Metal Shaping Processes. Vukota Boljanovic
Fig. 2.11 Steps in basic permanent-mold casting: a) mold is preheated and coated; b) mold is closed and molten metal poured; c) mold is opened and casting is ejected; d) finished part.
In preparation for casting, the mold is first preheated at 150 to 260°C (302 to 500°F); then a refractory washer mold coating is brushed or sprayed onto those surfaces that will be in direct contact with the molten metal alloy. The proper operating temperature for each casting is set. Cores, if applicable, are inserted, and the mold is closed manually or mechanically. The alloy is heated at the pouring temperature and is poured into the mold through the gating system. Unlike expendable molds, permanent molds do not collapse, so the mold must be opened before appreciable cooling contraction occurs in order to prevent cracks from developing in the casting.
It is desirable and generally more economical to use permanent steel cores to form cavities in a permanent mold casting. When the casting has re-entrant surfaces or cavities from which one-piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster, and other materials are used. This process then is called semipermanent mold casting. Sectional steel cores are used in some instances.
Advantages of permanent mold casting include the facts that cast surfaces are generally smoother than sand castings, and closer dimensional tolerances can be maintained. Permanent mold castings usually have better mechanical properties than sand castings because solidification is more rapid and fill is more laminar.
This process is used mostly for aluminum, magnesium, copper alloys, and gray iron because of their generally lower melting points. The process is not economical for small production runs and intricate shapes because of the difficulty in removing the casting from the mold.
2.4.2 Vacuum Permanent Mold Casting
Vacuum permanent mold casting (not to be confused with vacuum molding) is similar to low-pressure permanent mold casting, except for the step of filling the mold. In this case, the molten metal is sucked upward into the mold by vacuum pump. A schematic illustration of the vacuum casting is shown in Fig. 2.12.
Fig. 2.12 Schematic illustration of the vacuum casting process a) before flow up of molten metal, and b) after flow up of molten metal into cavity.
The permanent mold is enclosed in an airtight bell housing. The housing has two openings: the sprue at the bottom, through which molten metal enters the mold; and the vacuum outlet at the top. The sprue opening is submerged below the surface of the molten metal and the vacuum is drawn within the housing, creating a pressure differential between the mold cavity and the molten metal in the crucible. This pressure differential causes the molten metal to flow up the sprue and into the mold cavity, where it solidifies. The mold is removed from the housing, opened, and the casting ejected.
By controlling the vacuum, the pressure differential between the mold cavity and the molten metal can be varied, allowing for the differential fill rates that are necessitated by certain part designs and gating requirements. This results in tight control of the fill rate, which also directly influences the soundness of the casting. Through proper part design, mold design, and the use of the vacuum mold process, voids, shrinks, and gas pockets can be greatly reduced or eliminated in critical areas. Because the sprue opening is submerged beneath the surface of the molten metal, only pure alloy, free from oxides and dross, can enter the die cavity. This helps to produce clean, sound castings with minimal foreign materials that detract from strength, appearance, and machinability.
The mechanical properties of the vacuum permanent mold casting are 10 to 15% superior to those of the traditional permanent mold casting. Castings range in size from 200 g to 4.5 kg (6 oz to 10 lb).
Slush casting is a special type of permanent mold casting in which the molten metal is not allowed to completely solidify. In this process a metal mold in two or more sections is used. The mold is filled with molten metal. After partial solidification of the liquid metal on the surface in the desired thickness, the mold is inverted in order to drain out the still-liquid metal at the center, resulting in a hollow casting. The mold halves then are opened, and the casting removed.
Solidification begins at the walls because they are relatively cool; it then works inward, so the thickness of the shell is controlled by the amount of time allowed before the mold is drained. This is a relatively inexpensive process for small production runs and generally is used only for low-melting lead and zinc-based metals and to produce ornamental items that need not be strong, such as statues, lamp pedestals, and toys.
Pressure casting, also called low pressure casting, is the principle of pressing the molten metal through a refractory tube into the mold from below using an overpressure by gas of about 0.08 to 0.1 MPa (12 to 15 psi). The volume of the circulating material is widely reduced. Supported by the pressurization of the holding crucible, the molten metal is forced into the cavity from beneath so that the flow is upward, with low turbulence, a regulated casting pressure, and a controlled casting speed. The pressurization chamber with the crucible and the mold make one unit, connected by the refractory tube as illustrated in Fig. 2.13. Most crucibles are induction heated to keep the metal warm, but not to melt it.
Fig. 2.13 Schematic illustration of the low-casting process.
The advantages of low pressure casting are good fillability for thin-walled and large area parts and a close structure for pressurized parts. By maintaining several casting parameters, a very high constant quality in the serial production and process safety can be realized. The overpressure during the casting process and the following adjustable holding time acts in opposition to the mold shrinkage by the cooling down.
The low pressure casting process is a cost-effective production process (for medium-run series) that provides excellent mechanical properties of casting.
Die casting is one of the most important and versatile quantity production processes used in the metalworking industry. The traditional die casting process may be described as injection under high pressure, typically of 10 to 350 MPa (1450 to 50,000 psi) into a steel mold (otherwise known as a die) of a molten metal alloy. This solidifies rapidly to form a net-shaped part; then the die is opened and the casting is automatically ejected. After the casting has been removed, the die is closed, and the cycle is ready to be repeated. To run this cycle at a high rate of speed a die casting machine is used. As in other casting processes, after the casting is formed and removed from the die, the sprue and runners must be cut off.
Die casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique. The advantages of die-casting include the following:
High-speed production. Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required, and thousands of identical castings can be produced before additional tooling is required.
Dimensional accuracy and stability. Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat-resistant.
Strength and weight. Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin-wall castings