Introduction to Metal Casting Processes

Study Notes

Metal casting is a process where molten metal is poured into a mold cavity to form the final part upon solidification
Molds can be made from different materials, including expandable metals (sand, resin, ceramic, gypsum) and permanent metals (grey cast iron, steel, bronze, graphite)

Expandable casting processes use a pattern to make mold cavities, with materials like metals, wax, and polystyrene
Cores can be inserted into the mold cavity to make parts with internal holes, made of sand mixed with binder materials
Three types of casting processes will be covered in this experiment: sand, shell mold, and centrifuging casting

Mold is made of sand, with a pattern used to make internal die cavities and cores used to form internal holes in the final castings
Die is divided into two portions: cope (upper) and drag (lower)
Pattern is also divided into two portions, one for the cavity in the cope section and one for the cavity in the drag portion
Re-usable sand can be used to minimize overall process cost
Sand casting process can be used to cast almost any metal type with no limitations on part size and low tooling cost
Surface finish and dimensional accuracy of the final parts are low, requiring some finishing operations
Surface finish can be enhanced by using small sand particles to form the die, but this reduces die permeability and may cause casting defects

Expendable mold casting process, where molten metal is poured into a heat-cured mold that is non-reusable and in the form of a shell
Shell is made from silica sand with a resin binder
Two shells are held together and brokered when the final part is solidified
Superior surface finish and good dimensional accuracy are advantages of this process over sand casting
Can be used for a wider range of alloys with greater die design flexibility than die-casting and lower cost than investment casting process
Secondary machining operations are often not required, and casting cracks are minimized by mold collapsibility
Process can be mechanized for mass production
Limitations include high equipment and pattern cost, low part size, and formation of noxious fumes that must be extracted
Shell mold casting process is characterized by low capital outlay on sand preparation plant, good space utilization, low sand-metal ratio, and lower cleaning costs