The invention relates to a method for blow forming a container from a parison made of plastic. By producing signals which should be of substantial identical characteristics, deviations from the situation where the signals are of identical characteristics can be used to identify various types of asymmetry which are otherwise difficult to detect. In one embodiment the voltage detection device comprises a cylindrical housing containing a voltage sensor of the type capable of detecting the magnetic field generated by the flow of electrical current in a specific voltage range. Molded parts are prepared by extracting the dissolved polyamide with protic solvents, preferably water, a C1-4-alcohol or mixtures thereof, wherein the dissolved polymer is transformed to a solid or wax-like state by cooling and extracted after subsequent moulding.