If you're standing between two bottles of silicone mold making and the only difference on the label is a number — 0A or 15A — the choice can feel arbitrary. It isn't. That single number decides whether your mold captures every fine detail of your original, or whether it holds its shape when you pour into it.
Here's the thing most guides won't tell you plainly: these two aren't competing products. They're built for different jobs, and picking the right one is less about which is "better" and more about what you're actually casting.
The short answer
15A handles about 80% of mold making projects. It's the general-purpose choice — flexible enough to demold easily, firm enough to hold its shape. If you're new to mold making or you're not sure, this is the one.
0A is the specialist. Ultra-soft, best-in-class flexibility, and it captures the finest detail without damaging your original during demolding. Reach for it when your piece is small, intricate, or delicate.
The Only Spec That Actually Differs
This is worth showing plainly, because it makes the decision much simpler than it looks. Both silicones share identical technical specifications across the board — the same viscosity, the same tear strength, the same elongation, the same cure time. Only hardness is different.
| Specification | 0A Silicone | 15A Silicone |
|---|---|---|
| Shore hardness | 0 ± 2 Shore A | 15 ± 2 Shore A |
| Viscosity | 1500 ± 500 mPa·s | 1500 ± 500 mPa·s |
| Tensile strength | ≥ 2 MPa | ≥ 2 MPa |
| Elongation | ≥ 300% | ≥ 300% |
| Tear strength | ≥ 12 KN/m | ≥ 12 KN/m |
| Working time | 30 minutes @ 25°C | 30 minutes @ 25°C |
| Cure time | 4 hours at room temperature | 4 hours at room temperature |
| Mix ratio | 1A : 1B | 1A : 1B |
| Appearance | Semi-transparent | Semi-transparent |
| Shelf life | 12 months unopened | 12 months unopened |
Everything greyed out above is identical. Both mix 1:1, both cure in 4 hours at room temperature, both give you a 30-minute working window, and both are semi-transparent so you can spot trapped bubbles before they ruin a mold. The decision comes down to one number.
What 0A and 15A Actually Feel Like
Shore A numbers don't mean much until you connect them to something you've held. Here's the practical translation.
0A
Ultra-soft
Think jelly or a marshmallow. Press it and it gives way immediately. Stretch it and it flows into the tightest gaps. It will not hold its own shape if you build it tall.
15A
Soft but structured
Think a rubber band or a soft eraser. Still very flexible — you can bend and twist it easily — but it springs back and keeps its form when you set it down.
To put both in context, here's roughly where they sit on the wider Shore A scale used for mold making silicone:
Shore A hardness — quick reference
0A
Ultra-soft. Maximum flexibility, best detail capture, no structural support.
15A
Soft with body. The all-rounder — flexible demolding plus enough firmness to hold shape.
30A
Medium. More structure, less give — used for larger or heavier casting jobs.
Choose 15A When...
Start here, because for most people this is the answer. 15A Liquid Silicone Rubber is the balanced grade — it has enough flexibility to release cleanly and enough firmness that the mold doesn't sag or distort. It covers roughly 80% of mold making needs, which is why it gets called the general-purpose grade.
15A The all-rounder — start here if you're unsure
Best for
- Everyday craft casting — soap, plaster ornaments, resin crafts
- Moderately detailed pieces — simple-pattern ceramic reproductions, small to medium cement molds
- Molds that need to hold their own shape — anything with height or a flat open face
- First-time mold makers — its medium hardness and tear resistance are far more forgiving of small mistakes
Why it works
- Medium hardness means the mold walls support themselves instead of collapsing inward
- Good tear resistance means you can flex it firmly during demolding without worrying about ripping it
- Wide margin for error — you don't need perfect technique to get a usable mold
The keychain mold above is a good example of where 15A earns its keep: a shallow open-face design with a raised loop detail. The mold needs to sit flat on a table without warping, and it needs to flex just enough to pop the cured piece out. That's exactly the balance 15A is tuned for.
Choose 0A When...
0A Liquid Silicone Rubber trades structural support for something else entirely: unmatched flexibility and detail capture. When your original has fine texture, delicate protrusions, or narrow crevices, the extra softness is what gets you a clean copy — and gets your original back out undamaged.
0A The specialist — for fine detail and delicate originals
Best for
- Small, intricate pieces — jewelry, miniature scenery, anything with complex surface patterns
- Delicate originals — where a firmer silicone might chip or crack the master during demolding
- Very narrow gaps — excellent flow lets it fill crevices a thicker feel would bridge over
- Fidget and fussy-cutting pads — where the finished mold itself needs a soft, comfortable feel
Why it works
- Best-in-class flexibility captures the finest surface detail on small, complex objects
- The mold peels away from the original with almost no force — nothing gets stressed or broken
- Outstanding flow means it fills the tightest spaces without help
Where it falls short
- Not suitable for tall molds. Without structural support it will deform under its own weight.
- Insufficient support can cause distortion, which affects dimensional accuracy on larger pieces
That ornate frame is the case 0A was made for. Look at the scrollwork around the border — dozens of small undercuts and raised curves. A firmer silicone would grip those details on the way out and risk snapping fine points off the original. 0A releases with almost no resistance.
Project-by-Project: Which One to Grab
If you'd rather skip the reasoning and just find your project, here it is.
| What you're making | Pick | Why |
|---|---|---|
| Resin jewelry pendant | 15A | Needs a stable shape through repeated pours |
| Finely carved ornament | 0A | Captures texture; releases without damaging the master |
| Soap or candle molds | 15A | Standard demolding, no special requirements |
| Miniature scenery pieces | 0A | Very small and highly detailed |
| Plaster or cement casts | 15A | Needs to support the weight of the cast material |
| Ceramic reproductions | 15A | Moderate complexity, benefits from firmness |
| Deep narrow crevices | 0A | Superior flow and stretch during release |
| Tall or large molds | 15A | 0A would sag under its own weight |
| Fidget / fussy-cutting pad | 0A | Soft feel is part of the finished product |
| Your first mold ever | 15A | More forgiving of technique mistakes |
What Both Silicones Need to Cure Properly
This section applies equally to both grades, and it's the one that catches people out. Platinum-cure silicone is sensitive — certain materials touching it while it's still liquid will stop it from setting. The result is a mold that stays tacky or stays soft in patches, hours after it should have cured. It's not a bad batch. It's contamination.
Things that can stop your silicone from curing
⚠️ Keep these away from uncured silicone
🧤
Latex gloves. Swap them for nitrile gloves — the blue or purple ones. This is the single most common cause of a mold that won't set.
🎨
Some modeling clays and plasticines. Sulfur-based clays are the problem. If you don't know what your clay contains, test a small patch first.
🧪
Recently cured epoxy or polyurethane pieces. Give them a few extra days to fully harden before molding them.
🖌️
Certain paints, spray paints, and body fillers. Test a hidden corner before committing to the whole piece.
The three-step check before every pour
- Clean and dry your original. Dust, oil from fingers, and leftover moisture all interfere with curing. Wipe it down and let it dry completely.
- Test-patch anything you're unsure about. Mix a small amount of silicone, dab it on an inconspicuous spot, and check it in 4 hours. If it's fully set and not tacky, you're clear.
- Check your gloves. Nitrile, not latex. Takes two seconds and saves the whole batch.
Three more things worth getting right
- Measure the 1:1 ratio properly. Use measuring cups or a scale — don't estimate by eye. Off-ratio silicone cures soft or stays sticky.
- If you want to add color, use silicone-specific pigment only, at 2–4% of the total mix. More than that and it won't cure.
- To clean up uncured spills, use a hydrocarbon solvent like mineral spirits. Do not reach for rubbing alcohol or acetone — those make it worse, not better.
One reason semi-transparency helps: because both grades cure clear enough to see through, you can spot trapped air bubbles while the silicone is still liquid and pop them before they set into your mold wall.
Frequently Asked Questions
Which one is better for a complete beginner?
15A, without much hesitation. Its medium hardness and tear resistance give you a wider margin for error — the mold holds its shape while you're still developing technique, and it can take a firm flex during demolding without tearing. 0A is a better material for certain jobs, but it's less tolerant of mistakes because it deforms so easily. Learn on 15A, then add 0A when you hit a project that specifically calls for it.
My piece has deep, narrow crevices — which should I use?
0A. Its flow characteristics let it reach into gaps that firmer silicone would bridge over, leaving a void in your mold. And because it stretches so readily, you can pull the mold off a deeply undercut original without tearing the silicone or snapping detail off the master. The one caveat: if the piece is also tall, 0A may need external support to hold its shape while curing.
How long will a 0A mold last?
With reasonable care, both grades hold up well across repeated pours. Lifespan depends far more on how you treat the mold than on the hardness — clean it after each use, store it flat and out of sunlight, and don't stretch it further than it needs to go during demolding. That said, 0A is softer and deforms more easily, so it benefits from gentler handling and flat storage more than 15A does.
Why isn't my silicone curing?
Almost always one of three things. First, contamination — latex gloves, sulfur-based clay, fresh epoxy or polyurethane, or certain paints touching the uncured silicone. Second, an inaccurate mix ratio; the 1:1 needs to be measured, not estimated. Third, too much pigment — anything beyond 2–4% of the total mix will prevent a full cure. If your mold is tacky in patches rather than uniformly soft, contamination is the likely cause.
How long does silicone last once I've opened it?
Unopened kits carry a 12-month shelf life from the date of manufacture when stored below 35°C in the original containers. Once opened, reseal both bottles tightly right away and keep them somewhere cool and dry. Moisture and air exposure are what degrade silicone over time, so a tightly capped bottle stored properly stays usable considerably longer than one left loosely closed on a shelf.