Abstract: A compaction unit (12) having a frame (14) resiliently mounted to a rigid support (10), the frame rigidly supporting a molding flask (24) therein, and having at least one pair of synchronized motors (A) with eccentric rotors coupled to the frame for vibratory motion therewith. The motor pairs are synchronized in counter rotation so that a given pair produces a net force vector perpendicular to a line joining the motor pair. Preferably, three motor pairs (A, B, C) for driving the frame in three different axis, are controlled by a program which permits sequential specification of the duration and net vector acceleration, over a wide range of values. The motors are maintained in synchronized phase relationship during the changeout of flasks, so that the compaction vectors experienced by each flask begin from the same initial idle condition.