AU690478B2

AU690478B2 - Method and apparatus for reducing die drool in cable manufacture

Info

Publication number: AU690478B2
Application number: AU75378/96A
Authority: AU
Inventors: Kevin R Schicker
Original assignee: PACIFIC DUNLOP HOLDINGS N Z LI
Current assignee: Australian Cables Pty Ltd
Priority date: 1995-12-15
Filing date: 1996-12-16
Publication date: 1998-04-23
Anticipated expiration: 2016-12-16
Also published as: AU7537896A

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARDl PATENT Applicant(s): PACIFIC DUNLOP HOLDINGS LIMITED Invention Title: METHOD AND APPARATUS FOR REDUCING DIE DROOL IN CABLE MANUFACTURE The following statement is a full description of this invention, including the best method of performing it known to me/us: 4 4* *.e4 0* 90 4* 4 0 4 44 0 4 0*40 *4*0 4.

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TITLE

Method and Apparatus for Reducing Die Drool in Cable Manufacture FIELD OF THE INVENTION This invention relates to a method and apparatus for reducing die drool in cable manufacture.

The insulation of conductors for use in cable is conventionally carried by extruding a plastics coating often polyethylene onto a copper conductor. A known problem in this type of extrusion process is the build up of die drool. 'Die drool' is a name given to the formation of plastics which accumulates at the exit edge of an extrusion die when the wire is being coated. Although the drool itself does not affect the surface finish of the product, there is a problem that is caused by the fact that as the process continues the drool increases in size until it breaks away. This act of breaking away often tears the insulation and jeopardises the production quality.

Extrusion in water curable polyethylene is a particular problem. Very small fragments of plast.ics feed out of the die and hang onto the edge of the die. As time goes on, more fragments of plastics exit the die and they all stick together creating a large ring which eventually, due to the frictional force of the moving cable, is ripped off the die 25 either in totality or in part. As the large ring in totality or in part, falls off, it can cause a tear in the insulation which could result in electrical failure.

DISCUSSION OF THE PRIOR ART A number of attempts have been made to eliminate this problem with varying degrees of success. One known technique is tc endeavour to eliminate the reason the drool exists. Other means including modifying the extrusion tooling so that the drool does not become a problem. Most attempts have been to find a material to make or coat the staHf/n/keepppernPN7175CAP 13 1296 3 die so that the drool does not stick to the die and build up during the production. Thus, it is known to apply material such as ceramic or TEFLON to the outer surfaces of the die. However, both have limited success. Another practice is to provide a naked flame at the face of the die to burn off the drool. This also has limited success and is difficult to control and potentially hazardous to operating personnel.

U.S. Patent 5,008,056 discloses a process for reducing die drool in the manufacture of cable by incorporating into the jacketing insulation a resin compound which decreases the adhesiveness between the extrusion die surface and the plastic insulation. Japanese Patent Application J03026504 describes a method of releasing a moulded article by supplying air through relief holes which are blocked with pins during manufacturing.

Canadian Patent 2,056,324 discloses an extrusion apparatus for providing a jacket of polyethylene onto a cable featuring circumferentially extending teeth at the cable outlet region which break down die drool into particles.

It is these problems that have brought about the present invention.

.6o* SUMMARY OF THE INVENTION "According to one aspect of the present invention 25 there is provided a method of preventing the formation of die drool during extrusion coating, the method comprising supplying a current of air at the exit of the extrusion die, the current of air being directed at a position where formation of die drool takes place.

30 Preferably, an annular jet of air is supplied at the exit of the die to surround the extruded article. The current of air is preferably heated.

t'oo According to a further aspect of the present e** invention, there is provided apparatus for preventing the formation of die drool during extrusion coating, the staWnkeep/perrfPN7175.CAP 13.12.90 4 apparatus comprising an extrusion die, the die including means to extrude a coating of plastics and means to supply a jet of air at the exit of the extrusion die at a position at which the formation of die drool takes place.

DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic side on view showing a wire insulation extrusion process incorporating means to prevent formation of die drool, Figure 2 is a sectional view of the extrusion tip that forms part of the apparatus, Figure 3 is a cross-sectional view of the extrusion die forming another part of the apparatus, Figure 4 is a cross-sectional view of a die plate that forms an additional part of the apparatus, and Figure 5 is an assembly view of the tip, die and die plate.

20 DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in Figure A which is a schematic side-on view of a conventional wire insulation extrusion process, a cable core or cable wire 10 is drawn through an extrusion die assembly D during which suitable plastics material P is extruded onto the outer surface of the wire 10. The die assembly D includes an extrusion tip T which forms the mouth of the die assembly D and this is in the form of a frusto-conical metallic member that has a conically tapering outer surface. An extrusion die E fits on the S 30 front end of the extrusion tip T to define an annular gap G therebetween into which the plastics insulation material P i:t' is located and heated to a suitable temperature whereby it *t can be extruded over the wire 10 as it is pulled through the die E. In simplistic terms, this is a conventional way staWn/keeptperrrVPN7J75 CAP 13.1296 5 of extruding a plastics coating on the wire. It has been discovered that during the extrusion process, there is a build up of drool or fragments of plastics at the mouth of the die. The fragments tend to build up into a large ring before breaking away often causing damage to the cable insulation.

In the embodiment of the present invention shown in Figures 1 to 5, the damaging effect of the formation of drool is avoided by providing a small annular air current adjacent the mouth of the die assembly 1. The air current is supplied between 1 and 3 bar and has the dual role of preventing the fragments of plastics from curling back and thereby attaching themselves to the die and also cooling the fragments reducing their inherent stickiness which in turn reduces the likelihood of the fragments sticking to the die. The use of the air current causes the drool to fragment and blow clear and ensures that there is no build up of a large ring caused by fragments adhering to the mouth of the die. The compressed air is heated slightly to avoid over-chilling of the die.

The die assembly 1 shown in Figures 1 to comprises three main components which secure together to form the assembly 1 shown in Figures 1 and 5. A tip shown in Figure 2 supports a die 30 (Figure 3) which in 25 turn supports a die plate 40 (Figure A series of countersunk cap head screws (not shown) bolt the die plate against the die As shown in Figure 2, the die tip 20 has an !externally threaded shank 21 that is screwed into the end of the extrusion die apparatus (not shown). The external surface 22 of the die tip is of frusto-conical configuration ending in a cylindrical land 23 at the forward end. The interior surface 24 of the die tip 20 is conically tapered to merge into a cylindrical forward tip 35 As shown in Figure 3, the die 30 is a frusto staftnfkeeptpermIPN7176 CAP 13.1296 6 conical member that fits over the die tip 20, the inclination of the interior surface 31 of the die 30 is greater than the exterior surface 22 of the die tip 20 to define as shown in Figures 1 and 5 an annular tapering gap 32 between the die 30 and the die tip 20 through which hot plastics material flows during the extrusion process. The annular gap 32 terminates in an annular ring 33 between the land 23 on the die tip 20 and a similarly profiled forward mouth 34 of the die 30. The die 30 has a frusto-conical exterior surface 35 which is adapted to accommodate a die plate 40 (Figure 4) that is bolted thereto by the series of cap head screws (not shown) that engage internally tapped holes 36 in the forward face of the die 30. The outer edge of the die 30 is provided with an annular shoulder 37 and the die plate 40 is provided with an annular rearward flange 41 that fits closely onto the shoulder 37 to locate the plate 40. Six equally spaced 3mm holes 42 are drilled into the die plate at a 300 angle to communicate with an internal bore 43 of the die plate 40 that has a frustoconical wall structure 44 terminating in a stepped shoulder at the forward edge. The stepped shoulder 45 has a tapered profile 46 so that the interior surface of the front of the die plate 40 is in close abutting contact with the external surface of the die 30. By bolting the die 25 plate 40 onto the die 30, air can be passed into the six holes 42 to escape at the front of the die assembly at the narrow gap 48 defined between the interior front edge of the die plate 40 and the external front edge of the die The annular narrow gap 48 at the forward edge of t.he assembly 10 ensures that an air current of annular configuration escapes from the front of the assembly o*ooo: Compressed air is supplied at a pressure of between 1 to 3 bar and the temperature of the die assembly 1 is used to heat the air to a temperature that ensures o"o" 35 that the extruded plastics is not brittle but cool as it contacts the forward edge of the die.

stafV/nlkeepperrVPN7175.AP 131296 7 Over extensive testing, it has been discovered that the provision of the annular air jet at the mouth of the die prevents the drool turning up on itself and adhering to the front surface of the die. The drool therefore does not build up, and on regular basis, it drops off in fragments avoiding the damage that may be caused to the insulation when a large build up of drool is ripped or torn off.

The assembly described above is particularly useful in the extrusion coating of wire in polyethylene.

Although the method and apparatus is specifically useful in a wire coating process, it is applicable to other extrusion processes and it is understood that the invention has been adapted for use in any extrusion process where drool build up is a problem.

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Claims

1. A method of preventing the formation of die drool during extrusion coating, the method comprising supplying a current of air at the exit of the extrusion die, the current of air being directed at a position where formation of die drool takes place.

2. The method according to claim 1 wherein the current of air at the exit of the die is an annular air current.

3. The method according to either claim 1 or claim 2 wherein the current of air is heated by passing the air through the die body.

4. The method according to any one of the preceding 15 claims wherein the current of air is supplied at a pressure of between 1 and 3 bar.

Apparatus for preventing the formation of die drool during extrusion coating, the apparatus comprising an extrusion die, the die including means to extrude a coating of plastics and means to supply a current of air at the exit of the extrusion die at a position at which the formation of die drool takes place.

6. The apparatus according to claim 5 wherein means is provided to heat the source of air. S 25

7. The apparatus according to claim 6 wherein the die has a narrow annular passageway terminating adjacent the exit of the die, the passageway being coupled to a source of compressed air,

8. A method of preventing the formation of die drool substantially as described herein with reference to and as illustrated by Figures 1 to \\molb0\hoznS\arn\Keep\Tem.~p\h378 96 2Nf.doc 2) 02/98 9

9. Apparatus for preventing the formation of die drool substantially as described herein with reference to and as illustrated by Figures 1 to Dated this 23rd day of February 1998. PACIFIC DUNLOP HOLDINGS (NZ) LIMITED By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia e 00 S:0 0* 0 a os 0* e \\melb0l\hcme\ann\Kp\Tep\T 753 q6 2HN.doa 21/02/98 ABSTRACT A method of preventing the formation of die drool during extrusion coating, the method comprising supplying a current of air at the exit of the ext.u-ion die, the current of air being directed at a position where formation of die drool takes place. stattindkeegporrrVPN717S CAP 13,1296