A method and apparatus for removing hot volatiles and moisture from particulate plastic material being fed for injection or extrusion molding by a plasticizing screw which rotates in a heated barrel. Flow control means, such as a conveying screw or rotary valve, drop the particulate material downward through a vertical feed passage extending to an opening in the barrel. At the same time, suction is applied to remove hot volatiles which are released through the opening in the barrel as the particulate plastic material is gradually melted as it is compressed by the screw. The rate of feed of the particulate material is restricted so that the barrel of the plasticizing screw is only partially filled near the opening, thereby improving the withdrawal of the gases from the barrel. The incoming particulate material is preheated by a continuous flow of hot air through it as it passes through a tubular guide member. The air flowing to the preheater passes through a heat exchanger where it recovers some waste heat from the hot gases withdrawn from the barrel to improve efficiency and reduce power costs.