Analysis of Blocking Issues in Plastic Gravure Printing
I. Causes of Blocking: Blocking is primarily caused by insoluble substances such as solidified components, high-viscosity materials, or foreign particles clogging the screen holes in the ink.
Ink Solidification on the Plate: Residual ink in the screen holes can partially solidify due to various factors, such as increased ink viscosity. When the printing plate cylinder re-enters the ink tank, the solidified ink in the screen holes is challenging to dissolve completely, leading to a reduction in ink transfer rate. Over time, the screen holes become increasingly blocked, resulting in blocking issues. This type of blocking is related to factors like ink type, viscosity, solvent, drying speed, printing speed, and the construction of the printing machine.
Solvent Drying Too Fast: Rapid drying of the solvent can lead to the formation of a film on the ink before it transfers to the substrate, causing poor re-dissolvability and accumulation over time.
Direct Blowing of Hot Air from the Drying Box: Structural issues in the gravure press, such as the direct blowing of hot air onto the plate surface, can cause ink to solidify within the screen holes.
Inclusion of Impurities in the Ink: The high-speed nature of gravure printing, especially on flexible substrates like film, can generate static electricity, attracting dust and debris from the environment into the ink. Mechanical impurities become a direct cause of blocking in the screen holes.
Use of Filters: Installing a metal mesh filter (80–120 mesh) before the circulation pump can filter out insoluble impurities and prevent blocking.
Low Plate Quality: The gravure plate undergoes processes like surface corrosion, engraving, aluminum plating, and grinding. Irregularities produced during these processes, such as uneven walls in the screen holes or Ω-shaped holes, can affect ink transfer and lead to blocking.
Corrosion Techniques: Utilizing corrosion processes that modify the shape of screen holes can reduce dot loss, minimizing the chances of blocking.
Poor Ink Dissolution (Deterioration): Changes in ink flow, turbidity, or the introduction of water during printing can impact ink flow and cause pigment re-agglomeration, resulting in blocking.
Chemical Changes in Reactive Inks: Two-component or reactive inks can undergo chemical reactions over time, leading to increased viscosity and decreased ink transfer rates.
II. Strategies for Resolving Blocking Issues: Based on the analysis above, here are strategies for resolving blocking issues in plastic gravure printing:
Cleaning with Solvents: When blocking occurs, use solvents or specialized cleaning agents for thorough cleaning. Analyze and address the specific causes of blocking.
Optimal Printing Environment: Maintain suitable temperature and humidity in the printing workshop. Ensure that the drying speed of the solvent aligns with the printing speed and environmental conditions.
Short Distance Between Doctor Blade and Impression Roller: Keep the distance between the doctor blade and the impression roller as short as possible to prevent direct blowing of hot air onto the plate surface.
Use of Slow-Drying Solvents: Incorporate slow-drying solvents and adjust the printing speed accordingly to prevent premature drying of the ink.
Timely Replacement of Ink: Replace ink promptly when defects in ink performance are detected. Avoid combining inks with significantly different densities.
Regular Stirring of Ink: Regularly stir the ink in the ink tank to keep it in a fluid state. Add new ink or replace it in a timely manner to prevent ink film formation.
Addressing Plate Quality Issues: If blocking is caused by plate quality, consider re-plating or re-engraving to ensure smooth screen holes.
Pre-Printing Cleaning: Before starting or just after starting the press, use solvents to wipe the printing plate cylinder. Avoid unnecessary stops during printing, and clean the plate thoroughly if a long stop is anticipated.
Warm-Up of Fresh Ink: If crystals form in fresh ink due to temperature differences, warm the ink before use (40–50°C) to dissolve the crystals.
Proper Use of Solvents: Use proper, specialized dilution solvents. Thoroughly clean the ink tank and circulation pump when changing ink to avoid contamination.
By implementing these strategies, one can effectively address and prevent blocking issues in plastic gravure printing, ensuring a smoother and more efficient printing process.