3D Curing - UV Lamp Design

The variety of shapes and sizes, angles and curves, edges and recesses present in a 3-dimensional object requires that the lamps deliver UV energy to all the surfaces.

UV Lamp Design Basics

Typically a UV lamp will consist of a tubular bulb together with a reflector - usually elliptical in shape, sometimes parabolic - to collect and direct the UV light onto the part. With the bulb placed at the primary focus of the ellipse, the elliptical reflector focuses the UV light to a narrow band of high peak irradiance at the secondary focus of the ellipse. However, owing to the divergence of the UV rays the peak irradiance diminishes rapidly beyond the focal point. A parabolic reflector produces parallel rays of UV light over a wider area but with much lower energy (although the parallel rays can cause shadows in some 3D applications).

Challenges of Industrial 3-D Coating

Many applications in industrial three-dimensional coating, require curing of "optically thick" coatings; that is, coatings which have a high absorptivity of UV light. This has the effect of rapidly diminishing the energy density of the UV, as it travels through the coating. To ensure adequate curing, sufficient UV must be able to reach the bottom of the coating. Coatings, which are applied at high film weight, have high pigmentation or filler levels are those that present the greatest challenge for achieving good through cure. Delivering high peak irradiance UV to the coating will maximize through cure. On flat substrates this can be achieved by using elliptical reflectors and ensuring the coating runs through the area of highest peak irradiance at the focus of the UV. In curing coatings on 3-dimensional objects, this is much more difficult.

A compact, modular lamp system can be arranged to follow the contours of a part, or targeted at specific areas. By modifying the elliptical reflector cavity of the microwave powered lamp, the focus can be altered, pushing it further from the lamp face and widening the focal area. This reduces the high peak irradiance, but allows a more consistent irradiance level to be achieved over a longer distance, while still maintaining the advantage of converging and diverging rays to minimize shadowing. See Table 1 for a comparison.

Table 1 - Comparison of peak irradiance values in W/cm2, 240W/cm lamp (600W/in)

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