By 6CProto Engineering Team · Updated August 14, 2026
Urethane casting pours liquid polyurethane into a silicone mold to produce small batches of plastic parts with molded-like quality. CNC machining cuts plastic parts from solid stock with tight tolerances and no tooling. For runs of 1 to 100 parts, the choice depends on geometry, material, finish, and whether the part needs molded-in detail. Both processes deliver real plastic parts without production tooling, and the right route is found by answering three questions in order.
The Decision Path in Three Questions
- How many parts do you need? One to five parts favor machining; ten to one hundred parts with molded-like detail favor casting.
- What must the part look and feel like? Molded surfaces, texture, and soft-touch properties point to casting; precise bores, threads, and flat sealing surfaces point to machining.
- Is the design stable? If revisions are still coming, machining each iteration is safer, because a new master and mold are not needed for every change.
The three answers usually point to one route. When they conflict, design stability and the critical features decide, because they determine whether a master and mold are a safe investment.
Quantity: Where the Mold Cost Pays Back
Machining has no tooling cost and is ideal for one to a few parts. Casting involves making a master and a silicone mold, which adds upfront work but then reproduces each part quickly and consistently.
For ten to one hundred parts with molded-like detail, casting is usually more efficient. For one to five parts, or parts that will be machined in production anyway, machining is simpler and faster.
| Comparison | Urethane Casting | CNC Machining |
|---|---|---|
| Tooling | Silicone mold from master | Fixturing only |
| Best quantity | 10–100 parts | 1–10 parts |
| Geometry | Molded detail, textures, undercuts with mold design | Machined features, tool access limited |
| Materials | Polyurethane resins, rubber-like grades | Full range of engineering plastics |
| Tolerance | ±0.1–0.3 mm typical | ±0.01–0.05 mm typical |
| Surface finish | Molded finish, textured options | Machined, polished, or bead blasted |
For quantities above one hundred, the comparison changes again. Low-volume injection molding or rapid tooling may become competitive with casting, so revisit the economics when volume grows.
Geometry: Molded Detail vs. Machined Precision
Casting handles molded details, textures, inserts, and shapes that are difficult or expensive to machine, such as enclosures with draft, soft-touch surfaces, and complex curves. Machining handles precise bores, threads, flat surfaces, and features that require tight tolerance.
The master pattern is the quality driver in casting. If the master is well finished, every cast part inherits that finish; if it has marks, they reproduce too. Plan the master carefully, because it is the most important investment in the casting route, and spare masters are worth keeping for mold replacement.
Geometry also affects mold design. Undercuts that are impossible in machining may be cast with flexible silicone molds that peel away, but very deep undercuts can still complicate demolding and drive up mold cost. Part size limits casting too: large parts need larger molds, more resin, and longer cure times, which erodes the cost advantage.
Materials: Urethane Resins vs. Engineering Plastics
Urethane resins can mimic production thermoplastics, including ABS-like, rubber-like, and transparent grades, with properties tailored by the resin system. The chemistry is broad, from rigid and impact-resistant to soft and rubber-like, which makes casting useful for simulating production plastics and for soft-touch parts.
CNC machining uses standard engineering plastics such as ABS, PEEK, nylon, acetal, and PVC, with properties documented in manufacturer data sheets. If the production material is a specific thermoplastic, machined parts match it directly.
Confirm the required properties, such as shore hardness and temperature range, with the resin supplier. Transparent, flame-retardant, and high-temperature grades cost more and may have longer cure times, so match the resin to the actual application rather than choosing a generic grade.
Keep the production material in mind during prototype testing. A cast part that behaves differently from the final molded part can mislead the team, so document the resin used and the properties tested.
Tolerances and Surface Finish
Machined parts typically hold ±0.01–0.05 mm on critical features, with surfaces that can be polished or finished to Ra 0.8 µm or better. Cast urethane parts typically hold ±0.1–0.3 mm, with molded surfaces that replicate the master and can include texture.
For parts with critical mating dimensions, machining is safer. For cosmetic parts where molded texture and appearance matter, casting is often the better route.
If the part must seal or fit precisely, plan for machining on the sealing faces even when the body is cast. This hybrid approach is common and avoids the tolerance risk of as-cast critical surfaces. Think about inserts too: casting can embed metal inserts such as threaded nuts, but position accuracy depends on the mold and placement method, so for tight insert positions, machining after casting is more reliable.
Setup and Hidden Costs: Masters, Bubbles, and Fixtures
Machining setup includes programming and fixturing; cost per part scales with machine time. Casting setup includes the master and silicone mold; once the mold exists, each casting is relatively fast and cheap.
The crossover usually sits around ten to twenty parts, depending on part complexity and material. A DFM and quote for both routes will show which is cheaper for your specific part.
Watch the hidden costs. Casting requires the master pattern, mold materials, and cure time; machining requires programming, fixturing, and possibly multiple setups. Yield is a hidden factor in casting: air bubbles, incomplete fills, and cure issues can cause rejects, so ask about the supplier’s process control and what happens to rejected parts.
Time-to-first-part matters too. Machining starts as soon as the file is ready, while casting needs a master and mold first. If the schedule is tight, machining may deliver the first part days sooner; once the mold exists, however, casting can be faster per part than machining for complex shapes.
Casting With Machined Interfaces: The Hybrid Route
For a part that combines a molded body with a precise machined interface, the practical solution is to cast the body and machine the critical features as a secondary operation. This is common for housings with precision bores or sealing faces.
6CProto provides both urethane casting and CNC machining, so the route can be selected per part rather than forcing one process. The engineering team can review the geometry, quantity, and finish requirements together and recommend whether to cast, machine, or combine both.
If the part will eventually be injection molded, casting prototypes can be a useful bridge for market testing and field trials before committing to steel tooling, provided the resin is chosen to represent the production material.
Common Misconceptions
- Casting is always cheaper for small batches. Below roughly ten parts, machining is often cheaper because there is no master or mold cost.
- Cast parts are identical to injection molded parts. Appearance and texture can match closely, but material properties are similar rather than identical, so verify the resin against the production material for functional testing.
- Casting means no machining. Many cast parts still need machined critical faces, inserts positioned by machining, or trimmed gates and parting lines.
- Quantity alone decides the route. A changing design makes machining safer at any quantity, because every cast revision needs a new master and mold.
6CProto Expert Views
6CProto engineering perspective: Choose casting when molded detail and small-batch economics matter, and machining when tolerance and material fidelity matter. Review the part geometry, quantity, and finish requirements together, and remember that cast parts can be machined on critical surfaces afterward. A DFM review will show which route holds your critical dimensions.
Conclusion
Urethane casting and CNC machining serve different low-volume needs. Cast when you want molded detail, texture, and efficient batches of 10–100 plastic parts; machine when you need precision, specific materials, or immediate parts. Combine both when a part needs molded form plus machined accuracy.
Define the part’s critical requirements first, then compare both routes on cost, lead time, and quality. The right process is the one that holds the features that matter at the lowest total cost, and the comparison should be revisited as the project evolves from machined prototype to cast short run to molded production.
FAQs
Is urethane casting cheaper than CNC machining?
It depends on quantity. Below roughly ten parts, machining is often cheaper; above that, casting usually wins because the mold cost is amortized over the batch.
Can cast urethane parts match injection molded parts?
In appearance and texture, often yes. In material properties, they are similar but not identical, so verify the resin against the production material for functional testing.
Can I machine features on a cast part?
Yes. Machining critical surfaces as a secondary operation is a common way to combine molded form with precise dimensions.
Does urethane casting require a master pattern?
Yes. A master, usually machined or 3D printed, is used to create the silicone mold. The quality of the master directly affects the quality of every cast part.
What quantities are too high for urethane casting?
Above roughly one hundred to a few hundred parts, low-volume injection molding often becomes more economical. The exact crossover depends on part size, resin cost, and mold investment.
Sources
- 6CProto Urethane Casting Services
- 6CProto CNC Machining Services
- ISO 2768-1:1989 – General tolerances
- ISO 9001:2015 – Quality management systems