Making soft silicone starts with choosing a two-part RTV (room temperature vulcanizing) silicone kit and mixing the base with a catalyst at the correct ratio. The “softness” of your final product depends on the specific product you select, how accurately you measure, and whether you add a softening agent like silicone oil. Most soft silicone projects, from prosthetics to squishy mold inserts, use platinum-cure silicone rated on the Shore 00 hardness scale, which covers very soft rubbers and gels.
How Two-Part Silicone Works
Two-part RTV silicone consists of a base polymer and a curing agent (catalyst). When you mix them together, a chemical reaction called hydrosilylation begins at room temperature. The base contains long polymer chains with reactive points, and the curing agent provides a crosslinker that bridges those chains into a solid rubber network. No heat, oven, or UV light is required.
The ratio between Part A and Part B matters enormously. Some kits use a 1:1 ratio by volume, while others call for 10:1 by weight. Read the label carefully: ratios listed as 1:1, 2:1, or 3:1 are typically measured by volume, not weight. If you need to convert to weight-based measurement for precision, contact the manufacturer for the correct numbers. Getting the ratio wrong will leave you with silicone that’s either too sticky, too stiff, or that never fully cures.
Platinum Cure vs. Tin Cure
The two main curing systems produce noticeably different results. Platinum-cure silicone is the better choice for most soft silicone projects. It shrinks less during curing, lasts for decades without degrading, and is FDA-compliant for food and skin contact. That makes it the standard for prosthetics, food molds, and medical applications.
Tin-cure silicone is cheaper and more forgiving with contaminants, but it shrinks more and has a limited shelf life of roughly 12 months before the cured rubber starts to break down. If you’re making a quick prototype or a mold you’ll only use a few times, tin cure works fine. For anything meant to last or touch skin, go with platinum cure.
Choosing the Right Softness
Silicone softness is measured on the Shore durometer scale. For very soft, gel-like silicone (think: the squish of human skin or a gummy bear), you want products rated on the Shore 00 scale. A Shore 00 value of 10 to 30 feels like a soft gel, while 50 to 70 feels more like a firm gummy. Shore A is the next scale up, covering standard rubber hardness. A Shore A 10 silicone feels like a rubber band, while Shore A 40 feels like a pencil eraser.
The easiest way to get soft silicone is to buy a product already formulated at the softness you need. Smooth-On’s Ecoflex series, for example, ranges from Shore 00-10 (extremely soft) to Shore 00-50 (medium soft). Dragon Skin offers slightly firmer options. Choosing the right product from the start saves you from having to modify the formula yourself.
Making Silicone Even Softer
If your silicone isn’t soft enough out of the box, you can add silicone oil (also called deadener or softener) to the mix before curing. This works by reducing the density of the crosslinked network, producing a squishier result. Start with small additions, around 5% to 10% of the total mix weight, and test a small batch first. Too much softener creates silicone that weeps oil, feels greasy, and won’t hold detail.
Only use softeners specifically designed for the silicone system you’re working with. Adding random silicone oils or incompatible additives can prevent the silicone from curing at all. Platinum-cure systems are especially sensitive to contamination.
Step-by-Step Mixing Process
Gather your materials: the two-part silicone kit, a digital scale (for weight-based ratios) or graduated mixing cups (for volume-based ratios), mixing sticks, and disposable cups. Work in a well-ventilated area on a covered surface.
- Measure accurately. Pour Part A into your container first, then add Part B at the exact ratio specified. For a 1:1 by volume product, use graduated cups. For 10:1 by weight, use a digital scale that reads to at least 0.1 grams.
- Mix thoroughly. Stir for 3 to 4 minutes, scraping the sides and bottom of the container. Incomplete mixing leaves uncured spots in the finished piece. You should see a uniform color with no streaks.
- Add softener if needed. Mix in your silicone softener after combining Parts A and B, then stir for another minute.
- Remove air bubbles. Pour the mixed silicone into your mold slowly from a high, thin stream. This breaks many bubbles on the way down.
Removing Air Bubbles
Soft silicone traps air easily because of its low viscosity, and bubbles weaken the final product while ruining surface detail. The most reliable method is vacuum degassing. Place your mixed silicone in a vacuum chamber and pull at least 29 inches of mercury (about 1 bar of negative pressure). The silicone will rise dramatically as trapped air expands, then collapse back down as bubbles escape. Once it falls, run the vacuum for an additional 60 seconds to catch residual air.
If you don’t have a vacuum chamber, pour in a long, thin stream from as high as you can manage, and tap or vibrate the mold gently after pouring. You can also use a pressure pot during curing, which compresses any remaining bubbles to invisibly small size rather than removing them entirely.
Curing Time and Temperature
Most soft silicones cure at room temperature in 4 to 24 hours depending on the product. Warmer temperatures speed up both the pot life (your working time before it starts to thicken) and the total cure time. At 70°F (21°C), a typical platinum-cure silicone with a 45-minute pot life might fully cure in 16 hours. At 85°F (29°C), that same silicone could be ready in 6 to 8 hours.
You can gently heat-accelerate the cure by placing the mold in a warm oven (around 150°F or 65°C) after pouring, but only if the mold material can handle that temperature. Don’t apply heat during the pot life window or you’ll lose your working time before the silicone has settled into the mold properly.
Avoiding Cure Inhibition
Platinum-cure silicone is notoriously sensitive to contamination. Certain materials prevent it from curing entirely, leaving you with a sticky, uncured mess. The most common culprits are sulfur-based modeling clays (like many oil-based clays), latex gloves, tin-cure silicone residue, and certain 3D printing resins.
If your silicone won’t cure against a surface, sulfur is almost always the reason. Switching to a sulfur-free clay solves the problem immediately. For other materials, you can try applying a barrier coat or surface treatment designed to block inhibition, though with platinum silicone even sealers sometimes fail. The safest approach is to test a small amount of silicone against any new material before committing to a full pour. Use nitrile or vinyl gloves instead of latex when handling platinum-cure products.
Thickening for Brush-On Applications
If you need to apply soft silicone to a vertical surface, like brushing it onto a sculpt to create a skin mold, the standard pourable consistency won’t work. It will just slide off. Thixotropic additives thicken the mixed silicone to a paste-like consistency that stays where you put it. You can adjust the amount to get anything from a slightly thicker pour to a non-slumping paste, depending on how much you add.
Mix the thixotropic agent into the combined Parts A and B, then work quickly. These additives often shorten your pot life because you’re adding extra time to the mixing step. Apply in thin layers rather than one thick coat, letting each layer partially set before adding the next. This gives better detail and a more uniform thickness.
Common Mistakes That Affect Softness
The most frequent problem is inaccurate measurement. Even a small error in the mix ratio can shift the final hardness by several Shore points, and in extreme cases, the silicone won’t cure properly. Always use the measurement method (weight or volume) specified by the manufacturer.
Another common issue is adding too much softener. Beyond a certain threshold (usually around 20 to 30% by weight, depending on the product), the silicone becomes structurally weak, tears easily, and bleeds oil onto anything it touches. If you need something extremely soft, it’s better to start with a softer base product than to overload a firmer silicone with softener. Contaminating your workspace with sulfur or latex residue, using expired catalyst, and failing to mix thoroughly are the other top reasons for failed batches.