Polymers that get rigid due to curing are called thermosets.
In this process, molecules will crosslink (purple) themselves and form one single makro-molecule.
Because of very strong covalent bonds, the makro-molecule can not be melted again.
Even if high heat is applied, the only result will be degradation.
In contrast to a thermoplast material, the mold for thermoset materials is heated up.
Typical thermoset polymers are phenolics, epoxies and resins.