Copper Wire Rod Process

Graphite-Based
Technology in Continuous
Casting of Re-Draw Rod
for Multi-Wire Drawing
Machines
by Sir Michael Nairn - Chairman
Rautomead International Limited - Scotland, UK

Background prevents these from dissolving in the formation of oxides along the grain
copper matrix and adversely affecting boundaries which, as mentioned earlier,
Prior to the 1970s, most copper rod was conductivity and annealability of the is often a cause of wire breaks later on.
produced by a batch process involving rod and drawn wire. Conversely,
casting of wire bars, hot rolling, however, the presence of occluded Unlike Cu-ETP, which is cast as a billet
pickling and butt welding to form oxides in the copper wire rod, while and hot rolled, a modern oxygen-free
longer lengths. In the thirty subsequent solving one problem, can lead directly copper plant is generally designed to
years, this traditional process has been to another, as oxides tend to form hard cast directly at 8mm diameter, as the
progressively superseded by continuous particles and lead to wire breaks in fine accepted input size for wire breakdown
billet casting and hot rolling by the wire drawing. throughout the world. In terms of
familiar Contirod, Properzi and performance in subsequent wire
Southwire processes, as the accepted Oxygen-free drawing, it is significant that rolling is
means to produce electrolytic tough high conductivity - Cu-OF eliminated as a process stage.
pitch copper (Cu-ETP) rod. With good
process control and careful grading of Cu-OF and its sister material Cu-OFE Why oxygen-free ?
coils during the production run, the best have minimum copper contents of
quality Cu-ETP rod has conductivity 99.95% and 99.99% respectively. The question is often asked “Why
characteristics in excess of 101% IACS Alongside the advancing technology of oxygen-free copper ?” “What are the
and is capable of being drawn to Cu-ETP rod production, other benefits ?” “Who needs it ?” The facts
superfine wire with an acceptable rod- companies developed processes are that by comparison to Cu-ETP, the
break performance. Characteristically, for production of Cu-OF rod. market for Cu-OF is relatively small,
these are however big plants in the Characteristically, these types of plant but it is growing. Cu-OF will serve any
output range 50,000 to 200,000 tonnes are smaller than the well-known of the purposes fulfilled by Cu-ETP, but
per year, which must be run close to Cu-ETP plants, with outputs in the has some additional properties of
capacity to be economically viable. range 2,000 to 30,000 tonnes per year, importance in particular applications.
implying a significantly lower
Electrolytic tough pitch - Cu-ETP investment. As the name of the material The absence of occluded oxides at the
suggests, there is little or almost no grain boundaries in Cu-OF results in a
Cu-ETP has a minimum copper content oxygen present in the redraw rod. There more ductile material. This can be
of 99.90%. Oxygen is intentionally is thus little or no oxygen to react with important in such applications as in
alloyed with the copper in production of impurities which may be in the copper aerospace, automotive wiring
Cu-ETP and is controlled to around and to avoid these becoming dissolved harnesses, robot arms and other similar
200-400ppm. The oxygen acts as a in the matrix of the copper. Production applications, where the copper
scavenger for dissolved hydrogen and of oxygen-free copper is thus more conductor core of a wire or cable is
sulphur and will react also with most demanding of the quality of the in-going subjected to repeated flexing. The same
other impurities, to form insoluble cathode feedstock used in the process. characteristic of Cu-OF produces less
oxides at the grain boundaries. This Conversely, there will be little or no “noise” and is thus of increasing interest

EuroWire - November 2001

 Copper Wire Rod Process

in such applications as high quality The plants are smaller and more In reality, a wire break can be the result
sound recording systems and in versatile than the characteristic large of a combination of the two sources.
headphones for both military and Cu-ETP plants. Alloyed coppers can be Detailed studies into wire breaks
civilian use. The elimination of the risk produced, such as Cu-Ag, which is occurring in Cu-ETP in drawing to
of hydrogen embrittlement in copper often used in commutator section of 0.05mm have been carried out recently
through the use of Cu-OF is an electric motors on account of its higher in USA, involving examination and
important characteristic in welding softening temperatures; Cu-Sn, Cu-Cd categorisation of over 2,500 wire break
applications. Creep resistance of Cu-OF and Cu-Mg as trolley wire alloys. samples[1].
is also superior, which favours its use in Changeover is relatively simple and
such applications as trolley wire. downtime minimised. Rods of different This work established that over 90% of
Another important market trend which sizes can be made simultaneously. all wire breaks were particle failures
is favouring Cu-OF is the explosive Investment is much less than for a and that within this total, over 50%
growth of the market for electronic typical casting and hot rolling plant to were ferrous inclusions and over 30%
devices and the progressive produce Cu-ETP. refractory inclusions. The dominant
miniaturisation of electronic wire failure gauge occurs at around
components. This has resulted in a Wire breaks in drawing 0.1mm (38 AWG) and a defect particle
demand for ever finer wire gauges, size of 0.05mm seemed to be the
where now wires of 0.050mm diameter More and more attention is now being statistical mean causing this. Using
are not unusual. Demand is increasing given to the causes of wire breaks in scanning electron microscopy, the
for wires of 0.030mm diameter and less. drawing, as with miniaturisation of principal elements causing wire breaks
Fine wires are now being regularly electronic components, demand for in descending order were found to be:
drawn on multi-wire machines with up finer and finer wires increases. The use • Iron: .......................................25.6%
to 32 strands. Wire break performance of multiwire drawing machines, • H-13 tool steel: ......................11.0%
is critical to production efficiency. The processing up to 32 strands • Silicon, aluminium: .................9.5%
purity of the metal and the absence of simultaneously greatly increases the • Off-centre hollow: ...................8.3%
oxide particles in the structure become efficiency of the fine wire drawing • Silicon:.....................................7.6%
matters of serious concern. Here too, process, but only so long as wire breaks • Slag:............................................1%
Cu-OF has distinct advantages over Cu- can be routinely avoided. Conversely,
ETP. In production, the cast material the whole machine must be stopped and Discussing these in sequence
exits the casting die and cooler rethreaded when a wire break occurs.
assembly at around 80°C, well below iron occurs in many continuous casting
the surface oxidation temperature. Cu- It is common practice to divide wire plants, as well as in wire drawing
OF thus has a very thin layer of surface breaks into two basic sources, from rod machines, making it difficult to identify
oxides, significantly less than Cu-ETP. manufacturing and from wire drawing. the source. However, contact with
The product thus lends itself to It is not always a simple matter to ferrous materials in the Rautomead RS
production of fine magnet wire and as a identify the source, but it is important to system is limited to the profiled
feedstock for continuous extrusion. study the process sequence as a whole. withdrawal rolls and coiler rolls, both of
which are made in specially hardened
▼ Model RS 3000/8/8 copper rod casting machine - 6,000t./year
steel and very unlikely to contaminate
the surface of the copper rod. No hot or
cold rolling occurs and the rod is coiled
on wooden pallets. Wire drawing is also
a source of iron contamination, though
in superfine wire drawing equipment,
capstans are now often made in ceramic
materials and guide pulleys and contact
sheaves in plastic.

H-13 tool steel can generally be traced

back to rod mill rolls and guides in the
Cu-ETP plant and is likely to have been
introduced in the hot rolling process
stage. No rolling takes place in the
Rautomead RS process for production
of 8mm redraw rod, so that this source
of wire breaks is eliminated.

Silicon & aluminium are typical

refractory materials used in fabrication
of conventional induction furnace
linings and hot metal launders. It is a
unique advantage of the Rautomead RS
graphite furnace technology that
graphite takes the place of fritted

EuroWire - November 2001

Copper Wire Rod Process

alumina ceramic furnace linings and is A synthetic graphite is used, die is very important in continuous casting
thus substantially reduced as a possible manufactured from carbon based and is the most significant property
source of this major cause of wire breaks. materials, rather than the natural mined governing heat transfer.
More details of the graphite material and substance. Synthetic graphite is superior
its properties are given below. to the naturally occurring material, The grade of graphite commonly used
which can suffer from mechanical in the fabrication of dies has a thermal
Off-centre hollows is a condition in weakness associated with impurities conductivity value in the range
Cu-ETP production, where a small and ash content. Under non-oxidising 100-200W/m K.
particle of refractory is introduced into conditions, synthetic graphite is the
the molten metal flow, floating just highest temperature-stable elemental Reaction with liquid copper
below the surface as the metal solid known. This is related to its high
solidifies. A skin of copper is created binding energy and its heat of Copper (together with tin, gold, silver
over the defect, which then ruptures in sublimation. In manufacture, it involves and lead) is virtually inert relative to
drawing. Again, the graphite furnace powder processing technology on a graphite at the temperatures necessary
technology obviates this risk. large scale. Carbonaceous fillers are for continuous casting.
bonded with carbon-yielding binders,
Silicon and slag can collect at the cured at around 1,500°C and sintered Investigation has shown that the
surface of the melt in a Cu-ETP plant together at temperatures of the order of solubility in wt. % C, is about 0.0001 at
and wash into the cast. The principal 3,000°C. In the process, the temperature 1,100°C, 0.00015 at 1,300°C, 0.0005 at
components are copper oxides, with the of the baked carbon is gradually raised. 1,500°C, and 0.003 at 1,700°C.[2] As
inclusion of silicon, aluminium and Graphitisation commences at carbon does not diffuse through solid
iron. Again, in the Rautomead RS approximately 2,200°C. This process copper, solubility is exceedingly
process, the transfer from the melting slowly transforms the baked carbon, a small.[3] Claims that the copper
chamber through to the casting chamber very hard, abrasive material with low processed in the Rautomead system can
of the crucible is through the base, so thermal and electrical properties, into somehow become contaminated by
that this risk does not arise. graphite, an allotropic modification of carbon can thus be easily dismissed.
carbon, crystallising as hexagonal
The Rautomead platelets. This material possesses Graphite crucibles
RS Upwards Vertical Process excellent lubricity and high thermal and
electrical conductivity. Crucibles are machined from solid
Rautomead has been building blocks of vibration moulded graphite. In
continuous casting machines based on The bulk filler material is carbon, the case of integrated plants for the one-
the use of graphite furnace technology carefully milled to a specific particle step conversion of copper cathode
since the late 1970s. The technology was size and combined with an appropriate plates to Cu-OF redraw rod, these
adapted to production of Cu-OF in the volume of binder. The binder used in the crucibles are of a twin-chamber design,
early 1990s, when the company saw an graphite industry is coal tar pitch, the where the cathodes are lowered and
opportunity to improve on processes product of destructive distillation of melted in a melt chamber, with a bottom
available at that time and particularly to coking coal. The filler material is port to a separate casting chamber. The
use the naturally reducing characteristics calcinated petroleum coke. The precise significant mass represented by the
of graphite to best advantage. type and volume of filler, particle size heated crucible itself, forms part of the
and quantity of binder all have a marked potential energy of the furnace design
The process uses a single furnace in influence on the density and final and contributes significantly to furnace
which to melt, hold and cast the copper. physical properties of the product. The temperature stability.
This contrasts with other systems, which aggregate mix is ground to a specific
almost invariably use ceramic refractory particle size and either moulded or
lined, induction-heated furnaces, often extruded into “green” blocks. Vibration-
positioned in tandem, one for melting moulded blocks are now preferred for
and the second for casting, with the large crucible manufacture, being more
copper being poured from one to isotropic than extruded materials. High-
another. quality die grade graphite is made by
isostatic pressing.
Graphite
Properties
Characteristics which favour the use of
graphite include the purity of this The tensile strength and elastic modulus ▲ Graphite crucible
material as elemental graphitised of graphite increase with increasing
carbon, its machinability, its thermal temperature, up to 2,400°C. Graphite The flow of molten copper is thus
conductivity, its naturally reducing exhibits very high thermal shock necessarily first-in-first-out and the
function, whereby oxygen present in the resistance - orders of magnitude higher design incorporates a graphite filter bed
molten metal will react with the carbon than most high-temperature ceramic at the base of the casting chamber to
and be eliminated from the melt and its refractories. High-strength graphite ensure that the de-oxygenating process
excellent high temperature stability and with a bulk density of around is complete, before the metal enters the
strength. Graphite is not wetted by 1.8gm/cm3 has an open porosity of around casting dies at the top of the casting
copper in the molten state. 8%. Thermal conductivity in the casting chamber.

EuroWire - November 2001

 Copper Wire Rod Process

picked up and fed under control to the

melting chamber of the graphite crucible.
The cathode passes through a layer of
graphite flake, which acts as a hot metal
cover and protects the surface of the
molten copper from oxidation.

The liquid copper sees only graphite

contact surfaces. Crucible design is such
that the copper must follow a first-in-
first-out path, downwards to the transfer
port and then upwards through a graphite
filter arrangement to the casting dies. The
casting dies themselves are immersed
through the layer of graphite flake cover
in the copper, where the metallostatic
pressure created is used to channel the
liquid copper into the die and cooler
arrangement. LME grade A cathodes
entering the process may be expected to
contain about 60-80ppm oxygen. This is
reduced to 2-3ppm in the process. No hot
metal transfers between furnaces take
▲ Furnace cut-away elevation drawing place, thus avoiding the risk of hydrogen
pick-up at that stage of the process.
Moisture vapour entering such a gas
generator from the atmosphere, Furnace heating is by electric resistance
particularly in areas of high humidity, is by means of a chain of graphite heating
a major potential source of hydrogen in elements positioned adjacent to the wall
the melt and of consequent gas porosity of the graphite crucible, with the heat
in the rod. With no hot metal transfers being transferred to the copper by
taking place in the Rautomead system, radiation and convection. The This
this source of possible hydrogen gas results in a still metal bath and an ideal
inclusion is effectively eliminated. condition from which to cast.

As furnaces have become larger,

Rautomead has recently carried out
extensive finite element analysis work in
finalizing the design of graphite crucibles
in association with the University of
Strathclyde, Glasgow. This work has
enabled performance at temperature to be
carefully studied and any points of high
stress in the design to be eliminated.

▲ Finite element analysis crucible study Rautomead process design

A ceramic one-piece liner is fitted on A pick-and-place mechanism is used to

top of the crucible to protect this from lift the cathodes and to transfer them to a
erosion through exposure to atmosphere tilting table, which turns them to the
and from physical abrasion. A layer of vertical position, from where they are
high purity graphite flake is used to
protect the surface of the molten metal ▼ Cathode feed
▲ Heating element chain
and an inert gas atmosphere is
maintained inside the furnace to protect
the graphite hot-working parts from Casting is through water-cooled graphite
erosion. casting inserts and backer inserts.
Withdrawal is via two pairs of profiled
In other systems utilising separate nip rolls, using pneumatic pressure and
induction melting and holding furnaces, rapid pulse indexing drive. Separate
a charcoal-based carbon-monoxide gas drives are provided for either side of the
generator is often used to protect the machine, making it easy to cast different
molten metal from oxidation, as it is sizes of rod simultaneously and selecting
poured from one furnace to another. the casting speed best suited to each size.

EuroWire - November 2001

Copper Wire Rod Process

Rod surface temperature at the cooler

assembly exit point is less than 100°C.
The rods are then fed over nylon
support rollers and down to rod coilers
via a dancing arm speed control station.

The whole process is PLC controlled

and is designed to monitor and manage
all the major process variables without
the need for human interference.

From this brief description, it will be

clear that in the Rautomead process,
there is no exposure to alumina or other
types of frit ceramic furnace linings and
no pouring of liquid metal from one
furnace to another. On the other hand, a
completely still metal bath is created
from which to cast oxygen-free copper.
▲ Temperature trace

and heated to approximately 200°C developed in the early 1990s, the target
before being fed to the furnace. casting speed was 3.0m/min, to give
80kg/per strand per hour. This balanced
The effect of the cathode preheater is to with a melting capability of around
dry off any surface moisture, 700kg/per hour for the machine in an
condensation or surface-trapped eight strand configuration.
electrolyte and thus to ensure that
hydrogen from those possible sources is Recently, improvements in rod cooler
eliminated. It avoids metal splashing at design and in mechanical withdrawal
the feed point and contributes to the have permitted increases in casting
overall melting burden of the furnace speed to 5m per minute. An eight strand
itself. machine today is now rated at 500KVA
and is capable of over 1,000kg per hour.
Improved temperature control
Feedstock and impurities
Maintenance of stable production
parameters is one of the most important This paper would be incomplete
▲ Rod withdrawal criteria for production of good and without a section on the selection of
consistent quality copper redraw rod. feedstock. Except in the removal of
Cathode preheater oxygen, the Rautomead Cu-OF process
Among these parameters, none is more is not a refining one. Thus, the quality
A recent innovation in some of important than stable control of and cleanliness of the feedstock used is
Rautomead’s copper rod casting temperature at the point of casting. In of critical importance in production of
machines has been to incorporate a the Rautomead process, operating good quality copper redraw rod.
cathode preheater. This is part of the temperature is measured by two
cathode feed system and operates on the thermocouples, one of which is The specification set by Rautomead calls
principle of an electric toaster, whereby immersed in the molten metal, close to for the use of LME grade A cathode (Cu-
each cathode is lowered into an the casting dies and the other in the CATH-1). Impurities generally enter the
electrically powered preheat furnace furnace, in close proximity to production chain at source. Cu-CATH-1
the crucible wall. Using specification is, however, relatively
▼ Line sketch of cathode preheater a cascade temperature broad and permits the presence many of
control arrangement, this the elements which can be deleterious in
enables a tolerance of wire drawing. Maximum impurities
±3°C to be automatically permitted are 0.0065%, although most
maintained at the casting cathodes produced have significantly
dies. lower impurities at around 0.002%.
Therefore, it is not sufficiently precise
High-speed rod for the user to specify grade A cathode,
casting as the feedstock. Rather, a more detailed
appraisal of both nominal composition
When the RS upwards and of manufacturing consistency must
vertical casting process be made in the selection of suitable
for 8mm Cu-OF rod was cathode feedstock.

EuroWire - November 2001

 Copper Wire Rod Process

Composition - cathode cathode should be

and wire rod treated with caution.
There is no substitute
Within the total allowable level, for taking rod samples
individual impurities or groups of after casting. This takes
impurities may also be close to the into account any mixing
maximum permitted individual or group of cathode during the
levels (see Table 1). process.

Composition is influenced by the Hydrogen levels in

mining source of the copper ore and by molten copper are easily
the technology of the refinery where the raised where oxygen
cathode is produced. The most critical levels are low. The
impurities in relation to wire drawing, presence of hydrogen
graded as to their influence on wire may create voids in the
breaks are as follows: matrix structure, which
• Severe effect: Bi Te Se (cause grain may close down or may
boundary cracks) propagate during wire
• Deleterious: Pb As Sb S (cause grain drawing. It is important
boundary cracks) that dry cathode is
• Low effect: Cr Fe Sn P Si Ag (affect charged into any of the
annealability) production process. A
further source of
Hydrogen also accentuates the hydrogen may be from
detrimental effect of other impurities the electrolytic
which may be present at grain deposition process used
boundaries. Care must be taken to in cathode production.
eliminate as many sources of hydrogen Electrolyte can become
ion from the process as possible. trapped in the cathode
Cathode selection and handling, furnace and enter the melt. The
atmosphere, as well as melt surface electrolyte contains
protection, must all be carefully thiourea and glue, both ▲ Table 1
regulated to minimise hydrogen pick-up. of which are high in
hydrogen ions. Cathodes should be benefits of this rod production
Procedures for testing cathode for selected which have been shown to have technology will play an increasingly
impurities during production vary low levels of trapped electrolyte and important role in association with the
between cathode producers. Some take should be stored ready for use in a dry adoption by wire and cable
20mm diameter samples either from environment. Care must be taken to manufacturers of multi-wire drawing
each or from a proportion of cathodes. avoid surface condensation on cathodes machines and enable them to obtain the
The samples are melted and analysed. exposed to changes in atmospheric best efficiency from these machines. ■
However, chemical composition is temperature and humidity before use.
recognised as not necessarily consistent Acknowledgements
over the full area of a cathode. A true Summary
representative analysis requires a full 1] Masoud Garshasb and Timothy J Day,
“Quality Enhancement of Ultrafine Magnet
cathode to be melted. As this is rarely It can thus be shown that both in the Wire utilizing Failure Analysis”, Conference
done, quoted chemical compositions of selection of materials and in the process Proceedings of the Wire Association
design, the Rautomead International’s Wire & Cable Technical
RS upwards vertical Symposium, 2001, pp 220-226.
casting process 2] M. B. Bever and C. F. Floe, Trans AIME, 166,
1946, 128-141
effectively prevents Solubility of Carbon in Copper
many of the common 3] W. Baukloh and F. Springorum, Z. anorg.
rod manufacturing- Chem., 230, 1937, 315-320
related sources of wire Solubility of Carbon in Copper
breaks and offers users
real advantages in
processing to fine and
superfine wire. It does
not however eliminate
the need for careful Rautomead International Ltd
PO Box 100
selection of feedstock
Dundee, Scotland DD1 9QY - UK
cathode and similar Fax: Int’l +44 1382 622941
detailed attention to all E-mail: sales@rautomead.co.uk
stage of the wire Website: www.rautomead.co.uk
drawing process. The

EuroWire - November 2001