Urethane casting and injection molding both produce plastic parts from a mold, but they sit on opposite sides of the volume curve. Urethane casting pours two-part resin into a silicone mold, with low tooling cost and small batches; injection molding injects thermoplastic into a steel mold, with high tooling cost and low per-part cost at volume. The decision is a break-even calculation plus a material question: where the tooling costs cross, and whether the cast resin can stand in for the production thermoplastic. This guide works through both, so the process is chosen for the program's real quantity and material requirement.
The Two Cost Structures
Urethane casting costs little to start: a silicone mold is made from a master in days, and each part is cast by hand or semi-automatically, so the tooling investment is small and the per-part cost stays high. The mold life is limited, counted in tens of parts depending on the resin and geometry, so a larger batch needs multiple molds made from the same master, which keeps the per-part cost roughly flat.
Injection molding costs much more to start: a steel mold is machined, and the tooling cost is a real line item in the program. Once the mold exists, the machine cycles in seconds to minutes and the per-part cost drops with volume, so the unit price falls steeply as the tooling amortizes.
The two curves cross at the break-even quantity. Below it, casting is cheaper because the tooling is small; above it, molding wins because the per-part cost is low enough to pay back the mold. The crossover depends on the part size, the mold cost, the cavity count, and the material, so the honest answer comes from quotes at the real quantity, not from a rule of thumb.
| Cost factor | Urethane casting | Injection molding |
|---|---|---|
| Tooling | Silicone mold, low cost, days | Steel mold, high cost, weeks |
| Tool life | Tens of parts per mold | Hundreds of thousands of cycles |
| Per-part cost | High and roughly flat | Low at volume, falls with quantity |
| Lead time to first part | Days | Weeks after tooling |
| Material | Two-part polyurethane resin | Production thermoplastics |
| Part size flexibility | Practical for small and medium parts | Cavity size and press tonnage set limits |
The Break-Even Calculation
Run the numbers the same way as any make-or-buy decision. The casting route costs the silicone molds plus a per-part cast price; the molding route costs the steel mold plus a per-part molded price.
An illustrative example shows the shape. Suppose a housing casts at $22 per part with $600 of silicone molds, and molds at $4 per part with a $28,000 steel mold. The crossover solves 600 + 22Q = 28,000 + 4Q, or Q = 1,700 parts. Below about 1,700, casting is cheaper; above it, molding pays back the tool. The numbers are illustrative and your geometry will change them, but the structure of the calculation is the same: total delivered cost at the real quantity, including tooling and any secondary finishing.
The calculation should also include the practical constraints. Casting mold life means a 1,000-part order needs many silicone molds, which raises the cast tooling cost and can add lead time; molding has a minimum economic run because of setup and material, and the tooling lead time may set the schedule. The break-even is a starting point, and the schedule and the mold-life realities often decide the route before the crossover does.
Material: The Cast Resin Is Not the Production Thermoplastic
The second part of the decision is material equivalence. Cast polyurethane is not ABS, PC, or nylon: the strength, temperature limit, chemical resistance, and fatigue behavior differ, and the difference matters when the part carries load or sees heat. Urethane casting fits appearance samples, user testing, and low-volume products where the resin's properties meet the requirement; it fails when the part must behave like a specific production thermoplastic, because no finishing step can change the resin family.
When the production material is known and the volumes are low, the honest approach is to test the cast part against the same environment the production part will see. If the cast resin is a valid proxy, the casting route can carry the low-volume program; if not, the program needs the real material, which usually means molding or machining.
Which Process for Which Program
| Program situation | Lean urethane casting | Lean injection molding |
|---|---|---|
| Quantity | Tens to low hundreds | Thousands and up |
| Tooling budget | Low, fast to start | High, committed at design freeze |
| Material requirement | Resin properties meet the part | Specific thermoplastic required |
| Lead time | Days to first parts | Weeks after tooling |
| Design stability | Still changing | Frozen before tooling |
| Appearance | Molded-like with finishing | Production surface in the mold |
The table is a filter. A program with a few hundred parts, a stable enough design for a master, and a resin that meets the requirement is casting territory; a program with thousands of parts and a specified thermoplastic is molding territory, and the tooling lead time should be in the schedule.
The Bridge Route: Cast First, Mold Later
The two processes often combine in one program. Urethane casting carries the first batches, user testing, and market validation while the steel mold is being built, then injection molding takes over for production. The cast parts validate the appearance and assembly, the molding validates the production material and process, and the program never waits for the tool to see a part.
The bridge works when the master and the molded design are aligned: the geometry that casts well should be the geometry that molds well, with draft and uniform walls planned from the start. The cast phase answers the product questions; the molded phase answers the production questions, and the data from each phase feeds the next.
The material question cuts both ways. A cast part can also be over-specified: if the part only needs appearance and moderate service, paying for a steel mold and a production thermoplastic is wasted, and the cast route delivers the requirement at a fraction of the tooling cost. The program should separate what the part truly needs from what the production catalog suggests, and the material requirement should be written down before the process comparison, because the process follows the material.
Decision Checklist
- Quantity forecast set, including the realistic range, not a single number
- Tooling quotes obtained for both routes at the real quantity
- Mold life counted into the casting tooling cost
- Material requirement confirmed: can the cast resin meet it, or is a specific thermoplastic required?
- Lead time checked against the schedule, including tooling weeks
- Design reviewed for moldability, with draft and uniform walls, if molding is in the plan
- Bridge plan defined if casting will carry the program until the mold is ready
Conclusion
Choose urethane casting for low tooling, small batches, and molded-like appearance when the resin meets the requirement, and choose injection molding when volume or the material specification pays back the steel tool. Compute the break-even at the real quantity, count mold life into the cast cost, and verify material equivalence before committing. For many programs the answer is both: cast the early batches, mold the production, and let each process answer the question it is good at.
FAQs
At what quantity does injection molding become cheaper than urethane casting?
The crossover depends on the mold cost, the per-part costs, and the part geometry; it is often in the low thousands for small parts and higher for larger parts. Calculate it as total delivered cost at your quantity, with the silicone molds or the steel mold included, and check the schedule too.
Can urethane cast parts replace injection molded parts?
Only when the cast resin's strength, temperature, and chemical properties meet the requirement and the quantity stays low. Cast polyurethane is not the same material as a production thermoplastic, so material equivalence should be verified, not assumed.
Why is injection molding tooling so expensive?
The steel mold is machined with hardened cavities, cooling channels, ejection, and fit features, and it is built to cycle hundreds of thousands of times. The cost pays for repeatability and tool life, which is why molding wins only when the volume spreads the tooling across enough parts.
How many parts can a silicone mold produce for urethane casting?
Typically tens of parts, depending on the resin, the geometry, and the surface detail. Larger orders need multiple molds made from the same master, so mold life should be counted into the casting tooling cost and the lead time.
Sources
- 6CProto Urethane Casting
- 6CProto Injection Molding Services
- 6CProto Plastic Injection Molding
- ISO 9001:2015 – Quality management systems
- ISO 2768-1:1989 – General tolerances for linear and angular dimensions