What is vacuum investment casting?

Manufacturers in high-compliance industries such as aerospace, defense, and energy continually identify ways to improve the quality and precision of their metal components.  One such advanced method is vacuum investment casting.  If you are familiar with traditional metal casting processes but have not explored the potential of vacuum investment casting, this blog will offer insight into how it can improve the performance and quality of your parts.

GD-HUB contains other products and information you need, so please check it out.

What is Vacuum Investment Casting?

Vacuum investment casting is a specialized form of investment casting.  While the traditional investment casting process takes place in open-air conditions, vacuum investment casting incorporates the use of a vacuum to improve precision and material integrity.  The vacuum investment casting process is ideal for producing complex, high-quality metal parts with fine details, minimal defects, and superior mechanical properties.

In traditional investment casting, a wax pattern on a &#;tree&#; with identical patterns is coated with a ceramic slurry and stucco material to create a shell.  The wax is melted away from the shell, leaving a hollow mold. After preheating, molten metal is poured into this mold to form the desired part.  Vacuum investment casting enhances this process by using a vacuum to eliminate air and gas, reducing the risk of porosity and other casting defects.

How Does Vacuum Investment Casting Work?

The vacuum investment casting process follows a sequence of carefully controlled steps that improve upon the traditional investment casting process. Steps 1, 2, 3, and 6 below are identical to traditional investment casting. The critical differences are the vacuum furnace and vacuum chamber for casting.

1. Wax Pattern Creation: Vacuum investment casting begins with the creation of a wax pattern of the part to be produced. This pattern is mounted onto a &#;tree&#; with identical patterns.
   
   
2. Investment: Pattern is coated in multiple layers of ceramic slurry and stucco material to form a ceramic shell. This &#;investment&#; is allowed to dry and harden.
   
   
3. Dewaxing: The shell is placed in an autoclave, which melts the wax pattern away. This leaves a hollow mold in the shape of the part.
   
   
4. Preheating in a Vacuum Furnace: The mold is placed in a vacuum furnace, which removes air bubbles and gases.  This prevents porosity and other internal defects in the final part.
   
   
5. Casting in a Vacuum Chamber: The mold is transferred to a vacuum chamber for casting.  The vacuum helps molten metal flow uniformly, filling intricate details and thin walls with even greater accuracy than traditional investment casting.
   
   
6. Shakeout and Finishing: After solidification the ceramic shell is removed in a shakeout process, leaving the part.  Normal finishing processes such as machining, grinding, and surface finishing are often not needed after vacuum investment casting.
   
   

Casting in a vacuum helps molten metal flow uniformly, filling intricate details and thin walls with even greater accuracy than traditional investment casting.

Why Use Vacuum Investment Casting?

Vacuum investment casting offers several distinct advantages over traditional investment casting:

Reduced Porosity and Defects

One of the biggest challenges in most metal casting processes is controlling gas porosity, which can weaken parts and ultimately lead to failure over time.  The vacuum ensures that air and gas pockets are eliminated, resulting in more durable metal parts.

Superior Surface Finish

The vacuum investment casting process often results in an even smoother surface finish compared to traditional investment casting.  This reduces the need for post-casting machining and surface treatments, reducing lead time and costs.

Complex Geometries

With vacuum investment casting, manufacturers can produce highly intricate designs that would be difficult or impossible to achieve with other metal casting processes.  Vacuum investment casting excels at forming thin walls, sharp corners, and fine features, making it perfect for components with complex geometries.

Material Versatility

Vacuum investment casting is compatible with a wide range of metals and alloys, including stainless steel and nickel-based superalloys.  This versatility makes it ideal for applications where specific material properties are critical to performance.  Superalloys that contain reactive elements can easily be cast without being negatively affected by an oxygen rich environment.

Applications of Vacuum Investment Casting

The unique benefits of vacuum investment casting make it a preferred choice in industries where precision and material integrity are paramount. Common applications include:

Aerospace

Engine components and structural components often require high-strength, lightweight materials with intricate designs.

The unique benefits of vacuum investment casting make it ideal for demanding industries such as space exploration.

Defense

Critical components for naval systems and weapons systems benefit from the superior durability, precision, and performance offered by vacuum investment casting.

Energy

Components for gas turbines, pumps, and valves rely on the process for producing high-performance parts that operate in extreme conditions.

Is Vacuum Investment Casting Right for Your Application?

Traditional investment casting produces detailed metal parts with tight tolerances to customer specifications.  Vacuum investment casting improves on these benefits, offering higher-quality parts with reduced porosity and smoother surfaces.  This leads to better performance and durability. The process also enables the production of more intricate designs with greater precision, allowing customers to receive parts that meet tighter tolerances and more complex specifications.  Super alloys can be used to improve part performance at high temperatures. 

MetalTek&#;s Wisconsin Investcast Division uses both traditional investment casting and vacuum investment casting.  Not sure which process is best for your application? 

Contact us to learn more.

About the Author

Mike McCleary is Sales Manager for MetalTek&#;s Wisconsin Investcast Division.  He joined MetalTek in and previously served as a Sales Engineer at MetalTek&#;s Wisconsin Centrifugal Division.  Mike holds a BS in Mechanical Engineering from Marquette University.

Explore more:
Do uPVC doors look cheap?

Contact us to discuss your requirements of Precision Surface Finishing. Our experienced sales team can help you identify the options that best suit your needs.

Vacuum investment casting yields extremely high quality metal parts with fine detail and excellent surface finish. It&#;s also more complex than conventional investment casting and requires sophisticated melting and mold-filling equipment. As a result, it&#;s reserved for casting parts that need very high levels of structural integrity and where defects are expensive.

Why Use Vacuum?

Liquid metal tends to &#;churn&#; as it flows into a mold. This turbulence leads to two types of defects: oxides and porosity.

Oxides are formed when metal atoms bond with oxygen. This takes place at the metal-air where it forms an oxide layer. On some metals, such as aluminum, surface oxidation is beneficial because it creates a corrosion-resistant layer. However, it&#;s a problem during casting.

This is because, as the liquid metal churns, oxides form on the surface and get incorporated into the body of the cast part. Once the metal solidifies these discontinuities form areas of weakness.

Porosity is the formation of bubbles within the body of the metal. Like oxides, these reduce strength, especially in high stress areas when exposed by machining.

One way to minimize turbulence, oxidation and porosity is to fill slowly from the bottom of the mold. Most casting companies try to do this but part design and cycle time constraints often limit what&#;s possible. Another method is to extract air from the mold before the metal flows in.

The Vacuum Investment Casting Process

Investment casting is used to create complex metal parts that need little secondary machining. The mold is formed by coating a wax pattern with a ceramic slurry. After the slurry has dried to a hard shell the wax is melted out to leave a cavity for the metal to fill.

In vacuum investment casting, the mold is made the same way but then air is pumped out of the mold cavity, before it&#;s filled with metal. Since the objective is to prevent the liquid metal from oxidizing, during melting and pouring they are both done under vacuum. This takes place in a vacuum induction melting machine (VIM).

In one VIM configuration, the machine consists of two chambers arranged vertically.  Another VIM configuration contains an efficient horizontal three-chamber vacuum induction system.  The vertical two-chamber configuration has an upper chamber and lower chamber which are under vacuum. The ceramic investment casting shell is placed into the lower chamber then pumped down to a low micron level. Next, an isolation valve is opened and the mold is raised into a position, close to the melt crucible, for pouring.  Once full, with metal, the shell is removed from the mold vacuum chamber. When solidification is complete, the ceramic is broken away to reveal the cast parts.  The horizontal three-chamber VIM system has isolation valves which allow for vacuum level consistency between chambers. Efficiency is increased by crucible ramming and melting integral to the casting cycle providing for more melts/more pounds poured per hour while maintaining high vacuum levels and superior casting quality.

In a variation of this process the crucible chamber may be pressurized. (Using an inert gas avoids oxidation.) The pressure increases mold filling speed and helps ensure every part of the mold cavity is filled.

Benefits of Vacuum Investment Casting

The primary advantage of this process is the near total elimination of oxidation and porosity defects and flaws within the cast part. (Only &#;near&#; because it&#;s impossible to extract every molecule of oxygen from the mold.) Furthermore, it prevents non-fills caused by air pockets.

Other benefits are that the process can form very fine features and exceptionally thin-walled sections because the vacuum ensures the cavity is completely filled. In addition, surface quality is excellent.

Metals for Vacuum Investment Casting

Any metal suitable for investment casting can be processed under vacuum. However, the process is most often used with nickel, cobalt-based superalloys and titanium alloys as these metals have exceptionally high affinity for oxygen and form oxides readily. In addition, the high value of these metals and their use in complex, high-value products makes it beneficial to minimize scrap.

Should You Use Vacuum Investment Casting?

Drawing a vacuum on mold tools before metal is poured prevents air from creating defects within the body of a casting. Combined with investment casting, this technique enables production of high quality parts suitable for high-load and high-stress applications. Process economics make vacuum investment casting especially attractive for high-value metals like nickel, cobalt-based superalloys, and titanium alloys.

Impro casting specialists can help you decide if vacuum investment casting would be right for the parts you make. Contact us to start that discussion.