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 exactly the same or exhibit equal performance characteristics. This post is the 1st in a series to provide process advancements for industrial uv printer on plastics.
Until recently, UV LEDs are already faced with technical and economic barriers who have prevented broad commercial acceptance. High cost and limited accessibility to 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 some of the most significant breakthroughs in inkjet printing on plastics.
Simple to operate and control, UV LED curing has lots of advantages over mercury (Hg) vapor lamps. Small profile semiconductor devices are created to last beyond 20,000 hours operating time (about 10 times longer) than UV lamps. Output is incredibly consistent for too 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.
LED and Hg vapor bulbs have different emission spectra. Photoinitiators are matched to the lamp, monomers, speed and applications. To accomplish robust cure, LED requires different photoinitiators, and as a consequence, different monomer and oligomers inside the formulations.
Probably the most scrutinized areas of UV LED technology may be the maximum radiant power and efficiency produced. Ink curing necessitates concentrated energy to get delivered to the curable ink. Mercury Hg bulbs normally have reflectors that focus the rays so 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 and/or packaging. High-power systems utilize grouping arrays of LED die. Irradiance is inversely proportional to the junction temperature of your LED die. Maintaining a cooler die extends life, improves reliability and increases efficiency and output. Historical challenges of packaging UV LEDs into arrays have been solved, and alternative solutions are available, dependant on application. A lot of the development and adoption of LED technology has been driven by consumer electronics and displays.
First, formulating changes and materials are already developed, along with the vast knowledge is shared. Many chemists now understand how to reformulate inks to complement the lamps.
Second, lamp power has risen. 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 in the lamp face, or better, at the fluid.
Third, lenses on lamp assemblies focus the strength, so peak irradiance is higher. The mix of these developments is making LED directly competitive, otherwise superior, to Hg bulbs in many applications.
Depending upon the application and variety of inks, wavelength offerings typically include 365nm, 385nm and 395nm. Higher wavelengths are available for select chemistries. As wavelength boosts the output power, efficiency and expenses also scale, e.g., 365nm LEDs provide less output than 395nm LEDs.
The performance in the die is preferable at longer wavelengths, along with the cost per watt output is lower while delivering more energy. Application history demonstrates that often 395nm solutions can effectively cure formulations more economically than 365nm alternatives. However, in some instances, 365nm or shorter wavelengths are required 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 have the print heads assembled in modules and set up in overlapping rows. The compact, cool UV lamp fits nicely connected 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 definitely a efficient means of 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 where there is restricted space around the curing area. The drawbacks water cooling solutions dexjpky05 the heavier weight in the curing unit and added complexity and expenses for chillers and water piping.
The 2nd 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 successful air-cooling curing systems, typically around 12W per square centimeter. The advantages of air-cooled systems include easy integration, very light, lower costs without any external chillers.
Maximization of uv printer output power is critical. Via selective optics, the energy from LEDs might be delivered preferable to the substrate or ink. Different techniques are integrated into integrated systems which range from reflection to focused light using lenses. Optics can be customized to meet specific performance criteria. As the OEM (consumer) must not necessarily be worried about how the optics are provided from the UV LED lamp, they must recognize that suppliers’ expertise varies, and all of UV LED systems will not be created equal.