Analysis of Problems in Flexo Plate Making

August 21, 2019

In recent years, the quality of flexographic printing has been continuously improved, which has enabled it to develop rapidly in the paper packaging and printing market. The quality of flexographic plates determines the quality of flexographic products. This article focuses on the main points of the flexographic plates process and issues that should be noted.

Reliable factors of plate quality in flexographic printing

The factors that affect the quality of flexo platemaking mainly include the following aspects:

Choice of outlets

1. Choice of dot shape: Compared with traditional offset printing, the flexographic reproducible tone range is from 3% to 95%. According to the flexographic feature, circular dots are usually used in the plate making process to obtain better dot reproduction. effect.

2. The choice of dot number: The number of lines determines the accuracy of image copying. The higher the number of lines is, the higher the precision is, and the better the reproduction is. However, the number of high-adding lines cannot be blindly pursued. For variable carton printing, 100 lines/inch or less than 100 lines/inch of screen lines are sufficient.

3. Screen angle selection: Flexographic printing is performed by an anilox roller. In order to avoid the occurrence of moiré and other problems, the dot angle of the flexographic printing is different from that of the lithographic plate in each color. 7.5°. (See Table 1)

Table 1: Comparison of flex and lithographic dot angles

Green Edition (C) Black Edition (K) Magenta Edition (M) Yellow Edition (Y)

Flexo dot dot angle 7.5° 37.5° 67.5° 82.5°

Offset dot angle 15° 45° 75° 90°

With the development of anilox roller engraving technology and computer-based plate making technology, it is becoming a reality to realize four-color dot printing at the same angle on the flexographic printing plate. It can make printed graphics with softer colors and greater level contrast. At the same time, it also saves a lot of ink.

Color separation sheet size

The flexographic plate has elasticity, and after the plate is mounted on the cylinder, it is inevitable that a tensile deformation occurs in the circumferential direction of the cylinder. This deformation directly results in the printed article not correctly reproducing the original information. In order to compensate for the distortion of the printed image, the size of the corresponding graphic on the negative must be reduced. Before designing a manuscript or separating a plate, the amount of elongation of the plate should be taken into consideration, and the corresponding value in the circumferential length of the manuscript should be subtracted from the corresponding value in order to compensate, so that the printed product can meet the dimensional requirements.

The proportion of parts reduced in the original design is referred to as the reduction ratio. Reducing ratio and the radius of the plate cylinder, the thickness of the plate, and the thickness of the double-sided tape are calculated as follows:

Reducing ratio (percentage) = K/R × 100%

Where R represents the printing length of the plate cylinder; K is a coefficient, depending on the thickness of the plate used.

Flexographic production

At present, the plate used for flexographic printing is mainly a solid photosensitive resin plate, and its photosensitive mechanism is that under a certain amount of light irradiation of the photosensitive resin, the molecules rapidly decompose to generate active and unstable free radicals, and the free radical initiates the presence of unsaturated bonds. The resin undergoes polymerization and forms a graphic area.

From the perspective of the platemaking process, the flexographic printing process has its own unique features. The basic process flow includes the following processes:

1. Back exposure of the plate to determine the height and detail of the embossed on the plate, while also enhancing the plate base and improving the printing plate resistance. This process is commonly referred to as pre-exposure. The length of the exposure varies depending on the type of plate, the source of the plates, and the complexity of the graphics.

2. Place negative film negatives on the plate, use UV light for frontal exposure, form graphic portions on the plate, and cure. This process is called main exposure. Exposure quality is affected by negative film, exposure time, and platemaking equipment. Under normal circumstances, the main exposure time is higher than the line version, and the main exposure time of each color version is also different.

3. The unexposed resin layer on the plate is cleaned and removed, ie, the development process. The strict control of the development time is crucial, and it is related to the factors such as the development temperature and the concentration of the drug solution.

4. The developed plate is dried in a desiccator to allow the absorbed solvent to evaporate as quickly as possible to return to its original standard thickness.

5. Detackify the relief layer of the printing plate, that is, post-processing. Treatment methods include light, chemical, and dusting methods, which can be selected according to the original.

6. Perform a full, uniform exposure of the dried plate. This process is called post-exposure. The exposure time should be set according to the type of graphic text to ensure a high quality printing plate.

Problems with Plate Design

1. When designing an original, try not to use a gradation effect of 1% to 100%. Improve the midtones of the image, and give 2% to 3% of the protective dots at high light to avoid netting.

2 text specifications should not be too small, the width of the independent line should not be less than 0.2mm, especially for the Yin picture. Because when the printing pressure changes, the printed graphics and text will show a larger deformation, making the Yang Wenwen thicker, Yin Xie text thin or paste dead.

3. The engraving angle of the ink hole on the anilox roller in flexo printing is generally 45°. Therefore, when using an ordinary type of anilox roller, the printing plate should avoid the angle of 45° and avoid the appearance of prints. Turtle pattern.

4. In the case of sufficient flexo printing units, the same version of the cable screen version and field version should be produced separately, try to use spot color printing.

5. The flexible nature of the flexographic plate makes it easy to cause regular geometrical patterns to become irregular when it is deformed, so designing a large circular pattern should be avoided.

6. Try to avoid overprinting of large solid plots. If necessary, use large local blocks for overprinting text or patterns.

7. Avoid designing wide, long bars and solids in the horizontal direction of the print cylinder. This can cause machine vibration. Ideally, slashes, curves, wave lines, and other irregular curves.

Flexo plate making common faults and solutions

Relief shallower, base thickness inconsistent

Causes and solutions:

1. The back exposure time is excessive. Since the photosensitive resin of the cured plate material is formed on the back side of the plate during the main exposure, the thick substrate is formed when the back exposure time is long. This results in relatively shallow embossing. Solution: Reduce the back exposure time appropriately.

2. The exposed surface of the plate is reversed. Due to work inadvertently reversing the front and back of the plate, exposure was performed, which resulted in shallow relief and even no embossing. Solution: After cutting the plate, uncover a small corner of the plate's protective film and mark it to prevent mistakes.

3. Washing time is not enough. Due to insufficient washing time, the uncured photosensitive resin was not thoroughly washed out, which resulted in a shallow relief. Solution: Properly extend the washing time.

Lines are deformed and dots are lost

The main causes and solutions:

1. The main exposure, post-exposure and drying time are not sufficient. The exposure time is insufficient, the substrate of the printing plate is thinner, and the relief is too high, which may cause bending of lines and loss of dots. Solution: Adjust the main exposure and post-exposure time to an ideal value and extend the drying time appropriately.

2. Washing time is long. A long washing time will make the independent lines or independent meshes excessively inflated and easy to bend and wash away. Solution: Pay attention to controlling the wash time.

Shallow text plate

The main causes and solutions:

1. The main exposure time is excessive. If the exposure time used on the plate for solid and halftone images is the same, the text will be shallow. The solution is to use mask processing.

2. The original negative film density is too low. In the main exposure, the density of the film is not enough because the density of the film is too low.

3. The light-free photosensitive resin is not completely dissolved. Mainly due to insufficient washing time, it was unable to completely dissolve the photosensitive resin. In addition, dirty washing solvents can also cause this phenomenon. Therefore, it is required that the solution of the cleaning solution be frequently replaced.

Plate hardness is too high

High hardness of the plate will greatly reduce its service life. Mainly because the post exposure time is too long. Therefore, the post-exposure time should be properly controlled in the plate making.

Layout sticky

It is mainly caused by insufficient cleaning of the plate material, excessive use of the rinsing liquid, or insufficient active ingredients. Plates in production should be thoroughly cleaned, and attention should be paid to the timely replacement of rinses.

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