UV LED flood curing

Flood curing irradiates a whole area at once. Instead of following individual bond points, an area light source covers a complete part, a tray of parts or a coated panel, and cures everything within its field in a single exposure. It is the technique of choice for batches, larger bonds and coatings on stationary parts, and the basis of UV curing ovens and many conveyor systems.

How it works

An area light source is an array of UV LEDs behind a protective window, cooled by fans or by water. A controller sets the output and the exposure time, and starts the exposure from a button, a foot switch, a sensor or a PLC.

The part is placed under the source, usually in a fixture or on a conveyor, at a defined distance. Every point within the field receives light at the same time, so the whole part cures together. Many sources can be dimmed and switched in sections, so the exposure can be matched to different parts.

Irradiance, uniformity and working distance

An area source has to spread its output over a large field. Its peak irradiance is therefore lower than that of a focused spot head, and it falls as the emitting area grows. Small, compact heads reach the highest values; large fan-cooled panels deliver less per square centimetre; water cooling allows much more power in the same area.

For flood curing, uniformity matters as much as peak irradiance. Close to the window, the individual LEDs can produce a pattern of brighter and darker spots. A little further away, the light from neighbouring LEDs overlaps and the field becomes more even, but the irradiance falls. Area sources are usually specified at a working distance of about 10 mm.

The irradiance also drops towards the edges of the field. Choose an emitting area larger than the part, so that its corners and edges receive enough light.

Irradiance and dose

• Irradiance (mW/cm²) is the UV power arriving per unit area at the surface.
• Dose (mJ/cm²) is irradiance multiplied by exposure time. For example, 1,500 mW/cm² for 2 s gives 3,000 mJ/cm².

The material data sheet gives the dose and usually a minimum irradiance. A stationary flood exposure makes dose easy to control: set the time on the controller. If the irradiance is too low, a longer exposure may still leave a tacky surface, because oxygen inhibits the cure at the surface. In that case a more powerful source, a shorter working distance or an LED-optimised formulation helps.

Measure the irradiance across the field, not only in the centre, with a UV Radiometer when you set up the process.

Wavelength

Area sources are available at 365, 385, 395 and 405 nm. Use the wavelength your adhesive or coating is formulated for. Longer wavelengths penetrate further into thick, filled or pigmented layers, while 365 nm suits many general-purpose adhesives and formulations derived from mercury-lamp chemistry.

Typical uses

• Curing trays of small parts, such as bonded electronic components and medical device sub-assemblies
• Coatings and adhesives on panels, housings, glass and wood in Coatings and Adhesives
• Cemented lenses and small optical assemblies in Optics
• Conformal coatings on boards in Electronics
• Inks and varnishes on sheets and wide webs in Printing
• Laboratory samples and post-curing in a UV Curing Oven

Limits

• Lower peak irradiance than spot curing. If only a few small points need curing, a spot system delivers more irradiance with less power.
• Line of sight. Tall or complex parts cast shadows. Rotate the part, add sources from other directions or use a dual-cure material.
• Heat from large arrays. The LEDs emit no infrared, but a large, high-power array produces heat that has to be removed by fans or water cooling, and absorbed UV warms the part.
• Operator protection. An open area source exposes a large field. Shield the curing zone, or use an enclosed oven or conveyor.
• Moving products. For products that move continuously past the lamp, a narrow linear source or a conveyor is usually more efficient.

How to specify a flood system

1. Part or tray size, plus a margin, to set the emitting area.
2. Required dose and minimum irradiance, from the material data sheet, which give the exposure time and the power class.
3. Wavelength, from the material data sheet.
4. Working distance and uniformity required across the part.
5. Cooling: fan cooling for most sizes, water cooling for the highest irradiance.
6. Format: an open source for integration into a machine, an oven for batches and laboratory work, or a conveyor for continuous production.
7. Control: external triggering, PLC and RS232 interfaces, status and alarm outputs, and exposure modes.

Awellcure area curing systems

Awellcure UV LED Area sources range from 20 × 20 mm, with a peak irradiance of 5,000 mW/cm², to fan-cooled panels of 400 × 400 mm and 250 × 600 mm. Water-cooled sources reach 6 to 25 W/cm². Their controllers start exposure from external signals, a foot switch or a sensor, provide status outputs for start, stop, working and alarm, and can be programmed through RS232. The same technology is built into enclosed UV Curing Ovens and UV Conveyor systems.