Cause, high temperature level of hot air was effect to big hole and many crack in OPB from Fig.4-7.
This is because microwave drying to excitation of molecular vibrations on higher shrinkage and strength than Convection drying.
It is therefore, concluded that transverse and longitudinal moisture gradient of convection drying at high temperatures effect of much gap and checks in OPB. Although, these result were confirmed on SEM of studies the structural changes in OPB.