UV curing refers to the process by which UV inks and UV varnishes undergo a cross-linking and curing reaction under the radiation of UV lamps, thereby drying the printed material. When using UV inks and UV varnishes, a UV curing unit is required. When printing thick ink layers, built-in UV curing units need to be installed between each printing press unit. Sometimes, to ensure thorough drying of the printed material, a UV curing unit also needs to be installed in the extended delivery drying section.
1. Traditional UV curing equipment
Traditional UV curing equipment contains one or more mercury lamps with wavelengths ranging from 100 to 380 nm. Common mercury lamps generate high temperatures and ozone during operation, therefore UV curing equipment is also equipped with cooling and ozone removal devices. To fully utilize the energy of the mercury lamps and protect personal safety and the environment, the entire UV curing equipment is enclosed in a reflective chamber.
In the UV curing unit of a sheet-fed offset printing press, a gas mercury lamp with an output power of 100-120 W/cm is typically used. The energy utilization rate of this mercury lamp is only 25%, with the remaining 50% converted into infrared light and 25% into visible light. To prevent the printing material (paper) from overheating or coming into contact with the radiation device, a cooling device is required, and a protective plate (usually an infrared filter coated with quartz glass) is installed below the UV curing unit.
Traditional UV lamps typically use aluminum or glass reflectors. However, Heidelberg's DryStar3000 UV curing system and KBA's UV curing units now employ URS-coated reflectors that reflect blue light. The URS coating reflects UV light and absorbs infrared heat, which is then dissipated through a water-cooling system. Compared to aluminum or glass reflectors, this significantly reduces heat radiation to the substrate and prevents ink and dirt from burning on the reflector surface, thus maintaining its reflectivity and performance stability.
2UV-Quickstart Mobile Belt Curing Unit
Kuhnast Radiation Technology GmbH of Germany has launched the UV-Quickstart moving belt curing unit. Initially used on the RYOBI 520 offset printing press, it can also be used on Heidelberg GTO 52, PM 52, SM 52 offset printing presses and Komori Lithrone 20 offset printing presses. This UV curing unit is equipped with 1-2 UV lamps with a power of 181W/cm². Traditional UV curing units require 5-10 minutes to operate after startup and need to remain operational throughout the printing process. In contrast, the UV-Quickstart moving belt curing unit is operational within 3 seconds of startup, and the entire system shuts down when printing is interrupted. Experiments show that when printing the same number of prints, the actual operating time of the UV-Quickstart moving belt curing unit is reduced by more than half compared to traditional UV curing units. This saves energy consumption, reduces maintenance costs, and allows for the replacement of water-cooling systems with air-cooled systems.
3 Excitation Material Radiation Device
To address the drawbacks of mercury lamps, such as low energy efficiency and the generation of large amounts of infrared radiation and ozone, excitation radiation devices (low-temperature UV curing devices) have been developed. An excitation radiation device is a special curing apparatus that uses a monochromatic UV lamp (its main wavelength is 308nm). This type of device does not produce infrared light, the substrate is not heated, thus improving energy efficiency, and it does not produce ozone. Currently, a problem with excitation radiation devices is their low power; to improve the curing effect, the curing process needs to be carried out in an inert gas (such as nitrogen). Furthermore, the photoinitiator of the ink used in conjunction with it must be matched to the wavelength of the light emitted by the excitation radiation device.
Since current excitation radiation devices require an inert gas (such as nitrogen), curing necessitates a sealed space. However, achieving a sealed space is difficult in sheet-fed offset printing presses due to the presence of grippers. Therefore, this curing device is more easily implemented in web offset printing presses.
