I heat my plastic in the microwave, making sure to stop and stir it in the mid 200 range. It usually is still gelled but it seems to get the heat distributed. Then I zap it a bit longer aiming for a bit over 300.
I then add all ingredients. If I dally too long I give it another short zap so it pours well into the Presto Pot.
Sprues get zapped and added back. If using light colors, added heat often darkens or yellows the plastic so go easy.