IllustrationUrethane casting pours two-part polyurethane resin into a silicone mold made from a master pattern, often under vacuum. It suits about 1 to 100 enclosure parts in ABS-like, PC-like or rubber-like grades for EVT and early DVT units. Each silicone mold lasts a few dozen casts, and the parts are not the production thermoplastic.
Urethane casting sits between a 3D printed shell and a molded part. A team with a settled enclosure shape and a need for 10 to 50 units that look and feel close to production will often cast them. This guide explains the process, how long a silicone mold lasts, what the resins can stand in for, the tolerances to design to, and when to move to an injection mold.
Figures below come from published vendor pages read on November 6, 2026. Shops publish different numbers, so confirm them with whoever casts your parts.
What is urethane casting, and how does vacuum casting work?
Urethane casting makes plastic or rubber-like parts by pouring liquid polyurethane into a soft mold. Formlabs says vacuum casting is better known as urethane casting or polyurethane casting in the US, so the two names usually describe one process (Formlabs, as of November 2026). The steps run in this order.
- Master pattern. Xometry says the master is made by additive manufacturing and is typically hand-finished before the mold is made. Formlabs says SLA gives the smoothest master but needs sanding to remove support marks, while SLS masters can benefit from media blasting or tumbling. Every mark left on the master copies into every part.
- Silicone mold. Silicone is poured around the master and cured for several hours. The mold is then sliced into two halves and the master is removed (Formlabs).
- Casting. A two-part resin is mixed, degassed and poured. Formlabs preheats resins to 40 °C and says vacuum chambers suit resins that cure in twenty minutes or more, while pressure pots suit quick-setting resins. Xometry says the chamber typically pulls a vacuum for clear parts to cut bubbles.
- Cure and demold. Formlabs cures at 70 °C and then opens the mold. It notes that post-curing can take up to a week.
The slow part is the casting itself. Xometry says a single mold makes around 1 to 10 parts per day, and that production can take up to four weeks after a master and mold that take one to two days (Xometry comparison article, as of November 2026). Formlabs quotes about 10 days from master to cast parts.
How many parts does one silicone mold make?
Mold life is the number that shapes a casting plan, and published figures vary by source and by silicone type. Each row comes from one page, as of November 2026.
| Source | What it says | Link |
|---|---|---|
| Fictiv, soft and hard tooling article | Silicone molds can withstand about 25 shots per cavity. | Fictiv |
| Formlabs, vacuum casting guide | One silicone mold can produce roughly 50 casts. Molds made with HTV silicone or rubber can last as many as 300 to 500 casts. | Formlabs |
| Xometry, urethane casting capability page | No count. It says the mold wears out more quickly than a metal injection mold and lists casting for quantities from one to one hundred. | Xometry |
Plan by part, then by mold. A small wearable often has a top shell, a bottom shell, a button and a light pipe, and each needs its own mold. If the shop expects 25 casts per mold and you need 30 units, every part needs two molds. Ask the shop for its own count on your geometry, and check parts from the second mold against the first. Xometry's design tips warn that blind internal corners may not be possible and can add cost and lead time (Xometry design tips, as of November 2026).
Which urethane casting materials stand in for ABS, PC and TPE?
Cast resins are graded by hardness and by the thermoplastic they imitate. The table lists Xometry's published grades, as of November 2026 (Xometry urethane casting). The last column is our own example of a use on a device. Xometry does not give it.
| Grade | Hardness | What Xometry says | Example use on a device |
|---|---|---|---|
| General Purpose | Shore D 76 to 80 | Impact resistant and ABS-like. Xometry lists it for enclosures and concept models. | Main housing shells for looks-like units. |
| ABS-Like, FR | Shore D 78 to 82 | Flame retardant. Xometry describes it as UL Listed with a flammability rating at 1/16 in. (1.6 mm). | Shells near the battery or charger when the design calls for a flame-rated plastic. |
| Polycarbonate-Like | Shore D 82 to 86 | Stiff, high impact, high heat deflection. Simulates non-clear polycarbonate. | Stiffer frames and housings that will be PC in production. |
| Rigid Clear | Shore D 80 | Clear polyurethane. A matte finish looks frosted and high gloss raises transparency. | Light pipes, LED windows, display covers on looks-like units. |
| Rubber-Like Polyurethane | Shore A 25 to 95 | Elastomer with high elongation at break. | Straps, bumpers, button membranes, soft-touch bands. |
| Cast silicone | Shore A 10 up to the 50 to 60 range | Durable, stretchy, tear resistant, easy to clean, high temperature resistance. | Skin-contact pads, seals, soft covers. |
Formlabs lists a similar range, including ABS-like, PC-like, TPE-like, PMMA-like clear and fire-retardant resins. Xometry also lists polypropylene-like and glass-filled nylon-like grades. Ask for the datasheet of the exact cast resin.
What a cast resin cannot tell you
Formlabs says prototyping in the final injection-molded material is not possible with vacuum casting. Xometry adds that urethane can break down over time under extreme temperatures, UV and moisture (both as of November 2026). So a drop test, a sunscreen wipe or a week in a hot car on a cast part tells you how the urethane behaves. It says little about your PC/ABS. Use cast parts for fit, feel and appearance. Run material tests on parts molded in the production grade, as the prototype injection molding guide describes.
Flame-rated grades
Xometry states the flame rating of its ABS-Like FR grade at 1/16 in. (1.6 mm). If your design needs a flame-rated enclosure near the battery or charging circuit, compare that thickness with your wall, and ask for the listing behind the claim. The production resin needs its own rating check later; UL 94 flammability ratings explains how ratings tie to thickness.
Overmolding and inserts in cast parts
Formlabs says vacuum casting can combine flexible and rigid materials in an overmold and add metal parts through insert molding. Its example is an electronics housing for a wearable breath sensor with a silicone overmold. Xometry calls urethane molding an alternative to insert molding or overmolding in low volumes, and says cast parts can be painted, including EMI shielding, and fitted with inserts (both as of November 2026). On a device this covers a soft-touch band over a rigid shell, a button membrane, and brass inserts for screw bosses. Test how the soft layer bonds to the rigid part on the first parts, because a production overmold tool may bond it differently. Overmolding vs insert molding covers the molded versions.
What tolerances, walls and finishes should you design for?
Design the cast part to the production drawing, then check that the casting rules allow it. The rules below come from vendor pages, as of November 2026.
| Rule | Published value | Source |
|---|---|---|
| Typical tolerance | ±0.010 in. (0.25 mm) or ±0.003 in. per inch, whichever is larger. Irregular or very thick shapes may deviate. | Xometry capability page |
| Shrinkage | About 0.15%. Final size also depends on the accuracy of the master, the geometry and the resin. | Xometry capability page and design tips |
| Wall thickness | Formlabs suggests 1.5 to 4 mm with an absolute minimum of 1 mm. Xometry asks for at least 0.5 mm on small parts and 1 mm on larger ones, kept uniform so the resin cures evenly. | Formlabs guide, Xometry design tips |
| Ribs | Ribs can be thinner, at 60% of wall thickness. | Formlabs guide |
| Draft | Casting does not require draft. Long zero-draft features can break or warp when pulled from the mold, so a slight angle helps on larger jobs. | Xometry design tips |
| Sharp edges and text | Sharp corners and text may look slightly rounded. Xometry suggests a fillet or chamfer in place of knife edges. | Xometry capability page and design tips |
| Part size | Up to 30 in. (0.762 m) long at Xometry. Formlabs gives molds up to roughly 50 cm on the longest side and castings up to 1.5 kg. | Xometry capability page, Formlabs guide |
| Finish and color | Matte or frosted, semi-gloss, high-gloss, textured or custom. Black, color-matched, natural or clear. Polishing must be agreed when you order. | Xometry capability page |
Keep the draft you plan for the injection tool even though casting does not need it. Xometry's design tips say to consider later production processes, and a cast part without draft hides problems the tool will expose. Formlabs notes that silicone molds allow significant undercuts, and recommends sticking to the injection-molded design with its draft angles when you test for molding. A metal tool handles undercuts differently, as undercuts in injection molding explains, so a cast part can prove a shape the mold shop later rejects. Snap fits on cast parts show geometry and assembly order. Their retention force may change in the production resin; snap-fit joint design covers the sizing. For clear windows and light pipes, ask for the clear grade under vacuum and a polished master.
Urethane casting vs injection molding: when should you switch?
Xometry's capability page says quantity is the primary deciding factor between the two (Xometry, as of November 2026). The comparison below comes from its casting versus injection molding article (updated March 12, 2026). Its capability page gives a tighter typical casting tolerance, shown in the table above.
| Factor | Urethane casting | Injection molding |
|---|---|---|
| Output | Around 1 to 10 parts per mold per day, because the work is manual. | Hundreds or thousands of parts per day on an automated press. |
| Tooling | A silicone mold, much cheaper and faster to make than a metal tool. | A machined aluminum or steel tool with a much higher upfront cost. |
| Mold life | Silicone molds are not as durable as metal molds. See the mold life table above for published counts. | Hardened steel or aluminum molds can last decades. |
| Tolerance | Usually 0.015 in., per this article. | Often 0.005 in., and as small as 0.001 in. |
| Material | A limited set of curable urethanes. | An extensive variety of thermoplastics and thermosets. |
| Cycle | Curing takes a few hours. Some fast urethanes cure within half an hour. | Parts are ready straight after molding. |
For a device, quantity is rarely the only trigger. Move to a mold when any of these is true:
- You need drop, ingress, UV or chemical results that describe the plastic you will ship.
- A gasket or seal surface needs a tolerance tighter than casting holds.
- The count needs so many silicone molds that part-to-part variation becomes a problem.
- The outer shape is frozen and the next change would be a tool edit anyway.
The step after casting is often an aluminum bridge tool. Fictiv says soft tooling can support prototyping, bridge production and low-volume manufacturing, and puts urethane casting volumes at generally 1 to 100 (Fictiv, as of November 2026). Tool materials, cavities and shot life are covered in injection mold tooling explained.
Where does urethane casting fit between 3D printing and a mold?
Casting earns its place once the outer shape stops changing and you need more matching parts than printing gives you. Formlabs says that for larger parts it is much faster and cheaper than 3D printing, and that it suits the bridge from prototyping to production (Formlabs, as of November 2026). Before that point, printing is quicker for each CAD edit; SLA vs SLS for enclosure prototypes covers which printer suits a master or a test shell. For how printing, casting, CNC and quick-turn boards fit into one prototype plan, see rapid prototyping for electronics.
| Gate | Is casting worth it? | What cast parts can show |
|---|---|---|
| Works-like prototype (3–5 units) | Rarely. The shell is still changing, and a printed part from each CAD edit is faster. | Board, battery and button fit in printed shells. |
| EVT · first custom PCB (10–30 units) | Often, once the outer shape stops changing. One mold per part can cover a set of looks-like units. | Color, finish and feel for user trials and photos. Button feel through a cast membrane. Light leaks around LEDs. Assembly order. |
| DVT / PVT · small batch (10–1,000 units) | Sometimes, for early units while the tool is being cut. | Fit checks only. Drop, sealing, aging and antenna results come from molded parts in the production resin. |
Xometry says cast first-run parts can be done weeks before production tooling is ready, as of November 2026. That makes casting useful for early DVT units while the tool is cut. Keep those units out of the reliability results.
What should you ask your hardware team?
Bring these questions to the enclosure review before you order a cast set.
- How many casts does the shop expect from one mold for this part, and how many molds does our quantity need?
- Which cast resin stands in for our production plastic, and which properties will differ?
- Does the cast part keep the draft, wall thickness and rib sizes of the production design?
- Which dimensions are critical, and do they fit the casting tolerance and 0.15% shrink?
- How will the soft layer bond to the rigid part in an overmold, and how will we test that joint?
- Which tests will we run on cast parts, and which wait for molded parts?
- Is any conductive paint planned, and is it kept clear of the antenna area?
- Who owns the master and the silicone molds, and how long are they kept?
Enclosure work, from the first printed shell to tooling, follows our gated prototype development process.
Questions about urethane casting
Is vacuum casting the same as urethane casting?
Can I run drop or water tests on cast enclosures?
You can run early checks, but the results describe the urethane. Formlabs says prototyping in the final injection-molded material is not possible with vacuum casting, and Xometry says urethane can break down under UV, moisture and extreme temperatures, both as of November 2026. Repeat the tests on molded parts.
Can urethane cast parts ship in a sellable product?
Xometry describes urethane casting as an option for very low quantities of production-grade parts, as of November 2026 (Xometry). Before you ship them, check aging under heat, UV and moisture, any flame rating your design needs, and how many molds the quantity uses.
How could Duobay help with cast enclosure parts?
Enclosure work follows our gated process. The EVT gate includes a first enclosure, and the DVT / PVT gate includes tooling. Industrial design, tooling and production run at partner shops and factories, and molds you pay for are labeled with your name. The first call is free. Tell us about your device.
If you want a casting and tooling plan checked against your enclosure and unit counts, tell us about your device.
Sources. All pages accessed November 6, 2026. Vendor figures change, so confirm current values with your caster.
- Xometry, Urethane Casting Services capability page (link)
- Xometry, Urethane Casting vs. Injection Molding (published March 16, 2023, updated March 12, 2026) (link)
- Xometry, Top 5 Tips for Designing Custom Urethane Casted Parts (updated September 12, 2024) (link)
- Formlabs, Guide to Vacuum Casting (Urethane Casting) (June 5, 2023) (link)
- Fictiv, Soft Tooling and Hard Tooling for Injection Molding (September 28, 2022) (link)
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