Solutions for Ink Stripping and Dropping in UV Offset Inks

Despite the high viscosity and low fluidity of Uv Offset Inks, their viscosity can significantly decrease and fluidity increase after prolonged high-speed operation in high-temperature or low-humidity environments, leading to ink droplet splatter. When ink is excessively emulsified, the moisture content can affect the UV transmittance, reducing the bonding strength of the film layer. Sometimes, even if the ink appears cured, it may still contain moisture, resulting in the ink layer being pulled off when rubbed or when adhesive tape is used on printed patterns. Therefore, it is essential to control the room temperature, humidity, printing speed, and emulsification of the ink properly.

Ink Stripping Issues:

1. If the surface dyne value of the substrate is too high, it can affect the ink's adhesion, leading to stripping.
2. Ineffective drying of the ink surface during intermediate drying stages can cause stripping in subsequent printing.
3. Insufficient content of photoinitiators in the ink can also lead to this issue.

Ink Dropping Issues: This typically occurs in the white ink unit. To achieve a smoother white ink surface, mixing a thinner white ink may lead to leakage from the ink fountain, directly dripping into the fountain solution and causing contamination.

Addressing Severe Ink Emulsification Issues: When printing aluminum foil cardboard with UV offset ink, the printed dots may appear slightly rough and unclear, primarily due to an imbalance in water and ink. Compared to solvent-based inks, Uv Inks are supplied more slowly and printed in thinner layers, exacerbating the emulsification problem. Ink accumulation on the inking rollers and residue on the plate and blanket can blur the images. Adjusting the fountain solution concentration and ambient temperature can significantly impact ink emulsification. Reducing the printing speed to 5000-8000 impressions per hour and re-formulating the fountain solution can resolve this issue.

Regarding Ink Stripping and Dropping Phenomena: Although UV offset inks have high viscosity and low fluidity, prolonged operation in high-temperature or low-humidity conditions can lead to a significant decrease in viscosity and increased fluidity, resulting in droplet splatter. When ink is overly emulsified, the moisture content affects the UV transmittance, consequently reducing the bonding strength of the film layer.

Insufficient Ink Adhesion: Materials such as films, synthetic paper, metals, and plastics have poor permeability and weak ink adhesion, so even after UV curing, the ink may not adhere well, impacting print quality. Typically, surface treatment of these materials is necessary to improve ink adhesion, ensuring UV offset inks adhere effectively.

Incomplete Ink Curing Issues: In the UV offset printing process, greater printing pressure and thicker ink layers are often used to enhance color saturation, which complicates the curing of the ink. To ensure color fidelity and effective curing, it is advisable to slow down the printing speed. Adjusting the distance between the UV lamp and the substrate and increasing UV exposure intensity can accelerate the ink curing process, while also considering the potential impact of ink emulsification on curing.

Improper Color Sequence Issues: In UV offset printing, since the ink layer on the substrate is generally thin, achieving a thicker ink layer can be challenging. In four-color overprinting, the conventional sequence of black, cyan, magenta, and yellow may lead to reduced inking capability for subsequent colors and insufficient total ink volume, resulting in unsaturated colors. Taking overprint factors into account, it is better to print colors with smaller ink volumes first, followed by the larger black color, to maximize ink volume and color saturation. Care should be taken with the concentration of black ink to avoid excessive amounts that may affect the curing and adhesion of other colors.

Solutions: Conduct UV curing after the final color printing to minimize the effects of substrate expansion. In special cases, intermediate color groups may also briefly activate the UV lights, but exposure time should be kept as short as possible. Alternatively, the use of cold UV technology may be considered.