GB2217252A

GB2217252A - Adjusting separate extension channels, to maintain a constant total flow rate from a single extruder

Info

Publication number: GB2217252A
Application number: GB8907563A
Authority: GB
Inventors: Edward Thompson Lloyd; Barry Ronald Edgeler
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1988-04-11
Filing date: 1989-04-04
Publication date: 1989-10-25
Anticipated expiration: 2009-04-04
Also published as: GB8808403D0; GB2217252B; GB8907563D0

Abstract

In a process for extruding two layers of the same material simultaneously from a single extruder (2), the material is fed to the extruder head through respective flow channels (3, 4) incorporating plate valves (11, 12) for enabling the flow rates of the material in the channels to be adjusted in opposite senses to each other, in a manner such as to maintain the total flow rate of (11, 12) are disposed in the respective channels at 90 DEG to each other, mounted for simultaneous adjustment to a common manually actuated shaft (10), which is provided at its actuating end with an indicator (not shown), and is positively fixed by a pin (27). <IMAGE>

Description

Extrusion Process and Apparatus This invention relates to extrusion processes more especially, though not exclusively, in the manufacture of electric cables, and to apparatus for carrying out such processes.

In some designs of electric power cables there is a requirement for a thin layer of semi-conducting plastics material to be applied around the conductor followed by a thicker layer of insulating plastics material and finally a thin outer layer of semi-conducting plastics material over the insulation. These layers can be applied simultaneously in a 'three layer' extrusion head for example as described in Patent Specification No. 967567.

In this extrusion head the insulating layer is applied from one extruder and both semi-conducting layers from a second extruder.

The flow of material from this second extruder is divided, within the extrusion head, into two streams, to produce the inner and outer semi-conducting layers. The final thicknesses of the two semi-conducting layers are governed by the proportions of the original flow which extrude through the respective annular gaps formed between the tips and dies.

It will be appreciated that the thicknesses of these layers can only be adjusted by time consuming alterations to the tips and dies and/or by modifying the impedence of the flow channels within the head following repeated trial extrusion runs.

It is an object of the present invention to provide an adjustable flow proportioner within the head whereby the relative thicknesses of the two semi-conducting layers can be varied as part of the setting-up operation before a production extrusion run.

However the Invention could have other applications where it is desired to control the relative thicknesses of simultaneously extruded layers.

According to the invention in a process for extruding two layers of material simultaneously from a single extruder, the material which is to form the layers is fed to the extruder head through respective flow channels incorporating means for enabling the flow rates of the material in the channels to be adjusted in opposite senses to each other, in a manner such as to maintain the total flow rate of material to the extruder head approximately constant.

This is conveniently achieved by means of rotating plate valves in the two passages disposed at 900 to each other, and simultaneously adjustable to cause the cross-section of the flow path through one valve to decrease and the other to increase. By this means the proportioning of material between the two extruded layers can be varied without significantly altering the total cross-sectional area through which the material is fed. This is important because a change in the total cross-sectional areas of the flow passages would produce a change in impedance to flow which, in turn, would cause the extruder output to change. This is undesirable as it would complicate the setting-up operation and could in some cases cause overloading of the extruder drive.

Conveniently the two flow channels are disposed side-by-side and the plate valves mounted on a common shaft.

Thus a preferred form of apparatus for carrying out the invention comprises a pair of flow channels for feeding material separately to an extruder head for forming two separate extrudate layers, which flow channels are disposed side-by-side, and a pair of plate valves located one in each of the two flow channels and mounted on a common shaft at 900 to each other, so that, on rotation of the shaft, the cross-sectional area available for the passage of material past the valve in one flow channel increases whilst that available for the passage of material past the valve in the other flow channel decreases.

The material is conveniently fed to the flow channels by a common extruder.

One process for applying layers of semi-conducting plastics material to a cable in accordance with the invention, and an apparatus for carrying out the process, will now be described by way of example, with reference to Figures 1 to 3 of the accompanying drawing, in which Figure 1 represents a longitudinal section through the apparatus, Figure 2 represents a transverse section through part of the apparatus across the line A-A of Figure 1, and Figure 3 represents a section through the formed cable.

Referring to the drawings, the apparatus incorporates a flow proportioner 1 which is designed to be fitted between the barrel end 2 of an extruder, which is arranged to feed molten semi-conducting plastics material for forming inner and outer semi-conducting layers to a power cable, and the respective inlet ports 3, 4 of a three layer extruder head shown in part at 5, and which may, for example be constructed as described in Patent Specification No. 967567.

The proportioner 1 comprises a casing 6 having an entry passage 7, into which the molten plastics material is fed from the extruder, the passage then dividing into two separate flow channels 8, 9 lying side-by-side and parallel to each other within the casing. A valve shaft 10 extends transversely through both flow channels 8, 9 with its centre line passing at right angles through the centre lines of the two flow channels. A plate valve 11, 12 respectively is fixed rigidly to the valve shaft 10 within each of the two flow channels 8, 9, the plate valves being set on the shaft at 900 to each other, as shown more clearly in Figure 2, which illustrates a section through the shaft along the line A-A of Figure 1.With the vanes of the plate valve set in this way on the shaft 10, when the shaft is rotated, one of the flow channels will be progressively closed, and conversely the other flow channel will be progressively opened, thus changing the proportion of the total flow of molten semi-conducting plastics material directed through the flow channels to the respective extrudate layers 21, 22 (Figure 3).

The first 21 of these layers is extruded around the inner conductor 25, a layer 26 of insulation Is extruded around the layer 21, and the second 22 of the semi-conducting layers is extruded around the insulating layer 26.

The shaft 10 of the proportioner 1 is held within the casing 6 by means of a stud 13 which is screwed coaxially into one end of the shaft and urges a washer assembly 14 against the adjacent part of the outer surface of the casing, belleville washers 15 being interposed between the head of the stud and the washer assembly 14 as shown. This causes a collar 16 adjacent the opposite end of the shaft 10 to press a further washer 17 against the opposite side of the outer surface of the casing. O-ring seals 18 around the shaft 10 where it passes through the wall of the casing 6 prevent leakage of the molten plastics material, a heater 19 which surrounds the casing ensuring that the material is maintained in a suitably molten condition during its passage through the proportioner 1.

The protruding end 24 of the shaft 10 is hexagonal or square in section to facilitate the fitting of a spanner or similar device for adjustment of the position of the shaft 10, and hence of the plate valves 11, 12. The protruding end 24 of the shaft conveniently carries a suitable indication to show an operator the position in which the plate valves are set. A split, 'C' type of clamping ring 23 fitted around the end of the shaft 10 adjacent to the collar 16 and restrained from rotational movement by a pin or pins 27 extending from the body of the proportioner 1 fixes the shaft 10 in the required angular position when screw 28 is tightened.

The proportioner may be designed for use with materials which have a chemical catalyst added to them during extrusion to bring about a molecular cross-linking action within the material after extrusion. If small amounts of material were to stagnate in any of the channels with the flow proportioner, cross-linking of this material would take place whilst still within the channel. -If this now cross-linked material was subsequently dislodged it would be carried through the extrusion head by the melt flow and would appear in the extrudate as a gelled mass. This would cause a fault in the cable. The shape of the channels through the proportioner and the shape of the valve vanes which operate in these channels are designed to produce a flow of material across all surfaces at all positions of adjustment so that the possibility of stagnation is minimised.

This is achieved by making the channels across which the valve vanes are placed square or rectangular in section and by machining the valve vanes into the valve shaft which has a diameter equal to the depth of the channels across which it operates as indicated in Figure 2. The width of the valve vane is made equal to the width of the channel across which It operates. If this shaft is then placed with Its axis of rotation passing through and at right angles to the axes of the channels so that the vanes operate within the channels, there are no projections onto the passages other than the vanes and no undercuts in which material could lodge.

Although the invention has, as its main object the simultaneous extrusion of semi-conducting layers on to electric power cables, it will be appreciated that it could also be used to advantage in other applications where it is desired to vary the relative thicknesses of two extruded layers.

Claims

1. A process for extruding two layers of material simultaneously from a single extruder wherein the material which is to form the layers is fed to the extruder head through respective flow channels incorporating means for enabling the flow rates of the material In the channels to be adjusted in opposite senses to each other, in a manner such as to maintain the total flow rate of material to the extruder head approximately constant.

2. A process according to Claim 1 wherein adjustment of the flow rates of the material in the two channels is achieved by simultaneously adjusting rotating plate valves disposed in the two passages at 900 to each other so as to cause the cross-section of the flow path through one valve to decrease and that through the other valve to increase.

3. A process according to Claim 2 wherein adjustment of the plate valves is effected by rotation of a common shaft on which the plate valves are mounted.

4. A process according to Claim 1, 2 or 3 wherein the material is fed to the flow channels by a common extruder.

5. A method of applying inner and outer layers of semi-conducting plastics material to an electric power cable, wherein the material for forming the layers is extruded simultaneously by a process according to any preceding claim.

6. Apparatus for carrying out a process according to any one of Claims 1 to 4 comprising a pair of flow channels for feeding material separately to an extruder head for forming two separate extrudate layers, which flow channels are disposed side-by-side, and a pair of plate valves located one in each of the two flow channels and mounted on a common shaft at 90 to each other, so that, on rotation of the shaft, the cross-sectional area available for the passage of material past the valve in one flow channel increases whilst that available for the passage of material past the valve in the other flow channel decreases.

7. Apparatus according to Claim 6 wherein the shapes of the flow channels and of the vanes of the plate valves are such as to produce a flow of material across all surfaces, during operation thereof, at all positions of adjustment of the plate valves.

8. Apparatus according to Claim 7 wherein the cross-sections of the parts of the flow channels in which the plate valves are located are square or rectangular, and the valve vanes are formed in a common shaft having a diameter equal to the depth of said parts of the flow channels, the width of the vanes being equal to the widths of the respective parts of the flow channels in which they are located.

9. A process for extruding two layers of material simultaneously from a single extruder carried out substantially as hereinbefore described with refence to Figures 1 to 3 of the accompanying drawing.

10. Apparatus for extruding two layers of material simultaneously substantially as shown in and as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawing.

11. An electric power cable having inner and outer layers of semi conducting material which have been extruded by a process according to any one of Claims 1 to 4 or 9.