GB1179172A - Method and Apparatus for Measuring the Thickness of Sheet Material. - Google Patents

Abstract

1,179,172. Measuring thickness of tubular films. INDUSTRIAL NUCLEONICS CORP. 10 Jan., 1967, No. 1286/67. Heading B5B. [Also in Division G1] In apparatus for producing tubular films by the " blown film " method comprising a blown film extruder having a circular die including a plurality of adjustable peripheral die sections and a rotatable part which causes any variations in thickness of the tubular film formed by the extruder to spiral about the film, and means for flattening the tubular film to form a sheet film of double thickness there is provided a thickness measuring system comprising a stationary gauge positioned to measure the thickness of one edge of the travelling flattened film and means for deriving from the thickness measurement an indication of the single thickness across the sheet. As shown in Fig. 1 an extruder 10 receives a granular film-forming material in the hopper 12 and the motor-driven extruder screw extrudes the molten plastic through a rotating die member 16 provided with a number of adjustable die bolts 17 which extend radially from the centre of the die 16 and serve to vary the spacing of peripheral die sections from the core of the die. Air from a supply 18 is blown through the centre of the die 16 and inflates the extruded plastics tube to form a tubular film or bubble 20 which is flattened by the guide rolls 22 and pinch rolls 25 to form a flat sheet 24 of double thickness. In order to indicate variations in the thickness of the sheet a radiation gauge 26 comprising a radiation source 28 and a detector 30 is provided. The gauge operates on the principle of radiation absorption in which the detected radiation is a function of absorber thickness. As shown in Fig. 2, the gauge 26 comprises a U-bracket 32 having a lower and an upper arm for housing the source and detector elements 28 and 30 respectively. The bracket is mounted in a carriage 34 movable along a rail 36 by means of a motor 38. The gauge 26 measures the thickness of an area 40 which may be circular, elliptical or rectangular depending on the source geometry employed. To enable the gauge 26 to see a strip always located on the sheet of film, a sheet edge sensing and control device 42, 44 is provided, control unit 44 actuating the motor 38 continuously to position the carriage 24 to centre the sheet edge in the air gap of the sensing unit 42. As shown in Fig. 1, the thickness responsive output of the radiation detector 30 is received by a measuring servo 46 which drives a chart recorder 48 which records the profile information as a trace 50a in a moving chart 52. A profile-averaging computer 54 serves to integrate the thickness measurements of the radiation gauge 26. The computer may be read out as a trace 50b following the profile trace 50a. A timer-unit 56 determines the period of signal integration performed by the computer 54 which may be set for a duration equivalent to the time required for one revolution of the die 16. A control unit 58 provides the switching functions needed to display the derived process information. As shown in Fig. 7, a circular chart recorder 116 may be used in preference to the strip recorder 48 and provides a circular record 118 with the die sections numbered thereon. The operator can proceed manually to adjust the die bolts of each section as indicated by the actuator elements 144-174 (Fig. 6). The die bolts alter the spacing 178 between the core 180 of the die and the surrounding die block 182. By measuring the thickness of the flattened tubular film a true profile is obtained and by averaging the profile over one die rotation a significant figure for the average thickness of the single sheet of film is obtained.