How to choose and use UV ink well?

UV ink, with its vibrant colors, high gloss, and resistance to water, solvents, and wear, coupled with fast curing speed, is widely used in label printing. It's applied in various printing methods such as letterpress, flexography, and offset. Choosing and using UV ink well, especially fully understanding the factors affecting UV ink curing, determination methods, and solutions, is crucial for reducing printing problems and improving label printing quality.

01 The Application Status of UV Ink
1.  Uneven Quality of UV Ink
In the domestic label printing market, the quality of UV ink varies greatly. Common issues during printing, such as dry roller, ink piling, flying ink, and dirty plate, often stem from the use of low-end UV ink. These problems not only reduce printing efficiency but also result in label products that don't meet quality standards, such as poor ink adhesion, low gloss, and insufficient hiding power, with white ink having particularly low hiding power. To address these issues and meet the quality requirements of different materials, application fields, and printing processes, UV ink suppliers are continuously improving and launching new products.

2.  No Uniform Standard for UV Light Sources
The curing of UV ink is directly affected by UV light sources. However, there's no unified standard for the spectral range of UV light sources, leading to inconsistencies in the spectral range and power of available UV light sources. If the UV light source doesn't match the ink, UV ink may not cure thoroughly or may over-cure on the surface, both leading to adhesion problems.

02 How to Select UV Ink
Ink curing performance is key for UV printing quality. When selecting UV ink, ensure the UV lamp's emission wavelength matches the photoinitiator's trigger wavelength in the ink. Different UV ink models from various vendors have different curing speeds and adhesion properties. Generally, if black and white inks meet requirements, other colors won't pose significant issues, so focus on black and white inks.

1.  Performance Characteristics of UV Light Sources
UV light has a wavelength range of 200–600nm. However, ozone is generated at 254nm, and heat is produced above 400nm. Ozone pollutes the environment and harms workers' health, while heat can cause dimensional instability and even deformation of printed materials, as well as energy waste. Low-energy UV (HUV/LEC) lamps emit light in the 280–600nm range, avoiding the ozone band but still producing heat. In contrast, LED lamps offer a better solution. Single-wavelength LED lamps use wavelengths of 365nm, 385nm, and 395nm, avoiding both ozone and heat bands. With highly concentrated light energy, they enable efficient ink curing and safer use.

2.  Selecting UV Ink Based on Matching with UV Light Sources
UV ink curing requires UV light irradiation, and the photoinitiator in the ink must absorb the light's energy to generate free radicals. These free radicals facilitate the cross - linking reaction between resin and monomers in the ink, leading to curing. Traditional UV ink photoinitiators are triggered between 220–580nm, low - energy UV ink photoinitiators between 280–400nm, and LED - UV ink photoinitiators mainly absorb UV light between 365–405nm. When selecting UV ink, ensure its sensitive wavelength range matches the UV lamp's emission range and consider the actual curing energy of the UV curing system to determine the specific UV ink model.

3.  Selecting UV Ink Based on Black and White Ink Curing Performance
Color impacts UV ink curing speed due to different pigment absorption rates of various UV wavelengths. Among all colors, magenta ink cures the fastest, while black ink is the slowest. Although black theoretically absorbs all light and should cure quickly, pigment particles block light, hindering deep - layer curing. Thus, black and dark - colored inks require higher curing energy. White ink also needs high curing energy because its pigments reflect all visible light, making thorough curing difficult. If black and white inks in a series cure efficiently, other colors are likely acceptable.

03 Essential Knowledge for UV Printing
Selecting suitable UV ink is just the first step. Understanding its performance characteristics and factors affecting these properties is key to improving label UV printing quality. Here, we focus on composite color reproduction and determining whether the UV ink layer has fully cured.

1.  Why Composite Colors Differ in Printing
Strict color management is crucial for printing quality. During the process, even with the same Lab values for single - color inks, composite colors may vary. This is mainly due to different ink transparencies. In subtractive color mixing, the lower the transparency of the subsequently printed ink, the more it covers the previous color, and the more the composite color resembles the latter. For example, with similar Lab values for magenta inks from different vendors, composite colors can still differ significantly. Similarly, in special color matching, new inks from multiple colors have lower lightness but higher saturation, producing uniform and rich printed colors.

For instance, take the Lab value measurement of magenta ink from different vendors. Even if the Lab values of the magenta ink are similar, there can be significant differences in the composite colors obtained through overprinting. The principle is similar to that of special color matching. When multiple color inks are mixed to obtain a new ink, the resulting ink has a lower lightness but a higher saturation, making it easier to achieve a uniform and rich printed color effect.

2. How to Determine if UV Ink is Fully Cured
Whether UV ink is fully cured determines the adhesion, color reproduction, and gloss of the printed ink layer. It is therefore very important for production staff to quickly and accurately judge this on-site. In practice, people have summarized various testing methods, which can be categorized into three types: visual inspection, physical testing, and chemical testing.

Visual Inspection Method
The visual inspection method does not require any tools. It involves determining whether the UV ink layer has a shiny surface appearance. A fully cured UV ink layer will appear very glossy.

Physical Testing Methods
There are two commonly used physical testing methods.
Method 1:Place a piece of paper on the UV ink layer of the printed material and press it with your hand while rubbing the paper across the ink layer. If the ink layer is not fully cured, its surface may be damaged during this process.
Method 2:Scratch the surface of the UV ink layer lightly with a pin, then observe the scratch under a 200x magnifying lens. If the scratch appears jagged, it indicates that the UV ink layer has fully cured.
Chemical Testing Method

The chemical testing method requires the use of a solvent such as acetone. Drop some acetone onto the UV ink layer—if the ink layer dissolves, it indicates that the ink has not been fully cured.

Above, the author has shared key points regarding the selection and usage of UV ink from three perspectives: current application status, selection criteria, and essential knowledge. We hope this information is helpful to you.

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