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Cast urethane is a low-volume manufacturing process that produces plastic and rubber-like parts by pouring liquid polyurethane resin into a silicone mold. It occupies a specific and well-defined role in product development: quantities too small for injection molding, surface quality too high for 3D printing, and material flexibility that neither process can match.
How Cast Urethane Works
The cast urethane process begins with a master pattern, typically a high-quality 3D printed or CNC machined part that represents the final geometry. Liquid silicone rubber is poured around the master pattern and allowed to cure, forming a flexible mold. The master is then removed, leaving a cavity that precisely replicates its surface texture, details, and dimensions.
Once the silicone mold is ready, liquid polyurethane resin is mixed, degassed to remove air bubbles, and poured into the mold cavity under vacuum. The resin cures, the mold is opened, and the part is removed. The silicone mold is reusable for approximately 25 to 50 pours, depending on part geometry and resin chemistry.
The process from a finalized master pattern to first parts typically takes 7 to 14 days. Unlike injection molding, no metal tooling is machined, which keeps setup cost low.
What Materials Are Available
Cast urethane uses polyurethane resins formulated to simulate a wide range of production plastics and rubbers. Material selection is one of the key advantages of the process.
Rigid Urethanes
Rigid urethane resins simulate ABS, polycarbonate, and other hard thermoplastics. They are available in Shore D hardness ranges from approximately 60 to 85 and can be pigmented to match specific color requirements. Rigid urethanes are used for housings, enclosures, brackets, and structural components where dimensional stability and surface finish are priorities.
Flexible and Rubber-Like Urethanes
Flexible urethane resins cover the Shore A 20 to Shore A 90 range, simulating TPU, silicone-like materials, and natural rubber. They are the standard choice for overmolds, grips, seals, gaskets, and any component where compliance or tactile feel is part of the product specification. Achieving this range of hardness in injection molding requires changing both the material and the tooling design; in cast urethane it requires only a different resin.
Cast Urethane vs. Injection Molding
These two processes are frequently compared because they target overlapping applications at different volume thresholds. The table below shows where they separate.
| Factor | Cast Urethane | Injection Molding |
|---|---|---|
| Tooling cost | $200-$1,500 (silicone mold) | $1,000-$10,000+ (aluminum); $10,000-$100,000+ (steel) |
| Lead time | 1-2 weeks from approved master | 3-5 days (rapid); weeks (production) |
| Volume range | 10-300 parts | 50-100,000+ parts |
| Material options | Wide Shore A/D range; urethane resins | Full engineering thermoplastic library |
| Surface finish | Excellent; Class A possible | Excellent; texture in mold |
| Part strength | Good; process-dependent | Isotropic; production-grade |
| Design changes | New silicone mold; low cost | Aluminum insert or new mold; moderate cost |
| Best for | Consumer products, overmolds, pre-production samples | Functional testing, bridge and volume production |
The volume crossover is the most important line in the table. Below roughly 100 parts, cast urethane is almost always more cost-effective than injection molding because there is no tooling investment. Above 300 to 500 parts, the economics shift in favor of rapid injection molding: aluminum tooling is a one-time cost that drops the per-part price significantly as quantity grows. At that threshold, the part cost per unit from cast urethane typically exceeds what rapid injection molding can deliver.
What Cast Urethane Does Well
Surface quality is exceptional. Because the silicone mold captures the exact surface of the master pattern, cast urethane parts can achieve Class A cosmetic finish directly from the mold. This matters for consumer product development, where appearance prototypes need to represent the final product accurately at trade shows, in sales samples, or during customer testing. RPM Fast’s cast urethane service covers the surface finish options and master pattern requirements in detail.
Material range is broader than injection molding for low volumes. The ability to pour any Shore A or Shore D formulation into the same mold type means a single process can simulate soft grips, rigid housings, and flexible seals without requiring different tooling infrastructure for each.
Design iteration is low-cost. When a geometry change is needed, a new silicone mold is made from an updated master pattern. At $200 to $500 per silicone mold for most parts, this is an affordable way to incorporate engineering changes without the commitment of machining new aluminum tooling.
Where Cast Urethane Has Limits
Cast urethane is not the right answer for every low-volume situation, and being clear about its limits prevents misapplication.
It is not a thermoplastic process. Polyurethane resins behave differently from ABS, nylon, polycarbonate, and other production injection molding materials. This means cast urethane parts may not accurately represent how a production injection molded part behaves under load, thermal cycling, or chemical exposure. For regulatory testing that requires the exact production resin, injection molding is the appropriate process, even at low volumes.
Mold life is limited. At 25 to 50 shots per silicone mold, cost per part rises quickly above 200 to 300 units. rapid injection molding using aluminum tooling becomes more cost-effective at that volume threshold, and parts are produced in actual engineering thermoplastics.
Dimensional consistency is slightly lower than injection molding. Silicone molds are flexible and can distort slightly under repeated use. For parts with tight dimensional tolerances, injection molding or CNC machining is more appropriate.
Frequently Asked Questions
What is cast urethane used for?
Cast urethane is used for low-volume production of plastic and rubber-like parts, typically in quantities of 10 to 300 units. Common applications include consumer product housings, overmolded grips, enclosures, medical device prototypes, and automotive interior components. It is particularly useful when production-quality surface finish and material feel are needed before committing to injection mold tooling.
How does cast urethane compare to injection molding?
Cast urethane uses a silicone mold made from a master pattern and liquid polyurethane resin poured by hand. Injection molding forces molten thermoplastic into a machined metal mold under pressure. Cast urethane has a lower tooling cost and faster setup for very low volumes but does not scale economically and uses a different material family than most production injection molded parts. Injection molding is the correct process once volumes exceed a few hundred parts or production-grade thermoplastic properties are required.
How many parts can you get from a cast urethane silicone mold?
A silicone mold for cast urethane typically yields 25 to 50 pours before the tool degrades and parts begin to show surface defects or dimensional variation. Some simple molds with favorable geometry can yield up to 100 parts. This makes cast urethane economical for runs of 10 to 100 parts but increasingly costly per part as quantities grow toward 300 or more, where rapid injection molding becomes more cost-effective.
What Shore hardness options are available in cast urethane?
Cast urethane resins are available across a wide hardness range, from very soft flexible materials at Shore A 20 to semi-rigid parts approaching Shore D 80. This range covers soft grip materials, flexible seals, rigid enclosures, and most overmold applications. The ability to specify hardness makes cast urethane the standard process for simulating TPU, rubber, and rigid ABS-like materials before committing to production tooling.
Choosing Cast Urethane for Your Next Project
Cast urethane fits a precise window in the product development lifecycle: quantities between 10 and 300, surface quality requirements that 3D printing cannot meet, and applications where the flexibility to specify hardness across a wide Shore range is an advantage. Outside that window, either 3D printing or injection molding is typically the more appropriate process.
RPM Fast is ISO 9001:2015 certified and handles cast urethane alongside CNC machining, injection molding, and sheet metal fabrication under one roof. If you are evaluating cast urethane for a current project, request a quote from RPM Fast and we will review your geometry and volume requirements and recommend the right process.
