A light weight plate segment configured to be mounted on a disc of a disperser or refiner for comminuted cellulosic material including: a front face having disperser teeth or refining bars; a back face having a raised post surrounding a fastener attachment structure and a raised plate positioning section; side edges of the plate segment; and a radially outer edge and a radially inner edge extending between the side edges; wherein the back face lacks raised structures along the side edges.