← All guides
One Mold, Many Props: Silicone & Resin
Casting guide 🫗 · Workshop crew · Nov 2026
Why casting is the best-dollars-per-prop trick
Gems, sigils, badge emblems, gemstones, horns, anything you need twice or more pays for a mold. Sculpt once, cast in resin, and every future costume reuses it: cast a dozen gemstones and they match forever, which sculpting never does.
The master: sculpt to survive demolding
- Clay, foam or resin-sculpted masters all work, seal porous surfaces (clay needs a sealer spray or it sticks).
- No deep undercuts on a first mold: sloped walls and open shapes release cleanly.
- Mount the master in a small box (cardboard + hot glue) with the detail side up, the pour side needs the smoothest face.
The silicone pour
- Tin-cure RTV silicone is the beginner material: two parts, read the ratio on the product and mix by weight (the app's verified catalog carries OOMOO 25/30 at 100:130 by weight, 1:1 by volume, "weight and volume ratios are not interchangeable"). Stir slow and steady.
- Release agent on the master (petroleum jelly thinned or a spray wax) unless the silicone is designed not to stick.
- Pour in a thin, high stream, the fall pops most surface bubbles.
- Cure per the data sheet, typically 4–24 h at room temperature. Warm rooms cure faster; don't rush with high heat.
Safety: cure-time fumes want ventilation, gloves always, the chemistry is mild but the mess is permanent.
Casting the prop
| Material | Mix | Best for |
|---|
| Urethane casting resin | 1:1 by volume | Gems, emblems, rigid parts; dyes and mica powders color it |
| Clear casting resin | 2:1 by weight usually | Transparent gems (with translucent pigment) |
| Flexible rubber compound | 1:1 | Soft bits: straps, fake leather texture |
Pour low and slow into the deepest point, let it flow up. Cure times run minutes to hours, demold early-test a corner trim.
One gem, worked: a cabochon 30 × 22 × 10 mm masters in a box 60 × 50 × 25 mm. The cavity is the box minus the master: 75 − 6.6 ≈ 68 cm³ of silicone for the mold, and the gem itself needs 6.6 cm³ of resin per pour. The app's Silicone & resin mix tool turns those volumes into part-A/part-B amounts using the ratio the data sheet states — its verified catalog carries OOMOO 25/30 at 100:130 by weight (a very different scoop than 1:1 by volume), and the exact-basis rule is the whole point: weight is weight, volume is volume, the two are separate data.
Bubbles and other fixes
| Symptom | Cause | Fix |
|---|
| Clear resin has foam | Whipped air from fast stirring or a pour splash | Pour thin and high; rest the mix 2 min before pouring; pressure pot if it keeps happening |
| Cure stays tacky | Off-ratio mix or cold room | Measure volumes exactly; warm the room, not the resin |
| Mold tears early | Silicone too thin at stress points | Thicken key edges in the mold wall, not the detail face |
| The master stuck for good | No release on a porous master (clay drinks silicone) | Release agent is step 2 for a reason; seal clay first, and a stuck master becomes a pattern for a second cast |
| Mixed the whole batch, it cured in the cup | Urethane's pot life runs minutes, not hours | Small batches only; pour within the data sheet's working window |
| The cast came out slightly small | Master measured before sealing, or the scale percent never transferred | Scale the master with the app's X/Y/Z math (the prosthetics guide's example), then measure the finished master again before the mold |
The shelf version: the
build log tracks each pour and cure, and the
3D-print guide is the fastest route to a printed master for your first mold.
The app helps: the app's Silicone & resin mix tool does the box-minus-master math from this guide's shape (rectangle or cylinder, the 10% waste default, the part A/B split on the data sheet's basis), the casting material library carries the manufacturers' own numbers as a checked snapshot (2026-10-22) with the standing rule that the bottle's instructions win, and the two-stage timer runs the working window and the cure with its own honesty — a timer never confirms cure, a corner trim does. The build log keeps the pours' dates and the app's rows keep the exact volumes, so the second pour of a mold is a known quantity, not a re-lesson.