UV Curing: Process & dose, every question, answered

What's the difference between irradiance and dose?

Irradiance is the instantaneous power hitting the surface, in W/cm² or mW/cm²; dose, also called energy density, is irradiance integrated over time, in J/cm². Think of irradiance as brightness and dose as the total light energy delivered.

The two work together: an adhesive needs enough irradiance to cure well and enough total dose to cure fully.

How do W/cm² and J/cm² relate to each other?

Multiply irradiance by exposure time: 1 W/cm² held for 1 second delivers 1 J/cm². So a 2 W/cm² spot held for 5 seconds gives a 10 J/cm² dose.

Watch units, datasheets often mix mW/cm² and W/cm²; 1,000 mW/cm² equals 1 W/cm².

Does doubling exposure time double the dose?

Yes, dose scales linearly with time at constant intensity, so twice the seconds is exactly twice the J/cm². Just remember that doubling dose doesn't always mean a proportionally better cure; most adhesives plateau once fully crosslinked.

Why doesn't doubling cure time double bond strength?

Cure follows an S-curve, not a straight line: strength climbs quickly, then plateaus as the polymer network completes. Past full conversion, extra time mostly adds heat and, eventually, overcure effects like yellowing or embrittlement.

That's why we recommend finding the dose where properties plateau, then setting production time modestly above it rather than assuming more is better.

What is an intensity threshold in UV curing?

Below a certain irradiance, some adhesives never fully cure no matter how long you expose them, oxygen and stabilizers consume reactive species faster than the light creates them. The adhesive datasheet usually states a minimum intensity; stay comfortably above it.

Can I use lower intensity for a longer time instead?

Only within limits. Dose trades intensity for time in theory, but very low intensity can fall below the adhesive's cure threshold, and long exposures hurt cycle time. High-intensity spot sources exist precisely so you can cure in seconds, check the adhesive datasheet for its minimum irradiance.

What is reciprocity in UV curing?

Reciprocity is the idea that intensity and time trade off, half the intensity for twice the time gives the same dose. Real chemistries follow it only approximately: at very low intensity cure can stall, and at high intensity depth of cure often improves beyond what dose alone predicts.

My adhesive datasheet lists a dose. How do I set time?

Measure irradiance at your actual working distance with a radiometer matched to your source's band, then divide: time = required J/cm² ÷ measured W/cm². Confirm with your own functional checks before locking the recipe.

What's a typical spot-cure exposure time?

Most spot cures land between 1 and 10 seconds, tacking is often 1–4 s, structural bonds and potting 2–10 s. The right number depends on your adhesive, thickness, and measured intensity, so treat these as starting points, not answers.

How do I find the dose my adhesive actually needs?

Start with the datasheet's recommended dose, then run a ladder in-house: cure coupons at increasing exposures and test them until properties plateau. The knee of that curve is your minimum; production runs comfortably above it.

Why is my adhesive surface still tacky after curing?

Almost always oxygen inhibition: ambient oxygen scavenges radicals at the surface, leaving a thin uncured layer even when the bulk is hard. Higher intensity, broad-spectrum light with UVB content, or nitrogen blanketing all help.

Our broad-spectrum SunSpot 2 is the usual fix for chemistries that stay tacky under single-wavelength LEDs.

How do I get rid of surface tack?

Raise intensity at the surface, switch to a broad-spectrum source with short-wavelength content, choose an adhesive formulated for tack-free surface cure, or exclude oxygen with an inert purge. A quick IPA wipe removes a very thin inhibited layer but doesn't fix the root cause.

Does higher intensity improve depth of cure?

Generally yes. Higher irradiance drives more photons deeper before they're absorbed, so intense spot sources cure thicker sections than the same dose delivered slowly. That's one reason time-for-intensity trades aren't perfectly symmetrical.

How does adhesive thickness affect cure time?

Light is absorbed as it travels through the adhesive, so deeper material sees less intensity and needs more total exposure. Thick sections may need layered fills with a gel cure between passes, or a dual-cure chemistry, check the adhesive datasheet's depth-of-cure rating.

How deep can UV light cure an adhesive?

It depends on the formulation's absorbance, clear acrylates often cure several millimeters, filled or pigmented materials far less. With our broad-spectrum SunSpot 2, dam-and-fill encapsulation cures to roughly 5 mm; beyond that, fill in layers or use a dual-cure product.

Can I cure adhesive through a plastic part?

Only if the plastic transmits your cure wavelength. Many engineering plastics block deep UV but pass 405 nm reasonably well, so a 405 nm-curable adhesive plus a 405 nm source is the common recipe for through-plastic bonds. Verify in-house by measuring intensity through an actual part.

Can I cure through glass?

Usually yes, standard soda-lime glass transmits UVA well, though it attenuates it somewhat and blocks most UVB. Measure irradiance behind a real glass coupon and extend exposure to compensate; 365–405 nm sources work well here.

Why does wavelength matching matter for cure speed?

The photoinitiator only absorbs certain wavelengths, light it can't absorb contributes nothing, no matter how intense. Match the source to the datasheet's recommended band; 365 nm is the workhorse, and some initiators are tuned for 385, 395, or 405 nm.

The SkyBeam's four channels can each hold a different wavelength head (365, 385, 395, or 405 nm), so one controller covers mixed chemistries.

365 nm or 405 nm, which cures my adhesive faster?

Whichever your photoinitiator absorbs more strongly, there's no universal winner. 365 nm suits the most common initiators; 405 nm penetrates deeper and passes through more substrates. The adhesive datasheet is the tiebreaker.

What actually drives my total cure time?

Six things dominate: the adhesive chemistry, delivered intensity at the joint, wavelength match, adhesive thickness, bond-gap geometry, and any substrate the light must pass through. Improving the weakest of those usually cuts time more than pushing the others harder.

How do I develop a cure process window?

Fix distance and intensity, then step exposure time up in increments, curing several parts per step. Test each group in-house, bond strength, hardness, appearance, to find where results plateau (your minimum) and where degradation begins (your maximum).

Document the range between those two points as the window, along with every parameter that produced it.

Where in the process window should production run?

Comfortably above the minimum, run meaningful margin above the dose where properties plateau, so normal drift from lamp aging or small distance shifts never drops you below minimum before your next scheduled check catches it.

Is overcuring an adhesive a real problem?

It can be. Gross overexposure adds heat and can yellow, embrittle, or stress the adhesive and nearby plastics. Moderate margin is healthy; running at multiples of the required dose wastes cycle time and risks damage, define a maximum during qualification.

What are the signs of an undercured joint?

Surface tack, low hardness, a rubbery feel, visible liquid at fillet edges or in shadows, weak or inconsistent pull strength, and lingering adhesive odor. If any appear, verify output with a radiometer before touching the recipe, drift is more common than bad adhesive.

What are the signs of overcure?

Yellowing or browning, brittleness or cracking in the fillet, heat damage to surrounding plastic, and crazing on stressed substrates. If these appear at your validated settings, check that the timer and intensity haven't been altered.

What is a step or ramp cure?

Instead of one full-power blast, you cure in stages, a lower-intensity or very short first step to gel the adhesive gently, then a full exposure to finish. It reduces shrinkage stress and part movement in precision joints.

When should I use a ramped or stepped cure?

When shrinkage stress or movement during cure causes problems: optical alignments that drift, thin substrates that bow, or large fillets that crack. Gel first with a brief exposure, verify position, then complete the cure.

What is a tack-then-full-cure workflow?

You give each joint a short exposure that gels the adhesive just enough to hold position, do any remaining assembly or inspection, then apply the full cure. It decouples holding the part still from making it strong.

Why tack parts first instead of fully curing?

Tacking lets you verify alignment while rework is still easy, a gelled joint can often be corrected; a fully cured one can't. It also spreads heat input and lets one high-power station handle final cures for several tack stations.

How long should a tack exposure be?

Just long enough to gel, often a fraction of a second to a second or two on a high-intensity spot. Establish it in-house: step time down until parts no longer hold position, then add margin above that point.

Do parts need to stay clamped during the cure?

Yes, the joint must be motionless until the adhesive gels, which happens within the exposure itself. That's seconds on a spot system, which is why UV curing frees fixtures almost immediately compared with heat-cure clamping.

How soon can I handle a part after the exposure?

UV acrylates reach handling strength essentially at the end of the exposure, that's the appeal of spot curing. Full properties can take somewhat longer as the network finishes, so check the datasheet before applying full service loads immediately.

Does curing continue after the lamp switches off?

For standard UV acrylates, essentially no, polymerization stops within moments of losing light, so anything shadowed stays wet. Cationic epoxies are the exception: they continue a dark cure for minutes to hours after exposure.

How does working distance affect intensity?

Output from a light-guide tip diverges, so irradiance falls off quickly as you back away, small distance changes near the tip produce large intensity changes. Fix the distance mechanically and treat it as a controlled process parameter.

Whatever distance you choose, measure irradiance at that plane, not at the tip, and build the recipe on that number.

Does the inverse-square law apply to light guides?

Not exactly. Inverse-square describes a bare point source; a guide tip emits a diverging cone, so falloff near the tip is steeper and less tidy than the textbook law, and focusing optics change it again. The practical rule: measure at your real working distance rather than calculating.

What working distance should I use?

As close as the fixture and dispensing allow without touching adhesive, closer means higher intensity and shorter cures. Typical spot setups sit several millimeters to about a centimeter above the joint; our PCB tacking note uses 8–12 mm for a 5 mm spot.

How does spot size change with distance?

The beam diverges, so the spot grows as you pull back and the same power spreads over more area, intensity drops roughly with the square of the spot diameter's growth. Backing off is a legitimate way to cover a wider bond line, at the price of longer exposure.

Should the light guide be fixtured or handheld?

Fixtured, whenever the part allows it. A clamped guide gives the same distance and angle every cycle, which makes dose repeatable; handheld work adds operator variability. Reserve handheld curing, like our SkyWand, for rework and parts too large to fixture.

How do I fixture parts for repeatable curing?

Give the part hard locating features (nest, pins, stops) so the joint lands in the same place every cycle, clamp the guide tip at a fixed distance and angle, and make the exposure timer-controlled. Distance, angle, and time then stop being operator variables.

What angle should the beam hit the adhesive?

As close to perpendicular as geometry allows, delivered energy falls off with the cosine of the incidence angle, and steep angles stretch the spot into an ellipse. When walls shadow the joint, two guides at opposing 45° positions beat one extreme angle.

Why does shadowed adhesive stay uncured?

UV curing is line-of-sight: adhesive the light can't see receives no dose and stays liquid, except with dual-cure or cationic chemistries that have a secondary mechanism. Design fixtures and dams so the guide tip sees every fillet.

How do I cure adhesive hidden under a component?

You can't with light alone, pick a dual-cure adhesive whose secondary mechanism (moisture, heat, or anaerobic) finishes the shadowed regions, or redesign so light reaches the joint, for example through a transmissive substrate or from a second angle.

Can I cure several joints at the same time?

Yes. On the SunSpot 2, a multi-pole splitter divides one lamp into up to four balanced legs; on the SkyBeam, four independent LED channels each run their own timer. Aim one leg per joint and cure the whole pattern in a single cycle.

How do I balance a multi-point cure?

Use a balanced splitter, ours are matched to ±5%, set every leg to the same working distance and angle, then verify each tip with a radiometer. Time the exposure for the weakest leg so every joint meets minimum dose.

Which leg sets the timer on a splitter setup?

The weakest one. Measure irradiance at every tip at the working plane, compute time from the lowest reading, and every other joint simply gets a little extra margin. Re-check balance whenever you swap a guide or lamp.

Sequential or simultaneous cures, which is better?

Simultaneous (splitter or multi-channel) wins on cycle time and gives every joint an identical recipe; sequential with one guide is cheaper and fine at low volume. If joints interact mechanically, shrinkage pulling on alignment, simultaneous curing also balances the stresses.

How do I estimate parts per hour for a cure station?

Cycle time = load + position + exposure + unload. With a 4 s cure and 6 s of handling you're near 360 parts per hour on one station, and a four-point simultaneous cure multiplies joints, not time. Measure your handling honestly, it usually dwarfs the exposure.

What's the best way to cut cure cycle time?

In order of leverage: raise delivered intensity (shorter distance, smaller spot, fresher lamp and guide), cure multiple joints at once, and only then shave timer margin, never below your validated minimum dose. Handling time is often the true constraint, so fix that first.

How do I cure a long bead with a small spot?

Either step along the bead with overlapping timed exposures, spread the spot with optics or working distance to cover the width, or split the source and cure several segments at once. Keep overlap generous, the junctions between exposures are where undercure hides.

Bigger spot or multiple exposures for a wide joint?

A bigger spot (an 8 mm guide, or focusing optics spread wide) keeps one exposure and one recipe but lowers intensity; multiple placements keep intensity high but add cycle time and overlap risk. If the wider spot still exceeds your adhesive's minimum intensity, it's usually the cleaner process.

What causes a cure process to drift over time?

The big three: lamp or LED output declining with age, light-guide transmission degrading through end-face wear and solarization, and distance creep as fixtures loosen or get bumped. Adhesive lot changes and expired material round out the list.

Routine radiometer checks catch the optical causes early; lot logging catches the chemistry.

How does lamp aging affect my process?

Arc lamp output declines gradually across its life, so a time-based recipe delivers a little less dose every week. Trend tip irradiance on a schedule and replace the lamp when output approaches your alarm limit, before parts fall below minimum dose.

Keeping a spare lamp module on the shelf turns replacement into a minutes-long swap instead of a line-down event.

Do LED systems drift like arc lamps?

Far less, SkyBeam's LEDs are rated for 20,000 hours and decline slowly, but slowly isn't never. Keep the same radiometer schedule; the SkyBeam's per-channel life logs help you trend usage against measured output.

What is distance creep and how do I stop it?

It's the slow migration of the guide tip away from its validated position, loosened clamps, bumped fixtures, rebuilt tooling. Since intensity falls fast with distance, small creep quietly shrinks your dose.

Pin or hard-stop the guide mount, mark the validated position, and audit it during routine checks.

Can adhesive lot changes affect my cure?

Yes, photoinitiator concentration and viscosity vary slightly lot to lot, and expired or badly stored adhesive cures noticeably slower. Record lot numbers with your process data and run a first-article check whenever a new lot starts.

Can room lighting pre-cure my adhesive?

Bright daylight and some overhead fixtures emit enough near-UV and blue light to slowly gel exposed adhesive, especially 405 nm-sensitive chemistries. Keep containers closed, limit open time, and use UV-filtered task lighting at dispense stations if you see skinning.

Parts started failing after a lamp change, why?

A new lamp usually raises output, which changes delivered dose, any recipe tuned around a tired lamp is now off. Alignment of the new module can also shift coupling into the guide. Re-measure at the tip, reconfirm your window, and update the timer if needed.

What are the most common spot-curing mistakes?

Setting time by eye instead of from measured irradiance, no radiometer schedule, uncontrolled working distance, curing shadowed joints and hoping, ignoring the adhesive's minimum intensity, and never revalidating after lamp or guide swaps.

Every one of these is cheap to fix with fixturing, a measurement routine, and a written window.

What's the most common cause of weak UV bonds?

Under-dose, usually from output drift or distance creep rather than a bad adhesive, followed closely by poor surface prep and shadowed adhesive. Verify irradiance at the working plane first; it explains most sudden strength drops.

Should exposures be timed or operator-controlled?

Timed, always. A footswitch that opens a shutter for a programmed interval removes the single biggest human variable. The SunSpot 2 has an integral timer and shutter, and each SkyBeam channel runs its own programmable timer.

Do UV systems need a warm-up before production?

Arc lamps need a few minutes after strike to stabilize, so let the SunSpot 2 settle before measuring or curing; the shutter handles idle time between parts. LEDs reach full output effectively instantly, one reason automated lines like them.

Should I adjust intensity or time to tune a cure?

Prefer adjusting time at a fixed, known intensity, it keeps the dose math simple and preserves your margin over the adhesive's intensity threshold. Use LED intensity settings mainly to match a validated recipe across machines or to tame heat on delicate parts.

Does bond gap size change cure requirements?

Yes, a wider gap means a thicker adhesive section that needs more dose to cure through, and larger volumes shrink more as they cure. Keep gaps consistent with fixturing, and validate at your worst-case gap, not the nominal.

Does substrate color affect the cure?

Dark or absorptive substrates soak up light that would otherwise reflect back through the adhesive, so joints on black plastic often need slightly more exposure than the same joint on white or metal. Validate per substrate rather than assuming one recipe fits all.

Will spot curing overheat delicate components?

Rarely, that's the point of the technique. Energy is confined to a 2–12 mm spot, LED sources add almost no infrared, and exposures last seconds. For extremely sensitive optics, use the lowest validated intensity, break the cure into steps, and shield neighbors.

Can one machine run two different adhesives?

Yes, the cleanest way is the SkyBeam, where each of the four channels stores its own wavelength head, intensity, and timer, so adhesive A and adhesive B each get their own validated recipe on the same controller.

How do dual-cure adhesives change my process?

You still need the full UV step for everything light can reach, but shadowed or deep regions finish by the secondary mechanism, moisture, heat, or anaerobic, on its own clock, often hours. Plan downstream handling and your in-house strength checks around the slower mechanism's schedule.

How do I keep optics aligned while adhesive cures?

Use a gel-then-cure sequence: a brief low-dose exposure freezes the joint with minimal shrinkage, you verify alignment, then complete the cure. Symmetric multi-point exposure, opposing legs fired together, also balances shrinkage pull.

In what order should I cure multiple tack points?

Diagonally opposite pairs first, like torquing a wheel, it balances shrinkage so the part isn't pulled sideways. Firing opposing points simultaneously through a splitter or paired channels is even better.

What process parameters should I document?

Source and settings (model, wavelength, intensity), guide and optics part numbers, working distance and angle, exposure time, measured irradiance at the working plane, adhesive product and lot, and the date and radiometer used.

That record is what lets you rebuild or troubleshoot the process months later.

How do I set up a brand-new cure process?

Fixture the joint, set the shortest practical working distance, measure irradiance at that plane, and compute a starting time from the adhesive's datasheet dose. Then run a time ladder, test parts in-house to find the plateau, set production time with margin, and document the window.

How do I safely qualify a shorter cycle time?

Don't just turn the timer down, requalify: run a ladder between current and target times, test parts at each step, and confirm the shorter time still sits above your minimum-dose knee with real margin. Raising intensity via a closer distance or fresher optics is often safer than trimming margin.

How do I make handheld curing repeatable?

Add a mechanical standoff so the wand tip always sits at the same distance, mark or fixture the aim point, and use timed exposures rather than counting seconds. The SkyWand's constant-output design removes battery fade as a variable; the standoff removes the operator's arm.

What is energy density?

It's another name for dose, the total UV energy delivered per unit area, in J/cm², which equals irradiance in W/cm² multiplied by seconds of exposure. Adhesive datasheets use the two terms interchangeably.

Why do peak irradiance numbers matter?

Because cure chemistry responds to instantaneous intensity, not just accumulated energy, a high peak drives radicals past oxygen inhibition and deeper into the material. Two processes with equal J/cm² but different peaks can produce different cures.

Surface cure vs through-cure, what's different?

Surface cure is the skin the light hits first, where oxygen inhibition lives; through-cure (depth of cure) is how far polymerization reaches into the section. A joint can be hard on top and wet underneath, so your in-house checks should probe the surface and the depth separately.

Does higher intensity always mean a better cure?

No, past a point you're buying heat and surface stress, and very fast gelation can trap shrinkage stress in rigid joints. There's a sweet spot: enough intensity to beat inhibition and reach depth, with time set for the dose. That's exactly what a process window defines.

How much intensity margin should I keep?

Enough that normal drift never takes you below the adhesive's stated minimum between radiometer checks, many shops set limits so that even a 20% output decline still cures parts. The exact figure comes from your drift rate and check frequency, established in-house.

Can I split one lamp across several fixtures?

Yes, the SunSpot 2 accepts dual- and 4-pole splitter guides with legs balanced to ±5%, so one lamp cures up to four joints or four stations per exposure. All legs share one shutter and timer, so the recipe must be common to them.

How do I put spot curing on an automated line?

Fixture guide tips over the joint positions, trigger exposures from the line controller, and interlock the source with the guarding. SkyBeam is built for this, PLC I/O and RS-232 control with per-channel timers, so the cure becomes a logged station step like any other.

Does part temperature affect cure speed?

Somewhat, warm adhesive flows and cures marginally faster, cold parts slightly slower, and large swings can shift cure behavior. Normal shop-floor variation is rarely a problem, but validate at your process temperature and avoid curing parts fresh from a cold dock or a hot upstream step.

Can I rework a joint that was already UV cured?

Cured acrylates are thermosets, they don't remelt. Rework means mechanical removal or softening with heat or solvent per the adhesive datasheet, then cleaning, re-dispensing, and re-curing.

This is where a tack-first workflow pays off: catch alignment errors while the joint is still gelled, not fully cured.

How do I choose a spot size for my joint?

Pick the smallest spot that fully covers the adhesive with a little allowance for placement tolerance, smaller spots concentrate intensity and cure faster. Guide core diameter and working distance set the spot; our guides cover roughly 2–12 mm at the part, and we can spec focusing optics when a joint needs finer control.

Should adjacent stations share one cure recipe?

Only if measurement says they're equivalent. Match stations by measured irradiance at the working plane, not by knob settings, two identical systems can deliver different intensity through different guides. Same measured dose, then a shared recipe is legitimate.

Why validate a window instead of a single setpoint?

Because production drifts. A single passing setpoint tells you nothing about how much drift you can absorb; a characterized window with a minimum and maximum tells you exactly how far output or time can wander before parts are at risk, and where to set alarms.

How does a shutter help my process?

It turns an always-on arc lamp into a precisely timed exposure tool: the lamp stays hot and stable while the shutter meters identical bursts, part after part. The SunSpot 2's integral timer and shutter handle this automatically.

What's a good first exposure to try on the bench?

Start from the datasheet dose divided by your measured irradiance; if you have neither yet, begin in the 2–5 second range at a close working distance and adjust from your own results. Never ship on a guessed time, turn the guess into a measured window first.

Do thicker fillets shrink more during cure?

Yes, shrinkage scales with adhesive volume, so big fillets pull harder on parts and are more prone to internal stress and cracking. Keep fillets as small as the strength requirement allows, and use step cures or low-shrink formulations for large volumes.