How do multi-material or dual-durometer prototypes work in urethane casting?

The Overmolding Process Within a Single Mold

Multi-material or dual-durometer prototypes in urethane casting are achieved through a sequential overmolding process using a single, flexible silicone mold. Unlike injection molding which may require complex, multi-piece tools, this technique leverages the elasticity of the silicone tooling to create integrated parts with distinct material properties in different sections. The process is a cornerstone of advanced Rapid Prototyping for evaluating material interaction, ergonomics, and performance in a way that closely mimics mass-production overmolded parts.

Sequential Casting Steps

The process begins by casting the first, or "substrate," material into the silicone mold. This is typically the rigid core of the part, simulating a plastic like ABS or PC. The mold is then partially cured or a calculated pause is taken before the material fully sets. The partially cured rigid part is not demolded; instead, it remains in the silicone tool. The mold is then reopened to carefully expose the specific cavities or surfaces intended for the second material. The second, often softer, urethane resin is then poured or injected to bond with the first, creating a mechanical and chemical interlock at the interface. This allows for the creation of a single prototype with, for example, a rigid 70D body and a soft, grippy 40A overmold.

Design and Application Advantages

This method is ideal for creating prototypes that require integrated gaskets, soft-touch grips, shock-absorbing features, or seals. It eliminates the need for secondary assembling and ensures a perfect fit between the two materials. From a die castings Design service perspective, it allows for thorough validation of the bond integrity and functional performance of the multi-material design before committing to the high cost of multi-shot injection molding tooling. This is a key service for low-volume manufacturing of complex, multi-component assemblies.

Material Compatibility and Considerations

The success of dual-durometer casting hinges on the chemical compatibility and bonding capability of the selected Urethane Casting resins. Material suppliers provide specific pairs of rigid and flexible resins formulated to adhere to one another. A critical part of our die castings Engineering process is selecting the correct resin pair to ensure a strong bond that will not delaminate during functional testing, providing a prototype that is both visually and mechanically representative of the final production part.