Nice idea, Matt!

Something I will certainly keep in mind as I build the new layout. I'm an extruded-foam adherent when it comes to layout construction. So this is a timely idea!

spiffy!

Very Nice Matt!   I had never thought of using Great Stuff as foam adhesive until I saw Ken Pattersen use it in one of his What's neat episode.

Great Idea making a key-way like that.

excellent!

I tried to butt-join rigid foam for a small expansion piece.  Eventually i did - your method would have saved much time and annoyance.

Looking at time, cost and

Looking at time, cost and rigidity, why did you choose this method over a splice plate?  I've had good luck using another piece of foam for a plate, but I'd be willing to try this.

How do you...

keep the pieces from spreading apart as the foam expands?        John

Simplicity

beachbum, 

After rubbing my chin over that question for a few minutes, it comes down to a couple of things

• This seemed to be a simpler approach than using a splice plate since it required no support while it cured.
• I was aiming for something approximating a single piece
• While not at all a functional reason, the approach intrigued me - I wanted to see if it would work.

It was also very quick - maybe twenty minutes from plugging the hot wire cutter in to capping the Great Stuff - and used materials on hand.  I think it was a win on all three of your points.   

I have no doubt that a splice plate would work well, and I can always use it if this ends up failing. 

Spreading and Adhesiveness...

John, 

I thought this would be more of a problem than it turned out to be - the two pieces moved maybe 1/16" by the time the foam cured.  The sheets are eight feet apiece, so there was apparently enough inertia that the vent holes were an easier route for the expansion to resolve itself.  If I were to do it again, I would probably weight the two sheets and use fewer vent holes to ensure that the Great Stuff made it to all the nooks and crannies.  The less resistance this stuff has, the bigger the voids become - a problem for both strength in this case, and insulation for it's intended use.

Thanks for the question!

@Bill B

After half a dozen times of waiting for my skin to "grow" Great Stuff off my fingers, the light finally came on.

I've used it as an adhesive (intentionally) at least one other time.  I replaced my tank water heater with a tankless about fifteen years ago, and used Great Stuff to seal the old heater's flue/chimney with a 4" PVC cap.  It's still up there!

Paul,

Give it a try and let me know how it works.

Heat Proof

As an add on comment, I found when using Great Stuff to form the center ditch and some parts of my ballast shoulders that the material is extremely heat-resistant.  

If you end up using this approach in an area that you will later be carving down with such a knife, you will have to switch to a utility knife (or similar) where your Great Stuff "splice" is.

Interesting approach

Thats an ingenious solution but I wonder why not try yellow glue or even white glue, you'd be surprised at the "adhesiveness" of the stuff once dry. You already need to wait approx a day which would be same for the glue too. I wonder if this might not be a more cost effective solution too?

a simple way of stopping the spread is a couple of two inch masking tape prices across the joint too, but you weighting idea would work too.

love the idea though and great to see the lateral thoughts of people solving problems

Wish you had posted that 5 years ago!

It would have solved a few separations that occurred on my all-foam surface. 

BTW, white and yellow glue do not do well.  I skipped white glue because of using water-based ballast and scenery methods.  The yellow glue adheres, but easily pulls off with a micro-layer of pink foam attached. 

I used panel adhesive, and that worked "reasonably" well.  Some movement over time caused separations here and there that I think your method would have prevented.

Strength

Hi S. Brown,

I didn't consider any type of traditional adhesive since I was trying to avoid an edge to edge butt joint; there's simply not enough surface area to make a reliably strong joint. 

If I'd gone the splice plate route, I would have used a traditional adhesive, though I would probably have used something other than yellow glue, since it is geared for porous materials. 

"since I was trying to avoid

      I've edge glued foam for small boat cores using toothpicks to hold the edges in alignment. Of course they get glassed on both sides so the joint doesn't have to be that strong. I only use foam for non structural layout purposes as wooden framing is easier and more fun for me to work with , but your spray foam looks like a very good way to join thick foam sheets  ....DaveB

Non-Structural Foam

I'm on the same page on both counts.  The foam pictured here will support scenery, but not track.  In the next couple of months, I will be adding industry tracks through the pink wastelands, using my typical spline subroadbed. To accomplish that, I'll be building the spline on top of the foam, then cutting it away where the track will go. The broad expanse of pink that is there now will then become thin strips instead.  Not a very efficient usage of the material, but having the flat space gave me the opportunity to transfer my track plan to a flat surface for evaluation. That process may make it into another blog post.

Joint Cross Section

Since my original post, I've cut a slot in the foam for the industrial running track's 2" spline, which crossed the butt joint described in this post.  This cut provided the opportunity to see what the Great Stuff joint looked like in cross section.  

The top photo is a cross section through the seam, the bottom one through the seam and one of the expansion holes.  I'd been curious if the foam filled the slot completely (it did) and whether or not big voids had formed (one medium sized void in the vent, where it's less important).  Nice.

Model RR Geology...

cool!!  

Seriously, that is a very clever solution.

Safety caution re polyurethanes/CYANIDE release

Hi Matt:  As a long-ago chemistry student, I had a surviving memory neuron light up when I read your mention of trying a hot-knife.  "Great Stuff" is a polyurethane spray foam.  These contain substantial amounts of isocyanates, which release hydrogen cyanide when burned.  You can see this information in the SDS section of the product information.  So it's not a good idea to use a hot-knife on it.  Sounds like that didn't work well anyway.

Jim W

If this was Facebook...

...I'd hit the Like button!

Tom H.

Hot Knife

Jim said:

""Great Stuff" is a polyurethane spray foam.  These contain substantial amounts of isocyanates, which release hydrogen cyanide when burned.  You can see this information in the SDS section of the product information.  So it's not a good idea to use a hot-knife on it.  Sounds like that didn't work well anyway."

Hi Jim, 

I didn't use the hot knife on the Great Stuff - at least this time.  I tried it a few months back to see if I could use it to shape ballast shoulders and found that it was more or less impervious to heat.  The cuts shown in the last photo were done the old fashioned way - with a utility knife.

I see similar concerns posted about rigid insulation foam, but don't recall the details, so your input is welcome there.  The hot knife worked perfectly on that.  In any case, I'll at least put my painting respirator on next time.

Thanks for the concern and advice!

Relief holes are Key! Great Information

I used 1" foam over top of 1/2" plywood for my shelf layout.  I had a cavity under the foam where the elevation changed.  I used the Great Stuff to fill the caviity but didn't leave enough relief holes in the top of the foam.  It did ooze out the side.  It ended up bowing the top of the foam.  I didn't even notice it until running the trains over it and could notice the roller coaster track that I had created.

I wish I had seen this sooner.