a mold is described for the molding of substantially isotropic isocyanate - based polymer foam buns . the mold is a box mold which has sidewalls of which at least one pair disposed in parallel relationship is adapted to move outwardly when subjected to pressure exerted by the expanding foam during the molding operation . the outward movement of the sidewalls is controlled by stops provided on the periphery of the mold floor , said sidewalls being free to slide over the surface of the floor . the lid of the mold is supported in sliding engagement with the upper edges of the sidewalls . the mold and the process of fabricating foam buns therewith is particularly suitable for use in situations in which it is not possible or practicable to employ continuous bunstock machines . the mold is relatively inexpensive and readily dismantled for transportation to a fabrication site .