Metal Shaping Processes. Vukota Boljanovic
stronger and lighter than those possible with other casting methods. In addition, because die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process.
Multiple finishing techniques. Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation.
Simplified assembly. Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast.
There are two basic die casting processes, differentiated only by their methods of metal injection: hot chamber and cold die chamber.
a) Hot Chamber Process
The hot chamber process is only used for zinc and other low melting point alloys that do not readily attack and erode metal pots, cylinders, and plungers. Development of this technology, through the use of advanced materials, allows this process to be used for some magnesium alloys. The basic components of a hot chamber die casting machine and die are illustrated in Fig. 2.14. In this machine there is a main pot or crucible in which is immersed a fixed cylinder with a spout firmly connected against the die. A plunger operates in the cylinder. Raising the plunger uncovers or opens a port or slot that is below the molten metal level, and molten metal fills the cylinder.
When the plunger is forced downward, the metal in the cylinder is forced out through the spout into the die. The plunger is withdrawn as soon as the metal solidifies in the die. The die is then opened and the casting ejected. After that the die is closed and securely locked into position, and the casting cycle is repeated.
b) Cold Chamber Process
Cold chamber die casting cycle, Fig. 2.15, differs from hot chamber in that the injection system is not submerged in molten metal. Molten metal is still forced into the die by a hydraulically activated plunger.
Fig. 2.14 Schematic illustration of the hot chamber die casting process: a) die is closed, plunger withdrawn, and molten metal flows into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.
Fig. 2.15 Schematic illustration of cold chamber die casting process: a) die is closed, plunger withdrawn, and molten metal is poured into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.
However, in this process, the metal is poured into a “cold chamber” through a port or pouring slot with a ladle that only holds enough metal for one die filling or casting cycle. Immediately after the ladle is emptied the plunger advances, seals the port, and forces the molten metal into the die.
As the molten metal does not remain in the cold chamber very long, higher melting point metals like the copper alloys can be cast in this type of machine. It operates at a much slower cycle than the other machines in hot chamber process.
Extra material is used to force additional metal into the die cavity to compensate for the shrinkage that takes place during solidification.
In this group of processes, the molten metal is forced to distribute into the mold cavity by centrifugal acceleration. The process of centrifugal casting is long established, coming originally from a patent taken out by A. G. Eckhardt of Soho, England, in 1809. Centrifugal casting processes have greater reliability than static casting. They are relatively free from gas and shrinking.
There are three types of centrifugal casting processes: true centrifugal casting, semi-centrifugal casting, and centrifuging casting.
a) True Centrifugal Casting
The following operations are included in true centrifugal casting. One possible setup of true centrifugal casting is illustrated in Fig. 2.16. A mold is set up and rotated at a known speed along a horizontal axis; the mold is coated with a refractory coating.
Fig. 2.16 Setup for true centrifugal casting.
While horizontal, mold-rotating molten metal is poured into mold at one end. The high-speed rotation results in centrifugal forces that cause the metal to take the shape of the mold cavity. After the part has solidified, it is removed and finished.
The axis of rotation is usually horizontal but can be vertical for short workpiecers. The outside shape of the casting can be round or of a simple symmetrical shape. However, the inside shape of the casting is always round. During cooling, lower density impurities will tend to rise toward the center of rotation. Consequently, the properties of the casting can vary throughout its thickness.
Typically, in this casting process three structure zones may occur: the first zone is a layer of fine equiaxed structure that forms almost instantaneously at the mold wall. The second zone consists of directionally oriented crystals approximately perpendicular to the mold surface, and the third zone is nearest to the center and is characterized by a large number of uniformly grown crystals. The true centrifugal casting process is suitable for the production of hollow parts with large dimensions, such as pipes for oil, chemical industries, water supply, etc. Cylindrical parts ranging from 15 mm to 3 m (0.6 in. to 10 ft) in diameter and 15.5 m (50 ft) long can be cast centrifugally with wall thicknesses from 6 to 125 mm (0.25 to 5 in.). Typical metals cast are steel, iron, nickel alloys, copper alloys, and aluminum alloys.
Let us consider how fast the mold must rotate in horizontal centrifugal casting for the process to work successfully. Centrifugal force acting on a rotating body is defined by the following equation:
| (2.2) |
where
| Fc | = | centrifugal force, N (lb) |
| m | = | mass, kg (lb) |
| v | = | velocity, m/s (ft/sec) |
| R | = | inside radius of the mold, m (ft). |
Gravitational force is its weight,
| Fg = mg | (2.3) |
where
| F g | = | gravitation force, N (lb) |
| m | = | mass, kg (lb) |
| g | = | acceleration of gravity, m/s2 (ft/sec2) = 9.81 m/s2(32.2 ft/sec2). |
Velocity