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Old Feb 06, 2016, 01:54 PM
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Is hot glue considered "proper" for EPP construction? Fell apart in the air

Figured the scratchbuilders would know best about glues and foam.

I bought a large EPP plane that I did not build (Bormatec Maja). Everything seemed put together fine in initial flights.

I was practicing landing it and doing some go-arounds a few days ago when upon slapping the throttle open, I lost the entire motor mount/ESC/vertical stab section to motor torque. The prop made a mess of the back of the plane, the rudder ripped off and went through the prop, and the front half obviously landed fairly hard. Luckily only a few feet off the ground. Not impressed.

Upon recovering the pieces, I found the coroplast strip to which this section was mounted had been hot glued to the fuselage. Further inspection of other damage showed me that hot glue was actually used as the primary adhesive on the whole plane.

This seems awful weak to me, is this normal in EPP construction or did the guy I bought it off just do a crappy job? I've put all the pieces together with gorilla glue, including the mount that caused the failure. This plane is my first time working with EPP.
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Old Feb 06, 2016, 02:12 PM
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R’, There’s hot glue , and Hot glue properly applied .... You obviously got one that was not done right (" the guy I bought it off just do a crappy job?” ) ... Hot Glue has a “critical working Temperature , and all according to each type of foam used .... It’s like “welding” .... the glue has to be HOT enough to just barely melt the top surface of the foams being joined or else it will not hold ( penetration) ... Too Hot and the foam melts ... We always test on scrap first ...
Ps, A light dimmer works great on a Hot Glue Gun as they’re only about 40 watts .. that’s the same as a 40 watt light bulb ... Having an IR hand held thermometer really helps pinpoint the exact heat you need ...
Hot Glue is often blamed as a poor glue , but it is also the one most used improperly ... We’ve been using hot glue for all manner of foam models for many years now with excellent results ... Used Right , it’s joined Forever , and the foam always fails before the glue joint !!!

Gorilla Glue will work fine , bit messy , don’t get it on your fingers ...
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Old Feb 06, 2016, 04:56 PM
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Thanks, I use hot glue for a lot of things myself. Love it for hinges, servo mounting and gap filling but have never trusted it for structural work. I have a dual temp gun but have never thought of using a dimmer+IR gun! Good idea.

I just thought: does hot glue embrittle or detach at low flying temps? This incident occurred at about -10C.

As for Gorilla, I always have a damp paper towel and a dry one handy. Wipe squeeze out on foam with the damp one. Wipe plastic tools, spatulas etc on dry. Wipe glued fingers on damp to cause glue to set up a bit and detach from fingers, then rub firmly on dry.

Can you tell I spent a lot of time with my old trainer foamy and gorilla glue
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Old Feb 06, 2016, 05:07 PM
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-10C ... We’ve not had any trouble with the hot glue when COLD , but we have experienced it cold enough for the foam to shatter ... (Depron) We normally don’t fly when it’s very cold, but can assure if you use the Hi heat glue it will not melt on a summers day out in the sun , and we NEVER leave our planes in a closed up car , which can easily reach 140F ... and possibly might cause hot glue to go runny ....

JMHO , I believe your problem to be caused by insufficient glue heat when it was first applied .... You can check , by looking closely at the bond (ex bond) and see if it was “ Melted” into the foam or not .... That should leave a small furrow in the foam ...
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Old Feb 06, 2016, 05:26 PM
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Hot glue and Depron addict reporting in!

Like any adhesive it can be very durable and reliable when used correctly, and next to useless when not. It's a shame you got bitten by the previous user, it could be they were assembling the tail and did a substandard job and uttered the infamous line 'eh that'll do...' thinking they'd get away with it.

It is interesting the coroplast structure failed. I've found HG is very good at joining foam to itself or to wood, but plastic to foam has caused me trouble in the past. The problem seems to be that while the glue can penetrate into the cells of foam or wood, plastic is different and it's easy to end up with the glue sticking quite well to the foam and not at all to the plastic. I've found you need to apply very firm pressure when dealing with plastic to make sure it's sticking properly, and even then I'm not sure how much I'd trust it for a critical plastic part.

UHU POR has never let me down although it's very tedious to use as it has a 24 hour or so drying time before it's fully set. Often if there's a hot glue joint I'm nervous about I try to cover it with reinforced packing tape as a backup. Tape is very good in my experience at preventing complete failure if a joint lets go and it also lets you see for sure where the problem is.
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Old Feb 06, 2016, 06:10 PM
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The exact failure was a coroplast - coroplast bond, though upon the crash several other coroplast - EPP bonds failed on impact. These were the coroplast "clamps" that hold together the clamshell fuselage, for lack of a better word.

The glue was intact as were both pieces of coroplast. No visible melting of the coroplast. It's just like it came unstuck. Unfortunately there is no way to tape this particular joint as the coroplast holes are used to pin the motor assembly on, but hopefully the Gorilla will do the job. I will look into UHU POR.

On the coroplast - EPP bonds the glue was stuck firmly to the coroplast and detached from the EPP. No tear-out of the EPP at all. There was a very small groove in the foam such that the piece will register back where it came from - handy for when the Gorilla was applied.
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Old Feb 06, 2016, 10:03 PM
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Coroplast and maybe EPP, as well, may need to be roughened with sandpaper before bonding. There may be a mold release on Coroplast that could keep the hot glue from sticking. A clean solvent may take this off, but sandpaper might be better. EPP has a shiny side which might need roughing up too. Polypropylene sheet, think a blow molded solvent bottle, won't stick to much of anything unless it's mechanically roughened.
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Old Feb 07, 2016, 06:22 AM
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If this is the model you have it was probably assembled by the original manufacturer and not the hobbyist you purchased it from. The web site shows they sell fully assembled airframes.

http://bormatec.com/shop/product_inf...&products_id=3

As long as the power package used was within the allowable limits there may be some warranty coverage.

Regards,

Connie
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Old Feb 07, 2016, 02:08 PM
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Interesting that it may have been the manufacturer, though seeing how it has been through a few decidedly "non-warranty" crashes that could have loosened things up, I probably wouldn't bother them all the way over in Germany.

I got it all back together in flying condition, with little visible damage except for the rudder.
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Old Feb 08, 2016, 06:49 PM
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Hot glue can not be considered a good method of attaching coroplast together. Particularly in the cold. I used to build recumbent bicycle fairings and found the only positive method was to use small cable ties and stitch the panels together.

I have heard that you can first heat the area to be attached with a propane torch very lightly then use superglue. I have not tried it. The other method might be to use glue such as "Goop" or "Aquaseal".
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Old Feb 09, 2016, 05:17 AM
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Coroplast is polypropylene. It is made from closed chains at atomic level. That is why it is used as vandal proof because nothing wants to attach to it (especially paint) as the surface is somehow "sealed". If you want to glue it a mentioned flame torch method is breaking the chains with high heat. Once the surface is activated you can glue it. The mentioned silicon based glues like goop also work as basically they never "dry"
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