Lost Foam Casting Process: Steps, Benefits & Applications

Table of Contents

Origin of Casting

The origin of casting can be traced back to the late Neolithic period. At that time, people had already learned to use metals such as copper, gold, and tin, and by melting these metals and pouring them into simple molds, they were able to create tools and ornaments. A significant leap in casting technology occurred with the use of bronze (an alloy of copper and tin), marking the beginning of the Bronze Age (approximately 3000 BC to 1000 BC). During this period, casting techniques became more advanced, with the appearance of fine casting tools and complex mold-making methods.

Ritual Bronze from Sanxingdui Made Using Investment Casting, Before 110 BCE

Casting Methods

There are many types of casting methods. The method used for casting bronze was Lost Wax Casting (also known as Investment Casting in modern times). The principle behind this method is to first create a model that can melt, then encase it in a solid shell. After melting the model, molten metal is poured into the cavity, and once the metal cools, the desired casting is obtained. In addition to lost wax casting, other “lost mold” methods include lost sand casting, lost wood casting, and lost foam casting, all of which fall under the category of lost mold casting.

Lost Foam Casting Process

Create the Foam Model:The foam model is precisely processed by cutting and carving to ensure it meets the design requirements.
Coating the Model with Refractory Material:resistant material that can withstand high temperatures during casting and prevent chemical reactions between the casting metal and the mold surface.
Wrapping with resin Sand:The coated foam model is placed into a sand box, and dry or wet sand is used to encase the model.
Form the Basic Model:The sand is vibrated or compacted to ensure that it is tightly packed around the model.
Metal Pouring:Molten metal is poured into the cavity formed by the foam model and the shell-coated foam model is heated. The shell now withstands the high temperature of the molten metal, which fills the cavity to form the desired casting shape.
Removing,Cleaning and Finishing the Casting：Broken the Shell to remove the casting，then do Mechanical or manual cleaning to remove any residual sand, coating, or impurities,after that will be finishing processes to meet the desired dimensions and surface finish.

Characteristics of Lost Mold Casting

Lost mold casting is favored by many foundries due to its unique advantages. Compared to other casting techniques, the biggest benefit of the lost mold process is that once the white mold is buried in the sand, it no longer needs to be removed, simplifying the casting process and greatly improving the dimensional accuracy and surface quality of the castings. The appearance quality is comparable to precision castings made with investment casting.

The characteristics of lost mold casting include:

No need for core sand: Even for complex castings, a single white mold suffices.
No need for parting: The molding process does not require parting line design, making the process more flexible.
Flexible gating system: The gating system, including risers, can be easily designed to fit any location on the casting.
Use of dry sand molding: No binders are needed, which facilitates easy removal of sand and allows for a high sand reuse rate.
High production efficiency: It requires fewer workers, facilitates mechanization and automation, and requires a smaller initial investment with low labor intensity.
Increased casting density: The use of vacuum casting and solidification enhances the casting’s density.
Improved workshop environment: The vacuum casting process collects and expels gases efficiently, improving the workshop air quality.

Requirements for Lost Mold Casting

Lost mold casting has strict requirements for production conditions, as the quality of castings is sensitive to the performance and stability of raw and auxiliary materials. Since the process design is complex, each casting requires custom design. Thus, the following factors need to be ensured:

Stability of materials: Raw and auxiliary materials such as foam beads, adhesives, coatings, and dry sand should be stable in source, composition, and performance.
Vibration table performance: The vibration table must meet the requirements for vibrating dry sand to fill complex internal cavities of the white mold, and the white mold must maintain stable dimensions and shape.
Temperature and flow control: The cooling methods for old sand should prevent deformation of the white mold due to heat.
Precise alignment: The bonding of white mold pieces must be accurately aligned (a machine bonding system is recommended).
Coating thickness uniformity: The coating must have good adhesiveness, leveling properties, gas permeability, strength at both room and high temperatures, and collapse properties (using coating leveling machines is recommended).
Mold precision: The white mold should have high precision, hardness, wear resistance, and stability (using integral white molds and reducing the number of joints and seams is recommended).

Additionally, it is crucial to have a team of skilled engineers and technicians who understand the principles of lost mold casting and can design and manage operations according to the specific material, structure, and size of the casting.

Application Scope of Lost Mold Casting

It is important for manufacturing companies to recognize that lost mold casting is not a universal process. Given the current technological level in China, based on the basic principles, the flow patterns of molten iron, the gasification of the white mold, and the movement of gas residues, the lost mold process is best suited for the following types of castings:

Material: Gray iron castings are most suitable, followed by ductile iron, cast steel, and finally, cast aluminum.
Structure: Castings with uniform wall thickness, complex structures, and wall thicknesses ranging from 10-20mm are ideal, such as housings and shell-type castings.
Size: Castings weighing from several kilograms to several hundred kilograms are most suitable, with batch sizes ideally over 10,000 pieces. White molds are formed using molds, and pouring is done using production lines.
Cast steel parts: Best suited for non-machined or minimally machined cast steel parts that have fewer requirements for internal defects, such as wear-resistant, heat-resistant, and corrosion-resistant castings.
Large castings: Large castings made with resin sand molds, such as automotive body panels and machine tool beds, are well-suited for this process. White molds are made by cutting sheet material, CNC machining is used, and pouring is done using either atmospheric or vacuum pouring.

Although the lost mold process can be initiated with minimal investment, eliminating the cost of wooden molds and reducing dependence on skilled workers, for large foundries, it will not cause a significant disruption. This is because the production efficiency and pace of lost mold casting lag behind advanced automated vertical molding lines or static pressure molding lines, and large foundries typically produce certain types of standard products and have the capability to purchase automated molding lines.

Limitations of Lost Mold Casting

Despite the numerous advantages, lost mold casting has some shortcomings and limitations:

Molten iron flow issues: Irregular flow can lead to internal slag inclusion and gas entrapment.
Increased slag volume: The addition of a flame retardant in the white mold materials in China can increase slag volume.
Vacuum casting issues: The use of negative pressure can lead to more erratic flow of molten iron and hinder the removal of slag and gas.
Temperature differences: During pouring, the temperature differences between the leading edge and the rest of the molten iron can cause poor gasification of the white mold, leading to carbon build-up.
Coating quality dependence: The gases generated during pouring can only be expelled through narrow coating gaps, making the quality of the coating crucial.
Dry sand molding issues: Dry sand has lower thermal conductivity than wet sand, which may reduce the hardness of the casting.
Toxic gas emissions: The gasification of the white mold can release toxic gases, contaminating the air.
Aluminum casting issues: For aluminum castings, lower pouring temperatures can result in poor gasification of the white mold, leading to sand contamination and reduced permeability, which increases the likelihood of porosity.
Mold strength issues: The white mold’s low strength makes it prone to deformation during vibration, especially for complex internal cavities, which can lead to sand sticking in the cavities.

In conclusion, while the lost mold process offers high precision and flexibility, it is best suited for specific casting types. Each casting method has its own ideal applications, and the use of lost mold casting should be tailored to the characteristics of the enterprise and product requirements.