In plastic printing, controlling the viscosity of the ink is crucial. Especially in high-speed rotary gravure printing, whether the ink's viscosity is properly managed directly affects key quality issues such as pigment transfer, plate contamination, print gloss, ink adhesion, and static electricity. These factors significantly influence the final appearance, yield, and overall production efficiency. To ensure consistent quality, it's essential to understand how ink viscosity interacts with these common problems.
(1) Ink Viscosity and Pigment Transfer
Ink used for plastic film printing typically consists of organic solvents, resins, pigments, and additives. The effectiveness of pigment transfer during printing largely depends on the ink's viscosity. Research has shown that optimal performance occurs when the ink viscosity falls within a specific range (11–24 seconds using a 3# Zahn cup). Within this range, higher viscosity can lead to poor pigment transfer, as the ink becomes overly saturated. This causes pigments to clump together, making it difficult for them to flow into the printing cells. When this happens, the pigment cannot be transferred effectively, leading to what is known as "blocking." To avoid this, the ink system should remain in a saturated or unsaturated state, allowing pigments to disperse evenly and maintain a stable colloidal structure. This ensures smooth pigment movement in and out of the cells, improving print quality.
Some manufacturers suggest maintaining ink viscosity between 15–18 seconds (3# Zahn cup), but in high-speed gravure printing (typically 100–260 meters per minute), an ideal viscosity range of 11–15 seconds is often preferred to ensure long-term stability and better productivity.
(2) Ink Viscosity and Plate Contamination
Plate contamination is a common issue in gravure printing of plastic films. It occurs when the non-printing areas of the plate cylinder become stained with resin and pigment, resulting in unwanted marks on the printed product. This problem is closely linked to ink viscosity, along with blade hardness, pressure, contact angle, and the condition of the plate cylinder. If the ink is too viscous, the pigments and resins do not disperse well, leading to buildup. As the blade moves rapidly across the cylinder, this buildup can cause damage, resulting in broken lines or smears. Additionally, when the resin is in a supersaturated state, it tends to stick to the plate cylinder under blade pressure, causing contamination. Therefore, lower ink viscosity generally reduces the risk of plate contamination.
(3) Ink Viscosity and Print Gloss & Ink Adhesion
If the ink viscosity is too low, it indicates a high concentration of organic solvents and a relatively low amount of resin and pigment. This can prevent the formation of a smooth, even film after drying, resulting in a dull or white appearance. For prints requiring higher gloss, a higher viscosity (around 13–19 seconds using a 3# Zahn cup) is usually recommended, ensuring sufficient resin and pigment content to produce a glossy finish.
The adhesion of the pigment to the plastic film also depends on the ink's viscosity. A balanced ratio of resin and pigment is necessary to ensure strong adhesion. Too much or too little of either component can negatively affect the final result. Maintaining an appropriate viscosity helps achieve optimal pigment bonding and durability.
(4) Ink Viscosity and Static Electricity
Under normal humidity conditions, static electricity is rarely an issue when the ink viscosity is above 16 seconds (using a 3# Zahn cup). However, if the viscosity drops below this level, static phenomena such as whiskers, streaks, edge repulsion, flying ink, and irregular water stains may occur. These issues can worsen over time, affecting both print quality and machine performance.
To manage static electricity, it’s not always necessary to increase the ink viscosity. Instead, solutions like maintaining proper environmental humidity, adjusting the diluent composition, installing anti-static brushes, or using antistatic agents are often more effective. These methods help reduce static while maintaining print quality and production efficiency.
　 The content above is excerpted from the first issue of 2008, "Printing and Packaging," and other related journals. For more information, please refer to the journal section.
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