Liquid rubber: silicone or PU, what is the difference?

What is liquid rubber? A mould that gives you hundreds of casts, a seam that stays watertight for years, a flexible custom part? Liquid rubber is at the root of all of them. Liquid rubber is therefore not one product but a collective term. It covers roof coatings, PU rubber and silicone rubber. A roof coating is bought ready to use. The other two are raw materials, and that is what we specialise in. Two separate components that only become something the moment you pour them together. What exactly? You decide that, with the mixing ratio, the Shore hardness and the pot life. Below you can read what exactly happens during mixing, what makers use it for and why silicone has a place of its own within this family.

Silicone rubber

Liquid silicone flows into the finest details and irregularities, from the exact dimensions of a gear wheel to the grain in a piece of wood. In most cases it does not bond while curing. So the mould comes off in one go, without damaging the original. That is why the great strength of silicone is making true-to-life moulds.

There are exceptions, though. Silicone simply bonds to other silicone rubber, very clean glass and other materials containing silicon. On porous surfaces a mechanical bond also forms because the material is absorbed. If you are unsure whether your material is suitable for a mould, feel free to contact us.

On the packaging you will see this as RTV-2, room temperature vulcanising in two components. In other words it becomes rubber at room temperature, without an oven or press.

You will come across two types. That distinction drives almost every choice you make afterwards:

• Condensation-cure silicones, in practice called tin silicones after the catalyst that starts the reaction. They cure on almost any surface and are not bothered by a bit of contamination. The amount of catalyst controls how fast you work. Alcohol is released during curing, which makes the mould shrink slightly, up to about one percent.
• Addition-cure silicones, also called platinum silicones after the platinum catalyst. The ratio is fixed here and deviating from it gives sticky rubber, but in return you get dimensional stability, and food-safe versions are available. Watch out for inhibition, meaning the reaction is blocked: contact with sulphur-containing clay, latex or tin stops the rubber curing locally.

PU rubber

Flexible polyurethane is the powerhouse of the two. It is tougher, tears less easily and costs less per kilo, which makes it the logical choice for parts that have to absorb impacts and wear.

It does behave in exactly the opposite way, though. Where silicone releases, PU bonds to almost everything it touches, so without a release agent you will never get it out of your mould again. Moisture also takes part in the reaction, so a damp workshop or a plaster mould that is not yet dry gives you foam and air bubbles. And because most types cannot cope with UV, they discolour in full sun.

What is liquid rubber used for?

Applications range from a single cast in a shed to series rolling off the line in a workshop. Four uses come up most often.

Moulds and casts This is by far the largest application. You pour silicone over an original. What you then pour into that mould is up to you: plaster, concrete, epoxy, soap, wax or chocolate. For anything that goes in the mouth there are specially certified silicones, tested for what can be released on contact with food. The fact that silicone can withstand around 200 degrees helps here, because hot candle wax is no problem and neither is the heat that plaster gives off while setting. How long a mould lasts depends mainly on what goes into it: plaster is mild and gives dozens of casts, while more aggressive resins attack the inside more quickly.

Prototypes and small series

This is where the two materials work together. First you pour a silicone mould around your model and then fill it with PU resin: silicone provides the shape, PU the part. Within a day you have a series.

The alternative is an injection mould, the steel tool into which molten plastic is injected under pressure. You invest thousands of euros in it and wait weeks for delivery. With every design change that starts all over again. You only take that step when you are ready to scale up.

Flexible custom parts

Gaskets, edge protectors, rollers, bumpers, drive belts. Anything that has to bend, absorb impacts or dampen noise and is not in the catalogue, you cast yourself. PU rubber is often the obvious choice here, because it copes better with wear.

Sealing and bonding

On its own, silicone bonds to almost nothing, which is a blessing for moulds and precisely the problem here. With a primer, a thin pre-coat that makes the surface receptive, you can make it bond to glass, metal or plastic.

The best-known application is silicone sealant from a cartridge. That is the one-component version, which does not react with a hardener but with the moisture in the air, and therefore cures from the outside in. Roof coating works the same way. Both are bought ready to use, without any mixing.

What drives your choice

Whichever application you choose, you steer with three properties:

• Shore A, the measure of the hardness of the cured rubber. The lower the number, the softer and stretchier. Casting silicones are usually between 10 and 40. Soft rubber bends around undercuts, the protruding parts a stiff mould gets caught behind, while harder rubber holds its shape better on large or flat models.
• Pot life, the term for the working time after you have mixed the components. It is separate from the curing time: twenty minutes of pot life can easily go together with eight hours of waiting before you can demould. The stated pot life assumes an average room temperature, so a warm room shortens that time and a cool room extends it.
• Viscosity, the thickness of the mixed material. Thin material creeps into every detail by itself, thick material stays where you put it. Over a flat model you want thin, but on a wall, a fireplace or a statue it runs down, so you use a spreadable version.

What is the difference between liquid rubber and silicone?

Technically there is no contrast, because silicone is itself a liquid rubber. In everyday language it is different. There, liquid rubber usually means roof coating, and anyone searching for it will end up there first.

Silicone you mix yourself. That lets you determine the hardness, the working time and the thickness.

The choice is then between silicone and PU rubber. Silicone provides the shape, with fine detail reproduction, heat resistance up to around 200 degrees and a mould that comes off again without tricks. PU provides the part, tear-resistant and wear-resistant and cheaper per kilo. That lower price says little about your project costs, because a silicone mould lasts for dozens of casts and pays for itself over the series. Shelf life does differ: platinum silicones stay supple for years, while tin silicones slowly post-cure and become more brittle. PU is more demanding to process, because the fumes released during the reaction call for good ventilation. There are even silicone versions that may be used on the skin, which is why hand casts and body casting are made with them.

As a rule of thumb, choose silicone when it is about the shape and PU when the part has to withstand loads. In many projects you use both. Not sure which one suits your project? Just ask us, because you only notice you chose the wrong rubber once it has cured.