Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Thi.s invelltioD concorns i.mprovemell-ts re:Lat:i.n~
to the manufacture o~ covered electric cab~es Iwhich term
includes wires and/or conduccors theref'or) of the type in
which a coverin~, for example of el~stollleric and/or synthe-
tic plasti.cs material, is extruded directly onto the cable
core. Wel5. known examples of such coverillgs are na-tural
rubber compositions, synthetic rubber-l:il.e col~posi-tions such
as buty] rubber~ ethylene propylene rubber (~PR) and synthe-
tic plastics materials ~uch as cross-linked polyethylene.
The e:xtrusion of the coating onto the core is
elosely coupled with a trea-tmen-t proeess wllich reqllires the
application of heat for curing or strengthening the extruded
eoating and the application of pressure.to cause the extruded
~ eoating to collapse firmly onto the eore as it emerges from.~ 15 the extrusion machine. This treatment process is usually
carried out by passing the covered core continuously through
a treatment chamber hermetica:Lly sealed to the extruder
head. ~`his chamber commonly contains a fluid such as steam
at super-atomospheric pressure, typically of the order of
-20 1.7 MN/m (250 p.s.i.g.) for a period appropriate to the
', eomposition of the coating material. The treatment process
also includes a coo]ing stage which involves the application
of a cooling fluid such as water under correspondingly high
pressure.
For extrudin~ s-uch a coating onto a cable core,
. it has been conventional to employ an extrusion machine
having an inner male die and an outor fema:l~die ;jux-taI)osed
~ in the extruder head. Thec~b:Lo core i.s passo(l ttlroustl tho
`~ ma].e and female die orif:ices and the covoring matorial, that
~; 30 is the extrudate, is supp:l.i.ed to an ~Innul.us wh:i.ch is clefinecl
between th.e ma].e and remal.e dies by v.irtue Or their juxta-
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position~ The dies are silapod to support an~J loc~te ~hc
cable cn:re and t:o arranse ror thc di,s tr:ibution Or -the cover:ing
material around the core as it passes through the extruder
head.
As has been mentioned above, the extruded coating
on the core~is suhjected to a treatmcnt process ~s it
emer~es -from the extruder head, the trea*ment process including
conventiona~ly the introduction oi' the coated core into a
pressurised high temperature fluid. The pressure of the
fluid acts upon the covering materia~ as it emergr,s from the
extruder head, and, to ensure that the covering material is
not impeded and forced back through the orifice o~ the inner
male die between the die and the core, the covering is
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extruded under a pressure sufficient to withstand the pres-
surised fluid medium. To reduce the likelihood of the
-~ pressurised fluid breakin~ through the extrudate, the
''i~ clearance between the inner male die orifice and the core
~ itself must be minima] and, in the case of shaped cross-
'~! section cores, this has required the orifices of the male
~ 20 and female dies to be shaped to correspond exac-tly to the '
3 cross-sectional shape of the core. The prior art die
~ arrangements have had to be very carefully designed in order
,~ to take account of the design criteria which were, prior to
the present invention, commonly held to be relevant to the
- 25 design of cable coating dies. The die profile, its ~ow
, contour, the die entrance angles, their land leng-ths and
their clearances over the core havo beon common:l,y rogardecl
~ as critical features of dio desigrl. r~`urthermoro, whon
,j shaped cros-section cores arc being covorod, c:l,oso attolltion
has had to be paid to the relative ori,ontations o~ tllo innor
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and outer dies since this critically affects the conflguration of the
extruded coating obtained.
From the above considerations, it can be seen that the setting up
of a process for coating a cable core by extrusion has conventionally been
onerous. The dies have had to be made close tolerances with smooth finishes
and streamlined designs and, in order to accommodate unavoidable dimensional
variations between one length of core and another of the same nominal size
and even within the same length of core, it has been necessary to provide a
large range of dies of closely graded different sizes.
Additionally, the necessity of precisely orientating the dies
relative to one another has conventionally presented problems in regard to
minimising the setting up time of a production run and also presents problems
in regard to the maintenance of the dimensional stability of the extruded
covering throughout the course of a run. Thus, tooling costs, setting up
times and the cost of scrap produced by inordinately long setting up times
has conventionally considerably increased the overall cost of manufacture of
covered electric cables.
The present invention stems from efforts made to avoid or at least
significantly reduce the above mentioned problems and results from recon-
sideration of the design of the extrusion dies and adoption of die configur-
ations which, to a greater or lesser degree, depart from the established and
hitherto commonly held design criteria.
According to one aspect of the invention there is provided a set
of extrusion dies for use in the manufacture of covered electric cables of
the type having a core with a generally sector shaped cross-section by
extrusion of a covering onto the cable core as the core is passed through
orifices formed in the dies, said set of dies comprising a core die having
an orifice shaped to correspond closely to the cross-sectLonal shape Oe the
cable core to be coated, and a ring dle havlng a clrcular orlflce substantkll-
ly larger in diameter than the maximum dlmenslon of the core dle orlfLce,
said core die and ring die being adapted to be ~uxtaposed Ln an extruder
head with the die orifices in registry so as to enable a core to be threaded
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1~77~38~ "
through the die orifices and being shaped so as, when the two dies are thus
juxtaposed, to define between opposed surfaces of the two dies an annular
passageway serving, in use of the dies, for the delivery of covering material
to the cable core as it issues from the core die orifice, said ring die
orifice being defined at the juncture of a back surface of the ring die
which, in juxtaposition with the core die, defines said annular passageway
and a front surface of the ring die so as to have no extent in the direction
of the passage of the cable core through the dies when they are in use.
The invention, and advantages thereof, will be best understood
from consideration of the following description and discussion relating to
four exemplary embodiments of the invention contrasted with two prior art
arrangements all of which are shown in the accompanying drawings wherein:-
Figure 1 is a part cross-sectional view of a prior art die arrange-
ment employed for coating circular cross-section cable cores;
Figure lA is a part cross-sectional view similar to Figure 1 of a
prior art die arrangement employed for coating
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sectora:L cable cores;
~igurcxs 2A and 2J3.s}low respectively a part cross-
~i sectional ~ide elevational view and ~n end elev~tional view
- of a first embod:iment of the invention;
Figures 3A and 3B show views, .simila:r to those of
Figures 2A and 213, of a second embodiment of the invention;
igllra 1~ shows a third embocliment of the invent:ion
in part cross-secti.onal view; .and
~ Figures 5A and 5B show cross-sectional and endi lO ele~ational views of a fourth embvdimsnt of the invention.
- In the accompanying draw:ings, Figures l and lA
represent prior art arrangements shown in axial cross-section.
The arrangement of Figure l has been employed by us for
~ eoating a circular section aluminium conductor with an
J 15 extruded plastics material and the arrangement of Figure lA
``~ has been used for coating a sector shaped conductor. As
~ shown,- an inner male die l (hereinafter referred to as the
`~ core die) is juxtaposed with an outer female die 2 (herein-
. . after referred to as a ring die) .co as to define between the
x 20 surfaces of the two dies an annular sectioned conical
;q~ passage for admission of the extrudateO
~.
.Figure l represents a prior art arrangement which
. has been used by us for extruding a thermoplastics coating
onto a circular section aluminium core. The orifices of
male and female.dies l and 2 respectively are of circular
section, with the male die orifiee diameter made to closely
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aeeommodate the eore to a high degree of tolerance. The
female die is formed with a land lO (tl1at is, an extcn4:ion
~ beyond the end of the malo ~:io l wh:i.ch contactc; and shr1pes
; 30 the extrudate onto l;he core) wh:ich, :iXI somo ca,~os, has boon
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1~77~
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of considernblo lellgth. lt can be seen rrot~l the F:igure thrlt
the dies 1 ~nd 2 are ~esigr~ed for Laminar extrudate flow
with the entrance angle of the extrudate passing through
the flow passage 3 to tlhe core conventionalLy ~esigned for
a substantia]ly tansential or at least acute approactl of
the e~trud~te to -the core. Other details of the die
arrangement carl oe seen from the Figul~e and the criteria
cons:idered in de~iving this design have been men~ioned
! previous]y herein. Not only does the manufacture of the
dies involve close ~olerance machining, but also the setting
up of the dies for operation and the maintenance of the die
settings is a demanding process if consistent coating of
the core is to be obtained. As has been mentioned pre-
viously, these disadvantages are considered to be very
significant. The dLsadvantages are even more serious when
~ dies O:r the general type illustrated in Figure 1 are used
`3 for the coating of cable cores of other than circular c~oss~
sec-tion, for example sectoral cores; in these cases, it
has again been conventional -to form the die orifices as shown
in Figure lA to conform closely to the cross-sectional shape
of the core.
Described hereinafter, and illustrated in the
1~ - accompanying drawings, are exemplary die sets according to the
invention which have proven to provide improved results as
compared with the prior art dies just described. As will
become apparent from the following descriptions, the dies
.~ . .
according to the invention depart signif:icantly rrom the
; established design criteria prevLously disclls~socl in vnrLous
respects. ~or example, wllereaLt tho core dLe orLficc is
1 3O shaped to corrospond to the cross-f~oc-l;:ionil] shape Or I;IIc cal~:Le
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~:97~788~ . 1
core to be coatecl, for examp.Lc tllo core d:ie o:r:iri.co :is sector
shaped for a soc-tor sllaped core, the r:ing die ori~i.ce is in
eac1l case c:ircular regardless of -tllo shapo of the core die
orif:i.cc and is vory cons:i.derabl.y :Larser -tllan the core die
orifi.ce. Also, in the die sets accordin~r to the inventi.on
hore:inafter~descriibod, there is no question of the provis:ion
~of any ormation such as the land 10 i.n the prior art die
arrangement of Figures 1 and lA. ~d the:r the ring d:ie orifice
is defined, typically in the describod embodiments~ by the
junction between frusto-conical inlet and outlet sec-tions of
the ring die diverging in opposite diroctions from the die
crifice, the die orifice naving substantially no extent in
; the axial direc-tion of the frusto-conical sections. It will
be noted that in the embodiments of Figures 2A and 2B, and
Figures 3A and 3B the two frusto-conical sections defilling
the ring die orifice are both of the same transverse dimension
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at the orifice location, whereas, with the embodiments of
Figure 4 and Figures 5A and 5B, the corresponding dimensions
~ are diferent; ~n Figure 4, the outlet section of the ring
-.~ 20 die commences with a substantially larger d:imension than the
-, terminal dimension of the inlet section, and in the embodiment
of Figures 5A and 5B the opposite is the case. lt will be
- apparent from these differences, all of which exhibit sub-
stantial depart-ure from es-tablished prior art die design
cri-teria, that many modifi.cations and varia-tions are possib].e
. within the general ambit of the invention and that the
described and illustrated embodiments are i.llus-trative only
of the inventi.on and aro not intended -to :Lim:it tho scopo of
the invention.
I 30 I~ ures 2A and 2n show a firc;-t elnbo~:iment o:~ tho
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1~97781~9 ,
i.nvontion in crog~-sect:iona:L vi.ow, the dios il:lustratod bei.ng
- adapted :~o.r the co~tin~ of a sector shapcd ~luminium c~re.
It will be seen that the core die di~spenses wit~ the carefu:Lly
,angled entrance previously considerod essential f'or acllieving
. 5 ~aminar extrudate flow and, furthermore, in .t,he outer die
`" not only is the die land dispensed w:ith but in f~-,t a small
rel.ief an~le is provided on the downstrearr. side of the die
orifice. l'he spatial relationship of the two dies provides
, a ~low path fo:r the extrudate which is the antithesi.s of the
`lO stroamlined flow previously consiciored essential. A further
. considerable departure from 1;he priGr art resides in the fact
that whilst the orifi,ce of the core die is shaped to conform
`' to the sector shaped cross-section o-L the core, the outor
`~ die has a simple circular orifice which is substantially
,~ 15 larger than the core die orifice. Operational trials with
'i, the dies of Figures 2A and 2B have successfully produced a
~ covered core in which in dependence upon the ring die diameter
`d D and the pressure setti.ns P, the ratio of the thickness of, the coating at the corners of the core to its average
'~` 20 thickness was between 80 and 880/o.
j A modified form of the die shown in Figures 2A and
2B is illustrated in Figures 3A and 3B. The modified
arrangement employs an increased extrusion angle as compared
to the arrangement of Figures 2A and 2B and in trials with
this arrangement, the above mentioned ratio of corner thickness
. to averase thickness has been increased to 93.5%.
~ Figure l~ shows an arrangernent according to tho
,: invention for coat:i.nS a c:ircular ,soct:i.on co:ro. '~ :is li~:isure
i can usefully be contrasted wi.-th -the prlor art ~rransolnont
¦ 3 repre~ented in Fisure 1. Tri~ls w:i.th the d:ie.s shown :in
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1077889 ,,
Figure 4 have indicated that extended trouble frce production
runs are possib]e.
~ igures 5A and 5B show a further arrangemel1t for use
in coating a sector shaped core. The core dic orifice is
shaped to conform to the cross-sectional shape of the core,
but the outer die orifice is a simple circular shape consider-
ably largcr than the core die orifice. The departure of ~e
design of the dies shown in Figures 5A and 5B from the
criteria previously mentioned herein is self-evident.
In use of the dies according to the invention, the
basic princip]e is to extrude a tube of constant wall thickness
around the advancing core, and then to allow the fluid pres-
sure of an accompanying curing or strengthening stage ~ com-
pressthe ext~udate onto the core. Whereas with prior art
extrusion dies of the type shown in Figure 1 the orientation
of the outer die profile in relation to the inner die pro,file
is critical sincethe insulation thiclcness is totally con- ;
- trolled at the exit point of the covering material from the
dies, the method employing the improved dies of the invention
is subject to the form of the dies only in that the uniformi*y
of the coating depends upon the relative concentricity of the
in~er and outer dies. The thiclcness of the covering obtained
is dependent only upon the pressure of the extruded material
and the rate at which the material is drawn off by the mov;ng
cable core.
In use of the improved die arrangements accordin~
-to the invention, a controlled amount of draw down is desirable,
and with the pressure of the external fluid treatment medium
allowed to form the extrudate onto the core, it has been
found that the strength of the extrudate material is fully
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suf:ic:ient in it.seli'-t,o p:revent; any bac,lc f.l.ow (~f tlle ox-i,ru(late
betwe0n -the core alid the core d:ie. T'he prob].em of 'baclc
extrusion has been particularly seri,ous with pri.or ar-t die
arrangements and very complex and expeM.c,ive m~asureAq have
S been proposed for avoirlins this prob:l.eln. ~or example, a
! pressure differential techni.que has ~een evolved and is
~disclosed in British ~aten-t Specificati,Gn 1201528~ In
accordance with th,is technique, the tendency for extrudato
~ to be forced bac'k th:rough the core die, under the action o-f
j 10 the pressure applied at the outlet of -the extrudel^ head in
order to collapse the coating firmly npGn the core, is resis-ted
by injecting f]u:id into the core die so as to provide a back
. pressure. The fluid injected into the coredie is maintained
at a lo~er pressure than that of the f],uid at the extrusion
orifice, the magni,tude of the pressure differential thus
~, created being arranged -to be sufficient to collapse the
`~ co~ering material firrnly onto the core but to be insufficient
', to force the extrudate mat~rial baclc through the core die.
~ Whilst this method has been successful, it necessitates
.i 20 the provision of a pressure seal at the upstre~m end of the
core tube (leading to the core die) through which the core
to,be covered must pass with a consequent risk of sustaining
damage. It, further requ:ires the provision of a meansfor
injecting pressurised fluid into the interior of the core
tube and a means for controlling and maintaininS the pressure
' differential. The complexity and expense of the equipment
. ' required -to perform this technique is not insi.$ni.ficant.
It has beerl found that by u.q:i.n~ die arrangolnontC; accor(ling
to the invention increased clea:rances bo-twoen tho core die
! 30 and the cable core can safo:Ly 'be use(l w,i,thout :r:islc of l)ack
.
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extrusion. Th:i.s inc.reased c l.ea:rclnco :i.s add:it:iora'l.l.y
advantaseoll.s ln tha~ i-t groatly re~'luce.s the risk o~ tlle
'' core jammi.ng in -the core die and more readily allows tho
throughput of sligh1;1y distorted cores, welded ;joi.nts arld
materia], having slight'ly cli:rferi,ng pro.f.'i.lcs.
By use of the d:i.e arrangements accord:ing to the
~ invention as described above, ]ong con~inuous production
--I runs providing a cons:istently h:igh quality output can be
.
enabled. ~ sig:nificant reduction i.n machine down time and
in the producti.on of sub-standard product is obtainable.
' Furthermore~ by abandonmerlt of most of the established d.ie
design cr:iteria, the n.anufacture of dies is much facilitated
with consequen-t reduc-tion i.n tooling costs.
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