Halo Effect in Flexo Printing

This article is part of the #FlexoFlawFixing series

Flexo printing has carved out a niche in the high-speed printing industry thanks to its versatility and efficiency. However, perfecting flexo printing requires a thorough understanding of its potential flaws.

Flexo printing often faces a challenging defect known as the halo effect. It appears as an unwanted line or ring. This occurs around the screen dot where ink spreads too far. The article explores the basic causes of this effect. It also provides practical remedies to reduce its occurrence. The goal is to ensure pristine print quality.

Causes and Remedies of Halo Effect

Excessive Pressure

• Cause: Excessive pressure between the print cylinder and the substrate forces ink beyond the intended print area, creating the halo effect.
• Remedy: The solution is to reduce the applied pressure to a level that prevents the ink from spreading, while still providing sufficient contact for high-quality prints.

Uneven Pressure

• Cause: Uneven pressure distribution across the substrate causes uneven halo effects that affect print consistency.
• Remedy: Regularly monitor and adjust pressure across the impression cylinder to ensure consistent application and eliminate areas of excessive force.

Incorrect Pressure Settings

• Cause: Incorrect pressure settings, especially between the plate cylinder and substrate or the anilox roller and plate cylinder, can contribute to the halo effect.
• Remedy: Accurately calibrate pressure settings in these areas to maintain optimal ink transfer and print fidelity.

Excessive Hardness of Printing Forms

• Cause: The rigidity of printing forms can exacerbate ink spread and contribute to the halo effect.
• Remedy: Choosing softer printing forms and carefully selecting the hardness of the mounting tape can significantly reduce the risk of unwanted ink spread.

Cylinder and Web Speed Harmony

• Cause: A mismatch between the speed of the impression cylinder and the web can distort the printed image, causing variations similar to the halo effect.
• Remedy: Ensuring synchronous operation of the impression cylinder and web prevents ink misalignment and maintains the integrity of the printed design.

Unaligned Form Carriers

• Cause: Inconsistencies in the concentricity of the form carrier (sleeve or cylinder) can affect the distribution of ink on the substrate.
• Remedy: Regularly checking and adjusting the alignment of the form carrier will help ensure even ink application and reduce the halo effect.

Excessive Ink Transfer

• Cause: Excessive ink transfer, often caused by anilox cells that are too large in volume or low-viscosity ink, causes the ink to spread beyond the target area.
• Remedy: Selecting an anilox roller with an appropriate cell volume and adjusting the ink viscosity is critical to controlling ink transfer and preventing halo.

Conclusion

Flexo printing, while efficient, is susceptible to the halo effect, an imperfection that significantly degrades print quality. Understanding the root causes is key to prevention. These include excessive pressure and uneven pressure distribution. Ink transfer issues and speed mismatch are also critical.

By monitoring and adjusting press mechanics, high-quality flexo prints can be achieved. Adjusting ink transfer conditions is also necessary for quality prints. Remaining vigilant and adaptable is important in flexo printing. This ensures that halo and other defects are minimised. Such vigilance preserves the final product’s integrity and appeal.

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