You need to get a temperature gun, they got them at Harbor Freight or ebay,
For re-melt plastic add heat stabilizer, just a few drops per cup.
This will depend on the amount that you are heating & the power of your microwave. I use a 1200 Watt.
1 cup of plastic will heat pretty fast
1 minute stir
30 sec. stir
30 sec stir
20 sec stir & it's ready.
I heat to 325 to 340, do not overheat... If you burn plastic, you will know it. get it out of your shop.
Be well ventilated
ALL plastic smokes when heated...
Do not pour in shorts and flip flops, wear gloves & glasses. Be safe and enjoy your new hobby.