Abstract: In the manufacture of foamed plastic slab by introducing foamable fluid from a mixing head into a moving conveyor-type mold, the improvement wherein supplemental quantities of the fluid are conducted from the mixing head longitudinally to locations where the main body of the material has foamed to partly expanded condition and friction between the sides of the mold and expanding material has caused the upper longitudinal side edges of the expanding material to assume a rounded shape which curves away from the mold sides leaving a space. The supplemental material is introduced into the spaces while the mold moves and upon expansion substantially fills the spaces and thereby greatly reduces the amount of the slab which must be cropped off prior to cutting it to form saleable articles.