Oh god no, NEVER put sintra in an oven.
1. The Fumes are EXTREMELY POISONOUS. Chlorine Gas.
2. You're in an exclosed area, your kitchen.
3. The sintra is in the enclosed oven, the gas fumes will build up and might start fire.
If you EVER need to form Sintra. I recommend boilding water. Even a heat gun will release toxic fumes from the sintra. When I laser cut Sintra, we have the laser closed and the gas vacuumed outside, even the small amounts of gas that seep through can get you physically sick.