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Pinning Inks with UV LEDs

(April 2012) posted on Tue Apr 17, 2012

How UV-LED is used to gel ink droplets


By Paul Mills and Jennifer Heathcote

click an image below to view slideshow

Difference from UV
As the name implies, UV LEDs are an ultraviolet variety of light-emitting diodes. They differ dramatically from traditional UV lamps used for printing both in how they produce UV, and in their benefits. Traditionally, lamp suppliers have produced UV by energizing a small amount of mercury that is sealed in a quartz tube. When high voltage is applied to the lamps electrodes, (or with microwave lamps, when the tube is placed in a microwave generator), UV (along with a large amount of visible light and infrared heat) is created. So the output spectrum of mercury lamps contains a large number of peaks over a broad range of wavelengths that extend from 200nm to well into the IR region above 1000 nm.
UV LEDs, on the other hand, are semiconductor devices; modern cousins of the LED on your garage door opener except that UV LEDs emit light in the longer-wavelength portion of the UV spectrum. The exact output of the LED is chemistry dependent, but typically the output is a relatively monochromatic or narrow peak of energy somewhere between 365-405 nm
At first, the more specific, narrow bandwidth output of the LED caused UV chemists some fits as they scurried to adapt ink formulations to these new sources. But the benefits of UV LEDs for digital printing clearly made this effort worthwhile, because UV LEDs offer many advantages that arc lamps do not. For example, they do not generate dangerous short wavelength UV that can be a safety hazard, and they do not produce ozone. They do not contain mercury, another potential safety hazard, or use high voltages.
But LEDs are also much cooler than arc lamps, and they can be turned on and off instantly without the need for shutters. LEDs are compact in size, and produce more uniform light sources that last much longer than arc lamps. But it is the ability to precisely control the UV output of the light source, along with their compact size, and cool-running temperatures that limit heat build, that make UV LEDs ideal for pinning inks. Compact UV LEDs can be located in the tight confines between inkjet heads, a trick that would be impossible for arc lamps.
Figure 1 illustrates the ability of UV-LED pinning lamps to improve the quality of digital images by limiting drop gain on a difficult substrate. Without pinning, each color spreads as it is applied, mixing and muddling with other droplets. With pinning, the droplets remain distinct and sharp.
The use of state-of-the-art UV-LED technology to improve dot gain is advancing digital printing on two related fronts. While pinning helps to produce higher quality images with existing technology, it also makes it possible for press designers to achieve a higher perceived resolution with less expensive or complex jetting hardware. After all, resolution is not determined by the number of dots applied per inch alone, but also by the distinctness of those droplets to the reader’s eye. If ink droplets are fuzzy, or mingled with others, even a high density of droplets per inch can be a wasted investment.

In conclusion
While there is broad agreement that pinning improves image quality, there is no consensus yet regarding the ideal settings needed for optimal pinning. Ink formulators, UV-pinning-system manufacturers, and integrators are experimenting with UV settings, timing, and ink sets to find the answers. The activity is a strong indicator that more and more systems are, and will be, incorporating pinning into the inkjet printing process.

Paul Mills, Integration Technology, has been involved in UV curing since 1994. He has participated in a number of innovations in UV including UV powder coatings development, robotic UV curing and the development UV LED light sources. Paul holds BA and MA in economics, and has been a longtime member and board member of RadTech.

Jennifer Heathcote is general manager, North America (based in Chicago, Illinois), for Integration Technology. Heathcote received a B.S. in mechanical engineering from Purdue University and earned her MBA at The Ohio State University. Integration Technology, founded in 2000 and headquartered in Oxfordshire, England, is a supplier of both conventional mercury lamps and UV LED light sources to a wide range of industries including the global graphic arts and printing markets.

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