Patent GB-964563-A (GB)
Title: Production of synthetic organic thermoplastic polymeric film
IPC: B29C48/10
Filed: 1962-07-17
Abstract: 964,563. Producing tubular thermoplastic film. E. I. DU PONT DE NEMOURS & CO. July 17, 1962 [July 17, 1961], No. 27399/62. Heading B5B. As shown in the drawing, polymeric thermoplastic material is extruded from an annular die 11 in the form of continuous tubing 12 which is advanced over hollow cooling mandrel 13, having a smooth surface and a circular cross-section, by roller advancing means positioned downstream from the mandrel. The mandrel is adjustably suspended a predetermined distance from the face of the die 11 by a hollow support post 14 and is composed of a section 13a tapering towards the die and a cylindrical section 13b. Cooling fluid is passed through a coil 15 embedded in the wall of the mandrel by means of inlet tube 16 and outlet tube 17. Both the die and the mandrel are preferably rotated in the same direction at the same angular velocity to increase the uniformity of the wall thickness of the tubular film. Air or other gas is introduced through the feed line 18 at a pressure sufficient to expand the freshly extruded tubing to a diameter that is slightly larger (e.g. up to about 20 mils. larger) than the diameter of the section 13b of the mandrel. As the tubing is advanced over the mandrel excess gas is vented through the holes 19 located in a circumferential groove 20 in the wall of the mandrel and communicating with its interior. From the interior of the mandrel the gas passes through the post 14 to the atmosphere or other zone maintained at a slightly lower pressure than that within the portion of the tubular film intermediate between the die face and the mandrel. A temperature sensing signal-generating element 21, in this embodiment shown as a thermocouple junction in contact with the exterior of tubular film 12, is positioned to sense the temperature of the film at a point in its advance over the mandrel within the region or belt bounded on the upstream side by the junction of the sections 13a and 13b of the mandrel and on the downstream side by the circumferential line of the tubular film downstream from which the film is in thermal equilibrium with the surface of the mandrel, i.e. downstream from which the temperature of the tubular film is no longer decreasing due to the cooling action of the mandrel. Voltage signals from the element 21 are transmitted to a conventional electropneumatic transducer 22, which converts them to proportionate pneumatic signals which actuate a conventional temperature indicator 23 (which may be a recording indicator) and which simultaneously are fed to inverse derivative pneumatic conventional controller 24, the output signal from which is fed to a conventional proportionating and resetting controller 25. Pneumatic signals from the latter are applied to actuate diaphragm control valve 26 in gas supply line 18 in such a manner that an increase in voltage signal from temperature sensing element 21 will reduce the rate of gas flow through the line 18 and vice versa. Thus, an increase in temperature of the tubular film as sensed by the element 21 will bring about a proportionate decrease in the gaseous pressure within the portion of the tubular film at any moment between the die face and the cooling mandrel, said decrease in gaseous pressure permitting the tubular film to approach more closely the surface of the mandrel, thereby increasing the rate of heat transfer from the film to the mandrel and reducing the temperature of the film, which eliminates the signal that initiated the change described immediately above. Any decrease in the film temperature below the lower limit of a predetermined narrow range will be similarly automatically corrected, the gaseous pressure in this case being temporarily increased. The temperature sensing element 21 may be a non-contacting radiation sensing device, tuned for selective reception of and response to a particular wavelength of energy emanating from the tubular film. The temperature-sensing element may be positioned on the cooling mandrel itself. An electrically responsive control system may be used in place of the pneumatic system, the electropneumatic transducer being replaced by an electronic amplifying transmitter and the pneumatic temperature indicator by an electrically responsive temperature indicator. In the event of the automatic control equipment becoming inoperable, the control valve 26 may be temporarily manually operated in accordance with the readings of the temperature indicator. The invention minimizes the incidence of breaks in the tubular film resulting from over or under-expansion of the tubular film and is more sensitive than devices depending upon the sensing of the gaseous pressure within the tubular film. Specification 858,616 and U.S.A. Specification 2,987,765 are referred to.