I have now found it is called hydroforming with fluid or rubber-pad forming with "rubber", probably urethane. Thank you. I have found Gary Ash's posts etc.   So if I use rubber pad forming and I want to fit 0.6 mm aluminium onto a 4.5" diameter cylinder, how much elastic rebound should I expect? i.e. how big should the cylinder be? I expect it would be a little less than 4.5" dia..