What Is Vacuum Casting? A Complete Guide for Product Developers

Vacuum casting sits in a manufacturing gap that most product developers do not know exists. It is not as fast as 3Dプリンティング, not as precise as CNC加工, and not as cost-effective at high volume as injection molding — but it produces something none of those processes can: injection-molded quality surface finish, in production-equivalent materials, at prototype quantities and lead times.

For product developers who need functional prototypes, pre-production samples, or low-volume production parts that look and feel like the final injection-molded product, vacuum casting is often the right answer — and this guide explains exactly what it is, how it works, and when to use it.

How Vacuum Casting Works — Step by Step

Vacuum casting (also called polyurethane casting or PU casting) uses a silicone mold to produce parts from two-component polyurethane resins. The process unfolds in these stages:

1. Master pattern creation: A high-quality master — typically CNC machined or SLA 3D printed — is produced in the final part geometry. Surface quality on the master transfers directly to the castings, so the master finish is important.
2. Silicone mold making: The master is suspended in a frame and liquid silicone is poured around it. The silicone is cured and then cut open to remove the master, leaving a cavity in the exact negative of the master geometry.
3. Vacuum casting: Two-component polyurethane resin is mixed and placed in a vacuum chamber. The vacuum removes air bubbles from the resin. The silicone mold is placed in the chamber, the resin is poured or injected under vacuum, and the chamber returns to atmospheric pressure — pressing the resin into every surface detail of the mold.
4. Curing and demolding: The filled mold is cured at 40–70°C for 1–4 hours. After curing, the silicone mold is opened and the casting is removed. Secondary operations (painting, primer, hardware insertion) can follow.
5. Repetition: One silicone mold typically produces 15 to 25 castings before the mold degrades. Multiple molds can be made from one master to scale output.

Vacuum Casting vs CNC Machining vs Injection Molding

Vacuum Casting Materials — What You Can and Cannot Do

Vacuum casting uses two-component polyurethane systems that are formulated to simulate the properties of thermoplastic materials used in production 射出成形. Common options include:

• Rigid ABS-like resins: Hard, impact-resistant parts resembling production ABS. Shore D 80+. Used for enclosures, housings, structural components.
• Polypropylene-like (PP-like) resins: Semi-flexible, living hinge compatible. Used for snap-fit assemblies, container lids, flex components.
• Transparent/clear resins: Used for lenses, light guides, display covers, and aesthetic clear components. Optical clarity comparable to acrylic.
• Rubber and TPE-like resins: Flexible parts with Shore A 30–90 for grips, gaskets, overmolds, and ergonomic components.

What vacuum casting cannot do: produce parts in actual engineering thermoplastics (PEEK, nylon 66, POM) — only simulated equivalents. It cannot match the mechanical performance of production injection-molded parts in high-temperature or chemical-resistance applications.

When Vacuum Casting Is the Right Choice

Use vacuum casting when: You need 5–50 parts that must look and feel like the production injection-molded product for customer presentation, investor demos, or functional testing — and you need them faster and cheaper than cutting a production tool. You need to validate a design before committing to injection mold tooling. You need color-matched, painted, or textured surface finish quality that 3D printing cannot achieve without significant post-processing. Your volume is too low to justify injection mold tooling but too high for CNC machining to be economical.

よくある質問

How accurate are vacuum cast parts compared to the original master?

Vacuum cast parts typically achieve ±0.3mm to ±0.5mm dimensional accuracy relative to the master, with some shrinkage of 0.1–0.2% depending on the resin system. This is sufficient for form-fit-function testing and visual presentation, but not for parts with precision fits or tight mechanical tolerances. For critical tolerance features, plan for light post-machining on the castings.

Can vacuum cast parts be used for functional testing — not just appearance?

Yes, in many cases. Rigid polyurethane castings have mechanical properties close to engineering thermoplastics — tensile strength in the 50–70 MPa range, flexural modulus 2,000–3,000 MPa — suitable for many functional testing scenarios. They are not appropriate for heat resistance testing above 80°C or chemical resistance testing in aggressive environments where specific thermoplastic chemistry is required.

How long does it take to get vacuum cast parts from a finished CAD file?

A silicone mold can be made in 2 to 3 days from a master pattern. If we produce the master in-house (by CNC or SLA), add 1 to 3 days for master production. First castings are available 3 to 7 days from receipt of approved CAD. Each casting cycle after mold completion takes 4 to 8 hours depending on part size and resin curing time.