Why Is My Urethane Rubber Mold Shrinking?

Urethane rubber is often called an elastomer because it comes with elastic properties. These properties can be very beneficial, especially when casting and demolding concrete. However, they also come with downsides. One of these downsides is that it is susceptible to shrinking if you’re working in a cooler temperature.

All of VFI’s polyurethane rubbers have a dimensional stability of under 0.001 in/in at 77°F, which is the percentage of linear shrinkage when subject to changes in temperature or humidity during cure. This is tested using the ASTM D2566 method for thermoset casting systems.

With temperatures dropping rapidly across the US, more users may begin to have a temporary shrinkage problem due to the weather.

Testing for Rubber Shrinkage

We conducted a series of tests in our on-site lab to further prove our hypothesis that shrinkage happens due to the weather. We cast, cured, and demolded our own samples of pourable rubber along with competitors’ and observed what happened when we adjusted the room temperature.

First, we stuck the samples in a freezer at 20°F for 2 hours. Once they sat for the allotted time, we measured them and noticed that they shrunk 1-3% on each side.

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

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


Regardless of the temperature you plan to operate at, you’ll need to cast the liquid rubber in the same conditions. This works for people who cast urethane in warmer conditions, but if you’re someone who works in a colder climate, you may run into some issues. Urethane must reach a certain temperature for it to cure, so we do not recommend casting or using it in cooler conditions if you want to prevent shrinkage.

All VFI urethane molding rubbers, and most urethane rubbers on the market, must be allowed to sit at room temperature for 16-24 hours before demolding. A minimum of 3 days at room temperature is required before use. A full cure typically occurs after 7 days, and the rubber will develop full physical properties.

Another solution to prevent shrinkage from occurring if you can’t get around working in cold temperatures is casting the rubber on a rigid backing material. We typically recommend casting over some type of wood, like plywood. Urethane can form strong bonds with most surfaces, so it should have no trouble adhering to the wood.

When casting over a backing material, we recommend pouring it over the lip of the surface, allowing the urethane to grip onto it. Because the material has formed around the edges of the surface, it will have a much harder time shrinking. The plywood would have to break in order for any substantial shrinkage to occur.

Contact VFI if you have more questions on urethane rubber or need help finding the best material for your project.