There are a few reasons urethane rubber might not cure. If you’re seeing spots or streaks of uncured material, it’s likely that the material wasn’t thoroughly mixed before it was poured. The best way to ensure your material is fully mixed is by using a double-mix method. Once your components are measured by weight or volume, combine them in one bucket and mix. Before pouring into your mold box, pour the material into another container and mix again. This reduces the likelihood of seeing these streaks or spots of uncured material.

Another reason urethane rubber might not cure is due to an off-ratio mix. It is essential to follow the mix ratio of each material precisely, whether using volume or weight. These ratios are what help the material cure. If the mix ratio is not followed, it can result in a sticky, unusable mess.

One last thing to note is temperature. If your work environment, model, or material is too cold, the material will have a hard time curing. Urethane rubber is a room-temperature curing material, so we generally recommend using the material at 77°F for the best results.