BLOWING HEAD, METHOD FOR PRODUCING A BLOWN FILM AND BLOWN FILM INSTALLATION

Abstract

A blowing head, a method for producing a blown film, and a blown film installation include the use of plate spiral distributors in blown film installations for distributing melt steams arriving from extruders and for combining the melt streams to form an annular gap stream in several layers in an annular gap leading to a tubular die. The melt streams in a plate spiral distributor are initially bundled and then forwarded in a bundled manner to a common co-extrusion flow. The distributer has a plurality of spirals in superimposed layers, and the spirals of two layers have a junction for pre-combining the melt streams of two layers, and in that starting from the junction, a common guide leading to the annular gap is provided for combining there the pre-combined melt streams with the annular gap stream.

Claims

1. A blowing head for a blown-film plant, with a plate spiral distributor for distributing melt streams arriving from extruders and for combining the melt streams to form an annular gap stream in a plurality of layers in an annular gap leading to a tubular die, the plate spiral distributor having a plurality of spirals in superimposed layers, wherein the spirals of two layers have a junction for pre-combining the melt streams of two layers, and in that starting from the junction, a common guide leading to the annular gap is provided for combining the pre-combined melt streams with the annular gap stream.

2. The blowing head according to claim 1, wherein the spirals of two adjacent layers have a junction.

3. The blowing head according to claim 1, wherein the spirals of three layers have a junction.

4. The blowing head according to claim 1, further including a plurality of junctions having successive inputs to the annular gap.

5. The blowing head according to claim 1, wherein a plate has one spiral on a top side and one spiral on a bottom side.

6. The blowing head according to claim 5, wherein a plurality of plates have spirals on top sides and on the bottom sides.

7. A method for manufacturing a blown film with a blowing head according to claim 1, including the steps of conducting melt streams from extruders through the spirals, and pre-combining two melt streams in a junction, whereupon the pre-combined melt streams are conducted into the annular gap only in the further flow path and are merged there with the arriving annular gap stream.

8. The method according to claim 7, further including the step of pre-combining three melt streams in a junction, and providing an intermediate layer with two covering layers in the junction and conducting into the annular gap only in the covered state.

9. The method according to claim 8, further including the step of producing two covering layers with the same plastic.

10. A blown-film plant with extruders, a blowing head, a cooling device, a calibration unit, a lay-flat device, a reversing unit, if desired, and a winding station, wherein the blowing head is used and in that the blown-film plant is adapted to perform the method according to claim 7.

Description

[0033] In the following, the invention will be explained by means of an example of embodiment with reference to the drawing wherein

[0034] FIG. 1 schematically shows, in a longitudinal section, one half of a plate spiral distributor according to the state of the art for manufacturing nine layers in a film, and

[0035] FIG. 2 schematically shows, in an analogous view, a longitudinal section through a blowing head according to the invention.

[0036] The blowing head (1) (only partially shown) in FIG. 1 consists, among others, of a plate spiral distributor (2) with an extrusion axis (3). Nine spirals (A, B, C, D, E, F, G, H, J) are successively arranged on mutually parallel levels in the extrusion direction (4), with one spiral each being formed between a plate package 5 (numbered by way of example) of two superimposed plates (6, 7).

[0037] The spirals extend radially inward up to the extrusion axis (3) into an annular gap (8) leading to a tubular die (9).

[0038] During operation of the blowing head (1), melt streams produced by extruders (not shown) are conducted into the nine spirals, where they flow on one level, in parallel and separately from one another, towards the annular gap (8). In this way, a nine-layered film is produced in the annular gap (8). Starting with an input from the bottom spiral (J) into the annular gap (8), a one-layered melt stream is created which rises along the extrusion direction (4) through the annular gap (8). When the input from spiral (H) is reached, the melt stream supplied by spiral (H) is combined with the melt stream arriving only from spiral (J). Therefore, as of the input by spiral (H), there is a two-layered melt stream in the annular gap (8).

[0039] The two-layered melt stream flows further through the annular gap (8) in the extrusion direction (4), and when the level of spiral (G) is reached, a third layer is added, which is the melt stream flowing through the spiral (G).

[0040] This can already lead to a critical volume flow ratio; for two melt streams in the form of cylinder jacket-shaped layer streams arrive from below, whereas the melt stream coming from spiral (G) creates only one layer. In a simple case of identical volume flows of the melt streams through the spirals (G, H) and (J), the melt stream from spiral (G) thus merges with an arriving main melt stream with a volume flow which is twice as high.

[0041] This may in fact be desirable if the blowing head is suitably designed for a specific application; if, however, the melt stream arriving from spiral (G) is to be reduced e.g. for modification of the film or for an entirely different film, and if for instance in addition the volume flow of one or both melt streams from the spirals (H, J) is to be increased, e.g. because the layer created by spiral (G) is to become thinner and the other two layers are to become thicker, the possibilities of variation of conventional plate spiral distributors will quickly be exceeded.

[0042] The blowing head (10) according to the invention can be used much more flexibly:

[0043] The modified plate spiral distributor (11) disclosed herein is also adapted for outputting a nine-layered plastic film from the tubular die (9).

[0044] However, in this case, three superimposed spirals (A′, B′ and C′; D′, E′ and F′; G′, H′ and J′) each, forming parallel layers, are pre-combined before they reach the annular gap (8).

[0045] Thus, the innermost three layers of the final film are created during operation of the blowing head (10) according to the invention by the melt streams of the bottom modified spirals (G′, H′ and J′).

[0046] After a common spiral course (12) (which cannot be displayed, only numbered by way of example) of the spirals (G′, H′, J′), they have a first junction (13) forming three layers. From there, a common guide (14) leads to the annular gap (8).

[0047] This means that when the blown-film plant is operated with the blowing head (10) according to the invention, a three-layered melt stream is fed into the annular gap (8) at the very bottom.

[0048] This three-layered melt stream rises upward until it reaches a feeding height (15) of a next, second feed (16).

[0049] There as well, pre-combined melt streams from the spirals (D′, E′, F′) are input after they have already been combined to form three layers at a second junction (17).

[0050] Where the second feed (16) meets the arriving main flow in the annular gap (8), that is, at the feeding height (15), there is therefore an encounter of two three-layered streams. Modification of the volume flows of individual layers therefore has a much broader range of possibilities.

[0051] The same applies in an analogous manner to the last three melt streams from the spirals (A′, B′ and C′) which are also pre-combined at a third junction (18) before reaching the arriving main flow in the annular gap (8) via a third feed (19).

[0052] In the embodiment proposed here, first three melt streams each are pre-combined, and then the pre-combined three-layered melt streams are sequentially built up to form a nine-layered melt stream and consequently a nine-layered extruded film.

[0053] It is explicitly pointed out, however, that simpler or more complex junctions also form part of the inventive concept of first combining individual melt streams in a plate spiral distributor and then merging them to form a common co-extrusion flow.

[0054] Also, a blowing head according to the invention can be reduced in size as compared to the conventional form of construction.

[0055] This also reduces the length of the channel extending in the extruding direction and consequently the dwell time.

REFERENCE NUMERALS

[0056] A′J spiral [0057] A′-J′ modified spiral [0058] 1 blowing head [0059] 2 plate spiral distributor [0060] 3 extrusion axis [0061] 4 extrusion direction [0062] 5 plate package [0063] 6, 7 plates [0064] 8 annular gap [0065] 9 tubular die [0066] 10 blowing head according to the invention [0067] 11 modified plate spiral distributor [0068] 12 conventional spiral course [0069] 13 first junction [0070] 14 guide [0071] 15 feeding height [0072] 16 second feed [0073] 17 second junction [0074] 18 third junction [0075] 19 third feed