I'd add that small amounts of plastic are the hardest to work with since they will lose heat the fastest. Keep in mind that bulk holds its own heat.

Some people place their jars of hot plastic on hot plates or griddles, but for casual use I make a hollow in a small soft towel next to my injection station that fits the bowls I use and when the plastic is up to temp I put the bowl in that hollow and I can shoot for quite a while. The towel helps to insulate the plastic.