Valve unit for pneumatic operation in a blow-molding process, comprising a) a valve housing having a process pressure input line, a process pressure output line and a process guide shaft, b) an operating piston for closing and opening a connection between the process pressure input line and the process pressure output line, wherein the operating piston defines a longitudinal central axis, and c) a first and a second control line (50, 60) and a first and a second control chamber (40, 41) for controlling the operating piston, wherein the operating piston has a first control surface which faces the first control chamber, and wherein the operating piston has a second control surface which faces the second control chamber, wherein the two control surfaces (200, 240) are able to be subjected to a control pressure each, wherein the operating piston can be displaced in the axial direction within the process guide shaft in a sealed manner by means of a dynamic process seal and a) wherein the operating piston has a closing ring which in the closed state of the connection sealingly abuts a valve seat of the valve housing, or b) wherein the valve housing has a closing ring which in the closed state of the connection sealingly abuts against a valve seat of the operating piston, and wherein the closing ring is designed as a circumferential sealing edge, wherein the valve seat consists of a softer and more elastic material than the sealing edge abutting said valve seat, and wherein the circumferential sealing edge has a diameter which corresponds to a guide diameter of the dynamic process seal wherein at least one of the two control chambers (40, 41) is of cylindrical configuration and is arranged centrally to the longitudinal central axis by the operating piston, thereby comprising the longitudinal central axis, and wherein the operating piston has through-bores for connecting a valve chamber to a pressure compensation chamber. a) a valve housing having a process pressure input line, a process pressure output line and a process guide shaft, b) an operating piston for closing and opening a connection between the process pressure input line and the process pressure output line, wherein the operating piston defines a longitudinal central axis, and c) a first and a second control line (50, 60) and a first and a second control chamber (40, 41) for controlling the operating piston, wherein the operating piston has a first control surface which faces the first control chamber, and wherein the operating piston has a second control surface which faces the second control chamber, wherein the two control surfaces (200, 240) are able to be subjected to a control pressure each, a) wherein the operating piston has a closing ring which in the closed state of the connection sealingly abuts a valve seat of the valve housing, or b) wherein the valve housing has a closing ring which in the closed state of the connection sealingly abuts against a valve seat of the operating piston, and wherein the closing ring is designed as a circumferential sealing edge, wherein the valve seat consists of a softer and more elastic material than the sealing edge abutting said valve seat, and wherein the circumferential sealing edge has a diameter which corresponds to a guide diameter of the dynamic process seal wherein at least one of the two control chambers (40, 41) is of cylindrical configuration and is arranged centrally to the longitudinal central axis by the operating piston, thereby comprising the longitudinal central axis, and wherein the operating piston has through-bores for connecting a valve chamber to a pressure compensation chamber. and wherein the closing ring is designed as a circumferential sealing edge, wherein the valve seat consists of a softer and more elastic material than the sealing edge abutting said valve seat, and wherein the circumferential sealing edge has a diameter which corresponds to a guide diameter of the dynamic process seal wherein at least one of the two control chambers (40, 41) is of cylindrical configuration and is arranged centrally to the longitudinal central axis by the operating piston, thereby comprising the longitudinal central axis, and wherein the operating piston has through-bores for connecting a valve chamber to a pressure compensation chamber.