Expanding foam calculations

[Cbuf]

Before I add foam I figure I might as well run my math past the group.

I’m using the 2lb bbc 2part foam.

Length 34”
Height 4”
Width 10.5 = 1428 cubic in

1 cubic in = .554 us fl oz
1428 insq = 791.2727. Div by expansion of 20. Then divid by 2 for equal parts.

792/20 = 39.6 /2 = 19.8 oz of part a and 19.8 oz of part by.

So mix 19.8 oz of A and 19.8 oz of B to fill the void.

Does this sound right? Does the foam expand 20 to 1?

Any other tips for working with foam. I should be able to keep the garage at around 72-74 degrees.

Thanks

Cbuf.

[Fred in Wisc]

The 2lb foam is supposed to expand 30:1, but it never does for me.

A couple things I've found that might be helpful:

Wear gloves and a long sleeve shirt that you can throw away. This stuff is messy if you get it on you.

The hotter it is the better the foam expands. The temperature of the boat hull, foam A&B, and air temperature all seem to play into that. If you can get everything up to 90 degrees F, it's going to expand much bigger and faster than at 72-74.

You really don't get long to mix the stuff before it starts going off. I like to use 8oz Styrofoam cups- pour a row of cups a little under half full with part A and a row with equal amounts of part B. One of the parts is thinner than the other and pours better. Pour that part into the other cup, stir like mad for 15-30 seconds and once it starts to react dump it where you need it, FAST. I actually often drop the cup right into the compartment as well, the foam still inside it will expand and fill it.

I'd highly recommend working in smaller batches, it would be difficult to thoroughly mix a big batch and it's likely you'll waste more that way. Small batches gives you better control, and the stuff reacts and sets up quite quickly, so it doesn't take a whole lot longer that way. Plus it's super fun to watch it expand.

[txjm]

Another alternative calculation that worked real well for me was to use the 2#/cuft as the basis for your calculations. You know how much foam weight you have and you know that the free rise density is 2#/cuft so you know how many cuft of foam you have. You know the volume you need. Try using a Proportion. Volume of foam needed to fill cavity / total volume foam generated by your foam * gallons of foam. The issue is that the cavity doesn’t fill perfectly because of the muffin top.
At work we foam into fixtures so our calculations are easier. Volume of cavity * 2.5#/cuft / flow rate of gun.

[Dougster]

My experience is just like Fred's. I always need more foam than I first thought. Also, like Fred I mix small batches in cups, pour them, and then give that a few minutes to rise before the next pour. I never thought of tossing the cup in with it. Good idea!

Dougster

[Chuck H]

Since temperature, relative humidity, density altitude, mixing accuracy and whatever else are factors I’m going to say it’s incalculable.

I think the usual scientific method is to buy what the plans say plus 2 gallons and then see how much more you need when you get that poured.

Fred’s pretty smart, I wish I’d thought of using foam cups. I can’t find a “tip of the hat” emoji.

Chuck

[Cbuf]

Thanks for the replies, I’ll post my results. I have about 5 more cleats to put in before I crank the heat for a day or two to start foaming.

I’m going to get some foam cups.

Thanks

Cbuf

[OrangeQuest]

Your calculations are to much foam. You have less than a cubic foot to fill and it takes 16oz. of each part to make a cubic foot.

Because of the way the foam expands don't try to fill the void in one pour but smaller ones.

[Cbuf]

I did a test pour. The temp of the boat and foam was 77 degree. I got an expansion rate of about 10, not 20, and no where near 30. I can try to tarp off the boat and put a heater in there to bring up the temp. What is the risk of not full expansion? Will it float less weight? In a sealed compartment will the good pool noodles do about the same as the expanding foam? I like captain UBs noodle floatation.

Thanks

Cbuf

[Cbuf]

About 5 oz of a and 5 oz of b yielded the pic below.

[joe2700]

Note I’ve never used this stuff myself so just thinking out loud. I know people like small batches since the stuff goes off quickly, but if it’s a reaction that both produces heat and goes more quickly with heat, wouldn’t a larger batch get hotter and therefore expand more?

Anyone notice a difference with batch size. In applications other than this forum I’ve seen people mix in a 5 gallon bucket with a paint mixer on a drill, so the small cups are pretty different.