GB2286557A

GB2286557A - Method and apparatus for cooling the tubular web of a thermoplastic synthetic melt extruded from a film blowing head

Info

Publication number: GB2286557A
Application number: GB9425709A
Authority: GB
Inventors: Klemens Sensen; Gerd Kasselmann; Guenter Schmitt
Original assignee: Windmoeller and Hoelscher KG
Current assignee: Windmoeller and Hoelscher KG
Priority date: 1994-02-21
Filing date: 1994-12-20
Publication date: 1995-08-23
Also published as: HU9500255D0; GB9425709D0; CZ302994A3; DE4405463A1; ITBZ950006A1; JPH07251448A; IT1282296B1; FR2716403A1; CA2142405A1; ITBZ950006A0

Description

2286557 METHOD AND APPARATUS FOR COOLING THE TUBULAR WEB OF A

THERMOPLASTIC SYNTHETIC MIELT EXTRUDED FROM A FILM BLOWING HEAD The invention concerns a method for cooling the tubular web extruded from a film blowing head of a thermoplastic synthetic melt, wherein cooling air is blown in the direction towards, and/or parallel to the tubular web, out of at least one cooling ring with an annular outlet gap surrounding the annular nozzle gap of the blowing head andlor the tubular web. The invention also relates to an apparatus for operating this method.

The output of filin inflation-extrusion installations can be increased, not only by increasing the cooling air mass blown out per unit time, but also by lowering the temperature of the cooling air.

It is the object of the invention to create a method of the kind indicated in the opening paragraph above which makes it possible to increase the output of film inflationextrusion installations by the use of colder cooling air.

This problem is solved by one aspect of the present invention providing a method for cooling the tubular web extruded from a film blowing head of a thermoplastic synthetic melt, wherein cooling air is blown in the direction towards, andlor parallel to, the tubular web out of at least one cooling ring with an annular outlet gap surrounding the annular nozzle gap of the blowing head and/or the tubular web; wherein the tubular web is exposed to a dry cooling air flow at a temperature of less than O'C and is screened from the ambient air in the zone exposed to the flow; and wherein the cooling air is drawn off from the screened space, is cooled, and is fed again to the cooling ring.

If, for the purpose of increasing the thermal performance of a film inflation-extrusion installation, the temperature of the cooling air is lowered, the ensuing tendency of the water vapour of the ambient air to condense may result in the precipitation of droplets, snow or ice crystals in the zone where the cooling air comes into contact with the ambient air or is swirled. This precipitation may also be deposited on the extruded tubular web or on the inflated film bubble and may there cause damage.

The risk of condensation of water from the ambient air is particularly high when the cooling air flow blown out of the cooling ring is very cold and thus has temperatures of - WC and less. To avoid the risk of unwanted condensation of water from the ambient air when very cold air is used, the dry cooling air flow cooled below 0' C is, in accordance with the invention, screened from the ambient air, so that the cold cooling air flow is carried in a closed circuit and cannot come into contact with the ambient air containing moisture. Since, with to the method in accordance with the invention, the cold cooling air is carried in a closed circuit and therefore cannot come into contact with the ambient air, it is possible to lower the temperature of the cooling air flow to very low temperatures to increase the output of the flhn inflation-extrusion installation. Preferably the temperature of the cooling air flow lies in the range of -10 to -250C.

A second aspect of the present invention provides inflation extrusion apparatus for operating the bubble extrusion method, comprising a film blowing head having at least one outer cooling ring; a cooling ring surrounded by a casing delimiting a substantially closed chamber whose upper cover plate is provided with a circular opening for the passing of the tubular film web; and in a line for drawing off the cooling air from the chamber.

The extruded tubular film web to be cooled by an external cooling means thus passes through a substantially closed chamber where it is exposed to the flow of the very cold cooling air while being screened from the ambient air. The desired rate of flow of the very cold cooling air flow surrounding the extruded tubular film web is maintained in that the cooling air is drawn off out of the chamber at the appropriate time rate.

To allow the cooling air to be carried in a closed circuit, there are lines for returning 1.

71 the drawn-off cooling air to the cooling ring. In such an arrangement, a fan and a cooler are arranged in the lines for recooling the cooling air. To the extent that the cooling airflow has been reduced because of unavoidable leakage losses, an appropriate mass of fresh air is resupplied by the fan.

Expediently, a filter and/or separator are arranged in the suction line, ahead of the fan, to eliminate entrained particles or gases from the cooling air, so that unwanted substances or gases cannot accumulate in the cooling air.

In a preferred embodiment, the cooling ring may be surrounded in the chamber with an interspacing by a cylindrical pipe which terminates below the cover plate of the casing, and the suction line may be connected to the annular space between the cylindrical pipe and the casing surrounding it. In this way, it is possible to ensure that the cooling air surrounds the extruded film blowing head with a substantially laminar flow over the height of the cylindrical pipe, and that the cooling air flow is then drawn off via the annular space, while the desired rate of flow is maintained.

In a further preferred embodiment, the opening in the cover wall of the casing may be formed by an iris-diaphragm. In this way, the diameter of the opening can be adjusted quickly and in a simple manner to suit the diameter of the tubular film web being extruded at the time.

By way of example one embodiment of the invention will now be explained in greater detail below, with reference to the accompanying drawing in which the sole Figure is a longitudinal section through the cooling device in accordance with the invention, showing the cooling device combined with a film blowing head.

Figure 1 shows a conventional film blowing head 1 which is fed by an extruder 2 with a molten thermoplastic synthetic material. The tubular film web 3 is extruded from the annular nozzle of the film blowing head 1. This tubular film web is exposed to the flow of very cold dry cooling air from at least one cooling ring surrounding the annular gap of the filin blowing head in such a way that this air surrounds the extruded tubular film web and, while being in intimate contact with the tubular flIm web, flows alongside it. To this extent, the film blowing head and the external cooling means are of a conventional design which does not need to be described here in greater detail.

A bottom plate 4, surrounding the outer cooling ring or rings, is secured on the film blowing head 1 and is joined at its edge zone to a cylindrical casing shell 5. A section 6 of a cylindrical pipe is secured on the bottom plate 4 concentrically with the annular nozzle gap and the outlet gap for the outer cooling air, as well as with the casing shell 5. This section terminates with an interspacing below a cover plate 7 which is joined to the casing shell 5 and delimits a substantially closed chamber. The cover plate 7 is provided with an opening for the tubular film web 3 and the edge of this opening carries a conventional iris- diaphragm 8 that is adjustable by adjustment drives, not shown. By means of the iris-diaphragm 8, the diameter of the opening in the cover plate 7 can on each occasion be adjusted to the diameter of the extruded tubular film 3.

Directly above the bottom plate 4, the casing 5 is provided with a connector for a suction pipe 9. The suction pipe 9 leads via an interposed filter or separator 10 into the suction side of a fan 11. A line 12, connected on the pressure side of the fan, leads into a distributor device 13 for the very cold cooling air, and from this distributor device cooling air lines 14 lead to an outer cooling ring, not shown. A cooler 16 arranged in the line 12 recools the cooling air to the desired low temperature.

The diameters of the casing 5 and of the cylindrical pipe section 6 are adapted to each other in such a way that the desired substantially laminar flow of the very cold dry cooling air is maintained in the section 6, and that it can be drawn off via the annular space 17 with the required speed by means of the fan 11.

Since in essence it is necessary to remove from the cooling air only the amount of heat which the cooling air has just drawn off from the extruded tubular film web 3, the 1 f.

1 cooler only has to provide a cooling output which is economically adapted to the amount of heat to be removed.

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Claims

CLAINIS

1. A method for cooling the tubular web extruded from a filin blowing head of a thermoplastic synthetic melt, wherein cooling air is blown in the direction towards, andlor parallel to, the tubular web out of at least one cooling ring with an annular outlet gap surrounding the annular nozzle gap of the blowing head and/or the tubular web; wherein the tubular web is exposed to a dry cooling air flow at a temperature of less than O'C and is screened from the ambient air in the zone exposed to the flow; and wherein the cooling air is drawn off from the screened space, is cooled, and is fed 10 again to the cooling ring.

2. A method according to claim 1, wherein the drawn off cooling air is filtered.

3. A method according to either of claims 1 and 2, wherein fresh air is fed to the 15 drawn off cooling air flow for the replacement of flow leakage losses.

4. A method according to one of claims 1 to 3, wherein the temperature of the cooling air flow lies in the range of -10 to -25' C.

5. Inflation extrusion apparatus for operating the bubble extrusion method, comprising a film blowing head having at least one outer cooling ring; a cooling ring surrounded by a casing delimiting a substantially closed chamber whose upper cover plate is provided with a circular opening for the passing of the tubular filin web; and in a line for drawing off the cooling air from the chamber.

6. An apparatus according to claim 5, including lines for returning the drawn-off cooling air to the cooling ring; and a fan and a cooler arranged in said lines for the recooling of the cooling air.

7. An apparatus according to claim 6, including a filter andlor separator in the suction line ahead of the fan.

8. An apparatus according to any one of claims 5 to 7, wherein the cooling ring is surrounded in the chamber, with an interspacing. by a cylindrical pipe which terminates below the cover plate of the casing; and wherein the suction line is connected to the annular space between the cylindrical pipe and the casing surrounding 5 it.

9. An apparatus according to any one of claims 5 to 8, wherein the opening in the cover plate of the casing is formed by an iris-diaphragm.

10. A method of cooling the tubular web extruded from a film, inflation extrusion head, substantially as hereinbefore described with reference to the accompanying drawing.

11. Inflation extrusion apparatus constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.