[eric2011 (OP)]

What acid mixture can dissolve plastic when u sink the plastic in ?  Then after a bit it can hardfen again

[paul cotter]

This cannot be done. If a plastic dissolves in an acid, the polymer will have been disrupted and the plastic will not be reconstituted. The same goes for basic dissolution.

[evan_au]

The best way to soften plastics, and then later have them harden again is to melt them.
- Mild heat leaves the polymer chains intact, but temporarily mobilises them.
- Only works for thermoplastics...