Why Is My Urethane Rubber Mold Sticky?

Some urethane rubber users have come to us and are confused as to why their rubber is sticky after waiting for it to cure. Others have mentioned having a hard time removing it from the mold box or form as well.

If the material itself is sticky, it may not have been mixed thoroughly enough before it was poured. If it’s clinging to the mold box or form, then not enough mold release was used to prevent it from sticking to the surface.

Reasons Rubber Molds Might Be Sticky

Mixing is typically where the biggest mistakes are made. Just because the material might look fully mixed in the bucket doesn’t mean it is. Poor mixing generally occurs because users are concerned about the pot life of the material, so they’re not mixing for long enough.

Some mold makers use large amounts of rubber with short pot lives, and that causes them to rush and skip vital steps. They end up with a bad mix, which wastes time and material.

Also, scraping the sides of your mixing container while pouring the rubber into a mold can cause these sticky spots. It’s harder to thoroughly mix material that is stuck on the sides or at the bottom of the container, which creates excessive amounts of the A or B side material.

Excessive amounts of A side material will cause sticky or tacky spots that may dry eventually. However, if they dry, that doesn’t mean the rubber will last as long as it normally should. It might feel softer in these areas and could tear more easily in the casting process. Excessive spots of B side material will never dry.

On the other hand, if your material is sticking to the mold box or even the master models, it probably wasn’t released or sealed properly.

Non-porous surfaces always require the use of a release agent. If you’re casting urethane rubber over porous surfaces, those surfaces have to be sealed and released to prevent adhesion from occurring.

If adhesion does occur, it could potentially break the master. Keeping the master intact is important, especially if it is delicate. Using enough release should prevent this from happening. Release is also needed when casting into the urethane mold as well. Check out VFI’s release tech piece for more information.

Note: Cure inhibition can also cause stickiness, but it is unlikely that this is a cause for liquid urethane rubber molds. Cure inhibition is a much bigger problem for tin-cured silicone rubber.

However, one material that should be avoided when making any rubber mold is sulfur-based clay. Anywhere the clay touches will not dry, so it will be sticky.

Solution

If your initial mix has already been left to dry at room temperature for days and it’s still sticky, there’s nothing you can do to fix it. The best thing you can do is restart with a new batch of material.

The important thing is that sticky spots in your urethane rubber will typically not ruin the master models. If the rubber ruined your master, it would create even more work for you. It’s much easier to purchase more material and start over than it is to make another master.

Use the double bucket mix method. After mixing in one container, transfer the material into another clean mixing container to mix again. Doing this will ensure a thorough mix, so you won’t have to worry about your molding rubber being sticky.

Always scrape the bottom and sides of the container while you are mixing. Unmixed material can cling to those hard-to-reach areas.

For mixing larger amounts of material, use a mechanical mixer. If you try to hand mix more than a gallon of material, you will be unable to mix it thoroughly enough before the pot life ends.

Using a release agent will protect and extend the life of your mold. Always use the correct type of mold release and the right amount.

If you’re molding over a porous surface, VFI recommends using a mixture of 80% mineral spirits to 20% petroleum jelly by weight to seal it. Then, you should also spray the entire molding surface with a silicone-based release agent like Chem-Trend Mr-515 Aerosol to further prevent the rubber from sticking to the mold.

Contact VFI if you have further questions on preventing urethane rubber issues. If your urethane rubber mold is not drying, read our other tech piece here.