Never had that problem, nearly all my wheel arches are plasticard and I usually just bend it into shape without heating it.I've just given up with plasticard. Poured boiling water on it and left in bowl for 3 minutes. Wrapped it around a cigar tube to get desired circular shape, and held it firmly in place with elastic bands.
Removed hours later. It immediately reverted to its flat shape and snapped in two. This has happened twice.
Presume I'm doing something wrong?