How to 3D print a mold for silicone casting
This guide was drafted with AI assistance.
For a quick test, print a rigid two-part mold and pour platinum silicone straight into it, with release on every surface. For a smooth, repeatable result, print the shape as a master instead, sand and seal it, make a silicone mold of it in a printed shell, and cast into that with a silicone-on-silicone release. Either way: pour into the lowest point, vent every high point, and put the seam on an edge.
This is the practical half of casting soft silicone pieces: grips, squishy toys, prosthetic parts, and the custom adult toys that bring a lot of people to this technique. Which silicone to use, and why it has to be platinum-cure for anything that touches skin, is in platinum vs tin-cure silicone. The whole process from start to finish is in how to make body-safe silicone toys. This page is about the mold.
Two ways to do it
| A. Rigid printed mold | B. Silicone mold in a printed shell | |
|---|---|---|
| What you print | The mold halves | The master, plus a shell or housing |
| What you pour | Casting silicone only | Mold silicone once, then casting silicone |
| Surface of the cast | The inside of the print: layer lines unless sanded and sealed | The surface of your finished master |
| Time to first cast | One print | Print, finish master, make mold, then cast |
| Extra cost | Almost none | A mold silicone kit and a release |
| Choose it when | Testing a size or shape, or the texture does not matter | You want a clean surface, or you will cast it more than a few times |
A good pattern is both, in order: print a rigid mold to test the shape and size cheaply, then commit to route B once the design is right.
Route A: a rigid printed mold
Print it in PETG or PLA, on an FDM printer
Platinum silicone does not bond chemically to either. PETG handles warmth better, which helps if you speed the cure in a warm room. Use 3 or more walls so the halves do not flex and leak. A resin-printed mold is smoother inside, but uncured resin stops platinum silicone curing, so it must be fully washed, post-cured and tested with a small blob first.
Decide what to do about layer lines
The cast copies every line. Print the mold with the most visible surfaces facing up, at 0.12 mm layers or finer. For a smooth finish, sand the cavity and brush on a thin epoxy coating made for prints, then let it cure a full day and do a test blob, because some epoxies inhibit too.
Release, clamp and seal
Coat every surface that silicone touches with release, including the mating faces. Clamp the halves firmly, and run a bead of hot glue or clay along the outside of the seam: low-viscosity silicone finds any gap. Use sulfur-free clay; sulfur clay stops platinum silicone curing.
Route B: a silicone mold in a printed shell
This is how most people who cast regularly end up working. The silicone mold copies your master's finished surface exactly, and the printed shell around it (sometimes called a mother mold) holds the soft mold in shape while you pour.
Finish the master
Sand it and seal it. Every flaw you leave on the master appears on every cast. Add a short flat plug where the pour opening will be.
Pick the mold style
A glove mold is a thin, even layer of silicone (around 5 to 10 mm) in a close-fitting printed shell. It peels off the master like a sock, uses little silicone, and has no seam across the cast. A block mold fills a box around the master; you cut it open along a zigzag line to free the master, or pour it in two halves for complex shapes. Glove molds suit long, rounded shapes; block molds suit wide or complex ones.
Use a platinum mold silicone
A firmer platinum grade (Shore 15A to 30A, for example Mold Star or Rebound) makes a mold that holds its shape. Tin-cure mold silicone is cheaper, but its residue can stop the platinum casting silicone from curing against it.
Cast into it, with release
Put the silicone mold back in its shell, coat it with a silicone-on-silicone release, and pour. See release below: this is the step that ruins the most first attempts.
Pour opening and vents
- Put the pour opening at the flat end that gets trimmed or becomes the base, and stand the mold so that opening is the highest point.
- Vent every other high point. A raised detail that points upward traps air. A 2 to 3 mm vent channel from its tip to the top of the mold lets it escape.
- Make the opening wide enough to pour into, at least 10 to 15 mm, so you can pour a thin stream down one side without it bridging over.
- Pour in one continuous stream, slowly, against the wall. Stopping and starting folds air in.
- Leave a little extra at the top. Silicone settles slightly, and the excess is trimmed off.
Seams and undercuts
Wherever mold parts meet, silicone leaves a thin line, called flash. Where it sits is a design decision:
- Put the seam on an edge or along the sides, never across a face or through fine detail. A seam along a smooth side trims almost invisibly with small curved scissors.
- Split through the widest part of the shape, so each half can pull straight away.
- Undercuts are less of a problem than with rigid casts, because the soft cast stretches out. Deep hooks, thin necks and narrow tips still tear, especially in the softest grades.
Release, every time
| Mold surface | Release | Without it |
|---|---|---|
| Silicone mold | A silicone-on-silicone release (Ease Release 200 or similar), light coat, brushed into details. Thinned petroleum jelly is a common homemade option | The cast and the mold fuse into one piece. Nothing separates them |
| PLA or PETG print | A spray release, or a thin film of petroleum jelly | Silicone locks into the layer lines and tears on the way out |
| Epoxy-coated print | Same as a print | Usually releases, but not reliably enough to skip it |
Demolding
- Wait for the full cure time on the datasheet, not the demold time, and longer in a cold room.
- Open rigid parts first, then peel the soft mold away from the cast slowly, from one edge.
- Warm soapy water run under an edge breaks the grip if it sticks. Never use a metal tool near a silicone mold: one nick copies into every future cast.
- Wash the cast to remove the release, and clean the mold before storing it flat and dust-free.
Doing it with Meshcast
Designing a mold around an organic shape by hand, with a clean seam, a pour opening and vents in the right places, is the hard part. Meshcast does it from your STL, free, in the browser.
| You want | Use | What it does for you |
|---|---|---|
| Route B, a glove mold | Silicone mold maker, contoured | A printed shell that hugs the master at the silicone thickness you set, in 2 to 4 parts with printed clips and a pour funnel. Air-pocket pins flag trapped air; Turn for a cleaner seam and Best split place the parting |
| Route B, a block mold | Silicone mold maker, box | A printed housing that splits in two around the master, a silicone volume estimate before you print, and undercuts painted red on the model as you turn it |
| Route A, a rigid mold | Two-part mold maker | Two printed halves with alignment keys, a pour funnel you can move and a second pour hole if needed. It has no silicone preset: choose Resin, which gives the tightest fit for a runny liquid |
If your model is the wrong size when it loads, check the units: the tools show real millimetres, so read the dimensions rather than the preview. And if the file will not load, every error message is explained here.
Common questions
Can you pour silicone into a 3D-printed mold?
Yes. Platinum silicone does not bond chemically to PLA or PETG, but it flows into every layer line and locks in mechanically, so coat the mold with a release agent. The cast will copy the print's surface, so sand and seal the inside of the mold if you want a smooth result. Resin-printed molds must be fully post-cured and tested first, because uncured resin stops platinum silicone curing.
How do you stop silicone sticking to a silicone mold?
Silicone bonds to silicone, so a release agent is not optional. Use a release made for silicone-on-silicone, such as Ease Release 200, applied in a light coat and brushed into every detail. Petroleum jelly thinned with mineral spirits is a common homemade alternative. Test on a scrap first, because without a working release the cast and the mold fuse into one block.
Do undercuts matter when casting soft silicone?
Much less than with rigid casts. A soft silicone cast stretches out of moderate undercuts in a rigid mold, and a silicone mold stretches off the master. Deep hooks, thin necks and narrow tips can still tear, so place the seam through the widest part and give the cast a straight path out.
Where should the pour opening go?
At the flat end that you will trim or that becomes the base, with the mold standing so the opening is the highest point. The silicone then fills from the far end upwards and pushes the air out ahead of it. Any other high point in the cavity, such as a raised detail pointing up, needs its own small vent.
Made by meshcast.app. Mold not coming out right? Email us, discreetly, and we will help.