Methods of Manufacturing: Non-Expendable Mold Casting
By Mary Iannuzzi on April 8, 2020
Casting is a manufacturing process that forms parts by solidifying molten, liquid metal within a mold.
The two primary types casting used today are expendable and non-expendable casting. This blog focuses on non-expendable mold casting. (See blog on expendable mold casting)
Five common kinds of non-expendable mold casting are: permanent mold casting, die casting, semi-solid metal casting, centrifugal casting, and continuous casting.
Permanent Mold Casting
Pros: High strength, low porosity and lower tooling cost compared to die casting. Lower part cost and better finish than expendable molds.
Cons: Unusable with ferrous metals. Mold can be expensive and need replacing quickly. Slower process and a rougher surface compared to die casting,
Permanent mold casting is one of the simplest forms of non-expendable mold casting. The process works in the same way as a sand casting except the mold is made from steel and is reused. The cost of the mold can be a high investment, but will allow for a good surface finish and near-net-shape casting and can reduce the cost per piece. Molten metal is poured into the steel mold and fills the design by gravity. This slows down the production process but also reduces porosity.
Pros: Quick, excellent surface finish, can create near-net complex shapes with tight tolerances, including thin walls.
Cons: Unusable with ferrous metals, high tool cost, vulnerable to porosity and resulting weakness.
Die casting uses high pressure to force molten metal into machined dies. This allows for small, detailed designs, including thin walls, to be created with a high degree of accuracy. The surface finish is very fine and the piece hardens at near-net-shape. Using pressure speeds up the process which can reduce per piece cost; however, parts are vulnerable to porosity and resulting weakness.
Semi-Solid Metal Casting
Pros: Hi strength and ductility, tight tolerances, heat treating possible, minimum porosity, thin walls possible.
Cons: Costs can be prohibitive.
Semi-solid metal casting offers all of the benefits of die casting while eliminating the primary problem of porosity. It uses the same high pressure process as die casting; however, the metal forced into the mold is 50% solid and 50% liquid. This controls turbulence in the filling process that would otherwise cause porosity. The uncoated steel dies allow for rapid heat transfer and, in turn, stronger parts. The cost of these parts can be quite high, and semi-solid metal casting is often not the best choice as a result.
Pros: Controls impurities and eliminates defects and micro-porosity.
Cons: Size and expense limitations. Most sensible for cylindrical shapes.
Centrifugal casting can produce excellent parts that do not have the defects and porosity often seen with other methods of casting. This casting method works by depositing molten metal in to the center of a mold which spins continuously. The molten metal is pushed to the circumference by centrifugal force and solidifies there from the outside in. Because the part solidifies in one direction while under pressure, porosity is eliminated almost entirely. Because new molten metal is added continuously to the inner diameter, any shrinkage is filled naturally. In addition, any defects that are less dense than the metal collect at the inner diameter and can be machined off for superior purity.
Centrifugal casting is limited based on size, as well as a part's height to diameter ratio. If the height is more than twice the diameter, it becomes nearly impossible evenly distribute the molten metal up the entire height of the wall. Centrifugal casting is also unreasonable for parts with very small diameters, as the centrifuge would have to spin incredibly fast to produce the necessary forces at such a small radius.
Pros: High quality due to directional solidification. Large cost benefit for continuous, high volume production.
Cons: Only possible for constant cross-sections (usually simple bars).
Continuous casting is a popular process to make simple shapes with continuous cross sections such as round and cube bars. This non-stop process can be extremely cost effective, especially for large quantity orders. The parts solidify from the outside in, which maintains good properties. The strand is pulled from the mold and cut at predetermined lengths by torches or shears.
Michigan Manufacturing International provides casting services as well as many other methods of production to American OEMs. Our engineers are available to help you ensure the best production process for your custom part. Learn more on our website, or download our brochure.