Why Are There Bubbles in My Urethane Rubber?

We’ve come across urethane rubber users in the past who have wondered why there are bubbles in their molds. Bubbles can present themselves in urethane rubber for various reasons, with the biggest one being moisture.

Polyurethane is a very moisture sensitive material, so if it finds its way into your mold-making process, it can create issues. The good news is that most moisture sources that cause bubbles can be controlled.

Reasons Bubbles Form in Urethane Rubber

Bubbles that appear in your finished piece can diminish details and transfer over to future castings, which is why it’s important to prevent them from forming. The following should be avoided:

1. Humidity

Working in a hot, humid environment is not good for your rubber material. The second you open the A (Iso) and B side (Poly) containers, moisture will try to find its way in. The longer these materials are left to sit, even when unmixed, the more they will absorb moisture.

Humidity can be especially detrimental if the material has a long working time. If your rubber is left to cure in these conditions, the material will continue to react with the moisture in the environment, which will lead to foaming or bubbling. These defects will then remain in the rubber once it has dried.

2. Wet Molding Surface & Porous Masters

Again, because urethane is moisture sensitive, if you see bubbles on the surface of your rubber once you’ve demolded it, it means that something was on the surface that caused it.

If your molding surface is wet or you’re using porous masters (plaster, concrete, gypsum, etc.) without properly sealing them, you might discover bubbles on the surface of the rubber.

3. Wet Mixing Equipment

If your mixing equipment (buckets, mixing sticks, etc.) is wet when you pour your material into it, a moisture reaction will occur. You will notice air bubbles begin to form while you’re mixing.

Air bubbles can also form based on your mixing procedures. If you mix too quickly or for too long, you may introduce more air into your mix. If this happens, pour your material slowly in a thin stream to allow those air bubbles to pop, or degas the material before pouring.

4. Excessive Mold Release

If you see surface defects like tiny pinholes or champagne bubbles in your rubber, you may be using too much mold release. You may also not be waiting enough time for the release to dry before you start casting.

Alternatively, the mold release you’re using could be past its expiration date or poorly mixed, but this will appear as indentations in the rubber and not air bubbles.

Solutions

Always check the technical data sheet for recommendations before using the material. The best thing you can do to avoid bubbles in your material is to work in a room temperature-controlled (77°F) environment. You want the relative humidity to be as low as possible.

While vacuum degassing and pressure potting can be useful in certain mold-making scenarios, it’s not always possible, depending on how much material you’re casting and the size of the mold you are trying to make.

Before you mix and pour the material, make sure your forms and masters are properly released and sealed. VFI recommends sealing porous surfaces with a mixture of 80% mineral spirits to 20% petroleum jelly.

Some people have used polyvinyl acetate (PVA), or Shellac, to seal their masters. However, it is easy to remove it from the surface accidentally. So, if you forget to reapply it, your masters will not be sealed, leading to various issues.

Once any porous surfaces are sealed, you must apply a compatible release agent. VFI recommends Chem-Trend MR-515 or a similar silicone-based release for urethane rubber. Do not use a water-based release agent.

Avoid using too much release and allow it to dry before you pour your rubber. If you use too much release, bubbles may form, or the surface might appear glossy, which will then transfer over to future castings.

Use dry mixing equipment. Plastic and metal mixing equipment is less likely to hold moisture. Mix slowly so you don’t generate more trapped air in the mixture. Once fully mixed, pour it slowly in a thin stream into the lowest point of the form to minimize the formation of air bubbles. Let it flow naturally into the rest of the form.

Avoid repeated opening and closing of the containers as this can also introduce moisture into the A and B side materials. If you plan on storing unused material, you must nitrogen purge each material and close the containers as soon as possible after use. Store the containers in a cool, dry place.

Contact VFI if you have other mold-making issues or check out our other tech pieces.