LED vs. Traditional UV Curing

For many years UV technology has been a reliable method for the curing of photo-reactive chemicals. In response to increasing production speeds and new applications, e.g. in the field of 3D, UV lamp technology has also developed. Today, a great variety of different systems are available, each specific to the particular application. Users and providers of  chemistry are constantly developing new applications for UV curing. Their innovative ideas often mean increasing demands on UV curing devices – where sometimes conventional UV technology has reached its technical limits. Thus, within the recent years, a completely new branch of UV technology has developed: UV LEDs.  

 

This report provides the reader with an objective comparison between both technologies, UV and UV LED. It should help the user determine to what  extent LEDs can provide an alternative to conventional UV solutions.

 

Comparison of technologies

 

The operating technology of conventional UV lamps is based on plasma physics and optics, whereas UV LEDs are based on semiconductor technology and optics.

UV technology is based on a high-voltage arc between two electrodes leads to the vaporization of mercury and any optional doping within the lamp. A continuous UV spectrum between 200 nm and 450 nm is emitted.

 

LEDs are based on semiconductor technology. Specific wavelengths are directly emitted by the current input. Generally, UV-LED systems have a tolerance range of +/-5 nm and an nm output curve that is relatively narrow but spans approximately +/- 15 nm. The initiators selected for UV-LED inks target this range, but are also reactive outside this range. This means that UV-LED inks should be reactive to both UV-LED and mercury vapour curing systems. This may vary to some degree depending on the specific ink’s colour and pigment load

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With the introduction of 4+ watts, 395 nm UV-LED light sources, inks can effectively be cured at processing speeds typical for screen and digital printing. Systems that are below 4 watts would require a thinner ink deposit, lower pigmented ink, closer distance between the lamp and the printed ink and slower scanning / belt speed when curing.

 

Currently in the market, manufacturing a 4+ watts system requires access to higher quality UV-LED lamps with a more sophisticated cooling system. Accessing parts and assembling a lamp to gain effective curing remains out of reach for those thinking of assembling their own units.

 

Cooling Configurations

 

UV-LED lamps require cooling at the lamp and electronics to run efficiently and effectively. A good cooling system should have a lamp life of 20,000+ hours of continual usage. Two types of systems are used: air cooling and water cooling. Water cooling tends to be more effective and is not affected by the surrounding unit’s environment. However, a water chiller has a higher upfront cost, needs to be maintained on a regular basis, and has a potential for leaking. More UV-LED manufacturers are developing air cooled systems that are built into the lamp head itself. Air cooling systems are incorporated into the lamp head and tend to have less maintenance. The caution to using air cooling is that the environment should be clean and kept at room temperatures or cooler.

 

UV-LED to Mercury Vapour

 

UV-LED is unlike mercury vapour curing systems in that they are not focused or use reflectors; instead, the light directed at the substrate is diffused. Reference the illustration.

In addition, UV-LED systems are rated on their output or how much light is generated at the lamps. For example, a 4 watt UV-LED system refers to 4 watts of energy emitted from the lamps. Mercury vapour curing systems are rated based on their input of energy. For example, a curing system can be set to 200 or 300 watts. A radiometer is then used to measure the output of the system; typical output for a graphics screen printer is about 400 to 600 mW. 

 

Because of these differences, it becomes difficult to determine how to compare apples to apples energy consumption between systems. As a very general guideline, a 4 watt UV-LED system at ¼” to ½” is similar to a 300 watt mercury vapour system with typical lamp to print distances 6+ inches. In general, UV-LED provides a 30-50% reduction in energy consumption to run the lamp and cooling system. This does not take into account energy needed to cool or heat the displaced air pulled through a mercury vapour system.

 

UV LED measurement

 

UV measurement assures production process security and for research and development, reliable and repeatable test results in the laboratory. The market offers a selection of devices for measuring the intensity and/or the dose with different sensor geometries which can be easily matched to the specific application. The physical classification of the UV spectrum in the UVA region is from 400-320nm, UVB from 320-380nm and UVC from 280- 200nm, in most cases the spectral sensitivity of the sensor is adapted accordingly. The specific characteristics of a broad UV spectrum can therefore be analysed in detail. However, LED irradiation units do not produce a broad UV spectrum, but emit narrow bandwidths at specific wavelengths. Therefore, any intensity measurement of these bandwidths with conventional sensors is inaccurate.

 

To enable the measurement of LED UV units, a broad band sensor can measure the intensity and the dose across the bandwidth of all LED-wavelengths from 365 to 405nm - with only one sensor. The UV LED measuring sensor is attached to a standard UV meter which automatically indentifies the LED sensor when connected. The measured value for the intensity is displayed in W/cm² or mW/cm² with a maximum intensity of 20W/cm², for the dose it is displayed in J/cm² or mJ/cm².

Compared to conventional UV irradiators, it’s clear that LEDs can offer many advantages in a variety of coating applications. However there are some restrictions, governing or preventing the use of LEDs. The benefits of LED curing should be carefully considered, case by case.