ADHESIVES

Hold your hobbies together!

There are many different adhesives that you can buy today to assemble a kit or finish the assembly of an ARF. Without a doubt cyanoacrylate (CA) is the most widely used glue when building a balsa kit. CA is available in four different thickness, they are:

* Thin – This cures in a matter of seconds.

* Medium – This cures in 15-20 seconds allowing you to position parts before it sets up.
* Thick – This cures in a longer period then medium. Usually used for gap filling.
* Gel – This CA has the longest curing time of all of the CAs. It is used for special cases.

There are a few drawbacks to using CA. Number one the fumes that it emits can be very irratating. Some people are allergic to the fumes and must work in a well ventilated area. An alternative to using regular CA, if it really bothers you, is to use oderless CA. The oderless CA does not emit any noticable fumes, but it is a lot more expensive then regular CA. Another drawback to CA is that it will glue almost anything to anything else, including gluing your fingers to each other.

Many Mondays I would go to work and still be pealing pieces of CA from my hands from work that I did the day before. One other drawback to CA, is that it has a shelf life of about 4-6 months. You can extend the shelf life by keeping the CA in a refrigerator. Some manufacturers actually date stamp the CA so that you know that you are getting a fresh batch. As CA ages, it becomes thicker and thicker until it is useless. Finally, it is not recommended to use regular CA to glue on clear plastic parts such as canopies. The fumes from the CA can permanently fog the clear parts.

The two most widely used thickness of CA are thin and medium. Thin is used for most balsa to balsa joints, it sets up almost instantly. I use medium mostly for sheeting and rib caps. With medium CA, you can apply a bead of glue to what you want to bond together then put the pieces in place before it has had a chance to set up. Medium and thick CA can be used for gap filling (more on this later). If you need to speed up the curing process of medium or thick CA, you can use an accelerator that is available at most hobby shops. If you use an accelerator, be very careful on how you use it. In the process of speeding the cure time, the combination of the CA and the accelerator produces a lot of heat. If you have any uncured CA on a body part and the accelerator comes in contact with it you could end up burning yourself.

If you are using thin CA, I would highly recommend that you buy some applicator tips as shown in the image to the left. These tips allow you to accurately position and control the amount of glue that you are applying. Generally speaking, the lighter an airplane is, the better it will fly. This even applies to the amount of glue that you use. The tips also act as an extension that helps to get into tight spaces.

It is not imperative to use CA. There many types of glues available. You can even use something as common as Elmer’s® wood glue (white glue) or an aliphatic resin. There are some advantages to using something like Elmer’s®. They include the following:

* It is readily available. You can buy it at most hardware stores.
* It is a lot cheaper then CA.
* You can use soap and water for clean up.
* It has a very long shelf life.
* It gives a strong bond when cured, similar to CA.
* When cured, it is sandable.

The one drawback to using a product like Elmer’s® is that it has a long cure time. You must keep the parts pinned or clamped in place for at least a half of an hour. Total bonding strength only occurs after several hours (like overnight) of curing time.

There will be times when you will need to attach parts using two part epoxy. I highly recommend that you use epoxy with a cure time of 30 minutes or longer. In many cases you will be using epoxy to bond pieces of hardwood. Hardwoods are not nearly as porous as balsa. If you use a fast setting epoxy to join two pieces, the epoxy will not have a chance to penatrate into the wood. I have had bad experiences with 5 minute epoxy. Sometimes the glue joint will fail rather then the wood itself.

Editors Note: I once belonged to a flying club that had a building marathon. The effort was to have a bunch of people build a balsa trainer in a weekend. One of the better builders realized that he had the epoxied in the firewall upside down and backwards. This was several hours after the epoxy had cured. I would have panicked. What he did, was to take a heat gun and heat the firewall until the epoxy melted. He then removed the firewall, added some additional epoxy and and put it in the right way.

He learned this tip from an experience he had flying one day. Someone else had a model that had a black cowl. It was a really hot afternoon and the model had been sitting in direct sunlight for several hours. When the person who owned the model took off he was having real difficulty with it. The combination of the black cowl and the heat from the sun had melted the epoxy holding the firewall in place. The only thing that kept the engine attached to the model was the throttle linkage.

There may be times when you will want to “thin out” the two part epoxy. You can add a little acetone or isopropyl rubbing alcohol. I use this method when I am applying fiberglass cloth. I use a disposable brush to apply the “thinned out” epoxy, then when finished, I scrape off excessive epoxy using something with consistency of a business card. You can use either of the above mentioned solvents to clean up any excessive epoxy. If you thin out the epoxy, the cure time will be slightly longer.

Hint: Before applying fiberglass cloth to a model, I like to “tack” it into place. To accomplish this I use a product from 3M. It is called Super 77® spray adhesive. You merely spray the section of the model that you are going to cover with fiberglass, put the cloth into place, and rub it smooth. You have a couple of minutes to do this before the adhesive sets up. I use the same product to make my own sticky back sandpaper to be used on sanding blocks and T bars.
I mentioned earlier that it is not recommended to use regular CA for bonding clear plastic parts because of a potential fogging problem. There are special CAs that will not fog, or you may be okay using a CA formulated for bonding plastic. What I like to use is a product called Zap-A-Dap-A-Goo® (no kidding). It does not bond instantly like CA, but is easy to use and gives a very good bond.

Earlier I mentioned that medium and thick CAs can be used as gap fillers. Gaps in your work should be the exception rather then the rule. Extra care should be taken to be sure that you have joints that are butted right up against each other. An example of a good joint is shown in the image to the left. With a good joint it will only require a small amount of glue to get a strong and reliable bond. The image on the right illustrates what your joints should not look like. You can fill the gap with medium or thick CA, but it will not be as strong as the image on the left. Due to building contraints there will be times that you will have gaps to be filled, but not very often.

One final note in this section. While it may not be considered an adhesive, I recommend that you use Locktite® (sparingly) to help secure nuts and bolts on your model. With your engine pounding away at thousands of RPM the model vibrates a lot. For items that will not be taken apart oftern, or ever, I always use both a lock washer and Locktite® to make sure the item will not vibrate loose. An example of this would be mounting your engine mount to your firewall. Locktite® can be obtained at most automotive stores. There are different formulations of Locktite®, they vary on the size of the threads of the nuts and bolts that you are going to secure. What you want to use is the formulation that is for fine threads.

Other great site for adhesive info.

This to That (Glue Advice = which to use for each application)***