Control of Dot Increase in Flexographic Printing

Although flexographic printing has developed rapidly, it is not an omnipotent printing method. In actual printing samples, the highlights are lost and the shadows are printed on the ground. The lack of layers is a prominent issue in flexographic printing. This is because the dot gain in flexo printing is more serious. The so-called dot gain refers to that the area of ​​the printed matter of the printed matter is larger than that of the corresponding part of the printing plate. Theoretically speaking, the increase in network points includes the increase of mechanical network points (also known as the increase of physical network points) and the increase of optical network points. In practice, it usually refers to the mechanical dot gain.

Why is there a more serious dot gain in flexo printing? Because the flexographic printing plate material is soft and elastic, under the effect of printing pressure, the graphic size on the printing plate will become larger, ie, the dot area will become larger. Since the flexographic printing plate is a relief printing plate, when a low-viscosity water-based ink is used for printing, as long as the pressure between the printing plate surface and the printing material surface is slightly larger, the ink on the printing plate will spread to the surrounding, and the dots will form an enlarged one. The middle ink layer is thin, the edge ink layer thickness is similar to the hollow point, which is the increase of the mechanical dot occurred in the flexible printing.

There are many factors that cause the network to increase, and improper control of any link in the technology, process, equipment, or material may cause the network to increase. Here are five aspects of how to control the dot gain:

(1) Printing pressure: Printing pressure is high. There are more inks transferred from the printing plate to the substrate. The definition of the ink increases, but the pressure is too high, which will cause the network to increase, resulting in paste or ghosting. Pressure control is generally expressed in the amount of compression of the printing plate, and the compression of the printing plate should be strictly controlled within 0.1 mm. The pressure for fine screen printing is relatively small; when printing on the ground, the pressure is relatively large.

(B) printing plate: hardness is a key indicator of the performance of the flexographic plate, the contact between the plate and the substrate with a low hardness is more uniform, and the ink color printed out is also relatively uniform, but the reproducibility of the image layer is poor, and the dot gain is more significant. Therefore, the printing of surface smooth substrates requires a high hardness of the printing plate, and printing a rough surface of the substrate requires a low hardness of the printing plate.

The consistency of the thickness of the printing plate can ensure that the increase rate of halftone dots in the halftone printing is minimized, and the ability of tone reproduction is improved, thereby improving the reproducibility of the detail of the highlight portion and the dark tone portion.

For the Saili version, for thin plates with a thickness less than 3.18 mm, the allowable thickness error between each plate is ±0.025 mm, while the thickness error for each part of the same plate is ±0.013 mm. For thick plates thicker than 3.94 mm, the allowable thickness error between each plate is ±0.046 mm, and the thickness error between parts of the same plate is ±0.025 mm.

(c) Anilox Rollers: Anilox rolls with a high number of screens can be made thinner. A more uniform ink layer can meet the printing requirements of a richly-enriched print product, in particular meeting the needs of a high-light portion, and can reduce dot gain during printing and maintain a uniform and constant ink transfer amount.

However, the higher the number of lines is, the better the anilox roller is. One is that the anilox roller has a long processing time and the selling price is high; the other is that the amount of ink is insufficient when printing.

Ink distribution system, there are usually three kinds of anilox roller arrangement for measuring ink in flexo printing:

Rubber roller ink scraping ink metering system: This is a way to change the relative speed of rotation between the rubber roller and the anilox roller and the two rollers

The pressure between to control ink metering.

This method can only achieve limited control of ink metering, and as the printing speed increases, more ink is transferred to the anilox roller, which not only increases the ink density value, but also makes it difficult to control the ink metering accuracy at high speeds. Sexuality and dot gains increase.

2. Ink metering system with positive squeegee: The squeegee can scrape excess ink on the surface of the anilox roller and only store ink in the stencil. Therefore, ink metering is almost independent of the printing speed, and the printing speed is 200. In the case of ~300 meters/minute, the amount of ink can be well controlled, but when the printing speed is more than 300 meters/minute, the ink cannot be well controlled.

3. Ink metering system with reverse squeegee: This device still has good ink control at printing speeds up to 500 m/min. Therefore, many printers use this device for high-quality dot printing.

(5) Ink: The dot gain is related to many factors and properties in the ink, such as pigment, viscosity, viscosity, and drying speed. The ink has high coloring power, and the ink layer during printing can be thinner, and the dots can be reduced. The increase. Viscosity is the most important control index in flexographic ink applications. It directly affects the ink transfer transfer performance and flexographic product quality. The viscosity of the ink is high, and the dot gain is small, but the viscosity is too high, which can affect the transfer performance of the anilox roller, causing uneven ink, stencil, and poor drying.

Reprinted from: Paper Information