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I have a silicone mold that I want to use for a concrete piece. I want to add a piece of wood to that mold, which will leave a negative space in the concrete. Later, I will fill that with resin.

Is CA glue the best practice to stabilize the wood piece in the silicone mold? Or is there a better way?

I also want to be able to remove the wood from the mold if I decide not to use it, without damaging the mold.

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Silicone works well as a mold material because virtually nothing adheres to silicone other than silicone.

CA glue may not be a good choice, as it certainly will not adhere, but also may damage the surface of the mold.

Your best option is to attach the wood to an outside jig or bracket of some sort, which has reference points to the mold that are not adhesive dependent. For example, if you can encase the mold in a box of plywood and drill holes in the plywood for attachment, the insert with an extension arm would drop into the mold in the correct position once the holes on the outside align with the arm.

Without drawings of the mold and insert, I can't generate a mock-up drawing. Words will have to suffice.

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Get a set of magnets (I think 4 but more may be needed) and drill holes in the wood on the side it sits against the mold and insert (glue in) the magnets.
When you get ready to poor your concrete you place your wood and add the magnets on the outside of the mold, which should keep your wood in place.

The bigger the piece of wood, the more magnets you need due to the tendency of silicone to stretch, and it have to be strong magnets so it gets a strong bonding even through the extra layer.
Using magnets you do not need to glue to your mold and should not damage it at all.

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