a closure mechanism for a metallographic sample pressure molding system includes a mold cylinder having a bayonet socket for receiving a closure member which includes an upper ram mounted thereto for lockably enclosing the mold chamber defined by the cylinder . the closure member includes a cam and means for actuating the cam for moving the upper ram an incremental distance to break flashing from the molded metallographic specimen once cured for facilitating removal of the specimen and its surrounding thermoset or thermoplastic mount . in the preferred embodiment , the mold cylinder is demountable to a mold collar at an end remote from the closure member . a piston extends into the mold chamber through the collar and an alignment coupling joins the piston to a piston rod facilitating installation of the mold cylinder to the collar . the system provides a positive locking mold chamber with easy access and facilitates removal of the molded article by breaking the flashing from the article once molded .