Today, UV curing is being used very successfully on complex, three-dimensional parts including automotive lighting assemblies, plastic molded parts such as mobile phone cases and other electronic devices, contoured wood products, golf balls, electric motor housings and fiberglass composite parts.
UV Curing Three Dimensional (3D) Parts
UV curing of complex shaped parts can be a challenge because UV curing is "line-of-sight", meaning that every point on a surface must see the UV energy to cure the coating. Hidden or shadow areas will remain uncured (remain wet).
Can Your 3D Part be UV Coated?
Noblelight has many years of experience with 3D UV curing projects. The information below can help you determine whether UV curing may be suitable for your 3D product or part.
Chemistry
Applying coatings to 3D parts, including spray coating or flow coating requires low viscosity coatings that, in many cases, cannot be easily achieved using a 100% UV curable system.
Addition of small amounts of organic solvents is often used to meet viscosity requirements. While not ideal, manufacturers do not see this as an insurmountable problem.
Of course, 100% solids, sprayable or flow-coatable, UV curable coatings are the ultimate goal and some are available. UV powder coatings are also a route to 100% solids systems for 3D parts.
Hybrid UV/Thermal Coatings
Dual cure hybrid coatings (combination UV and thermal) are yet another option especially suited to coating parts that have shadow areas or for retrofit situations where thermal ovens already exist. The addition of UV to a thermal curing line can significantly increase line speeds and coating quality. These hybrid systems require much lower bake temperatures – and for shorter periods – than their 100% thermal counterparts. In addition, the part may be touch-dry in all areas after the UV exposure, thus retaining the advantage of less contamination risk. The hybrid systems retain all of the improved performance characteristics of 100% UV curable coatings in terms of improved scratch, chip, scuff resistance, etc.
Although Noblelight does not sell UV curable coatings, we can provide a list of companies that may have a coating to meet your needs.
UV Curing 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.
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 reach the bottom of the coating. Coatings with high film weight, pigmentation, or filler levels present the greatest challenge for achieving good through cure. Delivering high peak irradiance UV to the coating maximizes through cure. On flat substrates, using elliptical reflectors and ensuring the coating runs through the area of highest peak irradiance at the focus of the UV energy ensures adequate cure. When 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 UV 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, 240 W/cm lamp (600 W/in)
| Lamp Distance From Part | |||
|---|---|---|---|
| Reflector Type | 50mm | 100mm | 150mm |
| Standard Elliptical | 2.45 | 0.46 | 0.14 |
| Modified Elliptical | 1.63 | 0.65 | 0.21 |
Use of Robotics
Mounting UV curing lamps on robotic devices can produce great advantages for curing 3D parts. It works particularly well, and is most cost effective, when the production line contains a mix of shapes and sizes that are difficult to cover with a "static" array of lamps. Robots allow easier access to shadow areas that could be impossible to reach by fixed lamps.
Issues to consider when using robotics:
- The UV lamp must be sufficiently robust to withstand the acceleration and de-acceleration swings of the robot arm.
- The lamp must be able to operate efficiently and reliably in a variety of different positions.
- Ensure there is sufficient space to allow the robot movement to cover all areas.
- The robot must be programmed to ensure it delivers the correct UV energy to all parts. If the part has been robotically spray-coated, for example, the robotic curing system can follow essentially the same pattern and path as the coating robot.
Custom UV Curing Solutions for 3D Parts
A custom engineered solution for UV curing of 3-dimensional parts depends on several critical factors:
- The overall size and shape of the part, including what surfaces, edges and angles need to be cured, potential shadow areas, and the mix of parts traveling through the production line
- The transport mechanism of the part, including part orientation, how parts are held and presented to the UV lamps (single file or batches, hanging or on jigs, rotating or fixed)
- The speed or throughput on the line, including how much time will be needed to achieve the required UV energy, and any space restrictions
- The exposure requirements of the coating, including specifications of irradiance, wavelength range, time, and infrared exposure to produce the required physical properties
- The UV energy and irradiance, including the number of lamps needed, what lamp power, and lamp position to deliver the most uniform exposure to ensure the part meets the required end use specification
The UV energy and irradiance, including the number of lamps needed, what lamp power, and lamp position to deliver the most uniform exposure to ensure the part meets the required end use specification:
| UV Installation | Design Applications |
|---|---|
| Modular lamps with individual movement mounted on a fixed framework |
Polycarbonate headlamp lenses |
| Modular lamps with individual movement combined with rotation of the object | Powder coated motors |
| Coated part held by robot and rotated under a fixed lamp | Small, intricate parts used in consumer electronics |
| UV lamp mounted on a robot | Wide range of plastic boxes and frames |
| Fixed lamps and robot mounted lamps | Sealer coat on SMC body panels |