What Factors Will Affect The Printing Effect Of UV Ink On Paper?
Apr 23, 2025
1. The decisive role of paper properties on printing effects
2. Control of core parameters of UV ink formula
3. Coordinated optimization of printing equipment and processes
4. Precise control of UV curing conditions
5. Systematic influence of environmental factors
1. The decisive role of paper properties on printing effects
Ink absorption: The pore structure and surface polarity of paper determine the absorption capacity of ink. For loose paper (such as newsprint), it has larger and more pores inside, which form a capillary-like structure. When UV ink contacts the surface of newsprint, the binder in the ink will quickly penetrate into the paper driven by capillary action. This rapid absorption process can accelerate the fixation of the ink, so that the ink can reach a certain degree of dryness on the paper surface more quickly. However, due to the large amount of rapid absorption of the binder, the pigment particles in the ink are relatively more retained on the surface of the paper, and cannot form a uniform and tight arrangement, which may cause the gloss of the ink layer to decrease, presenting a duller visual effect.
However, compact paper (such as coated paper) has a coated surface with small and evenly distributed pores. During the printing process, the fine pores of coated paper allow low-molecular binders to penetrate into the paper, while high-molecular components are difficult to enter the pores due to their larger molecular size, and thus remain more on the surface of the paper. These polymer components retained on the surface undergo a curing reaction under the action of UV light to form a smooth, continuous and dense ink film, which makes the coated paper prints have high gloss and bright color performance.
In addition, for some non-absorbent film substrates (such as plastic films), due to their low surface polarity, it is difficult for ink to wet and adhere to their surfaces. Corona treatment is an effective improvement method. By applying a high voltage to the surface of the film, a corona discharge phenomenon is generated, the molecular structure of the film surface changes, and polar groups are introduced to increase the polarity of the film surface. In this way, the ink can better wet and spread on the surface of the corona-treated film, enhance the bonding force between the ink and the substrate, and improve the printing quality.
Surface roughness:
The surface roughness of paper has a significant effect on the printing effect of UV ink. The rough surface has more microscopic protrusions and depressions. When the ink contacts the paper, these protrusions can mechanically bite with the ink, so that the adhesion of the ink on the paper surface is enhanced. However, this rough surface also has certain disadvantages. When printing dots, the ink will fill more in the concave part, causing the dots to expand during the printing process, making the details of the printed images blurred and affecting the clarity of the printed products. When printing in the field, due to the uneven surface roughness, the distribution of ink on the paper surface will also be uneven, and the local ink layer may be too thick or too thin, resulting in inconsistent colors of the field printing, affecting the overall printing effect.
Taking uncoated paper as an example, its surface is relatively rough. In the printing process, in order to balance the transfer efficiency and detail restoration, the printing pressure and ink viscosity need to be reasonably adjusted. Appropriately increasing the printing pressure can make the ink better fill the concave part of the paper surface and improve the transfer efficiency of the ink, but excessive pressure may cause excessive expansion of the dots. At the same time, adjusting the ink viscosity is also very critical. Higher ink viscosity can reduce the diffusion of ink on the paper surface and help maintain the shape and size of the dots, but too high viscosity may cause difficulty in ink transfer and uneven ink layer.
pH value:
The pH value of paper is one of its important chemical properties, which has many effects on the printing effect of UV ink. Acidic paper (pH < 7) contains a certain amount of acidic substances, which will inhibit the oxidation filming process of the ink. In the drying process of traditional inks, oxidation filming is an important drying mechanism, and the acidic environment will slow down this process, thereby prolonging the drying time of the ink. For UV inks, although its main drying method is UV light curing, the acidic environment may still interfere with the curing reaction of the ink and affect the curing effect.
Alkaline paper (pH > 7) may cause ink discoloration or paper embrittlement. Alkaline substances may react chemically with certain components in the ink, causing the color of the ink to change, affecting the color accuracy of the printed product. At the same time, the alkaline environment will act on the paper for a long time, which will destroy the structure of the paper fiber, make the paper brittle, and reduce the physical strength and durability of the paper.
In the offset printing process, the pH value of the paper will also have an important impact on the balance of the fountain solution. The fountain solution plays an important role in offset printing. It can keep the blank part of the printing plate hydrophilic and prevent the ink from adhering to the blank part. However, the pH value of the paper will affect the pH balance of the fountain solution. If the paper is acidic, it may increase the acidity of the fountain solution and cause the ink emulsification. Ink emulsification refers to the mixing of ink and fountain solution to form an emulsion, which will reduce the viscosity and transferability of the ink, affect the printing quality, and cause problems such as light ink color and blurred dots. If the paper is alkaline, it may cause the alkalinity of the fountain solution to increase, making the ink adhesion on the printing plate worse, and causing sticky dirt, that is, traces of ink appearing in the blank part of the printed product.
Stiffness and thickness:
The stiffness and thickness of the paper are also important factors affecting the printing effect of UV ink. Thin paper is easy to deform during the printing process due to its thin thickness and relatively low physical strength. For example, in the process of multi-color overprinting, thin paper may bend, warp and other deformations due to factors such as printing pressure and ink drying shrinkage, which will cause inaccurate overprinting of subsequent colors and overprinting deviation, making it impossible to accurately align the graphics of the printed product, affecting the quality of the printed product.
Thick paper, due to its thickness, absorbs UV light more strongly. During the UV curing process, UV light needs to penetrate the ink layer and the surface of the paper to transfer energy to the photoinitiator in the ink, thereby initiating the curing reaction. When the paper is thicker, UV light will be absorbed and scattered in the process of penetrating the paper, resulting in a decrease in the UV light energy reaching the bottom ink. If the curing energy is not increased accordingly, the bottom ink may not be completely cured, thus affecting the ink's adhesion, wear resistance and other properties. Therefore, when printing on thick paper, it is usually necessary to increase the power of the UV lamp or increase the curing time to ensure that the bottom ink can get enough energy to cure and obtain a good printing effect.
2. Control of core parameters of UV ink formula
The chemical composition and rheological properties of ink directly determine the printability and curing effect.
2.1. Viscosity and thixotropy:
Viscosity affects the ink transfer efficiency and dot clarity. Low-viscosity inks (such as offset UV inks) are suitable for high-speed printing, but may cause water marks or light colors; high-viscosity inks (such as screen UV inks) are conducive to thick ink layer accumulation, but need to be matched with high-hardness scrapers to avoid plate sticking. Thixotropic inks thicken when standing, which can reduce ink dripping during printing.
2.2. Pigment concentration and dispersibility:
High-concentration pigments (such as white and black) absorb more UV light, resulting in incomplete curing. For example, opaque white ink is more difficult to cure than black, and the UV energy needs to be increased or the ink layer thickness needs to be reduced. Poor pigment dispersion may cause knife lines or hue deviations, which need to be optimized by dispersants.
2.3. Photoinitiator and resin system:
The absorption spectrum of the photoinitiator needs to match the UV light source. The type of resin determines the flexibility and chemical resistance of the ink layer. Acrylic resins cure quickly but are more brittle, while polyurethane modified resins can improve adhesion.
2.4. Addition of additives:
Leveling agents improve the flatness of the ink film, and inhibitors prevent the ink from curing prematurely during storage. Adhesion promoters (such as silane coupling agents) can enhance the chemical bonding between the ink and the paper.
3. Coordinated optimization of printing equipment and process
Equipment performance and operating parameters directly affect the transfer accuracy and curing efficiency of ink.
3.1. Printing method adaptation:
Offset printing: Suitable for high-precision graphics, it is necessary to control the water-ink balance to avoid UV ink emulsification.
Flexo printing: Quantitative ink supply through anilox roller, suitable for large-area solid printing, but attention should be paid to the matching of UV lamp power and printing speed.
Screen printing: Thick ink layer (10-12μm) can be achieved, but high-viscosity ink needs to be preheated to improve fluidity.
3.2. Ink supply system:
In gravure printing, the number of lines of the anilox roller and the shape of the ink hole determine the amount of ink transfer. High-line anilox rollers (such as 1200lpi) are suitable for fine printing, and low-line rollers (such as 300lpi) are used for solid color blocks.
3.3. Pressure and speed:
Insufficient printing pressure will lead to insufficient ink transfer, and excessive pressure may crush the paper fibers. When the printing speed is too fast, it is necessary to increase the UV lamp power or increase the number of curing units to ensure complete curing.
4. Precise control of UV curing conditions
The curing process is a key link in determining the performance of inks, and needs to be optimized from three aspects: energy, wavelength, and environment.
1. Selection of UV light source:
Mercury lamp: full spectrum output, suitable for multi-color inks, but high energy consumption and short life (about 1000 hours).
UV LED: narrow spectrum (such as 395nm), low energy consumption, long life (20,000 hours), but needs to match the ink photoinitiator, and may cause adhesion to decrease due to the characteristics of cold light source.
2. Energy management:
UV energy needs to be adjusted according to the type of ink and the thickness of the ink layer. For example, paper printing usually requires 800-1200mJ/cm², while film printing may require more than 1500mJ/cm². Insufficient energy leads to incomplete curing, and excessive energy may cause ink embrittlement or deformation of the substrate.
3. Curing environment:
Oxygen inhibits free radical polymerization, and the curing efficiency can be improved by nitrogen purging or reducing the oxygen content in the environment. In addition, high temperature environment (>30℃) may accelerate ink curing, but paper deformation should be prevented.
5. Systematic impact of environmental factors
Environmental variables such as temperature, humidity, dust and static electricity need to be included in the whole printing process control.
5.1. Temperature and humidity control:
Temperature: The viscosity of ink decreases with the increase of temperature, and 20-25℃ is the ideal working range. Too high temperature may cause ink to flow, and too low temperature may cause gelation.
Humidity: High humidity (>70% RH) may cause paper to absorb moisture and deform, affecting the accuracy of overprinting; low humidity (<30% RH) is prone to static electricity, adsorbing dust and interfering with ink transfer.
5.2. Cleanliness management:
The printing workshop needs to maintain a low dust level to avoid dust embedding in the ink layer to form white spots. Pollution can be reduced through air filtration systems and regular cleaning.
5.3. Static electricity elimination:
Static electricity may cause ink splashing or paper adhesion. Ion fans or humidification (humidity > 50% RH) can be used to reduce the risk of static electricity.
