An internal mixing apparatus for mixing plastic materials, especially raw rubber, has a mixing chamber defined by substantially parallel cylindrical individual chambers. Each of these chambers holds a rotating shaft and the two shafts rotate in opposite directions. In one embodiment of the invention, the shafts have a cylindrical outer surface on which is disposed at least one principal conveying and mixing vane. The vanes on the two parallel shafts are disposed in mirror symmetry so that the ridges of the vanes roll off against one another with or without contact. The principal vanes terminate ahead of the end faces of the mixing chamber, thereby defining free passages permitting the transverse motion of the material to be mixed. Within these passages, the shafts carry scraping vanes for preventing the accumulation of stationary material. In another embodiment of the invention, the vane cores are conical and the height of the vanes changes continuously so that the wall clearance between the ridges of the vanes and the internal walls of the mixing chamber change as a function of axial distance, defining high pressure and low pressure zones. In the high pressure zones, the wall clearance is large, permitting substantial tangential material flow. In a third embodiment of the invention, the vanes on the two shafts interleave, one solid vane engaging an interruption of the opposite vane.