Some people re-use silicone in this way; however, there are a few challenges with this method you should be aware of.
- Residue from release agents, etc. may contaminate the silicone, meaning the silicone would need to be thoroughly washed with a solvent before cutting it up.
- Cured silicone may not bond to new silicone, meaning silicone made incorporating old silicone will have a lower tear strength because the piece of cured silicone may not bond to the curing liquid.
- Inhibition risks are greatly increased when doing this, as there may be contaminants or incompatible chemical formulations at work.
In short, it is a risky proposition to incorporate old silicone into new.