Bentonite is primarily used in green sand molds as a binder.
In the foundry process, green sand is a mixture of sand, clay (typically bentonite), water, and sometimes other additives. The role of the bentonite clay is crucial for giving the sand mixture cohesion and plasticity, allowing it to be molded into the desired shape for casting.
The Role of Bentonite as a Binder
The main reason bentonite is incorporated into green sand is its ability to bind the individual sand grains together when mixed with water. This binding property is essential for creating a mold cavity that can hold its shape during the casting process.
- Cohesion: It holds the sand particles together, preventing the mold from collapsing.
- Plasticity: It allows the sand mixture to be shaped and compacted around a pattern.
- Strength: It provides sufficient strength to the mold to withstand the pressure of the molten metal during pouring.
As stated in the provided reference: "Green sand molds in foundry use bentonite as a binder."
Bentonite's Behavior During Casting
The reference also provides insight into what happens to the bentonite during the casting process itself: "During the casting process, the heat transferred to the surrounding sand in the mold leads to the degradation of bentonite structure, thereby forming new types of deposits on the surface of sand grain." While this describes the effect of heat on the bentonite, its initial inclusion is for its binding capability, which enables the mold to be formed in the first place.
In summary, bentonite acts as the critical binding agent that transforms loose sand into a moldable material capable of withstanding the casting operation.
