Method of controlling the position of an extruded preform in a blow mold

A method of extrusion blow-molding a hollow body comprising the steps of:

• (a) introducing thermoplastic material in an extruder head having a die discharge slit,
• (b) changing the slit width of the die discharge slit as a function of a wall thickness program for the hollow body, the wall thickness program including at least one critical cross section region,
• (c) extruding a tubular preformed article having at least one thickened wall portion corresponding to the critical cross sectional region of the wall thickness program,
• (d) expanding the preformed article in a blow mold to form a blown body therefrom,
• (e) ejecting the blown body from the blow mold,
• (f) dividing the blown body into at least the hollow body, an upper waste-portion and a lower waste-portion,
• (g) measuring the actual weight of the hollow body lower waste-portion to form a control value for position determination of the critical cross sectional region in the hollow body, and
• (h) adjusting the position of a subsequent tubular performed article as a function of the control value of step (g) in that during subsequent operation at least one of the width of the die discharge slit and the speed of extrusion are changed in such manner that when expanding the subsequent preformed tubular article at least one critical cross section region of the blown body corresponding to the last-mentioned thickened wall portion shall be moved to a desired position in the blow mold.

A method of extrusion-blow molding a hollow, thermoplastic body, comprising the steps of:

• a) introducing thermoplastic into an extrusion head which includes a die slit,
• b) changing the width of the die slit as a function of a wall-thickness program for the hollow body, said wall-thickness program including at least one critical cross-sectional region,
• c) extruding a tubular preformed article from the die slit of the extrusion head,
• d) expanding the preformed article in a blow mold into a blown body,
• e) ejecting the blown body from the blow mold,
• f) dividing the blown body at least into segments consisting of the hollow body, an upper waste-portion and a lower waste-portion,
• g) measuring the actual weight of at least one segment of the blown body different from the hollow body to form a control value for position-determination of the at least one critical cross-sectional region in the hollow body,
• h) adjusting the position of an at least one critical cross-sectional region in a subsequent hollow body as a function of the control value of step (g) in such a manner that, in accumulator-head operation, at least the width of the die slit, and in continuous extruder operation, at least an angular speed of a screw, is changed in such a manner that when expanding the preformed article, at least one critical cross-sectional region of the blown body is moved to a desired position in the blow mold.