CA1148319A - Extrusion of insulating plastic - Google Patents

Abstract

ABSTRACT
Molten plastic material is extruded uniformly about an advancing core by dividing a single stream of the plastic flowing to a die assembly transversely to the direction of advance of the core, into a plurality of streams, in which one half of the plastic flows along a first path spaced around the core and then towards the core; the other half of the plastic flows in a second path which is, in essence, a mirror image of the first path; the divided streams are reunited as a tubular sleeve of flowing plastic flowing in the direction of the advancing core, spaced around the core and flowing towards the advancing core.

Description

This invention is concerned with die assemblies, more especially with die assemblies for use with a screw extruder to provide an insulating plastic covering on a core' the invention is also concerned with a method of extruding an insulating plastic covering around an advancing core.
This application is a division of Canadian Patent Application S~No 354,140, filed June 17, 1980.
Electrical conductor wires in, for example, power cables and communication cables are electrically insulated by extruding a molten plastic around the wire as it advances through a cross-head die assembly and solidifying the plastic on the wire, The insulating covering, particularly in a com~
munication cable conductor wire, may be cellular or solid~
In the case of cellular coverings the molten plastic mate-rial contains gaseous material which expand~s as bubbles in the molten material as the coated wire emerges from the die assembly, The degree of expansion is controlled by the cooling to solidification, which is suitably achieved by advancing the coated wire through a water trough which moves in response to signals from a capacitance monitor, Similarly an insulating sleeve may be formed around a bundle of insulated conductors by extruding the molten plastic around the advancing bundle, In the case of communication cables the bundle suitably has a shield of aluminum wrapped around it, the shield conveniently having a synthetic coating of a carboxylated polymer which adheres readily to the molten plastic extruded around it, Insulating coverings particularly in communication cables are required to meet exacting standardsL particularly with regard to their thicknessv The thickness of the covering and the degree of cellularity in the case of 3~S~
cellular insulation, at any point along the insulated covering determines the electrical characteristics, for example, the dielectric properties of the insulated conductor. Variations in the e]ectrical characteristics along the insulated con-ductor can result in cross-talk in a telephone cable and this is difficult to eliminate.
It is generally desirable and necessary to form an insulating covering on a conductor particularly for a communication cable, which is uniform along the insulated conductorO
Generally the concentricity, rounaness and uni-formity of the insulating covering must be carefully controlled.
; In many cases it has~ in addition, been a practice to provide a thicker insulating covering than actually required so that at least a minimum insulation will be provided over the entire wire surface, however, this is - uneconomlc.
In the conventionally employed cross-head die assemblies molten plastlc insulating material is extruded ; 20 from a screw extruder into the die assembly, in a direction transverse to the direction of travel of the core advancing through the die assembly and onto the advancing core. This necessitates that the stream of molten plastic undergo at least one 90 change in direction in its flow path, which results in flow imbalance conditions in the molten plastic.
In particular a differential of pressure or flow is set up between the molten plastic in the outside regions of the bending stream and the molten plastic in the inside region of the bending stream.
The flow imbalances in the stream as it flows onto an advancing core cause eccentricity between the insulating ; covering and the core, and lack of roundness of the outer .

circumference of the extruded covering. This results in cross-sectional and longitudinal variations in the thickness of the formed covering, which, as indicated above, is unacceptable.
In U. S. Patent 3,860,686~ Daryl Lester Myers, issued January 14, 1975) a multi-path cross~head die assembly has been proposed. In the die assembly of Myers the molten plastic is subjected to a number of changes of flow in which the stream is divided and the direction of flow is changed. Unfortunately the Myers die assembly is impractical, particularly in that it is almost impossible to adequately clean the inaccessible, multiplicity flow channels which define a tortuous path for molten material in the assembly. It is necessary to thoroughly clean die assemblies, for example, to remove deposits of charred plastic which may collectl particularly in corners, and when the plastic being extruded is to be changed.
Thus the Myers die assembly does not provide a practical solution to the problem and does not appear to have been exploited commercially.
The present invention provides a die assembly which overcomes the prior difficulties and enables the formation of an insulating covering of an acceptable high standard of uniformity on an advancing core while at the same time the surfaces of the assembly exposed to molten plastic are accessible and can be cleaned without difficulty in a simple cleaning operation.
The die asse~lies of the invention can be employed to form insulating coverings on electrical conductor wires, as well as lnsulating sheaths around bundles of insulated conductor wires.

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There is disclosed a method of extruding an insulat-ing plastic covering around an advancing core which comprises:
guiding an advancing core through an extrusion die, forcing a stream of molten insulating material to said die, transversely of said advancing core, dividing said stream into a plurality of streams with at least two of the streams flowing in oppo-site directions, in paths spaced around the advancing core, directing said streams inwardly towards each other, deflec-ting said inwardly directed streams to form a tubular stream flowing in the direction of the advancing core and spaced around the advancing core, said tubular stream flow-ing towards said advancing core, contacting said advancing core and the flowing tubular stream to form a continuous coating of said molten material on said advancing core, and cooling said molten material to form a solidified covering of insulating plastic on said core.
The advancing core may comprise a bare electrical conductor wire, for example, a copper wire, or a bundle of insulated conductor wires wrapped ln a shield.
The molten insulating plastic may suitably comprise a polyolefin, for example, polyethylene including low density, medium density and high density polyethylene, polypro~ylene and mixtures thereof, or a halogenated polyolefin or copolymer of an olefin or haloolefin with a comonomer, for example, polyvinyl chloride.
There is also disclosed a die assembly for the extrusion o~ molten insulating plastic around an advancing core, including an extrusion die having a bore therethrough defining an inlet and an outlet, opposed to said inlet, for an advancing core, channel means spaced around said bore, for flow of molten plastic, a plurality of inwardly directed passages communicating said channel means with said bore, and a core guide having a continuous bore therethrough for an ad~ancing core adapted to be removabl seated in the die bore, said core guide including a deflect.i.ng surface which defines with said die bore an annular space, said deflecting surface being adapted to deflect molten pl.astic delievered through said passages along said annular space towards said die bore outlet.
In one aspect of the invention there is provided the die of the die assembly of the invention.
In accordance with this aspect there is provided an extrusion die for use in the extrusion of molten insulating plastic around an advancing core comprising a cylindrical body having a centrally disposed bore therethrough defining an inlet and an outlet opposed to said inlet, a circumferential channel means spaced around said bore, in an outer surFace of said ~ody for flow of molten plastic, and a pair of inwardly directed radial passages communicati.ng said circumferential channel means with said bore, said bore including a machined cylindrical surface extending from said inlet to said radial passages, adapted to mate with a core guide such that said cylindrical body supports said core guide in a fixed seated position, and said bore including a frusto-conical surface extending from said radial passages towards said outlet.
In another apsect of the invention there is provided the core guideof the die assembly of the invention.
In accordance with this latter aspect there is provided a core gui~efor guiding an advancing core through an extrusion die comprising:
a body adapted to be removarbly seated in a bore of a cross-head extrusion die, a continuous bore in said body for an advancing core, a core inlet end and a core outlet end :

opposed to said corè inlet end, said body including:
a cylindrical centering body portion at said core inlet end, adapted to fit the core guide centrally in the die bore, in a fixed-centred position, and a frusto-conical body portion including a deflecting surface to deflect molten plastic towards the core outlet end.
The invention is illustrated in particular and preferred embodiments by reference to the accompanying drawings in which:
FIGURE 1 illustrates schematically an apparatus for forming a cellular insulating covering on a conductor wire, including a die assembly of the i.nvention, FIGURE 2 is a front perspective view of a die assembly of the invention, FIGURE 3 which is alongside Figure 6, is a side perspective view of the die assembly of Figure 2, FIGURE 4 is a front eross-section along line 4-4 of the die assembly of Figure 2, FIGURE 5 which appears alongside Figure 2, is a plan cross-seetion along a line 5-5 of the die assembly of Figure 2, FIGURE 6 i5 a perspeetive view of the core tube of the die assembly of Figure 2, FIGURE 7 is a front cross-section of a die assembly of the invention in a different embodiment, and 33~

FIGURE 8 is a perspective view of the core tube of the die assembly of Figure 7.
With further reference to Figure 1 an apparatus generally designated 10, for forming a cellularly insulat~d electrical conductor wlre 26 includes a die assembly 12, a moving water trough 14, a capacitance monitor 16 and an extruder 28.
Wire 20 to be insulated is fed from a spool 18 around a roller 22, and through die assembly 12. The wire 20 is grounded at 24.
Moltan plastic insulating material containing a gas is extruded from extruder 28 to die assembly 12 in a direction transverse to the direction of the advancing wixe 20 and a coated wire 25, in which the coating is of the molten plast.ic, emerges from die assembly 1~, with the release oE the extrusion pressure the gas in the molten `plastic begins to expand as gas bubbles, and the coated wire 25 is fed through moving water trough 14 to solidify the molten material, the insulated wire 26 from trough 14 passes between rolls 30 and through capacitance monitor 16 and thence is conveyed to a collection spool (not shown) by rolls 32.
Trough 14 moves responsive to signals from capacitance monitor 16 to control the position at which coated wire 25 enters trough 14 and hence the degree o-f expansion of gas in the molten plastic, in a manner which is well known.
With further reference to Figures 2 to 6, a die asse~bly 12 comprises a die 34 and a core tube 36. Die 34 includes an entry port 38 for molten plastic, a circumferential channel 40 and opposed radial passages 42 and 44.

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Radial passages 4~ and 44, which are diametrically opposed, communicate at their inner ends with a machined die bore 46 which passes through the die 34 and defines an inlet 35 and an outlet 37.
The centre lines of circumferential channel 40 and passages 42 and 44 are contained in essentially the same planeO
The core tube 36, as is more particularly shown in Figure 4, is adapted to be removably seated in the bore 46 .10 of die 34 and includes a head 53, an upper cyli.ndrical centering portion 54, a generally frusto-conical lower portion 56 and a continuous bore 38 therethrough de:Eining a core inlet end 49 and a core outlet end 60.
Cylindrical portion 54 has a machined cylindrical surface 55 which mates with the wall of the die bore 46 with only a narrow clearance 50, whereby the core tube 36 adapts a fixed seated position in die 34, the fixed seated position being in particular a fixed-centered position, in which the core tube 36 is centrally disposed in the die 34.
Frusto-conical portion 56, which extends from centering portion 54 to core outlet end 60, includes a deflect-ing surface 58 which defines with bore 46 an annular space 62 of a cross-section which decreases in the direction of advance of wire 20, Core outletend 60 of core tube 36 is disposed within die 34 and spaced apart from outlek 37.
The junction 61 of cylindrical surface 55 and deflecting surface 58, i.e., the junction of centering portion ~:
54 and frusto-conical portion 56, is disposed on the central axis through opposed radial passages 42 and 44.
The bcre 46 of die 34 is defined adjacent outlet 37 by an insert 64 of material resistant to wear, and resistant --8~

to the molten plastic and the heat therefrom. In a parti cular embodiment insert 64 includes a carbide tip 64a and a body 64b of a metal alloy resistant to heat and wear' insert 64 conducts heat away rom the outlet 37, thereby avoiding excessive heat build-up at outlet 37~
A guide element 66 is disposed in bore 48 of core tube 36 to provide a fine clearance between bore 48 and wire 20. The guide 66 is suitably of a highly wear resistant mate-rial which can be bored to a fine and accurate tolerance, In particular guide 66 may comprise a diamond having a cylindrical bore lapped -therethrough~ Such a diamond is resistant to wear and has a long life even with wire speeds of the order of 5,000 feet per minute, and additionally can be lapped to provide a clearance with the wire 20 of about 0.0005 inches~
With reference to Figures 7 and 8 there is shown a die assembly 68 comprising a die 70 and a core tube 72.
Parts of die 70 common to die 34 are indicated by the same reference numerals. Thus die 70 includes an entry port 38, a circumferential channel 40 and radial passages 42 and 44.
Die 70 includes a die bore 71 therethrough defining an inlet 74 and an outlet 76. D.ie bore 71 includes a cylindrical surface 94, a generally frusto-conical sur-face 96 and an end cylindrical surface 98~
Core tube 72 has a continuous bore 73 therethrough between a core inlet end 75 and a core outlet end 77 and includes a head 78, a cylindrical centering portion 80, a generally frusto-conical portion 82 and a cylindrical end portion 84.

_ 9 Centering portion 80 includes a cylindrical surface 86 and a pair of continuous, spaced apart grooves 88 therein.
Frusto-conical portion 82 defines a deflecting surface 90 decreasing in cross-section to a cylindrical surface 92 in end portion 84.
As can be seen more clearly in Figure 7, the junction 91 of cylindri.cal surface 86 and deflecting surface 90 is disposed adjacent the uppermost walls of radial passages 42 and 44s An annular space 100 is defined in die 70 between deflecting surface 90 and cylindrical surface 92 of core tube 72 on the one hand, and frusto-conical surface 96 and cylindrical surface 98 of bore 71 on the other hand Core outlet end 77 of core tube 72 terminates at outlet 76 of die 70.
In the operation of tha die assembly 12 of Figures

2 to 6, molten plastic material, which may contain gas to be expanded to form cellular insulation, is extruded from an ~xtruder such as 28 in Figure 1, to entry port 38 of die assembly 12. At entry port 38, the stream of molten plastic is divided into two equal streams which flow in opposed circum-ferential directions, i.e., clockwise and anti-clockwise, towards radial passages 42 and 44 respectively.
At the outer extremities of radial passages 42 and 44 the two streams are bent inwardly through an angle of about 90 and flow towards each other along radial passages 42 and 44.
m e molten plastic material flows into bore 46 and is deflected by deflecting surface 58 to flow along the deflecting surface 58 in annular space 62 towards outlet 37. While being deflected by deflecting surface 58 the molten streams from passages 42 and 44 merge to form a tubular stream or flowing sleeve of molten plastic which is characterized by the absence of a merge or weld line. The tubular stream or sleeve of molten plastic flows downwardly in annular space 62 in the direction of advance of wire 20, along the deflecting surface 58 under the extrusion pressure from the screw extruder J such that the tubular stream converges towards wire ~0 until it is very close to wire 20, while still under the extrusion pres-sure, which may typically be of the order of 9,000 p.s.i Finally, the molten plastic flows over core outlet end 60 of core tube 36, while still under extrusi.on pressure, changing direction a final time to flow onto the advancing wire 20 as a continuous coating of molten plastic thereon~ The thus coated wire 25 continues to advance towaxds outlet 37 and then advances QUt of die 34 at outlet 37.
The small clearance 50 is kept to a minimum by the accurate machining of surface 55 and the mating portion of bore 46, whereby back flow of molten plastic between bore 46 and centering portion 54 is substantially avoided.
The combination of the continuous advance of wire ~0 in suhstantially the same direction as that of the flow of the tubular stream, in conjunction with the guide element 66 in core tube bore 48, which provides a very fine clearance, avoids any substantial back flow of molten plastic along bore 48 in a direction opposite that of the advance of wire 20~
In practice it is found appropriate to dimension the frusto-conical portion 56 and the adjacent portion of bore 46 so that the tubular stream flowiny over core outlet end 51 of core tube 36 is at an angle of 5 to 6 30' to the axis of wixe 20.
The deflecting surface 58 of core tube 48 prevents the molten plastic, under high pressure, from contacti.ng the advancing wire 20, until the molten plastic is travelling close to the advancing wire 20. If the molten plastic from passages 42 and 44 were to engage the advancing wire 20 directly on entering bore 46, the high extrusion pressure of the molten plastic would be sufficient to shear the wire 20 On the other hand, deflecting the tubular stream of molten plastic gradually towards the advancing wire 20 while still under the extrusion pressure, and within the die 34, until it ~ close to the wire 20 and travelling in the same general direction, ensures th~ the molten plastic in annular space 62 flows directly onto the advancing wire 20, without shearing the wire 20.
The final inward bending or convergence of the wall o-f the tubular stream which forces the stream onto the wire 20, thus takes place while the stream is under the influence of the high extrusion pressure.
The molten coating of the coated wire 25 advancing from outlet 37 can be solidified in the manner illustrated with reference to Figure 1.
The flow of the molten plastic material in die assembly 12 is indicated by the arrows in Figures 2 and 4.
The method described with reference to Figures 2 to 6 is essentially for extrusion coating bare wires, for example, copper conductor wires, where pressure exerted on the wire as the tubular stream is forced onto the advancing wire, will not deform or otherwise damage the wire However, where a bundle of insulated wires, i,e~, previously coated wires, is to be encompassed by a single sleeve, use of the pressure extrusion process described with reference to Figures 2 and 6 would damage the already insulated wires, for example, by deforming their coatings, and this would affect the electrical characteristics.

- 12 _ The die assembly illustrated in Figures 7 and 8 is employed for forming a covering or sleeve on a bundle of insulated wires and is described hereinafter:
Molten plastic is fed under the extrusion pressure into bore 71 of die 70 via an entry port 38, a circum~
ferential channel 40 and radial passages 42 and 44 in the same manner as described with reference to Figures 2 to 6.
On entering bore 71 from passages 42 and 44 the molten mate-rial is deflected by deflecting surface 90 to form a tubular stream of flowing molten plastic in annular space 100, The tubular stream flows along annular space 100 and converges towards core outlet end 79. A bundle of insulated wires (not shown) is advanced through bore 7.3 in the manner described for wire 20 in Figures 2 to 6. The cross-section of annular space 100 decreases towarcls outlet When the tubular stream reaches portion 84 the direction of flow of the tubular stream changes to flow along cylindrical end surfàce 92 such that the tubular stream is flowing in a direction substantially parallel with and circumferentially around that of the advancing bundle ~not shown).
Cylindrical end portion 84 texminates at outlet 76, the core outlet end 79 of core tube 72 being spaced inwardly of outlet 76 and the tubular stream of molten plastic emerges from outlet 76 and is spaced circumferentially around the advancing bundle.
In order to force the tubular sleeve onto the advancing bundle, a vacuum is applied to core tube bore 73 adjacent head 78. With the application of the vacuum, atmos-pheric pressure urges the tubular stream emerging from outlet 76 onto the advancing bundle.

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Once the process has been started and with the continuous application of the vacuum the tubular stream of molten plastic is forced onto the advancing bundle under pressure, as the tubular strearn and the aclvancing bundle emerge from outlet 76.
In this way the advancing bundle is not subjected to the high extrusion pressure as is the wire 20 in the embodiment of Figur~s2 to 6, and no deformation of the bundle occurs.
There is no guide element corresponding to guide 66 in core tube bore 73, and such a guide element is not required. Since the tubular stream of molten plastic is not under the high extrusion pressure when it is urged onto the advancing bundle, there is no tendency for backflow of molten plastic in bore 73.
The cylindrical surface 86 of centering poxtion 80 of core tube 72 and the cylindrical surface 94 of die bore 71 are accurately machined to provide a snug fit in which the core tube 72 is centrally fixed in die 70, and this minimizes backflow of molten plastic along bore 71 from passages 42 and 44. The grooves 88 are useful in increasing the easy removal of core tube 72 from die 70, and in addition serve as a trap for any molten material which might flow back between surfaces 86 and 94.
If found appropriate to control the process and the dimensions of the die assembly 68 such that the cross-sectional area of the covering formed on the advancing bundle is about 8~/o of the cross-sectional area of the annular space 100 at outlet 76~ This permits adequate shrinking of the molten material urged onto the advancing bundle by atmospheric pres-sure, under the influence of the vacuum applied to core tube bore 73, to permit a uniform covering on the advancing bundle.

If the amount of molten plastic material is increased above this ratio the additional plastic material may cause buckling and a non-uniform covering. If the amount of molten plastic material is decreased then the memory of the resulting cover-ing is deleteriously effected producing stxess in the cover-ing.
The advancing bundle which is covered employing the die assembly o-f Figures 7 and 8 suitably comprises a plurality of individually insulated conductor wires wrapped in an aluminium shield having an outer coating of a copolymer o-f ethylene and a carboxyl group containing monomer. Such a shield is available under the trade mark Zetabond from Dow Chemical. The copolymer coating provides good adhesion bekween the shield and the insulating plastic covering.
The die assemblies 12 and 68 which are suitably machined from tool steel can be readily cleaned by means of air Jets and wiping with an asbestos cloth. When the core tuhes are removed from the dies all the parts of the die and core tube which are contacted by molten plastic in its flow through the die assembly are readily accessible and there are no closed corners to act as collection points for charred plastic and debris from the cleaning operation.
Using the method and die assembly of the invention the molten plastic is formed into a flowing tubular stream or sleeve flowing in the same general direction as the advancing core, and is brought gradually closer to the advancing core be~ore being urged thereon.
The formation of the flowing tubular stream from opposed streams derived by division of a single flowing stream, and subjecting the divided streams to identical flow treatment along identical~ but opposite, flow paths, results 33~
in the formation of a uniform tubular stream or sleeve, in which the flow characteristics are the same in all cross-sectional portions, whereby a uniform covering of insulating plastic can be formed on an advancing core.
Stated otherwise, a single mass of molten plastic continuously delivered under extrusion to the die assembly of the invention, is divided in the die assembly into a plurality of streams. One half of the plurality is directed along a first flow path, and the other half is directed along a second flow path, which is a mirror image of the first flow path, such that the molten plastic in the two halves is subjected to identical flow txeatment along flow paths of identical length and character. The two halves of identical flow characteristics are finally brought together to form a flowi.ng tubular stream or sleeve spaced circumferentially around the advancing core and being concentric therewith, The uniform tubular stream flows towards and is then urged onto the advancing core to form a uniform molten coating, which is solidified to a uniform :~ 20 covering.

The tubular stream flows as an entering wedge towards the advancing core, as a result of the diminishing cross-section of the annular space in the direction of the advancing core; the pressure on the entering wedge in the annular space increases with the decrease in the cross-section of the annular space.
It will be understood that each half of the plurality ; comprises 50% of the mass or volume of the original single stream.

Preferably the single stream is divided 1nto an even number of streams, each stream flowing in the first direction ~ 3 :~L 9 having a counterpart identical stream flowing in the opposite direction, most preferably the single stream is divided into two equal streams flowing in identical, mirror image paths.
The invention may be employed to form both solid and cellular insulating coverings on an electrically conductive core.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An extrusion die for use in the extrusion of molten insulating plastic around an advancing core comprising:
a cylindrical body having a centrally disposed bore therethrough defining an inlet and an outlet opposed to said inlet, a circumferential channel means spaced around said bore, in an outer surface of said body for flow of molten plastic, and a pair of inwardly directed radial passages com-municating said circumferential channel means with said bore, said bore including a machined cylindrical sur-face extending from said inlet to said radial passages, adapted to mate with a core guide such that said cylindrical body supports said core guide in a fixed seated position, and said bore including a frusto-conical surface extending from said radial passages towards said outlet.

2. An extrusion die according to claim 1, wherein said radial passages are diametrically opposed, said circumferential channel extending from one radial passage to the other.

3. An extrusion die according to claim 1, wherein said frusto-conical surface includes an insert of mate-rial resistant to wear and resistant to molten plastic and the heat therefrom, said insert being effective to conduct heat away from said outlet.

4. An extrusion die according to claim 1, wherein said frusto-conical surface terminates in a cylindrical surface at said bore outlet.

5. A core guide for guiding an advancing core through an extrusion die comprising:
a body adapted to be removably seated in a bore of a cross-head extrusion die, a continuous bore in said body for an advancing core, a core inlet end and a core outlet end opposed to said core inlet end, said body including:
a cylindrical centering body portion at said core inlet end, adapted to fit the core guide centrally in the die bore, in a fixed-centred position, and a frusto-conical body portion including a deflect-ing surface to deflect molten plastic towards the core out-let end.

6. A core guide according to claim 5, wherein said centering body portion has a substantially cylindrical machined outer surface.

7. A core guide according to claim 5, wherein said frusto-conical body portion extends from said centering body portion to said core outlet end.

8. A core guide according to claim 5, including a guide element mounted in said continuous bore, said element having a cylindrical bore therethrough to provide a fine clearance with a core advancing therethrough.

9. A core guide according to claim 8, wherein said guide element is a diamond having a lapped cylindrical bore therethrough.

10, A core guide according to claim 6, wherein said frusto-conical body portion extends from said centering body portion to a cylindrical end portion at said core outlet end.