Market leading wide format and super wide format inkjet printer manufacturers are moving to UV curing using photopolymer chemistry. They move away from solvents containing VOCs (volatile organic compounds) requiring large, power hungry curing devices — furnaces.
The distinct advantages of UV LED curing over UV curing using traditional mercury lamps are manifold. The environmental factors alone are considerable as the inherent dangers in the building, handling, and disposing of a hazardous substance are well known.
Studies have also shown that in certain applications, replacing mercury lamps with UV LED lamps can lead to considerable CO2 reductions. In addition, the true Total Cost of Ownership (TCO) when taking into account cooling, venting, and maintaining mercury based systems versus UV LED based systems shows that 50-75% can be saved over the system’s useful life.
But not all UV LED curing lamp systems are created equal, and so it’s important to know what you’re buying. UV LED curing systems consist of multiple sub-components which taken together can be used to define the system’s overall performance.
The LED itself is the base building block; it is the first choice a UV LED lamp supplier has to make. It is a critical choice that impacts the remainder of the systems architecture and design. Key LED characteristics considered by each UV LED lamp supplier will include wavelength and UV output. UV LED output has improved considerably from 2005 to 2011 with a compound annual growth of 5-10%.
How the LEDs are combined, the number and type of LEDs chosen, the shape of the array, the method of electrically connecting the LEDs, and even their size will also have significant impact on the performance of the system.
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 1-2m wide. Therefore, in printers such as the VUTEk GS3250LX the engineers made sure the LED array can be uniformly scaled.
UV LED curing lamps can have a continuous scalable array that provides for better uniformity or a discrete array package that can be scaled, but doesn’t provide the same uniformity of output.
The next factor to consider is heat generation and dissipation. UV LEDs transfer 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 transferred as heat — LED arrays therefore need cooling.
UV LED arrays are cooled with either air or liquid. The higher output power, the more heat is generated. As the quality of LEDs improves and the irradiance increases, so does the need to remove the heat.
The final component and one of the most important differentiators is optics. Based on the end application the optical engineer has to decide what shape, form, and material best utilizes the LED’s unique characteristics. Next the engineer needs to take into account the fact the LEDs are a ‘flood’ type of light, unlike a focused mercury lamp where the light is captured by a reflector and directed to a specific point, at a specific focal length.
The end-user should not necessarily be concerned with how the optics are provided in the UV LED lamp, but they should understand if the supplier has the ability to improve their design for their specific application needs.
Regardless of the LED, array, thermal and optics design employed, the end result that matters to end-users is that their material is cured. The two measurement parameters for curation capability are Peak Irradiance and Dose.
Peak Irradiance is radiant power per unit area. Dose is radiant power per area per unit of time. What end-users should ask are the next two questions:
- Where is the peak irradiance specification point of reference?
- Over what area is the peak irradiance being delivered?
The reason the first question needs to be asked is that an end-user might be tempted to believe he has purchased a UV LED system that delivers for example 4 Watts/cm2 across the entire emission area when in fact only 2 Watts/cm2 are curing along the edges.
Dose being a function of exposure time, the faster the belt speed, the less dose is being delivered to the media even when the peak irradiance is the same. That’s why the second question is relevant. For a given media speed, altering the height of the UV LED light source from the media does not change the total amount of light (dose) delivered to the surface, but rather the peak irradiance decreases.
In general, however, inkjet manufacturers will take much care in assuring curing happens at an optimum. Failure to do so would result in bad press, bad customer experience and a quick drop in market share!