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In the past it has been established that medium viscosity CA (cyanoacrylate) is an appropriate glue for this event. However, I am uncertain of the best style of application. Is it better to put glue on both pieces and firmly push them together for a few seconds, or place both pieces together and 'wick' the glue into the joint?

From my experiences with Bridge, it doesn't seem that medium CA wicks very well, so you'd probably want to put it on one piece and stick the two pieces together. Thin CA will wick into the joint, but it's a little harder to deal with.

Since you're using that kind of CA, I know that you will have enough time after putting the glue on your piece to then place it on the one you're connecting it to. What you call 'wicking' using medium CA won't assure that the glue has seeped all into the correct places, nor as accurately, without using excess. Placing glue on both pieces even seems excessive to me; you'll end up using more than you need.

I've tried both methods for both thin and medium, i found wicking with medium to be an absolute no-no in most cases.

what about zap a gap glue I read on wiki to use it

Paradox21 wrote:Is it better to put glue on both pieces and firmly push them together for a few seconds, or place both pieces together and 'wick' the glue into the joint?

Here's a few of my observations in working with CA glues.

Surface area of a glue joint is one factor that determines the joint's strength. I'm not a fan of "wicking" the glue into the joints as there is no way to insure the glue penetrates the entire joint. If the glue doesn't penetrate the entire joint, it will be weaker

CA glue uses the moisture content in the wood to harden/cure. A thin layer of cured glue starts to form immediately when the glue is applied to the wood. Once the glue has hardened, it is somewhat impervious to moisture. If you put the glue on both pieces to be joined, the moisture cannot penetrate either layer, preventing the glue between the cured layers from hardening. It can then take quite some time (minutes and in worst case scenario, hours) for the glue to cure. I have seen pieces of wood that were clamped together overnight come apart the next day when the clamps were removed. The glue between them was still liquid. The best method is to apply the glue to one of the pieces and then press the two pieces together as quickly and tightly as possible. The smaller the gap between the pieces, the faster the glue will cure.

Gluing end-grain joints can be a problem. Due to the porous nature of the end-grain wood, the glue wicks out of the joint into the wood cells, leaving a dry joint with no glue to hold it together. One solution is to coat the end of one of the pieces of wood with glue, remove any excess (immediately), let the glue cure for a few seconds, apply more glue and then fit the joint together tightly. This seals the ends of the wood cells and prevents the second application of glue from being wicked out of the joint. If you are gluing end-grain to end-grain, you will encounter the problem mentioned in the paragraph above.

Here's a story about my experience with CA glue. We used the "thin" stuff in 1-liter wash bottles. One winter someone took a bottle of glue outside on a job and sat it down on a work bench when they returned. I Noticed it setting there and decided to put it away. What I didn't notice was, as the glue warmed, it expanded and was forced out of the wash bottle, completely covering the bottle with a thin layer of glue which ran down onto the bench. I grabbed the bottle firmly and the entire surface of my hand was instantly bonded to the bottle (which was firmly glued to the work bench). Every time I tried to remove my hand or get the bottle off the bench, another shot of the glue was expelled and wicked between my hand and the bottle, attaching the two together even more firmly. Finally I managed to wrench the bottle free, removing a large circle of paint from the bench in the process. I spent the next few hours running warm water over the bottle and my hand to get free (warm water softens the skin so that only a very thin layer comes off with the glue). To this day, I do not like the thin CA glue.

A number of years ago I wrote a long response on glues for Wright Stuff. It got cleaned up and posted on the WS resources page and migrated to the Helicopter Duration page this year.

In reality, it applies to ALL the SO building events. They all emphasize maximum strength with minimum weight. In summary, effective glue use is more about proper design for and use of your chosen glue than the glue itself. Here's where you can find it: http://soinc.org/sites/default/files/uploaded_files/glueweight.pdf

In ref your your question on Zap a Gap, it is just a brand name for cyanoacrylate glues. They sell all viscosities. Thin requires one approach, medium another. You can be successful with any of them with proper approach. Each has pluses and minuses.

Jeff Anderson
Livonia, MI

Edited by fleet130: fixed link

fleet130 wrote:

Paradox21 wrote:Is it better to put glue on both pieces and firmly push them together for a few seconds, or place both pieces together and 'wick' the glue into the joint?

Here's a few of my observations in working with CA glues.

Surface area of a glue joint is one factor that determines the joint's strength. I'm not a fan of "wicking" the glue into the joints as there is no way to insure the glue penetrates the entire joint. If the glue doesn't penetrate the entire joint, it will be weaker

CA glue uses the moisture content in the wood to harden/cure. A thin layer of cured glue starts to form immediately when the glue is applied to the wood. Once the glue has hardened, it is somewhat impervious to moisture. If you put the glue on both pieces to be joined, the moisture cannot penetrate either layer, preventing the glue between the cured layers from hardening. It can then take quite some time (minutes and in worst case scenario, hours) for the glue to cure. I have seen pieces of wood that were clamped together overnight come apart the next day when the clamps were removed. The glue between them was still liquid. The best method is to apply the glue to one of the pieces and then press the two pieces together as quickly and tightly as possible. The smaller the gap between the pieces, the faster the glue will cure.

Gluing end-grain joints can be a problem. Due to the porous nature of the end-grain wood, the glue wicks out of the joint into the wood cells, leaving a dry joint with no glue to hold it together. One solution is to coat the end of one of the pieces of wood with glue, remove any excess (immediately), let the glue cure for a few seconds, apply more glue and then fit the joint together tightly. This seals the ends of the wood cells and prevents the second application of glue from being wicked out of the joint. If you are gluing end-grain to end-grain, you will encounter the problem mentioned in the paragraph above.

Here's a story about my experience with CA glue. We used the "thin" stuff in 1-liter wash bottles. One winter someone took a bottle of glue outside on a job and sat it down on a work bench when they returned. I Noticed it setting there and decided to put it away. What I didn't notice was, as the glue warmed, it expanded and was forced out of the wash bottle, completely covering the bottle with a thin layer of glue which ran down onto the bench. I grabbed the bottle firmly and the entire surface of my hand was instantly bonded to the bottle (which was firmly glued to the work bench). Every time I tried to remove my hand or get the bottle off the bench, another shot of the glue was expelled and wicked between my hand and the bottle, attaching the two together even more firmly. Finally I managed to wrench the bottle free, removing a large circle of paint from the bench in the process. I spent the next few hours running warm water over the bottle and my hand to get free (warm water softens the skin so that only a very thin layer comes off with the glue). To this day, I do not like the thin CA glue.

And nor do i, for a very similar reason.

Way back when our team used mostly CA but also some Ambroid/Sigment to build the most efficient bridge at Nationals. CA is strong and fast but it has no solvent so it can be heavy (it does not lose weight while curing). If you can use a solvent type glue (Sigment/Ambroid, etc.) then do so, but it is impractical for every joint in a structure that has a lot of joints. I have never had the problem of slow curing or joints coming apart when using CA, but I use the accelerator if there is a large bondline (although you probably have a design issue if you end up with a large area of bonding). Accelerator is actually supposed to make the bond stronger. If you go to Locktite's website (they make a gazillion different glues for industry) they often give the strength of the bond with the use of accelerator because it is supposed to be used that way. The problem with accelerator for some CAs is that it can make the bond so fast that you don't have time to get the parts properly aligned. If you use accelerator with some thin CAs the reaction is so fast and exothermic that the glue will actually boil (not good for strength).

In our winning structures we found that glue typically amounted to about 10 - 15% of the mass of the structure, but keep in mind that works out to less than 0.5 gram of glue (or about 0.5cc of glue). That is a tiny amount of glue. A careless builder can easily use several grams of glue without even having big blobs all over the place.

Would you suggest buying a precision applicator (those long thin tubes)?