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, its not all LEDs are constructed the same or exhibit equal performance characteristics. This article is the first within a series to offer process advancements for industrial uv printer on plastics.
Until recently, UV LEDs are already faced with technical and economic barriers which may have prevented broad commercial acceptance. High cost and limited accessibility of LEDs, low output and efficiency, and thermal management problems – coupled with ink compatibility – were limiting factors preventing market acceptance. With advancements in UV LED technology, using UV LEDs for curing is arguably some of the most significant breakthroughs in inkjet printing on plastics.
Very 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 incredibly consistent for long periods. UV LED emits pure UV without infrared (IR), making it 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 towards the lamp, monomers, speed and applications. To achieve robust cure, LED requires different photoinitiators, and as a consequence, different monomer and oligomers inside the formulations.
Probably the most scrutinized parts of UV LED technology is definitely the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy being sent to the curable ink. Mercury Hg bulbs typically have reflectors that focus the rays so the light is most concentrated with 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 and/or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional for the junction temperature in the LED die. Maintaining a cooler die extends life, improves reliability and increases efficiency and output. Historical challenges of packaging UV LEDs into arrays are already solved, and alternative solutions are offered, based on application. Most of the development and adoption of LED technology has been driven by consumer electronics and displays.
First, formulating changes and materials are already developed, and also the vast knowledge has been shared. Many chemists now discover how to reformulate inks to complement the lamps.
Second, lamp power has risen. Diodes designs are improved, and cooling is much more efficient so diodes get packed more closely. That, subsequently, raises lamp power, measured in watts per unit area on the lamp face, or better, in the fluid.
Third, lenses on lamp assemblies focus the strength, so peak irradiance is higher. A combination of those developments is making LED directly competitive, or even superior, to Hg bulbs in many applications.
Depending on the application and selection 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 of the die is better at longer wavelengths, along with the cost per watt output is less while delivering more energy. Application history suggests that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, occasionally, 365nm or shorter wavelengths are required to achieve robust cure.
LED cure best complements digital inkjet printing. On reciprocating printheads, hot and high Hg bulbs require massive scanning system frames, that happen to be not essential 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 linked to a head module. Further, digital printing often is short run with frequent stops, so immediate “On/Off” yields greater productivity and revenue.
The two main implementations of thermal management: water and air-cooling. Water cooling is certainly a efficient approach to extracting heat, especially in applications in which high power densities are required over large curing areas. With water cooling, lower temperatures can be acquired with higher efficiency and reliability.
A 2nd benefit of water cooling may be the compact UV LED head size, which permits integration and then there is restricted space throughout the curing area. The drawbacks water cooling solutions dexjpky05 the heavier weight of your curing unit and added complexity and expenses for chillers and water piping.
The second thermal management solution is air-cooling. Air-cooling inherently is less effective at extracting heat from water. However, using enhanced airflow methods and optics yields very successful air-cooling curing systems, typically approximately 12W per square centimeter. The advantages of air-cooled systems include easy integration, very light, lower costs and no external chillers.
Maximization of uv printer output power is crucial. Via selective optics, the vitality from LEDs might be delivered easier to the substrate or ink. Different techniques are integrated into integrated systems ranging from reflection to focused light using lenses. Optics may be customized in order to meet specific performance criteria. Even though the OEM (end user) ought not necessarily be concerned with the way the optics are given inside the UV LED lamp, they need to notice that suppliers’ expertise varies, and UV LED systems are not made the same.