Measuring arrangement including a blow-molding extruder, method for measuring a molten tube or a blow-molding product as well as a blow-molding extruding method

A method for measuring a molten tube for an extrusion blow-molding process or a blow-molding product, the method comprising the steps of providing a THz measuring device including a THz sensor which comprises an optical axis, extruding a molten tube through a blow-molding extruder along an extrusion axis or positioning a blow-molding product determined by an extrusion axis, emitting a THz transmission beam from the THz sensor along its optical axis in such a manner that a front wall of the molten tube or the blow-molding product lies in a divergent beam area of the THz transmission beam behind the focus or in a convergent beam area before the focus, wherein the front wall is a set distance away from the focus, the set distance being above a Rayleigh length and up to a pre-determined multiple of the Rayleigh length, and wherein a focus lies in one or both of the following positions:

• (i) before the extrusion axis and (ii) before an axis of symmetry of the molten tube or the blow-molding product, receiving a reflected beam reflected from the molten tube or the blow-molding product and evaluating a measuring signal received, and adjusting an adjustment angle between the optical axis of the THz sensors on the one hand and the extrusion axis or an orthogonal to the extrusion axis on the other, depending on the measuring signal, in such a way that the measuring signal in the reflected beam is brought to a maximum, so as to attain perpendicular irradiation of the optical axis onto the front wall of the molten tube, and determining the wall thickness of the front wall.