PURPOSE: To dry a wet ionomer and thereby to enable thermal plasticization treatment thereof by injection-molding the wet hygroscopic ionomer directly by using a two-stage vented barrier screw having a barrier stage and a degassing zone subsequent to this stage.

CONSTITUTION: A screw is divided into a feeding part 16, a transitional barrier part 17, a degassing part 18, a second transitional part 19 and a metering part 20. Ionomer pellets are fed into the screw by a helical continuous pocket 21. The screw rotates in an injection barrel B and grains are advanced thereby toward a pocket wherein initial melting occurs. A main melting function is generated in the transitional barrier part 17, while a melting groove 26 is formed by a second helical barrier stage 23 divided from the leading edge of the final primary stage 25 of the feeding part and continues to the tailing edge of a transitional zone. The width of this groove increases from the beginning to the end of the barrier part. The barrier stage 23 has a smaller outside diameter than the one of a primary helical stage 14 and gives a gap for that molten resin between the barrel and it flows into the groove 26 and fills it partially. Therefore the molten material is degassed in a degassing zone and in the whole of the barrier part.