Investment Castings | What Is the Vacuum Casting Method?

Vacuum investment casting results in metal pieces that exhibit remarkable detail and superior surface finishes. The process is more complex than standard investment casting exporters, necessitating specialized equipment for melting and mold filling. Therefore, it is primarily employed for components requiring exceptionally high structural integrity, where costly mistakes are unacceptable.

Why Should You Vacuum?

When liquid metal is poured into a mold, it can generate turbulence, leading to two types of defects: oxides and porosity.

During this process, metal atoms may bond with oxygen, forming oxides, particularly at the metal-air interface, thus creating an oxide layer. While surface oxidation can be beneficial for certain metals like aluminum—providing a corrosion-resistant coating—it poses a challenge in casting. The turbulence that occurs during the pouring process can facilitate the absorption of oxides into the cast part, ultimately resulting in weak points once the metal solidifies.

To mitigate these issues, a gentle filling from the bottom of the mold is carried out to reduce turbulence, oxidation, and porosity.

The vacuum casting method is divided into five steps:

1. The Component 3D Model

The journey of producing any vacuum cast component starts with generating a 3D model. This model is developed according to the same design standards applicable to injection molding molds. Precise calculations are essential prior to creating the 3D model, as deformities can adversely affect the cast product. Consequently, it's crucial to follow design guidelines and adjust calculations as necessary.

2. Creating the Mold's Master Pattern

The master pattern is created using 3D rendering technology, serving as the primary mold for producing silicone molds utilized in casting. Constructed from robust materials such as metals or other polymers, it acts as a prototype for these silicone molds.

Previously, CNC or manual tools were employed to create master patterns, but advancements like 3D printing, stereolithography, and other technologies have enhanced cost-effectiveness and accuracy in making these patterns.

3. Silicone Mold Making

After the master pattern is complete, the next step involves creating silicone molds directly using the master pattern and a casting box. The casting box houses the master pattern, which is equipped with cores and inserts, as well as casting gates and risers for pouring molten silicone and extracting the final mold.

Hot, molten silicone is poured around the master design inside the casting box.

4. Making Polyurethane Resin

Before casting, the polyurethane (PU) resin mixture is prepared by combining the resin with pigments and other components, followed by heating. The choice of polyurethane with different material and structural properties depends on the intended finished product.

The preparation of the resin mixture involves heating the two resin components in a container to around 40°C while ensuring that color pigments are thoroughly blended into a homogeneous solution before pouring into the molds.

5. Demolding and Curing

Once the resin mixture has fully set in the mold and is securely sealed, it is then placed in the vacuum casting machine for casting. The required temperature and casting duration may differ based on the type of polyurethane used, yet the fundamental process remains consistent.

After casting, the mold remains in the casting machine during the curing phase, essential for maintaining structural integrity post-removal.

Conclusion

The vacuum investment casting method stands out as a cost-effective and efficient production technique. Compared to older methods, such as plastic injection molding and pressure die casting, vacuum casting offers much lower manufacturing expenses.