Key takeaways
- Washing and curing aren't cleaning steps — they create the mechanical properties. Every resin data sheet is measured on cured specimens.
- IPA has limits: as the resin concentration in the alcohol rises, it stops washing and just coats the part in thin resin.
- Parts must be fully dry before UV. Curing over a surface still wet with alcohol gives a dull face that holds dust.
- Over-curing is a real fault, not excess care: parts turn brittle and yellow, and thin parts warp.
With FDM, a finished print is a finished material. With resin it isn't: a part just off the plate is still covered in liquid resin and only partly polymerised. The next three steps — wash, dry, cure — are where the material actually forms, and skipping or rushing any of them leaves a measurable mark.
A fresh print isn't yet the material on the data sheet
The manufacturer says this plainly. Formlabs' Standard Resin data sheet has two columns for the same resin: the freshly printed part (green) and the UV-cured part (post-cured). They differ by nearly a factor of two in tensile strength. Read the wrong column and you've read the wrong material — and print without curing and you're holding exactly the lower column.
The same holds for engineering resins. The Tough 2000 values the materials page quotes were measured after 12 minutes in a Form Cure V2 at 70 °C; High Temp Resin goes from an HDT of 142 °C after UV cure to 238 °C after a thermal post-cure at 160 °C. All were read on 27/08/2026 and recorded on the sources page.
Four steps, in order
- 01
Remove the part and let it drain
Put on nitrile gloves before touching anything. Tilt the build plate so excess resin drains back into the vat for a few tens of seconds before removing the part — the resin you recover is resin you don't have to wash off or throw away.
- 02
Wash in IPA
Two tanks beat one: the first is the dirty tank and does most of the work; the second is clean alcohol for a final rinse. With hollow parts or pockets, watch for resin trapped inside: it will seep out after curing and leave streaks.
- 03
Let it dry completely
Blow it off with compressed air or leave it somewhere ventilated until the surface loses its wet sheen. Exposing a surface still wet with alcohol gives a dull face, sometimes a chalky white layer that won't sand off.
- 04
Expose to 405 nm
Turn the part, or use a turntable, so every face gets enough light. Shaded faces cure less than faces towards the lamp, and on a functional part the under-cured side is the one that deforms first.
| Resin line | Wash | Cure |
|---|---|---|
| Precision Resin | 3 min IPA | 2 min @ 405 nm |
| Tough Resin | 4 min IPA | 4 min @ 405 nm |
| High Temp Resin | 4 min IPA | 6 min @ 405 nm, then 120 °C for 2 h |
| Castable Resin | 2 min IPA, no soaking | 1 min @ 405 nm |
The last two rows are the two extremes. High Temp needs a whole extra oven step, and all of its heat resistance lives in that step. Castable is the opposite: the less cure the better, because its goal is to burn out cleanly in a casting mould, not to be strong — over-curing leaves ash in the mould.
IPA: when it stops working
Wash alcohol doesn't get used up; it just gets thicker with resin. Past a certain point, dipping a part only coats it in thin resin, and after curing the surface is slightly tacky and cloudy — a symptom very often blamed on cure time.
- Signs the alcohol is saturated: parts come out still greasy, take longer than usual to dry, or the cured surface is evenly dull rather than dull in patches.
- Separating resin from dirty alcohol: leave the dirty tank in the sun or under a UV lamp, and the resin in it sets into solid sheets you can filter out. Reuse the recovered alcohol in the first wash tank.
- Don't pour dirty alcohol down the drain. Uncured resin is toxic to aquatic life. Cure all the resin in it first, then dispose of the solids as ordinary waste.
- Water-washable resin too. "Water-washable" is about the solvent, not the toxicity — the wash water still contains uncured resin and still has to be cured before disposal.
Under-cure and over-cure: two opposite faults
| Symptom | Usual cause |
|---|---|
| Surface still tacky after curing | Not washed clean, or the alcohol is saturated — not under-curing |
| Part soft, deforms gradually over days | Under-cured core or shaded faces |
| Part brittle, shatters when dropped | Over-cured |
| Yellowing, especially clear resin | Over-cured, or sun used instead of a cure chamber |
| Thin parts warp after curing | Heat in the cure chamber plus cure shrinkage |
Safety: the non-negotiable part
Uncured resin is a skin irritant and sensitiser — meaning reactions can get worse with each exposure, even if you noticed nothing the first time. This is the biggest difference between resin and FDM workflows, and it applies to liquid resin, dirty alcohol, used gloves and wipes alike.
- Nitrile gloves, not latex, replaced when torn. Safety glasses when pouring and washing.
- Ventilation to outdoors, not a fan moving air around the room.
- Cure all waste before disposal: wipes, gloves, filtered residue, dirty alcohol. Cured resin is inert.
- Never share tools with food areas, and never wash hands with IPA — it drives resin into the skin rather than removing it.
The comparative risks of FDM and resin, and how to lay out a workspace, are in 3D printing fumes and ultrafine particles.
With resin, post-processing isn't finishing. It's the second half of making the material.
What comes after curing
A cured resin part is the easiest surface to finish of any technology: wet sanding works well, primer bonds very firmly, and there are no meaningful layer lines to fill. The order is the same as for FDM — cut support stubs, wet-sand, prime, paint — and is covered in post-processing 3D prints and how to paint 3D prints. The one difference: don't sand before curing. An uncured part is still soft, and the dust is uncured resin dust.
Frequently asked
How long should resin be cured?
It depends on the resin, the chamber's power and the part's thickness, so start at the low end of that resin's recommended window and work up. A practical check: print two identical test pieces, cure one for a short time and one for twice as long, then break both. The properly cured one is noticeably stiffer; an over-cured one shatters instead of snapping.
What strength of IPA washes best?
The higher the concentration, the faster it washes, because the water in dilute alcohol doesn't dissolve resin. 90 % or higher is typical; 70 % medical alcohol still works but more slowly and leaves more residue. Whatever the strength, the tank saturates over time and has to be replaced.
Does water-washable resin skip the cure step?
No. The two are independent: water-washable only means that resin's solvent is water instead of alcohol. The part is still only partly cured off the printer and still needs UV to reach its published properties. The wash water also still has to be cured before disposal.
Why does my resin part drift in size after a few weeks?
Almost always an under-cured core. The unpolymerised portion keeps reacting slowly and keeps shrinking, so the part drifts out of tolerance — most visible on fixtures and mating parts. For functional parts, cure longer, turn the part in the chamber, and prefer the tough resin lines designed for that use.
Does Castable Resin need curing?
Very little, and that's deliberate. Wax-filled resin exists to burn out cleanly during mould firing; over-curing makes it harder to burn out completely and leaves ash. With this line, the burnout schedule decides casting quality, not lamp time.