For this article, we decided to take it back to the basics and explain what metal casting is, and the various casting processes.
What is metal casting?
The casting process uses molten metal poured into a mold that contains a negative impression of the intended shape. When the mold and metal have cooled, the casting (the product) is removed from the mold. Metal casting is used for a variety of industries to make complex shapes that are difficult or not cost-effective to make by other methods.
All casting processes are not the same and will differ based on the size and complexity of the part being produced and the metal or material used. Casting processes are divided into two categories: expendable mold and non-expendable mold casting. Within these categories, processes are further divided based on mold material and pouring method.
Expendable mold casting
Expendable mold casting is a broad classification that includes sand, plastic, shell, plaster, and investment castings. This method of mold casting involves the use of temporary, non-reusable molds, hence the term “expendable.” Below are explanations of the more commonly used casting techniques.
Traditional casting techniques include lost-wax casting, which can also be called investment casting. This is one of the oldest known metal-forming techniques, having been practiced for thousands of years.
In the investment casting process, a wax pattern of the desired finished part is created and mounted onto a wax tree (or pattern). The wax tree is alternatively dipped into a ceramic slurry and sprinkled with stucco material in a controlled atmosphere and allowed to dry. This process is repeated until a thick shell is created over the wax pattern. The wax is melted out, leaving a cavity in the shape of the pattern. The cavity is subsequently poured with molten metal and allowed to solidify. After solidification, the shell is broken away, gating material is removed and the finished part remains.
Sand casting is another popular traditional technique used in metal casting and manufacturing. This process has been used for centuries and is cost-effective for producing small batches.
In the sand-casting process, a pattern of the desired finished part is constructed out of hardwood, urethane, metal, or foam. Sand containing bonding materials is packed around the pattern to form the mold. The pattern is removed from the bonded sand leaving a cavity in the mold that is the shape of the part. Internal passageways, including intricate structures, can be added to the mold by adding cores. Molten metal is poured into the cavity and the metal solidifies. The sand is removed through a shakeout process. The gates and risers (rigging) are removed, leaving the desired part for finishing.
Non-expendable mold casting
Non-expendable mold casting differs from expendable processes in that the mold does not need to be reformed after each production cycle. This form of casting delivers near-net-shaped results, reducing the need for surface finishing, thus reducing production costs.
The die-casting process forces molten metal into mold cavities. Most die castings are made from nonferrous metals, like zinc, copper, and aluminum-based alloys, but ferrous metal die castings are achievable. The die-casting method is especially suited for applications where many small to medium-sized parts are needed with good detail, fine surface quality, and dimensional consistency.
The centrifugal-casting process delivers products of very high material soundness and material properties and is the technology of choice for applications requiring the most stringent of product reliability.
In the centrifugal-casting process, molten metal is poured into a spinning die. The die can be spinning either on a vertical or horizontal axis depending on the configuration of the desired part. Ring and cylinder type shapes are cast vertically; tubular shapes are made with the horizontal centrifugal process. Either process may be used to produce multiple parts from a single casting. External structures and shaping can be cast in place to significantly reduce post-processing including machining or fabrication.
Continuous casting is a process that delivers cast products in a near-net shape, reducing machining costs. Premium quality continuous cast bars and shapes are straight, true, and concentric to keep high-speed bar machining centers running. Many non-traditional shapes are also available.
In the continuous casting process, molten metal is poured through a die of the desired outside profile. If an inner shape is also desired, the material may flow around a mandrel to create that profile. As the material flows through the water-cooled die, sufficient heat is extracted to solidify an outer shell. As the shape continues to solidify, the material is drawn continuously through pull rolls.
In essence, metal casting is a solidification process. The solidification phenomenon controls most of the properties of the casting. Additionally, a lot of the casting defects occur during solidification, such as gas porosity and shrinkage. We will discuss casting defects in a future article.
Our metallurgy experts consult with our clients on a project-specific basis to determine the best process for their product and deliver an engineered solution. If you’d like to discuss your project, contact us today to get started.
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