Polyurethane vacuum casting and injection molding both make plastic parts from a mold, and on a quotation sheet they can look interchangeable. They are not. The two processes sit on opposite sides of a cost, time, and property gap, and choosing the wrong one usually shows up as either an unbearably slow first article or a tooling bill you did not need yet. Here is how we draw the line for customers deciding between 10 and 1,000 pieces.
The fundamental difference
Vacuum casting pours liquid PU resin into a silicone mold under vacuum. The silicone master is cheap and fast — typically a few days from a 3D-printed or CNC-machined master pattern. Injection molding pushes molten thermoplastic into a machined tool: aluminum for prototype and bridge volumes, steel for production. The tool costs more up front and takes one to three weeks, but each part after that comes off faster, cheaper, and dimensionally tighter.
So the trade is: nearly zero tooling cost but high per-part cost (vacuum casting) versus meaningful tooling investment but low per-part cost (molding). Everything else follows from that.
Cost crossover: where molding wins
As a rule of thumb from our quoting history, a cast PU part typically runs several times the price of the same geometry injection molded in ABS once a tool exists — the resin alone is expensive, and cycle time is 20–60 minutes per part versus under a minute in a mold. The crossover depends heavily on geometry and finish, but for a palm-sized part it usually lands somewhere between 30 and 100 pieces. Below that, vacuum casting is almost always cheaper in total. Above a few hundred pieces, molding wins decisively and the tool pays for itself.
Two caveats. First, if the mold needs multiple inserts, complex slides, or aggressive textures, tooling cost rises and the crossover moves right. Second, if you will need design iterations anyway, keep the first runs in vacuum casting — changing a silicone mold costs a few hundred dollars, while welding and re-cutting a production tool costs far more.
Material properties: not the same family
This is where quotes mislead people. Cast PU resins mimic ABS, PC, or rubbery TPEs, but they are their own material family. Typical cast parts show lower heat resistance — most PU grades soften well below 100 °C — and slightly lower impact strength than their molded counterparts. If your part must survive an 85 °C test, validate with a real thermoplastic, not a PU stand-in.
Injection molding gives you the actual production polymer: ABS, PC, PA66-GF30, whatever the end product uses. Shrinkage is predictable (0.4–0.8% for ABS, more for PP), tolerances of ±0.1 mm are routine on small features, and mechanical test data is meaningful because the material matches production.
- Vacuum casting: good appearance, soft-touch and clear options, isotropic parts with no weld lines, but limited heat and long-term aging performance.
- Injection molding: true production material, anisotropic risks (weld lines, fiber orientation), tighter tolerances, and parts you can send straight into certification testing.
Dimensional reality check
Silicone molds are flexible. Wall thickness variation, resin shrinkage of 0.3–0.5% (grade dependent), and soft tooling mean cast parts typically hold ±0.2 mm on small features, looser over 100 mm. Fine for enclosures, covers, and marketing samples; risky for snap fits with 0.1 mm interference or sealing grooves that need gasket compression. Snap-fit beams can be cast, but expect to adjust them after the first batch — the good news is adjusting a silicone mold is quick.
Timeline comparison
Vacuum casting: 3–7 days for the first 10–20 parts, including master making. Injection molding: 1–3 weeks tooling plus days for molding. If your trade show is in two weeks, the decision makes itself. If your product launch is in three months and you need 500 units for early customers, start the mold now and use cast parts only to bridge the gap.
How we decide, in order
- Under ~30 pieces and non-structural: vacuum casting, every time.
- 30–200 pieces, or material properties matter (heat, impact, certification): aluminum injection mold.
- Design not frozen yet: stay in vacuum casting until it is.
- Production volumes beyond 500: production tooling, with bridge tooling if the schedule demands early units.
Most of our customers use both processes in sequence: cast parts for design validation and early demos, then a bridge tool for pilot builds. If you want a second opinion on your volumes, our vacuum casting and RIM team and our injection molding engineers quote both routes side by side so you can compare total cost rather than per-part price. For parts that also involve metal components, our CNC machining service covers the mating hardware.
Bottom line
Vacuum casting buys speed and flexibility; injection molding buys properties and cost at scale. Quote both with your real volumes and your real material requirements — not the optimistic ones — and the right answer is usually obvious.
