From molten copper to finished product: How many processes does a copper rod undergo?
Melting · Updraft casting · Continuous casting and rolling · Multiple passes of drawing · Tempering · Finished product control
A copper rod with a diameter of 8 mm may be continuously rolled by ten or more rolling machines, or it may be directly cast from molten copper; the same copper rod, if it is to be transformed into 1 mm copper wire later, will still have to undergo more than 98% reduction in cross-sectional area.
Therefore, there is no uniform answer to "how many processes does a copper rod undergo". One must first ask: Is it oxygen-containing electrical copper or oxygen-free copper? Is it to be delivered as an 8 mm round bar, or to be further processed into electromagnetic wire, stranded conductors, micro-wires? Should it use SCR, Contirod or updraft casting?
What is most worthy of attention is not the number of processes, but how each step changes impurities, oxygen content, cast structure, surface oxidation, plasticity and final electrical conductivity.
I. First, take a look at the overview: Three copper rod manufacturing routes, the differences are far greater than imagined.
Technical route
Typical Process
Key Differences
SCR Continuous Casting and Rolling
Electrolytic Copper → Melting → Wheel Belt Continuous Casting → Hot Rolling → Surface Treatment → Coiling
Continuous production of Cu-ETP electrical copper rods, with the cast billets being hot-rolled into shape
Contirod Continuous Casting and Rolling
Electrolytic Copper → Melting → Double Belt Continuous Casting → Hot Rolling → Surface Treatment → Coiling
Using the Hazelett double-belt continuous casting equipment, but still requires further rolling
UPCAST Up-Draw Continuous Casting
Electrolytic Copper → Melting/Heating → Up-Draw Crystallization → Traction → Coiling
Can directly cast out copper rods of 8 mm and other specifications without hot rolling
All three routes have mature industrial applications. The SCR system disclosed by Southwire includes vertical furnaces, troughs, insulation furnaces, wheel belt casting machines, casting bar preparation machines, rolling machines and coiling systems; Aurubis simultaneously uses Contirod and Southwire technologies; UPCAST states that its 8 mm products can be directly cast and formed without hot rolling. [1–3]
It must be emphasized: 8 mm is a widely used specification for electrical copper rods, not the only size for all copper rods. After changes in raw materials, products and equipment configuration, specific processes may be added, combined or omitted.
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II. The First Barrier: Not all "99.99% copper" can be directly used as high-end copper wire
1. Raw Materials: High purity is just the starting point
The main raw materials for electrical copper rods include qualified electrolytic copper anode plates. Some production lines also use recycled copper that has been strictly classified, cleaned, and quality-controlled. The factory should pay particular attention to impurities such as Bi, Pb, Sb, As, Se, Te, Fe, Ni, S, P, etc. that may affect conductivity, thermal processing, or drawing stability.
"High copper content" cannot be equated with "high conductivity and stable drawing of thin wires". The subsequent processing of the copper rods must also meet requirements such as uniformity, surface quality, oxygen control, and drawability. ASTM B49-20 specifically stipulates the requirements for copper rods for power supply conductors after processing, covering electrolytic duct copper, oxygen-free copper, and some refined high-conductivity copper. [4]
2. Melting: More than just temperature control is required
Vertical furnace melting is typically matched with continuous high-output continuous casting and rolling lines; induction melting/insulation furnaces are often used for oxygen-free copper and certain alloying or flexible production routes. It is necessary to control simultaneously the cleanliness of raw materials, furnace gas, copper liquid temperature, liquid level fluctuations, oxygen and hydrogen, insulation time, and the risk of inclusions caused by refractory materials.
Aurubis introduced a certain type of continuous casting and rolling production. After the cathode copper is melted in the vertical furnace, the copper liquid at approximately 1200°C enters the casting machine through a chute; this is only the public process description of the factory's route and cannot be regarded as the fixed pouring temperature for all factories. [2]
3. Oxygen-containing copper and oxygen-free copper: "Less oxygen is not necessarily always better"
Cu-ETP (electrolytic toughened copper) requires controlling an appropriate oxygen level. Its smelting, solidification and subsequent usage conditions are different from those of oxygen-free copper; Cu-OF focuses on reducing oxygen and impurities, and is suitable for subsequent processes that are sensitive to hydrogen embrittlement and other applications with specific purity requirements. UPCAST has publicly stated that its oxygen-free copper rod production can achieve an oxygen level of approximately 1–3 ppm. [3]
Note: 1–3 ppm are public indicators for a specific technology route/product and cannot be used to infer the oxygen content of all the drawn copper rods, nor can it be simply assumed that oxygen-free copper is necessarily superior to Cu-ETP in all cable products. The selection of materials should be based on a comprehensive judgment of conductivity, cost, subsequent heating atmosphere, surface requirements, and application. [3,5]
III. The second hurdle: How is an 8 mm copper rod formed on the continuous casting and continuous rolling line?
Route A: SCR, first cast into a strip, then continuous hot rolling
The key equipment for SCR is the wheel belt continuous casting machine: the cast wheel with grooves and the steel strip together form a continuously moving crystallization cavity, and the copper liquid enters and solidifies into a continuous cast strip; the cast strip is then processed, guided, and rolled through multiple stands to finally form a circular copper rod.
The production line is usually organized as "cathode copper feeding → vertical furnace smelting → chute and insulation → wheel belt continuous casting → cast strip processing → continuous hot rolling → surface cleaning/cooling → winding". The actual number of machine stands and die configurations in the rolling section are determined by the equipment specifications and cannot be uniformly described as a fixed number of passes. [1,6]
The advantage of continuous casting and rolling is not that "crystalline holes in the cast state will definitely be eliminated during rolling", but rather that it utilizes high-temperature deformation to break the original cast structure, reduce the cross-section, and achieve high-efficiency continuous production. For oxide film inclusion, hard inclusions, and severe internal porosity, folding, inclusions bands, or subsequent drawing breakage sources may still form after hot rolling.
Route B: Contirod, with a different casting method, the finished product still requires hot rolling
Contirod uses the Hazelett double-belt continuous casting machine to form a continuous casting space between two moving steel belts, obtaining continuous strip-shaped cast billets; subsequent processing through the rolling mill results in a copper rod with a diameter of approximately 8 mm. Sarkuysan's production description clearly gives its cathode copper smelting, Hazelett continuous casting, hot rolling, acid washing/deoxidation, cooling, and winding processes.
The main differences between the two continuous casting and continuous rolling routes lie in the casting equipment, melt transfer, billet size, and the integration method of the production line. It should not be simply assumed that one of these routes is inherently "zero-defect".
Why is the surface of hot-rolled products cleaned?
After hot rolling, the high-temperature copper rods may form surface oxides and fine adherents. If not treated, the oxides may be pressed into the surface layer during subsequent drawing, increasing friction, copper powder, and mold wear. Common production line configurations include online surface treatment, cooling, drying, and, if necessary, coating protective materials and continuous winding.
A study on the copper powder on the surface of SCR copper rods pointed out that the oxide film formed on the high-temperature cast billet during the air cooling stage of the production line is an important source of copper powder problems for the studied production line. Optimizing the casting machine, rolling, and surface treatment conditions can reduce copper powder. [8]
IV. The Third Route: Why can UPCAST not undergo hot rolling?
UPCAST uses a water-cooled crystallizer immersed in the copper liquid to allow the copper to continuously solidify upwards in the crystallizer. The solidified copper rods are periodically or continuously extracted by the traction device and enter the winding equipment. Its core is to directly cast continuous copper rods close to the finished product specifications.
When manufacturing 8 mm upper draw copper rods, the typical process is "raw material preparation → smelting/holding → protective atmosphere or oxygen isolation control → upper draw crystallization → traction → winding → inspection". Since the solidification stage results in a circular target cross-section, a continuous hot rolling unit like SCR is not necessary in principle.
The advantages of direct casting include reducing the equipment and contact links related to rolling, and being suitable for products such as oxygen-free copper; however, this does not mean that the product automatically has the best structure or ductility. If the heat flow of the mold, liquid level, pulling rhythm, or melt cleanliness fluctuates, surface longitudinal lines, cracks, pores, or internal structure problems may still occur.
The real engineering choice depends on production capacity, energy, first-time yield, material purity, product specifications, maintenance costs, and subsequent drawing breakage rate, rather than just comparing "fewer processes".
5. Copper rod leaving the factory is not the end: from 8 mm to 1 mm, how many more drawing steps are needed?
An 8 mm copper rod is usually just the raw material for drawing. After entering the drawing machine, the copper rod passes through a series of molds with gradually decreasing diameters, gradually reducing the cross-sectional area at room temperature to form medium and fine-sized copper wires. Depending on the final use, intermediate annealing, online continuous annealing, tin plating, twisting or enameled coating may also be carried out.
1. Calculate the total reduction rate: not 87.5%, but 98.44%
The cross-sectional area A of the round wire = πd²/4. The total area reduction ratio R from 8 mm to 1 mm = 1 - (1/8)² = 98.4375%. This means the final cross-sectional area is only 1/64 of the original. If the volume loss is approximately ignored, the wire length will increase to about 64 times the original.
The corresponding cumulative true strain (equivalent to the logarithmic strain of the single-axis length change approximation) ε = ln(A0/Af) = ln64 ≈ 4.159. One cannot mistake the 87.5% reduction in diameter as the reduction rate of the cross-sectional area.
2. The number of passes depends on the reduction rate of each pass
Assume the average single-pass reduction rate
Theoretical minimum number of passes required from 8 mm to 1 mm*
Engineering understanding
15%
26 lanes
There are a large number of lanes, and the deformation of each lane is relatively mild.
20%
19 examples
Suitable for demonstrating the calculation logic of typical multi-pass continuous drawing
25%
15 items
The number of items has decreased, but the control of each load and lubrication is now more crucial.
30%
12 passes
Higher single-pass deformation does not mean that all materials/sets can be used
* Reduce the cross-sectional area by no more than the given value for each pass, and calculate based on the final pass allowing for adjustments; this is not a recommended industrial profile table. Actual design should also consider the tower wheel speed ratio of the equipment, the mold angle, the wire condition, lubrication, tension, and the risk of wire breakage.
Calculation formula: n ≥ ln(Af/A0) / ln(1 - r), where r is the assumed single-pass cross-sectional area reduction rate. For example, with r = 20%, theoretically about 18.64 passes are required, so at least 19 passes should be arranged, and the final pass needs to be adjusted to the target 1 mm.
3. Why is annealing needed after drawing?
As the drawing process continues, the dislocation density of the copper wire increases, and the processing hardening intensifies: strength and hardness usually increase, but the subsequent plasticity decreases. For flexible conductors or products that require subsequent winding, twisting, or special fine wires, intermediate annealing or final annealing can be performed according to the regulations to restore sufficient elongation.
The annealing schedule is determined by the material state, wire diameter, wire speed, equipment heating method, and target performance. Online annealing is not "the hotter, the softer": overheating, oxidation, surface discoloration, and improper tension control can all damage the size and finished product performance.
Subsequent products may be classified according to their uses as enameled wires, tin-plated wires, twisted cable conductors, and special fine wires. IEC 60228:2023 stipulates the nominal cross-sectional area, structure, and resistance requirements for multiple types of insulated cable conductors, not the copper billet continuous casting process standard. [9]
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Six. What truly determines the quality rate is the following eight quality control points.
Crossing point
Main detection/monitoring objects
Priority for troubleshooting in case of abnormalities
1. Raw materials
Cathode copper purity, contamination, impurity spectrum
Mixing, oxidation/oil-contaminated recycled materials, impurity introduction
2. Melting
Temperature, oxygen/hydrogen, liquid level, flue gas
Atmosphere, moisture content of furnace materials, residence time of furnace lining and molten material
3. Continuous casting
Surface of ingot, cross-section, holes, cracks
Heat flow of mold, pulling, casting stability
4. Hot rolling (if applicable)
Temperature, deformation per pass, copper adhesion on rolls
Roll wear, mold, lubrication/cooling, guide vanes
5. Online surface treatment
Surface oxide film, residual liquid, copper powder
Cleaning/deoxidation, rinsing, drying and surface protection
6. Copper rod release
Resistivity, size, torsion/extension, surface
Process inheritance defects, batch differences, oxygen content
7. Wire drawing
Breakage rate, die wear, wire diameter, wire marks
In-line quality, mold, lubrication, tension, equipment alignment
8. Annealing and finished products
Extension rate, conductor resistance, surface, winding
Temperature/line speed matching, oxidation, finished product tension
The technical article on copper wire metallurgy by the American Copper Association points out that oxides, hot cracks and edge folding of cast bars involved in the continuous casting and continuous rolling process can be transformed into longitudinal defects in the subsequent drawing process; the die marks, scratches and metal debris pressing in caused by the wire drawing machine itself constitute another source of defects. [10]
Therefore, when there is a continuous breakage at the site, the broken sections and the segments before and after the breakage should be retained; for inclusions or oxide films, cross-sectional metallographic and SEM/EDS analyses should be conducted, and the corresponding copper rods' roll numbers and casting/rolling periods should be traced. Simply replacing the molds or increasing the lubrication amount may not solve the breakage caused by casting defects.
VII. Several concepts that are most prone to confusion
Misconception 1: All 8 mm copper rods must be hot-rolled. - The upper drawing continuous casting can directly produce 8 mm round rods; SCR, Contirod, etc. continuous casting and rolling must rely on hot rolling to reduce the cross-section.
Misconception 2: A copper rod with a conductivity rate of 101% IACS indicates no defects. - Conductivity rate cannot replace the evaluation of surface quality, inclusions, torsion, plasticity, and the stability of subsequent drawing.
Misconception 3: The larger the rolling reduction, the easier the casting holes will disappear completely. - Geometric compression does not equal achieving metallurgical bonding; the oxide interface may still remain.
Misconception 4: Oxygen-free copper must be hot-rolled to have good performance. - The direct casting route without hot rolling can be used; the final performance still depends on the material state and subsequent processes.
Misconception 5: Reducing the diameter from 8 mm to 1 mm only reduces 87.5%. - 87.5% is the diameter reduction, while the cumulative cross-sectional area reduction is 98.44%.
Misconception 6: The moment the copper rod is rolled up, the manufacturing process is over. - For finished conductors, further processes such as drawing, annealing, surface treatment, and twisting are still required.
VIII. How to choose the route if a new copper rod production line is to be established?
Final product/ business goal
Preferred evaluation route
Key accounting indicators
Large-scale general-purpose copper rods
SCR or Contirod continuous casting and rolling
Energy consumption per ton, production capacity, copper consumption, on-line oxide scale, subsequent breakage rate
Oxygen/hydrogen control, surface quality, casting rod stability, subsequent drawing performance
Small-batch special copper alloy wire
Alloy melting/cast + necessary hot processing or subsequent drawing
Alloy element loss, segregation, hot processing window, cost and yield
Final goal: 1 mm wire or finer products
Select the drawing + annealing production line based on copper rods
Molds and lubrication, breakage rate, line speed, elongation after annealing, resistance
Process development should not only compare the factory price per copper rod. For copper wire manufacturers, a more valuable indicator is "total cost of qualified terminal wire per ton", which includes raw materials, energy consumption, material loss, mold loss, breakage downtime, annealing, testing and scrap. A copper rod with a good appearance in the production line does not mean that it can still maintain a low breakage rate when drawn to a fine wire.
Conclusion: Be able to count the processes, but also understand the organizational traces left by each step
The manufacturing route of an 8 mm copper rod is not unique. SCR and Contirod produce round rods through continuous casting and hot rolling, and the upper drawing continuous casting can directly solidify the copper liquid into the target specification; subsequently, copper rods for different products may enter multiple drawing, annealing, coating or twisting processes.
Throughout the entire manufacturing chain, smelting determines the basis for impurity and gas control, continuous casting determines the initial solidification structure and pore risk, hot rolling determines how the cast structure and surface defects evolve, drawing magnifies the small defects into the risk of breakage, and annealing redistributes strength and plasticity.
Therefore, truly professional production line management is not about memorizing a string of equipment names.