Foundry Technology: Molds and Mold Making

Study Notes

To form a mold cavity, molding sand is compacted around a pattern in a flask using various methods, including hand ramming and machine-based packing.
The quality of a sand mold is determined by several factors:
- Strength: the ability to maintain its shape and resist erosion caused by molten metal flow.
- Permeability: the capacity to allow hot air and gases to pass through the voids in the sand.
- Thermal stability: the ability to resist cracking and buckling upon contact with molten metal.
- Collapsibility: the ability to give way and allow the casting to shrink without cracking.
- Reusability: the ability to reuse the sand from a broken mold to make other molds.

Green-sand molds: made of a mixture of sand, clay, and water, and possess sufficient strength, good collapsibility, good permeability, and good reusability.
Dry-sand molds: made using organic binders, baked in an oven, and provide better dimensional control, but are more expensive and have a lower production rate.
Skin-dried molds: combine the advantages of dry-sand molds, where the surface of a green-sand mold is dried to a depth of 10-25 mm, using special bonding materials.

The casting process involves pouring, solidification, and cooling of the cast part.
The gating and riser system must be designed to deliver liquid metal into the cavity and provide for a sufficient reservoir of molten metal during solidification shrinkage.
Air and gases must be allowed to escape during the casting process.
After solidification and cooling, the sand mold is broken away from the casting, and the part is cleaned and retrieved.

A mechanical drawing of the part is used to generate a design for the pattern.
Patterns are mounted on plates equipped with pins for alignment.
Core boxes produce core halves, which are pasted together.
The cope and drag halves of the mold are assembled, and the core is set in place.
The mold is closed, and the metal is poured into the mold.
After solidification, the casting is removed from the mold, and the sprue and risers are cut off and recycled.