illustration of the hot chamber die casting process: a) die is closed, plunger withdrawn, and molten metal flows into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.
Fig. 2.15 Schematic illustration of cold chamber die casting process: a) die is closed, plunger withdrawn, and molten metal is poured into chamber; b) plunger forces metal in chamber to flow into die cavity, maintaining pressure during solidification; c) plunger is withdrawn, die is opened, and casting is ejected; d) finished part is shown.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 v can be expressed as
| (2.4) |
where
| N | = | rotation speed, rev/min. |
G-factor is the ratio of centrifugal force divided by the gravitation force:
| (2.5) |
where
| GF | = | gravitation factor. |
Solving further, we get
| (2.6) |
where
| D | = | inside diameter of the mold, m (ft). |
If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced against the mold wall during the upper half of the circular path, but will drop inside the cavity. Too high a speed results in excessive stresses and hot tears in the outside surface—on an empirical basis of GF = 50 to 100 for the metal mold and GF = 25 to 50 for a sand cast mold.
True centrifugal casting is characterized by better mechanical properties of the cast than is true in conventional static casting: nonmetallic impurities that segregate toward the bore can be machined off; the casting is relatively
