The most effective way to make silicone softer is to reduce the density of its internal molecular network, either by adding silicone oil before curing, using less crosslinker in the formulation, or choosing a lower-hardness base material. If you’re working with silicone that’s already cured, your options are more limited, but heat soaking and mechanical working can provide modest results.
Why Silicone Has a Set Hardness
Silicone rubber gets its firmness from a web of chemical bonds formed during curing. The polymer chains have reactive sites (vinyl groups) that act like hooks. During curing, a crosslinker bonds those hooks on neighboring chains together, creating a tight molecular network. The denser that network, the harder the silicone. A heavily crosslinked silicone might land at 70A or 80A on the Shore hardness scale, while a loosely crosslinked one sits around 30A to 40A. Shore A is the standard measurement for rubber-like materials: lower numbers mean softer, higher numbers mean firmer. For reference, a rubber band is roughly 25A, a pencil eraser about 40A, and a car tire around 70A.
This matters because softening silicone essentially means disrupting or loosening that network. The approach differs depending on whether you’re mixing uncured silicone or trying to soften a finished piece.
Adding Silicone Oil Before Curing
The most common method for softening silicone is mixing in silicone oil (also called PDMS oil or silicone thinner) before you cure it. The oil molecules sit between the polymer chains without bonding into the crosslinked network, which spaces the chains apart and makes the final product softer and more flexible.
A low-viscosity silicone oil added at 10 to 15% of the total weight can drop Shore A hardness down to the 15 to 20 range, which is extremely soft and gel-like. Commercial silicone thinners are pre-formulated versions of this same concept, and manufacturers typically recommend a maximum of 15% by weight relative to the silicone rubber base.
There’s a real tradeoff here. Adding thinner decreases tear strength and tensile strength while increasing elongation. In plain terms, the silicone stretches more easily but also rips more easily. If you’re making a mold that needs to survive dozens of castings, heavy thinning will shorten its lifespan. If you’re making a prosthetic or cushioning pad where softness matters more than durability, the tradeoff is worth it.
How to Mix Silicone Oil Properly
Weigh your silicone base on a digital scale. Calculate the percentage of oil you want to add. Start at 5% if you want a modest softening effect, or go up to 10 to 15% for a dramatic change. Add the oil to Part A of a two-part silicone before you mix in Part B (the catalyst). Stir thoroughly for two to three minutes so the oil distributes evenly. Uneven mixing creates inconsistent hardness throughout the piece, with soft pockets next to firmer areas. Then add Part B and mix as you normally would.
Use only silicone-based oils. Petroleum-based oils, vegetable oils, or mineral oils will interfere with curing and can prevent the silicone from setting at all.
Adjusting the Crosslinker Ratio
If you’re formulating from scratch or using a platinum-cure system that allows ratio adjustments, reducing the amount of crosslinker (Part B in most two-part systems) produces a softer result. Less crosslinker means fewer bonds between chains, which means a looser, softer network.
This approach requires caution. Deviating too far from the manufacturer’s recommended mix ratio can leave the silicone undercured, sticky on the surface, or structurally weak. Small deviations of 5 to 10% less catalyst are usually tolerable in platinum-cure silicones, but going further risks a product that never fully sets. Tin-cure (condensation) silicones are less forgiving with ratio changes.
Choosing a Softer Base Material
Sometimes the simplest solution is starting with a softer silicone rather than trying to modify a harder one. Silicone rubbers are sold across a wide hardness range. If you’ve been working with a 40A silicone and need something softer, switching to a 20A or even a 00-scale silicone gel gives you a reliably soft result without sacrificing tear strength the way heavy thinning does. Specialty “super soft” silicones in the 00-10 to 00-30 range feel almost like skin and are used in prosthetics, special effects, and medical simulation.
This is the better path when durability matters. A silicone formulated to be soft from the start has its filler content and crosslink density optimized together, so you get softness without the mechanical property losses that come from dumping oil into a harder formulation.
Softening Already-Cured Silicone
Once silicone is fully cured, the crosslinked network is locked in place, and no additive will penetrate deeply enough to restructure it. Your options are limited, but not zero.
Heat exposure can provide a temporary softening effect. Silicone becomes more pliable when warm. Soaking a cured silicone piece in hot water (not boiling) for 15 to 30 minutes makes it noticeably more flexible. This is useful if you need to stretch a silicone item over something or adjust its fit. The effect reverses as it cools.
Repeated mechanical flexing can also break some crosslinks over time, marginally softening the material. This is essentially what happens to silicone products through normal wear. Kneading or repeatedly bending a cured silicone piece will make it slightly more pliable, though the change is subtle and you risk tearing thin sections.
Applying a thin coating of silicone oil to the surface of cured silicone makes it feel softer and slipperier to the touch without changing the bulk hardness. This works well for wearable items, handles, or anything where surface feel matters more than structural softness. The oil will need reapplication over time as it migrates off the surface.
Common Mistakes That Ruin the Cure
Adding non-silicone substances is the most frequent mistake. Baby oil, coconut oil, and petroleum jelly will inhibit platinum-cure silicones and leave you with a gummy, uncured mess. Even latex gloves can cause cure inhibition in platinum systems. Stick to silicone-compatible products.
Exceeding the 15% thinner threshold is another common error. Past that point, the oil starts to bleed out of the cured silicone over time, leaving an oily surface film and progressively weakening the material. If you need something softer than 15% thinner can achieve, switch to a softer base material rather than overloading with oil.
Inconsistent mixing creates parts that cure unevenly. If the thinner or catalyst isn’t distributed throughout the mixture, you’ll get hard spots and soft spots in the same piece. Use a flat mixing stick, scrape the sides and bottom of the container, and mix for longer than you think necessary.