Why Is My Urethane Rubber Mold Expanding?

Liquid urethane rubber molds shrink from low temperatures, but problems also arise if they are used in warm and humid climates. Pourable polyurethane rubber may expand in high temperatures due to its elastic properties. While these properties are beneficial, you must follow certain application procedures to ensure they work how they’re supposed to.

The dimensional stability of our urethane rubbers is tested using the ASTM D2566 method for thermoset casting systems. This is the percentage of linear shrinkage when subject to changes in temperature or humidity during cure. At room temperature, all VFI TDI urethane rubbers maintain dimensional stability below 0.001 in/in.

Once temperatures rise above 77°F, users may begin to see temporary expansion in their molds and forms due to weather.

Testing for Rubber Expansion

To prove that urethane expands when exposed to heat, we conducted a series of tests in our on-site lab. Using several samples of our own pourable urethane rubber and a few competitors’, we observed what happened when we adjusted the room temperature.

Once cast, cured, and demolded, we put the samples in an oven for 2 hours at 125°F. After the allotted time, we measured them and noticed they expanded by 1-3% on each side.

We then pulled the samples out of the oven and allowed them to sit for 2 hours at room temperature (77°F). We measured them again and noticed they returned to nearly the exact size of the molds they were cast in.

Results of our testing: If your mold or formliner has expanded due to a temperature change, it should return to its original size once it is brought back to room temperature.


We’ve seen this kind of dimensional change occur when people work in warm, humid climates. If you plan to operate at a warm temperature, you’ll need to cast the mold in the same conditions. So, the liquid rubber and the environment temperature will have to be relatively the same as the temperature you plan to use the cured mold in. Also, as a note, please see our urethane shrinkage blog if you are using molds at a temperature that has dropped substantially from what it was molded at.

Urethane rubber must sit at room temperature for 16-24 hours before it can be demolded to prevent deformation. Keep the material at room temperature for an additional 3 days before use so it has time to gain strength and properties. The rubber will typically develop full physical properties after 7 days.

Casting the rubber against a rigid backing material can also prevent it from expanding due to high temperatures. We typically recommend using some type of wood, like plywood. Urethane’s high adhesion properties will form a strong bond with the surface.

We recommend pouring the urethane over the lip of the surface, allowing the material to grip onto it. When formed around and bonded to something more dimensionally stable, it won’t expand unless the substrate expands. Expansion or contraction of a sturdy substrate like plywood is very unlikely.

Contact VFI if you require further assistance with urethane rubber issues. We can also help you find the best material for manufactured stone, cast stone, and concrete stamps.