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Processes - Non-Ferrous (Aluminum / Copper / Zinc / Lead / Magnesium): Material Processing - Casting

Casting molten non-ferrous material from a primary melting process into shapes for further processing, and/or re-melting, or direct into semi-finished parts (i.e.; die casting) encompasses a wide range of equipment.

Unlike steel where the majority of product is continuous cast, a sizable portion of aluminum and other non-ferrous materials are cast into ingots and then rolled or formed into sheet, plate, foil or other shapes.

General information for casting Aluminum, Copper, Zinc, Lead and Magnesium are included in this section. Melting, holding, and ladle or launder equipment descriptions and information that utilize natural gas are included in the equipment section.

Ingot casting is more common than continuous casting. Ingot casting, or sheet ingot, suitable for rolling, "T-ingot", pigs and sows for re-melting (by foundries), and forms of rod for drawing into wire are common shapes for casting aluminum. Ingots are also cast into round shapes (called billets) for extrusion into many products. Ingots require heating energy for reheating prior to rolling, forming or extrusion. These reheating furnaces are generally gas-fired although oil and electric induction systems exist. Aluminum ingot reheating furnaces (for plate, sheet, and foil) and billet heating furnaces (for prior to extrusion) are not covered in the module.

Vertical or horizontal continuous or semi-continuous casting apparatus systems are employed to produce a variety of bar shapes. A graphic showing the vertical type is shown below.

In this process, metal is poured into round 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. For horizontal casting (not shown) 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 (as shown), 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.

After casting, 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 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.

Three types of process are commonly used to cast zinc alloys: die-casting, permanent mould casting in steel or graphite moulds, and sand and plaster mould casting.

Several types of lead casting machines are utilized. Gravity billet casting machines (manual and automatic), ingot casting machines (also manual and automatic), rotary casting table for sections, continuous strand casting machines, vacuum billet casting machines, and continuous chain conveyor type billet casting machines.

Multi-purpose pressure die casting machines are also used to produce a variety of items including car wheel balance weights, fishing net weights and other small lead components (i.e.; battery terminals, etc). In addition, lead shot casting machines and powder solder machines are also offered.

Casting of magnesium presents a special environmental concern. Traditionally casting of magnesium ingot is completed using sulfur hexaflouride (SF6). This material protects molten magnesium from oxidation both in the melting equipment and during solidification. Replacement gases are being sought and the rapid projected growth of magnesium into automotive applications is said to hinge, in part, on finding an acceptable replacement gas for SF6.

Die Casting Note:
The basic idea of die casting is to force metal into a mold using high pressure. The metal then cools (often assisted by water cooling of the die). The mold is then opened, and the casting ejected. Molds for die casting are elaborate and are usually constructed of alloy steel in two pieces (called the cover and the ejector). The die must withstand high temperature and pressure, so the die is typically made from chromium or tungsten steel alloys. In order to increase die life, and improve throughput dies are usually cooled with water, air or nitrogen.

There are two major types of die casting machines: hot and cold chamber die casting machines. Die castings of aluminum, magnesium, brass and bronze are all made on cold chamber machines. In a cold chamber, the metal is fed first from the holding furnace into a chamber. The plunger then takes that metal and forces it into the die. Cold chamber machines are typically a bit slower than hot chamber machines.

Die casting of aluminum, copper, zinc, lead and magnesium (and other materials) account for a large percentage of the energy use in a facility. A die casting plant will utilize melting furnaces, holding furnaces, launders and / or ladles. Molten metal is fed to die casting machines either automatically (via a launder system) or manually. In some die casting facilities, metal is melted at the individual die casting machine.