Patent ID: 6258301
Filing Date: 2001-07-10
Classification: B29C

Abstract:
In a method of blow molding hollow articles of a thermoplastic synthetic resin, comprising the steps of:(a) extruding a tubular preform from a thermoplastic synthetic resin melt by forcing said thermoplastic synthetic resin melt through a nozzle gap having a variable gap width;(b) providing, as a predetermined wall-thickness program for controlling said gap width as a function of a length of said preform, a program curve which, plotted over n successive volume segments over the length of the preform, comprises a profile curve representing a preform thickness characteristic superimposed upon a base gap;(c) varying said gap width in accordance with said predetermined wall-thickness program thereby extruding successive volumes of extruded thermoplastic synthetic resin melt for said n successive segments over said length of the preform;(d) measuring at least one parameter of distribution of said extruded thermoplastic synthetic resin melt in said preform or distribution of thermoplastic synthetic resin in a blow-molded product fabricated from the preform to obtain a measured value and comparing said measured value with a control;(e) controlling the preform extrusion so that the preform reaches a predetermined desired position between mold halves of a blow mold(f) blowing said tubular preform in said blow mold to expand the tubular preform and produce a blow-molded article therefrom having waste portions;(g) removing said blow-molded article having waste portions from said blow mold and detaching said waste portions from said blow-molded article; and(h) additionally applying to gap width set points determined by said program curve of said wall thickness program corrective values influencing sag of said preform so as to eliminate deviations of said measured value from said control when the preform is in the desired position between the mold halves of the blow mold but a material distribution in the preform is unsatisfactory thereby compensating for visco-elastic changes in the preform during formation thereof, and adjusting the base gap width upon each said application of corrective values to the wall thickness program such that the melt quantity required for a particular preform is the same as a quantity required for a preform prior to correction.