Silicone
Silicone is a very versatile and easy product to use - ideal for taking mouldings from solid objects, it is less suitable for body moulding because silicone is susceptible to oil and water damage until cured,
although specialist body moulding silicones are available (Putty Silicone).
The due to the basic chemistry of silicones, there are rules which MUST be followed.
All of the silicones we supply are RTV Silicone (Room-Temperature-Vulcanizing silicone) which is a type of silicone rubber made from a two-component system (base plus curative; A+B)
available in a hardness range of very soft to medium - from 15 to 65 Shore hardness.
Applications include making moulds for reproducing cast, carved or modelled items including specialist products for low melt alloy casting.
Silicone moulding rubbers come in two cure types, curing is the process that takes place for the user to utilise the rubber to make moulds for application purposes.
Universally there are two types of silicone moulding rubber, Tin-cure commonly referred to as condensation cure and Platinum-cure commonly referred to as Addition cure.
At Tiranti we sell the following:
- RTV115 - Shore A hardness at full cure 15.
- RTV120 - Shore A hardness at Full cure ( 7 days) - 20.
- T25 - Shore A hardness at Full cure ( 7 days) - 25.
- RTV128 - Shore A hardness at Full cure ( 7 days) - 28.
- RTV65 - Shore A hardness at Full cure ( 7 days) - 65. For low melt alloy use only, maximum temperature 250°c.
- Putty Silicone - Shore A hardness at Full cure ( 1 hour) - 40.
- RTV4420 - Shore A hardness at Full cure ( 24 hour) - 20.
- RTV640 - Shore A hardness at Full cure ( 24 hour) - 40.
The chemical difference between tin-cure (condensation cure) and platinum-cure (addition cure) silicone mould making rubbers lies mainly in the metal used to catalyse or cure the base rubber. The metal, tin is used to catalyse or cure tin (condensation) silicone, and platinum is used to cure platinum (addition) silicone rubber. Of the two kinds, tin cure is much more forgiving in terms of the model materials which can be used to create moulds and in its curing process. Silicones actively repel both oils and water - all surfaces must be clean and dry before the moulding process can begin. Silicones react to heat and cold, so avoid extremes of both while mixing, pouring and moulding using silicone. Over-catalysing can also cause extreme issues, the silicone can react by breaking up (appearance similar to cottage cheese) or part catalysing (soft on the outside and hard in the middle).
Accordingly, tin-cure silicone rubbers are usually significantly less expensive than platinum-cure ones, and often have less restrictive curing environments. Both of these types of silicone rubber are very different from each other. More importantly, they are not compatible with one another either, and cannot be layered or combined in the mould making process.
| Condensation Cure Silicone | Addition Cure Silicone |
| Condensation cure silicones are more tear resistant. | If high temperatures are anticipated, then addition cure silicones are best - except with our low melt alloy metals. |
| Condensation cure silicone moulds tend to weaken or tear only after a year or two, depending on the amount of use. | Addition cure silicone is susceptible to some chemicals like nitrogen, sulphur, phosphorus, sulphur vulcanised rubbers and condensation cure silicone rubbers, Contact with these incompatible chemicals can inhibit their cure and result in a partially cured mould/cast. |
| Condensation cure silicone moulds cure with a slight degree of shrinkage. | Many addition silicones prove to be incompatible with polyurethane rubbers and epoxy resins too. |
| Condensation cure silicone is more economical and preferred for general mould making. | Addition cure silicones necessitate more careful and accurate mixing than condensation cure ones. |
| Condensation cure silicone moulds can capture fine and intricate details and are excellent for all kinds of polyester, epoxy, gypsum, wax, resin and plaster casting. | Addition cure siicones also capture fine and intricate details and can also be heat accelerated for faster curing. |
| Condensation cure silicones generally exhibit higher shrinkage over time depending on the type of mould rubber being used, material being cast into the rubber mould, mould configuration, and other variables. | Addition cure silicones generally have an almost negligible shrinkage rate. |
To sum up, both addition cure and condensation cure silicones have certain unique characteristics which should be carefully considered during product selection depending on the application required. Additional catalyst, fast catalyst and clear catalysts are also available
Usage instructions vary depending on the product you are using, please always refer to the instructions provided on the product in question or the downloadable TDS sheet information available from our website.
| Product Name | Pot Life | cure time (hours/days) | Shore A hardness @ full cure | shrinkage | Catalysed colour | Tear Strength | Thixo available? | Additional Catalyst |
| RTV115 | 95 | 36 hours | 13 / 15 | 1 | mint | 3 | YES | Green, fast & Clear |
| RTV120 | 100 | 36 hours | 13 / 21 | 1 | mint | 2.5 | YES | Green, fast & Clear |
| RTV128 | 90 | 36 hours | 18 / 28 | 1 | mint | 2.5 | YES | Green, fast & Clear |
| T25 | 30 | 30 hours | 25 +/- 2 | =0.2 | pink | MPa =26 | NO | Standard red |
| RTV65 | 45 | 15 hours | 65 | = 0.1 | deep red | 4 | NO | Standard Clear |
| RTV4420 | 11 - 15 mins | 3 hours | 19 +/- | = 0.1 | translucent | 4.5 | TBC | No |
| RTV640 | 90 | 11 - 13 hours | 40 / 40 | = 0.1 | translucent | =22 | TBC | No |
| Putty Silicone | 3 | 45 minutes | 45 | 0.9 | Bubblegum Pink | 0.9 | NO | No |