Excerpt from the September 2019 TheLibraryofManufacturing.com article.
Basics: Molds, Patterns, Cores and Gating
A mold is formed into the geometric shape of a desired part. Then, molten metal is poured into the mold, which 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. Closed molds contain 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 figure 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 castings. When considering manufacturing processes, there are advantages and disadvantages to both.
- Can produce only one metal casting
- 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 patterns. 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 oversized to compensate for shrinkage occurring in the metal during the casting's solidification in addition to any material machined off the cast part afterward. 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 compensates for unknown variables in shrinkage and reduces 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.
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