Metal Casting Processes: A Comprehensive Guide

Posted on Sep 2, 2024 in Technology

Metal Casting Processes

Molding Methods

Hand Molding

As its name suggests, all operations in hand molding are manual, requiring highly skilled personnel. This method is only suitable for a very small number of pieces or parts that are very complicated and cannot be produced using other methods.

Mechanical Molding

In foundries with high production volumes and series production, mechanical molding methods replace manual methods for the manufacture of molds and cores. This offers the following advantages:

  • Does not require highly qualified staff.
  • Allows for more rational use of specialized staff and frees them from auxiliary operations.
  • Enables operators to quickly acquire the ability to produce molds, while learning is slower with manual methods.
  • Allows for the production of complex pieces accurately and quickly.
  • Enables the creation of very thin pieces.
  • Results in molds with more uniform compactness and higher strength, leading to better-finished pieces.
  • Facilitates the stripping operation without damaging the mold, thus saving repair costs.
  • Decreases the number of defective parts and improves overall quality.

Metal Casting Molds

Metal molds, also called shells, advantageously replace sand molds when making large series of the same piece. They consist of two main parts: a cast body that gives the outer shape of the piece (always metallic) and male parts that reproduce hollow cores or incoming parts (can be metallic or sand).

Fine-grained cast iron is the most common material used to manufacture the mold body. The lifespan of the shell depends on the manufacturing material, the material being molded, and the care taken during handling. A well-made and properly used cast can often withstand 20,000 to 40,000 casts without major alterations.

The main advantages of die casting are:

  • Greater precision in part dimensions compared to sand casting.
  • Smaller linear shrinkage.
  • Reduced space requirements and material handling.
  • Cheaper than sand casting for series production of over 1,000 pieces.

Its main drawbacks are:

  • High cost of shells and accessories.
  • Time and cost associated with the development and manufacturing of each piece.

Pressure Die Casting or Die Casting

This method differs from gravity die casting in that the molten or semi-molten metal is introduced into the mold cavity under pressure. This facilitates rapid mold filling and faithful reproduction of fine details. It also ensures the elimination of porosity in solid sections of the piece. The pressure required is directly proportional to the alloy’s tendency to form pores and blowholes. After sprue removal, the pieces are completely finished and require no subsequent machining. The metal structure is fine-grained and exhibits high mechanical properties.

Special Casting

Shell Molding

This process involves creating a thin shell mold using silica sand bonded with thermosetting synthetic resins. The shell is then placed on a heated metal pattern plate.

The process typically involves the following steps:

  • Heating the model plate to 200°C.
  • Spraying the plate with a silicone release agent to facilitate demolding.
  • Placing the model plate on a partially filled tank containing the sand mixture.
  • Inverting the tank to deposit the sand onto the pattern plate from a height of 15 to 30 cm, initiating shell formation.
  • Rotating the tank back to its original position, allowing excess sand to fall back into the tank while the bonded sand remains on the plate, forming the shell.
  • Transferring the plate with the attached shell to an oven, where it hardens at a temperature between 350-450°C for 2 minutes.

Lost Wax Casting (Investment Casting)

This process is executed as follows:

  • A wax model of the object is created.
  • The model is coated with a thick layer of plaster and silica sand mixture.
  • After air drying, the coated wax model is heated in an oven, melting the wax and leaving behind a cavity within the coating. This cavity forms the mold, accurately replicating the wax model’s surface.

This process is widely used for manufacturing small parts in series, achieving excellent surface finish and accuracy, often eliminating the need for machining. Examples of parts made using this method include:

  • High-speed steel cutting tools
  • Steam turbine blades
  • Jet engine components
  • Surgical instruments
  • Small permanent magnets
  • Textile machinery parts
  • Firearm components
  • Combustion engine parts
  • Tools and gauges
  • Stamping dies
  • Household appliance parts

Limitations of this process include its relatively high cost and limitations on part size, typically less than 500 grams, with the majority not exceeding 30 grams. However, parts up to 20 kg have been produced.

Castability

Even with a perfectly designed gating system, the metal may begin to solidify before completely filling the mold, especially in thinner sections. To prevent this, the alloy should have adequate castability, defined as its ability to completely fill the mold cavity. Castability depends on the amount of heat the molten metal can lose before solidifying, and therefore, its pouring temperature. It also depends on the cooling conditions within the mold and the casting speed.

Casting Inspection

Common casting inspection methods include:

  • Visual Examination: Performed immediately after stripping to identify visible defects before cleaning.
  • Dimension Control: Using special gauges for large quantities or a layout table for smaller batches.
  • Soundness Testing: Involves striking the hanging piece with a wooden mallet and listening to the sound to detect internal cracks.
  • Nondestructive Testing: Employs methods like magnetic particle inspection, liquid penetrant testing, radiography, and ultrasonic testing.
  • Metallographic Examination: Determines grain size and microporosity.
  • Mechanical Tests: Includes hardness, tensile, compression, bending, and fatigue tests.
  • Chemical Analysis: Verifies that the material composition remains within the desired limits.
  • Surface Finish: Assesses surface quality using a profilometer.

Metrology in Precision Industry

The significant industrial development over the last two centuries stems from the mass production of quality products at affordable prices. This shift from traditional craftsmanship to mass production necessitates the interchangeability of parts for assembly and replacement.

Achieving interchangeability requires stringent quality control, including standardized design criteria, tolerance specifications, systematic monitoring of machinery and tools, and rigorous final inspection. Metrology, the science of measurement, plays a crucial role in this process, particularly in precision industries where dimensional accuracy is paramount.

Metrology encompasses a range of operations, including measurement and comparison against standards, to determine product acceptance or rejection based on quality requirements. Metrotécnia, a specialized field within metrology, focuses on dimensional measurements using appropriate instruments and gauges to ensure parts conform to specified tolerances.

Shaping by Casting

Casting, also known as founding, is a metal forming process that involves pouring molten metal into a mold cavity, where it solidifies to take the shape of the mold. This versatile and economical process enables the manufacturing of complex parts that are difficult or impossible to produce using other methods. It also allows for the use of metals and alloys unsuitable for forming or welding processes.

Fundamental Operations

Casting involves three fundamental operations:

  • Melting: Involves heating the metal or alloy to a molten state in furnaces specifically designed for the material and temperature range.
  • Molding and Demolding: Encompasses mold preparation, which typically involves creating a cavity that replicates the desired part shape. This cavity, called the mold, can be made from sand, metal, or refractory material. Once the molten metal solidifies, the mold is removed in a process called demolding.
  • Finishing: Includes cleaning the casting, removing excess material, and preparing it for further processing or use.

Sand Casting

Sand casting, a method dating back to prehistoric times, remains a prevalent casting process. It involves creating a mold cavity using a mixture of sand and binding agents. The molten metal is poured into this cavity, where it solidifies to form the desired shape.

Plate Model

Plate models, typically made from metal, wood, plaster, or plastic, are mounted on a metal plate and serve as the pattern for creating the mold cavity. Metal models offer durability, accuracy, and smoother surfaces.

Molding Sand

Silica-aluminous sands are commonly used for making molds and cores. These sands are composed of quartz, clay, lime, and feldspar. While affordable and readily available, specialized sands with higher temperature resistance and improved surface finish are increasingly used. These include olivine, chromite, zircon, and staurolite-aluminum silicate sands.

Moisture content in the sand is crucial. Ideally, it should be between 5% and 7%. Lower moisture content reduces the sand’s strength, increasing the risk of mold erosion and inclusions in the casting. Conversely, higher moisture content can generate steam during pouring, leading to defects.