Surface cure is easy; full-depth cure is where most UV processes fail. Light weakens as it travels through resin, and anything it cannot reach stays liquid or soft.
Q: Why Does UV-Cured Material Stay Soft Underneath?
A: Light Is Absorbed Before It Reaches the Bottom
Photoinitiators and pigments absorb UV as it enters the layer, so intensity falls with depth. Past a certain thickness, too little light remains to trigger polymerization. The remedies are a more penetrating wavelength, higher irradiance, thinner layers, or a secondary cure mechanism. For the broader system choices, see Incure’s overview of benchtop UV curing systems.
How Absorption Limits Depth
Intensity decays roughly exponentially through the layer. Pigments, fillers, and the photoinitiator itself all absorb. Strongly absorbing systems cure fast at the surface but shallowly; weakly absorbing systems cure deeper but slower. Formulators balance the two, so the data sheet’s stated cure depth is specific to a lamp output and wavelength.
Wavelength and Penetration
Shorter wavelengths are absorbed more strongly near the surface; longer ones travel deeper. In practice, 365 nm gives a hard surface on thin films, while 395–405 nm tends to reach farther into thicker or tinted layers. Incure’s L9000™ spot lamp is offered at 365, 375, 385, 395, and 405 nm, each unit set to one wavelength at the factory, so the choice must be made when specifying. Broad-spectrum arc sources such as the S20™ (275–650 nm) cover several bands at once.
Intensity Helps Depth
Higher irradiance pushes more light past the surface layer before it is absorbed. This is a reason to use concentrated sources on heavy sections: the L9000™ reaches 7,500 mW/cm² at a 9 mm focal point, and the water-cooled W44 reaches 8,100 mW/cm² across its area at 365 nm, compared with 3,100 mW/cm² for the air-cooled L44. The right comparison link is the L-Series flood lamp guide.
Shadow Areas
Light travels in straight lines. Undercuts, the underside of components, and areas beneath opaque parts receive none. Three approaches work:
- Redesign the joint so adhesive is exposed, or use a transparent substrate.
- Light from multiple angles, using several lightguide tips or a rotating fixture.
- Use a dual-cure material that adds a moisture, heat, or anaerobic secondary cure for the shadowed region.
Dual-cure adds a delay and a process step, so confirm the secondary cure completes before the part is loaded.
Email Us with the layer thickness, pigment level, and geometry, and Incure’s team can suggest a wavelength and lamp format.
Staged or Layered Curing
For deep sections, cure in layers: dispense, expose, add the next layer, and repeat. Each layer receives full-intensity light, and the exposed surface of the previous layer bonds chemically to the next. Typical practice limits each layer to the depth established by a test cure, then adds a margin.
Testing Depth
A simple test: fill a stepped mold or stack cured samples, expose at the process settings, remove uncured liquid with a solvent suited to the material, and measure the solid thickness. Repeat with different wavelengths or exposure times. This produces a depth-versus-dose curve for your own material rather than a catalog figure.
Heat and Shrinkage in Thick Sections
Fast cure of a thick section releases heat and shrinks the material. That can warp thin substrates or crack brittle ones. Lower irradiance with longer time, or layered exposure, spreads the stress. Incure’s Cast-Max™ dome coatings are an example of a product line specified by cavity depth and cure temperature.
Choosing Between the Fixes
Each remedy has a cost. A longer wavelength may change how the surface cures and can leave a slight tack from oxygen inhibition, which a short 365 nm finishing pass or a nitrogen blanket can address. Higher irradiance shortens the cycle but adds heat. Layering adds steps but gives the most predictable result for deep fills. Dual-cure chemistry handles shadows but adds a wait. Rank the options against the constraint that matters most on your line, whether cycle time, substrate heat tolerance, or joint geometry.
Documenting a Depth Process
Once the settings produce full-depth cure in test, record the wavelength, irradiance at the part, exposure time, layer thickness, and the measured solid depth. Re-run the depth test when the lamp, material lot, or pigment loading changes, because each of these shifts absorption and therefore penetration.
Quick Reference
Thin, clear films cure with nearly any matching lamp. Tinted layers need longer wavelengths and more irradiance. Deep fills need layering. Shadowed joints need a second cure path. Matching the fix to the situation, and testing it, avoids most soft-core failures.
Final Thoughts
Depth is a design variable. Match wavelength to thickness, raise intensity where needed, light the shadows or add a secondary cure, and verify with a depth test on your own material. Contact Our Team to plan a thick-section curing process.
Visit www.incurelab.com for more information.