Hot air gun / paint stripper for melting plastic
While the lower end hotair guns often only have one or two settings, the more expensive models offer variable increments (I've seen units that do 50 degrees and a commenter mentioned 10 degrees; each model will be different so review the specs).  The temperature spec gives the output temperature.

Start low and increase the temperature as needed to melt the plastic.  Do this in a well vented area as the fumes can be toxic.