Elastomers is short for elastic polymers, which are materials that typically have a low Young’s modulus and high failure strain. They are available with a wide range of material properties, such as tear strength, elongation at break (stretchiness), cure time, and durometer.
Many elastomers, including Elastosil, are hyperelastic, meaning that they can stretch a lot (i.e. Elastosil can stretch 700% of its original length) without breaking or permanent deformation, which leads to robust devices. Durometer is also an important consideration, indicating softness/hardness of material. It is measured by shore hardness scales:
Elastomers (and many other polymers) typically come in two parts: a base material containing free, non-crosslinked polymer chains, and a curing agent. To initiate curing, these two parts need to be mixed together thoroughly in the appropriate ratio. When this happens, the curing agent will cross-link polymer chains, hardening the material.
Since this is a chemical reaction, heat can accelerate curing, but sometimes you get incomplete curing/stickiness at mold-elastomer interface or unintended effects on material properties– in which case, try longer curing at room temperature.
If this doesn’t help, there may be a cure inhibition issue. Read elastomer manufacturer documentation for best practices.