A round top can be made out of the bottom from a PET bottle.
Pressurize the bottle up to 2 to 3 bars, i use a standard cap with a bicycle tire valve for this. Then very slowly and carefully heat the top with the paint stripper. Start with the sides and work your way slowly to the top, keep rotating the bottle to warm the bottle side as evenly as you can. It is good to stop at regular intervals and let the bottle cool down. This needs some practice and don't expect your first one to be perfect.
On the right you see the round top with the retaining tie wraps and a chock cord to pull the top off when it is released.
Then a warning, don't, I repeat DON'T, heat the bottle with hot water, although the heating will be much more equal over the bottle bottom, the risk that the pressure punches a hole in the bottle and blows the hot water in your face you just cant take. On the net I read that someone severely burned himself doing it this way!.
Let the bottle cool down to about room temperature before removing the pressure. Otherwise you probably will get the same modern
expression type off result as in the picture on the left. I over heated the PET material and it started to grow rapidly,
released the pressure and the whole thing collapsed.
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