Digital UV inkjet printing on three-dimensional plastic products is “ready for prime time.” Advancements in UV LED curing technology overcome many curing problems related to 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 all LEDs are constructed the same or exhibit equal performance characteristics. This article is the very first in a series to offer process advancements for industrial UV inkjet printing on plastics.
Until recently, UV LEDs have already been confronted with technical and economic barriers which have prevented broad commercial acceptance. High cost and limited availability of LEDs, low output and efficiency, and thermal management problems – combined with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, consumption of UV LEDs to treat could well be among the most significant breakthroughs in Coffee Printer on plastics.
Simple to operate and control, UV LED curing has several advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are made 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), rendering it process friendly to heat-sensitive plastic substrates. Reference Table 1 UV LEDs vs. Mercury Vapor Lamps.
UV LED early development factors
LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched to the lamp, monomers, speed and applications. To achieve robust cure, LED requires different photoinitiators, and as a result, different monomer and oligomers within the formulations.
Just about the most scrutinized regions of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to be shipped to the curable ink. Mercury Hg bulbs typically have reflectors that focus the rays and so the light is most concentrated on 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 UV Flatbed Printer by concentrating the radiant energy through optics and/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 purchased, based upon application. A lot of the development and adoption of LED technologies have been driven by electronic products and displays.
Recent significant developments
First, formulating changes and materials have already been developed, as well as the vast knowledge has been shared. Many chemists now discover how to reformulate inks to match the lamps.
Second, lamp power has grown. Diodes designs are improved, and cooling is a lot more efficient so diodes get packed more closely. That, subsequently, raises lamp power, measured in watts per unit area at the lamp face, or better, in the fluid.
Third, lenses on lamp assemblies focus the power, so peak irradiance is higher. The mixture of these developments is making LED directly competitive, otherwise superior, to Hg bulbs in lots of applications.
Based upon the application and choice of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are accessible for select chemistries. As wavelength increases the output power, efficiency and costs also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance from the die is 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, in some circumstances, 365nm or shorter wavelengths are required to achieve robust cure.
Integrated systems solutions
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and high Hg bulbs require massive scanning system frames, which are not necessary with LED. Fixed head machines hold the print heads assembled in modules and set up in overlapping rows. The compact, cool UV lamp fits nicely attached to a head module. Further, digital printing often is short term with frequent stops, so immediate “On/Off” yields greater productivity and revenue.
Thermal management and optics
The two main implementations of thermal management: water and air-cooling. Water cooling is definitely a efficient approach to extracting heat, specifically in applications in which high power densities are required over large curing areas. With water cooling, lower temperatures can be found with higher efficiency and reliability.
An additional benefit of water cooling is definitely the compact T-Shirt Printing Machine head size, which permits integration where there is limited space round the curing area. The drawbacks water cooling solutions would be the heavier weight from the curing unit and added complexity and expenses for chillers and water piping.
The 2nd 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 untyft effective air-cooling curing systems, typically approximately 12W per square centimeter. The advantages of air-cooled systems include ease of integration, light weight, lower costs and no external chillers.
Maximization of UV LED output power is crucial. Via selective optics, the energy from LEDs can be delivered better to the substrate or ink. Different techniques are integrated into integrated systems ranging from reflection to focused light using lenses. Optics may be customized to satisfy specific performance criteria. Whilst the OEM (end user) must not necessarily be concerned with how the optics are supplied within the UV LED lamp, they need to realize that suppliers’ expertise varies, and all of UV LED systems are not created equal.