Technically PTFE as Teflon (tm) is a trade name and tends to cost a bit more.
In casting and injection I make primarily aluminum molds. I do some steel press dies, and other things, but mostly I make aluminum molds. The cost for low to medium production is very desirable.
I have customers who cast lead, tin, and bismuth. (Tin sticks to aluminum). I'm playing with some aluminum die casting designs for steel molds, but that's personal.
I also make a lot of low pressure injection molds for a product called plastisol. Its a heat activated PVC product that comes as a liquid and cures as a rubber. Due to its nature it shrinks a great deal when cooling from a liquid to its semi sold state. Depending on the exact mold this can result in either drawing air into the core resulting in an unintentional hollow end product, or it can pull away from the cavity surfaces resulting in a surface dent in the finished injection.
The general process is to fill the mold with a hand injector that looks like a gigantic aluminum syringe. Well for most of my customers. There are machine processes also.
I often design larger body items with an excess capacity runner so the body cools it can draw from the reservoir this excessive size runner creates. It still can often result in a hollow from the sprue all the way thru the core of the body.
The general reason is that aluminum conducts heat away from the plastisol very quickly. This is desirable for cycle times allowing demolding rather quickly compared to other materials which may just build heat making each successive injection take longer than the last. Yes, secondary cooling is an option as well, but the extra cost is not "worth-it" for most of my market segment.
My thought is to use a piece of teflon tube inside the runner to reduce the cooling rate of only the material in the runner. This should allow more of it to remain liquid (syrup really) and fall inwards from gravity to prevent just a molten core of flowing and rather than becoming an undesirable hollow injection.
PTFE is a decent insulator, has a high degree of lubricity, and a relatively high working temperature. Plastisol runs from about 300-400 depending on the exact formula and working process to get good fills in a particular mold.