Optimal "Cure" time for Bridges

Aces
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Optimal "Cure" time for Bridges

Postby Aces » December 27th, 2014, 2:09 pm

Hi everyone,
So far, our team has broken 10 bridges in total, with a broad range of results. This is our first year doing the competition, so we are getting up a steep learning curve. We noticed that the bridges that "cured" for 48 hours generally performed better than the bridges that cured for only 24 hours. In your experience, how long do you wait to test the bridge after you complete gluing it? Is there an optimal waiting period based on experiments? As to the competition bridge, how much time in advance do you complete building it? We noticed that room temperature definitely changes weight, but not sure how it changes performance.

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Re: Optimal "Cure" time for Bridges

Postby iwonder » December 27th, 2014, 5:11 pm

It really depends on the glue you're using. Stock CA glue (superglue) seems to set in seconds but it takes about 24 hours in my experience to reach full strength. This really only matters if you have joints that are pushing the limit of the shear strength of the glue. It's possible, the base for boomilevers (since mine was ridiculously small) was a good experience for me. If I let is set for a day it was fine, anything less and it'd shear every time.

However, everything depends on humidity. CA sets because of humidity (i.e. theory says it sets faster in a humid environment than a dry one), and humidity is a huge factor in the weight of any balsa structure. (In fact what you're correlating to temperature is quite possibly a change in humidity instead). There's a bit of a curve to it, but typically a more moist structure will be less brittle or prone to just falling apart and crumbling, but might carry less load. A dry structure will be brittle and weak, and won't work well at all. From what I've noted the ideal is slightly more dry than typical (no numbers, sorry), sometimes I've actually blow dried structures before testing to minimize the water weight, without weakening it significantly.

I don't have a lot of experience with glues other than CA or a slow-cure version of CA I found. Urethane glues like Gorilla Glue are almost always too heavy to make it worth it and take forever to set. (IMHO, if you need to use gorilla glue on a joint, you need to redesign the joint)
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Re: Optimal "Cure" time for Bridges

Postby someusername » December 29th, 2014, 3:14 pm

iwonder wrote:
I don't have a lot of experience with glues other than CA or a slow-cure version of CA I found. Urethane glues like Gorilla Glue are almost always too heavy to make it worth it and take forever to set. (IMHO, if you need to use gorilla glue on a joint, you need to redesign the joint)


I have been using gorilla super glue for my bridges due to being able to use an accelerator to make it dry in seconds and keep strength. Are you suggesting that my glue is too heavy not good, because I am trying to loose a little bit of weight (current design always ends up at about 6 grams more or less)
simplicity is key...sometimes

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Re: Optimal "Cure" time for Bridges

Postby iwonder » December 29th, 2014, 3:43 pm

No, I meant the standard urethane gorilla glue, the stuff that expands into a white foamy looking solid. I'm pretty sure gorilla super glue is just the stock cyanoacrylate that most other superglued are... (Not sure, but most likely). Do you mean 6 grams in glue weight or 6 grams total, because if you're holding a sizable load with 6 grams you're on the right track.

Also, try not using the accelerator. It should dry in a few seconds anyways, and accelerators really effect the strength of the bond. Basically, it'll probably be more consistent without it.
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Re: Optimal "Cure" time for Bridges

Postby retired1 » December 29th, 2014, 6:31 pm

Most operations sheets for CA glue say that it takes 24 hours to achieve full strength. Thin sets super fast and gel is a lot slower. Somewhere I read that the accelerator very slightly weakens the glue joint. Some CA has to be a bit better than some others.
I have found that old opened superglue is not as good as fresher material.
I do not like CA because you can not glue CA to previous CA with much strength.
It does weigh a bit more than Ducco for the same strength joint.
Most builders either use too much or too little glue and all joints are not of the same quality. If it takes more than a thin film, the joint needs to be recut.
I have close to a dozen different glue in the glue box. Ambroid is my favorite, but it is out of production.

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Re: Optimal "Cure" time for Bridges

Postby iwonder » December 29th, 2014, 6:48 pm

Mechanically speaking, CA is a stronger glue, but I've only ever seen one structure (a series of booms I built a few years ago) that closed in remotely on the mechanical strengths of the glue (the wood is typically much weaker). There are a lot of things you can do to get light joints overall, see this paper...

http://soinc.org/sites/default/files/up ... weight.pdf

He makes a point about not using the 'gap filling' or thicker CA glues, I found when I did long laminations (45cm for a boom) I needed to use a slightly thicker CA because of the cure time, I just couldn't like up the parts in time. But that's about the only time I've deviated from it.

Cements like Ducco and Ambroid do have benefits, namely the ability to move joints after the glue is set, which I never found to be something I needed in structure events (also means I never really cared that I couldn't attach CA to CA). Flight events are a whole other story, I wouldn't do a flight event with CA personally.

In my understanding the accelerators speed up the process of polymerization of the acrylate monomer, but because the bonds don't form in the same way the resulting structure is more brittle than when set normally. (The other complaint is that the spray bottles you get it in are horrible ways of dispensing anything for a structure, a lack any amount of precision).
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Re: Optimal "Cure" time for Bridges

Postby azuritemalachite » December 29th, 2014, 7:27 pm

Aces wrote:Hi everyone,
So far, our team has broken 10 bridges in total, with a broad range of results. This is our first year doing the competition, so we are getting up a steep learning curve. We noticed that the bridges that "cured" for 48 hours generally performed better than the bridges that cured for only 24 hours. In your experience, how long do you wait to test the bridge after you complete gluing it? Is there an optimal waiting period based on experiments? As to the competition bridge, how much time in advance do you complete building it? We noticed that room temperature definitely changes weight, but not sure how it changes performance.

A) It really depends on glue and how much you put on it.
B) A good tip is to bake it. It may seem weird to some people but it is just taking the moisture out of the wood/glue. Last year before Nats our team baked it but there was delay in between testing and when it was baked. Also make sure you don't bake it too much otherwise it might actually be weaker or burnt.
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Re: Optimal "Cure" time for Bridges

Postby PatrickMcPherson » January 2nd, 2015, 4:13 pm

For the first year of boomilever I let my regional bridge sit for a week, and it did not do nearly as well as expected. I usually like to wait at least a full 24 hours. My booms last year used pretty low density wood, and CA gule (blue and purple), but every desinty type responds differently to dynamic humidty and temperature. So, I don't like to let my devices sit too long because they warp, but of course not too short either. I would absolutely advise waiting the full 24 hours though.


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