1 UV application in printing

In recent years, photosensitive resins that utilize light energy to produce changes in physical properties have been continuously developed and rapidly applied to printing inks, paints, adhesives, or photoresist inks. Along with this, the widely used UV technology is expanding into many fields. The UV irradiation device is completely different in irradiation method and irradiation condition depending on the type of the object to be irradiated and the conditions of the printing and coating.

Ultraviolet light is usually called UV, UV is Ultra Violet, it belongs to electromagnetic wave (light wave), generally refers to electromagnetic waves in the range of 10nm~400nm. Compared with visible light, photons have high energy and are irradiated onto the object to be irradiated. The energy can cut off the chemical bond or chemically bond.

The main advantages and disadvantages of UV inks used in printing are as follows:

Advantages: Instant curing of the ink (increased production efficiency), no spray powder;

The strength of the ink film is much higher than that of the oily ink;

Odorous gas emissions are small (no solvent).

Disadvantages: high ink prices;

Use special printing materials (ink roller, printing plate, cleaning agent, etc.);

The ink is irritating to the skin.

2 types of UV lamps

The UV lamp can be roughly classified into a low-pressure discharge lamp, a high-pressure discharge lamp, and an ultra-high pressure discharge lamp according to the pressure inside the lamp when lighting. Typical high pressure discharge lamps are described here, including mercury lamps and metal halide lamps.

1) Mercury lamp

The emitted light wave has a wavelength range of 200 to 450 nm, a dominant wavelength of 365 nm, and a strong line spectrum at 254 nm, 313 nm, 405 nm, and 436 nm.

2) Metal halide lamp

According to the difference of the iodide added to the seal in the lamp, it can be basically classified into Fe system (type A) and Ga system (type B).

Although the Fe-based metal halide lamp has substantially the same irradiation energy distribution as that of the mercury lamp, the irradiation efficiency is improved at 365 nm, and the irradiation efficiency is not as good as that of the mercury lamp at 300 nm or less.

The Ga system is designed to have a strong irradiation energy in the vicinity of 400 nm.

According to the material of the light-emitting tube, it can be divided into an ozone-type metal halide lamp and a non-ozone-type metal halide lamp. UV light of a wavelength below 220 nm encounters oxygen in the air to generate ozone. Since the material of the ozone-type metal halide lamp is a common quartz material, light having a wavelength of 220 nm or less can also pass. The non-ozone-type metal halide lamp is made of a special quartz material, and it is prohibited to pass light of wavelengths below 230 nm, so that ozone generation can be controlled. [next]

3 UV lamp selection

The shorter the wavelength of the UV light, the shallower the penetration depth, and the longer the wavelength, the deeper the penetration depth. In addition, the shorter the wavelength, the greater the energy.

When selecting a UV lamp, it is first necessary to confirm the main absorption wavelength of the UV-curable coating such as ink, paint, and binder used. Generally, a glaze lamp containing no pigment component is selected as a mercury lamp; an ink containing a pigment component is selected as a type A metal halide lamp; and a B-type metal halide lamp is often used for plate making and the like.

In the case of using a multi-turn UV lamp, in order to increase the surface tackiness of the coating material after UV curing, the printing surface may be irradiated with a mercury lamp after being irradiated with an A-type metal halide lamp.

In addition, ozone-type metal halide lamps which are advantageous for UV curing are generally used, and occasionally non-ozone type UV lamps are used (i.e., when ozone can adversely affect paints and substrates).

The above practice is in the general case. In recent years, various cured coatings have been developed, and different raw materials and curing conditions may occur differently from the above general conditions. In any case, the corresponding UV lamp must be selected according to the curing material used.

4 UV irradiation device

The UV irradiation device mainly comprises: 1 UV lamp; 2 UV irradiation lamp holder with UV reflector for reflecting UV lamp (reflector can effectively improve the irradiation effect of UV light on the illuminated surface); 3 power supply device for starting UV lamp; 4 Cooling device. In addition, a handling device as an accessory device is sometimes included.

1) UV irradiation lamp holder

The role of the UV illuminating lamp holder has been as described above. The choice of UV irradiation lamp holder should be determined according to the installation conditions of UV lamp, the intensity of UV light, the amount of integrated light, the temperature condition of the object to be irradiated, the processing speed, and the constraints on the setting of the UV device by the accessory equipment. What kind of reflector, illumination method, cooling method, and whether or not the shutter is configured (the role is to prevent UV light from being irradiated onto the substrate during standby).

The configuration of the UV device varies greatly depending on the irradiation conditions and the illumination cooling method. The types of reflectors are described later.

2) Power supply unit

There are two types of power supply units, namely the common copper-iron transformer power supply unit and the inverter power supply unit.

The former generally has a large weight and a large volume, while the latter has the characteristics of light weight, small volume, and continuous adjustment of light quantity, but is feared of dust and dust due to the use of electronic components.

3) Cooling device

The cooling device differs depending on the UV lamp and power source used, but the UV lamp operates at a high surface temperature and generates a large amount of heat. If the UV lamp and the irradiation lamp holder are not cooled, thermal deformation, breakage, and shortened life are required. Therefore, forced cooling is required.

Different cooling methods have different heat dissipation effects on the irradiated objects, and the cost is also different. The cooling method can be divided into the following three types. Regardless of the cooling method, each has its own advantages and disadvantages.

1) Air cooling method

The way in which the lamp and the lamp holder are vented or blown by the air pump (Fig. 4). In the case of blowing air cooling, a partial exhaust (emission odor, hot air) device is sometimes designed.

Advantages: Simple maintenance and low equipment.

Disadvantages: Sometimes it is necessary to find another countermeasure to control the temperature rise of the irradiated object.

2) Water cooling method

This is a way of inventing a sleeve on a UV lamp and indirect cooling by circulating cooling water through the casing. Although there is also a way of non-circulating cooling water, in recent years, cooling water circulation has generally been adopted. In order to prevent scale formation in the casing, the cooling water uses pure water. If scale adheres to the inner wall of the casing, the transmittance of UV light is significantly reduced. A visible cut filter is sometimes placed in the sleeve.

Advantage: Since the heat energy is absorbed by water, it is possible to suppress an increase in the temperature of the object to be irradiated.

Disadvantages: Due to the need to manage cooling water, equipment investment costs are higher than the other two methods.

3) Air-water cooling method

Water flows through the reflector holder to reduce the temperature around the lamp; in addition to the air pump air cooling method, a combined cooling method is formed (Fig. 6).

Advantages: It is possible to achieve miniaturization of the UV irradiation lamp holder.

Disadvantages: Compared with the water cooling method, there is no need for water cooling equipment, and the equipment cost is increased compared with the air cooling method. [next]

5 types of reflectors

Similar to the case of selecting an appropriate UV lamp, the reflector must also be selected depending on the curing material used, the heat resistant condition of the object to be irradiated, the use, and the like.

1) Aluminum mirror mode

Reflectors made of high-brightness aluminum with high reflectivity can be used in both air-cooled and water-cooled modes. Although it is used for general printing with high ink curing efficiency, the aluminum mirror reflector is most disadvantageous in temperature reduction of the object to be irradiated in the air cooling mode, and thus cannot be used for an irradiated substrate whose surface temperature must be lowered.

2) Cold mirror mode

This is a reflector made of a metal oxide film coated on glass, reflecting UV light and allowing a portion of visible light to pass. The reflector can be used to obtain high-intensity UV energy while reducing the temperature of the object to be irradiated.

3) Cold filter mode

This is a method in which a glass coated with a multilayer metal film (which can cut off infrared rays and visible light to pass UV light) is attached to the lower portion of the socket to suppress an increase in temperature of the object to be irradiated. When used in combination with a cold mirror method or the like, the temperature of the object to be irradiated can be further reduced. If there is installation space, it is easy to install as an optional component. However, unlike the reflector, the cold filter is placed in the vicinity of the object to be irradiated, and the ink mist and the gas from the object to be irradiated are easily attached thereto.

4) Metal cold mirror method

This is a reflector made of multi-layer coating on a metal reflector, which reflects UV light and absorbs light of unnecessary wavelength. It is suitable for applications where the cold mirror method cannot meet the requirements (high output low temperature irradiation). This method can obtain high ultraviolet irradiation intensity and reduce the temperature of the object to be irradiated, but the water cooling device is indispensable because the corresponding cooling method is limited to air-water cooling.

5) Double cold mirror mode

In this mode, the UV lamp is not directly illuminated, and the substrate is only exposed to indirect light from the UV lamp. In the air cooling mode, the reflector of this mode is most effective for controlling the temperature rise of the substrate. While the effect of lowering the temperature is significant, since there is no direct light, the intensity of UV irradiation is also greatly reduced. This is a method employed for substrates (heat sensitive paper, etc.) that must be irradiated at a low temperature. [next]

6 Comparison of temperature rise of irradiated objects

Comparison of the above different reflectors and cooling methods with corresponding temperature rises. However, in terms of temperature conditions, it is desirable to find a perfect standard because of differences in conditions such as the shape of the reflector and the irradiation distance.

7 Clarity of the conditions of the irradiated object

As described above, the introduction of the UV irradiation device must first be clear about the conditions (material, withstand temperature, size) of the object to be irradiated, processing conditions (printing speed, printing method, etc.), UV performance (UV intensity, integrated light amount), and the like. . Next, determine the UV device specifications: determine the configuration specifications based on the actual performance basis and existing data, etc., or perform a large number of tests for uncertain factors before determining the specifications.

In addition, in terms of the measuring instrument for the intensity of UV irradiation and the amount of integrated light, there is a difference in spectroscopic sensitivity due to differences in manufacturers and models. This difference occurs even when measured under the same irradiation conditions. Therefore, when the UV irradiation intensity and the integrated light amount necessary for ink curing are to be specified in advance, the manufacturer and model of the measuring instrument must be determined.

8 Determination of the specifications of the UV irradiation device

The UV light wavelength, the UV lamp output power, and the number of lamps are determined by the web of the object to be irradiated and the processing speed. In order to increase versatility, a combination of a mercury lamp and a metal halide lamp is generally used.

In addition, it is necessary to consider the temperature of the irradiated object, the size of the mounting position of the irradiated lamp holder, the irradiation distance, etc., in order to determine which reflector and cooling method to use. Even if it is based on the temperature condition of the object to be irradiated, the air cooling method can be used, but the water cooling method may have to be adopted due to the relationship of the installation space of the lamp holder.

When the UV irradiation lamp holder cannot be placed close to the substrate due to the installation space factor and the irradiation distance is too long, in order to avoid a large drop in the UV irradiation intensity, it is necessary to consider selecting the corresponding reflector; on the contrary, when the substrate is Irradiation can also be lengthened if it cannot withstand high temperatures. As a measure for heat resistance of a substrate, a cold air blower may be used for cooling. In many cases, the shutter is provided in the UV irradiation lamp holder. When the UV lamp is not turned off during standby, the UV light is not emitted on the substrate, but the UV irradiation lamp holder with the shutter is large, which will have a great impact on the installation space. .

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