Sandwich question

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Sandwich question

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Ngood	N Ngood posted 4 Years Ago HOT Topic Details
N Group: Forum Members Posts: 3, Visits: 20	Hello, i’m new to composites and i am making a kiteboard. I bought some pvc foam from easycomposites and have glued two sheets together a 20mm sheet and a 3mm sheet to give my required thickness. I used gorilla glue and applied it all over in a zig zag pattern. The bond is good apart from the edges where there are some bits that are lifting as they didnt have glue. I plan to vacuum laminate with carbon cloth and then apply a epoxy coat to finish. I am hoping that because i’ve got good bonding overall and because its the same material sandwiched together and i’m laminating around the edges it wont delaminate later. What do you guys think? should i do something to remedy or just crack on? thanks! Edited 4 Years Ago by Ngood
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Chris Rogers	C Chris Rogers posted 4 Years Ago HOT Post Details
C Group: Forum Members Posts: 120, Visits: 933	I don't mean to be that way... but how do you know the bond is good in the middle? Gorilla glue is great for this but it foams and presses back really hard. It will even fight back a vacuum bag. The best way to do it is to apply it all over one side of the joint in a very thin squeegeed coat and then clamp REALLY hard - like between two sheets of ply and in a vacuum bag. If you really want it to to foam you can (lightly) mist the non-glue side with water. I'm not sure what to do here - it will probably be fine but it is hard to be sure. Sometimes if your rub something over the foam you can hear voids inside but this is hard... if you hear any voidy spots you can drill a small hole through. It might be a good idea to drill small 1.5mm-ish holes through the whole thing every 50mm or so in a grid - so epoxy can get in there if need-be when you bag the skins. Even with some voids, you may as well make the board - squirt some epoxy in the gaps when you lay up and it will help. Are you doing it all at once (hard) or one side at a time? Keeping it flat can be an issue so lots of people bag it down to a flat (or shaped if you want any rocker) surface to keep it the right shape and then come back and do the edges later. Good luck - and be sure to show us all some pictures of the process!
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Ngood	N Ngood posted 4 Years Ago HOT Post Details
N Group: Forum Members Posts: 3, Visits: 20	+x Chris Rogers - 9/4/2020 8:12:14 PM I don't mean to be that way... but how do you know the bond is good in the middle? Gorilla glue is great for this but it foams and presses back really hard. It will even fight back a vacuum bag. The best way to do it is to apply it all over one side of the joint in a very thin squeegeed coat and then clamp REALLY hard - like between two sheets of ply and in a vacuum bag. If you really want it to to foam you can (lightly) mist the non-glue side with water. I'm not sure what to do here - it will probably be fine but it is hard to be sure. Sometimes if your rub something over the foam you can hear voids inside but this is hard... if you hear any voidy spots you can drill a small hole through. It might be a good idea to drill small 1.5mm-ish holes through the whole thing every 50mm or so in a grid - so epoxy can get in there if need-be when you bag the skins. Even with some voids, you may as well make the board - squirt some epoxy in the gaps when you lay up and it will help. Are you doing it all at once (hard) or one side at a time? Keeping it flat can be an issue so lots of people bag it down to a flat (or shaped if you want any rocker) surface to keep it the right shape and then come back and do the edges later. Good luck - and be sure to show us all some pictures of the process! Hi Chris, thanks for the reply. Yes youre right i dont know if the rest of it is well bonded. Like you say might as well carry on and cross my fingers! I’ll share progress as i go. Heres another photo. Cheers
	Reply Like 3
Lester Populaire	L Lester Populaire posted 4 Years Ago HOT Post Details
L Group: Forum Members Posts: 311, Visits: 13K	+x Ngood - 9/5/2020 7:19:16 PM +x Chris Rogers - 9/4/2020 8:12:14 PM I don't mean to be that way... but how do you know the bond is good in the middle? Gorilla glue is great for this but it foams and presses back really hard. It will even fight back a vacuum bag. The best way to do it is to apply it all over one side of the joint in a very thin squeegeed coat and then clamp REALLY hard - like between two sheets of ply and in a vacuum bag. If you really want it to to foam you can (lightly) mist the non-glue side with water. I'm not sure what to do here - it will probably be fine but it is hard to be sure. Sometimes if your rub something over the foam you can hear voids inside but this is hard... if you hear any voidy spots you can drill a small hole through. It might be a good idea to drill small 1.5mm-ish holes through the whole thing every 50mm or so in a grid - so epoxy can get in there if need-be when you bag the skins. Even with some voids, you may as well make the board - squirt some epoxy in the gaps when you lay up and it will help. Are you doing it all at once (hard) or one side at a time? Keeping it flat can be an issue so lots of people bag it down to a flat (or shaped if you want any rocker) surface to keep it the right shape and then come back and do the edges later. Good luck - and be sure to show us all some pictures of the process! Hi Chris, thanks for the reply. Yes youre right i dont know if the rest of it is well bonded. Like you say might as well carry on and cross my fingers! I’ll share progress as i go. Heres another photo. Cheers I honestly wouldn't risk it. The core will mainly work in shear which means you will have a lot of stress on a horizontal glue joint. Do you really need 23 mm of thickness? Seems super thick for a carbon sandwich panel (but i don't know much about foil boards).
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Hanaldo	Hanaldo posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 2.5K, Visits: 28K	There's nothing wrong with bonding core sections together to create a custom thickness core, but getting it void free is critical - any voids will create a massive weak spot under shear stress. Normally you would just coat both pieces of foam with laminating epoxy, and vac bag them together. This is very reliable and easy. I do feel Gorilla Glue is going to cause you problems here. Perhaps still worth giving it a go, as maybe there won't be enough shear stress in its application that it will fail, but on the other hand - the foam is the cheap part of a sandwich laminate. Perhaps worth starting again before putting too much more into it?
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Lester Populaire	L Lester Populaire posted 4 Years Ago HOT Post Details
L Group: Forum Members Posts: 311, Visits: 13K	+x Hanaldo - 9/6/2020 12:45:19 AM There's nothing wrong with bonding core sections together to create a custom thickness core, but getting it void free is critical - any voids will create a massive weak spot under shear stress. Normally you would just coat both pieces of foam with laminating epoxy, and vac bag them together. This is very reliable and easy. I do feel Gorilla Glue is going to cause you problems here. Perhaps still worth giving it a go, as maybe there won't be enough shear stress in its application that it will fail, but on the other hand - the foam is the cheap part of a sandwich laminate. Perhaps worth starting again before putting too much more into it? Maybe just get rid of the 3mm layer with a router and glue on another layer. And when you already glue on a 3mm layer anyway you could think about a layer of carbon or glass between the foam layers and form a micro sandwich that has a high bending stiffness and you could therefore maybe get away with a lighter foam for the base layer. Probably not gonna make a big difference as your board doesn't have as much volume as a wave board tho.
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Ngood	N Ngood posted 4 Years Ago HOT Post Details
N Group: Forum Members Posts: 3, Visits: 20	Thanks guys. Unfortunately i have gone a bit further today and laminated the bottom of the board Before reading your replies (thats the side that doesnt have the 3mm foam). I’ll see tomorrow how it looks and then make a decision. i’m not clear how to go about removing 3mm with a router when its all over? How would you do that? Maybe i could use an electronic plane? Thanks for the replies Edited 4 Years Ago by Ngood
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Rosta Spicl	Rosta Spicl posted 4 Years Ago HOT Post Details
Group: Forum Members Posts: 54, Visits: 250	From my point of view, Lester's idea is the best. Cut out the 3mm layer and bond it again properly. If the core isn't precisely bonded together, could starts inner delamination under the stress... For foam cores I use RG 5min epoxy or laminating epoxy filled by microballons to paste consistency and squeezeed on sufrace, vacuuming necessary for large surface cores just you have. Try cut it out manually, carefully using knife etc...
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