2 Part Expanding Urethane Foam

I want to try using urethane foam to glue fins into the fin can area. Fins, motor tube and body tube are all fiberglass. I have two questions #1 do I want to use 2 lb or 4 lb density? #2 what is the usual shelf life of the unmixed part A and B. I'm trying to figure out what quantity to purchase and keep on hand.

Bonus question, how do you figure out how much to mix up. In other words how much does this stuff expand or should I just mix a small test batch and see what happens?

Thanks, Kurt

I use the PML foam. Everything you want to know is here:

https://publicmissiles.com/product/adhesives

+1 PML, easy to use.
How much to buy? If you plan on building a bunch of 4 or 6 inch kits then I would consider getting a gallon. There are several resources that sell the part A & B 1 Gallon mix. I bought a gallon and will typically take out 16 oz at a time of each. I find it easier for me to control it usage that way. What I don’t use then remains in the two separate plastic containers. I like to use, used “dry” 16oz water bottles Please note: If you put a drop or two of water in the mix, it will easily triple or better in volume and cut down on lb density strength, so be sure to pour into a dry cup when mixing. I am not sure on shelf life, but I would imagine if you make an effort to keep the cap on when not in use then you should get at least a few years of shelf life. I also like to use a scale and solo cups when mixing. I put my cup on the scale, Zero it out, pour say 2oz of part A, then pour 2oz of part B then mix. It starts to expand slowly in roughly 20 seconds. After about a minute expansion its well on its way. The stuff is thermogenic, it will get hot to the touch, use disposable gloves and so be careful if you use it in something like a "plastic" nose cone. Within 2 minutes of the pour you will want to run cool water over the nose cone to keep it from bowing out. You will definitely want to make some practice pours. On your second practice pour try adding 3 drops of water and watch the difference and rate in expansion. And make sure your glue is cured before you pour, it can cause parts to shift around!

"Shelf life of unopened product approx. 1 year from date on label."

I've found that after a year it's a little more unpredictable. I have about half set on the shelf that's almost 2 years old. One of my goals this winter is to test it. If it's still good, I'll use it. I would suggest that you start off with a smaller kit of foam to get the hang of it. A little goes a LONG way, especially if you haven't done your math right.

Thanks everyone for your suggestions. I found some 4 pound density on line at a great price/quart bottles. I might use it all within a year. If not I'll store it in a cool dark dry place and hope for the best.

Uncrichie said:

Bonus question, how do you figure out how much to mix up. In other words how much does this stuff expand or should I just mix a small test batch and see what happens?

Thanks, Kurt

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test a bit. i found when filling fin cans and nose cones the best results come when i dont try and fill all in one pour. allow first pour to expand and cure then top off.
something a little extra i do when it comes to fin cans:
drill some holes in the fins where they will be inside the BT. allows foam to flow through and help lock them in.

2 lb. density is what most people use. That’s the least dense. It means that 2 lbs. of mixture will expand to fill a cubic foot, or said a different way, a cubic foot of cured foam will weigh 2 lbs. Obviously 4 lb. foam is twice as dense, meaning it expands half as much. That also means your rocket’s aft end will gain weight, exactly the wrong place to add weight. I think 2 lb. expands about 11 times.
Foam is a good adhesive for some purposes, and can make a large heavy rocket a little stiffer and heavier, but has very little strength in tension or shear. It’s no substitute for epoxy, especially in rockets where the diameter of the body tube is not much larger than the motor mount tube or where the fin tabs are small. Regardless, you will want to have fillets sealing the gaps between the fins and the body tube fin slots before pouring in foam or you will have an unholy mess.
Also, cured foam can only be removed by scraping or sanding and no matter how careful you are some will end up where you don’t want it. While it’s incurred acetone will remove it. Acetone is very flammable and if splashed in your eye will permanently cloud your corneas.
If you do use it, do it in small quantities and allow it to foam in place and cure fully before adding more.

It also heats up as it cures so doing a bit at a time controls the heat.

Thanks for the additional information. Particularly adding some holes in the fins inside the can, Tomsteve.

Steve, reference to 2 vs 4 pound. I was thinking 4 pound would have more adhesive strength being it would have less bubble content? I know there is the weight difference but didn't think it would be a deal breaker. I'll have to rethink that one. If one isn't stronger than the other I certainly will go with 2 lb density.

Reference to strength I was under the impression that it was just as strong as using epoxy inside the fin can area? I'm referencing "Wildmans" suggestion to using it instead of epoxy? (I hope I didn't read that article the wrong way) Please let me know if I shouldn't use it for that purpose? If thats the case I won't use it at all and save some money.

Kurt

I’ve never used it having said that I definitely wouldn’t use it in place of glue/epoxy. Even more so after having read some of the responses in this thread.

Uncrichie said:

Thanks for the additional information. Particularly adding some holes in the fins inside the can, Tomsteve.

Steve, reference to 2 vs 4 pound. I was thinking 4 pound would have more adhesive strength being it would have less bubble content? I know there is the weight difference but didn't think it would be a deal breaker. I'll have to rethink that one. If one isn't stronger than the other I certainly will go with 2 lb density.

Reference to strength I was under the impression that it was just as strong as using epoxy inside the fin can area? I'm referencing "Wildmans" suggestion to using it instead of epoxy? (I hope I didn't read that article the wrong way) Please let me know if I shouldn't use it for that purpose? If thats the case I won't use it at all and save some money.

Kurt

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Kurt,
You’re instincts are right; 4 lb. foam will be considerably stronger than 2 lb. Urethane is available up to 8 lb. The problem is that it’s adding weight in a bad place relative to stability.
Gorilla Glue is also urethane and I used it once to build a rocket as well. That rocket is still together. I think it actually does a better job of bonding to PML’s Quantum Tube than epoxy. It seemed to etch the surface and bond to it.
Is foam as strong as epoxy? It’s sort of apples vs. oranges. You would never totally embed a fin tab in epoxy. Nor would you build a fillet of urethane foam. If the area of the fin tab embedded in urethane is sufficient the total strength of the foam holding the fin in place can be equivalent to the bond strength of the much smaller internal fillets. Also, foam filling in a fin can helps fin tabs resist flexing, but do nothing to prevent the outside of the fin from flexing. You should look up the material properties of both to compare them.
I think foam has its place. You just have to be aware of how it works, which is different from epoxy.

Thanks Steve, I have a better understanding after your last post. This particular rocket has very little space between the motor tube and and body tube so I will stick with epoxy and give the urethane foam a try when I have a model with a more cavernous fin can.

Kurt

Kurt, when the space between the circumference of the motor mount and the air frame is limited I will do the following. Head to a tractor supply (tractorsupply.com) and pick up a 10cc syringe (body only) and then head to Walmart and grab some fish tank air hose. I cut off a piece of hose at long as one of the fins, then attach it to the syringe end. Take the plunger out of the syringe and fill up the syringe with epoxy and push down a controlled bead of epoxy. Sometimes I will warm up the epoxy first for better flow. I take a solo cup, mix Zap 30 Min Zpoxy in it, then in another solo cup pour in some hot water. Take the solo cup with the epoxy and set in the cup with the hot water. After it warms up, take the cup our, wipe the bottom of the cup dry, then pour into the end of the syringe. Put the plunger back in and push down the epoxy.

Mustang, thanks for the tip. I have a TS not too far away.

Kurt

Mustang67 said:

Kurt, when the space between the circumference of the motor mount and the air frame is limited I will do the following. Head to a tractor supply (tractorsupply.com) and pick up a 10cc syringe (body only) and then head to Walmart and grab some fish tank air hose. I cut off a piece of hose at long as one of the fins, then attach it to the syringe end. Take the plunger out of the syringe and fill up the syringe with epoxy and push down a controlled bead of epoxy. Sometimes I will warm up the epoxy first for better flow. I take a solo cup, mix Zap 30 Min Zpoxy in it, then in another solo cup pour in some hot water. Take the solo cup with the epoxy and set in the cup with the hot water. After it warms up, take the cup our, wipe the bottom of the cup dry, then pour into the end of the syringe. Put the plunger back in and push down the epoxy.

Click to expand...

That’s a great technique. Rather than putting the epoxy in a Solo cup (of any color) try putting it in an uncoated paper hot drink cup. If the epoxy does cook off it won’t melt through the bottom and ruin your day.