Help sought for flattening styrene sheet

[Rookie]

I'm not sure this is the right place to post this question, but I leave it to the moderator to move this topic.

A while ago I bought 0.5 x 1400 x 1000 mm styrene sheets. They were sent rolled up in tube and I was stupid enough to leave them in there.

Now I need them, but they refuse to take their original form (flat).

Does anyone know how I can straighten them again?

I can cut of portions, but the biggest size I will need to work with is about 400 x 150 mm.

Any help, hint or tip will be appreciated.

Willem

[Berman]

You could try placing the styrene between two minimum 5mm thick sheets of absolutely clean glass. Place in direct sunlight. Check every thirty to sixty minutes depending on the temperature.