there is no one substance labeled ‘plastic’ but more then a dozen, of only the thermoplastics, and they of course have different melting points; in fact they may have not a real melting point like water or metals but a softening & melting range.
And then you have plastics that don’t have a melting point or range at all, the thermo-hardening plastics. ( Bakelite etc ).
Temp use range :
Softening & melting points :