← All guidesOnce you have a good printed master die, you can make a reusable silicone mold from it and cast copies in resin - handy for a matched set, a run in a custom colour, or a batch of spares. The usual approach for a die is a two-part block mold in platinum-cure silicone. But there is one trap, specific to resin prints, that ruins more first molds than anything else, so this guide puts it front and centre. Need a master first? Design and export one with the free polyhedral dice generator.
Platinum-cure vs tin-cure silicone
Mold silicones come in two families, and which you pick matters more for a resin-printed master than for almost anything else:
Platinum-cure (addition-cure) is the higher-fidelity option: very low shrinkage (around 0.1%), crisp detail and dimensional accuracy, and a long library life (it keeps casting accurately for years). The catch is the one this whole guide is built around - its catalyst is easily poisoned, so resin prints (and sulfur, latex, ammonia, and some tapes and clays) can stop it curing against the master.
Tin-cure (condensation-cure) is the forgiving option: it is not inhibited by resin chemistry and shrugs off contamination, so it is the path of least resistance for a printed die - usually cheaper, too. The trade-offs are a little more shrinkage (around 0.3%, vs ~0.1% for platinum, so a fresh tin mold comes out slightly smaller than the master) and a shorter useful life - it degrades over time and with repeated casts, where a platinum mold keeps going for years.
For a one-off or a handful of dice off a resin print, tin-cure sidesteps the biggest headache. If you want dimensionally-tight molds that survive long runs - and you will prep the master carefully (below) - platinum-cure is worth the extra care.
The big one: resin prints can stop silicone from curing
Pour platinum-cure silicone over a resin-printed master and you may find it never fully sets against the print: it stays tacky, gooey, or liquid right at the surface that touches the die, while curing fine everywhere else. The detail you cared about is exactly the detail that will not set. This is called cure inhibition.
Why it happens
It affects platinum-cure (addition-cure) silicone. That silicone cross-links with the help of a platinum catalyst, and the catalyst is easily poisoned: any chemical that bonds to or disturbs it shuts the reaction down. Resin prints leach exactly those chemicals:
Leftover photoinitiators (such as TPO / phosphine oxides) that did not fully react under UV - the most common culprit, and highly reactive with the platinum catalyst.
Uncured monomers and oligomers - any thin film of liquid resin still on the surface is a guaranteed inhibitor.
Sulfur compounds, present in some resins (and in sulfur-based modelling clays), which are potent platinum poisons.
Solvent residue - isopropyl alcohol or cleaner left on the surface from washing.
The key point: it is not the cured plastic itself, but the unreacted residues sitting in and on it. A print that looks perfectly clean and fully cured can still inhibit silicone days later, because those residues keep migrating to the surface.
How to prevent it
Roughly in order of reliability - any one can be enough, and combining them is safest:
Sidestep it with tin-cure silicone. As covered above, condensation-cure silicone is not poisoned by resin chemistry, so inhibition essentially goes away - the simplest, most reliable route if you do not specifically need platinum (accepting its slight shrinkage and shorter library life).
Post-cure hard. More UV cure means fewer leachable photoinitiators and monomers. Cure the master thoroughly - many people cure it underwater in the UV chamber, which helps drive the surface photoinitiators off - and then let it outgas for a few days at room temperature (or a couple of hours warm in an oven) before molding.
Wash, then dry completely. Clean all surface resin off (IPA or your usual wash), then let the solvent fully flash off - residual IPA can itself inhibit. Never pour silicone onto a still-damp master.
Seal the master with a thin barrier coat so the resin and silicone never actually touch. Good options: Inhibit X (Smooth-On, made for exactly this - platinum silicone only), a clear acrylic spray such as Krylon Crystal Clear, a clear nitrocellulose lacquer, a thin clear epoxy coat (such as XTC-3D), or a dilute acrylic-in-acetone (PMMA) dip. Keep every coat thin and even: heavy coats fill in the engraved numbers and round off the crisp edges, which on a die defeats the purpose.
Always test first. Before committing a whole batch, dab a small blob of mixed silicone on a scrap print (or an inconspicuous spot) and check it cures hard and tack-free overnight. That test is far cheaper than a wasted master and a ruined pot of silicone.
Even done carefully, platinum silicone against a resin print is never a sure thing - the community reports the odd failure even with good prep. If repeatable, high-fidelity molds matter to you, tin-cure silicone or a reliable barrier coat is the safer bet.
Making the mold
Whatever style you pick, add a pour spout and a vent at the highest point so trapped air can escape as you cast, rather than collecting in a deep number recess. And if you do bed the master in clay, use sulfur-free (plastiline / oil-based) clay and sulfur-free tooling - sulfur inhibits platinum silicone the same way resin does.
Mixing the silicone
Silicone comes as two parts you combine in a fixed ratio, and you can measure that ratio two ways: by volume (graduated cups) or by weight (a scale). Weight is usually the better choice - all you need is an inexpensive scale, it is more precise and repeatable, and there is no eyeballing a meniscus or losing material clinging to a second cup. Reach for a 0.01 g jeweler's / pocket scale rather than a 1 g kitchen one: dice batches are small, and on a tight ratio the catalyst side can be a gram or two, where 1 g rounding is enough to throw off the cure.
Follow the manufacturer's instructions for that exact silicone. Mix ratios vary by product - 1:1, 10:1, 1:10 - and a product's weight ratio is not always the same number as its volume ratio, because the two parts can have different densities. Read the datasheet and use the ratio it gives for how you are measuring.
Be precise. Off-ratio mixing is the second-most-common way to ruin a mold after inhibition: too much or too little catalyst and it cures soft, tacky, or not at all. Measure carefully and mix thoroughly, scraping the sides and bottom of the cup so there are no unmixed streaks.
My own preference is to mix by weight, and reach for a 1:1 product whenever possible. A 1:1-by-weight silicone is the most foolproof - one number to remember, equal amounts on the scale, and far less to keep track of mid-pour.
Working time vs cure time
Two more numbers on the datasheet matter as much as the ratio: working time (pot life - how long after mixing before it thickens too much to pour) and cure time (how long until you can demold). A short pot life is the thing that makes mold-making stressful, and a longer one is usually worth the longer wait:
A longer working time lets bubbles escape. While the silicone is still fluid, trapped air can rise and pop on its own - so a 30-45 minute pot life does a lot of the de-bubbling for you, which matters most if you do not have a vacuum or pressure setup.
It also lets you pour a whole batch from one mix. A short pot life has you racing the clock and re-mixing for nearly every mold - a ~6-8 minute working time realistically gets you only two single-die molds before it is too viscous to pour. A ~45-minute working time lets you mix once and calmly fill ten or so single-die molds in one sitting.
Bigger molds need the long pot life even more. A multi-cavity party mold (a whole set in one block) takes far longer to fill and de-bubble than a single die, and a fast 6-8 minute silicone may thicken before you ever finish pouring it - so a big mold can be effectively impossible with a fast cure. A ~45-minute working time with a ~3-hour cure is a good middle ground: long enough to fill a large mold calmly, short enough that you still demold the same day.
The trade-off is that a longer working time usually comes with a longer cure before you can demold. I take that trade every time - for example a ~45-minute / 3-hour silicone over a ~6-minute / 1-hour one. Even if you are impatient, the calmer pour and bigger batches are worth waiting a couple more hours to unmold.
Pour a witness cup. Set aside a small cup of the same mix next to the mold; you can poke and pull at it freely to judge whether the silicone has actually cured without disturbing the real mold. When in doubt, leave it longer - under-curing ruins a mold, while extra time costs nothing. I routinely give silicone about twice the advertised cure time before demolding.
Slab mold vs sprue mold
A slab (block) mold is a simple two-part box poured in two stages. The common way to make one is to stick the master down by one face on double-sided tape at the bottom of the box (no clay needed - which also avoids the sulfur-clay inhibition risk): pour the first half of silicone, let it cure, brush the parting surface with mold release, then pour the second half on top. Add registration keys (or use a box with keyed walls) so the two halves seat back together exactly. It is forgiving - open it, pull one die, close it, pour again - and best for a single die or a few you want to finish by hand.
A sprue mold is a closed mold with a funnel (the sprue) feeding branching channels (gates) into several die cavities at once, with vents for air to escape. You pour or inject into the sprue and fill every cavity in one shot; the casting comes out as a tree of dice on a runner that you snip apart. More setup, far better throughput - this is how batches and commercial-style runs are made. Sprue molds almost always want pressure casting, since the resin has to travel through thin gates before it sets.
Getting the bubbles out: vacuum vs pressure
A die has deep recessed numbers and sharp corners that love to trap air, so bubble control is what separates a crisp cast from a mushy one. Vacuum and pressure are two different tools that attack the problem from opposite ends:
A vacuum (degassing) chamber removes air: under vacuum, dissolved air and mixing bubbles expand, rise, and burst, so you pour an already-degassed material. It is mainly used on the silicone before pouring the mold - the mix foams up dramatically, so you need a chamber with lots of headroom. It is less useful for fast casting resin, which can start to set before it finishes degassing.
A pressure pot does not remove bubbles, it crushes them: curing under roughly 3-4 bar (40-60 psi) shrinks any remaining bubbles until they are too small to see, and they stay that way once cured. It is the more broadly useful tool for crisp dice because it rescues the fast resin stage that vacuum cannot.
Pressure works for both stages, doing a different job each time - when you make the mold it sets the silicone bubble-free around the master, and when you cast a die it fills the cavity and crushes any bubbles in the resin. The reliable combination is: degas the silicone under vacuum if you can, then cure both the mold and every resin cast under pressure. If you only buy one tool, make it a pressure pot.
Cure the mold at the same-or-higher pressure than you cast - and never cast at a higher pressure than the mold was cured at. A common approach is to cure the mold near the top of the range (~4 bar / 60 psi) and then cast at or below that (~3-4 bar / 45-60 psi). The reason is the tiny bubbles still trapped in the cured silicone: if you later cast at a higher pressure than the mold ever saw, those bubbles get crushed and implode, and the collapsed dimples print onto the die surface as bumps and blurred detail. Curing the mold at the same or higher pressure keeps those bubbles already compressed, so the cavity stays true under the cast. Only ever pull vacuum or apply pressure in a vessel rated for it, and let the mold or casting fully cure before releasing.
With the mold made, you are ready to cast copies - colour and effects, pressure-casting, inking the numbers, and handling resin safely are all covered there.
Sources
This guide draws on my own mold-making experience, plus the silicone and resin makers and the wider mold-making community, who document the inhibition problem and its fixes well: