Curing UV Adhesive in a Recessed or Blind Hole
The hardest UV cure is the one the light can’t reach. Set screws, dowel pins, and potted connectors sit at the bottom of a bore where a standard guide throws most of its intensity onto the walls. The fix is more flux down the hole, a guide sized to the bore, and, when geometry truly blocks the light, an adhesive with a secondary cure mechanism.
UV only cures where photons land. Down a narrow, deep bore the working distance grows and the walls shadow the joint, so intensity at the bottom drops sharply. A larger-core, high-flux guide delivers more usable UV into the hole, and the broad-spectrum arc penetrates further into thicker adhesive films than a single narrow LED band.
The SunSpot 2 pairs the highest intensity we sell (>18,000 mW/cm² UVA) with a broad 275–650 nm spectrum that drives cure deeper into a thick or shadowed film. A high-flux 8 mm guide puts as much of that energy as possible down the bore.
Step by Step
- 1 Match the guide to the bore. Use the largest guide that clears the hole so the maximum flux enters instead of hitting the rim. The high-flux 8 mm guide is the workhorse for recesses.
- 2 Get the tip as deep as geometry allows. Lower the guide into the mouth of the bore so the exit face is close to the adhesive. Every millimetre closer meaningfully raises intensity at the joint.
- 3 Extend the exposure time. A shadowed joint needs more dose than a flat one. Dose is intensity multiplied by time, if you can’t add intensity, add seconds.
- 4 Verify the bottom actually cured. Probe the joint after cure. A skinned-over top with soft adhesive underneath means the light never reached full depth, increase time or switch to a dual-cure adhesive.
- 5 If light truly can’t reach, change chemistry. When the geometry fully blocks line of sight, no source can cure it optically. Choose an adhesive with a secondary cure (heat, moisture, or activator) so the shadowed volume still hardens.
Common Pitfalls
- Using a guide narrower than the bore, most of the light lands on the rim, not the joint.
- Trusting a cured-looking surface, the top skins first; the bottom is what fails.
- Fighting physics, if the joint has no line of sight to any light path, switch to a dual-cure adhesive rather than adding exposure forever.
Questions
Can UV light cure adhesive at the bottom of a deep hole?
Only as far as the light reaches. A high-flux, large-core guide lowered into the bore gets usable intensity surprisingly deep, and a broad-spectrum arc penetrates thick films better than a single LED band, but if the geometry fully shadows the joint, use a dual-cure adhesive that also sets by heat, moisture, or activator.
What light guide is best for recessed joints?
The largest guide that fits the bore. The high-flux 8 mm liquid light guide (UV0774) delivers the most flux into a recess; drop the tip into the mouth of the hole to shorten the working distance.
How do I know the adhesive cured all the way down?
Verify, don’t assume. A radiometer confirms real intensity at depth, and physically probing the joint after cure catches a skinned-over top hiding soft adhesive underneath.
