Copper tubing for radiators, heat exchangers and refrigeration circuits is produced through a small number of well-established process families. They can be grouped into two categories, hot processing and cold processing, and the choice between them determines the grain structure, the wall thickness tolerance, the achievable coil weight and the cost per metre of finished tube.
Two Families of Copper Tube Processing
Hot processing covers the operations that turn cathode copper into a billet: extrusion to produce a hollow shell, inclined roll piercing and continuous casting and rolling. All of them deform the metal at a temperature well above the copper recrystallisation temperature. Cold processing then takes that billet and reduces it at room temperature by cold rolling and cold drawing to the final tube dimensions. The traditional combination of hot extrusion billet plus cold finishing has been used for decades and remains the basis of most seamless copper tube production. Cold working also raises strength and improves surface finish and dimensional accuracy, because the metal is strain hardened and the tooling dimensions are stable at ambient temperature.
Extrusion-Based Billet Production
The classic extrusion route starts with electrolytic copper, which is melted and cast into a solid cylindrical ingot. The ingot is charged into a reheating furnace and brought to more than 850 C, then pushed one by one through an extrusion press to form a hollow billet. From the press the shell goes either to a cold rolling mill or to multi-pass straight-line drawing, followed by disc or coil drawing until the required dimensions are reached.
Melting and casting produce a solid ingot of controlled chemistry and low gas content.
Reheating above 850 C brings the copper into the range where it deforms readily and recrystallises during working.
Extrusion converts the solid ingot into a hollow shell in a single pass, so the process is fast and flexible in diameter.
Cold rolling or multi-pass drawing reduces the wall and diameter while improving concentricity and surface quality.
Both high-ratio and low-ratio extrusion produce refined grain structure and good surface quality, and the difference between them lies in how much reduction is taken in the press and how the shell is finished afterwards.
High-Ratio and Low-Ratio Extrusion Compared
The extrusion ratio is the ratio of the billet cross section to the extruded shell cross section. Selecting a high or low ratio changes the downstream equipment and the tolerance that can be held.
| Item | High extrusion ratio | Low extrusion ratio |
|---|---|---|
| Shell produced | Smaller diameter, thinner wall | Larger diameter, heavier wall |
| Billet size | Smaller ingot | Large ingot |
| Downstream processing | Finished directly on a continuous straight-line drawing machine or a disc drawing machine | Extended on a cold rolling mill before final drawing |
| Press tonnage | Higher for the same ingot size | Lower |
| Wall thickness tolerance | Wider, so it must be controlled at the drawing stage | Tighter after rolling reduction |
In practice the high-ratio route shortens the process chain because the extruded shell is close to final size, while the low-ratio route accepts a heavier shell and relies on cold rolling to bring the wall down. The grain structure and surface finish of both are acceptable, and the selection is made on product mix, press capacity and total conversion cost.
Continuous Casting and Rolling With a Three-Roll Planetary Mill
The continuous casting and rolling route is a newer billet method that appeared in the 1990s and has since become a mainstream supply route for thin-walled copper tube. Instead of casting a solid ingot and extruding it, the process casts a hollow billet horizontally, sizes it, cuts it to length and mills the surface, then feeds it directly into a three-roll planetary rolling mill that rolls the shell down to a smaller diameter and thinner wall.
Because the three-roll planetary mill works the tube without rotating it about its own axis, the emerging tube can be coiled directly on line. The advantages follow from that short process chain.
Reheating and extrusion are eliminated, so energy consumption falls.
Equipment investment is lower because the long press line is replaced by a compact rolling station.
Conversion cost per tonne drops accordingly.
Coil weight above 1500 kg per coil can be delivered, which the extrusion route cannot match at the same diameter.
Heavy coils create very favourable conditions for the downstream drawing operation, because fewer coil changes and fewer welded joints are needed and the drawing line runs more continuously. The route did not mature immediately: moving from initial commissioning to the point where it became the principal supply of thin-walled tube billet involved a series of technical problems, and the experience gained along the way is what made the present process stable. Continuous casting and rolling is now regarded as a mature route for volume production of thin-walled copper tube.
Downstream Finishing and Quality Control
Whichever billet route is used, the tube is finished by cold drawing, intermediate annealing and final annealing. Drawing reduces diameter and wall while generating a strain hardened structure; annealing restores the recrystallised grain structure and the required soft temper; final cleaning removes drawing lubricant and oxide.
| Stage | Purpose | Typical check |
|---|---|---|
| Cold drawing | Set final dimensions in strain hardened condition | Diameter, wall thickness, concentricity |
| Intermediate annealing | Restore ductility for further reduction | Grain size, mechanical properties |
| Final annealing | Deliver the specified temper | Hardness, tensile properties, bend test |
| Cleaning and drying | Remove lubricant and residual oxide | Surface cleanliness, internal dryness |
| Eddy current testing | Detect wall defects and cracks | Continuous test record |
| Packing | Protect the finished surface | Straight length or coil weight, marking |
Material designation follows the wrought copper chemistry system, in which T2 and the oxygen-free TU1 and TU2 grades are the common tubing choices, and refrigeration and air-conditioning tube is generally ordered to a dedicated copper tube specification such as the air-conditioning copper tube standard or its international equivalent for seamless refrigeration tube.
Frequently Asked Questions
Q: What is the difference between hot and cold copper tube processing?
Hot processing produces the billet by deforming copper above its recrystallisation temperature, for example by extrusion, inclined roll piercing or continuous casting and rolling. Cold processing then reduces the shell at room temperature by cold rolling and cold drawing to reach the final dimensions and temper.
Q: Why is the ingot heated to more than 850 C before extrusion?
At that temperature copper deforms easily and recrystallises during working, so the press can convert a solid ingot into a hollow shell in a single fast pass without the excessive force and cracking that cold extrusion would cause.
Q: What is the advantage of a high extrusion ratio?
The shell comes out with a smaller diameter and thinner wall, close to the finished size, so it can go directly onto a continuous straight-line or disc drawing machine and the process chain is shortened. The trade-off is that a larger press tonnage is required.
Q: How does the three-roll planetary mill achieve on-line coiling?
The three rolls work the tube without rotating it about its own axis, so the tube leaves the mill straight and untwisted and can be coiled immediately instead of being handled in straight lengths.
Q: Why does continuous casting and rolling cut cost?
It removes reheating and extrusion from the process chain, so energy consumption falls and equipment investment is lower. It also delivers coils above 1500 kg, which reduces coil changes and improves drawing line utilisation.
Q: Which copper tube grades are used for heat exchangers and radiators?
T2 tough-pitch copper is the common choice for heat exchange and radiator tube, while the oxygen-free TU1 and TU2 grades are selected where the tube will be brazed, welded or used in vacuum or hydrogen-containing service.
