How Copper Tube is Manufactured
The first step in the manufacture of copper tube is to cast molten and/or refined copper into large "logs." Casting can be done by either the continuous or semi-continuous method.
In continuous casting, metal is poured into round horizontal graphite molds which are water-cooled to freeze the copper as it passes through them. Gripping devices withdraw the copper from one side of the mold as more molten copper flows into the other end. Slowly, a solid log of pure copper about 12 inches in diameter is formed. A moving saw cuts the log into two-foot sections weighing approximately 400 pounds. These sections are known as billets.
When the casting process is carried out vertically, it is called semi-continuous casting because it has to be interrupted when the length of the billet reaches the depth of the pit beneath the molds. Some tube makers use molds containing a central water-cooled core so that the log emerges as a thick-walled pipe called a tube round.
If the 418,000 tons of scrap copper recycled each year were drawn into half-inch tube, the strand would circle the Earth more than 16 times!
The billets are next heated to approximately 1535°F (835°C) to make the copper pliable. A pointed rod called a piercing mandrel is driven lengthwise through the center of the billets to create what will eventually become the inside wall of the plumbing tube. This step isn't needed if the billets are cast as tube rounds. Piercing can take place either immediately before or concurrent with extrusion.
The billet, again heated to the proper hot-working temperature, is placed in the chamber of an extrusion press. The horizontally mounted chamber contains a die at one end and a hydraulically driven ram at the other. The face of the ram is fitted with a dummy block and, in some cases, with a piercing mandrel. If the billets are already hollow, the ram includes a rod (or mandrel) that is slightly smaller than the hole in the die at the opposite end of the chamber. As the ram moves forward, the copper is forced over the mandrel and through the hole in the die, causing a long hollow tube about 2 inches (70 mm) in diameter and 87 feet (26 m) long to squirt out of the extrusion press. It's just like toothpaste, only hollow.
Metal near the surface of the billet extrudes backwards over the undersized dummy block, forming a shell. This shell contains the oxidized surface layer of the billet. It is recycled to the refining furnace.
In some extrusion presses, the tube exits backwards through a hollow ram. This so-called "indirect" extrusion process offers certain manufacturing advantages. In either case, the extruded tube is cleaned to remove surface oxide scale and prepare it for the next stage in the tube-making process.
Drawing involves pulling the hollow tube through a series of hardened steel dies of gradually decreasing diameters. Before each step of the drawing process, the tube is pointed at one end to fit through the next smaller die. It is then gripped by automatic jaws attached to a rotating, 7-ft-diameter drawing machine called a bull block.
Before drawing, a tapered plug mandrel is placed inside the tube. (Floating plugs are used with bull blocks. Stationary mandrels are used for relatively short lengths of tube that are drawn on linear drawbenches.) As the tube is drawn onto the spinning bull block, the mandrel and die act together to reduce both the tube's outside diameter and its wall thickness. The mandrel also makes the tube's inside surface smoother.
Tube that is to be sold in straight lengths is passed through a series of straightening rolls. For tube sold in coils, the rolls are set so as to impart a bend of appropriate radius to the tube as it emerges.
Straight tube is sold in the as-drawn or hard condition. Coiled tube is passed through an annealing furnace operating at 1300°F (704°C). Annealing can also be performed in batches aptly called (for their shape) bell furnaces. Annealing softens the tube so that it can be bent to shape during installation. Annealed tube can be distinguished from hard-drawn tube by its matte surface finish. Aside from their appearance and stiffness, however, annealed and hard-drawn tubes have the same qualities and act identically when they are in contact with properly treated drinking water.
Finally, the tube is cleaned to remove any traces of drawing lubricants or other contaminants. Samples of the finished tube are analyzed at regular intervals to ensure that it meets all requirements of size, wall thickness and quality as required under NSF 61, ASTM B88 and other applicable standards.