Bob Smith 15-Minute Epoxy? 15 my arse... - RC Groups
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Oct 23, 2017, 05:45 PM
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PerryCodes's Avatar
Discussion

Bob Smith 15-Minute Epoxy? 15 my arse...


So I'll explain what happened and then bow to the great minds of the modelling world in hopes of help! Lots of help!

I'm assembling an E-flite J-3 Cub 450 ARF. Up to this point I've only been using CA, but the step I just murdered called for 15-minute epoxy (Bob Smith in my case). I'm combining the two wing halves with a center section and two balsa joiners. The instructions have me brushing the joiners with epoxy, using a popsicle stick to place epoxy inside the joiner pockets located in both the wing and center section - then taping the parts together while the epoxy sets up.

Well... I was about 5 minutes into the process of brushing and coating the pockets when I noticed the epoxy starting to get stringy. I actually had to have people clear the room because I started to get so pissed that I was yelling at them. Needless to say - this turned into a f*&#ing mess.

What in the world happened? I mixed 1/4 fluid oz or 2 drams according to the small mixing cups. 1:1 ratio. Is this amount too small? Did I mix them too long? How much time should I expect the epoxy to remain workable (able to be brushed on)? Wow - if I'm having this much trouble with an ARF, I should probably shelve the idea of doing this Seniorita kit I've pulled out!!
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Oct 23, 2017, 05:50 PM
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PerryCodes's Avatar
An auxiliary question: Is there ever any harm in using 30-Minute Epoxy when the instructions call for 15?

Regards ~ Perry
Oct 23, 2017, 07:50 PM
Registered User
The problem is that when using a mixing cup, is the epoxy starts to warm up and as it warms up it kicks off from the heat and just keeps getting warmer and warmer cutting it's working time. You can increase the working time by spreading the epoxy in a thin layer on some scrap wood after mixing it.

R
Oct 23, 2017, 09:01 PM
Registered User
What I do to keep mixed epoxy from curing too quickly is set the cup of epoxy in a larger cup with some cold water in the bottom. The water keeps the epoxy cool and useable for a longer period of time.
Oct 24, 2017, 08:30 AM
circle flyer
slow and low's Avatar
Bad advice from E-flite........ I use 15 minute only on small jobs and as you suggested, 30 minute on the larger jobs. If I only had 15 minute, I make three smaller jobs out of the one.

Mark this up as one of hundreds of little lessons you will learn in this hobby, some are more costly than others.

Larry
Oct 24, 2017, 10:54 AM
Arrowhead
Epoxy loves heat. It generates lots of it as it cures, and using it in a hot environment will shorten the curing time. Composite parts manufacturers actually bake their products in ovens. Doing so not only speeds up production, but heat curing strengthens the epoxy.

As others suggested, you can increase the working time by keeping the epoxy cool. Also, once you have the parts assembled (and clamped if appropriate), set the whole thing some place hot. In the summer Iíll use the back of my Terrain as a ďpoor manís curing ovenĒ.

One last thing about working with epoxy: wear nitrile gloves! That stuff is nasty.
Oct 25, 2017, 11:50 AM
Crash tests done dirt cheap!
Krumpel's Avatar
I use 30 minute for everything and even then if I mix too much at a time it will heat up and cure way too quickly. I find the quicker curing epoxys create joints that are more brittle than the slower cures.

Just my $ 0.02 worth. YMMV
Oct 25, 2017, 01:05 PM
Arrowhead
Quote:
Originally Posted by Krumpel
I use 30 minute for everything and even then if I mix too much at a time it will heat up and cure way too quickly. I find the quicker curing epoxys create joints that are more brittle than the slower cures.

Just my $ 0.02 worth. YMMV
Thatís a good point. The faster curing epoxies are great for low-stress joints, but longer cure like 30 minute is best for durable, long-lasting bonds.
Oct 29, 2017, 01:31 PM
Registered User
mixing in small plastic or aluminum tray so expoxy in thin layer.dont over mix dont go over two minutes then get the large fast to apply areas, hit first.try not to take more than 4 min to apply,
then join wing.pretty much a repeat of the how to do that whith 15 min epoxy.i would still use small tray and 30 min instead
Last edited by mdugas; Oct 29, 2017 at 01:49 PM.


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