Process for making MDI based flexible foam

Process for preparing a flexible polyurethane foam by reacting a polyisocyanate composition comprising polymethylene polyphenylene polyisocyanate and an isocyanate-reactive composition comprising at least one isocyanate-reactive compound comprising at least two isocyanate-reactive hydrogen atoms and using a blowing agent characterized in that the foaming is conducted in a closed, vented mould at a pressure of at least 0.001 to at most 0.5 bar above atmospheric pressure, wherein the process is conducted at an amount of overpack of 0.1 to 40% by weight.