the different requirements for the ink drying speed for different patterns, so as to make the prints clean and tidy (there is no paste version , hair pulling phenomenon). In addition, too coarse pigments can cause some wear on the anilox roller, and should also be noticed when choosing the ink.

In short, in the selection of raw materials, we should strive to save, but we can not only consider the price factor, we should consider from the perspective of comprehensive cost, according to the product pass rate. The rate of working hours, etc., is used for comprehensive evaluation and selection of suitable printing materials.

Printing environment requirements

Environmental factors are important when applying flexographic labels. We know that the graphic part of the label has a large area on the ground, as well as text, lines, and halftone images. Due to the limitations of these conditions, it will have a certain influence on the drying of the ink. Drying of ink on printed text, lines, etc. The speed will be accelerated due to the volatilization of ammonia and amines. At this time, the microenvironment of the ink needs to be improved. A humidifier is installed at the ink tank to suppress the volatilization of such substances, thereby controlling the drying speed.

In addition to controlling the micro-environment, the overall environmental control in the printing process is also very important. Flexo-printing is highly sensitive to the environment, printing different types of paper, different patterns (mainly based on the field, or mainly based on lines) When labeling, the choice of humidity is not the same. General waterproof paper printing requires higher humidity, and when printing large areas, the humidity is lower. High ambient temperature will affect the drying effect of inks (water-based inks, solvent-based inks), resulting in paste printing and other phenomena; if the temperature is too low, it will cause almost all kinds of ink viscosity value to increase, resulting in reduced ink fluidity. It is inconvenient to print, so it is necessary to keep the temperature and humidity of the printing environment within a certain range.

In China, the four seasons in the north have obvious changes in temperature and humidity, and the climate is mainly dry. It is best to seal the printing environment and install humidifiers, air conditioners and other equipment to create an environment suitable for flexo printing; The southern climate is unique for flexo printing, but due to the rainy weather in the south, workshop dehumidification should be used as a focal point for creating a flexographic environment. The ambient temperature and humidity range suitable for flexo printing is generally as follows: the temperature is about 25°C and the humidity is 50% to 60%.

The quality of the operator

In the label flexo, the human factor is also critical. Operators should mainly have the following qualities.

1. High sense of responsibility. Responsibility is the first, and if you don't have the responsibility to talk about anything, it's a loss.

2. Have a certain understanding of mechanical principles and electrical knowledge. When operating the flexographic printer, the mechanical and electrical characteristics of the printer can be fully taken into consideration to avoid unnecessary waste in printing, and to eliminate some problems existing in the printing press.

3, have a certain basis of chemical knowledge. In the adjustment of the ink can be considered from the physical and chemical angle, so that the ink to achieve the best results.

4. Master all kinds of printing processes. Only by mastering the various printing processes, can we feedback some problems on the design in time so that the label design is suitable for flexographic printing as much as possible, thus saving raw and auxiliary materials and avoiding waste.

5. Good at summing up. Only by summarizing and refining the problems that arise in the printing in a timely manner can one's own ability be continuously improved.

At present, flexographic printing is being favored by packaging and printing manufacturers because of its features of environmental protection, colorfulness, and stability. The proportion of label printing is also growing. Therefore, how to display its characteristics is becoming a general flexo work. One of the subjects. It is believed that flexo will occupy a more important position in label printing under the premise that the country vigorously advocates environmental protection.

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