Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Advancements in UV LED curing technology overcome many curing problems associated with traditional mercury vapor lamps. UV LED lamps are superior to treat low-viscosity UV inks on non-wettable, heat-sensitive polymeric and urethane/rubber substrates. However, not every LEDs are constructed the identical or exhibit equal performance characteristics. This information is the first in a series to show process advancements for industrial uv printer on plastics.
Until recently, UV LEDs are already faced with technical and economic barriers which have prevented broad commercial acceptance. High cost and limited accessibility to LEDs, low output and efficiency, and thermal management problems – put together with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, usage of UV LEDs for curing could well be one of the most significant breakthroughs in inkjet printing on plastics.
Easy to operate and control, UV LED curing has several advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are created to last beyond 20,000 hours operating time (about ten times longer) than UV lamps. Output is very consistent for very long periods. UV LED emits pure UV without infrared (IR), so that it is process friendly to heat-sensitive plastic substrates. Reference Table 1 UV LEDs vs. Mercury Vapor Lamps.
LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched on the lamp, monomers, speed and applications. To obtain robust cure, LED requires different photoinitiators, and as a result, different monomer and oligomers inside the formulations.
Just about the most scrutinized aspects of UV LED technology is the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to get sent to the curable ink. Mercury Hg bulbs normally have reflectors that focus the rays therefore the light is most concentrated in the ink surface. This greatly raises peak power and negates any competing reactions. Early LED lamps were not focused.
High power and efficiency are achievable with t-shirt printer by concentrating the radiant energy through optics or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional towards the junction temperature from the LED die. Maintaining a cooler die extends life, improves reliability and increases efficiency and output. Historical challenges of packaging UV LEDs into arrays have already been solved, and alternative solutions can be found, in relation to application. Most of the development and adoption of LED technology has been driven by consumer electronics and displays.
First, formulating changes and materials happen to be developed, as well as the vast knowledge has become shared. Many chemists now discover how to reformulate inks to complement the lamps.
Second, lamp power has increased. Diodes designs are improved, and cooling is a lot more efficient so diodes get packed more closely. That, therefore, raises lamp power, measured in watts per unit area on the lamp face, or better, on the fluid.
Third, lenses on lamp assemblies focus the energy, so peak irradiance is higher. The mixture of these developments is making LED directly competitive, otherwise superior, to Hg bulbs in many applications.
Depending on the applying and variety of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are accessible for select chemistries. As wavelength improves the output power, efficiency and costs also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance in the die is much better at longer wavelengths, and also the cost per watt output is lower while delivering more energy. Application history implies that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, sometimes, 365nm or shorter wavelengths are needed to achieve robust cure.
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and heavy Hg bulbs require massive scanning system frames, that are not necessary with LED. Fixed head machines get the print heads assembled in modules and positioned in overlapping rows. The compact, cool UV lamp fits nicely mounted on a head module. Further, digital printing often is short term with frequent stops, so immediate “On/Off” yields greater productivity and revenue.
There are two implementations of thermal management: water and air-cooling. Water cooling is definitely a efficient way of extracting heat, particularly in applications where high power densities will be required over large curing areas. With water cooling, lower temperatures can be had with higher efficiency and reliability.
A 2nd good thing about water cooling is the compact UV LED head size, which permits integration and then there has limitations space across the curing area. The drawbacks water cooling solutions dexjpky05 the heavier weight of the curing unit and added complexity and expenses for chillers and water piping.
Another thermal management option would be air-cooling. Air-cooling inherently is less efficient at extracting heat from water. However, using enhanced airflow methods and optics yields very effective air-cooling curing systems, typically up to 12W per square centimeter. Some great benefits of air-cooled systems include simplicity of integration, very light, lower costs without any external chillers.
Maximization of uv flatbed printer output power is vital. Via selective optics, the electricity from LEDs may be delivered easier to the substrate or ink. Different techniques are integrated into integrated systems ranging from reflection to focused light using lenses. Optics can be customized to meet specific performance criteria. Whilst the OEM (end user) should never necessarily be concerned with exactly how the optics are supplied within the UV LED lamp, they should realize that suppliers’ expertise varies, and all UV LED systems will not be created equal.