  The possibility in principle is shown that the noncompensated Casimir force
can exist in nanosized open metal cavities. The force shows up as time-constant
expulsion of open cavities toward their least opening. The optimal parameters
of the angles of the opening, of "generating lines" of cavities and their
lengths are found at which the expulsive force is maximal. The theory is
created for trapezoid configurations, in particular for parallel mirrors which
experience both the transverse Casimir pressure and longitudinal compression at
zero general expulsive force.
