Flexographic printing dot expansion? Blocking the board? Did you notice these details?

Flexographic printing dot expansion? Blocking the board? Did you notice these details?

In flexographic printing, dot enlargement and blockage are the two most common core issues affecting print quality. This not only causes loss of image layers but also affects print clarity and consistency, leading to a significant decline in print quality. So, what are the key factors causing the expansion of flexographic dots and plate blockage? In this article, the author analyzes feasible solutions from multiple perspectives, including prepress plate optimization and ink formulation adjustments, and printing pressure control, for reference by industry peers.

Flexographic dot size expanded

Dot enlargement is a common phenomenon in flexographic printing. Dot expansion usually occurs because when ink transfers from the plate drum to the substrate, the ink diffuses after absorption, causing the dots to further enlarge, which affects label detail and color saturation.

In flexographic printing, dot expansion is very common. For example, a dot point originally set at 50% may expand to 51% on the printing plate, and the dot point on the final printed product may even expand to 65% or even more. This phenomenon of dot expansion mainly stems from the characteristics of ink used in flexographic printing and the compressibility of the printing plate. It is worth noting that the specific degree of dot expansion varies depending on the type of printing press and the type of substrate.

01

Printing pressure

In flexographic printing, ink transfers under light pressure. During this process, the soft flexible plate experiences slight deformation and compression when the image transfer is pressed, which is the key reason for dot enlargement. If the printing pressure is too high, the deformation of the flexible printing plate will be greater, and the expansion of dots will become more severe.

02

Plate hardness

The hardness of the printing plate affects the degree of dot expansion. Harder plates deform less than softer plates, resulting in less dot expansion. To minimize dot expansion, it is recommended to prioritize thin overlays with hardness higher than the substrate (thickness 0.05~0.13mm). Secondly, compression tape or cushioning pads that absorb printing pressure can be used to secure the printing plate, which also helps control dot expansion.

03

Ink viscosity

Ink viscosity also has a significant impact on dot expansion. It is well known that high-viscosity ink diffuses more slowly than low-viscosity ink and is less likely to flow before drying. When using ink with lower viscosity for flexographic printing, the ink often diffuses or flows before drying after transferring to the substrate, causing dot enlargement. Therefore, keeping the ink viscosity of flexographic printing within a reasonable range is also one of the key factors in preventing dot expansion.

04

Substrate materials

The surface properties of the substrate directly affect the display effect of printed dots. Smooth non-absorbent films and coated papers often have stable ink adhesion, low diffusion, and can precisely maintain the original dot shape; Conversely, substrate materials with stronger absorption and uneven surfaces, such as uncoated paper and corrugated board linings, will absorb ink like wicks, causing ink to diffuse along the fiber direction beyond the expected dot shape and pattern range, making dot enlargement more likely.

Although dot enlargement occurs occasionally in flexographic printing, its pattern is usually consistent and predictable. To address this issue, current prepress treatment technologies offer the following solutions:

(1) Professional software calibration: Image processing and color separation software can adjust dot size according to the printing press's input characteristics.

(2) Standardized testing: Printing factories usually conduct plate testing and analysis to provide precise dot expansion parameters for color separators or designers.

(3) Closed-loop calibration: The dot settings of color-separated film are optimized and adjusted based on the actual printing effect of the standard test drawing.

(4) RIP: Using calibration functions from specialized devices such as RIP (Raster Image Processor), dot changes can be controlled more precisely.

Flexographic printing block plate

In flexographic printing, ensuring high-quality reproduction of the original is a core requirement. However, in actual production, a typical issue affecting printing quality is often encountered-ink accumulation at the edges of dots, known as flexographic plate blocking. This phenomenon affects the clarity and sharpness of printed images, leading to a decline in print quality.

The so-called flexographic printing blockage phenomenon occurs during flexographic printing where, for various reasons, ink cannot be fully transferred onto the substrate, causing ink to accumulate at the top and shoulders of dots. When this accumulation reaches a certain level, a small dot print can become large due to excessive ink transfer, or several dots may irregularly connect together, making the surface of the printed product appear dirty, affecting image print quality.

This issue often occurs in shallow network cable sections, especially in highlight areas. Because the highlight areas have a lighter hue, if the ink is too high, it becomes very noticeable. This is a key reason why the highlight areas are difficult to handle in flexographic printing.

01

The number of mesh lines on the anilox roller does not match the number of printing plate lines

The key difference between flexographic printing and other printing methods lies in the anilox roller. The matching of the number of lines on the anilox roller and the number of lines on the printing plate is crucial to ensuring printing quality. When the dots in the highlight part of the printing plate touch the mesh holes on the screen pattern roller, their diameter is smaller than the opening diameter of the hole opening. As a result, the dots fall into the holes during printing. When the dots and holes separate, not only the surface of the dots but also the surrounding areas are coated with ink, transferring to the substrate, resulting in the highlight area being printed into a large area, forming a screen phenomenon.

Most of the above issues occur in high-line printing. The main way to improve this problem is to ensure that the mesh hole size of the anilox roller is smaller than the diameter of the smallest dot on the printing plate.

02

Excessive ink supply on the anilox roller

The anilox roller is a metering roller for ink transfer; the type of anilox roller used determines the ink amount applied. When the anilox roller transfers excessive ink, it causes excessive ink on the printing plate. If not properly managed, this ink can accumulate around the dots, creating a hidden risk of clogging the plate.

The remedy is simple but effective. Using anilox rollers that can transfer smaller volume inks can significantly alleviate this issue, ensuring that the right amount of ink enters the plate and ultimately reaches the substrate.

03

Excessive printing pressure

If the ink pressure is too high, the dots on the printing plate will deform under pressure, causing too much ink to adhere to the dots and inevitably spill outward. At this point, if the printing pressure is not properly increased, the likelihood of blockage greatly increases. Adjusting the pressure setting to an appropriate level can prevent excessive ink compression, thereby reducing ink buildup and ensuring high-quality print results.

04

The ink has relatively high viscosity

Ink viscosity plays a crucial role in the quality of flexographic printing. High ink viscosity affects the quality of ink transfer, leading to ink accumulation around the dot. Adjusting ink viscosity to a lower and more suitable level can significantly reduce the risk of ink accumulation and lower the likelihood of ink clogging occurring.