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Flexographic printing is widely used but faces two core issues: dot gain and plate blocking. These problems result in loss of image layers, reduced clarity and consistency, and negatively impact printing quality. This article examines the causes of dot gain and plate blocking and offers solutions through pre-press plate optimization, ink formula adjustment, and printing pressure control.

Printing Plate of Stack Type Flexo Press for Coffee Paper Cup

What is the Causes and Solutions for Dot Gain?

Dot gain is a common issue in flexographic printing. As ink is transferred from the plate to the substrate, it diffuses and causes the dots to expand, affecting printing details and color saturation. For instance, a 50% dot on the plate may expand to 51%, and on the substrate, it could expand to 65% or more. The extent of dot gain varies depending on the printing machine and substrate type.

Effects of Dot Gain

Causes of Dot Gain

The main causes of dot gain are the following three points: printing pressure, plate hardness, ink viscosity, and printing materials.

Printing Pressure

In flexographic printing, ink is transferred under light pressure, causing slight deformation and compression of the soft printing plate, which results in dot gain. Excessive printing pressure leads to greater plate deformation and more severe dot gain.

Printing Plate in the CI Flexo Printing Machine for Wallpaper

Plate Hardness

The hardness of the printing plate affects the degree of dot gain. A harder printing plate will deform less than a softer printing plate, and thus the dot gain will be lower.

Ink Viscosity

Ink viscosity significantly affects dot gain. High-viscosity inks diffuse more slowly and are less likely to flow before drying. In contrast, low-viscosity inks tend to diffuse or flow after being transferred to the substrate, causing dot gain.

Water-Based Ink for Flexographic Printing Press

Printing Materials

The surface characteristics of printing materials directly impact dot presentation. Non-absorbent films and coated papers with smooth surfaces have stable ink adhesion and low diffusion, preserving the original dot shape. In contrast, uncoated paper and corrugated cardboard, with stronger absorption and uneven surfaces, cause ink to diffuse along paper fibers, leading to increased dot gain.

Mengji’s Centeral Drum Flexo Press for Coated Papers

Solutions of Dot Gain

The solutions for dot gain are mainly divided into prepress processing technology optimization and control during printing.

Prepress Processing Technology Optimization

Professional Software Correction: Adjust dot size using image processing and color separation software based on the printing press characteristics.

Standardized Testing: Conduct plate testing and analysis to provide accurate dot gain parameters for color separation and designers.

Closed-Loop Calibration: Optimize dot settings on the color separation film based on actual printing results from a standard test chart.

RIP Calibration Function: Use RIP (Raster Image Processor) calibration functions to accurately control dot changes.

Raster Image Processor Software

Control During Printing

Reasonable Setting of Printing Pressure: Adjust printing pressure based on the plate and substrate characteristics to avoid excessive plate deformation.

Select the Appropriate Hardness of the Printing Plate: Use a thin cover layer (0.05~0.13mm) with higher hardness than the substrate, and secure the plate with mounting tape or cushion pads.

Control Ink Viscosity: Maintain ink viscosity within a reasonable range to prevent ink diffusion or flow.

Adjust the Process According to the Substrate: Modify ink formulas or printing processes for highly absorbent substrates to reduce ink diffusion.

Flexo Plate Cushion Pad

What is the Causes and Solutions for Plate Blocking?

Flexographic plate blocking occurs when ink fails to fully transfer to the printing material, causing ink to accumulate on the dot tops and shoulders. As the accumulation increases, small dots may enlarge or several dots may merge, leading to a dirty appearance and compromised print quality. This issue is most common in the shallow mesh layers, especially in highlight areas.

Causes of Plate Blocking

Plate Blocking is mainly caused by the mismatch between the anilox roller lpi (lines per inch)and the printing plate lpi, excessive ink supply to the anilox roller, excessive printing pressure, and high ink viscosity.

Mismatch Between Anilox Roller LPI and Plate LPI

The key difference between flexographic printing and other methods lies in the anilox roller. Matching the anilox roller lpi with the printing plate lpi is crucial for print quality. In the highlight areas, the dots on the printing plate are smaller than the anilox roller cell openings, causing the dots to fall into the cells. When the dots separate from the cells, ink stains the dot surface and surrounding areas, leading to ink accumulation on the printed material and causing plate blocking.

Printing Plates With Different LPI

Too Much Ink on the Anilox Roller

The anilox roller meters ink transfer. If it transfers too much ink, excess ink can accumulate around the dots on the printing plate, potentially causing blockage if not properly managed.

Excessive Printing Pressure

If the inking pressure is too high, the dots on the printing plate will be deformed due to pressure, causing too much ink on the dots and overflowing to the surrounding areas. At this time, the greater the printing pressure, the greater the possibility of ink blocking the plate.

Plate Blocking in Flexo Printing

High ink viscosity

Ink viscosity is crucial for flexographic printing quality. High viscosity can hinder ink transfer, leading to ink accumulation around the dots and increasing the risk of ink blocking.

Solutions of Plate Blocking

Matching the Anilox Roller and the Printing Plate: Ensure that the cell size of the anilox roller is smaller than the diameter of the smallest dot on the printing plate to prevent the plate dots from falling into the anilox cells and reduce the occurrence of blockage.

Control the Ink Supply of the Anilox Roller: Use an anilox roller that can transfer a smaller volume of ink to ensure that the right amount of ink enters the printing plate and eventually reaches the substrate to avoid ink accumulation around the dots.

Anilox Roller of Inline Flexo Press

Adjust the Printing Pressure: Set the printing pressure to a suitable level to prevent excessive squeezing of the ink, reduce ink accumulation, and ensure high-quality printing results.

Adjust the Ink viscosity: Adjust the ink viscosity to a suitable level to reduce the risk of ink accumulation and the possibility of plate blocking.

Dot gain and plate blocking are key factors influencing flexographic printing quality. By analyzing their causes and addressing them through pre-press plate optimization, ink formulation adjustments, and printing pressure control, these issues can be minimized, enhancing both the quality and stability of the printing process.