Comprehensive Guide to Urethane Casting in Small Batch Production

Vacuum casting is a technology for bridge production in the rapid prototyping industry. It is the ideal choice for low-volume manufacturing before investing in expensive hard tooling.

Many industries, such as automotive, robotics, medical devices, and 3C electronics, rely on this process to transition a concept into a market-ready product. Traditional steel molds often cost between $10,000 and $100,000+ with lead times spanning several weeks even months. Any design flaw discovered at that stage leads to catastrophic financial and schedule losses. Vacuum casting provides a high-flexibility, low-risk validation gate to verify form, fit, and functional compliance.

What is vacuum casting?

Vacuum casting is a sophisticated copy molding process that utilizes liquid polyurethane resins and silicone soft molds to produce high-performance plastic parts.

The process begins with a Master Model, typically produced via CNC machining or SLA 3D printing, which serves as the dimensional anchor. This master is suspended in a mold box and encapsulated in liquid silicone rubber. Once cured, the silicone forms a flexible mold capable of replicating intricate textures, undercuts, and fine details with a dimensional tolerance typically held within 0.15 mm per 100 mm.

Because it bypasses the need for CNC-machined metal cavities, vacuum casting is significantly more agile and cost-effective than injection molding for batches ranging from 10 to 50 units.

Step-by-Step Vacuum Casting Processes

The whole vacuum casting process can be summarized into the following 4 steps.

1. Master Model Preparation

The foundation of a quality casting is the Master Model. Depending on the required surface finish, such as MT texture, high gloss, or VDI, the master is polished or textured. 3D drawings are reviewed for shrinkage compensation to ensure the final PU part accounts for the minor contraction of both the silicone and the resin.

2. Silicone Mold Construction (Tooling)

A technician prepares the mold frame and determines the Parting Line, which is the split point of the mold. Vents and gates (risers) are strategically attached to the master to allow air to escape and resin to enter. Liquid silicone is poured under a vacuum to ensure no air bubbles are trapped against the master’s surface. The mold typically cures for 8 to 12 hours.

3. Vacuum Casting and Degassing

The mold is placed in a Vacuum Casting Machine. The two-part polyurethane resin is mixed and degassed under high vacuum to eliminate voids. The resin is then gravity-fed or injected into the silicone cavity. By casting in a vacuum environment, the resin can penetrate complex geometries and thin walls down to 0.5 mm that would otherwise be blocked by air pockets.

4. Curing and Post-Processing

The mold is moved to a pre-heated oven, typically set at 60°C to 70°C. After initial curing for 30 to 60 minutes, the part is demolded. To reach peak mechanical properties, including HDT and tensile strength, parts undergo Secondary Curing (Post-Curing) at 70°C to 80°C for several hours. A single silicone mold typically yields 15 to 20 high-quality shots before the silicone begins to degrade due to chemical attack from the resin.

Materials for vacuum casting

Vacuum casting processes are valued for their dimensional stability, as silicone molds exhibit negligible shrinkage during the curing phase. This ensures the master’s geometry is captured faithfully. The process is also highly versatile, capable of achieving optically clear transparency, over-molding for soft-touch grips, and custom color matching via Pantone or RAL systems. However, users must note that mold life is sacrificial, and while PU resins simulate engineering plastics, they possess slightly different molecular structures than mass-produced injection-molded thermoplastics.

To meet diverse engineering requirements, a wide range of polyurethane resins is used to simulate standard plastics:

• ABS-like resins: These are the most common choice, offering high impact resistance and excellent machinability for housings and structural covers.
• PC-like Resins: These provide high transparency, UV stability, and heat resistance, making them ideal for lenses, light pipes, and clear enclosures.
• PP/PE-like resins: Known for high elongation and fatigue resistance, these materials are perfect for simulating living hinges and snap-fit assemblies.
• POM-like resins: These exhibit high stiffness and low friction coefficients, suitable for functional gears and mechanical linkages.
• Nylon-like resins: These are often reinforced with glass fiber to enhance rigidity and heat deflection temperatures for structural rig components.
• Elastomeric resins: Available in various Shore A durometers from 30A to 90A, these simulate rubber and TPU for gaskets, seals, and ergonomic grips.

Applications

Vacuum casting is the industry standard for low-volume production where the geometry is complex or the design is still subject to minor iterations. It is widely applied in:

• Medical device: Housings requiring biocompatibility or flame retardancy.
• Automotive: Interior Trim for ergonomic testing and aesthetic approval.
• Robot components require high strength-to-weight ratios.
• User-interface prototypes in 3C electronics.

How LKprototype Can Help?

Established in 2009, LKprototype provides a comprehensive one-stop solution for your manufacturing needs. LKprototype specializes in high-precision CNC machining, vacuum casting services, small to medium batch injection molding, sheet metal fabrication, and 3D printing. Our wide range of capabilities and ISO-certified quality control system allows us to serve diverse industries across the globe, delivering quality products that meet stringent international standards.