Device for injection moulding of plastic parts made of thermoplastic material, wherein the device comprises: a screw cylinder with a plastification and injection screw which is arranged in the cylinder and which is rotatory as well as translational movable in the same for the production of thermoplastic plastic melt and means for injection of a fluid at an axial injection position of the screw cylinder, wherein the plastification and injection screw can be arranged in the screw cylinder in such a manner that at least temporary screw channels are arranged at the axial injection position, wherein the plastification and injection screw is designed as multi zone screw and wherein the plastification and injection screw comprises a screw core around which at least one screw flight extends in a helical manner, wherein the plastification and injection screw comprises, seen from its driven side, at least four zones (a, b, c, d) which join in axial direction, wherein the first zone (a) is a feed zone which serves for feeding of plastic material, wherein the second zone (b) is a compression zone which serves for melting and compression of the plastic material, wherein the third zone (c) is a metering zone which serves for homogenisation of the produced plastic melt, wherein in the metering zone at least in sections a radial distance (t c) is given between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core, wherein the fourth zone (d) is a decompression zone, wherein in the decompression zone the radial distance (t d) is given between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core, wherein the radial distance (t c, t d) between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core in the region of the decompression zone (d) is at least in sections larger than in the region of the metering zone (c), wherein the plastification and injection screw is arranged movable in such a manner that it can take such an axial position in the screw cylinder that the means for injection of a fluid are arranged in the region of the decompression zone (d), wherein the metering zone (c) merges with the decompression zone (d) via a rounded region of the screw core, wherein the rounded region of the screw core extends along an axial distance (x) between 10 % and 100 % of the screw diameter, measured from the radial outer surface of the screw flight and wherein the region of the metering zone (c) and the region of the decompression zone (d) are free from mechanical blocking devices for influencing of the flow of the plastic melt. a screw cylinder with a plastification and injection screw which is arranged in the cylinder and which is rotatory as well as translational movable in the same for the production of thermoplastic plastic melt and means for injection of a fluid at an axial injection position of the screw cylinder, wherein the plastification and injection screw can be arranged in the screw cylinder in such a manner that at least temporary screw channels are arranged at the axial injection position, wherein the plastification and injection screw is designed as multi zone screw and wherein the plastification and injection screw comprises a screw core around which at least one screw flight extends in a helical manner, wherein the plastification and injection screw comprises, seen from its driven side, at least four zones (a, b, c, d) which join in axial direction, wherein the first zone (a) is a feed zone which serves for feeding of plastic material, wherein the second zone (b) is a compression zone which serves for melting and compression of the plastic material, wherein the third zone (c) is a metering zone which serves for homogenisation of the produced plastic melt, wherein in the metering zone at least in sections a radial distance (t c) is given between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core, wherein the fourth zone (d) is a decompression zone, wherein in the decompression zone the radial distance (t d) is given between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core, wherein the radial distance (t c, t d) between the outer radius (r S) of the at least one screw flight and the radius (r K) of the screw core in the region of the decompression zone (d) is at least in sections larger than in the region of the metering zone (c), wherein the plastification and injection screw is arranged movable in such a manner that it can take such an axial position in the screw cylinder that the means for injection of a fluid are arranged in the region of the decompression zone (d), wherein the metering zone (c) merges with the decompression zone (d) via a rounded region of the screw core, wherein the rounded region of the screw core extends along an axial distance (x) between 10 % and 100 % of the screw diameter, measured from the radial outer surface of the screw flight and wherein the region of the metering zone (c) and the region of the decompression zone (d) are free from mechanical blocking devices for influencing of the flow of the plastic melt.