Abstract: Blow molding apparatus for the manufacture of hollow bodies from a thermoplastic, in which a screw-extruder forms a parison into a receiving station. A blow mold is movable to and fro between the receiving station and a mold release station, the blow mold consisting of at least two parts which can be opened so that the blow mold encompasses the parison in the receiving station. The length of the parison extruded is detected when it reaches a given length corresponding to the length of the mold. Control means are provided, these being operable only by the means for detecting the parison length, and cause the mold parts to close on the parison. Time dependent open-loop control means control the movement of the blow-mold so that it is in the open position in the receiving station before the length of the parison has reached the given length, so that it is ready to close immediately the given length is reached.