As a printing consumable tailored to the flexographic process—characterized by anilox-cell metering and high-speed, low-pressure printing—the formulation of flexographic ink directly determines its transfer efficiency, curing performance, and final print quality. Under increasingly strict environmental regulations and diversified application requirements, ink systems have evolved from traditional solvent-based formulations toward more eco-friendly and functionalized systems.

A scientifically designed ink formulation ensures smooth ink transfer, strong adhesion, and regulatory compliance even at printing speeds of 200–300 m/min. This article breaks down the four core components of flexographic ink, explains their mechanisms, and provides guidance for proper selection.

1. Core Component: Resin

Resin is the film-forming component of flexographic ink, typically accounting for 30–50% of the formulation. It determines adhesion, flexibility, and weather resistance of the ink film. Flexo’s low-pressure printing and wide substrate compatibility require resins with low viscosity and flexible chain segments.

Main Resin Types and Application Scenarios

• Polyurethane Acrylate Resin: The mainstream choice for film substrates. With flexible polyether/polyester segments and polar groups, it offers strong adhesion on PP and PE films (up to 5B cross-cut), ink film elongation ≥20%, and excellent fold resistance. Ideal for soft packaging and nonwoven applications.
• Acrylic Resin: Available in water-based and solvent-based versions. Water-based acrylics have very low VOCs (≤15 g/L), making them suitable for food packaging. Their viscosity can be adjusted to 100–200 mPa·s for anilox transfer. Solvent-based grades provide better alcohol resistance for personal-care labels.
• Polyamide Resin: Used for special polar substrates such as nylon film and aluminum foil. Hydrogen bonding improves adhesion, but the cost is higher, making it suitable for premium laminated packaging.
• Bio-based Resin: An eco-friendly option made from modified soybean oil or rosin. The share in some high-end inks has reached 35%, with a carbon footprint 40% lower than conventional resins.

Key resin-selection principle: PU acrylate for non-polar films, water-based acrylic for food packaging, and polyamide for special polar substrates.

2. Core Component: Pigments

Pigments (10–25%) provide color and opacity. Particle size, dispersion, and lightfastness directly affect color saturation and printing stability. The anilox transfer mechanism requires fine, well-dispersed pigments to avoid cell-clogging.

2.1 Coloring Pigments

• Organic Pigments: Includes azo (yellow/red) and phthalocyanine (blue/green). With particle sizes of 100–300 nm, they offer vibrant colors, high transparency, and excellent dot reproduction up to 175 LPI. Lightfastness is typically 3–4 but can reach ≥5 with UV absorbers.
• Inorganic Pigments: Includes titanium dioxide, carbon black, and iron oxides. With larger particle sizes (500–1000 nm), they deliver strong hiding power and excellent lightfastness, suitable for solid areas. However, they provide lower color saturation.
• Food Packaging Compliance: Pigments must meet standards such as GB 4806.14-2023. Heavy metals (Pb, Cd) must not exceed 0.1 mg/kg. For example, modern phthalocyanine blue pigments have migration levels ≤0.008 mg/kg per SGS testing.

2.2 Extender Pigments

Also known as fillers (e.g., calcium carbonate, talc), used at ≤5%. They help adjust viscosity and enhance abrasion resistance. Excessive addition may cause graying and reduced adhesion.

3. Core Component: Solvents / Diluents

Solvents or diluents (20–40%) regulate ink viscosity (typically 100–300 mPa·s) and drying speed. With growing environmental requirements, the industry has shifted from traditional solvent systems toward water-based and UV-reactive diluent systems.

Three Main Systems

• Water-based System: The mainstream choice today. Deionized water accounts for 60–80%, supplemented by trace food-grade co-solvents. VOCs are ≤12 g/L. Drying relies on water evaporation with 45–60°C hot air. The best option for food packaging and children’s products.
• Solvent-based System: Now declining. Common solvents include ethyl acetate and isopropanol with VOCs of 50–100 g/L. Fast drying (1–2 seconds) but high environmental impact. Mainly used for industrial packaging with no strict environmental requirements.
• UV-Reactive Diluent System: Preferred for UV flexo. Uses diacrylates such as TPGDA or DPGDA. Zero VOCs and ultra-fast curing (≤0.3 seconds) under UV or LED-UV. Ideal for high-speed label production lines.

Key selection rule: Water-based for food packaging, UV-reactive for high-speed presses, solvent-based only for low-cost industrial applications.

4. Core Component: Additives

Though used in small amounts, additives are critical for solving printing defects such as foaming, poor leveling, or insufficient adhesion.

• Defoamers: Typically polyether-modified siloxanes added at 0.1–0.3% to eliminate bubbles during anilox transfer and prevent pinholes.
• Leveling Agents: Acrylic copolymers added at 0.5–1% to reduce surface tension, improve lay-down uniformity, and avoid orange peel or anilox patterns.
• Adhesion Promoters: Silane coupling agents added at 1–2% to enhance adhesion on non-polar substrates (PE/PP), improving cross-cut performance from 3B to 5B.
• Functional Additives: Antibacterial agents, antistatic agents, or UV absorbers to expand application scenarios and enhance durability.

5. Component Synergy

Flexographic ink performance results from the synergy of all components rather than any single ingredient. For example:

• PU acrylate resin + titanium dioxide + water-based solvent + defoamer → ideal for PE food packaging films.
• Acrylic resin + organic pigments + UV-reactive diluent → suitable for personal-care labels requiring bright colors and fast curing.

Conclusion

Flexographic ink is an integrated system of resins, pigments, solvents, and additives. Proper selection and proportioning of each component must align with printing processes, substrate characteristics, and application requirements. As environmental and performance demands increase, future flexographic inks will move toward higher bio-based resin content, wider use of low-migration pigments, and integrated multifunctional additives.