Silicone mold not curing: sticky, tacky or soft
Look at where it is sticky. If the mold is cured everywhere except the face that touched your master, that is cure inhibition: something on the master poisoned the reaction. If the whole batch is uniformly soft, soupy or oily, it is a mixing problem. These have nothing in common and nothing about the fixes overlaps, so read the next section before you do anything.
Which failure do you have?
| What you see | What it is | Go to |
|---|---|---|
| Back face cured, master face tacky or gooey | Cure inhibition (contamination) | Cure inhibition |
| Tacky in patches, in the shape of where you handled it | Cure inhibition, from a glove or a fingerprint | Cure inhibition |
| Whole batch uniformly soft but not sticky | Under-catalysed, or still curing in the cold | Whole batch soft |
| Whole batch soupy, oily, never firmed at all | Wrong ratio, or part B never mixed in | Whole batch soft |
| Cured but with soft streaks and swirls through it | Incomplete mixing, unmixed pockets | Whole batch soft |
| Firm but weeping a clear oil | Normal on some tin-cure silicones, not a failure | Nothing to fix |
Cure inhibition, the one that surprises people
Platinum-cure silicone (also sold as addition-cure, and what most modern kits are) cures through a platinum catalyst that a short list of everyday substances shuts down on contact. When that happens the silicone touching the contaminated surface never finishes curing. It stays tacky forever. It does not eventually get there.
Nothing is wrong with your silicone, and adding more catalyst will not help. The contamination is on the master, or on your hands, or on the container.
| Inhibitor | Where it comes from | Risk |
|---|---|---|
| Sulfur-containing modelling clay | Plastilina and most cheap oil-based clays, used for parting walls | Very high |
| Uncured photopolymer | A resin (SLA/DLP/MSLA) print that was not fully washed and post-cured | Very high |
| Latex and natural rubber | Latex gloves, rubber bands, some elastics | Very high |
| Cyanoacrylate | Superglue used to fix or assemble the master | High |
| Tin-cure silicone | An older mold, or residue in a reused container or stir stick | High |
| Amine-cured epoxy | Epoxy putty, filler or adhesive on the master | High |
| Some spray paints and lacquers | A painted or primed master that has not fully off-gassed | Medium |
| Certain tapes and adhesives | Masking tape holding a mold box together | Medium |
| PLA, PETG, PP (FDM prints) | A normal filament print | Very low, these are fine |
If you print in resin, this is almost certainly your cause
Resin prints are the number-one inhibition source for 3D printing makers, and it is worth being specific about why: a print can feel completely dry and hard and still be leaching uncured monomer from just below the surface. The surface cured, the inside did not finish, and it keeps migrating outward for days.
The fix is process, not product:
Wash properly
Fresh isopropyl alcohol, long enough to remove every trace of surface resin, then a second wash in clean IPA. A dirty wash bath just redistributes resin over the print.
Post-cure fully, then over-cure a bit
Cure longer than you would for a display print, and rotate it so no face is shadowed. Under-curing is invisible and is exactly what poisons the silicone.
Let it rest
Give a resin master a day or two in open air before pouring silicone on it. This alone resolves a lot of cases.
Seal it anyway
Even a well-cured resin master benefits from the barrier coat below. On anything you cannot afford to redo, do not skip it.
The barrier coat
A barrier coat is a thin sealing layer between a suspect master and the silicone. It is the standard answer when you cannot identify or remove the contaminant, and it takes about ten minutes.
| Barrier | How to apply | Notes |
|---|---|---|
| Clear acrylic spray sealer | 2 to 3 light coats, fully dry between, then a full day to off-gas | The usual choice. Do not rush the off-gassing, the solvent itself can inhibit |
| Shellac or sanding sealer | Brush or spray a thin coat, dry hard | Excellent on sulfur clay and on porous masters |
| PVA / mold-release lacquer | Brush thin, let dry to a film | Doubles as a release layer |
| Dedicated inhibit-barrier product | Per the datasheet | Made for exactly this if you do it often |
When the whole batch is soft
If the silicone failed uniformly, contamination is not your problem. Work down this list.
The ratio was measured by volume instead of weight
Cause Many silicones are specified by weight, and part A and part B do not have the same density. Measuring cups therefore give you the wrong ratio even when they look right.
Fix Use a scale with 1 g resolution and follow the datasheet's units exactly. A 1:1 by weight product and a 1:1 by volume product are different products.
Part B settled in the container
Cause The catalyst side separates on standing, especially in a part-used tub that has sat for months. You then pour off the thin top layer and get almost no catalyst.
Fix Stir each part thoroughly in its own container before measuring anything. This is the most-skipped step in the whole process.
It was not mixed all the way to the sides and bottom
Cause Unmixed material clings to the corners of the cup and to the stir stick, then gets poured in as a streak that never cures.
Fix Scrape the sides and the bottom deliberately, then transfer to a second clean cup and mix again. Double-potting like this removes almost every soft-streak failure.
The room was too cold
Cause Cure rate falls off sharply with temperature. Below roughly 18 °C a platinum silicone can take several times its stated time, and a cold garage in winter can stall it long enough to look like a failure.
Fix Give it more time at room temperature before declaring it dead. Warmth speeds cure, so a warmer room is the correct lever. Never add extra catalyst to compensate: with platinum silicone that does not work, and on some products it makes the result worse.
The silicone was past its life
Cause Silicone has a real shelf life, usually around a year unopened and less once opened, and the catalyst side degrades first. Old stock cures soft or not at all.
Fix Check the date. If it is old, test a small batch before committing it to a master you care about.
Can you rescue a tacky mold?
Honest answer: usually not. Inhibited silicone has not simply cured slowly, the reaction was stopped, and it will not restart. What you can do is stop it costing you a second pour.
Get all of it off
Scrape every trace of uncured silicone off the master. Anything left behind will inhibit the next pour too.
Clean the master hard
Isopropyl alcohol and a stiff brush, then dry completely. On a porous master, expect the contaminant to have soaked in, which is why the next step is not optional.
Barrier coat, then test
Seal it as above, let it off-gas properly, and run the 60-second test before you pour a full batch.
The 60-second test that prevents all of this
It costs a few grams of silicone and one cure cycle, and it is the difference between finding out now and finding out after you have poured half a kilo over a master that took six hours to print. Do it every time the master is new, resin-printed, painted, puttied or made of clay you did not buy specifically as sulfur-free.
Getting this right on the first pour is mostly about the master, not the silicone. If yours is a printed part, the surface prep in best print settings for molds and the release choices in mold release agents cover the rest of the setup. And if the mold cured fine but the cast came out wrong, that is a different page: casting troubleshooting.
Made by meshcast.app. Still tacky? Email a photo and I'll help you debug.