permanent mold casting involving die casting and centrifugal casting

1. Lecture No.4
Permanent Mold Casting Processes
By
Ass. Lect. Ali H. Almaily
enginerali1989@gmail.com

2. 4.1 Die Casting
• A permanent mold casting process in which molten metal is injected into mold
cavity under high pressure
• Pressure is maintained during solidification, then mold is opened and
part is removed.
• Molds in this casting operation are called dies; hence the name die
Casting.
• Use of high pressure ( 7- 350 Mpa ) to force metal into die cavity is what
distinguishes this from other permanent mold processes.
Die Casting Machines
• Designed to hold and accurately close two mold halves and keep them
closed while liquid metal is forced into cavity.
• Two main types:
1. Hot-chamber machine
2. Cold-chamber machine

3. 1. Hot-Chamber Die Casting
Metal is melted in a container attached to the machine, and a piston injects liquid
metal under high pressure ( 7 to 35 MPa ) into the die. The casting cycle is summarized in
Figure 4.1.
• High production rates - 500 parts per hour common.
• Applications limited to low melting-point metals that do not chemically attack plunger
and other mechanical components.
• Casting metals: zinc, tin, lead, and magnesium.
2. Cold-Chamber Die Casting
Molten metal is poured into unheated chamber from external melting container, and a piston
injects metal under high pressure(typically 14 to 140 MPa) into die cavity. The production cycle
is explained in Figure 4.2
• High production but not usually as fast as hot-chamber machines because of pouring step.
• Casting metals: aluminum, brass, and magnesium alloys.
• Advantages of hot-chamber process favor its use on low melting-point alloys (zinc, tin,
lead).

4. Figure 4.1: Cycle in hot chamber casting: (1) with die closed and plunger withdrawn, molten metal
flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure
during cooling and solidification; and (3) plunger is withdrawn, die is opened, and solidified part is
ejected. Finished part is shown in (4).

5. Figure 4.2: Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is
poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during cooling
and solidification; and (3) ram is withdrawn, die is opened, and part is ejected.

6. Molds for Die Casting
• Usually made of tool steel, mold steel, or maraging steel.
• Tungsten and molybdenum (good refractory qualities) used to die cast
steel and cast iron.
• Ejector pins required to remove part from die when it opens.
• Lubricants must be sprayed into cavities to prevent sticking.
Advantages and Limitations
 Advantages of die casting:
• Economical for large production quantities, high production rates.
• Good accuracy and surface finish.
• Thin sections are possible.
• Rapid cooling provides small grain size and good strength to Casting.
 Disadvantages:
• Generally limited to metals with low metal points.
• Part geometry must allow removal from die.

7. Die-casting dies (Fig. 4.3) may be single cavity, multiple cavity
(with several identical cavities), combination cavity (with several different cavities), or
unit dies (simple small dies that can be combined in two or more units in a master
holding die).
The dies usually are made of hot-work die steels or mold steels. Die wear increases
with the temperature of the molten.
Because the die materials have no natural porosity and the molten metal rapidly
flows into the die during injection, venting holes and passageways must be built into
the dies at the parting line to evacuate the air and gases in the cavity. The vents are
quite small; yet they fill with metal during injection.
This metal must later be trimmed from the part. Also, formation of flash is common
in die casting, in which the liquid metal under high pressure squeezes into the small
space between the die halves at the parting line or into the clearances around the
cores and ejector pins. This flash must be trimmed from the casting, along with the
sprue and gating system.

8. Figure 4.3 Various types of cavities in a die-casting die.

9. 4.2 Centrifugal Casting:
Centrifugal casting refers to several casting methods in which the mold is rotated
at high speed so that centrifugal force distributes the molten metal to the outer regions of
the die cavity. In true centrifugal casting, molten metal is poured into a rotating mold to
produce a tubular part. Examples of parts made by this process include pipes, tubes,
bushings, and rings. One possible setup is illustrated in Figure 4.4.
Figure 4.4 Setup for True Centrifugal Casting.

10. A. Horizontal centrifugal casting:
One possible setup is illustrated in Figure 4.4. Molten metal is poured into
a horizontal rotating mold at one end. In some operations, mold rotation commences after
pouring has occurred rather than beforehand. The high-speed rotation results in centrifugal
forces that cause the metal to take the shape of the mold cavity. Thus, the outside shape of
the casting can be round, octagonal, hexagonal, and so on. However, the inside shape of the
casting is (theoretically) perfectly round, due to the radially symmetric forces at work.
Centrifugal force is defined by this physics equation:
………....4.1
where F = force, N ; m = mass, kg ; v = velocity, m/s ; and r = inside radius of the mold,
(m). The force of gravity is its weight W= mg, where W is given in kg , and g = acceleration
of gravity = 9.8 m/s2 . The so-called G-factor GF is the ratio of centrifugal force divided by
weight:

11. ………4.2
………….4.3
Velocity v can be expressed as:
…… 4.4
Where N=rotational speed, rev/min. Substituting this expression into Eq. (4.3), we obtain:
……….. 4.5
Rearranging this to solve for rotational speed N, and using diameter D rather than radius in
the resulting equation, we have:

12. where D = inside diameter of the mold,( m).
Example 4.1: A true centrifugal casting operation is to be performed horizontally to make
copper tube sections with OD = 25 cm and ID = 22.5 cm. What rotational speed is
required if a G factor of 65 is used to cast the tubing?
Solution: The inside diameter of the mold D=OD of the casting=25 cm=0.25 m.

13. B. vertical centrifugal casting:
The effect of gravity acting on the liquid metal causes the casting wall to be thicker at the
base than at the top. The inside profile of the casting wall takes on a parabolic shape. The
difference in inside radius between top and bottom is related to speed of rotation as follows:
…………. 4.7
Example 4.2: A vertical true centrifugal casting process is used to produce bushings that are
200 mm long and 200 mm in outside diameter. If the rotational speed during solidification is
500 rev/min, determine the inside diameter at the top of the bushing if the inside diameter at
the bottom is 150 mm.
Solution: L = 200 mm = 0.2 m. Rb = 150/2 = 75 mm = 0.075 m.
Rt = .08399 m = 83.99 mm.
Dt = 2(83.99) = 167.98 mm.

14. 4.3 Cleaning of Castings:
After the casting has solidified and been removed from the mold, a
number of additional steps are usually required. These operations
include:
1. Trimming (involves removal of sprues, runners, risers, parting-line
flash, fins, chaplets, and any other excess metal from the cast part)
2. Removal of core, feeder etc.
3. Surface cleaning.
4. Finishing,( the end product final finish is provided by machining,
polishing, buffing, chemical treatment, heat treatment, etc).

15. 4.4 Inspection of casting:
Generally, the inspection of castings is carried out to ascertain the required surface finish,
dimensional accuracy, various mechanical and metallurgical properties and soundness.
Various tests used for inspection of castings are:
1. Measurement of the final dimensions
2. Measurement of Surface finish such as roughness tester,
3. Non-destructive testing :visual inspection to detect obvious defects such as misruns, cold
shuts, and severe surface flaws; pressure testing—to locate leaks in the casting; radiographic
methods (X-rays), magnetic particle tests, the use of fluorescent penetrants, and supersonic
testing to detect either surface or internal defects in the casting, as well as to metallurgical,
chemical, physical, tests .
4. Destructive testing (mechanical testing) to determine properties such as tensile strength
and hardness.