What are the stages of the casting process?

Metal casting processes—also known as liquid forming—refer to a shaping method where metal is melted by heat and then poured into pre-made molds (transforming the molten metal into a solid, mold-shaped structure). After solidifying, the metal takes on a specific form and exhibits desired properties, resulting in finished metal products. Liquid forming is one of the earliest manufacturing techniques. Although centuries of casting history have demystified this process, it remains a fundamental shaping technology. Castings boast advantages such as low production costs, high process flexibility, and excellent adaptability, making them ideal for producing components of varying materials, shapes, and weights—and particularly suited for mass production. However, the process also has drawbacks: it typically involves large tolerances (defined as the difference between the maximum and minimum allowable dimensions), is prone to internal defects, and often requires more materials and equipment than other methods. Additionally, casting operations can generate dust, harmful gases, and noise, leading to environmental pollution.

The casting process typically involves the following three stages:

1. Mold Preparation: Molds can be categorized based on the materials used, including sand molds, metal molds, ceramic molds, clay molds, graphite molds, and more. When a mold is removed from the casting, it typically gets damaged. The mold cavities are usually made from materials such as wax, wood, plastic, or metal. While molds are relatively expensive, they can be reused multiple times. However, due to their high cost and the significant expense of making on-site modifications—sometimes even rendering them irreparable—extra time is required for thorough design preparation. Moreover, the quality of the mold directly influences the overall quality of the castings.

2. Melting and Pouring of Cast Metals: Cast metals (casting alloys) primarily include cast iron, cast steel, and cast non-ferrous alloys;

3. Casting Treatment and Inspection: Casting treatment includes removing cores and foreign materials from the casting surface, eliminating sprues and risers, cutting off burrs and weld seams along with other protrusions, as well as performing heat treatment, shaping, rust-proofing, and rough machining.

There are many types of castings, typically categorized as follows: conventional sand casting, which includes wet sand molds, dry sand molds, and chemically hardened sand molds.

1. Wet sand molds and dry sand molds

A wet sand mold is created by compacting moist sand into a wooden or metal cope and drag within a molding box. Once assembled, the mold can be used for casting—either with or without cores—into the cavity, followed by pouring molten metal into the mold. After the part is removed, the mold itself is typically destroyed. In contrast, the moisture content of the sand used in dry sand molds is slightly higher than that of wet sand molds. After the mold is prepared, its cavity surface should be coated with a fire-resistant material to prevent the sand from overheating. This refractory coating not only safeguards the mold but also allows it to be dried thoroughly in an oven. Once cooled, the mold is ready for shaping and pouring. However, clay sand molds take significantly longer to dry, consuming substantial amounts of fuel in the process. Moreover, during drying, these molds are prone to warping or deformation, which can compromise casting accuracy and ultimately affect the quality of the final product.

2. Chemically Hardened Sand Mold

The molding sand used for chemically hardened molds is called chemically hardened sand. Its binder is typically a substance capable of undergoing molecular polymerization, which then transforms into a three-dimensional structure under the action of a hardening agent—commonly including various synthetic resins and water glass. The key features of the chemically hardened sand casting process are:

(1) The casting mold accurately reflects the dimensions and contour shape of the pattern, resulting in castings with high dimensional precision.

(2) This equipment features a lightweight design, low power consumption, and high productivity, allowing for a simplified sand-processing section.

(3) When using chemically hardened sand for molding, pattern materials such as wood, plastic, and metal can be selected according to production requirements.