How to cast resin in a 3D-printed mold
Resin needs a flexible mold so undercuts and fine detail demold without breaking. This guide walks you through casting silicone around a 3D-printed master, then pouring resin into that silicone - silicone choice, vacuum degassing on a budget, mixing ratios, demolding, and how to fix the bubbles that ruin a perfect pour.
What you'll need
PLA filament ~$20
For the master + the outer shell. PLA is fine - silicone cures around it at room temperature, no heat issues.
Buy →Platinum-cure silicone ~$35
Two-part platinum (addition) cure tears less and copies detail better than tin (condensation) cure. 30A is the sweet spot - flexible enough to demold undercuts, firm enough to hold its shape.
Buy →Epoxy or polyurethane resin ~$18
Epoxy is clearer and pours slower (more working time). Polyurethane demolds in 30 minutes but yellows in UV. Pick by speed-vs-clarity priorities.
Buy →Silicone-safe mold release ~$12
Used between the master and the silicone, then again between the silicone and the resin. Check the can - generic "mold release" often inhibits platinum-cure silicone.
Buy →Disposable mixing cups + sticks ~$8
Graduated cups make ratio-mixing painless. Wooden sticks (popsicle / tongue depressor) - paper or plastic bend at the bottom.
Buy →Nitrile gloves + respirator ~$18
Skin contact with uncured resin can sensitize you for life - once your immune system flags it, you're done. Always wear nitrile (not latex - resin eats latex) and an organic-vapor respirator if you're indoors.
Buy →Optional but recommended: a vacuum chamber (~$100) or pressure pot (~$150). Without one, you can still get great casts - see step 4 for the budget trick.
Print & Prepare
Print the master + outer shell
In Meshcast, switch to Silicone mold type. Generate a master + outer shell that hugs it with a ~10 mm silicone gap. Slice both at 0.2 mm layers, 4 walls (so resin pressure can't crack the shell), 20% infill, PLA at 210°C.
Sand the master with 400 → 800 → 1500 grit. Silicone is a perfect mirror - every layer line and every fingerprint comes through to your final cast.
Coat with mold release
Spray two thin coats of silicone-safe mold release on the master AND inside the shell. Let each coat flash off for 5 minutes - wet release inhibits the silicone cure.
Cast the Silicone
Mix the silicone
Weigh part A and part B on a 0.1 g scale. Mold Star 30 is 1:1 by weight; Smooth-Sil is 1:1 by volume but 10A:9.7B by weight - always read the data sheet.
Stir slowly with a flat stick for 2 minutes. Scrape the sides and bottom of the cup every 30 s - unmixed silicone in the corners stays sticky. Avoid whipping.
| Silicone | Pot life | Demold time |
|---|---|---|
| Mold Star 15 | 50 min | 4 hours |
| Mold Star 30 | 45 min | 6 hours |
| Smooth-Sil 940 | 20 min | 16 hours |
| Dragon Skin 10 | 30 min | 5 hours |
Degas (or fake it)
If you have a vacuum chamber: pull 28 inHg for 60–90 s. The silicone foams up, peaks, then collapses back. Hold for another 20 s after the collapse, release.
Pressure-cast instead? Pour first, then put the whole shell into a pressure pot at 40 psi for the full silicone cure. Bubbles get crushed to invisibility.
No chamber, no pot? Pour from 30 cm up in a thin pencil-thin stream into the corner of the shell, letting gravity self-degas. Tap the shell against the table 3–4 times after pouring. You'll get 90 % as good a result for most shapes - undercuts are where bubbles love to hide, so over-pour into them first.
Pour silicone & cure
Pour into the shell with the master held in place by its own pour-stub. Leave the top vent open. Let cure at room temperature per the spec chart above. To speed up: 60 °C oven for 30 minutes (only after the silicone has set enough not to slump).
When cured, peel the shell off the silicone, then peel the silicone off the master. You now have a flexible mold ready for resin.
Cast the Resin
Mix and pour resin
Read the resin's data sheet - ratios vary wildly:
| Resin | Mix ratio | Pot life | Demold |
|---|---|---|---|
| Epoxy (Art Resin) | 1:1 by volume | 45 min | 24 hours |
| Epoxy (deep pour) | 2:1 A:B | 3 hours | 48 hours |
| Polyurethane (Smooth-Cast 300) | 1:1 | 3 min | 30 min |
Weigh, stir for 3 minutes scraping the sides, pour into the silicone cavity through the pour vent in a thin stream. For polyurethane you have 3 minutes - pour fast.
Demold the cast
Wait for full cure (table above). Peel the silicone gently - flex it, don't pry with a knife. Trim the sprue with a hobby blade. For epoxy, optionally cure another 24 hours at room temperature for full hardness.
Troubleshooting
The silicone never cured - it's still sticky
You hit cure inhibition. Common causes: sulphur in modeling clay, latex gloves touching the master, tin-cured silicone tools, residual mold release that didn't flash off, or unflashed paint. Throw out the bad cast; re-prep the master with proper materials.
The cast has surface bubbles
Pour thinner and slower from higher up, or pressure-cast. For deep parts, paint a first layer of resin into the cavity with a brush, then pour the rest - the brushed layer fills tiny detail.
The resin won't demold from the silicone
Either you skipped release on the silicone surface or the cure isn't complete. Wait another 12 hours for epoxy. Next time: light spray of release inside the silicone before each pour.
The cured resin is cloudy/milky
Either off-ratio (too much B), under-stirred, or moisture in the resin. Use a fresh batch, mix exact ratios on a scale, store resin sealed and warm.
The silicone tore on demold
Cuts at sharp inner corners - flex slower next time, or step up Shore hardness to 40A. For deep undercuts, plan a parting cut into the silicone before pouring.
The resin is sticky and never fully hardens
Off-ratio. Even 5 % off makes epoxy stay tacky. Throw it out, weigh more carefully next time. Cold temperatures (< 21 °C) also slow cure - warm the bottles before mixing.
I can smell something strong - should I worry?
Yes. Resin VOCs sensitize over time, and exposure compounds. Always work in a ventilated space and wear an organic-vapor respirator. If you start getting headaches or rashes, you're already overexposed - stop and ventilate.
Made by @meshminds3d. Some product links are affiliate links - they cost you nothing extra and help keep this tool free. Got stuck? Email me a photo and I'll help you debug.