When it comes to selecting a silicone for your mould, there are a few questions you may want to ask yourself:
What will be cast into the silicone?
What shore hardness do I need?
Are there undercuts I need to work with?
How long have I got to work with/demould?
But if you’re completely new to silicones let me quickly make sure everything in this blog makes sense!
Types of silicones for moulds
Firstly and most importantly… Platinum vs Tin.
Platinum, also known as addition cure silicone and Tin, also known as condensation cure.
The difference is really only each take a different metal to catalyse (cure) the base, can you guess?
Tin silicones uses Tin salts to catalyse which makes Platinum silicone use Platinum to catalyse.
Next, Shore Hardness. We are likely to mention this scale throughout any blog concerning casting or moulding materials… (read our blog post on Shore Hardness to familiarise yourself with the conversion chart used).
It’s the scale of solidity of a material, often you see a material end in a number. For example; PlatSil Gel 25. This is to identify the material and it’s shore hardness. Polytek make this super easy for us!
Plat (platinum) Sil (silicone) 25 (25 shore)
And finally, inhibition.
This is something either on your master, ancillaries or PPE that disrupts the cure of the silicone.
Your master may be a clay sculptures which contains sulphur, this will inhibit the cure of both plat and tin silicones, or you may be wearing latex gloves. I’ll explain this in detail.
But for now, these are the 3 main factors you must consider when starting to use silicones.
Lets run through the part these factors play specifically for mould making.
Both silicones are RTV which means Room Temperature Vulcanisation; they cure at room temp. They are each preferred for specific applications. High temperatures, longevity and quicker working times? Platinum (addition cure) is for you. Generally cheaper and chemically resistant? Tin (condensation cure)!!
Condensation cure silicones
Condensation cure silicones are cheaper and often used for larger general mould making applications, prototyping, industrial, ornamental plastering, and of course throughout special effects workshops. The large amount of silicone used in these sectors make this the economically friendly option.
They’re typically (not always) a 10:1 ratio and require accurate measuring, often by volume or weight. Condensations cure silicone catalysts are often pre-pigmented, but the addition of silicone pigments and filler additives can be used without inhibition.
Choosing to work with a tin silicone, you must be aware its library life is significantly shorter than a platinum silicone and will not perform as well as a platinum when casting vast amounts.
You may have as little as 12 months usage from a tin mould as opposed to decades from platinum. The wear and tear after this length of time can literally cause tears in your silicone making your mould completely unusable. Your catalyst becomes unstable and you may find leaching from this mould.
However, you will find inhibition resistance in tin silicones. Meaning, they are likely to cure at room temperature over virtually any surface.
You will also find you have a much longer working time than a platinum silicone. This is helpful when you are mixing a large batch and waiting for it to de-gas. You then have enough time to pour, allow remaining bubbles to rise to the surface before it kicks.
Work exceptionally well when casting resins; polyester, epoxy or polyurethane as well as plaster and candle waxes.
Warning! It gets trickier when you want to cast silicone into silicone. If you remember our first question we ask ourselves when moulding – “what will be cast into the silicone?”
This is because a platinum silicone pour can only be cured into a platinum mould. Conversely, tin silicone will cure over platinum. Released, of course. Silicone WILL adhere to silicone!
Addition Cure Silicones
Addition cure silicones are typically skin safe and often used for life-casting or prosthetic appliances.
Addition silicones are great multipurpose material, as above you can take life-casts with specific silicones for a longer lasting detail capture (an alginate alternative, as this is a one use throw away “mould”). Prosthetic appliances are typically Super Baldiez / Baldiez (cap plastic) encapsulated platinum silicones; the translucency of platinum silicones makes for exceptional skin replications. There’s such a wide range of additives for most platinum silicones making it such a versatile range of silicone – accelerator, retarder, thickener, thinner, softener, hardener and pigments. Following instructions with these additives is important.
The possibilities are quite endless with platinum, one material can be your mould and cast (if that’s what you wanted!) otherwise, they work JUST as well with casting resins with little to no release agent required.
However as discussed, they are a pricier option; here’s why…
There’s virtually no shrinkage upon a cured platinum mould, you are ensured exceptional detail replication with extreme high tear strength and a mould guaranteed to last you many, many casts. YEARS worth!
They are however slightly more susceptible to inhibition; so here are the no-no’s..
What to look out for:
- Latex – avoid cross contaminating any silicones (condensation also included) with latex. opt for nitrile or vinyl gloves, and definitely no casting latex into a mould or moulding latex. Don’t even keep these materials in the same room!
- Sulphur – Clays can often contain sulphur, your master containing sulphur will stop your silicone from ever curing. Always look out for Sulphur-Free labels!
- Condensation Silicones – as briefly mentioned, silicone can inhibit silicone. If there’s ever tin silicone residue over a master for example, let’s say you made a condensation cure mould of the same master, and attempted to mould again in platinum – it won’t cure.
You’ll find platinum silicones to have a much shorter working and demould time than a tin silicone, commonly the working time is too quick and retarder is required to allow enough time for mixing, de-gassing, pouring or even injecting! Heat is also a factor that speeds up working/cure times dramatically; RTV being room temperature curing means a warm environment will really kick the silicone! So a slowing additive or just chilled materials will give you some extra minutes to work with.
This however may benefit you! Again, choosing a material is all according to purpose.
- No shrinkage
- Multipurpose usage, can be used as moulding or casting material
- Exceptional library life
- Versatility – a wide range of additives to manipulate the material according to requirements
- Widely used for SFX artists and medical professions to replicate skin, the low shore and additives available create this look and feel exceptionally well
- Some systems can be used directly onto the skin for effects or lifecasting.
- Inhibition possibilities
- Inhibition resistance - cure upon almost anything.
- Versatile – can be used for general
- Tin silicone have a reduced library life – months, with deterioration after multiple casts
- Begins to leech over time thus not safe for skin contact