Digital printing has become a trend. Currently, more and more label printing companies are putting investment in digital printing equipment on the agenda. However, facing the endless variety of digital label printing equipment in the market, how should label printing companies choose equipment that is suitable for their own needs? This article provides a comparative analysis of the principles of digital printing, the properties of consumables, and the adaptability of products for your reference. It is hoped that it can help label printing companies purchase the desired label digital printing equipment.

According to imaging principles, the mainstream digital printing technologies in the market can be divided into two categories: digital toner electrostatic imaging technology and digital inkjet imaging technology.

Digital toner electrostatic imaging technology

Digital toner electrostatic imaging technology can be divided into two types: dry toner technology (mainly represented by the Saikang CX series equipment) and wet toner (electronic ink) technology (mainly represented by HP equipment).

1. Composition and characteristics of dry carbon powder

Dry toner typically consists of the following components: (1) pigments, colorants used to achieve the desired color; (2) Resin, mainly polyester, is a high molecular organic polymer that is solid at room temperature. This resin surrounds pigment particles and forms the main part of the carbon powder; (3) Filler, a charge control agent dispersed in the resin, to accelerate or, if necessary, slow down the charging rate and maintain the charging characteristics of carbon powder and adhesive additives; (4) Surface additives or external additives to further improve the performance of carbon powder; (5) Additives for specific applications to achieve the special properties and characteristics of carbon powder.

The particles of dry carbon powder are relatively fine, ranging from 6 to 9 μ Between m, usually size 8 μ M. When using dry toner printing, once the image is transferred to the substrate, heat is released to fuse the toner with the substrate. Heat will cause the carbon powder particles to solidify (i.e., the resin melts), forming a uniform solid polyester film.

Under normal circumstances, the average thickness of a single layer image printed with dry toner is about 4 μ M. By adjusting the amount of imaging light without any impact on production efficiency, thicker image layers can be achieved, usually for opaque white ink layers or color 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 label digital printing machines on the market include: (1) pigments, colorants used to achieve the desired color; (2) Modified polyethylene resin with low glass transition temperature, resembling rubber at room temperature. During the manufacturing process, the pigment is rubbed into the polyethylene resin, and the pigment particles are reduced by fragmentation, forming a unique star shaped carbon powder particle; (3) Carrier liquid, a mineral oil, can be partially dissolved in the pigment resin liquid due to its high chemical compatibility with polyethylene resin, changing the viscoelastic properties of the resin, so that it can be transferred to the final substrate in a molten state; (4) Organic dispersant deposited on the surface of wet carbon powder particles, used to stabilize and charge the carbon powder particles (due to the addition of metal salt complexes); (5) Additives, additional components added to the carrier liquid, can ensure the electrical neutrality of the toner system when charged toner particles move to the photosensitive drum.

The pigment particle size of wet toner (electronic ink) is about 2 μ m. It is much smaller than dry toner. Normally, the thickness of a single layer image printed with wet toner is about 1.5 μ M.

During the printing process, before transferring the image to the substrate, heating is required, the carbon powder melts, most of the carrier liquid evaporates, and the carbon powder particles solidify to form a uniform flexible film on the substrate. After image transfer, the evaporation process continues, and the remaining carrier liquid will evaporate completely within a few days, allowing the polyethylene polymer to return to its normal state at room temperature.

Digital inkjet imaging technology

The commonly used ink for digital printing equipment using digital inkjet imaging technology includes UV and water-based ink.

1. Composition and characteristics of UV ink

The typical composition of UV inkjet ink mainly includes: (1) pigments, ground to below 150nm, used for stabilizing dispersants to maintain long-term stability of dispersions; (2) Carrier liquid, an active solvent, usually acrylate, contains monomer (chemicals with simple molecular structure, which can combine with other similar molecules to form polymers), and is a mixture of photoinitiator and synergist; (3) Monomers with single functional active vinyl groups can be selected from a wide range of candidate materials to ensure good adhesion, flexibility, weather resistance, shrinkage, etc; (4) Monomers with bifunctional active vinyl groups (acrylic acid or Enol ether) can ensure effective curing; (5) Photoinitiators and synergists provide ink sensitivity to a range of wavelengths, ensuring good curing performance of the entire ink layer, as oxygen in the air slows down the curing rate of the substrate surface or ink layer surface. (6) Surfactants control the static and dynamic surface tension of ink, ensuring the formation of uniform ink droplets (without satellite ink droplets), and ensuring good, fast, and controllable wetting effect when ink droplets reach the surface of the substrate.

Under the influence of UV light, ink is solidified, which means that the photoinitiator forms free radicals and undergoes cross-linking reactions with other ink components (monomers) to form a polymerized or cured film. After the cross-linking reaction is completed (i.e. all components are cross-linked), the ink achieves complete drying.
The thickness of a single layer image printed with UV ink is approximately 4-6 μ M. Due to the chemical composition required for curing, UV ink has a higher viscosity compared to water ink, but at the same time, it is about 6 times lower in viscosity than UV offset or UV flexographic ink. This can have several consequences, which will be further discussed below.

2. Composition and characteristics of water-based ink

The water-based ink used in existing label digital printing machines on the market usually consists of the following parts: (1) water-based carrier media, which constitutes 60% to 90% of the ink's composition; (2) Pigment, a colorant used to achieve the desired color, dispersed in a carrier; (3) Dispersant is a dispersion that stabilizes pigments for a long period of time; (4) Moisturizing agent to prevent the moisture in ink from evaporating when the nozzle is not closed or idle; (5) Surfactants promote the formation of ink droplets (without forming satellite ink droplets) and improve the wettability of non paper substrates; (6) Fungicides that prevent biological growth; (7) Buffer, controlling the pH value of the ink (CO2 dissolved in the air will affect the pH value of the ink); (8) Other additives, such as chelating agents, defoamers, and solubilizers.

After complete drying, the thickness of a single layer image printed with ink is usually 0.2-0.4 μ M. Due to the lowest viscosity of ink, it is more suitable for high-speed inkjet printing. However, low viscosity water-based inks also have a drawback, which is that they cannot guarantee sufficient dispersion of heavy particles, such as TiO2 in white ink, making it difficult to achieve sufficient dispersion.