UV Curing

For UV inks to cure properly, they must not only be exposed to the right wavelengths of UV energy, but also to the right amount of energy delivered at the correct intensity. The amount of energy arriving at the surface of the printed substrate is called the dose and is measured in millijoules (mJ)/sq cm. The dose of energy a UV print receives is affected by the conveyor speed and number of lamps in the curing unit, as well as the number of times that it is exposed to the curing environment (such as in the case of multicolor UV prints).

The intensity of energy emitted by curing lamps is known as irradiance and is measured in watts (or milliwatts)/sq cm. Irradiance is directly related to electrical power, lamp condition, and the geometry of the reflector that directs and focuses the lamp output. Irradiance does not vary with exposure time.

The depth of cure achieved in the ink film is directly influenced by the irradiance level of the lamp. Delivering higher, more intense energy at the surface of the ink will allow more energy to penetrate the thickness of the ink film.

In curing units, the lamp and reflector assembly that delivers the UV light is call an irradiator. The lamps used in curing units generally consist of mercury vapor contained in a transparent quartz envelope. The lamps are powered in one of two ways. One type has electrodes at each end. When electrical current is applied, an "arc" is formed within the lamp, causing it to emit UV energy. The other type of lamp is electrodeless and ignites the gas with microwaves. In this system, a microwave-generating unit is placed above the irradiator and the microwaves are directed to the lamp by waveguides.

The medium-pressure mercury lamp is the most common lamp used for curing UV screen-printing inks. Argon gas and an exact amount of mercury (to create the right pressure when vaporized) are captured in a tube of fused silica. As Figure 2A shows, vaporized mercury emits ultraviolet energy at certain wavelengths so that when UV inks with the proper photoinitiators are exposed, cross-linking (curing) results.

Research has revealed that the addition of compounds called metal halides to the mercury enhances the spectral output of the lamp. This enhanced output is advantageous for curing opaque ink systems and those containing the white pigment titanium dioxide (TiO2). Two such "doped" lamps include the iron and gallium varieties. Most curing units offered today are designed to allow lamps to be changed in a matter of minutes.

One factor that has the greatest impact on the curing efficiency of the lamp is the amount of electrical current that flows through it during operation. According to Ohm's Law, volts x amps = watts. However, to calculate the actual power that the curing unit is receiving, watts must be multiplied by the "power factor" for the particular area (this factor can be supplied by the local electric company), and divided by the length of the lamp. The result is the exact power that the lamp is receiving, provided that there are no power fluctuations.

Curing units employ reflectors with different physical geometries to focus or scatter the lamp output across the print. The specific configuration of the reflector influences how the ink film is cured. Three main reflector geometries are used today:

elliptical, which concentrates energy at the print surface with a narrow focal width parabolic, which reflects energy at a slightly wider focal width multifaceted, which reflects energy at the widest focal width

One final consideration about curing equipment concerns heat management. High temperature created by the IR energy UV lamps emit is an unfortunate byproduct of the curing process. Preventing this heat from building up and damaging sensitive substrates is a critical concern in UV curing. Systems for heat management in modern curing units run the gamut from water cooling systems and fans to dichroic mirrors (which reflect UV but absorb IR energy) and automatic shutter systems.

What processing issues must be considered when using UV?

In screen printing UV inks, controlling ink-film thickness is of paramount importance. Mesh type, screen-tension level, emulsion thickness, and related screen making factors all contribute to the amount of ink deposited on the substrate. These variables have to be controlled in order to consistently achieve the ink-film thickness recommended by the ink manufacturer. Similarly, press-setup parameters must also be controlled, including squeegee hardness, screen off-contact, and squeegee / floodbar speed and pressure.

The type of substrates you are printing on also influences the results you can expect from UV curing. On polycarbonate substrates, for example, UV energy at shorter wavelengths (below 300 nm) tends to affect the substrate surface. This phenomenon, known as photo-oxidation, appears as a gradual yellowing of the polycarbonate surface that occurs with repeated exposure to the UV energy. The subtle change in the polycarbonate surface affects ink adhesion, especially if multiple colors are printed. However, the use of gallium lamps, which release UV energy in the shorter wavelength range, has been shown to minimize this photo-oxidation of polycarbonate.

Polyester is another material commonly printed with UV. The main factor to keep in mind when working with polyester is that this material needs to be pretreated (flame or corona treatment) or coated with some form of primer to achieve good ink adhesion.

Click here to read about the Precautions using uv while making inks.