Making flexible moulds with silicone

Two-component systems, RTV, condensation or addition curing

Our silicones are two-component systems, i.e. you always need a silicone and a suitable crosslinker. They are also RTV silicones, which means that the silicones cure at room temperature (Raum-Ttemperature-V curing).

Basically, silicones are also differentiated according to the type of chemical reaction during crosslinking: there are condensation-crosslinking and addition-crosslinking silicones. No decomposition products are produced during addition curing, which means that the shrinkage of addition-curing silicones is generally even lower than that of condensation-curing silicones. They are therefore particularly suitable for extremely detailed mouldings (e.g. of human skin). With bildhau, however, the condensation-curing silicones also have a very low shrinkage. Please pay attention to the information on the respective products!

For silicones with polyaddition crosslinking, it is also important to ensure that the quantity ratios are adhered to very precisely and that the two components are weighed very carefully. Even small deviations can disrupt the crosslinking and curing of these silicones. Polycondensation-curing silicones are somewhat easier to process.

Also make absolutely sure that the silicone compound and crosslinker are compatible. Polyaddition silicones and polycondensation silicones also do not mix with each other!

Measure precisely: silicone, crosslinker, thixotropic agent

As silicone is a two-component system, for moulding silicone you need a cross-linking agent in addition to the liquid silicone mass itself to ensure that the silicone sets. There are also different variants here: There are crosslinkers that allow the silicone mass to set more quickly or more slowly. When mixing carefully, pay close attention to the specified proportions. It is best to use a scale to measure exactly – otherwise the silicone may not harden. This also applies to the two-component silicone modelling compound, which has a different consistency from the outset.

If you want to fill the silicone rather than pour it, you can thicken it with thixotropic agent, or thixo for short. This makes the mixture thicker. Pay attention to the exact quantities here too and mix carefully!

Potting time and demoulding time

The pot life is the time you have to process the silicone, i.e. from mixing to setting. Demoulding time, on the other hand, is the time it takes for the silicone to dry completely so that you can remove the silicone mould from your master model. Pot life and demoulding time can vary greatly depending on the silicone compound and crosslinker, but also on the room conditions.

Humidity, room temperature, protective clothing when working with silicone

There are ideal conditions under which silicone is best processed: around 23 °C room temperature and medium humidity. The manufacturer's specifications for pot life and demoulding time usually refer to these ideal conditions. Significant deviations, e.g. high or very low humidity, extreme heat or cold, affect the silicone's ability to cure and can result in the silicone not setting or, conversely, having a much shorter pot life.

As some silicone components contain harmful substances, we recommend protective clothing with safety goggles , dust mask , gloves and apron .

Flexible moulds need support moulds

If you are building a multi-part casting mould from silicone, you will usually need support moulds that can be made very well from plaster . Support moulds give the casting mould stability and prevent it from collapsing, for example.