Patent GB-1065518-A (GB)
Title: Method and apparatus for stretching a web
IPC: B29C48/92
Filed: 1965-05-19
Abstract: 1,065,518. Stretching films. E. I. DU FONT DE NEMOURS & CO., May 19, 1965 [May 20, 1964], No. 21255/65. Heading B5B. [Also in Division G3] In film or web stretching apparatus which includes stretching means and a plurality of heaters, there are provided (1) means for measuring the film tension as it is stretched and for signalling any difference between the value measured and a reference signal related to the desired film tension; (2) a film thickness detector arranged to scan repeatedly across the film in segments corresponding to portions of the film width being affected by each heater; (3) control means responsive to a signal from the thickness detector relative to a designated film segment whereby a correction signal is provided relative to the designated segment by comparison with a reference signal; (4) actuating means for the heaters whereby the outputs of all the heaters are changed uniformly in response to a signal from the tension measuring means and the output of the particular heater corresponding to the designated film segment whose thickness is being measured is changed in response to a signal from the said control means. The stretching means may be two pairs of rollers rotating at different speeds. A film 10 (Fig. 1) is cast on to wheel 24, heated by a plurality of parallel radiant heaters 13 and stretched by pull rolls 14, in front of which is placed means 15 for measuring the film tension. An X-ray gauge 16, located after the pull rolls, measures the thickness of the film in longitudinal zones corresponding to the area of film heated by any one of individual heaters 13. The thickness in a particular zone is measured for a length of time corresponding to one revolution of casting wheel 24, the X-ray gauge then transversely scanning the remaining zones. The primary feedback signal thus generated is sent to a controller 18, and may also be kept on recorder 17. The controller computes an actuating signal for each zone of the film by composing the primary feedback signal with a reference input signal derived from circuit 20, adding to the difference a feedback signal from memory circuit 19, and transmitting the resulting signal through a restriction 26 which serves to integrate with respect to time the algebraic sum of the signals. The memory feedback permits the controller to produce an output signal equal to the memory signal for that particular zone when there is no error of thickness represented in the primary feedback. The final integrated signal is transmitted to control element 21, which activates one or more of the heaters 13 if the output signal differs from the signal from the memory circuit in any particular zone. A second primary feedback derived from the tension-sensing means 15, is transmitted to controller 22, which compares it with a fixed set point which represents the predetermined stretching force desired. The amplified and integrated difference is transmitted to control element 21 and activates that part of element 21 that maintains total heater input without affecting distribution of heat in the various zones. The control system may utilize pneumatic or electronic devices. In the pneumatic system (Fig. 2, not shown) the primary feedback signal is produced by an electric to pneumatic transducer (34) and is transmitted to a proportional plus reset pneumatic controller (18). The reference input signal is derived from a circuit of flow valves (35) and volume chambers (36, 37) while the memory circuit is an arrangement of volume chambers (44) connected to a common manifold (45) through a series of micro-valves (46) each of which is actuated by e.g. a rotating cam assembly. The final integrated signals are transmitted to pneumatic actuators (47) wherein the movement of a piston determines the setting of an autotransformer corresponding to each heater (13). The total film tension is measured by an idler roll having hydraulic/pneumatic transmitting load cells (49) at each end. In the electronic system (Fig. 3. not shown) the primary feedback signal derived from the X-ray gauge is received by a summer (63) where by comparison with a reference voltage (from circuit 71) an error voltage is developed. This voltage is amplified and integrated in a circuit (65) which also contains the memory means, consisting of a capacitor for each heater (13). The difference between the integrated signal and a signal voltage that represents the reference setting of an autotransformer for the particular zone being scanned is transmitted to a chopper amplifier (68), the output of which operates motors (69) which are mechanically linked to auto-transformers (48). The second primary feedback, from tension measuring device (49), is compared in summer (76) with a reference signal obtained from manually set constant signal means (82). Any error voltage is amplified at (77) and compared on summer (78) with a feedback voltage representing the setting of rheostat (54). The difference voltage is transmitted to chopper amplifier (79) which operates motor (81) mechanically linked to potentiometer (80) and rheostat (54).