Basic Principles and Practical Applications of UV Inks
1. How does viscosity affect printing?
Viscosity is critically important in inkjet printing. To achieve optimal performance, the ink viscosity must remain within a defined range. If the viscosity is too high, the ink will clog the nozzles; if the viscosity is too low, air will flow back into the nozzles, causing ink leakage.
Maintaining ink viscosity within a specific range ensures maximum optical density without the occurrence of ink flowing or leaking.
2. Relationship between temperature and viscosity
An increase in ink viscosity is usually caused by operating the inkjet printer in a cold environment. When the ink cools down, it becomes thicker and more viscous.
A simple solution is to increase the ambient temperature and keep the workspace above 20°C.
Low temperatures also weaken the ink’s adhesion to the substrate, resulting in dot gain or dot spreading.
3. How do excessively high or low viscosity levels affect printing?
* If viscosity is too high:
You will begin to notice reduced print density because fewer ink droplets are ejected. If the ink becomes too thick, droplets cannot exit the nozzles at all, causing printing to stop.
* If viscosity is too low:
Ink will flow out of the printhead, and air may be drawn back into the ink chamber, causing print defects or interruptions.
4. Relationship between surface tension and inkjet printing
Surface tension refers to the attractive force between molecules on the surface of a liquid. Water molecules cluster tightly, forming a hemispherical shape.
Pure water has high surface tension (about 75 dyn/cm), whereas many organic solvents have low surface tension (around 25 dyn/cm). This is why solvent inks generally perform better.
However, water-based inks can still be used because water’s hydrogen bonding causes high surface tension. By adding "wetting agents" to water-based inks, hydrogen bonding is disrupted, reducing surface tension so the ink can wet the substrate properly. Alcohol-type additives can also reduce surface tension in water-based inks.
5. What is added to water-based laminates (protective films)?
Water-based laminates contain alcohol components.
Alcohol has three functions:
1. Reduces surface tension
2. Improves compatibility between water-based and solvent-based laminates
3. Accelerates drying
Ink clogging rate and ink formulation factors
Since neutral inks must not chemically react with media, they must rely on the coating layer to adhere.
Water-based pigment inks contain UV-resistant additives to improve outdoor durability.
However:
👉 The amount of UV additive is directly proportional to the nozzle clogging rate.
If UV content is too high, clogging will occur.
Therefore, pigment inks require a UV-protective laminate to maintain outdoor durability. UV additives must stay within a controlled ratio to avoid clogging.
Once pigment inks achieve water resistance, they can typically last up to 8 months outdoors without clogging, but beyond that, a UV coating is required.
Solvent inks require reduced UV additives to avoid clogging, so their outdoor durability often does not exceed 2 months.
Weather resistance (Lightfastness) differences between dye and pigment inks
* Dye inks have much lower weather resistance than pigment inks.
* Magenta is the weakest
* Cyan, Yellow, and Black are stronger
* Pigment inks have excellent weather resistance, typically rated 6–8 on the lightfastness scale.
However, besides the colorant type, the ink formulation itself is key.
Adding the following can significantly improve weather resistance:
* Heat stabilizers
* UV absorbers
* Antioxidants
* Free radical scavengers
The coating material used on the media also affects weather durability.
Environmental conditions such as humidity, sunlight exposure duration, the direction the print is facing, and local climate all affect outdoor lifespan.
Therefore:
👉 Weather-resistance guarantees can never be absolute, as too many environmental variables are involved.