A device for thermally molding a molten polymer or a resin by injection molding consists of a cylindrical melting chamber equipped with a heater, and a piston disposed therein so as to be translatable along the axis of the chamber. The melting chamber and an injection mold are situated inside of a separable sealed casing, the inner cavity of which is connected to a fore-vacuum pump. The inner cavity of the injection mold communicates with the inner cavity of the sealed casing, and the melting chamber, the piston, and the injection mold are made of an aluminum-bearing alloy. The injection mold consists of a casing with a vertical plane of separation, a lower cover with an aperture for admitting the molten polymer and with a quick-release means of being fastened to the melting chamber, and an upper cover with an aperture for releasing excess polymer. Both covers are conical inside of the mold cavity and at their perimeter are disposed in annular grooves formed in the casing, and the separable sections of the casing are joined by at least one adjustable clamping band situated on the outer surface of the casing.