Platinum vs tin-cure silicone for mold making

Platinum-cure silicone makes the most accurate, longest-lasting molds and comes in food- and skin-safe grades, but it can fail to cure against certain materials. Tin-cure silicone is cheaper and far less fussy about what it touches, but it shrinks more and its molds wear out sooner. Here's how they differ, and how to choose for what you're casting.

Step 1 — Know the two chemistries

Both are two-part liquid silicones that you mix and pour around a model. What differs is the catalyst that cures them:

• Platinum-cure (addition cure) — cured by a platinum catalyst. Sold as "platinum" or "addition cure" silicone, usually mixed 1A:1B by weight or volume.
• Tin-cure (condensation cure) — cured by a tin catalyst. Often mixed 10:1 or 100:x by weight with a small amount of catalyst.

Step 2 — Compare them side by side

• Accuracy: platinum molds have very low shrinkage. Tin-cure molds generally shrink more, and continue to shrink over time.
• Mold lifespan (library life): a platinum mold can last decades on the shelf. Tin-cure molds have a limited library life — often around a year — before they dry out and lose detail.
• Cure inhibition: platinum silicone is very sensitive to sulfur and to some other materials — certain clays, latex gloves, tin-cure silicone residue and some 3D printing resins can stop it curing where it touches. Tin-cure silicone cures over almost anything.
• Food and skin: food-safe and skin-safe grades are platinum silicones. Don't use tin-cure silicone for food molds or skin contact.
• Cost: tin-cure silicone is usually noticeably cheaper.

Step 3 — Choose by what you're casting

• Food (chocolate, ice, candy): platinum, in a food-grade product only.
• Resin, plaster, concrete, jesmonite, wax, soap: either works. Choose platinum for a mold you'll reuse for a long time, tin for a cheaper short-run mold.
• An original you can't seal (sulfur-based clay, an unknown material): tin-cure is the safe bet.
• A 3D printed master: both can work. FDM prints in PLA or PETG often cure fine with platinum silicone, but some filaments and many resin (SLA) prints can inhibit it. Test first, or seal the master.

Step 4 — Test before you pour

With platinum silicone, a small test saves a whole batch. Mix a spoonful, dab it on the master (or a scrap of the same material) and let it cure fully. If it stays tacky where it touched the master, seal the master with a barrier coat — clear acrylic spray or a sealer the silicone maker recommends — and test again. If it still won't cure, switch to tin-cure. The causes and fixes are covered in why your silicone mold won't cure.

Step 5 — Build the mold box and size the pour

Whichever silicone you choose, the mold box is the same. You can build one in CAD if you have the time, but I use the 3DGEN silicone mold box generator — upload the master, set the walls, and the printable box is ready in seconds in the browser, no CAD.

1. Upload the master and orient it.
2. Set Silicone wall thickness and Top margin. Thicker walls make a stiffer mold; thinner ones save silicone.
3. Click Build the box and read the Material estimate panel: it shows the silicone the box will hold in milliliters and grams.
4. Adjust for your product. The generator converts milliliters to grams with a typical silicone density; use your silicone's specific gravity from its data sheet for an exact weight, and add 5–10% for the cup.

For a multi-cavity silicone tray, the casting tray generator in Silicone-pour mold mode does the same for the whole trough.

Step 6 — Mix and pour carefully

• Weigh both parts on a digital scale using the ratio on the data sheet. Tin-cure kits use a small amount of catalyst, so accuracy matters even more.
• Keep the two chemistries apart. Use separate cups and stir sticks — tin-cure residue on a tool can inhibit platinum silicone.
• Wear nitrile gloves. Latex gloves can inhibit platinum silicone.
• Pour in a thin stream into one corner and let the silicone flow over the master to push air out.
• Cure at room temperature for the time on the data sheet before demolding.

FAQ

Which silicone is better for resin casting?

Both work. Platinum molds keep their size and detail longer, which matters if you'll pour many castings; tin-cure is cheaper for a short run.

Will platinum silicone cure on a 3D print?

Often on PLA and PETG, but some filaments and many resin prints inhibit it. Always test on the master first, and seal it if the test stays tacky.

Is tin-cure silicone food safe?

No. For food molds, use a platinum silicone that is specifically rated food grade.