Vacuum Casting (Urethane Casting)
Vacuum casting, known in the US as urethane casting, pours polyurethane resin into a silicone mold under vacuum to copy a master pattern for short runs.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Plastics and Rubber
- Typical tolerances
- ±0.3% of the nominal dimension, with a minimum of about ±0.010 in (±0.25 mm). Accuracy depends on the master pattern and drifts slightly as the silicone mold ages.
- Surface finish
- Reproduces the master exactly — polished masters give gloss, bead-blasted masters give an even matte, and light textures transfer directly
- Typical volumes
- 10–100 parts; 15–25 parts per silicone mold before it must be replaced
- Lead time
- 5–15 working days from master pattern to first parts, including silicone cure; a few parts per mold per day thereafter
- Materials
- Plastic
What it is
Vacuum casting — called urethane casting in the US — pours a two-part polyurethane resin into a silicone rubber mold under vacuum, so the resin fills every detail without entrapped air. The silicone mold is itself cast around a master pattern, almost always a stereolithography or CNC-machined part, which means the process reproduces a single prototype into a small batch of production-like copies.
It occupies a narrow but important window: 10–100 parts, delivered in one to two weeks, with the appearance, feel, and approximate mechanical behavior of injection molded parts. Resins are formulated to mimic ABS, PP, PC, and filled compounds, plus rubber-like grades from roughly 30 to 90 Shore A and optically clear grades.
The constraint is mold life. A silicone tool yields roughly 15–25 parts before it degrades, so larger batches mean more molds, not more shots.
How it works
- Build the master. A stereolithography, PolyJet, or machined pattern is finished to the exact surface you want on the final parts — the silicone reproduces everything, including layer lines and sanding scratches. Any texture, polish, or lettering has to exist on the master.
- Gate and box the master. The pattern is suspended in a casting frame with sprue and vent risers attached, and a parting line is marked on it with tape to plan where the mold will be cut open.
- Cast the silicone. Two-part RTV silicone is degassed and poured around the master, then cured — typically several hours in an oven at around 105–160 °F (40–70 °C). Cure time is the long pole in the first-article schedule.
- Cut and extract. The cured block is cut open along the planned parting line, usually by hand with a scalpel in a deliberately jagged path so the two halves key back together precisely. The master is removed, leaving a cavity that is an exact negative.
- Cast the part. The mold is closed, taped, and placed in a vacuum chamber. Polyurethane A and B components are metered, mixed, and poured under vacuum so no air is trapped in the resin or the cavity. Pot life is short — often only a few minutes — so mixing, degassing, and pouring are one continuous operation.
- Cure and demold. The filled mold goes into an oven, commonly 160–175 °F (70–80 °C) for 30–60 minutes depending on the resin, then the flexible silicone is peeled back off the part.
Why can silicone molds handle undercuts?
Because the mold itself stretches. Silicone elongates enough to release moderate undercuts, snap features, and even light textures that would require a side action in a steel tool. This is the main geometric advantage over injection molding, and it is why vacuum casting can produce a finished-looking prototype with living hinges and integral clips in one piece.
Design guidelines
Wall thickness
Minimum around 0.030 in (0.75 mm) for a small part; 0.060–0.120 in (1.5–3 mm) is the comfortable range. Thin sections cool and cure unevenly and are prone to short fills at the extremities of a long flow path. Unlike injection molding, thick sections are tolerated — but heavy masses exotherm, which can discolor the resin and increase shrinkage locally.
Draft
1–3° is preferred and makes demolding cleaner, but zero draft is workable because the silicone flexes. This is a genuine design freedom: parts intended for eventual injection molding should still carry production draft so that the prototype validates the real geometry.
Undercuts and detail
Moderate undercuts, snap hooks, and light textures release directly from the silicone with no tooling action. Deep, sharp undercuts still risk tearing the mold and shortening its already limited life.
Tolerances
The standard figure is ±0.3% of the nominal dimension with a minimum of about ±0.010 in (±0.25 mm) — so a 100 mm dimension carries about ±0.3 mm. Accuracy depends on the master, on silicone shrinkage, and on how many shots the mold has taken, since dimensions drift slightly as the tool ages. Where a part must be dimensionally critical, take it from the first parts out of a fresh mold.
Text, texture, and finish
Whatever the master carries, the parts carry. A polished master gives gloss parts; a bead-blasted master gives an even matte. Molded-in text down to small point sizes reproduces well. Parts can also be painted, primed, and pad printed exactly like injection molded ABS, which is why vacuum casting dominates pre-production appearance models.
Material selection
Resins are specified by target behavior rather than by polymer name — an "ABS-like" grade matching approximate stiffness and impact, a "PP-like" grade for living hinges, rubber-like grades at 30–90 Shore A, optically clear grades for lenses. They approximate but do not equal the real thermoplastic, particularly in long-term heat and UV performance. For the properties of the production polymers being simulated, see /charts/material-properties.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Wall thickness | 0.060–0.120 in (1.5–3 mm) | 0.030 in (0.75 mm) | Thin walls short-fill at the end of a long flow path |
| Draft | 1–3° per side | 0° | Silicone flexes, but production draft validates the real design |
| Undercuts | Moderate, released by mold flex | Deep sharp undercuts | Tearing shortens an already short mold life |
| Tolerance | ±0.3% of dimension | Minimum ±0.010 in (±0.25 mm) | Master accuracy plus silicone shrinkage |
| Parts per mold | 15–25 | — | Silicone degrades from heat and resin chemistry |
| Surface | Whatever the master carries | — | The silicone reproduces every scratch and layer line |
Cost drivers
The cost structure is dominated by labor and by mold life. Each silicone mold is hand-cut, hand-taped, hand-poured, and hand-demolded, so per-part cost stays roughly flat with quantity instead of falling — the opposite of injection molding. The master pattern and the first silicone tool are the fixed costs, and they are small compared with any steel tooling.
Volume breakpoints:
- 1–5 parts: 3D print them directly. A silicone tool is not worth building.
- 10–100 parts: the vacuum casting window, using one to five molds.
- 100–1,000 parts: possible, but you are now paying for five to fifty silicone tools and the labor of running them. Compare hard against aluminum bridge tooling for injection molding.
- Over 1,000 parts: injection molding, essentially always.
Cost reduction:
- Design the parting line into the master. A clean, planar split cuts silicone consumption and makes each demold faster and less damaging to the mold.
- Keep the part small. Silicone is billed by volume and a large casting frame consumes a great deal of it. Splitting a large part into bonded sections is often cheaper than one big mold.
- Batch color into the resin. Pigmenting the shot avoids painting every part, which is usually the largest secondary-operation line item.
- Reuse one mold for the whole batch where quality allows. Parts 20–25 out of a mold are dimensionally and cosmetically the weakest; if the application tolerates it, you avoid building a second tool.
- Get the master right first. Every defect on the pattern is reproduced on every part in the batch, and fixing it means a new mold, not a process adjustment.
Questions
6 questionsHow many parts can you get from one silicone mold?
Roughly 15–25, depending on part geometry, resin chemistry, and cure temperature. The silicone degrades from repeated heat exposure and from the polyurethane's reaction, so larger batches require building additional molds rather than taking more shots from one.
What tolerance can vacuum casting hold?
The standard figure is ±0.3% of the nominal dimension with a minimum of about ±0.010 in (±0.25 mm), so a 100 mm dimension carries roughly ±0.3 mm. Accuracy inherits from the master pattern and drifts slightly as the mold ages, so take dimensionally critical parts from the first shots out of a fresh mold.
Is vacuum casting the same as urethane casting?
Yes — they are the same process under two names. European and Asian suppliers generally say vacuum casting, describing the vacuum chamber used to fill the mold air-free; US suppliers generally say urethane casting, describing the polyurethane resin. Silicone tooling, RTV casting, and polyurethane casting all refer to the same method.
Can vacuum cast parts have undercuts?
Yes, and this is one of the process's main advantages. The silicone mold flexes enough to release moderate undercuts, snap hooks, living hinges, and light textures that would require a side action in a steel injection mold. Deep, sharp undercuts still risk tearing the mold and shortening its already limited life.
When should I choose injection molding over vacuum casting?
Above roughly 100 parts. Vacuum casting cost stays nearly flat per part because every mold is hand-built and hand-run, so a batch of 500 means building and running ten or more silicone tools. At that point an aluminum bridge tool for injection molding is usually cheaper and gives real production material properties.
Do vacuum cast parts have the same properties as the plastic they imitate?
They approximate them. Polyurethane resins are formulated to match the stiffness, impact behavior, and feel of ABS, PP, or PC closely enough for fit, function, and appearance testing. They differ meaningfully in long-term heat resistance, UV stability, and chemical resistance, so do not use them to qualify a part for those requirements.