Introduction
Gravity casting, also known as gravity pouring casting, is a fundamental and widely - used metal casting process. In this process, molten metal is poured into a pre - heated mold cavity and fills the cavity solely through the force generated by the Earth's gravity, without the application of additional external forces like high - pressure as in die - casting. This method has a long history and remains an important choice in the manufacturing industry due to its unique characteristics and wide range of applications.
Types of Gravity Casting
Sand Casting (as a form of gravity casting in a broad sense)
- Process: Sand casting involves shaping a mold from a sand mixture. A pattern, which is a replica of the desired casting, is used to create an impression in the sand. The sand is compacted around the pattern, and then the pattern is removed, leaving a cavity in the sand mold. Molten metal is poured into this cavity, and after solidification, the sand mold is broken away to retrieve the casting. Sand casting is often considered a form of gravity casting as the metal flow into the mold is mainly driven by gravity.
- Advantages: It has a relatively low cost, especially for small - scale production, as the sand molds are inexpensive to make. It is highly versatile and can be used to produce complex - shaped castings of various sizes. Sand casting is also suitable for a wide range of metals.
- Disadvantages: The dimensional accuracy of sand - cast parts is generally lower compared to some other gravity - casting methods like metal - mold gravity casting. The surface finish is also not as smooth, often requiring more extensive post - processing. The sand molds are expendable, meaning they can only be used once, which can increase the overall production cost for high - volume production.
Metal - Mold Gravity Casting (Narrow - defined Gravity Casting)
- Process: In metal - mold gravity casting, the mold is made of a metal, usually a heat - resistant alloy. The molten metal is poured into the metal mold under the influence of gravity. The metal mold can be reused multiple times, making it more suitable for medium - to high - volume production.
- Advantages: Metal - mold gravity casting offers higher dimensional accuracy and better surface finish compared to sand casting. The production efficiency is relatively higher as the molds can be reused. The castings tend to have better mechanical properties due to the faster cooling rate in the metal mold, which results in a finer grain structure.
- Disadvantages: The initial investment in metal molds is higher compared to sand molds. The design and manufacturing of metal molds are more complex and time - consuming. There are also limitations in the complexity of the shapes that can be produced compared to some other casting methods.