Vacuum Casting(Cast Urethane): Process, Steps, Advantages, and Disadvantages

Introduction

Vacuum casting(Cast Urethane) is a widely used manufacturing process for producing high-quality plastic or rubber parts in small to medium batches. It is particularly popular in prototyping, low-volume production, and customized part manufacturing. The process involves pouring liquid resin into a silicone mold under vacuum conditions to eliminate air bubbles and ensure precise replication of the master pattern.

This article provides a detailed step-by-step explanation of the vacuum casting process, along with its advantages and disadvantages.

Step-by-Step Process of Vacuum Casting(Cast Urethane)

1. Master Pattern Creation

The process begins with the creation of a master pattern, which is typically made using CNC machining or 3D printing (SLA, SLS, or FDM). The master pattern must be highly accurate since it determines the final part's quality.

2. Mold Making (Silicone Mold/Tooling Preparation)

• The master pattern is placed in a casting box.

• Liquid silicone rubber is poured over the master pattern and degassed in a vacuum chamber to remove trapped air.

• The silicone is then cured in an oven at a controlled temperature (usually around 40-60°C) for several hours.

• Once cured, the silicone mold is cut open, and the master pattern is removed, leaving a precise cavity.

3. Resin Preparation

A two-part polyurethane (PU) resin is selected based on the desired material properties (e.g., flexibility, strength, transparency). The resin is mixed with additives (such as colorants or fillers) and degassed under vacuum to eliminate air bubbles.

4. Casting Process

• The silicone mold is reassembled and placed in the vacuum casting machine.

• The prepared resin is poured into the mold under vacuum to prevent air entrapment.

• The mold may be rotated or tilted to ensure even resin distribution (for complex geometries).

5. Curing

• The filled mold is placed in an oven to cure the resin at a controlled temperature (typically 60-80°C).

• Curing time varies (usually 1-4 hours) depending on the resin type and part thickness.

6. Demolding and Finishing

• After curing, the silicone mold is carefully opened, and the cast part is removed.

• Post-processing (such as trimming, sanding, or painting) may be applied to improve surface finish.

7. Reusing the Mold

A single silicone mold can produce 15-25 parts before degradation occurs, making vacuum casting ideal for small-batch production.

Advantages of Vacuum Casting(Cast Urethane)

1. High Accuracy & Detail Reproduction

• Vacuum casting can replicate fine details, textures, and complex geometries with high precision.

2. Fast Production for Prototypes & Small Batches

• Compared to injection molding, vacuum casting is quicker and more cost-effective for low-volume production.

3. Material Variety

• Different polyurethane resins can mimic engineering plastics (ABS, PP, PC) or rubber-like materials.

4. Low Tooling Cost

• Silicone molds are cheaper than metal molds used in injection molding.

5. Excellent Surface Finish

• Parts require minimal post-processing, making them suitable for visual prototypes and functional testing.

6. Reduced Waste

• Unlike subtractive methods (CNC machining), vacuum casting(Cast Urethane) is additive, minimizing material waste.

Disadvantages of Vacuum Casting(Cast Urethane)

1. Limited Mold Lifespan

• Silicone molds degrade after 15-25 uses, making the process unsuitable for mass production.

2. Lower Strength Than Injection-Molded Parts

• PU resins are generally weaker than industrial thermoplastics, limiting structural applications.

3. Size Limitations

• Large parts are difficult to produce due to vacuum chamber size constraints.

4. Manual Labor Dependency

• The process requires skilled operators for mold preparation and resin handling.

5. Longer Cycle Time for Large Batches

• Each part requires individual casting and curing, increasing lead time for high quantities.

Conclusion

Vacuum casting(Cast Urethane) is an excellent solution for producing high-quality prototypes and small batches with complex geometries. Its ability to use various resins, fast turnaround time, and cost-effectiveness make it a preferred choice in industries like automotive, medical, and consumer goods. However, its limitations in mold durability and part strength restrict its use for large-scale production.

For applications requiring precision, aesthetic appeal, and quick iterations, vacuum casting(Cast Urethane) remains a highly valuable manufacturing method.