A formed rib, boss, wall, or deep pocket can lose shape after the pressure differential is removed. Collapse may occur on the tool, during air eject, immediately after demolding, or later as the part cools.
Thin local walls, trapped heat, inadequate support, excessive eject pressure, and insufficient geometric stiffness are common causes. A feature that formed incompletely may also appear to collapse when it was never fully defined.
Diagnose the timing
Observation should establish whether the feature reaches full shape under vacuum or pressure. If it relaxes before tool opening, cooling or pressure hold is inadequate. If it fails during release, eject force, sticking, or undercut is involved. Delayed deformation points to residual stress or uneven cooling.
Cooling and pressure hold should be extended only enough to reach stable stiffness. Local material distribution, ribs, radii, and tool temperature may need revision. Blowing more release air into a weak feature can make the failure worse.