1. 4.5 Permanent-Mold Casting Processes
In permanent-mold casting, the mold is reused many times.
Disadvantage of expandable mold processes is that a new mold is required for every casting.
4.5.1 The basic Permanent-Mold Processes
Use a metal mold constructed of two sections that are designed for easy, precise opening and closing.
These molds are commonly made of steel or cast iron.
Metal commonly cast in permanent molds include aluminum, magnesium, copper-base alloys, and cast iron.
Cores can be used in permanent molds to form interior surfaces in the cast product.
If withdrawal of a metal core would be difficult or impossible, sand cores can be used, in which case the casting process is often referred to as semi permanent-mold casting.
Advantages is good surface finish and close dimensional control.
Typical parts include pistons, pump bodies, and certain missiles.

2. Figure 4.14: Steps in permanent mold casting: (1) mold is preheated and coated; (2) cores (if
used) are inserted and mold is closed, (3) molten metal is poured into the mold,
where it solidifies; (4) mold is opened; (5) finished part

3. 4.5.2 Variations of Permanent-Mold Casting
Slush casting is a hollow casting is formed by inverting the mold after partial freezing at the surface to drain out the liquid metal in the center. Typical parts such as statues, lamp pedestals, and toys.
Low-pressure casting, the liquid metal is forced into the cavity under low pressure – approximately 0.1 MPa – from beneath so that the flow is upward.
Figure 4.15: Low-pressure casting. The diagrams shows how air pressure is used to force the
molten metal in the ladle upward into the mold cavity. Pressure is maintained
until the casting has solidified
Vacuum permanent-mold casting is a variation of low-pressure casting in which a vacuum is used to draw the molten metal into the mold cavity.

4. The molten metal is injected into the mold cavity under high pressure.
4.5.3 Die casting
The molten metal is injected into the mold cavity under high pressure.
The pressure is maintained during solidification, after which the mold is opened and the part is removed.
Modern die-casting machines are designed to hold and accurately close the two halves of the mold, and keep them closed while the liquid metal is forced into the cavity.
There are two main types of die-casting machines; (1) hot-chamber and (2) cold-chamber, differentiated by how the molten metal is injected into the cavity.
Figure 4.16: General configuration of a (cold-chamber) die-casting machines

5. In hot-chamber machines, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die.
Typical injection pressure are 7 to 35 MPa and production rates up to 500 parts per hour.
Limited to applications to low-melting point metals that do not chemically attack the plunger and other mechanical components.
The metals include zinc, tin, lead, and sometimes magnesium.
In cold-chamber machines, molten metal is poured into an unheated chamber from an external melting container, and a piston is used to inject the metal under high pressure into die cavity.
Injection pressure are 14 to 140 MPa and production rate are not usually as fast because of the need to ladle the liquid metal into the chamber from an external source.
The metals include aluminum, brass, and magnesium alloys.
Low-melting point alloys can also be cast on cold-chamber machines.

6. Figure 4.17: 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, solidified part is ejected; and (5)
finished part

7. Figure 4.18: 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

8. Molds use in die-casting operations are usually made of tool steel, mold steel, maraging steel, tungsten and molybdenum with good refractory are also being used.
Ejector pins are required to remove the part from the die when it opens.
Venting holes and passageways must be built into the dies at the parting line to evacuate the air and gases in the cavity.
Advantages of die-casting include:
~ high production rate
~ economical for large production quantities
~ close tolerance for small parts
~ good surface finish
~ rapid cooling provides small grain size and good strength to the casting

9. 4.5.4 Centrifugal Casting
Refer 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.
The group includes (1) true centrifugal casting, (2) semi-centrifugal casting, and (3) centrifuge casting.
True centrifugal casting (Figure 4.19), molten metal is poured into a rotating mold to produce a tubular part.
Typical of parts is made like pipes, tubes, bushings, and rings.
Orientation of the axis of mold rotation can be either horizontal or vertical.
Centrifugal force is defined by physics equation:
F = mv2/R
The so-called G-factor GF is the ratio of centrifugal force divided by weight:
GF = mv2/R = mv2/Rmg = v2/Rg 4.2
Velocity (v), can be expressed as 2RN/60 = RN/30. Substituting this expression into Equation 4.2, we obtain

10. GF = [R (N/30)2]/g 4.3
Rearranging this to solve for rotational speed (N), and using diameter D rather than radius in the resulting equation, we have
N = (30/)√(2gGF/D) 4.4
Example Calculation Rotation Speed in True Centrifugal Casting
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, we can compute the required rotational speed as follows:
N = (30/)√[(2)(9.8)(26)]/0.25 = rev/min
In vertical centrifugal casting, rotational speed as follows:
N = (30/)√(2gL/Rt2 – Rb2) 4.5

11. Figure 4.19: Setup for true centrifugal casting
Figure 4.20: Semi-centrifugal casting

12. Typical parts is made like wheels and pulleys.
Semi-centrifugal casting (Figure 4.20), centrifugal force is used to produce solid casting, rather than tubular parts.
The rotation speed in semi-centrifugal casting is usually set so that G-factors of around 15, and the molds are designed with risers at the center to supply feed metal.
Typical parts is made like wheels and pulleys.
Centrifuge casting (Figure 21), the mold is designed with part cavities located away from the axis of rotation, so that the molten metal poured into the mold is distributed to these cavities by centrifugal force. The process is used to smaller parts.
Figure 4.21: (a) centrifuge casting – centrifugal force causes metal to flow to the mold
cavities away from the axis of rotation; and (b) the casting part

13. 4.6 Casting Defects
There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product
The defects can be classified as follows:
Defects common to all casting processes
Defects related to sand casting process

18. 4.7 Metals for Casting
Most commercial castings are made of alloys rather than pure metals
Alloys are generally easier to cast, and properties of product are better
Casting alloys can be classified as:
Ferrous
Nonferrous

19. Ferrous Casting Alloys: Cast Iron
Most important of all casting alloys
Tonnage of cast iron castings is several times that of all other metals combined
Several types: (1) gray cast iron, (2) nodular iron, (3) white cast iron, (4) malleable iron, and (5) alloy cast irons
Typical pouring temperatures ∼ 1400°C (2500°F), depending on composition

20. Ferrous Casting Alloys: Steel
The mechanical properties of steel make it an attractive engineering material
The capability to create complex geometries makes casting an attractive shaping process
Difficulties faced by the foundry working with steel:
Pouring temperature of steel is higher than for most other casting metals ∼ 1650°C (3000°F)
At these temperatures, steel readily oxidizes, so molten metal must be isolated from air
Molten steel has relatively poor fluidity

21. Nonferrous Casting Alloys: Aluminum
Generally considered to be very castable
Pouring temperatures : low – melting temperature of aluminum Tm = 660°C (1220°F)
Properties:
Light weight
Range of strength properties by heat treatment
Ease of machining

22. Nonferrous Casting Alloys: Copper Alloys
Includes bronze, brass, and aluminum bronze
Properties:
Corrosion resistance
Attractive appearance
Good bearing qualities
Limitation: high cost of copper
Applications: pipe fittings, marine propeller blades, pump components, ornamental jewelry

23. Nonferrous Casting Alloys: Zinc Alloys
Highly castable, commonly used in die casting
Low melting point – melting point of zinc Tm = 419°C (786°F)
Good fluidity for ease of casting
Properties:
Low creep strength, so castings cannot be subjected to prolonged high stresses

24. 4.8 Product Design Considerations:
1. Geometry simplicity
Although casting can be used to produce complex part geometries, simplifying the part design will improve castability
Avoiding unnecessary complexities:
Simplifies mold‐making
Reduces the need for cores
Improves the strength of the casting

26. 4.8 Product Design Considerations:
2. Corners
Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracks
Generous fillets should be designed on inside corners and sharp edges should be blended

29. 4.8 Product Design Considerations:
3. Draft Guidelines
In expendable mold casting, purpose of draft is to facilitate removal of pattern from mold (1° for sand casting)
In permanent mold casting, purpose is to aid in removal of the part from the mold (2° to 3° for permanent mold processes)
Similar tapers should be allowed if solid cores are used

30. 4.8 Product Design Considerations:
4. Dimensional Tolerances and Surface Finish
Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process:
Poor dimensional accuracies and finish for sand casting
Good dimensional accuracies and finish for die casting and investment casting

31. 4.8 Product Design Considerations:
5. Machining Allowances
Almost all sand castings must be machined to achieve the required dimensions and part features
Additional material, called the machining allowance, must be left on the casting in those surfaces where machining is necessary
Typical machining allowances for sand castings are around 1.5 to 3 mm (1/16 to 1/4 in)