Can foam vs. two part foam

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Feb 8, 2004
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I am looking to fill my fin can with some foam. I was wondering if it is ok to use the can foam that you find at Home Depot or Lowes? How does that compare to the two part foam you can get from PML? Anyone tried the can foam and what was your results?


I personally have no experience, but I believe the PML 2 part foam is much better. As it is 2 part, it does not require oxygen to cure (which is very scarce in a closed off region, like inbetween your fins and motor tube) while the canned foam will not cure completley. You may get a hard shell with the canned stuff, but it is not likley cured at the front of the motor tube.
I have not used the two-part (expensive) stuff, but have used insulating foam-in-a-can for lots of projects. I even landscaped the layout for my slot-car track with that stuff!

Tips and recommendations: Don't get in a hurry with this stuff.

Place the foam into the back corners of your cavity, and let it expand toward an exit port (probably the same hole you reached through to squirt in the foam). Otherwise you will end up with un-foamed voids that you cannot reach later. Fill only about 1/4 to 1/3 of the cavity with 'fresh' foam at first and let that much sit for a day or two (it don't take much!). The foam will expand significantly and that initial amount may end up being plenty. If not you can add a squirt later.

Leave vent holes. If you try to squirt this foam into a cavity and then seal it tight before it is cured, it can burst through a cardboard body tube or push a well-glued centering ring off the end of an assembly. I think these foams need open air circulation anyway to cure so you may not get a good result in a sealed compartment anyway. Plan for vent holes in a location that can be cleaned up afterwards.

Let the excess expanded foam spill out the vent holes but try to catch it on a sheet of paper or something so it doesn't fall back onto your work. If excess foam does somehow get onto the wrong places, LEAVE IT ALONE. It is easier to break off & clean off AFTER it has cured than to try to wipe it off while still gooey (boy, is that stuff sticky!).

Clean off cured excess foam with careful use of X-acto blades and with sandpaper.

Before using foam in contact with plastics check for chemical compatibility with scrap pieces. Your brand of foam may 'eat' some types of plastic, especially if the plastic is in the form of thin sheet material.

Plan ahead as to how you are going to seal the partly-used foam can (the stuff costs too much to buy a $4 can for only a two-second squirt) or else have a bunch of 'needs foam'-type projects lined up on your work bench ready to get squirted until the can runs empty.

Hey, solrules-
What part of SW Wisconsin? I still have family in Davenport, De Witt, and Madison, and drive from Dubuque to Madison about once a year. Are you anywhere close to the highway?
I have use foam in a can and it worked amazingly well, everyone told me it wouldnt work but it is great, in my experience. I foamed the fincan of my V2 with it, I poured it in and let it dry and then I put the top CR on and I havent had a problem and the fins are amazingly stiff and I have had no problems with them.

I also did a nosecone I drilled a small hole in the top then stuck the nozzle through and filled it up, I didnt have a problem, though, because it was a closed area I thought I might. A long time later I cut that nose and dug out the foam to put an altimeter bay in and it was definantly dry and hard to get out, I broke a pair of scissors trying to pry it out.
I have used both 2-part from Giant Leap and the canned stuff. I have never had a problem with the 2-part, and have had mixed success with the canned. Like solrules said, the canned stuff requires moisture to react, whereas the 2-part does not. The typical problem is that the canned foam may continue reacting for some time as more moisture and/or heat is encountered. I have also found that the 2-part is easier to apply as the canned tends to start reacting right away. powderburner's suggestions are all good ones if you decide to go that way. I'd also suggest that you experiment on something other than your expensive, prize rocket kit.

Having read *many* threads on this, I would say my experience is about typical.
I ahven't used either, but I've read in a number of places that 2 part is better, as it doesn't require oxygen to cure while the canned does. There might be spaces in the canned foam that never cure because they don't get any air.
Originally posted by rstaff3
. . . the canned tends to start reacting right away.

He is right, the canned stuff pretty much starts expanding the instant it comes out of the can. Nothing explosive or anything like that, but it does start 'growing' quickly.

In my experience, this stuff is 99.99 percent finished after about one day. It is cured all the way through. I have never had a case where it was still gooey in the middle, but others may have somehow run into this.

I have put large quantities of this stuff into single applications without having any problem. I filled a four inch NC with the stuff (in stages) and had absolutely no problems. If you are unsure, find a small box in the trash and squirt it----wait to see how the stuff expands and behaves, do an autopsy the next day, and see for yourself.

I've foamed most of my fin cans with PML 2-part foam. It works very nicely. I've never used the canned stuff cause of the horror stories of it having a tendency to expand in your enclosed fincan on a hot day. (never my experience only what I've read)

The 2-part is a little pricey but I still have some left after five rockets so it goes a long way. The expansion rate is phenominal, 25 times its liquid volume. I also do several seperate pours because I can never seem to gauge just how much I'll need. I've tested the bonding strength of multiple pours and found that the foam bonds almost seamless to itself. Be sure to cover what you don't want the foam to get on because it is very hard to remove.

My plan is to use 2-part foam as primary bonding agent in place of epoxy for internal fillets on a future project. The foam bonds the fincan components into one very solid assembly.
What I suspect is really going on here is something like this:

The two-part specialty stuff probably works better (whatever that means) but only comes in a fairly big size for a fairly big price. The hardware-store stuff works OK, is also quite sticky when 'wet' and cures to a hard set, but you can get it $4-5 at a time without waiting for shipping.

I would guess that there are a lot of people who have used one and not the other (like me) who don't know much about the other. As in many things, a body of 'knowledge' (read: rumors and old wive's tales) builds up about the unknown item.

The foam-in-a-can has worked just fine for me in many different kinds of uses, including model rocketry. I have never had a batch remain soft or un-cured in the middle. The stuff is 90% finished in a couple hours and completely cured the next day. The biggest problem I have with the stuff is that once you begin using a can it is pretty hard to store a partly-used can-----best to try to use the whole thing in one go.

I used a few cans of the stuff once to help a friend fill in some storm damage on his roof. When the roofers got on the job, it took them about two hours to tear out the foam (with much muttering about the idiot who put it in there).

The two-part stuff is probably very very good, and I encourage you to try that too if you get a chance.
Thanks for all the insight!

I think I will try to experiment with the can foam on a couple of scrap pieces of body tubes and see if I like it. I just wanted something that was easy to go out and buy at the local Home Depot or Wal Mart. I hate to wait on parts to come in the mail. However, I do not want to screw up my rocket. I think the can foam will do for what I am building if I apply it in layers and don’t get to carried away. It is going to be used to fill the fin can in a mid power rocket that at the most will see a G64.

For my next scratch build I will place a order for some two part foam
Looks like you just volunteered to become our TRF resident expert in comparative foam applications.

Welcome to TRF!
I think some of the 1-part foams will absorb water over time. they were unsuitable for marine applications so we always used 2 part foams. but my quick check on the web shows most modern 1 part foams advertise "closed cell, very low water absorbtion".

still if you care about weight then maybe 2 part would be better. but I guess if you care about weight then you would be building out of carbon fiber or S-glass and not foaming the fin cans.
I've used both 1-part and 2-part foam (even two types of 1-part foam--Great Stuff and DAP). I tested both on nosecone fills and fin cans. My advice--DO NOT USE 1-part foam. It will deteriorate over time and/or never fully cure. This happened on two designs. Made a mess and had to be flushed out.

2-part foam is fast and easy. It cures immediately and fills small spaces much better (a liquid, not sticky foam on application). I believe it also has better heat characteristics after curing.

You'll pay $14.99 for PML foam at Magnum or the stuff sold at Giant Leap but its worth it. And really the stuff goes a long way as it has a huge expansion ratio.
If dixon has had bad luck with the one-part stuff, then this may be the first specific instance we have documented.

OTOH, I have used one-part foam dozens of times and never had it stay gooey or deteriorate. In fact, I have a 20-yr-old application in my garage right now that has not broken down.

I still say, try 'em both, see for yourself what the pros and cons are
Wish I had the pictures...

I think it specifically occurs in closed cavities such as a nosecone or fin casing after the aft centering ring is installed. First bad experience came on a nosecone on an upgraded Estes Eliminator. I applied the foam a day or two after epoxying sinkers for nosecone weighting. First day it stood up fine. By the second day I noticed a rattle--one sinker came loose. This sometimes happens and you just reglue it. But I thought it strange that it was moving and hitting the nosecone sides with the foam installed. I took a pencil and punched through the hardened foam at the open top end (aft end) of the nosecone. Inside was a gooey mess.

Later I read a detailed account of the Aurora development ("Goddess of the Sky" from LDRS coverage on Discovery Channel) and guess what, they had experienced the same thing with 2-part foam in one of their mock ups. I'll try to find the article and post the specific section of the report.
Found the reference...

It is in the Dec '03 issue of Extreme Rocketry starting on the bottom of page 20. The Aurora team's experience was as follows:

"A note about foam filling: When we built the rocket, we also built an additional quick and dirty nosecone for testing, so we wouldn't scratch up our baby during our tests. In the top of it we wanted a foam cushion, so instead of using the PML pourable foam we'd normally use, we shot in the canned foam as you would find at the hardware stores. It was quick and easy. We ended up with a nice protruding oval nose for a cushion. Several days later, we noticed that it began to shrink. This slowly continued for two weeks, shrinking about 20%. It was a good test of the fallacy of using such products for the interior of rockets, or even for sealing the windows, walls, etc. of a house."
I would guess a couple of factors may come into play.
I have used 1 part foam, Great Stuff to be presise with great results and no problems whatsoever.
It may be the brand used and maybe even the average climate where you are using it.
Humidity, atmosperic pressures, or maybe just the way I stick my tounge out the side of my mouth when I am putting the foam in place?
I did finally break down and buy some PML foam I will be using in the future (simply because it can be used in small batches easily)