Determination of flow rates in different melt strands during multi-strand plastic extrusion involves measurement of pressure difference in each strand and calculation by ratio of melt flow in each strand

摘要

Plastic melted in an extruder(6) forms individual strands(8,9). A measuring section(10) is provided for each strand in which pressure difference between two points is determined. Comparison of the pressure differences enables a ratio calculation to determine the division of the total melt flow to be made. Each strand(8,9) forms a line in which an extrusion die(4), calibrator and take-off unit are located. Each line has a measuring unit(5) which can be calibrated in relation to measuring units in the other lines using the determined melt flow division. Melt flow in each strand can be adjusted by either mechanical, motor driven or thermal means so that the melt flow rate can be equal or different in each line. Total melt flow may be determined by weight and/or by a melt pump. Take-off speed can be controlled to change the weight/unit length of each strand. Alternatively a constant wall thickness is achieved. For this each strand is calibrated in such a way that an ultrasonic measuring head permits an absolute wall thickness measurement. A single take-off unit may haul-off two or more strands simultaneously. An independent claim is included for equipment for determination of the division of material flow during multi-strand extrusion into two or more flows and includes an extruder(6) which produces two lines of melt(11) in each of which a measuring unit is located for determing pressure difference betwen two points along a measuring section(10).