Methods of Manufacturing: Expendable Mold Casting
By Mary Iannuzzi on March 26, 2020
Casting is a manufacturing process that forms parts by solidifying molten, liquid metal within a mold.
Casting is most often used for intricate shapes would that would be too expensive to make with other methods. Casting is also economical for low production volumes and for parts that require a short manufacturing lead time.
There are two main classes of casting in use today: expendable and non-expendable. This blog focuses on expendable mold casting. Expendable mold casting is a casting method that uses temporary or single-use molds. The most common forms of expendable mold casting are sand casting, plaster casting, investment (lost wax) casting, and evaporative pattern casting.
Pros: Cheap, quick, versatile, good for low volume orders.
Cons: Rough surface finish, often requires additional machining.
One of the oldest methods of casting, sand casting is still popular today. Simple and quick, sand casting allows production of low-quantities, often with little to zero up-front cost. It is versatile, able to handle most types of metal at sizes ranging from small hand-held items to massive locomotive and rail parts. Although the mold is expendable, the sand itself can be easily recycled, keeping production costs low. The surface finish of a sand casting is quite rough (similar to packed, fine sand on a beach). Many sand cast parts need to be partially or fully machined after casting to achieve certain tolerances or aesthetic expectations.
Pros: Cheap, quick, excellent surface finish, complex shapes.
Cons: Unusable with ferrous metals, limited to smaller lighter parts (~100lb max).
Plaster casting is the same process as sand casting but with a mold made of plaster of paris rather than sand. Plaster can produce very intricate near net shapes, or castings with required surface finishes and tolerances straight out of the mold. This can eliminate the need for additional machining. The mold is easy to make, usually taking under a week, but the rate of production is slightly slower than sand molds. Plaster molds can’t handle extremely large, heavy parts nor high temperatures, and are therefore limited to non-ferrous metals with low meting points.
Investment (Lost Wax) Casting
Pros: Very precise, usually net shape, finish, and tolerance as-cast. Good for high-temperature casting.
Cons: High upfront costs, only economical for large quantities.
Investment molds are molds made from wax that are invested, or surrounded by, refractory materials. The wax itself is formed into the final shape of the desired cast product. This is done using a permanent mold made from metal. The molded wax shape is then covered in a refractory material such as ceramic to make a shell of the shape. The wax is melted out of the refractory shell and this shell is used as the mold for the final casting. After the casting is solidified, the refractory material is broken off. This allows for a very smooth, net-shape finish and helps avoid parting lines seen with permanent molds. Investment casting is very versatile and allows for nearly every type of metal. There is often little to no post-production finishing needed, even with tight tolerances and fine surface finishes. Because the process requires so many steps, it can be very costly compared to sand or plaster casting. The upfront costs can be high due to the need for a master die. This type of casting is more cost effective for very high quantity production.
Evaporative Pattern (Lost Foam) Casting
Pros: Simple and relatively inexpensive. Can make complex shapes without cores. Good surface finish and dimensional accuracy.
Cons: Pattern costs for low-volume production and tooling cost for high volume dies can be expensive. Patters can be damaged or distorted if recklessly packed.
This concept is similar to lost-wax casting but uses a foam model instead of wax. Where wax needs to be melted out of a refractory material before use, foam patterns remain inside of the mold material (usually sand) and instantaneously evaporate when molten metal is poured onto them. This process can be applied to a wide variety of ferrous and non-ferrous metals. There are no parting lines in this process and complex shapes can be made without the need for cores. While higher production orders could necessitate an injection molded foam model, lower quantity batches can be made by CNC machining foam block. This is a very simple method and relatively inexpensive. There can be high upfront costs if a metal die is needed to make the foam pattern. The process must also be performed with care as the foam models can be easily damaged.
A few of the most common problems or defects seen with cast parts are shrinkage defects, porosity, and mold material defects.
Shrinkage always occurs during casting. This is because the metal is changing from a less dense, liquid form to a more dense, solid form. In other words, if a mold were 100% full of liquid metal, it would not be 100% full once the metal solidified. To combat this issue, risers are added. Risers are additional cavities in a casting mold that allow extra liquid metal to be available to fill in the gaps caused by shrinkage. These risers must be strategically placed to allow proper directional solidification.
Porosity is the formation of gas bubbles within a solidified part. These bubbles form as the metal cools into a solid because the solid form cannot hold dissolved gas like a liquid can. These gas bubbles can form on the surface of the metal or they can form within the center of the material. This leads to a reduction in strength around the bubbles. Common methods for preventing porosity include melting the metal in a vacuum or in a flux that prevents air contact.
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.