Lost foam casting is a type of investment casting where a polystyrene foam pattern is used to create a mold for metal casting. The foam pattern is vaporized during the pouring of molten metal, leaving a precisely shaped metal casting.
Here's a breakdown of the process:
-
Pattern Creation: Pre-expanded polystyrene beads are injected into a die (mold) that is shaped like the desired casting. This is often done using heat and low pressure. Steam is then applied to further expand the polystyrene beads and completely fill the die. This results in a lightweight foam pattern. The final pattern is mostly air (approximately 97.5%) with a small percentage (2.5%) of polystyrene. Multiple foam pieces can be glued together to form a more complex pattern.
-
Pattern Coating: The foam pattern is then coated with a refractory slurry. This coating creates a barrier between the foam and the molten metal, preventing the metal from penetrating the foam too quickly and resulting in a rough surface finish. It also adds strength to the foam pattern during handling.
-
Pattern Embedding: The coated foam pattern is placed inside a casting flask and surrounded by loose, unbonded sand. This sand supports the pattern during the pouring process and helps to maintain the shape of the mold cavity. The sand is typically compacted by vibration.
-
Metal Pouring: Molten metal is carefully poured directly onto the foam pattern. As the metal comes into contact with the foam, the heat vaporizes the polystyrene. The metal then fills the space previously occupied by the foam, perfectly replicating its shape. Gases produced during vaporization escape through the permeable sand.
-
Cooling and Shakeout: The metal is allowed to cool and solidify. Once cooled, the casting flask is broken open, and the casting is removed. The sand is reusable.
-
Finishing: The casting undergoes typical finishing processes, such as removing gates and risers, cleaning, and machining, if required.
Key Advantages of Lost Foam Casting:
- Complex Shapes: Allows for the creation of intricate and complex shapes that are difficult or impossible to achieve with other casting methods.
- High Accuracy: Produces castings with excellent dimensional accuracy and surface finish.
- Reduced Machining: Often requires less machining than other casting processes, saving time and material.
- No Draft Angles: Draft angles are not required on the pattern, allowing for more flexibility in design.
- Reduced Core Usage: Eliminates the need for cores in many applications.
Applications:
Lost foam casting is used to produce a wide range of metal components, including:
- Automotive parts (engine blocks, cylinder heads)
- Pump housings
- Valve bodies
- Machine tools
