Why Is My Urethane Foam Not Expanding / Collapsing?

If you’re familiar with rigid or semi-flexible urethane foam, you know that each product has its own approximate expansion rate. The lower the foam density, the more it will expand. You’re trusting that it will expand to a specified amount multiplied by its volume, as stated on the technical data sheet (TDS). So, when it doesn’t do what’s expected, you’re probably wondering what went wrong.

In some cases, it might seem like the foam reaches the desired expansion rate, but then it will deflate before it cures. Urethane foam might not expand or will rise and then deflate for several reasons, with the most common being contamination or application issues.

Reasons Urethane Foam Doesn’t Expand

1. Contamination

Some materials are not compatible with urethane foam and will cause expansion problems. Silicone creates the most common issues because it is a low surface tension contaminant. If you have excessive amounts of silicone on your molding surface, your foam may rise but then it will collapse. When this occurs, it will kind of look like a deflated lung. This typically happens when using silicone-based release agents or new tin-cured silicone molds that haven’t fully cured.

2. Mixing Issues

As two-component materials, if you don’t mix the correct amount of the A (Iso) and B (Poly) sides, problems may occur. All urethane foams have a different mix ratio by weight, while most are 1A:1B or 1A:2B by volume. If you use too much of one component, it’s not going to react the way you’re expecting it to. It may never expand, or it will not develop a uniform cell structure.

Problems can also occur if you are not thoroughly mixing the two components. Expanding foams can be tricky to work with because of their extremely short pot lives and fast rise times. Mistakes with mixing occur because people rush to pour the material before it starts to expand.

3. Cold Temperatures

Since urethane foam is a water-based material, you need to consider the temperature of the environment, the material, and the molding surface. If you are working in cold temperatures or if your molding surface is too cold, the foam might experience less expansion or collapse as it cures due to separation of the A and B sides from the low temperatures. Temperatures below 60°F can cause these problems and there is also a risk that the material may freeze.

4. Curing Issues

If you move the foam or touch it while it is still expanding, this interference could cause cells to collapse. The area you touched will deflate and create a mass that will be hard to remove from the foam.

Also, if you allow it to expand but remove it from the mold too soon, you are not giving it enough time to gain strength. Even if it appears cured and firm on the surface, it probably hasn’t cured internally. Eventually, it will deform or deflate without a structure to hold its shape.

Solutions

Always follow the manufacturer’s instructions before working with urethane expanding foams for the best results. Temperature is one of the most important things to consider when using any urethane material. Working at higher temperatures can help to increase expansion. It’s recommended to bring the material temperature to a minimum of 65°F, work at room temperature (77°F), and use a heated mold.

You can heat the mold up to 120°F, but doing so will decrease the already short pot life. If your material is too cold, bring it up to room temperature before dispensing.

If you need to prevent adhesion from occurring, avoid using silicone-based release agents. The most recommended release agent for foam is a wax-based, silicone-free release agent. These release agents have proven effective in releasing urethane foam from various surfaces.

If you use a tin-cured silicone mold, you must ensure it is fully cured. You can do this by waiting for the allotted cure time or post-curing it in an oven. Often, it is best to avoid using a silicone mold and just use a metal mold when possible.

Thoroughly premix the B side before combining it with the A side. Ensure you are using the correct mix ratio as stated on the manufacturer’s technical data sheet. Once combined, mix quickly and thoroughly with a drill or power mixer. Drill mixing introduces more air into the system and will cause the foam to expand more. Mixing by hand will not provide you with an adequate mix before the pot life expires. The faster and harder you are able to mix, the better it will expand. For uniformity, make sure to scrape the sides and bottom of the mixing container as well.

Avoid moving or disturbing the foam while it cures. Allow it to remain in the mold or form until it has fully cured to prevent deformation. This will typically be 2 hours when left at room temperature. Cure times depend on the material, so always check the TDS. Thinner pours will require a longer cure time.

Contact VFI if you have other issues with urethane expanding foam. We’ll be happy to help you find solutions or alternatives to your problems.