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.

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

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.

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

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.

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

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

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

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 4.1 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

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 = 681.7 rev/min In vertical centrifugal casting, rotational speed as follows: N = (30/)√(2gL/Rt2 – Rb2) 4.5

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

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

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

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

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

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

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

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

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

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

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

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

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

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)