In-depth Analysis! Toner vs. Inkjet Imaging Technology: Do You Really Understand the Difference?
Digital printing has become an unstoppable trend. At present, more and more label printing companies are putting investment in digital printing equipment on their agendas. However, faced with the constantly emerging types of digital label printing equipment in the market, how should label printing companies choose equipment that suits their own needs? This article provides a comparative analysis from multiple aspects such as the principles of digital printing, the characteristics of consumables, and product adaptability for reference, hoping to help label printing companies purchase the desired digital label printing equipment.
According to imaging principles, mainstream digital printing technologies in the market can be divided into two categories: digital electrostatic toner imaging technology and digital inkjet imaging technology.
Digital Electrostatic Toner Imaging Technology
Digital electrostatic toner imaging technology can be further divided into dry toner technology (mainly represented by Seiko CX series equipment) and wet toner (electronic ink) technology (mainly represented by HP equipment).
1. Composition and Characteristics of Dry Toner
Dry toner typically consists of the following components:
(1) Pigments, which serve as colorants to achieve the desired color;
(2) Resin, mainly polyester, a high-molecular organic polymer that is solid at room temperature. This resin surrounds the pigment particles and forms the main body of the toner;
(3) Fillers, charge control agents dispersed in the resin, which accelerate or, if necessary, slow down the charging speed and maintain the charging properties of the toner and adhesion aids;
(4) Surface additives or external additives, which further improve the performance of the toner;
(5) Additives for specific applications, which provide the toner with special properties and characteristics.
The particles of dry toner are relatively fine, ranging from 6 to 9 μm, with a typical size of 8 μm. When printing with dry toner, once the image is transferred onto the substrate, heat is applied to fuse the toner with the substrate. The heat causes the toner particles to solidify (i.e., the resin melts), forming a uniform solid polyester film.Under normal conditions, the average thickness of a single-layer image printed with dry toner is about 4 μm. Without affecting production efficiency, a thicker image layer can be achieved by adjusting the imaging light dose, typically applied to opaque white ink layers or colored layers that require tactile thickness.2. Composition and Characteristics of Wet Toner (Electronic Ink)The main components of wet toner (electronic ink) used in existing digital label printers in the market are as follows:(1) Pigments, which serve as coloring agents to achieve the desired color;(2) Modified polyethylene resin with a low glass transition temperature, rubber-like at room temperature. During manufacturing, pigments are kneaded into the polyethylene resin, then broken down to reduce the pigment particle size, forming characteristic star-shaped toner particles;(3) Carrier liquid, a mineral oil, which partially dissolves into the pigment resin liquid due to its high chemical compatibility with polyethylene resin, altering the resin's viscoelastic properties so it can be transferred in a molten state to the final substrate;(4) Organic dispersants deposited on the surface of wet toner particles, used to stabilize and charge the toner particles (due to the addition of metal salt complexes);(5) Additives, additional components added to the carrier liquid to ensure the electrostatic neutrality of the toner system as charged toner particles move to the photoconductor drum.The pigment particles of wet toner (electronic ink) are about 2 μm, much smaller than dry toner. Typically, the thickness of a single-layer image printed with wet toner is about 1.5 μm.
During the printing process, before transferring the image onto the substrate, heating is required to melt the toner. Most of the carrier liquid evaporates, and the toner particles solidify, forming a uniform flexible film on the substrate. After the image transfer, the evaporation process continues, and any remaining carrier liquid will completely evaporate within a few days, allowing the polyethylene polymer to return to its normal state at room temperature. Digital Inkjet Imaging TechnologyDigital printing equipment using digital inkjet imaging technology commonly uses two types of ink: UV ink and water-based ink.1. Composition and Characteristics of UV InkTypical UV ink for inkjet printing mainly includes:(1) Pigments, ground to below 150 nm, stabilized with dispersants to maintain long-term stability of the dispersion;(2) Carrier liquid, an active solvent, usually an acrylate, containing monomers (chemicals with simple molecular structures that can combine with other similar molecules to form polymers), which is a mixture of photoinitiators and enhancers;(3) Monomers with a single functional active vinyl group, selected from a wide range of candidates to ensure good adhesion, flexibility, weather resistance, and shrinkage properties;(4) Monomers with a bifunctional active vinyl group (acrylate or enol ether), which ensure effective curing;(5) Photoinitiators and enhancers, providing the ink with sensitivity to a range of wavelengths to ensure good curing performance throughout the ink layer, as oxygen in the air can slow the curing rate on the substrate or ink layer surface;(6) Surfactants, which control the static and dynamic surface tension of the ink, ensuring uniform ink droplets (without satellite droplets) and good, fast, and controllable wetting when the droplets reach the substrate.Under UV light, the ink is cured, with photoinitiators generating free radicals that react with other ink components (monomers) to form cross-linked polymers or a cured film. Once the cross-linking reaction is completed (i.e., all components are cross-linked), the ink is fully dried.
The thickness of a single-layer image printed with UV ink is approximately 4–6 μm. Due to the chemical components required for curing, UV ink has a higher viscosity compared to water-based ink, but at the same time, its viscosity is about six times lower than that of UV offset or UV flexographic inks. This leads to several consequences, which will be discussed further below.2. Composition and Characteristics of Water-Based InksWater-based inks used in commercially available digital label printers typically consist of the following components:(1) Water-based carrier medium, which makes up 60%–90% of the ink;(2) Pigments, which provide the desired color and are dispersed within the carrier;(3) Dispersants, which stabilize the pigment dispersions for long periods;(4) Humectants, which prevent the water in the ink from evaporating when the printhead is not sealed or is idle;(5) Surfactants, which facilitate droplet formation (preventing satellite droplets) and improve wetting on non-paper substrates;(6) Biocides, which prevent microbial growth;(7) Buffers, which control the ink's pH (dissolved CO2 in the air can affect the ink's pH);(8) Other additives, such as chelating agents, defoamers, and solubilizers.Once fully dried, the thickness of a single-layer image printed with water-based ink is typically 0.2–0.4 μm. Because water-based ink has the lowest viscosity, it is well suited for high-speed inkjet printing. However, the low viscosity of water-based inks has a drawback: it cannot ensure adequate dispersion of heavy particles, such as TiO2 in white ink, making thorough dispersion difficult.