Hei-Cast 8400 is not a silicone it is a three component polyurethane for vacuum casting/molding.
One of those components is an isocyanate and that is likely to be toxic.
One of the other components is a polyol which is harmless.
The third component is probably a plastisier as adding more of it reduces the hardness of the finished article. Some plastisiers can be a health issue.
I would not put a product made by you using this in my mouth.