How To Solve The Problem Of Pinholes Caused By Poor Leveling Of The Ink Film During Packaging Printing

Shrinkage is one of the defects that occurs in packaging printing ink films due to poor leveling. After ink is printed, the wet ink film may retract during the leveling process, forming small round spots that expose the substrate material (bottom) or the previous ink layer. The primary reason for shrinkage is the difference in surface tension between the upper and lower parts of the printed ink film. Some believe that a wet ink film can be regarded as a double-layer liquid ink film. During film formation, shrinkage occurs when the surface tension of the upper wet ink layer is lower than that of the lower wet ink layer, due to the interaction with the substrate material. Sometimes, just as this phenomenon is about to occur, the wet ink film loses its mobility, which coincidentally suppresses the occurrence of shrinkage. For example, in certain gravure or offset printing operations on metal, as the temperature rises, the wet ink film regains mobility, possibly resulting in secondary shrinkage.

Theoretically, the remedy is to improve the flow and leveling of the wet ink film. The ink's surface tension should be low, and the substrate material should have good wettability. Because there is a surface tension difference at the interface between the upper and lower wet ink layers, the system's surface tension decreases after packaging printing, allowing the ink to spontaneously spread into a complete film. The specific measures to address shrinkage defects are:

① Properly add leveling agents to the printing ink;

② Use solvents with low surface tension;

③ Treat the substrate surface to improve wettability (e.g., wipe the substrate with a solvent before printing);

④ Control the environmental temperature and humidity during printing operations;

⑤ Select appropriate dispersing agents;

⑥ Reduce the surface tension between the printing ink and the substrate to ensure good wettability and prevent a surface tension gradient that causes shrinkage;

⑦ Adjust the solvent evaporation rate, reduce viscosity, improve ink flow, and extend leveling time;

⑧ Form an ultra-thin monomolecular layer on the surface of the packaging printing ink film to provide stable and uniform surface tension.

To correctly select anti-shrinkage agents for printing inks, various types of anti-shrinkage agents can be chosen:

Resin type: Generally includes cellulose acetate butyrate, especially shrinkage-resisting resins of polyacrylate with limited compatibility. Usage amounts are: acrylic resin 1%–1.5%, epoxy resin 0.5%–1%, polyester resin about 1%.

Organosilicone resin: Its functions include preventing shrinkage, leveling, anti-floating color, and defoaming. It helps promote ink flow, leveling, and gloss.

In addition to the above anti-shrinkage agents, there are non-silicone fluorinated surfactants, which are high-molecular surfactants. They are also very effective in preventing ink film shrinkage, enhancing leveling, and improving the gloss of printed (graphic) ink films.

Preventing shrinkage of packaging ink films is an important measure to improve the decorative effect of ink on packaging. Using different anti-shrinkage and leveling agents aims to enhance print quality and extend the grade of the printed (surface ink film).

Although there are many types available, their usage is limited. Inappropriate types or dosages may not achieve the desired effect and may even cause side effects.

Years of practice have shown that before use, different types and dosages should be rigorously screened and tested to determine the best type and most suitable dosage. Additionally, batch comparison tests should be conducted to select the most scientific process flow, including the methods of adding the agents during the printing process. Only in this way can the quality of packaging printing products in China be continuously improved, and a more abundant theoretical knowledge be accumulated through practice.