Vacuum Casting: Process, Pros & Cons, and Applications

Welcome to IDEAL's ultimate guide to Vacuum Casting: Process, Pros & Cons, and Applications!

Our blog is a comprehensive resource for anyone interested in learning more about this versatile and affordable option.

What is Vacuum Casting?

Vacuum casting, also called urethane casting, is a manufacturing process used to create small quantities of high-quality plastic or metal parts. It involves creating a mould of the desired part and then injecting liquid plastic or metal into the mould under a vacuum. This creates a uniform and detailed part with an excellent surface finish and dimensional accuracy.

Let’s see what are the fundamentals of vacuum casting, its applications and its limitations.

How Does Vacuum Casting Work?

Vacuum casting is a manufacturing process used to create high-quality prototypes and small batches of parts, typically from elastomers or resins, by replicating a master model in a silicone mold. The process uses a vacuum to remove air and ensure that the material fills the mold properly. Here's how it works step by step:

Step 1: Create the Master Model:

First, a master model of the part is made, often using 3D printing or cnc machining. This master can be made from materials like resin, metal, or plastic, depending on the application. The model should be highly detailed since the final product will be a direct replica. 

The master model is then placed inside a box, and liquid silicone rubber is poured over it. The silicone mold is left to cure, which takes a few hours. Once cured, the mold is flexible and can be opened to remove the master model. The mold captures all the details and features of the master model.

Step 2: Prepare the Casting Material:

A casting material, such as polyurethane resin, silicone, or wax, is chosen depending on the desired properties of the final product (like flexibility, strength, or temperature resistance).

The casting material is mixed, sometimes with additional pigments, fillers, or hardeners if needed.

Step 3: Make Silicone Rubber Molds

The silicone mold with the prepared material inside is placed in a vacuum chamber. The vacuum removes any air bubbles in the mold or material. This step is crucial for ensuring that the material fully fills the mold, especially for complex geometries, and for avoiding air pockets that can weaken the final cast.

The vacuum process also helps remove any trapped air in the material, ensuring the highest-quality finish.

Step 4: Mix the Polyurethane Resin

Various types of polyurethane (PU) resins are available with varying material and structural properties relative to the type of components to be manufactured. The polyurethane resins used in vacuum casting are usually heated to 40ºC before mixing. A two-component casting resin and any preferred pigment are combined in precise volumes and poured into a container to use the machine. The mold is then returned to its position while the pouring gates are fastened to the mixing and pouring vessels.

Usually, the vacuum-casting resins and color pigments are mixed continuously and deaerated for about 50 to 60 seconds in a vacuum during the auto-pouring process. Then, the liquid material is poured into the mold under a vacuum pocket to prevent air pockets. Vacuum technology ensures bubble-free molds and eliminates possible flow resistance caused by air bubbles inside the mould tool, and gravity ensures the tool gets filled properly.

Step 5: Casting & Demolding

Once the air is removed, the casting material is poured into the mold under vacuum pressure. This ensures the material flows into all the tiny cavities and details of the mold. After the material is in place, the mold is left to cure (depending on the material, this could take a few hours or more).

Once the casting material has cured, the silicone mold is carefully opened, and the cast part is removed. The final product is often very close to the master model, with high fidelity to its original details. Depending on the use case, the part may need to be cleaned, painted, or undergo further finishing steps, like sanding or polishing, to get the desired surface texture.

Designing Parts for Vacuum Casting

In vacuum casting, you can expect the general shrinkage rate — the difference between the master pattern and the cast part — to be approximately 0.15 to 0.25 per cent. This is less than the general shrinkage rate for injection moulded plastics.

While the process has many advantages, it also has its limitations. Simple design modifications such as adjusting rib thickness can help minimize sink effects and ensure better part quality. Check out how to design bosses, undercuts and more in our Vacuum Casting Design Guide.

Vacuum Casting Process Considerations

Shrinking

Shrinkage can cause irregular, overly thin, and thick features to deviate from normal tolerance, just like injection moulding. For vacuum casting, consider a 0.15% shrink rate.

Surface Quality

Even the tiniest detail will be replicated in the silicone mould. Hence for better results, it is essential to get an impeccable finish on the master model.

Silicone

Silicone moulds depreciate with use. Therefore, depending on factors such as the complexity of the part and material choice, the mould can only last around ten runs on average.

Vacuum Casting Design Guidelines

Wall Thickness: The minimum wall thickness of 0.75 mm is necessary to ensure even filling of the mold. However, at least 1.5 mm wall thickness is recommended to achieve the best results.

Bosses: Minimum height of 1.0mm; minimum diameter of 1.0mm

Wall Thickness: For Bosses No more than 60% of wall thickness

Rib Thickness: No more than 60% of wall thickness

Rib Height: No more than 3x of rib thickness

Engraved and Embossed Details: Minimum depth of 1.0mm; minimum width of 1.0mm

Accuracy：± 0.3% (dimensions below 100 mm bear a lower limit of 0.3mm)

Advantages and Disadvantages of Vacuum Casting

Vacuum casting is a versatile production process perfect for small-batch production runs of prototypes and productions. Below are the significant advantages of vacuum casting:

- Polyurethane casting helps to achieve parts with complex shapes and features that are nearly impossible with other rapid prototyping methods.

- There are various resins compatible with vacuum casting.

- Vacuum casting is well-suited for producing high-quality cast parts in small batches.

- The vacuum casting process is relatively low cost than other alternatives like CNC machining because its molds are reusable.

- Components of an assembly created using the vacuum casting process often fit perfectly without post-processing operations like drilling or sanding.

Although vacuum casting offers numerous advantages, its common setbacks include:

- The vacuum casting process is only compatible with specific materials like thermoplastics and rubber.

- It is unsuitable for high-temperature applications.

- Vacuum casting is a slow process, which may not be ideal for high-volume production runs.

- The mold type used in vacuum casting may affect the quality of prototypes.

- Shrinkage may occur, leading to overly thick or thin features in cast parts.

General Applications of Vacuum Casting

Vacuum casting is a versatile process with many applications in different industries. The ability of the process to create intricate and precise components makes it an ideal choice for high-quality results.

Medical implants: Vacuum casting is widely used in the medical industry to fabricate complex parts and components. It can be used to make implants and prosthetics.

Automotive industry: Automotive parts are often manufactured through vacuum casting due to the process’s ability to create highly detailed components. Automobile parts such as intake manifolds, exhaust systems, and body panels can benefit from the superior accuracy and repeatability of silicone vacuum casting.

Food Industry: The process is often used to manufacture intricate parts for the food industry. It can create moulds, packaging components, and other complex shapes needed in food production operations.

Aerospace components: This process can be used to fabricate precision aerospace components. Due to its superior accuracy, repeatability, and ability to create intricate detail, components such as air ducts, fuel systems, and even some parts of aeroplanes’ exteriors are often created through vacuum casting.

Consumer goods: Vacuum casting can manufacture complex consumer goods such as toys and sporting equipment. The process can create highly precise parts that are perfect for consumer use.

Conclusion

Vacuum casting is a cost-effective method for making detailed plastic and rubber components in small batches. Product teams often use vacuum-cast parts for function testing during the product development stages. Since silicone vacuum casting is a cheaper manufacturing solution that provides excellent results, it has a wide range of applications.

Contact IDEAL whenever you need help assessing the manufacturability of your product designs.