okay, equipped both sides with thin galvanized metal sheets and rubbed them with silicon oil. Also lowered the temperature to 200 degree, the baking time to 1,5 hours and place a small indicator sheet with flakes on the mold to get an impression of the melting process. 

This solution worked well, the plasticsheet was easy to remove and the surface of side 1 was very good. Unfortuanetly the amount of flakes was not sufficient to fill the whole mold with molten plastic. So side 2 was not so nice and looked like small trees would come out of the surface. 

Then.....Fourth try will be with weighing the needed plastic amount in advance. Stay tuned