Metal Casting, The Basics
Casting is essentially using a mold to define the shape of a final product. The fabrication of parts from casting involves setting molten into a mold. When the molten metal cools it solidifies into the desired shape. The procedure for making parts from casting begins with pattern development, then mold preparation. Once the mold is ready, the metal is melted and poured into the mold, where it cools and solidifies. The part can then be removed and excess metal cleared. Usually a heat treatment is applied next, and any finalization process such as machining is carried out.
Casting can be divided into two main kinds, which depict the mold used. Molds can be expendable or non-expendable. Expendable casting involves sand, plastic, clay or metal molds, which are discarded once finished with. Non-expendable casting uses more permanent molds intended for long-term usage. Aside from the molds, casting is divided into further categories depending on the material used to make the molds. Let’s take a look at the different types in more detail.
Expendable mold casting includes sand casting, plaster mold castings, shell molding, investment casting, and full-mold casting.
Sand Casting uses molds created from polymer mixed with sand, or chemicals. Compared to other molds, this one is more durable, and permit larger runs created with a single mold. This method has been used for decades and is very versatile producing many cast geometries. It is cost-effective, and can be used for small volumes as the cost of tooling can be minimal or even removed totally by using 3D sand printing or direct machining of the mold.
Plaster mold castings, although a little less durable, are faster to make, and have a gypsum plaster base. Shell molding comprises resin-treated sand, and offer a thinner mold than traditional sand casting. The thinness enables more precision and detailed geometries to be formed from the molds.
Investment casting, also known as lost-wax casting, is more durable and stable than typical wax composites by using technologically advanced waxes. If the part has complicated alloys and geometries, investment casting can work well here. This is a good choice if you have thin walls, tight tolerances, or shaped or non-symmetrical parts with intricate detailing. Tooling for this type can be costly, so is typically used for larger volumes, though 3D printing techniques can bring this cost down so smaller volumes are possible.
Full-mold casting uses molds made from durable polystyrene foam covered by sand, which forms the shell of the mold. Molten metal going into the mold makes the foam evaporate, which enables the sand to control the geometry of the part.
Non-expendable mold casting includes die-casting, permanent mold casting, centrifugal casting, and semi-solid metal casting.
Die-casting is a method where die is made which has a horizontal pattern. Typically, a bar metal or something similar is fed through the die, creating the bar, then pieces are cut out in segments. The mold in permanent mold casting is made from metal, making it less expendable that other types, however, it is reusable. Centrifugal casting employs a rotating drum to make centrifugal force moving the molten inside the mold. Centrifugal casting delivers a material cleanliness that is often better to other processes. Semi-solid metal casting falls into the die-casting category, where softened metal is pressed through the die.
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