GB2225744A

GB2225744A - Controlling extrudate flow through an extrusion coating head

Info

Publication number: GB2225744A
Application number: GB8927244A
Authority: GB
Inventors: Robert Galletly
Original assignee: CALEDONIAN CABLES Ltd
Current assignee: CALEDONIAN CABLES Ltd
Priority date: 1988-12-01
Filing date: 1989-12-01
Publication date: 1990-06-13
Also published as: GB8828077D0; GB8927244D0

Abstract

The head comprises a cross-head (16) provided with a die (26) for coating at least one elongate metallic element (20) within a matrix of extrudate. A guide member (32) for the or each element is adjustably displaceable in a wall of the extruder head (16) in order to provide adjustment of flow of extrudate through a gap (28) between the elongate element (20) and the die (26), according to the spacing (39) between the guide member (32) and the die (26), the displacement either being effected linearly by hydraulic jacks, or by manual or stepping motor rotation at a threaded engagement (40) between the guide (32) and head (16). The extruded article may comprise a cable, or a fencing member having reinforcements (20), the individual adjustment of each guide (32) also enabling the speed through the die of each reinforcement (20) to be separately adjusted by alteration of the extrudate drag flow. <IMAGE>

Description

EXTRUDING APPARATUS AND METHOD This invention relates to extruding apparatus and method, more particularly apparatus and method for producing a composite extruded product.

Composite extruded products often compromise one or more elongate metallic elements embedded in a matrix of plastic material. Examples of this type of product include insulated cables, in which the metallic element serves as a conductor, and structural members such as fencing panels, in which the element serves as a reinforcement.

Apparatus for producing composite extruded products generally comprises an extruder machine which feeds molten extrudate to an extruder head.

The extruder head carries a die which has a shaped opening therein. In use, the metallic element is passed at a steady rate through the opening in the die and is supported such that a gap is maintained between the element and the edge of the opening.

Molten plastic material is supplied by the extruder machine to the extruder head and passes through the gap, in contact with the element, whereby the element is embedded in the plastic material. The shape of the opening defines the cross-sectional shape of the finished product.

Methods for producing composite extruded products generally include a process wherein the apparatus described above is used. Although essentially simple, the method of extrusion is subject to many variables which, if not carefully controlled, can lead to poor results. For instance, the viscosity of the extrudate may vary markedly in response to small changes in temperature or minor variations in its composition. The problems are still greater in the production of composite products, in view of the need to control, and match, the feeding speed of the metallic element and the flow rate of the extrudate. This is particularly important where the extruded product includes two or more separate metallic elements.

Existing extruding apparatus and methods cannot readily compensate for changes in the variables which may affect the extrusion process.

This means that great effort must be made to ensure that conditions are kept as steady as possible during production. However, this ideal is rarely obtainable and quality suffers as a consequence.

Another disadvantage of existing extruding apparatus and methods is a lack of adaptability.

Specifically, existing apparatus and methods cannot be readily adapted to produce different products or to suit different extrudate compositions. Adaptation usually involves changing the die and other components which may involve considerable cost and loss of production time.

A further drawback of existing apparatus and methods is that the metallic element is often fed parallel to the axis of the extruding machine.

This is an awkward arrangement because the extruding machine is bulky and may obstruct the passage of the metallic element, which makes control of the production process still more difficult.

The principal object of this invention is to overcome the drawbacks of existing devices and processes with an improved extruding apparatus and method.

According to one aspect of the invention there is provided extruding apparatus, for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the apparatus comprising an extruder head having a die, means for receiving the elongate element for passage through the die with extrudate, and means for controlling flow of said extrudate.

Preferably, the control means comprises two spaced apart members, the spatial separation of which may be varied to vary the flow of extrudate, and one of the said members may be the die.

Advantageousl the receiving means includes a guide member for guiding the elongate element towards the die, and the guide member forms the other of the spaced apart members. The elongate element is thus guided accurately through the die by the guide member.

In the preferred arrangement the guide member is movable relative to the die, and may be threadingly engaged with the extruder head.

Therefore, by rotation of the guide member about its axis it may be moved relative to the die to control the flow of molten extrudate through the die. This is a particular advantage due to the precision of control and the simplicity of use, thereby improving the quality and consistency of the extruded product.

According to another aspect of the invention, there is provided an extruding apparatus for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the apparatus including an extruder head having a die, and at least one hollow guide member for receiving and guiding the or each said element in use, wherein the guide member and the die are spaced apart from one another to define a gap through which molten extrudate can flow in use, and wherein the size of the gap can be varied to vary the flow of extrudate therethrough.

It is advantageous that the invention includes means for providing molten extrudate to the extruder head through a duct, the direction of which is transverse to the direction of passage of the or each elongate element through the extruder head. This is particularly advantageous as the supply of extrudate does not interfere with the feeding of the or each elongate element into the extruder head.

Conveniently, the extruding apparatus includes means for cooling the extruded product.

This is especially useful, as it prevents the undesirable deformation of the product under the force of gravity.

According to a further aspect of the invention, there is provided an extruding method for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the method comprising the steps of passing molten extrudate to an extruder head along a first axis, feeding the or each elongate element into the extruder head along a second axis which is transverse to the first axis, guiding the or each elongate element towards a die by means of a guide member, varying the spatial separation of the guide member and the die by movement of the guide member relative to the die and extruding the extrudate with the or each elongate element from the die to produce said composite extruded product. Thus it is possible to precisely control the flow of molten extrudate, thereby improving the quality and consistency of extruded products.

In preferred embodiments for use in manufacturing products having two substantially parallel spaced elements, two individually movable guide members may be employed.

Embodiments and aspects of this invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic plan view showing the lay-out of the extruding apparatus and method of this invention; Figure 2 is a part-sectioned plan view showing a detail of Figure 1; Figure 3 is a part-sectional perspective view showing an extruded product; and Figure 4 is a plan view showing details of a preferred embodiment of this invention.

Referring to Figure 1 of the drawings, extruding apparatus (10) according to this invention includes an extruder machine (12). The extruder machine (12) has a hopper (14) at one end and an extruder head (16) at the other end. The extruder head (16) and the extruder machine (12) lie on a common longitudinal axis viewed from above.

The hopper (14) receives plastic material in powder or pellet form which is melted within the extruder machine (12) to form a molten extrudate.

The extrudate is expelled at a controlled rate into the extruder head (16).

Referring also now to Figure 2, the extruder head (16) receives an elongate metallic element (20) in the form of a high-tensile steel wire.

The element (20) is drawn from a supply reel (22) and passes through the extruder head (16) at a controlled rate in a direction transverse to the above-mentioned longitudinal axis. In the usual way, the element (20) emerges from the extruder head (16) through an opening (24) in a removable die (26). The opening (24) is slightly larger than the element (20) in cross-section, so as to define a gap (28) through which extrudate flows to emerge from the extruder head in contact with the element (20).

As is common in the art, cooling means (30) may be provided to cool and set the extruded product as it emerges from the extruder head (16).

This prevents undesirable deformation of the product under the force of gravity. After cooling, the product may be cut into lengths and/or wound onto drums as appropriate.

The extruder head (16) contains an elongate hollow guide member (32) of generally tubular form, which receives and supports the element (20) within its internal cavity. The element (20) enters the cavity through an opening at one end (34) of the guide member (32). This end (34) of the guide member (32) protrudes through the wall of the extruder head (16) and has an hexagonal head (36) whereby the guide member may be turned about its longitudinal axis. The other end (38) of the guide member (32), from which the element (20) emerges, is situated close to the opening (24) in the die (26). However, the end (38) and the die (26) remain spaced from one another so as to leave a small opening through which the extrudate flows into gap (28).

The guide member (32) has an external thread (40) which engages with a corresponding internal thread in a wall (42) of the extruder head (16).

As a result, when turned about its longitudinal axis, the guide member (32) also moves linearly along this axis. The linear movement varies the distance between the end (38) of the guide member (32) and the die (26), that is, the linear movement varies the size of the opening.

As will be clear, the size of the opening between the guide member (32) and the die (26) is a factor in the flow rate of extrudate through the gap (28). Consequently, the ability to vary the size of the opening allows fine control over the flow of extrudate.

The apparatus of this invention enjoys major advantages over existing arrangements which have fixed guide members and which give little or no fine control. In particular, variations in the viscosity of the extrudate can be readily compensated by moving the guide member (32) until satisfactory results are obtained. This operation can be continuously performed without replacing the expensive die or otherwise disrupting production, and may be automated to allow continuous adjustment in response to quality monitoring.

The embodiment illustrated is particularly suitable for manual operation. However, means such as stepper motors could be used to turn the guide member (32). Moreover, it will be clear that the guide member (32) need not necessarily have thread means and could simply be moved linearly by means such as hydraulic jacks.

The advantages of this invention are particularly evident in the manufacture of structural members having two or more metallic elements spaced apart and joined by a web of plastics material. An example of such a product, which is particularly suitable for use a fencing member, is shown in Figure 3.

the nanufacture of extruded products having spaced allic elements is subject to the many problems of extrusion described above, but suffers from still more drawbacks. Perhaps the most serious problem is that it is very difficult to synchronize the speeds of the metallic elements as they pass through the extruder head. It is believed that the problem is caused by drag forces causing uneven stretching of the metallic elements as they are brought into contact with the extrudate in the extruder head.

The synchronization problem is serious because any difference in the speeds of the metallic elements is likely to cause unacceptable deformation of the finished product.

It has been found that the speed of a metallic element (20) through the extruder head (16) can be varied to a small but significant degree by moving the guide member (32) as described above. Accordingly, if one guide member (32) is provided for each metallic element (20), as shown in Figure 4, the speed of each element can be independently adjusted whereby synchronization can be achieved and maintained.

Another problem where two or more metallic elements (20) are employed is that the temperature, and therefore the viscosity, of the extrudate may not be exactly the same adjacent each element This may lead to poor quality.

The temperature difference arises because the extruder head (16) may have relatively hot or cool regions, despite the incorporation of heaters embedded in the walls of the head. The arrangement shown in Figure 4 overcomes this problem because each guide member (32) may be independently adjusted to suit the local temperature of the extrudate.

Claims

1. Extruding apparatus, for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the apparatus comprising an extruder head having a die, means for receiving the elongate element for passage through the die with extrudate, and means for controlling flow of said extrudate.

2. Extruding apparatus according to claim 1, wherein the control means comprises two spaced apart members, the spatial separation of which may be varied to vary the flow of extrudate.

3. Extruding apparatus according to either claim 1 or 2, wherein one of said members is the die.

4. Extruding apparatus according to claim 3, wherein the receiving means includes a guide member for guiding the elongate element towards the die, and wherein the other of said members is the guide member.

5. Extruding apparatus according to claim 4, wherein the guide member is movable relative to the die.

6. Extruding apparatus according to claim 5, wherein the guide member is threadingly engaged with the extruder head.

7. Extruding apparatus for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the apparatus including an extruder head having a die and at least one hollow guide member for receiving and guiding the or each said element in use, wherein the guide member and the die are spaced apart from one another to define a gap through which molten extrudate can flow in use, and wherein the size of the gap can be varied to vary the flow of extrudate therethrough.

8. Extruding apparatus according to any preceding claim, further including means for providing molten extrudate to the extruder head through a duct, the direction of which is transverse to the direction of passage of the or each elongate element through the extruder head.

9. Extruding apparatus according to any preceding claim, including means for cooling the extruded product.

10. Extruding apparatus for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the apparatus including means for providing molten extrudate to an extruder head along a duct, wherein the or each said element is passed through the extruder head in a direction which is transverse to the direction of said duct.

t1. Extruding apparatus constructed and arranged substantially as described herein with the reference to Figure 1, Figure 2, Figure 3, and Figure 4 of the accompanying drawings.

12. A method for producing a composite extruded product having at least one elongate element and a matrix of extrudate, the method comprising the steps of passing molten extrudate to an extruder head along a first axis, feeding the or each elongate element into the extruder head along a second axis which is transverse to the first axis, guiding the or each elongate element towards a die by means of a guide member; varying the spatial separation of the guide member and the die by movement of the guide member relative to the die; and extruding the extrudate with the or each elongate element from the die to produce said composite extruded product.

13. A method of producing a composite product according to claim 12, wherein the spatial separation is varied during extrusion of the extrudate.

14. A method of producing a composite extruded product substantially as hereinbefore described with reference to and as illustrated by Figure 1, Figure 2, Figure 3, and Figure 4 of the accompanying drawings.