It's not working...

I got a vacuum former last week- and I can't get it to work properly yet. I did a tester with some acrylic that came with it and I didn't let it get hot enough so that was a fail...

Then I thought maybe material would make a difference. Since I wanted to make moulds from polyethylene anyway, I waited till my delivery of 1mm HDPE came and then this happened...

I don't know if it got too hot, or whether it's not suitable for making bangles. Maybe I need LDPE so it's more flexible? But then I have an area where the sheet has pulled away from the clamps which suggest it is too hot (or the clamp is faulty), and it would only get worse if I used a softer plastic surely? Is there a better way of doing things?

*sigh*