Brief Analysis of the Causes and Remedial Measures for Pinholes in Ink Films

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Pinholes are common defects in packaging printing ink films caused by poor leveling. After printing, the wet ink film retracts during the leveling process, forming small circular voids that expose the substrate or underlying ink layer. The primary cause of pinholes is the difference in surface tension between the upper and lower layers of the printed ink film. Some experts consider the wet ink film as a dual-layer liquid structure-upper and lower. During film formation, pinholes occur when the surface tension of the upper layer is lower than that of the lower layer, typically due to substrate wetting issues. Occasionally, before this phenomenon fully manifests, the wet ink film loses fluidity, preventing cratering. For example, in certain gravure or tinplate offset printing operations, rising temperatures may restore fluidity, potentially causing secondary pinholes.

From a theoretical perspective, the solution involves improving the flow and leveling properties of the wet ink film. The ink's surface tension should be low, while also ensuring good wettability of the substrate. Due to interfacial tension differences between the upper and lower wet ink layers, after printing, the system's surface tension decreases, prompting the ink to spread evenly into a continuous film. Specific corrective measures for shrinkage cavity defects include:

① Adding an appropriate amount of leveling agent to the ink;
② Using solvents with low surface tension;
③ Pre-wetting the substrate surface (e.g., wiping with a solvent before printing);
④ Controlling the temperature and humidity of the printing environment;
⑤ Selecting suitable additives;
⑥ Reducing the surface tension between the ink and the printing carrier to ensure good wettability and prevent the formation of tension gradients that lead to pinholes;
⑦ Adjusting the solvent evaporation rate, reducing viscosity, improving ink fluidity, and extending leveling time;
⑧ Forming a thin monolayer on the surface of the ink film to maintain stable and uniform surface tension.

To correctly select anti-pinhole agents for printing inks, the following provides a brief overview of commonly used types.

Solvent-based agents: Typically refer to high-boiling-point mixed solvents that serve both as effective solvents and excellent pigment wetting agents. However, solvent evaporation can degrade the solubility of the ink (film-forming) base, potentially causing craters, boiling marks, or blistering-particularly in baked tinplate offset inks.