The skin starts to burn after approx. 3 min./ 300 °C. After approx. 5 min. the skin has almost disintegrated and partly separated from the core.
The heat presses the moisture towards the cold side.
The increasing moisture on the cold side causes a swelling of the core.
Thereby the gaps between core, stiles and frame close. As the glue line between skin and core is still intact the door leaf can only destore towards the fire.
The destortion is reduced when leaving a gap of 1-2 mm between core and stiles.
The moisture has evaporated on the cold side too. The door leaf flatens again.
Existing gaps between core, stiles and frame open again.
The intumescents can develop to a full extent.
All the components shrink due to the continued combustion.
The door leaf bows outside.