Urethane casting (also called vacuum casting) is the classic answer to one of product development's most frustrating gaps: you need thirty or fifty identical units, but injection mold tooling isn't economically justified yet. The process produces small runs of high-quality plastic parts from a flexible silicone mold, at a small fraction of the cost of steel tooling and in days instead of months. In practice, it is one of the fastest routes to units that look and behave like a finished product — which is why it anchors so many prototype development programs.
How the Process Actually Works
Everything starts from a precise master pattern — usually a resin 3D print, hand-finished to a smooth surface. The master is suspended in a frame, liquid silicone is poured around it, and the silicone cures for about a day. The cured block is then cut open along a planned parting line and the master removed, leaving a hollow silicone mold that reproduces every surface detail.
For production, a two-part polyurethane resin is poured into the mold inside a vacuum chamber. The vacuum is the heart of the method: it pulls trapped air out of the material and the cavity, preventing the bubbles and voids that would otherwise scrap the part. After casting, parts cure in an oven, and the cycle repeats. Each silicone mold typically survives roughly fifteen to twenty-five casts before it degrades, so larger runs simply use several molds in parallel.
Materials and Finishes You Can Get
Casting resins mimic familiar plastic families — exactly the advantage over standard 3D printing:
- ABS-like resins — rigid and impact-resistant, for enclosures and housings.
- Polypropylene-like resins — more flexible, for living hinges and snap fits.
- Elastomers in various Shore A hardnesses — for seals, grips, and button covers.
- Clear resins — for display windows and light pipes, after polishing.
- Heat-resistant and flame-retardant grades where standards require them.
Resin can be pigmented to a full custom color, threaded metal inserts can be cast in, and surface texture transfers directly from the master. The result is close enough to a retail product for trade shows, user testing, and marketing photography. If you are still weighing prototype technologies, see FDM vs SLA 3D printing and the broader survey of rapid prototyping techniques.
When It Pays Off — and When It Doesn't
The simple rule is quantity. Up to a few dozen units, urethane casting is almost always cheaper and faster than tooling. Into the many hundreds or thousands, the per-unit cost of casting stays relatively high while injection molding's economics take over — compare against the numbers in injection molding costs.
The second limitation is tolerances. Cast material shrinks slightly, so dimensional accuracy is good but not machining-grade. Very large parts or geometries with extremely thin walls may need master-pattern adjustments. Long-term mechanical durability is also below molded plastic, so don't build a product meant for years of hard field use on cast urethane.
Timeline and Cost Structure
A typical schedule: one to three days to build and finish the master, a day for the silicone to cure, then one to three hours per cast including oven time. A run of twenty simple units usually ships within one to two weeks. For comparison, even a simple steel mold commonly takes six to ten weeks just to build, before the first shot.
Cost splits in two: mold preparation, a one-time expense driven by part size and parting-line complexity, and per-unit price driven mostly by material volume and finish level. Full-color pigmenting, cast-in inserts, and optical clarity add meaningfully. When you request quotes, ask for tooling and per-unit costs separated — that is the only fair way to compare against alternatives and to see when hard tooling starts to win. It also puts the numbers in context against overall prototype costs.
How to Prepare Your Design
Design the part with slight draft angles, decide where the parting line may run, and flag the surfaces that must be flawless. Freeze the revision before casting: any change to the master means a new mold. For runs headed to early customers, treat casting as a planned bridge stage on the way to mass production — part of the transition mapped in from prototype to production — not a final destination.
Find Out If Casting Fits Your Run
Projects House uses urethane casting to deliver pilot runs, demo fleets, and first-customer units without premature tooling spend. Want to know whether it suits the series you are planning? Send us your model and quantities through the contact form and we will come back with a practical recommendation and clear numbers.