Foundry Technology: Molds and Mold Making

Molds and Mold Making

• 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.

Types of Sand 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 Operation

• 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.

Steps in the Casting Process

• Place a pattern in sand to create a mold.
• Incorporate the pattern and sand in a gating system, and remove the pattern.
• Fill the mold cavity with molten metal.
• Allow the metal to cool.
• Break away the sand mold and remove the casting.

Sequence of Operations in Sand Casting

• 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.