Casting Processes Flashcards
What is casting?
Casting is a process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity.
What is an ingot?
The term ingot is usually associated with the primary metals industries; it describes a large casting that is simple in shape and intended for subsequent reshaping by processes such as rolling or forging.
What are advantages of the casting process?
- Casting can be used to create complex part geometries, including both external and internal shapes
- Some casting processes are capable of producing parts to net shape. No further manufacturing operations are required to achieve the required geometry and dimensions of the parts.
- Casting can be used to produce very large parts. Castings weighing more than 100 tons have been made
- The casting process can be performed on any metal that can be heated to the liquid state
What are disadvantages of the casting process?
They include limitations on mechanical properties, porosity, poor dimensional accuracy and surface finish for some casting processes, safety hazards to humans when processing hot molten metals, and environmental problems.
What is the difference between an open mold and a closed mold
In an open mold, the liquid metal is simply poured until it fills the open cavity. In a closed mold, Figure 2.1b, a passageway, called the gating system, is provided to permit the molten metal to flow from outside the mold into the cavity. The closed mold is by far the more important category in production casting operations.
What are the two main categories of casting?
expendable mold casting and permanent-mold casting
What is an expendable mold?
An expendable mold means that the mold in which the molten metal solidifies must be destroyed in order to remove the casting.
What is a permanent mold?
A permanent mold is one that can be used over and over to produce many castings.
What are benefits of expendable molds?
More intricate parts can be cast.
What are advantages and disadvantages of permanent molds?
Part shapes in the permanent-mold processes are limited by the need to open the mold. On the other hand, some of the permanent mold processes have certain economic advantages in high production operations.
What does Chvorinov’s rules refer to?
Equates solidification time to the volume and surface area of the casting, with a mold constant.
What does Chvorinov’s rule indicate
Chvorinov’s rule indicates that a casting with a higher volume-to-surface area ratio will cool and solidify more slowly than one with a lower ratio. This principle is put to good use in designing the riser in a mold. To perform its function of feeding molten metal to the main cavity, the metal in the riser must remain in the liquid phase longer than the casting.
What are the three steps during shrinkage?
(1) liquid contraction during cooling prior to solidification.
(2) contraction during the phase change from liquid to solid, called solidification shrinkage.
(3) thermal contraction of the solidified casting during cooling to room temperature.
What is the name of the cavity created by shrinkage?
a pipe
What are the parts of a sand casting mold?
The mold consists of two halves: cope and drag. The cope is the upper half of the mold, and the drag is the bottom half. These two mold parts are contained in a box, called a flask, which is also divided into two halves, one for the cope and the other for the drag. The two halves of the mold separate at the parting line.
What are the steps involved in sand casting.
A pattern is used to imprint a void into the mixture of sand and binder, the pattern is oversized to account for shrinkage. Internal surfaces are formed by a core. The metal flows into the cavity via the gating system and hot air escapes through the gaps between sand particles.
What makes up the gating system of a mold?
the gating system typically consists of a down sprue, through which the metal enters a runner that leads into the main cavity. At the top of the down sprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into the down sprue.
What are the steps in investment casting?
A wax pattern is coated with refractory to make a mold. When the refractory is set the wax is melted out leaving a cavity. The mold is heated to a high temperature to eliminate contaminates. The molten metal is poured, sets and then the mold is broken away.
What are the advantages of investment casting?
(1) parts of great complexity and intricacy can be cast;
(2) close dimensional control-tolerances of ±0.075 mm are possible;
(3) good surface finish is possible;
(4) the wax can usually be recovered for reuse;
(5) additional machining is not normally required-this is a net shape process
Name some other expendable mold casting processes.
plaster-mold casting, ceramic-mold casting, expanded polystyrene process and vacuum molding
What is the most common material of a permanent mold?
steel or cast iron
What materials are often formed by a permanent mold?
Aluminum, magnesium, copper-base alloys, and cast iron. However, cast iron requires a high pouring temperature, 1250◦C to1500◦C, which takes a heavy toll on mold life.
What are the 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
(4) mold is opened
What are advantages of permanent mold casting?
Advantages of permanent-mold casting include good surface finish and close dimensional control, as previously indicated. In addition, more rapid solidification caused by the metal mold results in a finer grain structure, so stronger castings are produced.
