Silicone moulds have a lifetime of 20 to 30 casts. Over that time the surface of the tool will be attacked by the hardener and will become brittle and hard.

Brush silicone oil over the surface of your silicone tools and leave them overnight; the oil will react with the silicone and extend its useful life. Silicon spray or an oil bleed silicone may also extend the life.

Eventually a new tool will be required.