Structural Foam for chassis stiffening

Most of the foam in new cars is for energy absorption (occupant protection in a collision) and sound control.

Different densities of foam are used. For occupant protection it's ridged foam. For sound and panel flutter it's often a flexible foam.

The plastic gussets mentioned above, snap into the inner structure and work as a barrier to control the foam when it is applied. They prevent the foam from going where you don't want it it go.