What is polyurethane rubber?

Article courtesy of Smooth-On, Inc.

Polyurethane rubbers are two-component systems (base plus curative; A+B) that cover a wide variety of applications at a relatively low cost. They are available for making molds that are poured, brushed or sprayed onto a model.


  • Polyurethanes are easy to use, with many having a simple mix ratio by volume (i.e. 1A: 1B) – no scale required.
  • Flexible urethanes are available in a wide hardness range from gel-like to harder than a car tire and everything in between. [Related: Durometer Shore Hardness Scale Guide]
  • Urethane rubbers have relatively low viscosity and "de-air" themselves – no vacuum degassing required.
  • Urethanes have good abrasion resistance and are used to cast abrasive materials like concrete.
  • Polyurethane rubbers are less expensive than silicones and polysulfides.


  • As silicone rubber has the best release properties, urethane rubber has the worst release properties and will adhere to just about anything. Thorough model preparation is essential to successful mold making with urethane rubber. [Related: Sealer and Release Agent Reference Guide]
  • Urethanes are moisture sensitive and may bubble if exposed to too much moisture (making molds outside on a very humid day, for example).
  • Polyurethanes have a limited shelf life after opening – remaining product may be affected by ambient moisture in the air. (Smooth-On makes a product called XTEND-IT™ that greatly extends the shelf life of unused urethanes).

See Smooth-On's Polyurethane Rubber Selection ›


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.