A Practical Look at Mix-and-Pour Foam

Mix-and-pour polyurethane foam is not as intimidating as it seems—although watching it react is almost like watching a scary science fiction movie:The Blob that Ate San Francisco…two liquids went in…one foaming mass pushed its way out, covering everything in its path…

While it’s very entertaining, it’s more importantly a very practical product for use with composites. So let’s dispel some of the confusion related to mix-and-pour with some simple facts and explanation.

Why do fabricators use mix and pour foam?

BoatStringerFoamIt is most frequently used to fill cavities for reinforcement, insulation, or flotation. It is often used to make buoys and for filling in pontoons and stringers in boat making. (Just for reference, a 2 lb. density foam will provide 60 pounds of buoyancy per cubic foot.) Once cured, it can also be easily shaped and carved, so it’s used to make plugs for mold making and for sculpting purposes.

What should I expect when I order mix and pour foam?

When you order from Fibre Glast, mix and pour foam comes in a two-part kit of equally sized cans. If you only get one can, you’re missing something—or you have the wrong product. Each can is full of viscous liquid which is stable on its own. Each part only begins to react chemically when mixed with the other. The two parts are designed to be mixed in equal portions (1:1), by volume.

What’s the difference between two-pound and six-pound foams?

CoastGuardBuoyMix-and-pour foam is available in a variety of densities. Fibre Glast carries two selections: a 2 lb. density and a 6 lb. density. What does this mean to you? In a roundabout way, it’s all about air.

During its chemical reaction, the foam creates cells which close up and trap in air. Compared to the 6 lb. foam, the 2 lb. variety traps a greater amount of air inside each cell. So overall, it weighs less—if you were to cut a one-foot square cube from a mass of this foam, it would weigh two pounds. It’s also less dense and rigid.

Likewise, 6 lb. foam is closer to a solid, with smaller cells containing less air—a square cube weighs six pounds. A practical illustration of the difference between the two: you can push your thumb into cured 2 lb. foam and it will make a slight indention. You can stand on a piece of cured 6 lb. foam without changing its overall shape. For this reason, 6 lb. foam is more suited for structural support, sculpting, and holding fine detail.

How much will I need? (Doing the math.)

First, calculate volume (that’s “length x width x height” for the container into which you’ll pour both parts).

Second, convert to cubic feet. If you’re working with inches, you’ll need to divide your total by 1,728.

Third: if you’re working with 2 lb. foam, multiply the volume by 2.5 to determine the number of pounds of foam you’ll need; if you’re working with 6 lb. foam, multiply by 7.5.

How do I mix foam? What will happen?

This is a critical step, so pay attention.

There’s some irony involved: mix-and-pour foam is ideally suited for flotation ONCE it’s cured. However, PRIOR to cure, its components are highly sensitive to moisture. In fact, just drops of moisture can keep your foam from curing properly. Avoid using paper mixing containers and wooden stirrers as they can introduce moisture. Use plastic or metal when possible.

Like all resins, heat and humidity play a factor. While mixing, try to keep your environment at room temperature (approximately 77 degrees), and keep humidity to a minimum (urethanes may absorb moisture from air). The hotter the room, the faster foam will react and begin to cure.

As mentioned, mix parts equally by volume or weight. Mix thoroughly! Scrape the sides of containers, mix vigorously. For amounts larger than a cupful, consider using a Jiffy Mixer (that’s a long metal attachment for a hand drill, powered to spin rapidly (you can find those in Mixing Supplies at FibreGlast.com).

Be prepared—foam will expand rapidly and with force (remember “The Blob that Ate San Francisco”? It really happens that way.). Two-lb. foam expands 30 times its liquid volume, 6 lb. foam about 10 times. Use sturdy containers—if walls are very thin, they may distort, or buckle. Make sure at least one end of your container is open, as a “release” measure for the foam. (Larger amounts of mix-and-pour foam have been known to blow the deck off of boats when repairs have been made without properly preparing for expansion.)

How much working time do I have?

Not much. As soon as its individual components come into contact, the clock begins. For 2 lb. foam, that’s about 45 total seconds before the solution begins to foam. For 6 lb., foam begins to form at about one minute. Keep this in mind, mixing time is included in this. Get it mixed and poured into your container within this time.

How much time before it quits expanding? Before it cures?

ClockBoth foams have a “rise time” of about five minutes—that means it should be finished expanding in this time. Remember, as mentioned, however, that factors like temperature, humidity, and adequate mixing can play into this factor.

Time can be an issue when your application requires a large volume of foam. Rest assured, foam can be poured in layers, waiting 15–20 minutes between layers. In this case, be sure to clean up thoroughly between layers if using the same containers, for example. Even the smallest amount of cured foam left behind (or water) can affect the cure of the next batch.

Also be sure to protect elements outside the pouring area—mix-and-pour foam has been known to stain gel coat on other surfaces, and would need to be sanded or scraped from unwanted “splash or drip” areas.

While foam may seem fairly hard to the touch in less than an hour, all resins—including mix-and-pour polyurethanes—should be allowed a full 24 hours for complete cure.

What can I expect from cured mix-and-pour foam?

Once cured, mix-and-pour foam is closed cell and rigid.

1) It will not absorb water—it will float.

  • It will resist gas and oil—it is durable.

    2) You can laminate directly onto foam using polyester or epoxy resin.

    3) Mix-and-pour foam can be colored using urethane pigments (add pigment to part #24 before combining #24 and #25; add to part #624 before combining #624 and #625); it can also be painted using latex paint.

    4) If used in mold, it will release easily when a layer of #2 Parting Wax is applied to the mold.

    To get a look at mix-and-pour in action, watch this product spotlight video. You’ll also get more detailed specifics on our website when you visit product pages for both two-pound (#24/25) and six-pound (#624/625) foams.


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