Patent GB-1290160-A (GB)
IPC: B29C48/92
Filed: 1969-11-12
Abstract: 1290160 Improving uniformity of shrinkage of polyester film E I DU FONT DE NEMOURS & CO 12 Nov 1969 [13 Nov 1968] 55317/69 Heading B5B Biaxially-oriented, heat-set polyethylene terepthalate film is treated to improve the uniformity of its latent shrinkage characteristics by passing it through a radiant-heating zone wherein longitudinal edge portions or lanes of the film having a higher latent shrinkage than the intermediate portion(s) or lanes of the film are heated to a temperature higher than that of said intermediate portion(s) while the film is tensioned between restrained and minimum relaxation, the minimum temperature in said zone being a temperature sufficient to effect relaxation on the portion(s) or lane(s) exposed thereto and the maximum temperature being below the melt temperature of the film. The term "restraint" is defined as meaning that the peripheral speed of a pair of nip rollers by means of which the film is withdrawn from the heating zone is not higher than that of a pair of nip rollers by means of which the film is advanced into the heating zone. The film is thus prevented from sagging between the first and second pair of nip rollers but is not permitted to be drawn between the first and second pair of rollers. The expression "minimum relaxation" is defined as meaning that the second pair of nip rollers are driven at a peripheral speed sufficiently below that of the first pair of nip rollers to permit a relaxation of the film between the first and second pair of nip rollers equal to the smallest shrinkage value across the cross section of the film. As shown in Fig. 1 of the drawings, a film 10 to be treated is advanced between a pair of rollers 11 and 12. The latter can be cooled by cooling means 13 to quench the film if it has come from a heat-setting operation. The film then passes beneath three banks of radiant-heating devices 14 to 16 powered from a source 18. A reflector 17 is positioned below the film. As the film advances through the radiant-heating zone it passes beneath a row of optical temperaturesensing probes 19 individually coupled through multiconductor cable 20 to a panel indicating the temperatures of respective heated lanes of the film. The radiant-heating devices in banks 14 to 16 are positioned so as to supply a uniform gradient of heat across the width of the film sheet 10. Power is supplied to the individual heating units in such a manner that the film temperature profile is symmetrical about the centre of the film. As shown in Fig. 2, the centre heaters 34 to 36 are interconnected to the same power source 41, heaters 33 and 37 to power source 42 which maintains them at a somewhat lower temperature than that of heaters 34 to 36. Pairs of heaters 32, 38 and 31, 39 are connected to power sources 43 and 44, respectively, with the latter being held at the lowest temperature. The output of power sources 41 to 44 is controlled by auto transformers and powered by line power 45. The heaters of banks 15 and 16 are wired and controlled in a similar manner. The film 10 ultimately passes through the nip of rollers 50 and 51. The relative peripheral speeds of rollers 11, 12 and 50, 51 are such that the film is tensioned between restraint and minimum relaxation.