Hi Aaron – is that 215 degrees C or F? Assuming it’s Celsius, that should be plenty hot – maybe too hot, by the sounds of it. Maybe try around 200 C. You should be able to heat a small area, remove the iron, press on it with a rag for a few seconds to allow the weld to set, and then it should stay like that. You may see an occasional small bubble, as in the picture below, but larger bubbles might be caused by too high heat and/or trying to heat too large of an area at once. Here’s a picture showing what it should look like after heat sealing 210D (yellow) to 1000D (darker 210D fabrics aren’t as translucent, so you can’t see the weld like this).
The pieces cannot be peeled apart without destroying the TPU. Once the temperature is dialed in, you shouldn’t have to repeat the sealing process – just heat an area for a few seconds (pressing firmly and sliding the iron a bit), remove the iron, and press on the weld with a rag while it cools for a few seconds, and then move on. If it lifts or bubbles after you remove the rag, press a bit longer to allow it to cool more or use a lower temperature. (When lowering the temperature, check to make sure the weld is still sound.) Practice with scrap material until it’s dialed in.
Cheers!