I use a lot of different types of foam for all kinds of projects. Of course, this means that I also have an equally diverse array of adhesives for foam. My workshop has an entire shelf full of epoxies, cyanoacrylates (CA, a.k.a super glue), polyurethane glues, hot glues, aliphatics, you name it. I've gradually sorted out my favorite applications for each type of sticky stuff.
Since all of my foam-bonding needs have been covered, I've been resistant to experiment with anything new, until recently. A unique repair job spurred me to try Beacon Adhesives' Foam-Tac, a contact cement intended for use on foam. It provided exactly what I needed, a strong, yet flexible bond. None of my other glues would have worked in this situation. Consequently, my glue collection just expanded.
Molded foam RC airplanes often use what is called a "live" hinge on the control surfaces. It's basically just an area of thin foam that can flex freely. Live hinges are convenient, but not always very durable. In fact, one of my recent model airplane acquisitions had shipping damage that completely ripped apart the entire rudder hinge. This particular model is made of Expanded Polyolefin (EPO) foam.
My normal process for repairing this kind of damage would have involved several steps. First, I would have trimmed back the jagged parts of the ripped foam with an X-Acto knife and/or sandpaper. The next step would be to reattach the rudder using mechanical hinges. This involves cutting aligned slits in both foam parts and gluing the hinges into place with CA. Lastly, I would find a decent color match in my paint stash and touch-up the cut/sanded areas of foam. None of these steps are difficult, but it all takes time and effort.
Several of my RC buddies have suggested Foam-Tac for these types of repairs in the past. I'm glad I finally listened. The repair process is much faster.
To prepare for using the Foam-Tac method, I verified that the hinge could be realigned along the ripped edge. The two parts mated together well. Following the instructions on the bottle of Foam-Tac, I applied a thin bead of glue to both parts. I then pressed the parts together and immediately pulled them back apart slightly. Numerous strings of glue bridged the gap between the two parts. After about five seconds of separation, I combined the parts again.
I could already tell that the glue was getting very sticky. So, I quickly smeared any blobs of glue on both sides of the hinge using a dry paper towel. For all intents and purposes, the job was done at this point. Total elapsed time to join the parts was about 1 minute. No sandpaper or touch-up paint was required. It's not even obvious that the hinge was repaired. I set the parts aside for a couple of hours to let the glue fully cure.
The attribute that allows Foam-Tac to work in this application is that it remains flexible when it is cured. Using a hard glue such as epoxy or CA would have resulted in a brittle and fragile hinge. When I examined the Foam-Tac-repaired rudder hinge, it did not seem to have any higher resistance to movement than other live hinges on the same model. At the same time, the rudder is very solidly attached. I would have assumed that those two goals are mutually exclusive. This experience has shown me otherwise.
Following my initial positive use of Foam-Tac, I experimented with other applications. My primary goal was to determine if the glue's solvent was incompatible with any of my usual types of foam. I glued together pieces of extruded polystyrene in varying densities. I also tried expanded polystyrene and other foam scraps that I don't even know the genesis of. There were no instances of the Foam-Tac dissolving any of these materials.
With each of my test samples, I laminated two pieces of the foam together. There was no flexibility of the glue joint in this configuration…at least not that I could tell. This suggests that Foam-Tac could also be used for rigid structures in addition to intentionally-flexible joints.
One of the problems that I often run into when working with foam is that the adhesive is harder than the foam itself. This is especially true with epoxy and CA. When I sand the completed part, the foam surrounding the glue erodes faster than the glue. Creating a smooth surface can be a real challenge.
I sanded a couple of my Foam-Tac test samples to see what effects the glue joint would have. I admit that it wasn't a very comprehensive evaluation. I basically used 150-grit paper to round off edges along the glue joints. Overall, the results were encouraging. The glue joint appears to lose material at the same rate as the foam. There were a few areas where the glue balled up when sanded. But we're talking about tiny balls of glue…well within my tolerance level.
More to Come
Obviously, I'm still feeling out where Foam-Tac fits into my glue lineup. It's definitely the top contender for flexible joints. Well, the only contender, actually. It may or may not beat out some of my old favorites for other applications.
I don't know if there are any glues under different names that are equivalent to Foam-Tac. On the flip side, I discovered a couple of other products from Beacon Adhesives that I hope to experiment with soon. Next on my list is Foam Finish, which appears to be a surface coating that can be used alone or with fiberglass cloth. More to come on that. You'll also see more of the repaired model very soon.
If you have favorite adhesives or techniques for working with foam, please share them in the comments section.
Terry is a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.