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Old Aug 29, 2012, 09:06 PM
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United States, CA, Lake Forest
Joined Feb 1999
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To clarify a couple things, spray insulation foam and the two-part castable foams may be similar in that they are two part polyurethane based but they are very different in terms of physical properties and practical use (especially in this case). The insulation foams are typically 1.5lb/cu" density (24kg/cubic meter) or less and quite spongy/brittle. The equipment used is typically a high pressure impingement spray gun that forces pre-heated A&B together just in front of the gun so it mixes in the air. It's ridiculously fast stuff. The equipment for two part castable foams is lower pressure but uses a mix tube for proper blending. You can hand mix or machine dispense castable PU. You cannot realistically hand mix spray foams.

The key thing to note when comparing the GP330 (PU based) to the Ampreg (epoxy based) used by Timbuck is density. We are talking about a 2lb foam vs a 12 pound foam. This presents a huge difference in physical properties in the cured foam. Also, the lower density PU castable foams will begin to "cream" or react faster than a higher density castable PU foam. So, you get more working time with a 6lb foam than a 2lb foam. There are also "slow rise" formulations of castable PU foams but they do not typically work well when cast in thin sections. They are designed to do larger pours and control the exotherm in mass.

So, my suggestion would be to look at either a 4 or 6lb density foam (64-96kg/cubic meter) to get a little more working time plus some added strength. I have seen 10lb PU foam used as filler in hollow spots to allow a vac-bag pre-preg repair for aviation skins. It's very strong stuff.

Temperature does affect the reaction time because it is a thermoset system just like epoxies, polyesters, and other polyurethanes. But, you will not get the same expansion rate if you purposely cool down the material before mixing. You also raise the viscosity and potentially make it harder to mix (to a certain degree).

Also, expanding foams are tricky to use if you do not have the tool set up properly. Pouring it on the open molds and brushing it around is going to be tricky as well (you found out why) and will be hard to get consistent results. Typically close molds are the best way to control the density and promote consistent cell structure. If your mold has gaps and the foam can go wherever it wants, then it will not produce a high quality product. Ideally you want a mold that has carefully placed vents (and even the vent size is critical) while keeping the rest of the mold sealed. This often creates a lot of pressure on the tool and a flimsy mold won't hold up well to the pressures involved.
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Last edited by Troy; Aug 29, 2012 at 11:55 PM.
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