How To Make Soft Silicone Mix Cure And Degas?

Making soft silicone involves three separate jobs: mixing the two parts, removing trapped air, and letting the material cure. The mix is a two-part platinum-cure (addition-cure) silicone, usually labeled Part A and Part B. You combine them at the ratio the manufacturer states, stir slowly and thoroughly, degas the bubbles under vacuum, then pour and let it cure at room temperature or with gentle heat. The single most common reason a soft silicone turns out hard, sticky, or full of holes is an inaccurate mix ratio or air that was never removed.

None of those three steps is hard on its own. What trips people up is treating them as one step. Mixing, degassing, and curing each have their own rules, and breaking one ruins the result even if the other two were done perfectly. This guide walks through each stage in plain terms.

What Is Soft Silicone and Why Does the Mix Matter?

Soft silicone is a platinum-cure (addition-cure) elastomer that stays flexible after it sets. Its softness is measured on the Shore A scale, where lower numbers mean softer material. Skin-like silicones often fall in the very low Shore A range, while firmer mold-making silicones sit higher.

That softness is set by the formula, not by how you mix it. You cannot make a hard silicone soft by adding less Part B. The A-to-B ratio is a chemical requirement, not a strength dial. Part A carries the silicone polymer and platinum catalyst. Part B carries a crosslinker and often a inhibitor. When they meet, the platinum triggers a reaction that links the polymer chains into a flexible network.

Get the ratio wrong and the network never forms properly. Too little Part B leaves unreacted polymer, so the silicone stays tacky or never fully sets. Too much Part B can leave excess crosslinker that may interfere with the cure and can make the surface feel oily or greasy. Either way, the softness you wanted is gone.

This is why the ratio printed on the container is the one number you do not guess. Common ratios include 1:1 by weight, 10:1 by weight, and 100:10 by weight. They are not interchangeable, and they are usually by weight, not volume. Part A and Part B often have different densities, so measuring by volume can throw the ratio off. A scale is the safer tool.

How Do You Mix Soft Silicone Correctly?

Weigh each part separately, then combine and stir with patience. Rushing the stir is what drags air into the mix, and air is the enemy of a clean pour.

• Work in a clean, room-temperature space. Cold slows the cure; heat can shorten your working time.
• Zero the scale with your mixing cup on it, then weigh Part A first.
• Add Part B to the exact weight the ratio calls for. Do not eyeball it.
• Stir slowly and steadily, scraping the sides and bottom of the cup as you go.
• Keep stirring well past the point where the color looks even. Streaks hide at the bottom.

The color change is a useful clue but not a guarantee. Some silicones shift from one shade to another as the two parts blend. Even after the color looks uniform, unmixed material can cling to the cup walls and the bottom corner. Scrape those areas into the center and fold them through.

Stirring slowly matters more than people expect. Fast whisking whips in air that you then have to remove. A slow figure-eight motion with a flat stick or spatula folds the parts together with less turbulence. If your mix looks cloudy with tiny bubbles right after stirring, that is normal. The next step handles it.

How To Make Soft Silicone Mix Cure And Degas Without Bubbles

Degassing removes the air you stirred in, and it is the step most beginners skip. The standard method is a vacuum chamber. You place the mixed silicone under vacuum, and the drop in pressure pulls trapped air to the surface, where it bubbles out and pops.

The process looks alarming the first time. The mix rises and foams as the air escapes, sometimes doubling in volume. That is expected. Keep the vacuum running until the foam collapses back down and the surface looks smooth and still. If bubbles keep rising, keep going. When they stop, the mix is degassed.

A few practical points help:

• Use a container several times larger than the silicone volume so it can foam without overflowing.
• Degas soon after mixing, before the silicone starts to thicken. Once it gels, trapped air cannot escape.
• Work within the pot life, the window before the mix becomes too thick to pour. This varies by product and is usually listed on the label.
• Pour slowly from a low height to avoid reintroducing air.

If you do not have a vacuum chamber, you can reduce bubbles by mixing slowly, pouring in a thin stream, and tapping the mold to help air rise. These steps help but do not match vacuum degassing. For critical work where bubbles ruin the result, vacuum is the reliable method.

How Long Does Soft Silicone Take to Cure?

Cure time depends on the specific product, the temperature, and the mass of silicone. Thin sections cure faster than thick blocks because the reaction generates its own heat and thick pours hold that heat longer. Most platinum-cure silicones list a cure time on the label, often in the range of several hours to a full day at room temperature, with faster set times at elevated temperatures.

Temperature is the biggest lever you control. Warmer conditions speed the cure; colder conditions slow it and can even stall it. A silicone that feels set on the surface may still be soft underneath. Full cure, where the material reaches its final strength and softness, takes longer than the initial set.

Two things can stop a platinum-cure silicone from curing at all:

• Contamination. Platinum catalysts are sensitive to certain substances, including sulfur, some tin-based materials, and certain latex and modeling clays. These can poison the catalyst and leave the silicone permanently sticky.
• Wrong ratio. Off-ratio mix, especially too little Part B, prevents the network from forming.

If your silicone stays tacky well past its stated cure time, contamination or a mix error is more likely than a bad batch. Check what the silicone touched and how carefully you measured.

What Temperature Should You Cure Soft Silicone At?

Room temperature works for most soft silicones, but gentle heat speeds the process. Many products allow curing in a warm oven or under a heat source at a moderate temperature, and the manufacturer’s data sheet gives the exact range. Follow that number rather than guessing.

More heat is not better. Excess heat can cause the silicone to cure too fast, trap bubbles, or in some cases degrade the material. It can also shorten pot life so much that the mix thickens before you finish pouring. If you use heat, use the temperature the product specifies and no higher.

For thick pours, the reaction’s own heat can build up inside the block. This internal heat can be much higher than the room temperature and can affect how evenly the piece cures. Pouring in layers can help manage heat in large pieces.

Why Did My Soft Silicone Turn Out Hard or Sticky?

A hard or sticky result almost always traces back to the mix or to contamination. The material itself rarely fails on its own.

Hard, brittle silicone usually means too much Part B or a product that was never meant to be soft in the first place. Sticky, tacky silicone that never sets points to too little Part B, an off ratio, or catalyst poisoning from a contaminant. In both cases, the fix is the same: measure by weight, follow the exact ratio, and keep the silicone away from sulfur, tin, and incompatible materials.

One detail worth knowing: platinum-cure silicone and tin-cure silicone are different chemistries. Tin-cure silicones are more forgiving of mix ratio but can be inhibited by different substances, and they generally cannot be used to make molds for casting platinum-cure silicone. Mixing the two systems or using the wrong one for a job can cause cure problems that look like a mixing error but are really a compatibility issue.