Expanding foam calculations

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pee wee
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Re: Expanding foam calculations

Post by pee wee »

joe2700 wrote: Mon Oct 07, 2019 10:50 pm 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.
Yes, the stuff is temperature sensitive, and small batches will not expand as much as large ones. The warmer the temperature is, the faster the reaction will kick off, so I wouldn't try really large batches until you have experience- if it's not mixed well it won't expand well, if it kicks off in the bucket things get exciting. Seventy eight degrees is warm, but Cracker Larry preferred to pour foam when the temps were over ninety. I did my foam work with temps in the nineties, and also got a space heater to warm the compartments up even more before I started mixing; I got excellent expansion. With enough heat you have to work really fast, but you get good expansion. With less expansion, you'll be adding more weight to the boat per cubic foot, but it will also be more solid.
Foam will keep water out of a compartment even if there's a hole through the ply, pool noodles will limit how much water can enter. Foam will make a more solid hull, but it has weight and it costs money (pool noodles aren't free, either). Your choice.
Hank

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Re: Expanding foam calculations

Post by Dougster »

Low expansion just means you'll use more foam and it will be a bit heavier. No loss of flotation. The flotation is the volume of the space, the foam just means the space can't fill with water. Warmer temp will get you some more expansion. If you have the foam I'd say just use it.

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Fred in Wisc
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Re: Expanding foam calculations

Post by Fred in Wisc »

I haven't used pool noodles to replace foam but I've used sheets of closed cell Styrofoam to "stretch" my expanding foam when it looked like I wouldn't have quite enough to finish filling a boat. Cut sheets of pink or blue foam, lay them in flat areas of the boat that you are filling, then foam around and over them with the expanding foam. Leave a good sized gap around the edges for pouring in the expanding foam, an inch or two works good. If the expanding foam gets under the sheets it'll push them up with surprising force, so it really works best when you can place the sheets against a flat surface, whether it's the bottom of the compartment or the stringers. You can hold it in place with a little hot glue while you're pouring foam, once it expands that stuff will be locked in tight. Sheet foam is a bit heavier than expanding foam, but it won't absorb water.

Don't use white Styrofoam, if it's exposed to water it will absorb some between the foam beads. That can add a lot of weight and nastiness.

Fred in Wisc
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Re: Expanding foam calculations

Post by Fred in Wisc »

Reply for Joe about big batches-

Good idea on using the exotherm heat for additional expansion. I've never used real big batches, but that does make sense. The difficulty lies in getting it mixed completely- this stuff is a really horrible mess if it's not mixed well and doesn't react completely.

Also it tends to pull away from the walls of the area you pour it in when there is a lot of expansion and motion in even medium sized batches, I've found small ones are easier to bet a 100% gap free fill. That was a terrible sentence, I hope you get what I'm trying to say.


If it were confined in an area, I bet it would fill to the walls better and exotherm more, but that's a scary thought. Foam exerts a tremendous amount of pressure as it expands and you could easily distort the shape of a boat putting it in a confined compartment.

An interesting demonstration is to take a couple sheets of plywood about 12-18" square, pour a few ounces of mixed foam on one, place the other over it, and stand on it. In a couple minutes it'll lift you right up.


Summary- big batches probably works real well if everything goes according to plan. In my world, that doesn't happen reliably enough for me to take the chance. I just ain't that brave. Or foolish.

Cbuf
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Re: Expanding foam calculations

Post by Cbuf »

I have a heater set to 85 in the front of the boat and a fan on the back pulling air through, by tonight everything sh ok old be in the 80 range. I have to pour most locations today as I'm running out of thing I can do.

I will post more pics when the foam is in
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pee wee
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Re: Expanding foam calculations

Post by pee wee »

Good luck with it! Remember, while the foam expands quickly it will continue imperceptibly to grow a little for a day or two. Let it finish all it's going to do before you cut off excess or glue panels over it. Also, as discussed recently, a seal coat of neat epoxy is recommended for the cut surfaces of the foam.
Hank

Fred in Wisc
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Re: Expanding foam calculations

Post by Fred in Wisc »

I didn't notice before, but glad to have another Wisconsin guy on the board. I'm only about an hour east of you. And that's a nice looking boat so far!

Cbuf
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Re: Expanding foam calculations

Post by Cbuf »

Getting everything in the boat up to 80-82 degrees was the key. I had a rate of about 15-20:1. The cup trick was perfect. I used 16 oz cups and put about 6 oz per cup and reused the A cup over and over again. After the first couple batches I didn’t even need the timer because it would have the initial expansion in the cups then it was time to pour.

Here are the final steps if others want to follow.

1. Make sure the boat, mix a and b, cups, and tools are above 80. In my case I had to lay packing blankets over a temp roof made out of the decks and sole in order to keep heat in it for 12 hours. I used a temp controlled space heater set to 85.

2. Buy about 50 16 oz cups per 2 gallon kit.

3. Don’t use mixing sticks, instead make yourself a paddle out of one of you off cuts of 1/4 plywood. Make is wedge shape so that it is easy to hold at the top but tapers down to 3/8 at the bottom. Mine was a little over a foot which was about right. You need the length, for mixing, scraping the sides, and getting out the extra after each dump.

4. Look for a common marking inside the cup use that as a fill reference point. One of my cups had a half way ring, and the other had a bar code marking. Put a shop light the the counter where you are mix to that the light shines through the cup to confirm your levels are exact.

5. By filling each cup to 6 oz of A and B, and combining you enough room to mix the foam vigorously. After you see the foam grow about 1/2 in the cup while stirring, dump it immediately. However time your first couple to get a understanding of the reaction. It starts to grow at 50 seconds.

6. When pouring try to pour it on the seems and have it expand out from there. If you just put it in the middle it doesn’t always grow the way you want it to. For example if you are filling a 36” x 12” rectangle. It will probably take 4 separate pour using the 12 oz batches. Pour the first batch on the seems where the floor meets the wall it will expand up and into the middle of the cavity. The next do the other long side dumping it in a line touching the stringer and floor. Repeat for the next 2 pours.

7. After the pour, using the the mixing stick, while holding the cup upside down at a 45 degree angle spin the cups on the stick to get out the remaining heel in the cup. It doesn’t all come out in the pour and if you don’t scrap the cup you will waste a bunch of foam.

8. Lastly, clean the edge of you stick on the cups you just poured. Make sure you get off as much foam as possible so you can reuse it.

9. Foam is sticky, have plenty of space in a garbage can for the cups after each pour. Have plenty of gloves on hand. Don’t try to clean drips until they harden. It just smeers them and makes it worse.

10. Wait 2 days before trimming access foam. Use a vibrating cut off saw to to trim over expanded foam, it works much better than sanding it, knifing it, or rasping it.

11. Seal cut foam with epoxy to reseal the foam.

I’m not saying this is the best process but it worked well for me.

Thanks
Cbuf.
Last edited by Cbuf on Wed Oct 09, 2019 11:38 am, edited 1 time in total.
See my LM 18 build here.
viewtopic.php?t=64254
See my LM 18 Questions here.
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Finished Boat Here
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Jeff
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Re: Expanding foam calculations

Post by Jeff »

Cbuf, Good receipt for buoyancy foam!! Thank you for putting this on the Forum!!! Jeff

Fred in Wisc
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Re: Expanding foam calculations

Post by Fred in Wisc »

Indeed, detailed step by step instructions like that are super valuable to the next guy with the same question. Especially with the very specific helpful hints you included. Next time around, I'm using that "start at the corners" thing- I usually start in the middle and end up filling cracks around the outside of the compartment.

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