GB2286555A

GB2286555A - Cooling a tubular thermoplastic web extruded from a film blowing head

Info

Publication number: GB2286555A
Application number: GB9425597A
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: 1994-12-19
Publication date: 1995-08-23
Also published as: DE4405464C1; ITBZ950007A0; HUT71306A; HU9500256D0; GB9425597D0; JPH07251449A; DK18695A; CZ302894A3; CA2142179A1; IT1282297B1; FR2716404A1; ITBZ950007A1

Abstract

In a process for cooling the parison of thermoplastic melt extruded from a film blowing head, cooling air is blown out in the direction of the parison from at least one cooling ring surrounding the annular die gap of the blowing head. To be able to increase the output of blown-film installations by using colder cooling air, the parison is blown with a primary cooling-air stream at a temperature below 0 degrees C and this cooling-air stream is enveloped by a secondary air stream of dry air at a temperature of above 0 degrees C.

Description

2286555 Wethod and gpparatus for cooling the tubular web of a

thermoplastic synthetic melt 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 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 or the tubular web.

The thermal output of film blowing installations can be increased by increasing the mass flow rate of the cooling air blown out, and it can also be increased by lowering the temperature of the cooling air.

It is the object of the invention to create a method of the kind indicated above, which makes it possible to increase the thermal output of film blowing installations by the use of colder cooling air.

Accordingly, one aspect of the present invention provides a method for cooling the tubular web of a thermoplastic synthetic melt extruded from a film blowing head, wherein cooling air is blown in the direction towards 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 or the tubular web, and wherein the tubular web is exposed to a primary cooling air flow at a temperature of less than O'C, and this cooling air flow is surrounded by a secondary air flow of dry air at a temperature of at least O'C.

If, for the purpose of increasing the output performance of film blowing installations, the temperature of the cooling air is lowered, droplets, snow or ice crystals may be precipitated 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 primary cooling air flow is very cold and thus has temperatures of minus MC and less. To avoid the risk of undesired condensation of water from the ambient air when very cold cooling air is used, the cold primary cooling air flow is, in accordance with the invention, surrounded by a secondary air flow of dry air with a temperature of over O'C. This secondary air flow surrounding the cooling air prevents the cold cooling air flow from coming into contact prematurely with the ambient air at a time when it still has such a low temperature that water is condensed out of the ambient air. This secondary air flow at first screens the cooling air flow from the ainbient air, so that the cooling air flow cannot condense any moisture out of the secondary air flow, since the latter consists of dry air and since, in the boundary zone between the secondary air flow and the ambient air, no moisture is condensed either, because the dry secondary air flow can absorb a certain quantity of moisture from the ambient air, and a mixing of the cooling air flow and of the secondary air flow surrounding it only occurs at a time when the cooling air flow has been warmed to such an extent that it cannot condense any moisture from the ambient air to a detrimental extent. Because of the dry secondary air flow surrounding the cooling air flow, in accordance with the invention, the temperature of the cooling air flow may be lowered to very low temperatures to increase the thermal output. Preferably the temperature of the primary cooling air flow lies in the range of minus 10 to minus 25C The temperature of the dry secondary air flow should be chosen such that, on the one hand, the cooling air flow is efficiently protected from the ambient air and, on the other hand, the formation of any condensate is avoided between the secondary air flow and the ambient air. Expediently, the temperature of the secondary air flow lies in the range of S to 15 C and preferably at approximately lTC.

In a further development of the invention, provision is made for the secondary air flow to be separated off from the primary air flow and to be warmed to the higher temperature of the secondary air flow. Admittedly, because of this separation of air from the primary air flow a proportion of the cooling air is lost. On the other hand, because of the waiming of the air separated from the cooling air flow a dry secondary air flow is obtained, which meets the requirements to be placed on the secondary air flow.

A second aspect of the present invention provides an inflation extrusion head for use in a bubble extrusion process, including a lower cooling ring with an annular outlet gap for cold primary cooling air flow, a cooling ring disposed on top thereof with a separate annular outlet gap for a warmer secondary air flow, and means for ensuring that the temperature of the secondary air flow is higher than that of the primary cooling air flow.

In this arrangement, both outlet gaps are arranged with respect to each other in such a way that the warmer secondary air flow concentrically surrounds the inner 5 cooling air flow.

Lines may be provided for separating off a proportion of the cold primary cooling air flow and for feeding the separated cooling air flow to the annular outlet gap for the warmer air flow, and at least one wall may delimit the outlet gap for the warmer air flow to be heated. Because of this heated wall, the separated cooling air flow is 10 heated to the desired temperature of the secondary air flow.

Preferably, the lines for separating a proportion of the cooling air flow consist of bores passing through the ring that separates the air flows from each other.

The wall delimiting the outlet gap for the warmer air may be formed by a ring provided with heating elements.

According to an inventive further development, provision is made for the film blowing head to be provided with a cylindrical tubular section surrounding the extruded tubular web, which section is provided with a heating element warming the web and with a perforated wall. This cylindrical wall that is heated and provided with passages forms, as it were, a heating grill through which ambient air is sucked in from the cooling air flow; in this heating process the ambient air is dried to such an extent that no water is any longer condensed out of it.

Expediently, the cylindrical perforated tubular section consists of a material with good heat conducting properties, for example copper.

The annular element warming the secondary air flow may be subdivided into ring sectors whose temperature can be controlled individually. In this way, it is possible to influence the temperature profile surrounding the extruded tubular film web.

The annular outlet gap for the cold primary cooling air flow may be subdivided into individual outlet segments whose outlet cross-sections can be controlled individually. By a suitable control of the outlet crosssections, it is also possible to produce a desired temperature profile over the circumference of the extruded tubular film web.

Exemplary embodiments of the invention will be explained in greater detail below, with reference to the drawings in which: - Fig. 1 is a partial longitudinal section through a first embodiment of a film blowing head; Fig. 2 is a partial longitudinal section through a second embodiment of a film 5 blowing head; and Fig. 3 is a partial longitudinal section through a third embodiment of a filin blowing head.

Figure 1 shows a filin blowing head 1 which is of a conventional design and is provided with an annular nozzle gap 2 from which there emerges the tubular web 3 of a synthetic melt which is inflated into a film bubble from which the laid flat tubular filin web is then drawn off by conventional nip rollers.

A first cooling ring 6, preferably made of a material with poor heatconducting properties, is mounted on the outer ring 4 of the film blowing head 1 above an insulating gap 5. This ring 6 delimits, together with a second cooling ring 7, an annular outlet gap 8 for the well cooled primary cooling air flow, which gap points substantially in the direction along the extruded tubular flhn web 3. An annular chaniber 9 is disposed ahead of the outlet gap 8, and has the dry cold cooling air fed to it via a vertical connecting pipe 10.

The cooling ring 7, which is threadably mounted on a median holding and separating plate 11 of the outer cooling unit 12, forms together with an upper cooling ring 13 an outlet gap 14 which also points along the direction of the extruded tubular film web 3 and carries the warmer secondary dry air. The annular outlet gap 14 has a larger diameter than the annular outlet gap 8 for the primary cooling air, so that when viewed in a top plan view the annular outlet gap 14 would concentrically surround the annular inner outlet gap 8. The warmer secondary air is fed to the outlet gap 14 via an annular chamber 16 disposed ahead of it. The annular chamber is provided with a horizontal intake pipe 17 for the warmer dry secondary air.

In the embodiment shown in Figure 2, the film blowing head 1 corresponds to the design shown in Figure 1. A first outer cooling ring 20, made of a material with poor heat conducting properties, is secured on the outer ring 4 of the film blowing head 1 with an insulating gap 21. The cooling ring 20 delimits, together with a second cooling ring 22 arranged above it, an annular outlet gap 23 for the well cooled cooling air, which gap 23 points along the direction of the extruded tubular film web- 3. On the cooling ring 22 there is secured a third ring 24 which is provided with heating elements 25 and which delimits, together with the median cooling ring 22, an outlet gap 26 pointing along the direction of the extruded tubular film web 3. The annular chamber disposed ahead of the outlet gap 26 is connected by bores 27, passing vertically through the cooling ring 22 to an annular chamber 28, by means of which the well cooled primary cooling air is fed to the inner outlet gap 23. The outlet gaps 23 and 26 are disposed concentrically with respect to each other, in the same way as the 10 outlet gaps 8 and 14 in Figure 1.

The mass of air separated off by the bores 27 from the cold primary cooling air flow is warmed by the heated ring 24, so that through the annular gap 26 there emerges a warmer air flow surrounding the cooling air flow emerging from the annular gap 23.

The embodiment shown in Figure 3 is distinguished in essence from the embodiment of Figure 2 in that the upper ring 24 warming the separated cooling air flow carries a cylindrical wall 30, made of a material with good heat conducting properties, which is provided on its lateral surface with bores 31 in the manner of a sieve. Provision is moreover made for a ring 32 provided with a heating element 20 which heats both the cylindrical section 30 and the ring 24.

Ambient air is sucked in through the bores 31 by virtue of the fluid entrainment pump (ejector pump) principle, because of the concentric air flows, emerging from the annular gaps 23 and 26, warming and drying the ambient air so that the air flows swirl together above the cylindrical section in one zone and at temperatures preventing 25 formation of condensates.

Claims

0 1. A method for cooling the tubular web of a thermoplastic synthetic melt extruded from a film blowing head, wherein cooling air is blown in the direction towards 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 or the tubular web, and wherein the tubular web is exposed to a primary cooling air flow at a temperature of less than WC, and this cooling air flow is surrounded by a secondary air flow of dry 10 air at a temperature of at least WC.
2. A method according to claim 1, wherein the temperature of the primary cooling air flow lies in the range of minus 100 to minus 250C.
3. A method according to claim 1 or 2, wherein the temperature of the secondary air flow lies in the range of 5 to WC
4. A method according to claim 3, wherein the temperature of the secondary air is approximately WC.
5. A method according to one of claims 1 to 4, wherein the secondary air flow is separated off from the primary cooling air flow and is warmed to the higher temperature of the secondary air flow.

7
6. An inflation extrusion head for use in a bubble extrusion process, including a lower cooling ring with an annular outlet gap for cold primary cooling air flow, a cooling ring disposed on top thereof with a separate annular outlet gap for a wanner secondary air flow, and means for ensuring that the temperature of the secondary air flow is higher than that of the primary cooling air flow.
7. An apparatus according to claim 6, including lines for separating off a proportion of the cold primary cooling air flow and for feeding the separated cooling air flow to the annular outlet gap for the wanner air flow, and wherein at least one wall delimiting the outlet gap for the wanner air flow is heated.
8. An apparatus according to claim 7, wherein the lines consist of bores passing through the ring and separating the air flows from each other.
9. An apparatus according to any one of claims 6 to 8, wherein the upper wall delimiting the outlet gap for wanner air is formed by a ring provided with heating elements.
10. An apparatus according to any one of claims 6 to 9, wherein the film blowing inflation extrusion head is provided with a cylindrical section surrounding the extruded tubular web, which section is provided with a heating element warming it and with a perforated wall.
An apparatus according to claim 10, wherein the cylindrical section consists -8of a material with good heat conducting properties.
12. An apparatus according to clahn 11, wherein said material is copper.
13. An apparatus according to any one of claims 6 to 12, wherein said cooling ring for the warm secondary air flow is subdivided into ring sectors whose temperatures can be controlled individually.
14. An apparatus according to any one of claims 6 to 13, wherein the annular 10 outlet gap for the cold primary air flow is subdivided into individual outlet segments whose outlet cross-sections can be controlled individually.

is. A method of cooling the tubular web of a thermoplastic synthetic melt, substantially as hereinbefore described with reference to the accompanying drawings.

is 16. An inflation extrusion head for use in a bubble extrusion process, constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, Figure 1, or Figure 2, or Figure 3, the accompanying drawings.