In flexographic printing, the viscosity of water-based inks is one of the critical factors affecting print quality and the smoothness of the printing process. Excessively high or low viscosity can impact ink fluidity, coating uniformity, image and text clarity, and the operational stability of the printing press. The viscosity range of flexographic water-based inks typically needs to be determined based on different printing processes, printing substrates, and specific ink formulations.

1. Viscosity Range of Water-based Inks
   The viscosity of flexographic water-based inks usually falls between 15-40 seconds (Ford Cup 4), depending on application requirements and ink types. For different application areas, the common viscosity ranges are as follows:

For Zhongzhixing flexographic water-based inks used in packaging printing such as plastic films: The viscosity is generally between 20-30 seconds (Ford Cup 4). This viscosity range suits most standard flexographic printing presses and common substrates like paper or plastic films.

For high-quality printing or printing with high detail requirements: To ensure clear images and sharp edges, the viscosity may be slightly lower, typically between 15-18 seconds. This maintains ink fluidity and avoids blurred or uneven patterns.

For large-format or rougher substrates: For rougher substrates (such as corrugated paper, certain non-woven fabrics, etc.), a slightly higher viscosity is required to ensure uniform ink coating and sufficient adhesion. The viscosity may range from 22-30 seconds.

2. Methods for Controlling Water-based Ink Viscosity
   Maintaining appropriate viscosity is crucial for ensuring smooth flexographic printing. The following are commonly used methods for controlling and adjusting viscosity:

• Temperature Control
• Temperature has a significant impact on the viscosity of water-based inks. As the temperature rises, the viscosity of water-based inks decreases; conversely, as the temperature drops, the viscosity increases. To maintain stability during the printing process, a constant temperature is usually maintained in the printing workshop.

Excessively high temperatures: Decreased viscosity may lead to excessive ink fluidity, resulting in unclear printing or even ink overflow.
Excessively low temperatures: Increased viscosity may lead to insufficient ink fluidity, causing clogging, stringing, and other issues during printing.

Control Methods:
Use constant temperature equipment (such as heaters, constant temperature tanks) to keep the ink within an appropriate temperature range.
Ensure the temperature control system of the printing press is stable to avoid excessive viscosity changes due to temperature fluctuations.

• Water Addition
• The water content in water-based inks directly affects their viscosity. Appropriate water addition can reduce ink viscosity and improve ink fluidity.

Control Methods:
Adding water: Adjust the viscosity by controlling the amount of water added. Too much water can lead to excessive ink fluidity, affecting image clarity; too little water can make the ink too viscous, causing uneven coating.
Using water-based diluents: Using specialized water-based diluents can more precisely control viscosity, avoiding other performance changes caused by simply adding water.

• Thickeners
  When ink viscosity is too low, it may lead to excessive ink fluidity, making it difficult to control the coating effect. In such cases, thickeners are used to increase viscosity, ensuring printing stability and image and text clarity.

Common thickeners:
Natural gum thickeners: Such as hydroxypropyl methylcellulose (HPMC).
Synthetic thickeners: Such as polyurethane thickeners or acrylate thickeners.

• Rheological Control Agents
  Rheological control agents can not only adjust the viscosity of inks but also control their rheological properties during the printing process, ensuring uniform ink coating.

Leveling agents: Improve ink fluidity and reduce coating unevenness during printing.
Anti-sagging agents: Adding anti-sagging agents to high-viscosity inks prevents ink sagging during high-temperature or long-duration printing.

• Continuous Monitoring and Adjustment
  Since the viscosity of inks may fluctuate over time and due to external factors during flexographic printing, it is necessary to regularly test the viscosity of inks and adjust them as needed.

Control Methods:
Regularly use a viscometer (such as Ford Cup) to test ink viscosity.
Monitor ink fluidity in real-time during the printing process and add water or adjust other additives as needed.

3. Relationship Between Water-based Ink Viscosity and Print Quality
   Too low viscosity: Can result in ink being too thin, unable to provide sufficient coverage, and may lead to issues such as uneven ink layers, blurred details, and ink overflow.
   Too high viscosity: Can lead to poor ink fluidity, potentially causing scraping, incomplete printing, and blurred patterns.

This is about the viscosity control of flexographic water-based inks. In actual printing, the viscosity of flexographic water-based inks needs to be precisely controlled based on actual conditions to ensure the stability of the printing process and high-quality output.