Metal Casting Processes: Techniques and Applications

Posted on Jul 1, 2025 in Industrial Chemical Process Engineering

Casting Processes: Manufacturing Complex Geometries

Casting is a fundamental manufacturing process used to create complex geometries and components, often with internal holes. It consists of pouring molten metal into a mold, allowing it to cool and solidify, and then extracting the finished casting from the mold.

1. Non-Permanent Mold Casting

These processes utilize molds that are destroyed or consumed during the casting process, meaning a new mold is required for each casting.

1.1. Sand Casting

Sand casting is the most widely used casting process. Almost all metals can be sand cast (e.g., steel, nickel, titanium, etc.), and it can produce small or large pieces, from single prototypes to millions of units.

Steps:
  • Prepare the mold.
  • Pour the molten metal into the mold.
  • Allow it to solidify.
  • Break the mold to remove the casting.
  • Clean and inspect the casting.
  • Apply heat treatment to the casting if necessary.
Key Components:
  • The Pattern: A full-sized model of the part, made slightly bigger to account for shrinkage and machining allowances in the final casting.
  • Core: A full-scale model of the interior surfaces of the part. It is inserted into the mold before pouring, and chaplets are necessary for holding it in place.

1.2. Flaskless Processes

These are variations of sand casting where the mold is not contained within a traditional flask, often used for automated production.

1.3. Shell Molding

In shell molding, the mold is a thin shell of sand held together by a thermosetting resin binder.

Steps:
  • A metal pattern is heated and placed over a box containing a sand and resin mixture.
  • The box is inverted, causing a layer of the mixture to partially cure on the surface, forming a hard shell.
  • The box is repositioned again so uncured particles drop away.
  • The sand shell is heated for several minutes to complete curing.
  • The shell mold is stripped from the pattern.
  • Two halves of the shell mold are assembled, supported by sand or metal shot.
  • The finished casting is removed (sprue removed).
Advantages:
  • Smooth cavity surface permits easier flow of molten metal and better surface finish.
  • Good dimensional accuracy.
  • Machining is often not required.
  • Mold collapsibility avoids cracks in the casting.
  • Can be mechanized for mass production.
Disadvantages:
  • More expensive metal pattern.
  • Difficult to justify for small quantities.

2. Casting Processes in Permanent Molds

These processes use molds that are designed for repeated use, typically made from metal or refractory materials.

2.1. Gravity Casting

Gravity casting uses a metal mold typically made of two sections. Molds for lower melting point metals are made of steel or cast iron. Molds for casting steel must be made of refractory materials. The mold can be reused.

Steps:
  • The mold is preheated and coated.
  • Cores are inserted, and the mold is closed.
  • Molten metal is poured into the mold.
Advantages:
  • Good dimensional control and surface finish.
  • More rapid solidification leads to stronger castings.
Limitations:
  • Limited to metals with low melting points.
  • Simple part geometries compared to sand casting.
  • High cost of the mold.
Applications:
  • High-volume production.
  • Automotive pistons, pump bodies.
  • Commonly used with aluminum, magnesium, cast iron, etc.

2.2. Injection or Die Casting

Injection or die casting is a permanent mold process where molten metal is injected into the cavity under high pressure. Pressure is maintained during solidification, which distinguishes it from other permanent mold processes.

Die Casting Machines:
  • Hot-Chamber Machine: Metal is melted in a container, and a piston injects liquid metal under high pressure into the die. Capable of producing up to 500 parts per hour. Limited to low melting point metals (e.g., zinc, lead, magnesium).
    Steps:
    • With the die closed, molten metal flows into the chamber.
    • A plunger forces the metal in the chamber to flow into the die.
  • Cold-Chamber Machine: Molten metal is poured into an unheated chamber from an external melting container, and a piston injects the metal under high pressure into the die cavity. Offers high production rates, though not as fast as hot-chamber machines. Suitable for aluminum, brass, etc.
    Steps:
    • With the die closed, molten metal is poured.
    • A ram forces the metal to flow into the die.
Advantages:
  • Economical for large production volumes.
  • Good dimensional accuracy and surface finish.
  • Thin sections possible.
  • Rapid cooling.
Limitations:
  • Primarily for low melting point metals.
  • Geometry must allow for easy removal from the die cavity.

2.3. Low-Pressure Die Casting Process

This process uses gas at low pressure to push molten metal into the mold cavity. Molten material is pushed into the mold exceptionally cleanly.

Advantages:
  • Very little turbulence, which minimizes gas porosity and dross formation.
Disadvantage:
  • Cycle times are longer than gravity permanent molds.

3. Special Casting Processes

These processes offer unique advantages for specific applications, often involving more complex techniques or materials.

3.1. Lost Wax Process (Investment Casting)

The lost wax process, also known as investment casting, uses a pattern made of wax and is known for its precision casting capabilities.

Steps:
  • Wax patterns are produced.
  • Several patterns are attached to form a pattern tree.
  • The pattern tree is coated with a thin layer of refractory material.
  • The full mold is formed by covering the coated tree with additional refractory material.
  • The mold is inverted and heated to melt the wax and drain it out.
  • Molten metal is poured into the mold and solidifies.
  • The mold is broken away to reveal the casting.
Advantages:
  • Complex parts can be cast.
  • Close dimensional control and good surface finish.
  • Wax can be reused.
  • Machining is often not required.
Disadvantage:
  • Many processing steps.
  • Expensive process.

3.2. Centrifugal Casting

In centrifugal casting, centrifugal force distributes molten metal to the outer regions of the cavity, often used for cylindrical parts.

3.3. Lost Foam Casting

Lost foam casting involves a mold of sand packed around a polystyrene foam pattern that vaporizes when molten metal is poured into it.

Steps:
  • The foam pattern is coated with a refractory compound.
  • The foam pattern is placed into a box, and sand is packed around it.
  • Molten metal is poured into the pattern, causing the foam to vaporize.
Advantages:
  • The pattern does not need to be removed from the mold.
Disadvantage:
  • A new pattern is required for every casting.

3.4. Vacuum Casting

Vacuum casting is a process that uses vacuum pressure to draw molten metal into the mold cavity, often for intricate parts or to minimize gas defects.