UV LED curing is making strides in the flexo sector, but there is a risk of users imagining that all such technology is the same. Andy Knaggs discovers there is much more to it.
UV LED curing has been the coming technology for several years and seems to now be gaining some critical mass in the flexo industry. Vendors for the technology are proliferating, cost and environmental arguments and comparisons have been made, and ink suppliers are ramping up R&D to provide ink products that can be cured using UV LED or are ‘dual cure’ so can be cured by either LED or traditional UV.
The obvious conclusion is that the future is UV LED, with a gradual takeover of the flexo print curing market from the traditional technology. However, all UV LED is not equal. There is a lot happening when an LED (diode) tries to cure a printed ink, and users need to make sure that they have made the right choices in specifying the curing system for their needs.
Why? Because none other than the foundation of their reputation is at stake. ‘When it comes to print quality, if the wrong UV LED choice is made, the printer may have issues with under cured or over cured material that can lead to press downtime and excessive scrap,’ says Stacy Hoge, Phoseon marketing manager for Excelitas Technologies. ‘If there is not enough energy density from the UV LED curing system, it can result in tracking (uncured ink present on idle rollers), and in contrast, if the ink, coating or adhesive is exposed to more energy than expected, it can result in over cured materials, which may cause the material to become brittle or have bonding issues.
Good science
It is important, therefore, for printers to understand what makes for a ‘good’ UV LED cure – what factors are at play and how they should approach their investment in, and integration of, this curing technology. A little science is needed.
Firstly, the reactivity of the ink used has a huge bearing on the required output of a UV LED system. Additional key factors include the speed of the printing machine, and the nature of the application, including its end use.’
Peaks and doses
In terms of the curing systems themselves, many contributors highlight two factors in particular that must be considered for optimising the UV LED cure: peak irradiance (expressed in Watts/cm2); and energy density (expressed in Joules/cm2).
‘Identifying this process window will result in the most durable and desirable finish, as well as acceptable adhesion and surface cure,’ says Ms Hoge.
She explains irradiance as ‘the punching power to kick start the polymerisation process’, in which photo-initiators cross-link acrylate monomers and oligomers in UV curable adhesives, coatings and inks. ‘There is a minimum threshold irradiance value needed to start this process, and anything greater than the minimum threshold will turn into heat. It is not a given that higher irradiance will enable faster cure speeds,’ she says.
Energy density is also commonly referred to as dose. This is explained as the total number of light photons that hit a specific area within a specific duration of time. Energy density is the key to ensuring adhesion and cure at high speeds.’
Perfect combination
To pick the bones out of this, it seems that there is a combination of peak irradiance, energy density (or dose) and dwell time, that needs to be perfected to cure the ink being used in any given application. However, cooling systems, optics, wavelength, mounting location, machine set up, and ingress protection to keep out contaminants such as dirt, ink and moisture, are also important considerations.
It is, therefore, highly technical, and flexo printers need to be able to trust their UV LED supplier, to ensure that everything that needs to be factored in is included.
Tags: FlexoTech MagazineCategories: Printing, Flexo | UV LED Curing by Phoseon
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