Dual-density silicone casting: firm core, soft skin
This guide was drafted with AI assistance.
Cast a firm silicone core first (Dragon Skin 20 or 30), then hold it in the middle of a slightly bigger mold and pour a soft silicone around it (Ecoflex 00-30 or 00-50, or Dragon Skin 10). Platinum silicone bonds to clean, cured platinum silicone, so the two become one piece. Keep the soft layer at least 4 to 5 mm thick, put no release agent on the core, and keep everything platinum silicone.
What it is, and why
A single silicone is always a compromise: soft enough to feel good is often too floppy to hold its shape, and firm enough to hold its shape does not feel soft. Dual density splits the job. The core gives the piece structure and spring; the skin gives it the feel. It is the construction behind many commercial grips, prosthetic parts, realistic special-effects pieces and premium adult toys, and it is very doable at home once you can make one mold.
If you have not cast silicone before, start with a single-density piece: the whole process is the same, just once.
Which silicones to pair
| Feel you want | Core | Skin |
|---|---|---|
| Firm inside, very soft outside | Dragon Skin 30 (Shore 30A) | Ecoflex 00-30 |
| Firm inside, soft outside, tougher skin | Dragon Skin 30 | Ecoflex 00-50 or Dragon Skin 10 |
| Medium inside, soft outside | Dragon Skin 20 (Shore 20A) | Ecoflex 00-30 or 00-50 |
| Subtle difference, very durable | Dragon Skin 20 | Dragon Skin 10 |
For comparison, commercial dual-density products are usually a Shore 30A to 50A core with a 10A to 20A outer layer. The numbers behind each grade are in Ecoflex vs Dragon Skin. Stick to one manufacturer's platinum line for both layers where you can, so you know the two are compatible.
Method 1: cast a core, then the skin
The most controllable method. You need two molds: one for the core and one for the full outer shape.
Make the core model
Shrink the outer model inward by the skin thickness. Do it as an offset, not a scale: in Blender, the Displace modifier with a negative strength, or Shrink/Fatten (Alt+S) in edit mode, moves every surface in by the same distance. Scaling a long shape down gives a thin skin at the ends and a thick one in the middle.
Give the core a stem
Extend the core at its base end with a short stem, 8 to 10 mm across, that reaches out through the outer mold's pour opening. That stem is how you hold the core in the centre while the skin is poured. It becomes part of the base and is trimmed flush afterwards.
Make both molds and cast the core
Print both masters and make a mold of each. Cast the core in the firm silicone, let it cure fully, and demold it. Do not coat it with anything.
Centre the core and pour the skin
Wipe the core with isopropyl alcohol and let it dry. Release the outer mold as normal, then hang the core in it by the stem: a printed bridge across the pour opening that grips the stem works well. Degas the soft silicone and pour it slowly down the side of the mold, not onto the core, so it rises evenly around it.
Method 2: brush a skin, then fill
Only one mold, at the cost of less even thickness. The soft layer goes in first, as a brushed lining, and the firm silicone fills the middle.
Thicken the soft silicone
Mix the soft grade with a thickener made for it (THI-VEX for Smooth-On silicones) until it stays put on a vertical surface.
Brush it into the mold in coats
Brush a thin first coat into every detail, let it cure until it is tacky but no longer transfers to a gloved finger, then add coats until the lining is 4 to 5 mm thick. Keep the pour opening's edge clean.
Fill with the firm silicone
Degas the firm grade and pour it into the lined mold while the last coat is still fresh enough to bond. It cures into the lining from the inside.
This works best in an open, one-piece glove mold where you can reach every surface with a brush. In a closed two-part mold, you can line each half separately, then close the mold and fill it.
Making the layers bond
- No release on the core, ever. The same property that makes silicone stick to a silicone mold is what joins your layers. Release on the core means two loose pieces.
- Clean, not sanded. A wipe with isopropyl alcohol is enough. Sanding adds dust that weakens the bond.
- Same cure type. Both layers platinum. Tin-cure residue on a core stops the platinum skin curing at the interface.
- Test it. Pour a scrap of skin silicone onto an offcut of the core silicone. Once cured, pull them apart. If the silicone tears rather than peeling at the join, the bond is good.
When it goes wrong
The layers separate
Cause Release agent on the core, a dusty or old core surface, or oils from handling.
Fix Handle the core with nitrile gloves, wipe it with isopropyl alcohol, pour the skin within a day, and never let release touch it.
The core ended up off-centre
Cause The core shifted or tilted while the skin was poured, or the pour pushed it.
Fix Hold it by the stem in a printed bridge rather than by hand, and pour down the mold wall instead of onto the core.
Bubbles along the join
Cause Air trapped against the core surface as the skin silicone rose around it.
Fix Degas the skin silicone, pour slowly, and brush a thin film of the skin silicone onto the core just before it goes into the mold.
The skin is tacky only where it touches the core
Cause Cure inhibition at the interface: a tin-cure core, or something on the core surface.
Fix Use platinum silicone for both layers. Silicone not curing lists what else inhibits.
Where Meshcast fits
Dual density needs two molds that match: the core's and the outer shape's. Meshcast makes both from your STLs, free in the browser.
- Silicone mold maker, contoured mode: a printed shell that hugs each master at the silicone thickness you set, with clips and a pour funnel. Generate it once for the core master and once for the outer master.
- Box mode: a classic printed housing for a silicone block mold, with an estimate of how much silicone it takes.
- Air-pocket pins in the silicone tool mark where air would be trapped during a pour, which matters twice as much when a core is in the way.
Make the core model in your 3D software first (the offset in step 1 above); Meshcast then works on each model exactly as it does on a single one.
Common questions
What is dual-density silicone?
A piece made of two silicones of different hardness bonded into one: a firmer inner core that gives it shape and support, and a softer outer layer that gives it a soft feel. Commercial products are typically a Shore 30A to 50A core with a Shore 10A to 20A outer layer; at home, a Dragon Skin 20 or 30 core with an Ecoflex or Dragon Skin 10 skin is a common pairing.
Will new silicone bond to cured silicone?
Yes. Platinum silicone poured onto clean, cured platinum silicone bonds to it, which is exactly why a release agent is needed when you cast silicone into a silicone mold. For a dual-density piece you use that on purpose: no release on the core, keep it clean, and pour the outer layer within a day or so of the core curing.
How thick should the soft outer layer be?
As a rule of thumb, at least 4 to 5 mm everywhere, and more for the softest grades. Thinner than that and the firm core is felt as a hard edge through the skin, and a thin spot is where the skin will tear first.
Can I put a 3D-printed or rigid insert inside the silicone?
Not for anything that touches skin. If the silicone ever tears, the insert is exposed, and a printed part cannot be cleaned. Keep both layers platinum silicone so the piece is one material all the way through.
Made by meshcast.app. Stuck on the core? Email us, discreetly, and we will help.