Why Is My Urethane Rubber Mold Not Drying?

VFI has had customers using two-component urethane rubber come to us saying their material is not drying or curing. From experience and testing, we know this is typically an issue with mixing, mix ratios, or temperature.

To dry or harden liquid urethane, you must measure and combine a precise amount of each liquid component (A and B sides) before mixing thoroughly. The exact combination of materials is the mix ratio, which also helps the material achieve its formulated properties. It can be expressed by weight and/or by volume.

After mixing and once poured into a mold box or form, the material should be left to dry for at least 16 hours at room temperature (77°F) for best results.

Reasons Liquid Rubber Might Not Cure

Do not assume all mix ratios and mixing instructions are the same. The mix ratio varies per product because each material’s precise mixing formula provides the desired performance characteristics and allows it to dry.

If you mix with too little Poly (B side), the material can feel brittle and not as strong. If you mix with too little Iso (A side), the material can feel too soft, tacky, or gooey to the touch.

A change in final properties may occur from using too much or too little of one material. The off ratio mix may also be softer (lower hardness) than it was formulated to be.

Insufficient mixing can also cause drying issues. While some areas of the rubber may harden like normal, you may see tacky or soft streaks that are excessive amounts of A side material or excessive amounts of B side material. Also, it’s important to always premix your B side material to ensure you have an even weight per gallon. This makes it easier to achieve a uniform mix overall.

Also, temperature can play a huge role in the material’s ability to dry. Urethane rubbers cure best at room temperature (77°F). If your material, work area, or molding models are too cold, the mold rubber will take more time to dry. In some cases, it may not dry at all.

It’s important to read the product technical data sheets (TDS). The TDS includes mix ratios, mixing instructions, and molding guidelines. This information helps ensure you’re using the correct amount of each component, working in the best conditions, and mixing thoroughly to prevent drying issues.

Solutions

The best thing you can do if your urethane rubber isn’t drying is to start over. If it hasn’t dried after at least 3 days sitting at room temperature, odds are it never will.

One of the things you can do to limit issues is to pour your A side (Iso) first. It’s better to add more Iso than it is to add too much Poly (B side). If your mix is heavy on the Poly, you’ll never get the rubber off the master or out of the mold box.

An important thing to note is that tacky or sticky material should not ruin your master if the rubber is Iso-rich, and it will be reusable. You won’t have to restart the entire mold making process, which is a win.

To ensure that your material will dry in your second attempt, adhere to precise mix ratios. If you deviate from the suggested mix ratios, whether by volume or weight, your material will not solidify or reach its formulated properties.

Depending on your application, you may benefit from using a mix ratio by weight over a mix ratio by volume, or vice versa. Measuring by weight can have more accurate results as long as you use a working scale.

Bring the material to at least 65°F before use. Work in room-temperature environments with room-temperature equipment when possible.

VFI also recommends using the double bucket mix method when combining liquid rubber. Rather than mixing in a single container and then pouring, you should mix in one container and then pour the material into a clean container to mix again. This is the best way to ensure that unmixed material in the first mixing container will not appear in your final product.

The material should be left for a minimum of 16 hours before demolding. After demolding, allow the rubber to sit for 3 days before use. It will take at least 7 days for the material to develop final formulated properties. If the material is left to dry at lower temperatures, it may take longer.

Sticking to these guidelines will provide the best, most consistent results when pouring urethane molding materials. Plus, it will save you time and money if you don’t have wasted, unusable material.

Contact VFI if you have any further questions on the urethane rubber drying process. If you’re wondering why your urethane rubber mold is bubbling, check out our tech piece here.