Solutions to Bubbles, Doctor Lines, and Scumming in Gravure Printing

In the gravure printing process, issues such as bubbles, doctor lines, and scumming often plague production personnel. Many users suspect the ink itself is the problem, but overlook key factors like ink formulation, equipment parameters, and substrate compatibility. Based on Shunfeng's years of industry experience, these problems often require a multi-dimensional comprehensive solution, rather than relying solely on ink adjustments. Below are the causes of these issues and corresponding solutions.

Printing Conditions

Problem Manifestation: A large number of bubbles are generated in the ink during printing, leading to poor doctor blade performance, dot loss, and even foam splashing that contaminates the printed product. At the same time, irregular doctor lines appear on the print surface, or solid areas show scumming and blurred text.
Solutions: Optimize the ink circulation system to reduce the ink return height and minimize air entrainment; add an appropriate amount of defoamer to eliminate bubbles; adjust the doctor blade angle to 60-65°, ensuring the blade edge is sharp and pressure is uniform; increase ink viscosity, optimize the matching between printing speed and drying speed to avoid scumming caused by excessive drying.

Ink

Ink Formulation and Dispersion Performance:
The dispersion and leveling properties of ink largely depend on the resin's ability to wet and encapsulate pigments. Bubble problems are usually caused by trapped air in the ink or improper addition of surfactants in the formulation. Doctor lines and scumming are mostly related to high ink viscosity, coarse pigment particles, and insufficient dispersion. In water-based systems, more pigment particles can easily lead to doctor blade wear and scumming. For example, nitrocellulose resins tend to produce doctor lines at high viscosities, while acrylic resins provide better leveling properties.
Solutions:
Select appropriate resins and dispersants, optimize the pigment dispersion process to reduce interparticle forces, thereby enhancing ink flowability and anti-foaming capabilities. For inks prone to doctor lines, add an appropriate amount of dispersant to improve pigment wettability, or replace with finer ink. For scumming issues, add a retarder to slow down drying and adjust the diluent ratio.

On Substrate

Adhesion and Wetting Performance:
In gravure printing, the surface tension of the substrate directly affects the wetting and spreading of the ink. Low surface tension can cause ink to bead up, forming pinholes or bubbles. Impurities or uneven treatment on the substrate surface can lead to doctor lines and scumming. Based on our application experience, we recommend the following wetting agents to address wetting issues on different substrates.
Solutions:

• For high surface tension substrates (such as treated PP and PE films), use standard substrate wetting agents.

• For low surface tension substrates (such as untreated BOPP), when standard wetting agents are insufficient, high-performance fluorocarbon-based substrate wetting agents can be used.

• For particularly difficult-to-wet substrates (such as poorly treated PET), we recommend silicone-based substrate wetting agents.