Controls on extrusion-blow moulding - automatic system uses weight of tubular parison as datum value and adjusts extruded die gap as required to keep prod. within tolerances

摘要

Control on extrusion-blow moulding machines is provided w.r.t. wt. measurements. In the process the cycle time and the wt. produced are measured, from which the wt. of the parison is obtd.; this wt. is compared with a target value and is adjusted by changing the target value of the wall thickness; the corresp. changed cycle time is fed back to the target time by modifying the speed of the extruder. ADVANTAGE - When the parison has the required profile its target wt. is set by the extruder die gap; the cycle time measured is that between closing the mould of one cycle and the same pt. during the next cycle, and this is triggered by a photoelectric cell. The speed of extruding the tubular parison should be that which ensures that the blow mould only waits briefly before receiving the tubing. The feed hopper of the extruder has a balance to weigh the supply, and a second hopper above the first has a control damper which remains open until the max. wt. of filling is reached. The max. and min. wt. values are the set target wts. The overall system produces precise measurement of the throughput.