This article is part of the CNC Machining Processes Guide: Milling, Turning, EDM & Multi-Axis on CNX Precision.
Vacuum casting turns a master model into dozens of high-quality plastic parts in days. The process uses a silicone mold and polyurethane resin. Therefore, it suits prototypes, pilot runs, and low-volume production. CNX Precision offers this process alongside its CNC machining services. This guide explains how the process works, when to use it, and what results you can expect.
This guide covers vacuum casting from master pattern to finished part. You will learn the step-by-step workflow, key benefits, and ideal batch sizes. In addition, you will see how the process fits into a complete product development timeline. As a result, you can decide quickly whether this process fits your project.
What Is Vacuum Casting?
The process is a low-pressure casting method for plastic parts. First, a master model is created, usually by CNC machining or 3D printing. Then, silicone rubber is poured around the master to form a soft mold. After curing, the mold is cut open and the master is removed.
Next, polyurethane resin is mixed, de-gassed, and poured into the mold under vacuum. The vacuum removes trapped air. Consequently, the resin fills every detail, including thin walls and sharp corners. The part cures at low temperature and is demolded when solid.
Each silicone mold produces roughly 10 to 100 parts before surface quality declines. Therefore, this method fills the gap between a single prototype and injection molding. It delivers real production plastics without the cost of steel tooling.
The silicone mold also allows molded-in inserts. Metal threads, bushings, and pins can be positioned in the cavity before pouring. As a result, the finished part gains strength and reduces secondary assembly.
The Vacuum Casting Process Step by Step
The casting workflow follows clear stages. Each stage affects quality, so experienced technicians control every variable.
- Master Model. CNC machining or 3D printing creates the master with the exact geometry you need.
- Silicone Mold. Liquid silicone cures around the master, capturing fine surface detail and undercuts.
- Mold Cutting. The technician cuts a parting line, removes the master, and prepares the cavity.
- Resin Mixing. Polyurethane resin and hardener are weighed and mixed in precise ratios.
- Vacuum Pouring. The mixed resin is poured into the mold inside a vacuum chamber, which removes air bubbles.
- Curing and Demolding. The mold rests at a controlled temperature until the resin solidifies, then the part is released.
- Finishing. Excess material is trimmed, edges are cleaned, and the part is painted or textured as required.
The cycle from master to first parts usually takes 7 to 15 days. Because the silicone mold is soft, parts need some care during demolding. Thin features should be designed with draft or extra support.
Temperature control matters at every stage. The master should be stable and dry before molding. The silicone cures at room temperature or in a gentle oven. Finally, the resin cures under controlled conditions to reach full hardness.
Why Choose a Vacuum Casting Service
A professional vacuum casting service adds real value to product development. Below are the main advantages.
- Speed. You receive functional parts in days, not months, compared with injection molding.
- Cost. Silicone tooling costs a fraction of steel molds, so small batches stay affordable.
- Real Materials. Polyurethane resins simulate ABS, PP, PC, nylon, and rubber-like materials.
- Surface Detail. The vacuum process captures fine textures, logos, and draft angles.
- Flexibility. You can change the design between batches by making a new master and mold.
Another benefit is finishing flexibility. Cast parts can be painted, textured, and fitted with inserts. Therefore, they arrive closer to a production-ready component, saving your team assembly time.
Vacuum casting suits batches of 10 to 100 pieces. For 1 to 10 parts, CNC machining or 3D printing is often faster. For thousands of parts, injection molding becomes more economical. Therefore, it is the sweet spot for pilots, market tests, and niche products.
The table below compares typical tolerances and surface quality for cast parts.
| Feature | Typical Result |
|---|---|
| Linear Tolerance | Plus or minus 0.15 to 0.3 mm |
| Wall Thickness | 1 to 10 mm recommended |
| Surface Finish | Fine texture, smooth or matte |
| Hardness Range | Shore A 30 to Shore D 80 |
| Color | Natural, clear, or custom tinted |
When to Use Vacuum Casting vs Other Methods
Choose this process when you need multiple parts that feel like injection-molded products. This includes user-testing devices, functional prototypes, and pre-production samples. In addition, the process supports low-volume production for medical housings, automotive interior parts, and consumer electronics.
Think about geometry, not just volume. Deep ribs, fine textures, and undercuts cast well in silicone. In contrast, the same features complicate steel tooling. Therefore, complex plastic parts often move faster through this route at low volumes.
Compare it with CNC machining for small batches. Machining works best for structural brackets and metal parts. Casting works best for enclosures, covers, and complex plastic geometries. In many projects, the two methods work together. For example, you can machine aluminum inserts and then cast them into a polyurethane assembly.
Finally, compare it with injection molding. Injection molding wins at very high volumes. However, the steel tool takes weeks to build and costs thousands of dollars. Casting avoids that investment. Therefore, it is the smart choice for pilot production and market validation.
FAQ
How long does vacuum casting take?
A typical vacuum casting order takes 7 to 15 days from approved master model to finished parts. The silicone mold is ready in 1 to 2 days, and each casting cycle takes about 4 to 6 hours. Ask your supplier for a firm schedule based on part size and batch count.
What materials does vacuum casting use?
Vacuum casting uses two-part polyurethane resins. These resins can simulate ABS, polypropylene, polycarbonate, nylon, and flexible rubber. Shore hardness ranges from soft Shore A 30 to rigid Shore D 80. Your supplier can tint the resin to match a production color.
Can vacuum casting hold tight tolerances?
Yes, within practical limits. Expect plus or minus 0.15 to 0.3 mm on most dimensions. Silicone molds flex slightly, so very long parts need careful support. Therefore, review critical dimensions with the casting engineer before tooling begins.
Conclusion
Vacuum casting bridges the gap between prototype and production. It delivers functional plastic parts quickly, affordably, and with excellent detail. Therefore, it deserves a place in every product development toolkit.
For reference, machining standards such as the ISO 9001 quality management standard define the quality and tolerance requirements we follow.
CNX Precision combines casting with CNC machining under one roof. We are ISO 9001 certified, and we handle master models, molds, casting, and finishing in-house. Contact us today with your 3D model, and we will recommend the fastest path to your next batch.