“Cures in seconds” is printed on nearly every UV resin glue, but that number describes a lamp, a distance, and a clear substrate you may not have. The bond line only cures on the energy that actually arrives there.
Q: What is UV resin glue, and how fast does it really cure?
A: UV resin glue is a one-part, light-curing adhesive — usually an acrylate — that stays liquid indefinitely until exposed to ultraviolet or near-visible light, then hardens in seconds. In industrial terms:
- Wavelength: most grades respond between 365 nm and 405 nm. The lamp’s output must overlap the resin’s absorption band.
- Irradiance: measured in mW/cm² at the bond line, not at the lamp face.
- Dose: irradiance × time, expressed in mJ/cm². This is the number that determines whether the resin cures through.
- Substrates: it bonds glass, metal, ceramic, and many rigid plastics — provided at least one side lets enough light through, or the joint edge is directly exposed.
The “5–10 second” claim holds when a strong lamp sits close to an exposed bead. Put a tinted plastic or a thick glass pane between the lamp and the resin, and the same resin may need several times longer — or never fully cure.
The Three Numbers in the Worksheet
Delivered dose at the bond line comes from three factors multiplied together:
Delivered dose = irradiance at the surface × substrate transmittance at the lamp wavelength × exposure time
Each factor is measurable, and each one is routinely assumed instead.
Irradiance at the surface depends heavily on working distance. Incure’s L9000™ UV LED spot lamp publishes its irradiance curve at 365 nm: 7,500 mW/cm² at a 9 mm focal distance, 5,000 mW/cm² at 10 mm, 2,300 mW/cm² at 17 mm, 1,200 mW/cm² at 20 mm, 450 mW/cm² at 25 mm, and 223 mW/cm² at 30 mm. Moving the lightguide from 10 mm to 25 mm cuts delivered irradiance by roughly 90%. A fixture that drifts a few millimeters changes cure results more than most people expect. The L9000™ lightguide and working-distance guide covers how to set that distance repeatably.
Substrate transmittance is the factor most often ignored. Clear soda-lime glass passes a large share of 365–405 nm light at typical thicknesses, but transmission falls off sharply below about 320 nm. Many polycarbonates and acrylics contain UV stabilizers that absorb strongly below roughly 380–390 nm, which can reduce 365 nm transmission to a small fraction while 405 nm still passes. Tints, coatings, and anti-reflection layers change the picture again.
Exposure time is the only factor the operator controls directly, which is why it gets over-adjusted to compensate for problems in the other two.
Working an Example
Consider a clear acrylic cover bonded to a glass window, cured through the acrylic. Suppose the resin’s data sheet calls for 2,000 mJ/cm² at 365 nm for full cure at the bond-line thickness in use.
- The lamp sits at 20 mm: 1,200 mW/cm² at the acrylic surface.
- A radiometer placed behind a coupon of the same acrylic reads only 10% of the open-air value at 365 nm, because of the UV stabilizer package.
- Delivered irradiance at the bond line: 120 mW/cm².
- Time to reach 2,000 mJ/cm²: about 17 seconds — not the 2 seconds an open-air calculation would suggest.
Now switch the same setup to a 405 nm lamp. If the acrylic passes 80% at 405 nm, and the resin is formulated to respond at that wavelength, the delivered irradiance becomes far higher and the cure time drops sharply. The wavelength decision is a transmission decision as much as a resin decision.
How to Measure Transmittance on the Bench
A handheld UV radiometer matched to the lamp’s wavelength makes this a ten-minute test:
- Measure irradiance at the planned working distance with nothing in the path.
- Place a coupon of the actual substrate — same material, thickness, and coating — directly over the sensor and measure again.
- Divide the second reading by the first to get effective transmittance.
- Repeat at each candidate wavelength if more than one lamp is available.
Keep the coupon readings on file. Material suppliers change stabilizer packages, and a lot-to-lot drop in transmittance will show up as undercured parts long before anyone suspects the substrate.
If your substrate transmittance comes out low and you want a second opinion on wavelength and grade, Email Us with the readings.
Matching Resin Viscosity to the Light Path
Once enough light reaches the bond line, resin viscosity decides how evenly it fills the joint — and a thin, uniform bond line cures more consistently than a thick, uneven one. Incure’s Uni-Weld™ glass-and-metal bonder line shows the range available: grade 1910 runs at 30–60 cP for capillary wicking into pre-assembled glass joints, grade 1931 at 90–180 cP, grade 2204 at 1,200–2,400 cP, and grade 2204VT at 18,000–36,000 cP for gap filling and vertical faces. Thicker beads need more dose to cure through their full depth, so a high-viscosity grade in a deep bead pushes the worksheet’s required time upward.
For plastic-to-plastic joints, where transmittance limits bite hardest, the Uni-Weld™ plastic bonder grade guide maps grades to substrate and flexibility. For a broader comparison of when a light-cured resin beats a conventional adhesive at all, see UV resin vs. traditional glue.
Signs the Dose Is Short
Undercured UV resin glue rarely announces itself at the line. Watch for:
- A tacky surface film on exposed fillets, beyond normal oxygen-inhibited surface tack.
- Bonds that pass immediately but lose strength after a day or two.
- Softening or haze when the part sees 60–80°C.
- Strength that varies with part position on the fixture — a working-distance signal.
Each of these traces back to one of the three worksheet factors. Recalculate before adding more exposure time blindly; excess exposure can yellow some resins and heat-sensitive substrates without fixing a transmission problem.
To set up a dose worksheet for your own substrate, lamp, and resin combination, Contact Our Team.
Visit www.incurelab.com for more information.