Introduction
UV curing is a photochemical process that changes liquid ink into a solid, thereby fixing it in place. Instead of using heat to drive off the solvent or water to dry an ink, UV energy causes fixation of the specially formulated photosensitive chemistry in a very short time, from a fraction of a second to a few seconds depending on the situation.
An efficient UV curing process needs to match the output of the UV lamps to the absorption characteristics of the UV chemistry in terms of the energy dose (mJ/cm2) and wavelength range (nm) needed, as the ink photoinitiator chemistry and the UV lamp form a matched pair.
UV Lamps: LED or arc lamp?
Until the last few years, UV curing systems have used mercury arc lamps that use excited gases to emit UV wavelengths, but recently a solid state alternative has been introduced using UV light emitting diodes (LEDs). UV LEDs offer potential advantages over arc lamps, with smaller size and lower weight, rapid start up and lower power usage. LED options are typically more expensive at present, but as costs come down, the UV LED is becoming a more common option in UV curing systems.
UV LEDs offer other advantages as they have a longer lifetime (typically more than 10,000 hours compared with 500-2000 hours for an arc lamp bulb), and show more constant output over this life. They also generate far less ozone in operation, and do not contain mercury. A key advantage of UV LEDs is the lower operating temperature (typically 60°C versus more than 350°C for an arc lamp) leading to less heat damage to the substrate and reduced need for shielding. This also makes the system safer as there is less chance of substrates catching fire.
We will look at the four key components of LED curing systems and how they compare with arc lamps.
LEDs – The base building block
Wavelength
The wavelength emitted from an LED is controlled by altering the chemical composition of the semiconductor material used in the manufacturing of the LED. A UV LED generates UV energy by applying an electric current to a semiconductor diode, causing it to emit UV energy in a very narrow band of 10-20 nm. The figure compares the output of a UV LED lamp with a typical mercury arc lamp that tends to have a much broader range of emission frequencies. This makes it easier to match with a photoinitiator – more care needs to be taken with LED curing to ensure the photoinitiator will respond to the narrow range of wavelengths emitted.
Comparison of the output from a mercury arc lamp and a number of LED options
UV Output
UV LED output has shown considerable improvement in recent years. Prices for UV LED lamps continue to come down even as their output increases, making additional applications feasible both technically and economically. UV LED system output is four to six times what it was just four years ago.
Array – Grouping of LEDs
The grouping of individual diodes into arrays is influential on the performance on the final device, due to the number and type of LEDs chosen, the shape of the array and their electrical connections.
Most applications require UV LED curing systems that consist of more than one LED or LED array in order to achieve not only the desired throughput but to meet the demands for curing applications where the media can be 1-2 m wide.
Thermal management – Keeping it Cool
The third component is cooling. UV LEDs convert about 15-25% of the received electrical energy into light. While this is significantly better energy efficiency than mercury lamps, the remaining 75-85% is converted to heat that needs to be removed to keep the LEDs at their preferred operating temperature.
Currently, UV LED arrays are cooled with either air or liquid. Air cooled devices tend to be cheaper, but larger and less powerful due to the lower cooling efficiency leading to higher operating temperatures, which reduces efficiency. Liquid-cooled devices are more compact and tend to have higher output, but are more complex and therefore more expensive.
As LEDs emit higher output power, they generate more heat. Thus in the race to build ever higher irradiance products, the ability to control and remove heat has become crucial to building reliable systems.
Optics – Guiding the Light
The final component and an important differentiator between products is the optics used to extract light from the LED and guide it to the substrate in the most efficient manner. Based on the application the optics are designed to have the correct shape and form to give uniform illumination of the substrate. Arc lamps typically use a dichroic reflector to focus the emitted light into a line of intense UV light that maximises the curing efficiency. In contrast the output from an LED is a more uniform ‘flood’ illumination, giving different curing behaviour.
Measuring the Differences between UV LED Curing Systems
Regardless of the LED, array, thermal and optics design employed, the end result that matters to end-users is the amount of UV power being supplied to the ink. This is generally parameterised using peak irradiance (radiant power per unit area, in W/cm2) and/or dose (radiant power per unit area per unit time, in Joules/cm2). The peak irradiance depends on the output from the UV unit and the distance from the substrate, while the dose also depends on the amount of time the ink is exposed to the UV light. While the curing requirement is usually quoted as a dose, in practice there is often a difference between a dose provided over a very short time (at a high power) and the same dose over a longer time. The reason for this is that oxygen inhibition, a process where interaction of the activated photoinitiator with oxygen atoms from the air inhibits the curing reaction, has more time to take effect with a lower power. The short wavelength emission from arc lamps also tends to cure the ink surface quickly, which helps with overcoming oxygen emission, so the required dose number needs to be treated with care when designing a curing system.
Summary
While ink compatibility and cost issues hold back implementation of UV LEDs in some cases, as the performance of LED UV sources increases and their cost decreases (with arc lamps being a ‘static target’) the adoption of UV LEDs is increasing. The prospect is that this will accelerate in the coming years.
Phoseon Technology are one of the presenters of the Inkjet Drying & Curing course at IMI Europe’s Inkjet Summer School event in Ghent, Belgium from 12-16 June 2017. The course covers the key questions around drying and curing of inkjet inks in much greater detail, with the opportunity to ask questions and discuss your requirements.