A mold is formed into the geometric shape of a desired part. Molten metal is then poured into the mold, the mold holds this material in shape as it solidifies. A metal casting is created. Although this seems rather simple, the manufacturing process of metal casting is both a science and an art. Let's begin our study of metal casting with the mold. First, molds can be classified as either open or closed. An open mold is a container, like a cup, that has only the shape of the desired part. The molten material is poured directly into the mold cavity which is exposed to the open environment.
This type of mold is rarely used in manufacturing production, particularly for metal castings of any level of quality. The other type of mold is a closed mold, it contains a delivery system for the molten material to reach the mold cavity, where the part will harden within the mold. A very simple closed mold is shown in 2. The closed mold is, by far, more important in manufacturing metal casting operations.
There are many different metal casting processes used in the manufacture of parts. Two main branches of methods can be distinguished by the basic nature of the mold they employ. There is expendable mold casting and permanent mold casting. As the name implies, expendable molds are used for only one metal casting while permanent molds are used for many. When considering manufacturing processes, there are advantages and disadvantages to both.
Can produce one metal casting only
Made of sand, plaster, or other similar material
Binders used to help material hold its form
Mold that metal solidifies in must be destroyed to remove casting
More intricate geometries are possible for casting
Can manufacture many metal castings
Usually made of metal or sometimes a refractory ceramic
Mold has sections that can open or close, permitting removal of the casting
Need to open mold limits part shapes
Expendable molds require some sort of pattern. The interior cavities of the mold, in which the molten metal will solidify, are formed by the impression of this pattern. Pattern design is crucial to success in manufacture by expendable mold metal casting. The pattern is a geometric replica of the metal casting to be produced. It is made slightly oversize to compensate for the shrinkage that will occur in the metal during the casting's solidification, and whatever amount of material that will be machined off the cast part afterwards. Although machining will add an extra process to the manufacture of a part, machining can improve surface finish and part dimensions considerably. Also, increasing the machine finish allowance will help compensate for unknown variables in shrinkage, and reduce trouble from areas of the metal casting that may have been originally too thin or intricate.
The material from which the pattern is made is dependent upon the type of mold and metal casting process, the casting's geometry and size, the dimensional accuracy required, and the number of metal castings to be manufactured using the pattern. Patterns can be made from wood, like pine (softwood), or mahogany (hardwood), various plastics, or metal, like aluminum, cast iron, or steel. In most manufacturing operations, patterns will be coated with a parting agent to ease their removal from the mold.
For metal castings with internal geometry cores are used. A core is a replica, (actually an inverse), of the internal features of the part to be cast. Like a pattern, the size of the core is designed to accommodate for shrinkage during the metal casting operation. Unlike a pattern, a core remains in the mold while the metal is being poured. Hence, a core is usually made of a similar material as the mold. Once the metal casting has hardened, the core is broken up and removed much like the mold. Depending upon the location and geometry of the core within the casting, it may require that it is supported during the operation to prevent it from moving or shifting. Structural supports that hold the core in place are called chaplets.The chaplets are made of a material with a higher melting temperature than the casting's material, and become assimilated into the part when it hardens. Note that when manufacturing a metal casting with a permanent mold process, the core will be a part of the mold itself.
A typical mold is shown in 3.
When manufacturing by metal casting, consideration of the mold is essential. The pattern is placed in the mold and the mold material is packed around it. The mold contains two parts, the drag (bottom), and the cope (top). The parting line between the cope and drag allows for the mold to be opened and the pattern to be removed once the impression has been made.
The core is placed in the metal casting after the removal of the pattern. 5 shows the pattern impression with the core in place.
Now the impression in the mold contains all the geometry of the part to be cast. This metal casting setup, however, is not complete. In order for this mold to be functional to manufacture a casting, in addition to the impression of the part, the mold cavity will also need to include a gating system. Sometimes the gating system will be cut by hand or in more adept manufacturing procedures, the gating system will be incorporated into the pattern along with the part. Basically, a gating system functions during the metal casting operation to facilitate the flow of the molten material into the mold cavity.